Fabricating designed fullerene nanostructures for functional

Transcription

Fabricating designed fullerene
nanostructures for functional
electronic devices
Christian Larsen
Department of Physics
Umeå 2014
© Christian Larsen
ISBN: 978-91-7601-187-4
Cover art: An almost PCBM transistor
Electronic version available at http://umu.diva-portal.org/
Printed by: Print & Media
Umeå, Sweden 2014
"Shiny"
Abstract
A long-term goal within the field of organic electronics has been to develop
flexible and functional devices, which can be processed and patterned with
low-cost and energy-efficient solution-based methods. This thesis presents a
number of functional paths towards the attainment of this goal via the
development and demonstration of novel fabrication and patterning
methods involving the important organic-semiconductor family termed
fullerenes.
Fullerenes are soccer-shaped small molecules, with two often-employed
examples being the symmetric C60 molecule and its more soluble derivative
[6,6]-phenyl-C61-butyric acid methyl ester (PCBM). We show that PCBM can
be photochemically transformed into a dimeric state in a bi-excited reaction
process, and that the exposed material features a significantly reduced
solubility in common solvents as well as an effectively retained electron
mobility. This attractive combination of material properties allows for a
direct and resist-free lithographic patterning of electronic PCBM films down
to a smallest feature size of 1 µm, using a simple and scalable two-step
process constituting light exposure and solution development. In a further
development, it was shown that the two-step method was useful also in the
area-selective transformation of fullerene/conjugated-polymer blend films,
as demonstrated through the realization of a functional complementary logic
circuit comprising a 5-stage ring oscillator.
In another project, we have synthesized highly flexible, single-crystal C60
nanorods with a solution-based self-assembly process termed liquid-liquid
interfacial precipitation. The 1-dimensional nanorods can be deposited from
their synthesis solution and employed as the active material in field-effect
transistor devices. Here, it was revealed that the as-fabricated nanorods can
feature an impressive electron mobility of 1.0 cm2 V-1 s-1, which is on par with
the performance of a work horse in the transistor field, viz. vacuumdeposited amorphous Si. We further demonstrated that the processability of
the nanorods can be improved by a tuned light-exposure treatment, during
which the nanorod shell is polymerized while the high-mobility interior bulk
is left intact. This has the desired consequence that stabile nanorod
dispersions can be prepared in a wide range of solvents, and we anticipate
that functional electronic devices based on solution-processable nanorods
can be realized in a near future.
i
ii
Sammanfattning
Ett stående mål för utvecklingen av organisk elektronik är att kunna
realisera flexibla och funktionella elektroniska komponenter genom
kostnads- och energieffektiva tillverkningsmetoder. Denna avhandling
presenterar möjliga vägar för att kunna uppfylla detta mål genom nya
tillverkningsprocesser baserade på en viktig grupp organiska halvledare,
fullerenerna.
Fullerener består av små fotbollsformade och kolbaserade molekyler där de,
inom den organiska elektroniken, mest använda varianterna är C60 och dess
lättlösliga derivat [6,6]-phenyl-C61-butyric acid methyl ester (PCBM). Vi
demonstrerar att PCBM kan transformeras till dimerer genom en fotokemisk
bi-exciterad reaktionsprocess genom enkel ljusexponering. Dimeriserad
PCBM visar en attraktiv kombination av egenskaper genom bibehållna
elektroniska egenskaper samt en minskad löslighet i vanliga lösningsmedel.
Dessa egenskaper möjliggör en direkt och resist-fri mönstringsprocess av det
aktiva lagret i organiska fälteffekttransistorer genom att inkludera en
tvåstegsprocess bestående av exponering samt framkallning av det aktiva
lagret. En mönsterupplösning på 1 µm demonstreras genom mönstring med
UV-ljus. Den enkla tvåstegsprocessen kan även användas för att skapa
funktionella komplementära kretsar vilket demonstreras genom tillverkning
av en 5-stegs ringoscillator.
I ett annat projekt tillverkar vi enkristallina, men flexibla, nanostavar av C 60
med hjälp av en uppskalningsbar lösningsmedelsbaserad metod. De 1dimensionella nanostavarna har deponerats och använts som det aktiva
materialet i fälteffekttransistorer. Vi visar att dessa nanostavar uppvisar en
imponerande elektronmobilitet på 1.0 cm2 V-1 s-1, vilket är jämförbart med
vakuum-deponerat amorft kisel som är den nuvarande arbetshästen för
kommersiella tunnfilmstransistorer. Vi demonstrerar att processningen av
nanostavar kan förenklas genom en ljusexponering som transformerar C 60 i
nanostavens skal till en polymerisk struktur, medan stavens kärna förblir
intakt. Denna transformation resulterar i att nanostavarna kan användas i
ett stort antal lösningsmedel, vilket i sin tur öppnar upp möjligheter för
storskalig och lösningsmedelsbaserad tillverkning av funktionella
elektroniska komponenter baserat på nanostavar av C60.
iii
List of Publications
I. Direct UV Patterning of Electronically Active Fullerene Films
Jia Wang, Christian Larsen, Thomas Wågberg and Ludvig Edman
Advanced Functional Materials, 2011. 21(19): p. 3723-3728.
http://dx.doi.org/10.1002/adfm.201100568
II. Complementary ring oscillator fabricated via direct laserexposure and solution-processing of a single-layer organic film
Christian Larsen, Jia Wang and Ludvig Edman
Thin Solid Films, 2012. 520(7): p. 3009-3012.
http://dx.doi.org/10.1016/j.tsf.2011.12.048
III. On the fabrication of crystalline C 60 nanorod transistors
from solution
Christian Larsen, Hamid Reza Barzegar, Florian Nitze, Thomas Wågberg
and Ludvig Edman
Nanotechnology, 2012. 23(34): p. 344015.
http://dx.doi.org/10.1088/0957-4484/23/34/344015
IV. Solution-Based Phototransformation of C60 Nanorods:
Towards Improved Electronic Devices
Hamid Reza Barzegar, Christian Larsen, Ludvig Edman and Thomas
Wågberg
Particle & Particle Systems Characterization, 2013. 30(8): p. 715-720.
http://dx.doi.org/10.1002/ppsc.201300016
V. Palladium nanocrystals supported on photo-transformed C60
nanorods: Effect of crystal morphology and electron mobility on
the electrocatalytic activity towards ethanol oxidation
Hamid Reza Barzegar, Guangzhi Hu, Christian Larsen, Xueen Jia, Ludvig
Edman and Thomas Wågberg
Carbon, 2014. 73(0): p. 34-40.
http://dx.doi.org/10.1016/j.carbon.2014.02.028
VI. An Arylene-Vinylene Based Donor-Acceptor-Donor Small
Molecule for the Donor Compound in High-Voltage Organic Solar
Cells
Javed Iqbal, Jenny Enevold, Christian Larsen, Jia Wang, Thomas
Wågberg, Bertil Eliasson and Ludvig Edman
Submitted
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Publications not included in this thesis:
Improving the Performance of Light-Emitting Electrochemical
Cells by Optical Design
Nikolai Kaihovirta, Christian Larsen and Ludvig Edman
ACS Applied Materials & Interfaces, 2014. 6(4): p. 2940-2947.
http://dx.doi.org/10.1021/am405530d
v
List of Acronyms
AFM
BHJ
CV
DCFL
DSC
EDP
EQE
ETL
fcc
GND
hcp
HOMO
HRTEM
HTL
IQE
ITO
LLIP
LUMO
MPP
NR
OFET
OLED
OPV
OSC
PCE
PDN
PDP
PNR
PUN
R2R
RFID
SAED
STEM
TCO
TEM
TGA
XRD
Atomic force microscopy
Bulk heterojunction
Cyclic voltammetry
Direct-coupled FET logic
Differential scanning calorimetry
Energy delay product
External quantum efficiency
Electron transport layer
Face-centered cubic
Ground
Hexagonal close-packed
Highest occupied molecular orbital
High-resolution transmission electron microscopy
Hole transport layer
Internal quantum efficiency
Indium tin oxide
Liquid-liquid interfacial precipitation
Lowest unoccupied molecular orbital
Maximum power point
Nanorod
Organic field-effect transistor
Organic light-emitting diode
Organic photovoltaic
Organic semiconductor
Power conversion efficiency
Pull-down network
Power delay product
Polymerized nanorod
Pull-up network
Roll-to-roll
Radio-frequency identification
Selective area electron diffraction
Scanning transmission electron microscopy
Transparent conductive oxide
Transmission electron microscopy
Thermal gravimetric analysis
X-ray diffraction
vi
Table of Contents
1. Introduction
1
2. Organic semiconductors
2
2.1. The fullerenes
4
3. Electronic devices
6
3.1. The field-effect transistor
7
3.1.1. The inverter
11
3.1.2. The ring oscillator
15
3.2. The photovoltaic cell
16
3.2.1. The bilayer structure and operation principles
17
3.2.2. The bulk heterojunction solar cell
20
3.2.3. The OPV performance
21
4. Fullerene photochemical transformation
24
4.1. Raman spectroscopy
24
4.2. Fullerene patterning by resist-free lithography
25
4.3. Patterning efficiency
27
4.4. The photochemical reaction mechanism
29
4.5. PCBM OFETs
31
4.6. Realizing complementary logic circuits with resist-free patterning
33
4.6.1. The inverter
34
36
5. Fullerene C60 nanorods
38
5.1. Synthesis
38
5.1.1. Polymerization
40
5.2. Characterization
41
5.2.1. As-grown
41
5.2.2. Annealing
43
44
5.3. The C60 nanorod transistor
46
5.3.1. Nanorod deposition
47
5.3.2. Transistor performance
49
6. Material design for improved photovoltaics
52
6.1. A small-molecule donor compound for OPVs
52
6.2. The C60 nanorod OPV
56
7. Conclusions
58
Acknowledgements
59
References
61
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viii
1. Introduction
The field of organic electronics includes the design, synthesis,
characterization and application of organic compounds with electronic
properties. These organic compounds are small molecules or polymers with a
conjugated, carbon based, backbone. Since carbon is an abundant material
on earth, these materials promise to be produced and recycled inexpensively
at a very large scale. The chemical tailoring possibilities of conjugated
materials also allow for synthesis of materials with a wide variety of
properties, making them suitable for both electronic applications (such as
transparent, large surface-area electrodes and transistors) and photonic
applications (such as light-emitting devices and solar cells) [1-4]. Conjugated
materials can also be tailored for improved processability in e.g. lowtemperature solution processes. This subsequently allows the use of
unconventional flexible substrates, such as plastic and paper, which are
compatible with roll-to-roll fabrication, allowing for high throughput device
fabrication [5-9]. Organic conjugated materials are already used in
commercial applications, with the most noteworthy being the organic lightemitting diode (OLED) displays. Possibilities for future applications include
intelligent packaging, flexible displays [10], radio-frequency identification
(RFID) tags [11], transparent or paintable solar cells [12] and light-emitting
wallpaper [5].
This thesis is structured such that the basics of organic semiconductors
(OSCs) are covered in chapter 2. In chapter 3 the device physics of organic
field-effect transistors (OFETs) and organic photovoltaic (OPV) cells are
disclosed. Chapter 4 describes the process of photochemical transformation
of fullerene molecules to create dimers/oligomers/polymers, and how this
transformation can be used to realize efficient electronic devices and
complementary logic circuits. Chapter 5 presents a solution-based method
for the synthesis of crystalline C60 nanorods, and describes how such
nanostructures can be practically implemented as the active material in
OFETs. Finally, chapter 6 presents an innovative solution to the detrimental
issue of fullerene aggregation in OPVs in the form of a morphology-locked
active material based on C60 nanorods.
1
2. Organic semiconductors
Figure 1. (a) The electron orbitals of a sp2 hybridized carbon atom, with the
three sp2 orbitals positioned at a 120° angle with respect to each other, and
the 2pz orbital positioned in a direction perpendicular to the plane spanned
by the sp2 orbitals. (b) The overlap of two 2pz orbitals on neighboring
carbon atoms in a conjugated molecule will give rise to π-orbitals (red), as
shown for ethene.
Organic conjugated materials comprise a molecular backbone of carbon
atoms connected through alternating single and double bonds in a planar
configuration. The planarity originates from the sp2 hybridization, where the
2s, 2px and 2py orbitals of the carbon atom hybridize into three degenerate
in-plane sp2 orbitals, positioned at a 120° angle with respect to each other.
The 2pz-orbital of the carbon atom is positioned in a perpendicular direction
to the plane spanned by the three sp2 orbitals, as shown in figure 1(a). Strong
covalent σ-bonds are formed between two neighboring sp2 hybridized carbon
atoms through the overlap of two neighboring sp2 orbitals. The two
neighboring pz orbitals will also overlap to give rise to a new π-orbital, which
contains delocalized π-electrons. These π-orbitals provide the rigidity of a
conjugated molecule, since they need to be broken in order to rotate the
molecule. The σ- and π-bonding of an ethene molecule (C2H4) is shown in
figure 1(b).
For a molecule in the ground state, the most energetic electron is positioned
in the highest occupied molecular orbital (HOMO). The next available
electron state is termed the lowest unoccupied molecular orbital (LUMO),
and the energetic difference between the LUMO and HOMO levels is termed
the energy gap (Eg). In long-chain conjugated molecules, the most energetic
2
π-electrons can delocalize over several repeat units, and the length of this
delocalization is called the conjugation length. By increasing the conjugation
length, the energy gap between the LUMO and HOMO levels will decrease in
a monotonic fashion, as shown schematically for an increasing number of
connected thiophene monomers in figure 2 [13]. During this process, the
LUMO+1, LUMO+2 levels, etc. can become so close in energy that it, in some
cases, make sense to term them a LUMO band or conduction band; the same
argumentation for the HOMO levels motivates the formation of a HOMO
band or valence band. As the ground-state configuration (at zero Kelvin) for
a conjugated compound is a filled HOMO band and an empty LUMO band,
these materials are appropriately classified as semiconductors.
Figure 2. Electronic structure as a function of number of thiophene rings.
At sufficiently long conjugation length, the discrete energy levels become
degenerate and form a band-like structure.
An organic semiconductor (OSC) can be reduced (oxidized) through the
addition of an electron (hole) into the LUMO (HOMO) band. Once injected,
the electron is free to move within the π-states, and this intra-molecular
transport is commonly facile [14]. An inter-molecular transfer step is
however necessary in order for the electrons to propagate beyond the
conjugation length, and this is a comparatively slow process. The ease with
which charge carriers move is quantified by the mobility (µ, unit: cm2 V-1 s-1),
which can be calculated as the drift velocity of the charge carrier (v, unit: cm
s-1) divided by the driving electric field (E, unit: V cm-1).
3
When OSC molecules are in close proximity of each other, as in a solid film,
the π-orbitals of different molecules can overlap. This overlap can result in a
more efficient inter-molecular charge transport, and molecular ordering,
through crystallization, can increase the molecular π-orbital overlap and the
mobility significantly [15, 16].
The conductivity (σ) of a semiconductor is given by the equation
  qnn  qpp
(1)
where n and µn are the concentration and mobility of the electrons and p and
µp are the concentration and mobility of holes.
2.1. The fullerenes
An exceptionally interesting type of organic semiconductor is the fullerene. A
fullerene comprises at least 20 carbon atoms arranged in hexagonal and
pentagonal rings (the smallest fullerene C20 only comprises pentagonal
rings), and it can form spherical, cylindrical, ellipsoidal and many other
shapes. The most common fullerene is C60, which is built up from 60 carbon
atoms in alternating hexagonal and pentagonal rings to form a shape similar
to a football, as shown in figure 3.
Figure 3. The C60 fullerene (left) with alternating hexagonal and
pentagonal rings of conjugated carbon. The shape can be directly
compared to a football (right).
C60 and several of its derivatives are excellent organic semiconductors, with
some rather unique features, such as high electron mobility and low LUMO
level. They have therefore been used extensively as the active material in
OFETs, and as the electron acceptor material in OPV devices. The symmetric
form of the spherical C60 molecule makes it prone to crystallization.
4
Moreover, the π-orbitals of C60 are positioned around the entire molecule
and inter-molecular π-orbital overlaps are accordingly possible in all
directions. This combination of properties is the origin to why C60 features
one of the highest values for the electron mobility among OSCs [17].
Additionally, different C60 molecules can be made to chemically connect in a
photo-chemical process, so that dimers/oligomers/polymers can be created
[18].
One drawback with C60 is that its solubility in organic solvents is limited,
making thermal evaporation the deposition method of choice. This
deposition method results in high quality films, but suffers from being very
time-consuming and expensive as compared to solution-based processes.
The solubility issue can however be resolved through chemical tailoring. By
adding a side chain to the C60 molecule to produce the derivative [6,6]phenyl-C61-butyric acid methyl ester (PCBM), the solubility is drastically
improved.
5
3. Electronic devices
Figure 4. The basic structure for electronic devices consisting of two
electrodes connected by a thin film of an organic semiconductor in a planar
(a) or sandwich (b) configuration.
Since their discovery, OSC materials have been introduced into a wide
variety of applications, notably transistors, solar cells and light-emitting
devices. Although the operational mechanisms differ, these devices share a
common feature in that they comprise a thin OSC film contacted by two
electrodes. The electrode configuration can be either planar (figure 4(a)),
with the current flowing along the substrate, or vertical or sandwich (figure
4(b)), with the current flowing in a direction perpendicular to the substrate.
A schematic energy level diagram for the constituent components of a 2electrode structure at open circuit is shown in figure 5(a). As intrinsic
organic semiconductors by definition are undoped, efficient charge injection
can be achieved by minimizing the injection barrier (qΦ) at the
electrode/OSC interface. For electron injection, this injection barrier
corresponds to the energy difference between the cathode work function (EC)
and the OSC LUMO level (ELUMO), and for hole injection it corresponds to the
mismatch between the anode work function and the OSC HOMO level
(EHOMO). In figures 5(a-c) the electrodes were chosen such that the left
electrode, the cathode, has a low work function, hence favoring electron
injection, while the right electrode, the anode, has a high work function that
favors hole injection.
6
Figure 5. Schematic energy level diagram of a 2-electrode device structure
at (a) open circuit, (b) short circuit, (c) during ambipolar hole and electron
injection, and (d) during unipolar electron injection.
The use of electrodes with different work functions introduces an asymmetry
into the device, which results in a built-in-field (Vbi) in the active material
under short-circuit conditions, as indicated in figure 5(b). By applying a
forward bias (VF) larger than the Vbi, the injected charges will be driven by
the electric field towards the opposing electrode, as seen in figure 5(c). If the
device is biased in the reverse direction, the injection of electrons and holes
will be severely limited by the large injection barriers. The described device
is thus a diode, which rectifies the current in the forward direction [19-21]. If
one instead uses the same material for both electrodes, it is possible to
achieve efficient injection of only one carrier type, and the operation of an
electron-only device is depicted in figure 5(d).
3.1. The field-effect transistor
The realization of the transistor and the integrated circuit sparked a
technical revolution, which has resulted in faster, smaller and more efficient
electronic devices. Following the demonstration of the metal oxide
semiconductor field-effect transistor (MOSFET), the efficiency of the
integrated circuits was further increased, eventually leading up to the high-
7
performance electronic tools and toys we have around us today. However,
the processing of the constituent inorganic semiconductor materials, most
commonly Si, is expensive, energy consuming, and executed at high
temperatures not compatible with flexible substrates. OSCs, on the other
hand, can due to their solubility and intrinsic flexibility offer a
complementary category of low-cost and flexible electronic devices, which is
not available for their inorganic counterparts.
OFETs are fabricated in a planar geometry, with the electrodes in contact
with the OSC termed source and drain. The OFET structure also comprises a
third gate electrode separated from the OSC and the source and drain by an
insulating dielectric layer [22-25]. The role of the gate is to capacitively
induce charges in the OSC channel next to the dielectric through the
application of a gate voltage. The OSC conductivity is thus controlled by the
gate voltage, as described by equation (1).
Figure 6. Schematic representation of the four different OFET geometries,
with the definition of the gate length, LG, and the drain width, WD, included
in the bottom-gate, bottom-contact OFET in (a).
There are four OFET geometries, which are distinguished by the position of
the active OSC layer and the electrodes, as schematically depicted in figure 6.
The bottom-gate (BG) geometry has the gate electrode located directly on the
substrate and below the active layer1, whereas the top-gate (TG) geometry
has the active material directly on the substrate with the gate on top.
1 The BG geometry is commonly used in research since doped Si with a thermally grown SiO 2 layer is a
substrate which simultaneously fulfills the role as both gate electrode and dielectric layer, making fabrication
procedures simple.
8
Furthermore, the source and drain electrodes can be either in top-contact
(TC) or in bottom-contact (BC) depending on whether they are deposited on
top or below the active material. The schematic figure of the BG-BC
geometry, shown in figure 6(a), also includes the definitions of the gate
length (LG) and the drain width (WD).
Figure 7. Schematic presenting the operation of an OFET (a) in the linear
region, (b) at the pinch-off point and (c) in the saturation region. The dark
red color indicates the charge accumulation in the channel. (d) Typical
output data with the different regions indicated.
The OFET operation can be separated into three regimes: the sub-threshold
region, the linear region and the saturation region. The threshold voltage
(VT) is defined as the gate-source voltage (VGS) at which all immobile trap
states in the OSC channel are filled. Hence, in the sub-threshold region,
when VGS < VT, effectively no mobile charge carriers exist in the OSC channel
and only leakage currents can be measured. When VGS > VT, mobile charge
carriers are induced in the channel and the current through the device is
greatly increased. In the linear regime, when VDS < VGS - VT, the potential in
the entire channel (from source to drain) is above VT, and the drain-source
9
current (IDS) is increasing linearly with increasing VDS, as shown in figure
7(a). When VDS = VGS - VT the channel potential at the drain electrode equals
VT, and this is the pinch-off point, as shown in figure 7(b). When VDS > VGS VT, the device operates in the saturation region, as shown in figure 7(c). In
this region the IDS is limited by the amount of charges injected into the
channel from the source electrode and is hence dependent on VGS - VT [26].
The IDS in the linear and saturation regimes can derived by the long-channel
approximation, and the corresponding analytical expressions are [27-29]:
I DS, l i n 
I DS, sa t 
 C GWD 
LG
 C GWD
2 LG
V2 
 VGS  VT VDS  DS  , VGS > VT ; VDS ≤ VGS - VT
2 

(2)
VGS  VT 2 , V
(3)
GS
> VT ; VDS ≥ VGS - VT
Here, CG is the gate capacitance, which depends on the permittivity and
thickness of the dielectric layer, and µ is the charge-carrier mobility. Note
that the mobile charge-carrier in the OFET channel can either be a hole or an
electron, and that the OFET accordingly is termed p-type or n-type. In some
cases, both n-type and p-type operation can be measured on the same OFET
device, and such OFETs are termed ambipolar.
Figure 8. (a) 3D representation of OFET characteristics, with the output
data indicated by black lines and the transfer data indicated by grey lines.
(b) The square root of IDS as a function of VGS, with which it is possible to
extract VT and the mobility, µ, through the relationship in equation (3).
Typical OFET characteristics are shown in the 3D plot in figure 8(a), with the
output data indicated by the black lines and the transfer data indicated by
10
the grey lines. There are a several ways to determine the charge carrier
mobility of the OSC with an OFET, but perhaps the most straightforward
method is to plot the square root of IDS as a function of VGS at saturation
conditions, i.e. at VDS > VGS - VT. By applying a linear fit (solid line in figure
8(b)) it is possible to determine VT as the voltage where the extrapolation of
the linear fit crosses the x-axis (i.e. when IDS = 0). With the aid of equation
(3) and information on the geometrical parameters CG, WD and LG, it is then
possible to also determine the charge carrier mobility [29].
3.1.1. The inverter
The inverter, which is designed to invert an incoming signal, is based on
transistors and is the simplest digital logic element in integrated circuits. It
is therefore used extensively to show that a new transistor technology is
viable for integration into digital integrated circuits. Several inverter designs
exist, and the most suitable for a given transistor technology is set by the
properties of the specific constituent transistors.
Figure 9. Three schematic alternatives to realize an inverter: (a) a resistive
PUN design, (b) a DCFL design and (c) the superior complementary design.
The simplest design is using resistors in a pull-up network (PUN), i.e. the
part of the circuit responsible for pulling the output high (VDD), and an ntype transistor in a pull-down network (PDN), i.e. the part of the circuit
responsible for pulling the output low (GND, ground), as show in figure 9(a).
A careful selection of RPUN is important in order to achieve fast and robust
operation. It is however notable that this design will always suffer from a
high supply current (IDD) at high input voltage (Vin), and consequently a low
output voltage (Vout), due to the direct current path between VDD and GND
when the n-FETPDN is on. The requirements on RPUN in order to achieve
robust operation will also result in a slow low-to-high Vout switching speed.
11
Another option using only n-type transistors is the direct-coupled FET logic
design (DCFL), where the PUN comprises an FET with the gate and drain
connected2, as shown in figure 9(b). In this design the n-FETPUN will operate
similar to a diode, and a careful design of the WD for both the n-FETPUN and
the n-FETPDN is important to achieve fast and robust operation. Due to the
nonlinear behavior of the PUN in this design it is possible to achieve faster
switching speeds, but the design will still suffer from a direct current path for
IDD at high Vin. Both designs can also, with minor changes, be realized using
p-type FETs [30].
Figure 10. Inverter stability diagram showing Vout (solid line), its inverse
(dotted line), and IDD (dashed line) as a function of Vin. The different
operation regimes for the constituent OFETs are indicated and marked
with the open circles and the letters (a)-(e).
If a transistor technology can integrate both n- and p-type FETs into an
integrated circuit then it is possible to achieve the complementary inverter
design, as shown in the schematic in figure 9(c) [31]. In this design the PUN
comprises a p-type OFET, while the PDN comprises an n-type OFET. This
design is the superior alternative, since only one FET at a time is conducting
2 It is also possible to connect gate and source in the PUN, although this requires a transistor with negative VT
in order to operate at a decent speed.
12
during static operation. This greatly reduces the power consumption of the
inverter circuit, since only leakage currents through FETs operated in the
sub-threshold region will be dissipated during steady-state operation.
The principles of static operation of a balanced complementary inverter (see
figure 9(c)) is presented in the stability diagram in figure 10, which includes
Vout (solid line) and IDD (dashed line) as a function of Vin. The shape of the
curves is dictated by the operation regimes of the constituent OFETs 3: (a)
When Vin is low, the p-FET operates in the linear regime, while the n-FET is
in the sub-threshold region, i.e. off. At this point IDD is limited by the low nFET sub-threshold current. (b) With increasing Vin, the VGS for the n-FET
will eventually reach and exceed VT. This corresponds to the entering of its
saturation regime, and the point where the channel of the n-FET starts to
become conducting. At this point Vout will begin to decrease and IDD will start
to increase, since both FETs are now entering an on-state operation regime.
(c) By further increasing Vin, the p-FET is also pushed into saturation, thus
further increasing the IDD and allowing a very rapid change in Vout. The slope
of the stability curve, denoted as the gain, reaches a maximum at this point.
This peak value is an interesting metric of the inverter as it describes how
sharp the switching from low to high (and vice versa) is. (d-e) A further
increase in Vin will move the FETs through the reverse behavior, as observed
before the switching threshold.
A functional inverter thus features a high Vout when Vin is low, and vice versa,
as well as a sharp switching threshold which is quantified by the gain, g =
dVout/dVinmax. An indication of the robustness of the inverter is obtained by
plotting the inverse of Vout (dotted line). A large area between Vout and its
inverse is preferable, since this indicates high robustness against noise
caused by e.g. interconnections and device-to-device variations [32].
The description so far have only concerned the steady-state operation of the
inverter. In order to evaluate the dynamic behavior it is important to
consider the charging of capacitances (both intrinsic and parasitic), when
charge carriers move through the device. As a first-order approximation it is
common to sum up all capacitive elements into a fixed single-load
capacitance between the output node and ground (CL), as shown in figure
11(a). The capacitance originating from the overlap between the input and
output electrodes is known as a Miller capacitance and should be counted
twice in CL [32].
3 This description assumes that the n-FET have a positive VT and that the p-FET have a negative VT which is
the most common case for undoped OFETs.
13
Figure 11. (a) Circuit model describing the dynamic behavior of a
complementary inverter and (b) the corresponding dynamic Vout response
following a step change in Vin.
The dynamic operation of the inverter can be pictured as the charging and
discharging of the load capacitance, and the temporal response of Vout to a
step in Vin for a balanced complementary inverter (see figure 10) is plotted in
figure 11(b). First, the inverter input is held at Vin = GND (= low), so that Vout
is high and CL charged. Following a Vin step from GND to VDD (i.e. low-tohigh), the n-FETPDN is forced to open and the p-FETPUN forced to close. This
opens up a path to ground through the n-FETPDN for the charge stored in CL,
and this dissipation will begin to lower Vout. The time it takes for Vout to reach
VDD/2 is defined as the high-to-low propagation delay (tpHL). Eventually all
CL charges will be dissipated, and Vout has now reached the low output. When
Vin is switched back to GND, the n-FETPDN closes and the p-FETPUN opens,
resulting in a charge flow through the open p-FETPUN so that CL becomes
charged and Vout increase. The time for Vout to reach VDD/2 is defined as the
low-to-high propagation delay (tpLH). The total propagation delay (tp) is
defined as the mean of the high-to-low and low-to-high delays, and can be
estimated with the following equation:
tp 
1
tpHL  tpLH   1  C LVDD  C LVDD 
2
2  I DS,PDN I DS,PUN 
(4)
where IDS,PDN and IDS,PUN is measured at |VGS| = |VDS| = VDD for the n-FETPDN
and the p-FETPUN, respectively. It is usually desirable to have inverters with
similar values for tpHL and tpLH, and according to (4) this can be achieved by
balancing the IDS,PDN and IDS,PUN of the constituent OFETs e.g. by varying
their WD [32].
14
The average power consumption (Pav) is another important metric for an
inverter technology and it comprises three contributions:
Pav  Pdyn  Pdp  Pstat
(5)
where Pdyn is the dynamic power consumption, Pdp the direct-path power
consumption and Pstat the static power consumption. The Pdyn contribution is
due to the power dissipated through charging and discharging of CL, and is
hence dependent upon the frequency of switching. Pdp originates from the
direct path current which arises if both the n-FETPDN and the p-FETPUN are
on at the same time during a dynamic switching event. This contribution also
depends on the switching frequency. However, this effect can be reduced by
increasing the size of CL [32]. Pstat corresponds to the steady-state leakage of
the inverters. This contribution is significant for organic FETs, due to their
characteristic high sub-threshold currents [33].
Figure 12. (a) Schematic showing a 5-stage (N=5) ring oscillator and (b)
typical data based on a 5-stage complementary ring oscillator with Vout
(thick line) and IDD (thin line).
A simple circuit can be realized by connecting an odd number (N) of
inverters in series, with the output of the last inverter connected to the input
of the first, as shown in figure 12(a) [34, 35]. Due to the odd number of
inverters, this circuit will have no stable operation point, and a voltage
transition will propagate through the circuit and looped as fast as the
inverters allow. The voltage at each output node will thus oscillate with a
characteristic frequency. This circuit is hence referred to as a ring oscillator.
Due to its simplicity, the ring oscillator is commonly used to evaluate the
performance and functionality of the inverter technology. A successful ring
15
oscillator operation requires that each inverter is able to switch and drive the
subsequent inverter in the chain. With a successful ring oscillator at hand,
the realization of larger logic circuits is often simply a matter of circuit
design [36-39].
A typical measurement on a complementary ring oscillator is plotted in
figure 12(b), and it presents Vout from one of the output nodes (thick line)
and the supply current (IDD) consumed by the oscillator (thin line) as a
function of time. The time period (T) for a full oscillation of Vout can be
estimated as N × 2tp. The frequency of the oscillation (f) is the inverse of T. It
is also possible to determine Pav for an inverter in the ring oscillator by
integrating IDD over one time period:
T
Pa v 
VDD
I DD t dt
NT 0
(6)
By multiplying Pav with tp the power delay product (PDP) is obtained. The
PDP represents a measure of the average energy consumed per switching
event. By subsequently multiplying the PDP with tp the energy delay product
(EDP) is obtained, and the EDP thus represents a combined energy and
performance metric. These metrics allow for a comparison of different
inverter technologies, as they describe the performance and efficiency of
each technology in a quantitative manner [32, 40].
3.2. The photovoltaic cell
Our modern lifestyle consumes an increasing amount of energy each year.
Historically, this energy has been almost exclusively supplied through fossil
fuels (coal, oil and gas), a non-renewable energy resource. Along with the
problem of exhausting earth's fossil energy resources comes the release of
the greenhouse gases (CO2, CH4, etc.), the inevitable product from fossil fuel
combustion which, once in the atmosphere, contributes to global warming.
Renewable energy alternatives are thus necessary in order to alleviate these
problems. One such alternative is the photovoltaic (PV) cell in which solar
energy is converted to electrical energy. The current generation of
commercial PV cells comprises inorganic Si, and such cells can feature a high
efficiency thanks to the maturity of the Si technology. The refining and
processing of the Si is however an expensive process, both in terms of price
and energy.
In 1992, an ultra-fast photo-induced electron transfer from a conjugated
polymer to C60 fullerene molecule was demonstrated [2]. This discovery of
16
efficient charge separation in organic materials started the development of a
new generation of light-energy-harvesting devices, organic photovoltaics
(OPVs), which offer a low-cost and easy-to-fabricate alternative to the
existing Si-based PVs [3].
3.2.1. The bilayer structure and operation principles
Figure 13. The (a) device structure and (b) energy diagram for a bi-layer
organic solar cell.
In order to understand the operation principles of an OPV cell it is
educational to start with the simple bi-layer structure. This structure is
depicted in figure 13(a) and comprises two organic semiconductor layers
sandwiched between two electrodes, of which at least one needs to be
transparent. Transparent conductive oxides (TCO), such as indium tin oxide
(ITO), is usually used as the transparent electrode, although significant
efforts are put into finding replacements based on more abundant, low-cost
and flexible materials [41-43].
A thin film of a donor (D) material is first deposited onto the TCO. The role
of the donor is to absorb the incoming photons to excite electrons from the
HOMO into the LUMO to create excitons, viz. bound electron and hole pairs.
An effective donor material should thus feature a high absorption coefficient
(~105 cm-1) to allow for complete light absorption for a film with a thickness
on the order of 100 nm. Since the exciton is a neutral particle it will diffuse
randomly in the donor, independent on the electric field, until it either
recombines or dissociates into a free electron and hole. Due to the low
permittivity of organic materials, the binding energy of the exciton (Eexciton >
17
100 meV) is much larger than the average phonon energy at room
temperature (~25 meV), implying that the exciton will recombine unless a
driving force will cause it to dissociate [44].
Figure 14. Schematic highlighting the desired (green solid arrows) and
non-desired (black dashed arrows) processes related to (a) photon
absorption, (b) exciton dissociation, (c) charge transport and (d) charge
collection in an OPV at short-circuit condition.
18
A driving force for exciton dissociation can however be an interface with an
acceptor (A) material with lower ELUMO and EHOMO than the donor, so that
the electron, but not the hole, finds a more favorable state in the acceptor.
The bound exciton can thus dissociate into a free electron located in the
acceptor LUMO and a free hole located in the donor HOMO at the D/A
interface. Once separated, the charges are free to move in an external field.
By selecting a top electrode material with a lower work function than the
TCO, it is possible to create an internal electric field which will act as a
driving force on the electrons and holes towards the top cathode and the
bottom TCO anode, respectively. The successful collection of the charge
carriers at the respective electrode results in a PV current. A schematic
energy diagram for a functional bi-layer solar cell is shown in figure 13(b).
In order to maximize the performance of an OPV it is important to
understand and minimize the loss factors. The efficiency with which the OPV
converts incoming photons into current exiting the device is given by the
equation for the external quantum efficiency (EQE):
EQE   a bs  IQE   a bs   ed   ct   cc
(7)
The internal quantum efficiency (IQE) refers to the efficiency with which the
absorbed photons, i.e. those photons that are not reflected or transmitted out
of the cell, can generate current. ηabs describes the efficiency of absorption,
and this process is presented in figure 14(a), where the green solid arrow
indicates the desired events (in this case photon absorption to create an
exciton) and the black dashed arrow indicates the non-desired event (a nonabsorbed photon reflected from or transmitted through the device). ηed
represents the exciton dissociation efficiency, as schematically shown in
figure 14(b). Once created through absorption, the exciton will diffuse in the
organic semiconductor until it is either separated into free charge carriers at
an interface (green solid arrow) or recombined under the generation of heat
or light (black dashed arrow).
ηct describes the charge transport efficiency, as depicted schematically in
figure 14(c). Once separated, the free charge carriers need to be transported
to the correct electrodes in order to be collected (green solid arrows). This
process requires a driving force from an electric field or a charge
concentration gradient, as well as uninterrupted pathways for the charge
carriers. The free electrons and holes can, however, also recombine at a D/A
interface before reaching the electrodes (black dashed arrow). ηcc is the
charge collection efficiency at the electrodes, as described in figure 14(d),
where the non-preferred process is charge collection at the wrong electrode.
19
For bi-layer devices, the good news is that ηabs is efficient at a film thickness
of ~100 nm, ηct can be high due to the good mobility in the pure donor and
acceptor materials, and ηcc is close to unity since the electrodes are only
contacting materials carrying a single charge-carrier type. ηed is, however,
greatly limited by the short diffusion length (~10-20 nm) of excitons in OSCs
[45]. As a result, only photons absorbed within an exciton diffusion length
away from a D/A interface will contribute to the total EQE, thus greatly
limiting the total current produced in a bi-layer device.
3.2.2. The bulk heterojunction solar cell
Figure 15. (a) A schematic of the BHJ-OPV device structure, with ETL and
HTL layers, and (b) the corresponding energy diagram.
The bulk heterojunction (BHJ) OPV cell is designed to alleviate the problem
of inefficient exciton dissociation. In a BHJ device the donor and acceptor
are intimately mixed in a single active-layer film. In such a film, the donor
and acceptor materials are able to form interfaces on length scales of only a
few nm, i.e. well below the exciton diffusion length, which allows for a high
ηed. However, with a randomly intermixed material network come charge
transport and collection problems. Since both donor and acceptor materials
are in contact with the electrodes in a BHJ device, it is important to ensure
that only the correct type of charge carrier is collected at the electrodes.
Therefore, a hole-transport layer (HTL), designed to block electrons while
efficiently transporting holes, and an electron-transport layer (ETL),
designed to block holes while transporting electrons, can be deposited in
20
between the active layer and the anode and the cathode, respectively. The
inclusion of a HTL and an ETL minimizes charge collection losses, i.e. keeps
ηcc close to unity. A typical device structure for a BHJ-OPV cell is shown in
figure 15(a) and the accompanying energy diagram is shown in figure 15(b).
Maximizing the charge transport efficiency ηct in a BHJ cell is a delicate
matter. In an intimately mixed network the interface surface area is greatly
increased, resulting in increased exciton dissociation but also higher
recombination probability [46, 47]. The D/A random network (as shown in
figure 15(a)) formed during thin-film deposition can also comprise onephase material islands, resulting in interrupted pathways for charge
transport. This random D/A network is however not permanent and it can be
adjusted after deposition in order to optimize the film morphology. Methods
for optimizing the film morphology include thermal annealing [48-50],
solvent treatment [51] and light exposure [52, 53]. Still, the overall aim is the
same, i.e. controlling the phase separation to reduce charge recombination
while maintaining efficient exciton dissociation.
3.2.3. The OPV performance
Figure 16. (a) Energy diagram highlighting parameters vital to the OPV
cell performance. (b) The AM1.5G irradiance spectrum showing the
spectral irradiance as a function of wavelength and energy. The total
irradiance is 1 kW m-2.
Various material and device parameters determine the maximum
performance of an OPV cell, with the energy levels of the D/A materials
playing an important role. The maximum voltage is determined by the
difference in energy between the acceptor LUMO and donor HOMO, Emax =
21
ELUMO,A - EHOMO,D, as this represents the energy difference between the
separated electrons and holes, as seen in figure 16(a). To maximize the
voltage, the EHOMO,D is increased and/or the ELUMO,A is decreased. In order to
allow for efficient exciton dissociation, and therefore a high current density,
the driving force for exciton dissociation, ΔE = ELUMO,D - ELUMO,A, must be
larger than Eexciton. Moreover, a lower donor energy gap (Eg = ELUMO,D EHOMO,D) results in the absorption of a larger portion of the solar spectrum,
as shown in figure 16(b). It is thus clear that a tradeoff exists between the
maximum voltage and the maximum current density from an OPV [54].
Figure 17. Typical data obtained from a J-V measurement on an OPV cell
with the dark current indicated by the black line and the AM1.5G
illuminated indicated in red. The Jsc, Voc and MPP are marked as a
reference.
In order to generate power, the device needs to be operated in the forward
bias regime. In this regime, the solar-induced current will compete with the
dark current (as measured on a non-illuminated dark OPV and indicated by
the black line in figure 17), which is due to charge injection in the diode
structure created by the asymmetric electrodes. A typical current density –
voltage (J-V) measurement performed on an AM1.5G illuminated organic
solar cell is shown in figure 17 (red curve). This current density represents
the net of the generated and the dark currents.
The point where the generated and dark currents balance, and no net current
is produced by the device, is termed the open-circuit voltage (Voc).
Consequently, when the voltage measured over the device is zero, no net
current is injected from the electrodes. The current density measured at this
22
point is termed the short-circuit current density (Jsc). The total generated
power is the product of the current and voltage; hence no power is generated
by the device at Jsc or Voc. Instead, the maximum power point (MPP) defines
the point where the product between current and voltage is maximized, as
visualized by the green rectangle with dashed borders in figure 17. In an ideal
device, the MPP would equal the product of Jsc×Voc (red square with solid
borders in figure 17) but because of, e.g., the dark current the measured MPP
is reduced. The ratio between the MPP and the ideal (Jsc×Voc) is defined as
the fill factor (FF).
In the end, the most important metric for a PV cell is its total power
conversion efficiency (PCE), as calculated by
PCE 
J sc  Voc  FF
Pi n
(8)
where Pin is the total power density of the of the light impinging upon the
surface of the PV cell, which commonly is 1 kW m-2 in experiments [45]. A lot
of effort has been invested on optimizing materials and processes in order to
maximize the PCE of BHJ-OPV cells [55]. Record values for the PCE are
however almost exclusively measured during a short time period following
device fabrication, but a practical device should maintain this level of
performance for an extended period of light illumination. This requirement
is not simple for a BHJ-OPV cell, since the temperature increase during
continuous operation can drive the phase separation beyond its optimum,
and in the process reduces the device efficiency. A fabrication process aimed
at locking the film morphology at its optimum is thus essential for long-term
continuous operation. Several methods have been suggested with varying
success and chapter 6.2 will present a fully solution-processable alternative
for solving this problem.
23
4. Fullerene photochemical transformation
Figure 18. The photochemical dimerization of PCBM.
The fullerene C60 has been demonstrated to photochemically transform into
dimers, oligomers and polymers during exposure to light [18, 56]. It has also
been shown that the C60 derivative PCBM can be dimerized during exposure
to green laser [57] and UV [58] light. The two fullerene monomers are
connected to a dimer in a [2+2] cycloaddition dimerization reaction, as
depicted in figure 18.
4.1. Raman spectroscopy
Figure 19. Raman spectra showing the Ag(2) vibrational mode for PCBM
films. While the pristine film (a) indicates a fully monomeric film, both the
green laser exposed (b) and UV exposed (c) films shows a clear downshift,
indicating the presence of PCBM dimers.
24
The presence of dimerized PCBM in a film can be established with Raman
vibrational spectroscopy. In this technique, the sample is exposed to a
monochromatic excitation light, which is scattered by the molecules in the
sample. A small portion of the excitation light will interact with the
vibrational modes (phonons) of the molecule, resulting in an inelastic
scattering process where the scattered photon is shifted either up or down in
energy by an amount corresponding to the phonon energy. So by measuring
the scattered light spectrum it is possible to obtain information about the
vibrational modes of a molecule.
The Ag(2) pentagonal pinch mode gives valuable information about the
degree of polymerization of fullerenes. It is located at 1469 cm-1 for monomer
C60, but is down-shifted by ~5 cm-1 for each chemical connection to the C60
cage. For the C60 molecule, one downshift thus indicates the presence of
dimers, two downshifts indicate linear oligomers/polymers, and three
downshifts indicate a branched polymeric structure [18, 59-62].
Since a solubilizing side-group is chemically attached to the C60 cage in
PCBM, the Ag(2) mode of the PCBM monomer is located at 1465 cm-1, i.e.
downshifted by 4 cm-1 in comparison to the C60 monomer [57]. The Raman
spectrum for a 100 nm thick pristine PCBM film is shown in figure 19(a), as
obtained using a Raman excitation wavelength of λRaman = 780 nm at a power
density of PRaman < 10 mW cm-2.
The photochemical transformation of the PCBM film was performed by
exposure to either a green laser (λ = 532 nm) or a hand-held UV lamp (λ =
365 nm), and the corresponding Raman spectra are presented in figure 19(b)
and figure 19(c). Both spectra show a clear downshift of the Ag(2) mode by 5
cm-1 as highlighted by the two dotted vertical lines, which indicate the
formation of PCBM dimers in both films. Interestingly, no further degree of
polymerization is observed in the exposed films. The lack of a higher degree
of polymerization of PCBM is motivated by the steric hindrance of the PCBM
side-group [57].
4.2. Fullerene patterning by resist-free lithography
An exciting and useful property of the photochemically transformed PCBM
dimers is that their solubility in organic solvents, as compared to the PCBM
monomers, is drastically reduced. Thus, by selectively exposing certain
regions of a pristine PCBM film, it is possible to create lateral regions of
lowered solubility which can be developed by immersion into a tuned
solution [57, 58, 63-65]. This process is known as photolithography.
25
Figure 20. Schematic representation of the key steps in the resist-free
photolithographic patterning of PCBM: (a) film deposition, (b) light
exposure through a shadow mask, (c) development in a tuned developer
solution, and (d) the patterned PCBM dimer film. The optical micrographs
in (e-g) display (dark brown) patterned PCBM, in the form of lines with
different line widths and a square 2D structure.
The resist-free photolithographic patterning of a PCBM film is schematically
depicted in figures 20(a-d). The process begins with the deposition of a
PCBM film through spin-coating, as shown in figure 20(a). The as-cast film
is then exposed to light through a shadow-mask, featuring the pattern to be
transferred as the open areas, as shown in figure 20(b). The exposure will
transfer the pattern of the shadow mask into PCBM dimer regions with
reduced solubility. By immersing the exposed film into a carefully tuned
developer solution (e.g. chloroform:acetone in a 1:7 volume ratio), as shown
in figure 20(c), the monomer PCBM is washed away, leaving only the
patterned dimers on the substrate, as shown in figure 20(d).
Optical micrographs of patterned (dark brown) PCBM films on SiO2
substrates are shown in figures 20(e-g). The line width of the patterned
PCBM lines decreases from 100 µm to 10 µm, in steps of 10 µm, in figure
20(e), and from 10 µm to 1 µm, in steps of 1 µm, in figure 20(f). Figure 20(g)
shows a 22.5 µm square pattern. The successful patterning of the smallest 1
µm sized lines in figure 20(f), and the sharp corners in figure 20(g),
demonstrate the method’s capacity for high resolution. Moreover, the
26
measured line-edge roughness of 56 nm, in combination with that the
smallest feature on the shadow-mask was 1 µm, indicates that an even
smaller feature size of <1 µm should be achievable.
4.3. Patterning efficiency
Figure 21. (a) Optical micrograph of a patterned PCBM film, with only the
left part of the film being exposed. (b) An AFM height scan was performed
in a direction perpendicular to the patterned interface, as indicated by the
dotted line in (b). (c) The UV-vis absorption spectrum of a pristine PCBM
film. (d) The normalized retained PCBM film thickness as a function of
exposure dose, as extracted from AFM measurements. A mercury lamp
(solid squares) and a green laser (open circles) were used for the exposure
in (d).
In order to evaluate the photochemical dimerization efficiency for PCBM, a
set of pristine PCBM films were exposed to light in varying doses. Half of the
sample was masked during the exposure, resulting in a step profile following
the development step. The normalized retained thickness of the developed
27
film was measured by atomic force microscopy (AFM), as depicted in figure
21(a) and figure 21(b). Since the developer solution is assumed to selectively
remove the PCBM monomers, the normalized retained thickness can be
viewed as an estimate of the dimerization fraction in the exposed and
developed film.
Due to the wide absorption of PCBM, as shown in the UV-vis absorption
spectrum in figure 21(c), a wide range of wavelengths are able to
photochemically transform PCBM monomers. From a resolution
perspective, it is preferable with a short wavelength since the minimum
feature size in photolithography is dependent on the wavelength of the
exposure light. Additionally, PCBM shows a significantly higher absorption
coefficient in the UV region than in the visible region, with the anticipated
consequence being that the dimerization process should be more efficient
with UV-light exposure. A hand held mercury UV-light source (λ = 365 nm, P
= 8.4 mW cm-2) and a longer wavelength green laser (λ = 532 nm, P = 61.3
mW cm-2) were chosen to investigate the effect of the wavelength on the
process efficiency.
The normalized retained PCBM film thickness as a function of exposure dose
and wavelength is shown in figure 21(d), with the exposure dose being
controlled by the exposure time. We quantify the process efficiency with the
dose required for 50% retained thickness. It is clear that the UV exposure
(solid squares) requires less energy to fully dimerize the PCBM film than the
green laser (open circles), as quantified by that the 50% dose for the former
process is 3.57 J cm-2 and 26.1 J cm-2 for the latter process.
Some assumptions are necessary in order to transform the measured dose
into a more conceptually accessible “dimerization events per absorbed
photon” as represented by the symbol Φ. First, the intensity of the exposure
light is uniform throughout the film thickness, and no reflections at the film
or substrate surface are present, implying that the incoming light passes
through the film once. Second, the absorption coefficient remains constant
(equal to that of pristine PCBM) throughout the entire exposure step. Third,
a successful dimerization event requires that two neighboring PCBM
monomers are simultaneously excited by one photon each. Based on these
assumptions it is possible to formulate an expression for Φ based on a
modified version of Beer-Lambert's law:
28
N
Φ  di m 
N a bs
VN A
E tot

M
hc
(9)
1  e 
d
where Ndim is the minimum number of photons necessary to fully dimerize
the PCBM film, i.e. assuming one absorbed photon per PCBM monomer. It is
given by the density of PCBM (ρ = 1.5 g cm-3), the volume of the dimerized
PCBM film (V), Avogadro's constant (NA) and the molar weight of PCBM
(M). Nabs represents the measured number of absorbed photons for full
dimerization. It is given by the total energy of the incoming light (Etot), the
wavelength of the exposure light (λ), Planck's constant (h), the speed of light
in vacuum (c), the absorption coefficient of PCBM at the exposure
wavelength (α), and the PCBM film thickness (d).
By including the relevant numbers into equation (9), notably Nabs,365 =
2.4×1019 cm-2 and Nabs,532 = 1.3×1020 cm-2, we find that the photon-todimerization efficiency is: Φ365 = 4.2×10-4 and Φ532 = 7.4×10-5. The fact that
Φ365 > Φ532 highlights the advantage of UV light over visible light as it both
results in a more efficient dimerization process and a higher resolution.
4.4. The photochemical reaction mechanism
Figure 22. (a) Jablonski diagram showing the PCBM energy levels and the
proposed reaction pathways for PCBM dimerization. (b) Normalized
residual film thickness of exposed and developed PCBM films, as a function
of the exposure dose and intensity. The exposure was effectuated by a high-
29
power UV-diode (λpeak = 365 nm) using different intensities and exposure
times.
Understanding the underlying mechanism of the PCBM photo-dimerization
reaction is crucial in order to optimize the exposure conditions. The energy
levels of PCBM and the possible reaction pathways during electronic
excitation, relaxation and dimer formation are presented in the Jablonski
diagram in figure 22(a). The energy levels are the singlet ground state (M),
the excited singlet state (1M*), the excited triplet state (3M*), and the PCBM
dimer state (D) [18, 66], and the transitions between the different states are
marked with arrows and associated rate constants, k.
The PCBM fluorescence (1M* → M) is in comparison to the intersystem
crossing to the triplet state (1M* → 3M*) a very slow process. It is thus
reasonable to state that k2 is negligible [18, 66]. This assumption has the
consequence that all excited electrons end up in the 3M* state, and this entire
transition can be described by the combined rate constant k13. C60 dimers
begin to dissociate into monomers at a high temperature of 400 K [18, 62],
and we therefore assume that the PCBM dimer dissociation rate k6 can be
neglected during an exposure step executed at room temperature. Once a
PCBM monomer is excited to the 3M* state, it can either relax to the ground
state (3M* → M) with a rate of k4, or form a dimer. The latter dimerformation process is described by two alternative pathways, the uni-excited
reaction and the bi-excited reaction, as described by the rate constants k5
and k’5, respectively. In the uni-excited reaction, a dimer is formed by the
combination of one fullerene monomer in the triplet excited state and
another neighboring fullerene monomer in the singlet ground state, i.e. 3M*
+ M → D. Whereas in the bi-excited reaction, two neighboring monomers
need to be simultaneously excited in order to form a dimer: 3M* + 3M* → D.
A Raman spectroscopy study by Eklund et al. came to the conclusion that the
uni-excited reaction described the dimerization rate of C60 fullerenes [18]. It
is however here contradicted by the data in figure 22(b), which shows that
higher-intensity exposure light results in a lower exposure dose for the
dimerization of a PCBM film. The uni-excited reaction pathway, which only
requires that one monomer is excited in order to form a dimer, will show an
intensity-independent dose, and it can therefore not be the accurate
description of the behavior seen in figure 22(b). The bi-excited reaction
pathway, which requires two neighboring monomers to be simultaneously
excited, will feature the observed intensity-dependence of the dose. It is
therefore concluded that the bi-excited reaction pathway describes the
photochemical dimerization process of PCBM.
30
The detailed understanding of the reaction processes for PCBM dimerization
results in the practical conclusion that high-intensity UV-light is an ideal
light source for the exposure step. The short wavelength will ensure high
resolution, strong absorption and low dose. For example, short wavelength
UV-light, with a wavelength of 365 nm and an intensity of 8000 mW cm-2,
will result in an exposure time of 1 mere second. This short exposure time
fits well with large-scale processes, such as a roll-to-roll (R2R) fabrication.
4.5. PCBM OFETs
Figure 23. Transfer (left) and output (right) data for typical PCBM OFETs,
as measured at different instants during the patterning process. The
transfer data were measured at VDS = 60 V. The output data were
measured in VGS steps of 20 V, starting from 0 V (lower trace) and ending
at 80 V (upper trace).
31
The impact of UV-patterning on the electronic transport properties of PCBM
was investigated with BG-TC OFETs, comprising a PCBM film as the active
material, Au as the source and drain electrodes, and SiO2/p:Si as the gateoxide/gate; the experimental details are presented in Paper I. The transfer
and output characteristics are presented in the left and right part of figure
23, respectively, as measured on: (i) pristine PCBM, (ii) UV-exposed PCBM
(λ = 365 nm, P = 8.4 mW cm-2, t = 15 min), (iii) UV-exposed and developed
PCBM, and (iv) unexposed and developed PCBM.
Treatment
(cm2
µn
V-1 s-1) [a]
VT
(V)
Ion/Ioff
(a.u.)
Pristine
1.8(±0.6)×10-2
2
2.0×104
Exposed
1.6(±0.5)×10-2
7
1.2×104
Exposed + Developed
1.2(±0.2)×10-2
10
1.2×104
---
---
---
Solely Developed
[a]
Mean value with standard deviation in parenthesis
Table 1. Performance metrics of PCBM transistors as measured during the
UV-patterning procedure.
The transistor data demonstrate well-behaved n-type OFETs for (i)-(iii),
with monotonously increasing IDS with increasing VGS in the transfer
characteristics, and clear saturation in the output characteristics. As
expected, the solely developed sample shows no OFET characteristics,
simply because the unexposed PCBM were dissolved and removed by the
developer solution. Both µn and VT were extracted from the transfer data, as
described in section 3.1, and the Ion/Ioff ratio was calculated as the ratio of IDS
at (VDS = VGS = 60 V) and (VDS = 60 V, VGS = 0 V). The average values from
>60 characterized OFETs are summarized in table 1. To our satisfaction,
PCBM features a respectable mobility following patterning, although it is
slightly reduced in comparison to the pristine state. We tentatively attribute
the decrease to a reduction of intermolecular π-orbital overlap following the
transformation of some sp2 orbitals to sp3 orbitals; see figure 18.
Interestingly, the dimerization also makes PCBM less sensitive to ambient
air. By exposing the OFETs to ambient air for 24 h prior to the FET
characterization, we found that the mobility of pristine PCBM dropped to a
mere 1 % of its initial value, whereas the UV-patterned PCBM retains 39 % of
its initial mobility. We speculate that this improvement is due to a more
32
dense and possibly more crystalline film, which could act as an improved
barrier towards O2 permeation into the active layer [58, 67].
Most importantly, the electronic mobility of PCBM (and C60) is only slightly
reduced by the photochemical transformation [57, 63]. This fact indicates
that high-resolution and resist-free patterning of the fullerene-based active
material in organic electronic devices could be possible. In OFETs, this
advantage could be used to reduce inter-device and fringe-field leakage
currents, and to realize advanced circuits such as memory devices.
4.6. Realizing complementary logic circuits with resist-free
patterning
The resist-free patterning technique can also be utilized to realize
complementary circuits with a single-layer active material, comprising a
blend of n-type PCBM and p-type poly(3-hexylthiophene-2,5-diyl) (P3HT).
By selecting an active material with high PCBM content, a predominantly ntype device is obtained. Interestingly, it is possible to transform this device
into featuring p-type behavior, by immersing the device into a developer
solution tuned to selectively dissolve (non-exposed) PCBM monomers, but
not (exposed) PCBM dimers and P3HT. By using this single-layer blend film
as the common active material for an array of transistors, exposing some of
the transistors to high-intensity light, and then developing the entire array
an interesting and attractive result is obtained. The exposed and developed
transistors remain n-type, as the active material is left effectively intact by
the developer solution, while the solely developed transistors have
transformed into being p-type, following the removal of the (non-exposed)
PCBM monomers [64].
33
4.6.1. The inverter
Figure 24. (a) Photograph and (b) schematic diagram of the PCBM:P3HT
complementary inverter fabricated with the resist-free patterning
technique. The output data of (c) the p-type FET and (d) the n-type FET,
with VGS changed in steps of 10 V starting from 0 V and ending at  50 V
and VDS swept at a rate of 2 V s-1, as indicated by the arrows. (e) The static
and (f) the dynamic inverter characteristics, as recorded with VDD = 50 V
and VDD = 44 V, respectively.
34
A schematic of the complementary inverter circuit, comprising a p-type and
an n-type FET, is depicted in figure 24(b). The FETs were fabricated in a BGTC geometry, comprising a common active material of PCBM:P3HT in a
mass ratio = 5:1, Au as the source and drain electrodes, and SiO2/p:Si as the
gate-oxide/gate. The active material in the upper and lower FETs was then
tuned to be p-type and n-type, respectively, following the procedure outlined
in the previous section. More specifically, a green laser (λ = 532 nm, P = 60
mW cm-2, t = 30 min) selectively exposed the active material in the FET
designed to be n-type, where after the entire active material film was
developed in the developer solution. Figure 24(a) shows a photograph of two
inverters, with the exposed dimer-PCBM:P3HT and the unexposed P3HT
regions distinguished by their dark blue-green and red-brown appearances,
respectively.
A typical output device performance of the p-type and n-type OFETs are
shown in figure 24(c) and figure 24(d), respectively. Both devices show wellbehaved operation with clear current saturation and low hysteresis. A slight
ambipolarity of the n-type FET is observed as a slight increase in IDS at high
VDS as VGS approaches 0 V. The µ and VT extracted from the transfer
measurements on 5 pair of transistors are shown in table 2.
Device
µ
(cm2 V-1 s-1) [a]
VT
(V) [a]
p-FETPUN
1.3(±0.4)×10-4
-14.8(±2.5)
a-FETPDN
1.7(±0.8)×10-4
19.4(±1.7)
[a]
Mean value with standard deviation in parenthesis
Table 2. Performance metrics of the two OFETs forming the inverter.
Figure 24(e) shows the static inverter characteristics, as recorded at VDD = 50
V. The inverter features a sharp switch of Vout (blue solid line) at Vin ≈ 30 V,
as quantified by a high gain of g = 11 ±1. The IDD (red dotted line) shows a
peak at the switching threshold, as expected, a low leakage current of <1 nA
at Vin > 35 V, and a larger leakage current for Vin < 25 V. The latter is an
effect of the slightly ambipolar character of the n-type FET, which results in
a direct current path from VDD to GND.
Figure 24(f) shows the dynamic inverter behavior, as measured at VDD = 44
V. The high-to-low transition (tpHL = 2.9 s) is delayed by an overshoot of Vout,
as it is pulled towards GND by the n-type FET. The low-to-high transition
(tpLH = 0.6 s), in contrast, shows an ideal behavior as Vout is pulled towards
35
VDD by the p-type FET. A total propagation delay of 1.75 s is measured at a
VDD of 44 V.
Figure 25. (a) Schematic of a 5-stage ring oscillator realized with the
complementary PCBM:P3HT inverters, and (b) the corresponding
temporal behavior of Vout and IDD at VDD = 50 V. (c) the delay time and the
average power consumption as a function of supply voltage, and (d) the
PDP and EDP as a function of the supply voltage.
A 5-stage ring oscillator was fabricated by connecting 5 inverters in series,
with the output of the last inverter connected to input of the first, as depicted
schematically in figure 25(a). When the supply voltage was connected, the
circuit began to oscillate, and temporal data on Vout and IDD at VDD = 50 V are
shown in figure 25(b). The frequency of operation is 10 mHz, and the circuit
consumes an average IDD ≈ 15 nA. Interestingly, the measured Vout only
oscillates between 25-50 V and thus never operates in the ambipolar region.
36
Figure 25(c) shows that the delay time, tp, decreases and the average power
consumption, Pav, increases with increasing VDD, as expected, for the 5-stage
ring oscillator. The superlinear increase of Pav with increasing VDD indicates
that the power consumption can become an issue at high VDD.
The dependence of the power delay product, PDP, and energy delay product,
EDP, on VDD are shown in figure 25(d). A PDP minimum of 2.0 µJ is found at
VDD = 46 V, indicating that this is the most efficient VDD for operation.
However, no minimum is found for EDP in the measured interval, since the
reduction of tp is greater than the increase of Pav as VDD increases. This
indicates that the supply voltage for optimum operation will be found at VDD
> 50 V.
In summary, it has been shown that functional complementary inverters and
ring oscillators can be fabricated with a facile photolithographic procedure
developed in-house, without the use of a sacrificial photo-resist material. By
considering the high values for the intrinsic capacitance (Ci = 14.7 pF) and
the extrinsic capacitance (CL = 1.44 ±0.15 nF) for the inverters in this
particular circuit, it is clear that further improvements in terms of speed and
power consumption by several orders of magnitude can be achieved by
reducing these parasitic capacitances.
37
5. Fullerene C60 nanorods
For facile electron transport, a crystalline material is often preferred over its
amorphous counterpart [14-16, 68]. Self-assembly is a common method for
the fabrication of crystalline organic materials with controlled size and
shape, and crystalline C60 structures ranging from 0-D dots, through 1-D
rods, to 2-D discs have been realized using such methods [17, 69-73]. A
notably straightforward solution-based self-assembly fabrication method is
the liquid-liquid interfacial precipitation (LLIP) method, which allows
synthesis of crystalline 1-D C60 nanorods (NRs) with sub-µm diameter [7480].
5.1. Synthesis
The LLIP synthesis of C60 NRs is a 3-step process: i) interface formation, ii)
nucleation, and iii) crystal growth. An example of a fabrication process is
detailed below: C60 is dissolved in a good solvent (1 mg ml-1 in mdichlorobenzene, m-DCB) and 1 ml of the solution is added to a glass vial. 10
ml of a poor C60 solvent (ethanol) is gently added to the vial, so that an
interface is formed between the good and poor solvents; see left vial in figure
26(a). The two-component solution is ultrasonicated to induce partial
mixing of the two solvents at the interface. The mixing results in local
oversaturation of C60, so that nucleation and precipitation of C60 seed
particles can take place, as manifested in the greenish interfacial region in
the middle vial in figure 26(a). The hermetically closed vial is then left at
room temperature, and after 2 days C60 NRs are observed to begin to deposit
at the bottom of the vial; see right vial in figure 26(a). After 3 days the crystal
growth is terminated by vigorously shaking the vial and the resulting asgrown NRs are ready for use.
An optical micrograph and a transmission electron microscope (TEM) image
of a batch of as-grown NRs are shown in figure 26(b) and figure 26(c),
respectively. The NRs feature a hexagonal cross section, lengths of ~1 mm,
an aspect ratio of >103, and they are highly flexible [81]. The size of the NRs
is controlled by the number of crystal seeds formed during the nucleation
step. This number can be controlled by the solvent intermixing during the
ultrasonication, and an increased ultrasonication power results in enlarged
intermixing and an increased number of crystal seeds, which in turn results
in smaller NRs [82]. We have utilized two different ultrasonication powers
for the fabrication of two types of NRs with: (i) a mean diameter of Ø1 = 250
nm, within a range of 100-400 nm (Paper III); (ii) Ø2 = 172 nm, within a
38
range of 50-360 nm (Paper IV). The size distribution of the latter is
presented in figure 26(d).
Figure 26. (a) Photograph of vials comprising solutions in the different
stages of the LLIP process: (left) interface formation, (middle) nucleation,
and (right) crystal growth. (b) Optical micrograph and (c) TEM image of
the as-grown NRs. (d) Diameter distribution of NRs with an average
diameter (Ø2) of 172 nm.
39
Figure 27. Schematic representation of the photochemical transformation
of NRs into PNRs. The photograph in the inset shows vials containing
dispersions of as-grown NRs (left) and PNRs (right), depicting the color
shift from green-brown to red-brown following the exposure.
Exposing the as-grown NRs to high-intensity light transforms the
monomeric C60 in the as-grown NRs into polymeric C60. The exposure was
performed by directing a laser beam (λ = 532 nm, P = 15 mW mm-2, t = 20 h)
into the NR dispersion through the bottom of the vial, as depicted in figure
27. The width of the laser beam was adjusted to match the width of the vial
(~1 cm), and the sides of the vial were covered in Al foil to trap the light
inside the vial. The exposed NRs featured a color shift from green-brown to
red-brown, as seen in the photograph inset in figure 27, while the NR
diameter distribution was found to be unaffected. The photochemically
transformed NRs will hereafter be referred to as polymerized nanorods
(PNRs). It should be noted that the exposure was performed on NRs in
dispersion, making it a scalable process.
40
5.2. Characterization
The NR structure was characterized by TEM, high-resolution TEM
(HRTEM), scanning TEM (STEM), Raman spectroscopy, x-ray diffraction
(XRD), and selective area electron diffraction (SAED), while the thermal
characterization was performed using thermal gravimetric analysis (TGA).
5.2.1. As-grown
Figure 28. Characterization of as-grown NRs with (a) XRD and (b)
HRTEM, with the (top left) inset showing a SAED image.
Figure 28(a) shows the XRD of as-grown NRs, with the peak pattern
indicating a hexagonal close-packed (hcp) structure, with a unit cell size of a
= b = 23.71 Å and c = 10.18 Å. The sharp distinct peaks indicate a singlecrystal structure, which is supported by both the HRTEM image in figure
28(b), in which both crystalline faults and dislocations are absent, and the
SAED pattern in the inset in figure 28(b). It is thus concluded that the asgrown NRs feature a single-crystalline hcp crystal structure.
TGA of as-grown NRs reveals a linear weight loss of 10.7 % between 85-171
°C, as shown in figure 29(a). This weight loss is assigned to the evaporation
of guest m-DCB molecules from within the NRs. The weight loss with an
onset of ~400 °C is due to the sublimation of C 60, which features a
sublimation temperature of 390 °C in the solid state.
The Raman spectra of as-grown NRs and pristine C60 are displayed in the
upper red curve and lower blue curve in figure 29(b), respectively. The asgrown spectrum features 10 characteristic C60 peaks (8 Hg and 2 Ag) and 3
additional peaks, as marked with asterisks, and these additional peaks can be
41
removed by annealing at 200 °C. Furthermore, the Hg(1) and Ag(1) modes of
the as-grown NRs are downshifted in comparison to pristine C60 (see inset
in figure 29(b)). The new and shifted Raman features of as-grown NRs are
assigned to the existence of solvated m-DCB guest molecules in the NR
lattice [72, 83, 84].
Figure 29. (a) TGA of as-grown NRs. (b) Raman spectra of as-grown NRs
(i, red upper curve) and pristine C60 (ii, blue lower curve). The inset in (b)
highlights the downshift of the Hg(1) and Ag(1) modes for as-grown NRs (i)
in comparison to pristine C60 (ii).
42
5.2.2. Annealing
Figure 30. (a) XRD patterns of annealed NRs (i, red upper curve) and
pristine C60 (ii, blue lower curve). (b) STEM, (c) HRTEM, and (d) SAED of
the annealed NRs, which reveal the existence of a porous and
polycrystalline structure.
A vacuum annealing step (T = 200 °C, p <1 kPa, t = 24 h) was introduced in
order to remove the solvated m-DCB molecules, and the upper red curve in
Figure 30(a) shows the XRD of annealed NRs. Interestingly, the crystal
structure has transformed from hcp to face-centered cubic (fcc), with a unit
cell size of a = b = c = 14.13 Å; i.e. the same crystal structure as pristine C60
(blue lower curve) [70]. Further inspection of the annealed NRs by STEM
(figure 30(b)), HRTEM (figure 30(c)) and SAED (figure 30(d)) reveals the
existence of a porous polycrystalline structure, with clearly distinguishable
crystal grain boundaries.
43
Figure 31. The Ag(2) Raman mode of PNRs (upper panels, solid line) and
pristine C60 (lower panels). The deconvoluted peaks of the PNRs (dotted
lines) are indicated in the upper panels. The excitation laser wavelength
was 514 nm in (a) and 785 nm in (b), with the longer wavelength probing
deeper into the sample.
Figure 31 shows the Ag(2) Raman mode of PNRs (upper panels, solid line)
and pristine C60 (lower panels), with the deconvoluted PNR peaks indicated
by dotted lines in the upper panels. The excitation wavelength was λRaman =
514 nm and λRaman = 785 nm in figure 31(a) and figure 31(b), respectively.
Since the C60 absorption is significantly higher at 514 nm than 785 nm, the
former will probe the surface and the latter the bulk. A comparison of the
deconvoluted peaks of the Ag(2) mode shows a relative larger downshift at
514 nm, which is assigned to a higher degree of polymerization on the
surface than in the bulk. In other words, the PNRs can be described as
comprising a highly polymerized shell surrounding a mainly monomeric
core, as schematically indicated in the cartoon above the figure.
In order to investigate the effects of polymerization on the solubility, both
as-grown and PNRs were filtered out of the synthesis dispersion and added
to pure m-DCB in a concentration of 1 mg ml-1. The as-grown NRs were
observed to readily dissolve in the good C60 solvent, as evident by the
formation of a characteristic clear purple solution; see left vial in the
photograph in figure 32(a). Interestingly, the PNRs remained intact and well
dispersed in the m-DCB, as evident by the opaque brown dispersion in the
44
right vial in figure 32(a). In fact, subsequently recorded TEM images of the
PNRs reveal that these feature a completely smooth surface and are
completely unharmed by the good C60 solvent (see figure 32(b)). Similar tests
using common hydrophobic solvents showed similar results, indicating that
the PNRs can be processed in a wide range of organic solvents without
damage.
Figure 32. (a) Vials containing dissolved as-grown NRs (left vial with
purple solution) and dispersed PNRs (right vial with brown dispersion) at a
concentration of 1 mg ml-1 in m-DCB. (b) TEM image of PNRs, with the
inset showing a single PNR at higher magnification.
45
5.3. The C60 nanorod transistor
Figure 33. Schematic showing the NR FET structure, with the NRs aligned
in the shortest sourced-drain direction, i.e. perpendicular to the sourcedrain interfaces.
OFETs with NRs as the active material were realized in order to evaluate the
electronic transport properties of the NRs in the pristine state and following
the structural changes induced by annealing and photo-polymerization. A
schematic of the employed BG-TC device structure is shown in figure 33,
with p:Si/SiO2 (200 nm) as the gate/gate-oxide, and Cr/Au (15/15 nm) as the
source-drain electrodes. In order to accurately measure µn with equation (3),
it is important to accurately estimate the effective WD and LG of the NRs
involved in the transistor operation i.e. the NRs bridging the gated channel
between the source and drain electrodes. The WD can be readily calculated as
the product of the mean NR diameter (Ø) and the observed number of NRs
crossing the channel, while the estimation of LG benefits from the alignment
of the NRs in the shortest source-drain direction; see figure 33. This
geometry will also produce the largest source-drain current.
46
5.3.1. Nanorod deposition
Figure 34. Schematic representation of the drop-casting of as-grown NRs
from an ethanol/m-DCB dispersion onto a transistor structure,
highlighting (a) the initial ethanol evaporation and (b) the subsequent mDCB evaporation and the formation of a random network of NRs and
needle-like C60 crystals.
The NRs can conveniently be deposited by drop-casting, whereby a (~25 µl)
drop of NR dispersion was applied onto the transistor structure. The
dispersion comprises a solvent mixture of m-DCB and ethanol, with the two
solvents differing greatly in vapor pressure (0.252 kPa versus 7.87 kPa at 25
°C) [85]. As a result, the high-vapor pressure ethanol will evaporate much
faster during drying, leaving the slow-drying m-DCB on the substrate, as
depicted in figure 34. As a consequence, some of the NRs will dissolve in the
remaining m-DCB solvent, while the remaining undissolved NRs will deposit
randomly on the substrate. The dissolved C60 is observed to exclusively
deposit on the Au electrodes in the form of needle-like structures,
presumably due to the large surface energy difference between the bare SiO2
channel and the Au electrodes. The random positioning of the NRs have the
unfortunate consequence that the effective LG of drop-cast deposited NRs is
difficult to determine with good accuracy.
To circumvent these issues, the method of controlled dip-coating was
utilized. The transistor substrate was immersed into the NR dispersion with
the long side of the transistor channel parallel to the solution interface, and
then slowly withdrawn at a fixed angle of 70° and a constant rate of 3 cm
min-1; see schematic in figure 35(a). A photograph of an aligned NR in the
transistor channel is shown in the photograph in figure 36(a). The
corresponding angular distribution of the dip-coated NRs is shown in figure
35(b), where the measured distribution of the drop-cast NRs is included for
comparison as the dashed blue line. It is clear that dip-coating results in a
47
good alignment of the NRs in the transistor channel and that the issue of C 60
needle formation is effectively resolved. It is thus reasonable to approximate
LG with the shortest source/drain electrode distance in the calculation of the
mobility in transistor measurements on dip-coated NRs.
Figure 35. (a) Schematic of the dip-coating procedure and an optical
micrograph of dip-coated NR on a transistor structure (the light and dark
regions correspond to the Au electrodes and the SiO 2 channel, respectively).
(b) Angular distributions of the dip-coated NRs and drop-cast NRs, relative
to the channel.
48
5.3.2. Transistor performance
Figure 36(a) presents typical transfer characteristics of as-grown NRs (Ø1 =
250 nm) deposited by drop-casting, before and after annealing. The
undesired large hysteresis is suppressed after the annealing, and this
improvement in performance is assigned to the removal of the chargetrapping solvent molecules from the NRs. We have chosen to not include
data on the absolute mobility of the drop-cast NRs, due to the difficulty in
establishing the correct value for the gate length, LG, in drop-cast NR-based
OFETs, as discussed in detail in section 5.3.1.
Figure 36. Transfer characteristics of (a) drop-cast and (b) dip-coated NR
OFETs in different states. (c) Mobility distribution of as-grown NRs and
PNRs. (d) Output characteristics of a dip-coated PNR OFET.
Figure 36(b) shows the transfer characteristics of OFETs based on dipcoated NRs (Ø2 = 172 nm) as the active material, with the NRs either being
characterized as-grown or after photo-induced polymerization. Quantitative
performance data for dip-coated NRs as a function of nanorod diameter,
annealing and photo-polymerizaton are summarized in table 3. We find that
the highest mobility is recorded on as-grown NRs with the lower average
diameter of 172 nm. Both the annealing and photo-polymerization
49
procedures result in a significant drop for both the mobility and the on/off
ratio. We assign this reduction to the introduction of grain boundaries
during annealing and to the formation of lower-mobility C60 polymers during
photo-polymerization.
Treatment
Ø
(nm)
µn
(cm2 V-1 s-1) [a]
VT
(V) [a]
Ion/Ioff
(a.u.)
As-grown
250
4.4(±2.7)×10-2
17.4(±3.5)
103
Annealed
250
8.8(±2.6)×10-3
18.9(±1.7)
102
As-grown
172
3.0(±2.9)×10-1
21.5(±3.8)
104
Polymerized
172
4.7(±3.9)×10-3
20.4(±5.5)
103
[a] Mean
value with standard deviation in parenthesis
Table 3. Transistor data on dip-coated NR OFETs.
We also point out that the data in table 3 are average values (and standard
deviations), and that the variation in performance can be rather significant,
as demonstrated by the mobility distributions for as-grown and polymerized
NRs in figure 36(c). In fact, the best-performing NR-OFET, comprising asgrown NRs with an average diameter of 172 nm, did break the mobility
"unity" barrier at 1.0 cm2 V-1 s-1. This proves that solution-processable NRs
can compete with state-of-the-art vacuum-deposited C60 and a-Si [6, 86-88].
We further note that the drop in mobility following photo-polymerization is
in agreement with previous studies on the effects of chemical transformation
of C60 [63, 79]. In this context, it is, however, relevant to remind ourselves
that the charge accumulation and charge transport in an OFET take place in
the first few monolayers of the OSC deposited on the gate insulator, which
for PNRs correspond to the highly polymerized shell [19, 28, 89]. It is thus
reasonable to assume that the highly monomeric PNR core could exhibit a
higher mobility, similar to what was measured in as-grown NRs.
Typical output characteristics of a dip-coated PNR OFETs are shown in
figure 36(d). The devices exhibits a good IDS saturation at high VDS, but the
exponential increase at low VDS indicates a Schottky-type injection barrier at
the source electrode interface [90]. This observation, in combination with
the utilized simplification that all NRs are perfectly aligned in the transistor
channel, implies that the documented values for the mobility are
underestimating the true value [91, 92].
50
In summary, we have shown that single-crystal NRs can be synthesized in
solution using the LLIP technique, and that such NRs can be applied as the
active material in OFETs with a high electron mobility. A light-exposure step
can transform the NR surface to become polymeric, so that the resulting
PNRs can form a stabile dispersion in common hydrophobic solvents. With
these properties demonstrated, the NRs can be exploited in several
applications, and we mention the decoration of PNRs with Pd catalyst
particles, which resulted in an improvement in the oxidation of ethanol
(Paper V).
51
6. Material design for improved photovoltaics
The efficiency and practical usefulness of BHJ-OPVs are influenced by a
wide range of factors, and dedicated work in the field has resulted in the
demonstration of small-area devices with a PCE >10 % [55, 93, 94]. These
improvements are stemming from a reduction of the Eg of the constituent
compounds, so that the active material can absorb more of the incoming
solar radiation, and an improved active-material phase morphology as
manifested in an increased Jsc [55, 95-98]. It is also possible to attain a high
output voltage from BHJ-OPVs, but such reports are comparatively sparse
[55, 99]. Moreover, a well-documented issue with current BHJ-OPVs is the
poor morphological stability of the active material [52, 53, 100], and the
development of functional methods for the resolution of this setback is
highly needed.
6.1. A small-molecule donor compound for OPVs
Figure 37. (a) Molecular structure of ZOPTAN-TPA. (b) UV-vis absorption
spectra of ZOPTAN-TPA in solution (open circles) and as a thin film (solid
squares).
The BHJ-OPV commonly comprises an active-material blend of conjugated
polymer as the donor and a small-molecule fullerene as the acceptor. An
important advantage with small molecules is that they are comparatively
easy to synthesize and purify, and it is therefore of interest to employ smallmolecules also for the donor compound. For the attainment of a small
molecule donor compound with appropriate energy levels for high OPV
performance, we choose to synthesize a compound with an internal donoracceptor-donor (D-A-D) structure. By chemically connecting a (2Z,2Z)-2,2´(2,5-bis(octyloxy)-1,4-phenylene)bis(3-(5-bromothiophen-2-yl)acrylonitrile)
52
acceptor unit with two peripheral 4-(diphenylamino)phenylboronic acid
donor units, we attained the small-molecule donor compound ZOPTANTPA, with its chemical structure presented in figure 37(a); for a description
of the synthesis procedure, see Paper VI.
ZOPTAN-TPA was thermally characterized with thermogravimetric analysis
(TGA) and differential scanning calorimetry (DSC). The TGA showed that
the material is stable (under inert atmosphere) up to 410 °C, whereas the
DSC revealed that ZOPTAN-TPA can crystallize and that its melting point is
145 °C. Figure 37(b) shows the UV-vis absorption spectra of ZOPTAN-TPA in
solution (open circles) and as a thin film (solid squares). The film features a
red-shifted absorption onset in comparison to the solution, which is
positioned at ~600 nm; the latter corresponds to an optical energy gap of
Eg,OPT = 2.1 eV. The HOMO and LUMO levels of ZOPTAN-TPA were
established with cyclic voltammetry (CV), and these data and the
electrochemical energy gap (Eg,EC) and Eg,OPT are presented in table 4.
Compound
ZOPTAN-TPA
HOMO
(eV)
LUMO
(eV)
Eg,EC
(eV)
Eg,OPT
(eV)
–5.2
–3.0
2.2
2.1
Table 4. Measured energy levels and energy gaps for ZOPTAN-TPA.
By comparing the LUMO level of ZOPTAN-TPA with the LUMO level of
PCBM (-3.7 eV), we find that ΔELUMO = 0.7 eV in a PCBM:ZOPTAN-TPA
blend. Thus, it is expected that excitons, generated in the ZOPTAN-TPA
phase, will be efficiently split up at the ZOPTAN-TPA/PCBM interfaces in a
BHJ-OPV device. Moreover, as the difference in energy between the LUMO
of the PCBM acceptor and the HOMO of the ZOPTAN-TPA donor is notably
large at 1.5 eV, and this value dictates the maximum for the open-circuit
voltage, Voc, (see section 3.2.3), we also anticipate the opportunity for a high
output voltage.
BHJ-OPV cells were fabricated with an ITO/PEDOT:PSS anode and an Al
cathode sandwiching a PCBM:ZOPTAN-TPA active layer (see Paper VI for
the experimental procedure). The average photovoltaic performance of these
7×3 mm2 devices as a function of active material composition is presented in
figure 38(a)-(e). As anticipated, a high Voc was attained, with the maximum
of 1.03 V obtained for a PCBM:ZOPTAN-TPA mass ratio of 1:1. The Jsc, and
accordingly the EQE, featured a peak in performance for the 2:1 mass ratio.
The FF is relatively modest, with a maximum of 32 % for the 1:1 mass ratio.
In order to investigate whether recombination at the cathode could be the
53
culprit to the low FF, we also fabricated and tested devices with a 2nm LiF
electron-transport layer inserted between the cathode and the active
materials. As this procedure did not result in a significant improvement in
performance, we exclude cathodic recombination as the culprit. With the
information of Voc, Jsc and FF at hand, it is straightforward to calculate the
PCE, which features a peak value of 1.17 % for the 2:1 mass ratio.
Figure 38. (a) The open-circuit voltage, (b) the short-circuit current
density, (c) the fill factor, (d) the power conversion efficiency, and (e) the
external quantum efficiency for ITO/PEDOT:PSS/PCBM:ZOPTAN-TPA/Al
solar cells as a function of the PCBM:ZOPTAN-TPA mass ratio in the active
material.
We also wish to emphasize that the data in figure 38 represented the average
values (and standard deviations) derived from characterization of 14
independent devices, and that it is common that the performance of BHJOPV can vary significantly between different devices. We have therefore
chosen to include results from our “champion” device, being a non-annealed
ITO/PEDOT-PSS/PCBM:ZOPTAN-TPA/Al
OPV
cell,
with
a
PCBM:ZOPTAN-TPA mass ratio of 2:1. The champion device featured the
following performance metrics: Jsc = 6.01 mA cm-2, Voc = 0.98 V, FF = 32 %,
and PCE = 1.86 %.
The OPV performance as a function of annealing is presented in figure 39(a).
The 2:1 mass ratio was selected for this study, since it featured the best
performance (see figure 38). While only minor changes in Voc and FF were
found (data not shown), the Jsc, and consequently the PCE, showed major
drops in performance following annealing. For instance, a 60 s annealing at
70 °C under inert N2 atmosphere resulted in a 15 % and 19 % drop in Jsc and
PCE, respectively, and an increase in temperature to 90 °C resulted in a drop
of 26 % and 33 %, respectively.
54
Figure 39. Relative change of (a) Jsc and (b) PCE as measured on
PCBM:ZOPTAN-TPA (2:1) devices following annealing at 70 °C (solid line)
and 90 °C (dotted line) for varying time. AFM micrographs of similar (c)
as-prepared and (d) annealed (90 °C for 5 min) films.
AFM micrographs of the same type of PCBM:ZOPTAN-TPA blend film
directly after spin-coating and after a 5 min thermal annealing at 90 °C
under N2 atmosphere are displayed in figure 39(c) and figure 39(d),
respectively. The as-prepared film comprises small depressed features of
unknown origin, with a typical vertical size of <20 nm, on an otherwise
smooth film surface. With thermal annealing, the depressed features vanish
and are replaced by protruding features. The size and density of these
protruding features are observed to increase with both temperature and
annealing time. Based on the observation that ZOPTAN-TPA can crystallize
(with a melting temperature of Tm = 145 C, as established with DSC), we
attribute the observed growth of the protruding features to the enrichment
and subsequent crystallization of ZOPTAN-TPA within the blend film. As the
lateral size of these features of ~100 nm is much larger than the typical
exciton diffusion of ~10 nm in organic semiconductors, it is straightforward
to assign the observed decrease of Jsc with thermal annealing to a decrease in
the area of the critical charge-generating donor/acceptor interfaces in the
active-material film when (too) large ZOPTAN-TPA crystallites form.
55
6.2. The C60 nanorod OPV
Figure 40. Schematic device structure of an OPV based on C 60 polymerizednanorods.
One severe stability issue with BHJ-OPVs originates in the propensity of the
active material constituents to phase separate over time, with a reduction of
the exciton dissociation efficiency, and a lowering of the Jsc and PCE being a
common consequence [101, 102]. An example of this detrimental phaseseparation process was presented in the last paragraph in the previous
section, where an elevated temperature, as often encountered during OPV
operation, was shown to drive the undesired phase-separation process. Here,
we propose that the issue with an unstable phase morphology in BHJ-OPVs
could be alleviated by employing the polymerized nanorods as the acceptor
material for the realization of the device schematically depicted in figure 40.
We anticipate that not only will the charge-separation process be stabilized
in such a device, but also the subsequent charge-transport processes be
improved if the nanorods can be aligned with their long axes in the
interelectrode direction.
In order to investigate the feasibility of the outlined concept, we have
fabricated OPVs with a blend of PNR:P3HT. The as-grown NRs were first
ultrasonicated for 1 h to decrease their size to a length of 1-3 µm and a
diameter of 170 nm, thereafter photo-polymerized, and finally dispersed in a
P3HT-in-m-DCB solution, at a PNR:P3HT mass ratio of 1:2. This blend
solution/dispersion was spin-coated onto a PEDOT:PSS/ITO/glass
56
substrate, whereafter an Al cathode was thermally evaporated on top. The
performance of the OPV device was modest, but it is straightforward to
realize that a significant improvement should be attainable via a decrease in
NR size and through an alignment of the NRs, so that exciton split-up and
charge transport can be improved. In this context, it is interesting to note
that NRs with a small diameter of 50 nm can be synthesized; see figure
26(d).
57
7. Conclusions
We demonstrate that the fullerene PCBM can be photochemically
transformed into a dimeric state in a bi-excited reaction process, and that the
exposed material features a significantly reduced solubility in common
solvents as well as an effectively retained electron mobility. This attractive
combination of material properties allows for a direct and resist-free
lithographic patterning of electronic PCBM films down to a smallest feature
size of 1 µm, using a simple and scalable two-step process constituting light
exposure and solution development. In a further development, it was shown
that the two-step method was useful also in the area-selective transformation
of fullerene/conjugated-polymer blend films, as demonstrated through the
realization of a functional complementary logic circuit in the form of a 5stage ring oscillator.
In another project, we have synthesized highly flexible, single-crystal C60
nanorods with a solution-based self-assembly process termed liquid-liquid
interfacial precipitation. The 1-dimensional nanorods can be deposited from
their synthesis solution and employed as the active material in field-effect
transistor devices. Here, it was revealed that the as-fabricated nanorods can
feature an impressive electron mobility of 1.0 cm2 V-1 s-1, which is on par with
the performance of a work horse in the transistor field, viz. vacuumdeposited amorphous Si. We further demonstrated that the processability of
the nanorods can be improved by a tuned light-exposure treatment, during
which the nanorod shell is polymerized while the high-mobility interior bulk
is left intact. This has the desired consequence that stabile nanorod
dispersions can be prepared in a wide range of solvents, and we anticipate
that functional electronic devices, including organic solar cells, based on
solution-processable nanorods can be realized in a near future.
58
Acknowledgements
First and foremost I would like to thank my brilliant supervisor Ludvig
Edman for giving me the chance to come to Umeå and be a part of the
Organic Photonics and Electronics Group. I have truly learned a lot during
the last 5 years and much of it is thanks to you.
Next up I would like to thank the incredible people in the OPEG group,
starting with the office squad Amir Asadpoordarvish and Andreas
Sandström; may the science you are throwing at the wall be vitorious.
Nikolai Kaihovirta, good luck in Turku and may your frolf discs fly far and
true. A special gratitude is aimed towards my permanent collaborators Jia
Wang and Jenny Enevold for countless discussions about solar cells,
fullerenes, and other things. Special thanks also to the ink-maestro Shi Tang,
Mattias Lindh, Petter Lundberg and Ning Sun. I would also like to
acknowledge the previous group members Nahid Morshed, Jimmy
Granström, Antoni Munar and Piotr Matyba. Our lab-technician Roushdey
Sahl also deserves a big thanks for making all of our lives easier by keeping
the labs running.
I also owe gratitude towards my collaborators outside of the group, with
special mentions of the brilliant Hamid Barzegar and his equally brilliant
supervisor Thomas Wågberg. It was a true pleasure to work together with
you. I am also grateful for the science done and discussed together with
Javed Iqbal, Bertil Eliasson, Florian Nitze, Guangzhi Hu, Xueen Jia, Tiva
Sharifi, Srikanth Revoju, Eduardo Gracia, Nathaniel Robinson, Deyu Tu and
Robert Forchheimer.
I would also like to specially thank Patric Stafshede for offering me the
opportunity to join the LunaLEC crew. Together we will do great things.
I would also like to thank the people I got into contact with during my
teaching endeavors: Jens Zamanian, Leif Hassmyr, Hans Forsman, Gabriella
Allansson, Sune Pettersson, Ludvig Lizana, Lars-Erik Svensson, Patrik
Norqvist, Jonas Larsson, Isak Silander, Mats Forsberg and Michael Bradley.
A big thanks also to all students.
A big thanks to Jörgen Eriksson, Katarina Hassler, Lilian Andersson, Lena
Burström, Karin Rinnefeldt and Nils Blix for all the help with my various
silly questions and problems. Special thanks to the "bluecoats" of the
workshop Lena Åström, Tomas Gustafsson and Peter Wikström for very
professional help with complex demands. I would also like to acknowledge
59
the remaining staff at the department of physics for making it a great place
to work.
I am also grateful for the support from my friends from the south: Anil Dey
(although I suppose it is a bit more west than south now), Fredrik and Veddy
Hägerström and Tim Larsson. I moved 1000 km north but still you did not
get rid of me. I would also like to thank all my relatives for their
encouragements.
Finally, I am infinitely grateful for my brothers and sister Mattias, Felicia,
Kenny and Sebastian. And last, but certainly not least, my parents Marianne
and Lennart. Thank you for everything.
60
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