Anisotropy in Organic Single-Crystal Photovoltaic Characteristics

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DOI: 10.1002/adma.200701374
Anisotropy in Organic Single-Crystal Photovoltaic
Characteristics**
By Ricky J. Tseng, Ryan Chan, Vincent C. Tung, and Yang Yang*
–
[*] Prof. Y. Yang, Dr. R. J. Tseng, R. Chan, V. C. Tung
Department of Materials Science and Engineering
University of California, Los Angeles
Los Angeles, California 90095 (USA)
E-mail: [email protected]
[**] We acknowledge the financial support from the National Science
Foundation (NSF) and Air Force Office of Scientific Research
(AFOSR). We also acknowledge helpful discussions from Alejandro
Briseno and Prof. Zhenan Bao.
Adv. Mater. 2008, 20, 435–438
cating the tetracene crystal is c-oriented, with molecular-plane
growth along the vertical direction.[9,10]
The crystal data obtained for tetracene confirm a C18H6
molecular formula with a molecule weight of 222.23 g mol–1.
The lattice constants are a = 6.02 ± 0.025 Å, b = 7.77 ±
0.032 Å, c = 12.46 ± 0.054 Å, a =101.11 ± 0.078°, b = 99.41 ±
0.092°, and c = 94.40 ± 0.088°, and tetracene crystallizes with
a triclinic crystal structure in space group P
1. The coordinates
for tetracene can be obtained in the crystallographic information file (CIF) format from the Cambridge Crystallographic
Data Center (CCDC), and the above crystallographic data
are consistent with the data in the CCDC. Both the strong
birefringence and the XRD results indicate these organic single crystals are of high quality.
A schematic structure of a single-crystal solar cell is shown
in Figure 2a. The device structure comprised a poly(3,4-ethylenedioxythiophene): polystyrenesulfonate (PEDOT:PSS)coated indium tin oxide (ITO) substrate, a tetracene crystal,
evaporated thin films of C60 and bathocuproine (BCP), and
an aluminum thin-film electrode. We note that thin crystals
(ca. 200 nm) used in this study conformed better on substrates
(a)
(b)
(001)
(c)
(002)
Intensity (a. u.)
Organic single crystals have been well researched for many
years.[1] Typical vapor-phase growth of organic crystals developed from vertical[2] to horizontal growth[3] in order to
achieve improved crystalline quality. Recently, a field-effect
study on these single-crystal semiconductors demonstrated
high carrier mobilities, up to ca. 15 cm2 V–1s–1, along with anisotropic charge-transport properties.[4,5] These single-crystal
transistors were usually fabricated with rigid and thick crystals
(tens of micrometers to millimeters), which were fragile and
difficult to process because of poor mechanical properties.
Recently, a growth method affording thin, 150 nm thick, organic crystals demonstrated the processability of single-crystal
transistors on flexible substrates,[6] and these organic crystals
could be patterned on individual channels for transistors.[7]
However, in addition to transistor studies of these organic
crystals, other types of electronic applications, such as two-terminal devices, have not yet been realized, mainly because of
the difficultly processing thick crystals. In this Communication, we report organic single-crystal photovoltaics fabricated
from single pieces of thin tetracene crystals on bilayer heterojunctions with fullerene (C60) thin films. These organic singlecrystal devices exhibited excellent diode behavior with rectifying ratios of 105 and an external power conversion efficiency
(PCE) of ca. 0.34 %. By employing these high-quality single
crystals in two-terminal devices, high-performance optoelectronic devices, such as organic diodes, photovoltaics, and
photodetectors, become possible alternatives for large-area,
low-cost flexible electronics.
The quality of the single crystals was examined by cross-polarized microscopy,[8] X-ray powder diffraction (XRD), and
single-crystal X-ray diffraction. The optical microscopy images (Fig. 1a and b) recorded at 0° and 90° from the entrance
polarizer and exit analyzer, respectively, show large birefringence, confirming the anisotropic crystalline nature of the
tetracene crystals. The XRD data exhibit strong and narrow
first (001) and second order (002) reflections (Fig. 1c), indi-
5
10
15
2θ
Figure 1. Optical microscope images for a tetracene crystal under crosspolarized light recorded at a a) 0° and a b) 90° angle difference. c) XRD
pattern for a tetracene crystal.
© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
435
2
Current Density (mA/cm )
(b)
436
www.advmat.de
Absorbance
EQE (%)
2
Current Density (mA/cm )
COMMUNICATION
Organic solar cells based on singlecrystals/C60 adopted a bilayer structure
comprising an organic donor–acceptor
2
10
heterojunction. The employment of a
single crystal as a donor layer should
0
tetracene
10
improve the charge collection, as the
carrier mobility is much higher in sin-2
gle-crystal thin films than in amorphous
10
thin films.[4] The field-effect mobility of
-4
tetracene in single-crystal form[12] is
10
four times larger than in thin films[10]
because there are fewer structural de-2
0
2
4
6
fects and grain boundaries. Under solar
Bias (V)
illumination equivalent to one sun, the
device exhibited a short-circuit current
(c)
(d)
density (JSC) of 1.12 mA cm–2 and an
0.4
0.4
18
C60
open-circuit voltage (VOC) of 0.57 V.
Photocurrent
16
Dark current
Tetracene
The fill factor was 54 %, as defined by
0.3
14
0.0
VOC × JSC/VMJM, where VM and JM are
12
the voltage and current for the maxi10
0.2
-0.4
mum power output, respecticely
8
(Fig. 2c). The resulting PCE was
6
0.1
-0.8
4
0.34 %, defined as PCE = VOCJSCFF/
2
Iph, where Iph (photocurrent) is 100 mW
0.0
0
cm–2 of the incident photon density and
-1.2
-2
FF is the fill factor.
-0.2
0.0
0.2
0.4
0.6
350 400 450 500 550 600 650 700
Bias (V)
Furthermore, the absorption of these
Wavelength (nm)
crystals was not as strong as in their
thin-film counterparts because of the
Figure 2. Current–voltage (I–V) characteristics and PCE of tetracene-single-crystal/C60 solar cells.
a) A schematic device structure. b) The I–V curve shown on a log scale from –2 to 6 V. c) The I–V
semitransparency in these thin crystals.
characteristics for the device in the dark and under solar illumination equivalent to one sun. d) The
Figure 2d shows the absorbance spectra
absorbance and EQE in a tetracene-crystal/C60 device as a function of wavelength.
(tetracene crystal and C60) and external
quantum efficiency (EQE) as a function
than thick crystals (>1 lm), displaying better interfacial conof wavelength. The tetracene single crystal exhibited an abtact for electrical properties. The thinness of the crystals made
sorption range of approximately 400–540 nm with an absorpthem naturally bendable and electrostatically bonded (van
tion edge at 550 nm. The C60 had an absorption peak maxider Waals forces) to the PEDOT:PSS/ITO substrate. In conmum at 440 nm and a characteristic absorption at 335 nm.
trast, thick crystals > 1 lm did not adhere to the substrate very
The EQE displayed a maximum of 6.6 % at ca. 540 nm, givwell because of their relatively rigid form.
ing an internal efficiency of ca. 28.7 %. This value of internal
Typical diode electrical behavior in the dark was examined.
efficiency was comparable to values for thin-film systems.
Organic diodes with a rectifying ratio of approximately 105 at
There were also two local peak maxima of approximately 6 %
2 V and a current density of 102 mA cm–2 are shown in Figcentered at 455 and 490 nm. These local peaks were also obure 2b. The device also displayed a low leakage current denserved in other types of organic thin-film photovoltaics.[13,14]
–3
–2
sity of 10 mA cm under a reverse bias condition. This wellWith a decrease in wavelength, the EQE gradually increased
defined diode behavior indicates the crystal was in close conto 16 %, and a peak value of 10 % was observed at 385 nm.
tact with the two electrodes. The serial resistance (RS) of the
Comparing the absorbance spectra and EQE, the local abdevices near 13 X cm2 can be calculated from RS = DV/I in
sorption peaks in tetracene were responsible for the three
the semilogorithic plot of the dark current, where V and I are
peaks in the EQE. Overall this photovoltaic device showed
the voltage and current, respectively. In comparison, Hiramopromising result from a thin-crystal/C60 junction. This PCE
to and co-workers demonstrated perylene crystals (ca. 5 lm)
was still low compared to the 2–4 % small molecules in the
fabricated as organic light-emitting diodes by microtone slievaporated thin-film (ca. 50 nm)[13,14] or polymer blend sys[11]
cing. A relatively thick crystal was sandwiched in the device
tems,[15] mainly because of a relatively thick and highly transshowing very large operation voltages and low output current
parent crystal. The thicker crystal layer compared to the thinand rectifying ratios. Thus, the thin crystals in the present case
film system was larger than the exciton diffusion length,
provided advantages of easy processing and superior perforrestricting more charge collection. Nevertheless, this result is
mance in the two-terminal devices.
a proof-of-concept for utilizing large-area single crystals for
(a)
Adv. Mater. 2008, 20, 435–438
VOC (V)
Fill Factor (%)
Efficiency (%)
2
JSC (mA/cm )
(a)
COMMUNICATION
result of displaying better electrical contact. If the crystal
thickness could be further reduced, a higher efficiency is likely
achievable. Additionally, considering the stability of the crystal and devices, we found no performance degradation for
these crystal devices stored in a glovebox without encapsulation for more than six months.
The polarized absorption spectra are depicted in Figure 4a.
The spectra were obtained with a UV-vis spectrophotometer
equipped with a fixed polarizer by rotating the crystals. The
spectrum for a crystal with the b-axis parallel to the polarizer
exhibited nearly twice the absorbance of the spectrum for a
crystal with the b-axis perpendicular to the polarizer. This difference arises from the anisotropic crystal structure along the
a- and b-axes because of its molecular packing. This anisotropic absorption resulted in an anisotropic photocurrent and
photovoltaic efficiency of the device. The measured efficiencies along the b- and a-axes were ca. 0.09 % and 0.05 %,
respectively (Fig. 4b). We note that the efficiency data was
verified by rotating the polarizer 360° several times and was not considered a loss because of polarizer absorption. Anisotropic
(a)
(b)
charge transport in organic single crystals
0.6
has been demonstrated.[4] The anisotropic
-1
60
10
absorption and electroluminescence in
1
aligned polymers[16,17] or in organic single
-2
10
crystals[18] have also been reported else0.4
40
0.1
where. To our knowledge, this is the first
-3
10
time that anisotropic photovoltaic efficiency has been discovered in organic photovol0.01
-4
0.2
20
10
taic devices. The result is attributed to the
high-quality, thin, tetracene single crystals
1E-3
400
800
400
800
that we were able to fabricate into two-terCrystal Thickness (nm)
Crystal Thickness (nm)
minal devices. Yet, the efficiency was not
high, although they could be useful in optiFigure 3. Thickness-dependence of tetracene-crystal/C60 devices. a) Short-circuit current and
cal sensor applications for anisotropic deefficiency as a function of crystal thickness. b) Fill factor and open-circuit voltage as a function
of crystal thickness.
tection.
organic photovoltaics in a two-terminal device structure. The
efficiency could be further improved by using crystals covering wider absorption ranges.
The single-crystal photovoltaic devices exhibited PCE as a
function of crystal thickness. Whereas the crystal thickness decreased from 1000 to 220 nm, the PCE was enhanced by three
orders of magnitude, mainly because of the increase in JSC
(Fig. 3a). The increase in FF suggests the interfacial contact
was improved by thinner crystals because of better conformity
on the substrate, contributing a small part to the improvement
in the PCE (Fig. 3b). The VOC showed little dependence on
the crystal thickness, suggesting these devices had similar
photovoltaic effects (Fig. 3b). This increase in efficiency reflects the lowering of the serial resistance in the devices from
203 to 13 X cm2 for thicknesses of 720 and 220 nm, respectively. Thinner crystals gave more efficient charge collection
from traveling less distance. Another reason is that thinner
crystals better adhered and conformed to the substrates as a
(b)
90
0.10
120
b-axis
60
0.08
30
Efficiency (%)
150
0.06
0.04
180
0.04
0
a-axis
0.06
330
210
0.08
0.10
240
300
270
Figure 4. a) The absorbance of a tetracene single crystal with the crystal b-axis parallel or perpendicular to the polarizer. b) PCE in a tetracene-crystal/
C60 device as a function of polarizer rotation.
Adv. Mater. 2008, 20, 435–438
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In summary, organic single crystals as thin as 200 nm were
used to fabricate photovoltaic cells using a bilayer heterojunction of tetracene single crystals and C60 thin films. An external
PCE of 0.34 % was achieved under AM 1.5 at 100 mW cm–2
with a high open-circuit voltage of ca. 0.57 V, a short-circuit
current density of ca. 1.12 mA cm–2, and a fill factor of ca.
54 %. These single-crystal/C60 solar cells showed an increased
efficiency as the crystal thickness decreased. These devices
also exhibited very interesting anisotropic photovoltaic efficiency as a function of the crystallographic axis. The performance was due to the thinness of the high-quality crystals,
which were demonstrated to have applications other than
charge transport in transistor devices.
Experimental
Crystal Growth: The growth of tetracene crystals followed a typical
horizontal vapor-phase growth in a gradient sublimation furnace with
argon flow [3]. Details regarding crystal growth are reported elsewhere [6]. The tetracene source material was placed in the hottest
zone at approximately 170 °C, whereas single crystals were grown in a
cooler zone at 150°C. It is noted that the growth temperature varied
with different furnace instruments and different materials. This method yielded tetracene crystals with thicknesses from 200 to 1400 nm
and areas as large as ca. 1 cm2. This unique growth method allowed us
to fabricate two-terminal devices with thin crystals, which was different from the bulky crystals grown by traditional supersaturated solution methods [19]. Considering the single-crystal quality of tetracene,
a diffraction experiment was run on a Bruker Prospector APEX single crystal X-ray diffractometer at 100 K to collect a sufficient number
of reflections.
Device Fabrication and Characterization: The device fabrication began by carefully placing a thin tetracene crystal on top of a substrate.
The substrates were prepared by spin-coating a 30 nm layer of PEDOT:PSS at a speed of 4000 rpm on ITO-coated glass. The substrates
were then baked at 120 °C for 30 min. The PEDOT:PSS layer has
been widely used between ITO and organic layers for charge injection
and collection mconsider referencingm. A polydimethylsiloxane
(PDMS) stamp was used to provide pressure on top of the crystal surface to raise adhesion between the crystal and PEDOT substrate.
Then C60, BCP, and aluminum were thermally evaporated on the crystal surface with thicknesses of 40, 12, and 120 nm, respectively, under
a background pressure of 5 × 10–6 Torr (1 Torr = 133.32 Pa). The
evaporation rates for the above layers were ca. 0.4, 0.2, and 3 Å s–1.
The BCP was reported as a hole-blocking layer, which was helpful for
electron collection on the cathode side.
The crystal and film thicknesses were confirmed by a profilometer
(Dektek 3030). The areas of devices were defined through the shadow
mask and carefully examined under the microscope between
0.0017 and 0.043 cm2. The I–V characteristics were measured by a
Keithley 2400 source measurement unit. The photocurrent was ob-
tained under AM 1.5 solar illumination at 100 mW cm–2 supplied by a
xenon-lamp-based solar simulator (Oriel 96000 150 W solar simulator), and light intensity was calibrated with a Newport 818T-10 silicon
photodiode. The EQE was determined on a homemade system with
assistance from the National Renewable Energy Laboratory (NREL)
[20]. The system comprised a standard reference Si photodiode
equipped with a lock-in amplifier and a solar lamp. The absorption
spectra were recorded with a Varian Cary 50 UV-vis spectrophotometer. All the anisotropic data were collected by the insertion of a linear
polarizer (Chroma Technology, 21003b) in front of the device or crystal. The loss of photovoltaic efficiency and absorption because of
polarizer absorption were not considered.
Received: June 8, 2007
Revised: August 22, 2007
Published online: January 9, 2008
–
[1] W. Warta, N. Karl, Phys. Rev. B 1985, 32, 1172.
[2] Ch. Kloc, P. G. Simpkins, T. Siegrist, R. A. Laudise, J. Cryst. Growth
1997, 182, 416.
[3] R. A. Laudise, Ch. Kloc, P. G. Simpkins, T. Siegrist, J. Cryst. Growth
1998, 187, 449.
[4] V. C. Sundar, J. Zaumseil, V. Podzorov, E. Menard, R. L. Willett,
T. Someya, M. E. Gershenson, J. A. Rogers, Science 2004, 303, 1644.
[5] V. Podzorov, E. Menard, A. Borissov, V. Kiryukhin, J. A. Rogers,
M. E. Gershenson, Phys. Rev. Lett. 2004, 93, 086 602.
[6] A. L. Briseno, R. J. Tseng, M. M. Ling, E. H. L. Falco, Y. Yang,
F. Wudl, Z. Bao, Adv. Mater. 2006, 18, 2320.
[7] A. L. Briseno, S. B. Mannsfeld, M. M. Ling, S. Liu, R. J. Tseng,
C. Reese, M. E. Roberts, Y. Yang, F. Wudl, Z. Bao, Nature 2006,
444, 913.
[8] J. Vrijmoeth, R. W. Stok, R. Veldman, W. A. Schoonveld, T. M. J.
Klapwijk, J. Appl. Phys. 1998, 83, 3816.
[9] H. Akimichi, T. Inoshita, S. Hotta, H. Noge, H. Sakaki, Appl. Phys.
Lett. 1993, 63, 3158.
[10] D. J. Gundlach, J. A. Nichols, L. Zhou, T. N. Jackson, Appl. Phys.
Lett. 2002, 80, 2925.
[11] K. W. Yee, M. Yokoyama, M. Hiramoto, Appl. Phys. Lett. 2006, 88,
083 511.
[12] R. W. I. de Boer, T. M. Klapwijk, A. F. Morpurgo, Appl. Phys. Lett.
2003, 83, 4345.
[13] S. Yoo, B. Domercq, B. Kippelen, Appl. Phys. Lett. 2004, 85, 5427.
[14] P. Peumans, S. R. Forrest, Appl. Phys. Lett. 2001, 79, 126.
[15] G. Li, V. Shrotriya, J. Huang, Y. Yao, T. Moriarty, K. Emery, Y. Yang,
Nat. Mater. 2005, 4, 864.
[16] P. Dyreklev, M. Berggren, O. Inganäs, M. R. Andersson, O. Wennerström, T. Hjertberg, Adv. Mater. 1995, 7, 43.
[17] H. Yanagi, T. Morikawa, S. Hotta, K. Yase, Adv. Mater. 2001, 13, 313.
[18] W. Xie, Y. Li, F. Li, F. Shen, Y. Ma, Appl. Phys. Lett. 2007, 90,
141 110.
[19] M. MAs-Torrent, M. Durat, P. Hadley, X. Ribas, C. Rovira, J. Am.
Chem. Soc. 2004, 126, 984.
[20] V. Shrotriya, G. Li, Y. Yao, T. Moriarty, K. Emery, Y. Yang, Adv.
Funct. Mater. 2006, 16, 2016.
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Anisotropy in Organic Single-Crystal Photovoltaic Characteristics

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