Embedding metal electrodes in thick active layers

Embedding metal electrodes in thick active
layers for ITO-free plasmonic organic solar cells
with improved performance
Sangjun Lee,1 Daniel R. Mason,2 Sungjun In,2 and Namkyoo Park2,*
2
1
Department of Electrical and Computer Engineering, Ajou University, Suwon 443-749, South Korea
Photonic Systems Laboratory, School of EECS, Seoul National University, Seoul 151-744, South Korea
*
[email protected]
Abstract: We propose and numerically investigate the optical performance
of a novel plasmonic organic solar cell with metallic nanowire electrodes
embedded within the active layer. A significant improvement (~15%) in
optical absorption over both a conventional ITO organic solar cell and a
conventional plasmonic organic solar cell with top-loaded metallic grating
is predicted in the proposed structure. Optimal positioning of the embedded
metal electrodes (EME) is shown to preserve the condition for their strong
plasmonic coupling with the metallic back-plane, meanwhile halving the
hole path length to the anode which allows for a thicker active layer that
increases the optical path length of propagating modes. With a smaller sheet
resistance than a typical 100 nm thick ITO film transparent electrode, and
an increased optical absorption and hole collection efficiency, our EME
scheme could be an excellent alternative to ITO organic solar cells.
©2014 Optical Society of America
OCIS codes: (350.6050) Solar energy; (040.5350) Photovoltaic; (250.5403) Plasmonics;
(310.6860) Thin films, optical properties.
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(C) 2014 OSA
30 June 2014 | Vol. 22, No. S4 | DOI:10.1364/OE.22.0A1145 | OPTICS EXPRESS A1145
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1. Introduction
High optical transparency and large electrical conductivity of top-loaded electrodes in solar
cells ensures efficient transmission of light into the active layer and efficient transport of
collected charge carriers. While indium tin oxide (ITO) has been widely adopted as an
excellent transparent electrode for organic solar cells [1, 2], its high fabrication cost,
mechanical brittleness, limited supply, and problematic contamination of organics [2–4] has
prompted a search for ITO-free electrode schemes. Previously considered ITO alternatives
include an un-patterned (flat) thin metal film [5, 6], randomly [7, 8] or periodically [9–13]
patterned metal layers, conducting polymers [14], carbon nanotubes [15], and graphene [16].
Among these, nano-patterned metal electrodes which further serve as the functional element
in plasmonic solar cells [17–22] have offered improved or comparable performance to their
ITO-counterparts owing to their strong plasmonic enhancement of optical absorption (i.e.,
electron-hole pair production) in the active layer [9–13]. For example, the organic solar cell
with top-loaded nanowire electrode has demonstrated exceptional performance due to strong
plasmonic coupling between the electrodes and metallic back-plane [9–12, 22]. But an
inherent drawback of this scheme is that its performance is strongly dependent on the active
layer thickness (i.e., the distance between electrodes and back-plane) which determines the
plasmonic coupling strength, placing an upper limit on the active layer thickness, beyond
which performance becomes worse than its ITO-counterpart [9–11]. The potential
performance benefits associated with increased active layer thickness, such as increased
optical path length of propagating modes in the active layer, thus become inaccessible.
In this paper, we propose and theoretically investigate the optical and electrical
performance of a novel implementation scheme of metal nanowire electrodes designed to
overcome the performance limitations of a conventional plasmonic organic solar cells with
top-loaded metallic gratings (denoted ‘conventional metal electrode’ (CME, Fig. 1(a)) [9–
#210672 - $15.00 USD Received 22 Apr 2014; revised 24 May 2014; accepted 27 May 2014; published 10 Jun 2014
(C) 2014 OSA
30 June 2014 | Vol. 22, No. S4 | DOI:10.1364/OE.22.0A1145 | OPTICS EXPRESS A1146
13]). Our proposed structure, consisting of a periodic Ag nanowire anode (rectangular
element size wAg x dAg) embedded with an index matching SiO2 element (wAg x dSiO2) is
denoted ‘embedded metal electrode’ (EME, Fig. 1(b)). As a reference, both CME and EME
are compared to a typical ITO-based structure with top-loaded 100 nm thick ITO film (Fig.
1(c)). Our results show that, contrary to the CME and ITO reference, the EME exhibits an
optical absorption that increases monotonically with the active layer thickness owing to its
preservation of optimal plasmonic coupling between the electrode array and metallic backplane, at the same time as the optical path length is increased in the thicker active layer.
Furthermore, embedding of the electrodes in our EME scheme preserves the hole path length
(to the anode) of the optimal CME despite a two-fold increase in active layer thickness. On
top of its improved optical performance, our calculations of the EME sheet resistance suggest
it as a suitable replacement of ITO as a transparent electrode in organic solar cells.
Light (AM 1.5G)
(a)
y
z
-+
x
wAg
Glass
MoO3
CME
dHTL
Period
Glass
MoO3
wAg
+
-
SiO2
dAct
EME
-+
Light (AM 1.5G)
(c)
Glass
dSiO2
dAct
dAg
dCav P3HT:PCBM
Al (Cathode)
Light (AM 1.5G)
(b)
dAg
ITO(100nm)
MoO3
dHTL
dHTL
dAct
dCav P3HT:PCBM
Al (Cathode)
P3HT:PCBM
Al (Cathode)
Period
Fig. 1. Schematic diagrams of solar cells with (a) conventional metal electrodes (CME), (b) our
proposed embedded metal electrode scheme (EME), and (c) 100nm thick ITO-based reference.
Note that dashed arrows in the active region show examples of transport pathways of holes ( +
) and electrons (-).
2. Performance improvement by embedded metal electrode (EME) scheme
2.1 Device architecture and suggested fabrication method
The suggested EME based solar cell structure (Fig. 1(b)) could be easily fabricated, first by
sequential deposition of SiO2 and Ag on a glass substrate, followed by lithographic patterning
to form the periodic structure. Sequential deposition of the hole transporting layer (MoO3),
active layer (P3HT:PC61BM(1:1)), and Cathode (Al reflector) completes the device. A 10nm
thick conformal deposition was assumed for MoO3 [23]. Continuous deposition of the active
layer is enabled by annealing under optimal conditions to ensure a flat surface at the Al/active
layer interface. We note that the CME is a special case of EME where dSiO2 = 0 [9–13]. All
material properties were taken from experimentally measured complex and dispersive optical
constants (P3HT:PC61BM [1], Ag and Al [24], ITO [25]). The refractive indices of Glass and
SiO2 are fixed at n = 1.5. Note that our choice of SiO2 as dielectric element (insulator)
provides index matching with the glass substrate to simplify the analysis.
2.2 Optical absorption and electrical properties of EME solar cells
The sheet resistance Rs of EME is calculated using Eq. (A3) in Appendix to determine the
nanowire dimensions at which Rs is comparable to an ITO transparent electrode (i.e., an ITO
film) with the typical uniform thickness of 100 nm. Figure 2(a) shows Rs as a function of wAg
for Ag nanowires with dAg = 10, 20, and 30nm, and with the optimal Period of 300nm (see
below); Rs is inversely proportional to wAg. To ensure a better conductivity than the ITO film
(i.e., Rs<11Ω/sq – see gray shaded area on Fig. 2(a)) [6], it is evident that metal electrodes
with wAg = 300nm (termed ‘Flat’, corresponding to the uniform Ag film) should be chosen
when dAg = 10nm, wAg>~85nm when dAg = 20nm, and wAg>~45nm when dAg = 30nm. All
considered EME structures satisfy these geometrical constraints.
#210672 - $15.00 USD Received 22 Apr 2014; revised 24 May 2014; accepted 27 May 2014; published 10 Jun 2014
(C) 2014 OSA
30 June 2014 | Vol. 22, No. S4 | DOI:10.1364/OE.22.0A1145 | OPTICS EXPRESS A1147
0.46
dCav (nm)
140
120
0.42
100
0.38
80
0.34
40
0.30
20
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
60
100
140 180
wAg (nm)
220
260
100
125
dAct (nm)
150
(e)
175
dAct (nm)
0.52
0.51
0.5
0.49
0.48
0.47
0.46
0.45
0.44
200
TM pol.
ITO(100)
CME(100)
CME(200)
EME(200)
350 400 450 500 550 600 650 700 750
Wavelength (nm)
(d)
EME(dAg=20)
EME(dAg=30)
FOM
FOM
ITO
Flat(dAg=10, wAg=300)
CME(dAg=20, wAg=120)
EME(dAg=20, wAg=120, dCav=80)
CME(dAg=30, wAg=100)
EME(dAg=30, wAg=100, dCav=70)
75
110 120 130 140 150 160 170 180 190 200
300
(c)
50
Absorption Efficiency
0.50
(b)
60
20
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
160
ITO
dAg=10
dAg=20
dAg=30
Absorption Efficiency
Sheet Resistance (Ω/sq)
(a)
FOM
50
45
40
35
30
25
20
15
10
5
0
1
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
1.5
2
nDie
(f)
2.5
3
TE pol.
ITO(100)
CME(100)
CME(200)
EME(200)
350 400 450 500 550 600 650 700 750
Wavelength (nm)
Fig. 2. (a) Sheet resistance (Rs) as a function of wAg for silver nanowire electrodes with
different thickness (dAg). (b) FOM as a function of dAct and dCav for EME with dAg = 20nm,
wAg = 120nm. (c) FOM as a function of dAct for solar cells with indicated parameters. The
black dashed lines in (a) and (c) correspond to the ITO reference structure. (d) FOM with
varying index of the dielectric element for optimized EME (dAct = 200nm, dCav = 80nm for
dAg = 20nm, wAg = 120nm). Absorption spectra for (e) TM and (f) TE polarizations in CMEs
with dAct = 100nm (optimal) and 200nm, and optimized EME. The black dashed line
corresponds to ITO structure with dAct = 100nm.
To serve as a useful comparison to our proposed EME scheme, we first geometrically
optimize the CME with respect to the Figure of Merit (FOM). The FOM is defined as the ratio
of the number of photons absorbed in the active layer to the total number of incident photons
(see Appendix). We find the optimal parameters wAg = 120nm, dAct = 100nm, and Period =
300nm, when dAg = 20nm (see Appendix for details), and similarly wAg = 100nm, dAct =
100nm, and Period = 300nm, when dAg = 30nm. The EME nanowire dimensions (dAg, wAg,
Period) were chosen to be the same as those of optimized CMEs at respective Ag thicknesses
to ensure identical sheet resistance. Subsequently, the EME structure was optimized over dAct
and dCav (see Fig. 2(b)). All of the EME structures with dAct>100nm exhibit a maximum in
the FOM at dCav corresponding to that of the optimized CME (dCav = 80nm for dAg =
20nm, and dCav = 70nm for dAg = 30nm). This fact is due to preservation of strong
plasmonic coupling between the electrodes and metallic backplane, as we discuss later.
Figure 2(c) shows FOMs as a function of dAct for optimized CMEs and EMEs with dAg =
20nm and 30nm, as well as flat metal electrode with dAg = 10nm (wAg = 300nm), and 100nm
thick ITO reference structure. Sheet resistances of each of the considered EME are marked by
circles in Fig. 2(a). A key aspect of Fig. 2 (c) is that a thicker active layer does not guarantee a
larger FOM in CME when dAct>100nm. Indeed, in contrast with the ITO reference, we see
#210672 - $15.00 USD Received 22 Apr 2014; revised 24 May 2014; accepted 27 May 2014; published 10 Jun 2014
(C) 2014 OSA
30 June 2014 | Vol. 22, No. S4 | DOI:10.1364/OE.22.0A1145 | OPTICS EXPRESS A1148
that beyond the optimal thickness dAct~100nm, a further increase in dAct leads to serious
reduction of the CME FOM, which is consistent with previous studies [9, 10]. This reduction
of FOM is associated with a weakening of plasmonic coupling between the electrodes and
metallic back-plane due to the increase in dCav (defined as cavity thickness between Cathode
and Anode – Fig. 1 (a)) at longer wavelengths. Meanwhile, in the case of EME structures, we
find that FOM linearly increases with dAct (see circular data points in Fig. 2 (c)).
Figures 2(e) and 2(f) show absorption efficiencies in the active layer (as defined in
Appendix) as a function of wavelength in CMEs with dAct = 100nm (optimal) and 200nm,
and optimized EME with dAct = 200nm, dCav = 80nm (for dAg = 20nm, wAg = 120nm), for
TM and TE polarizations, respectively. It is noted that the absorption efficiencies for the
optimized EME are systematically larger than the optimized CME over the entire wavelength
range regardless of polarization.
CME (TM)
dAct=200nm
Glass
200
(b)
CME (TM)
dAct=200nm
Glass
200
CME
λ =500nm
CME
Y (nm)
CME
λ =600nm
(c)
CME (TM)
dAct=200nm
Glass
200
CME
Glass
1.0
CME
P3HT:PCBM
0
2.2
2.0
1.5
100
P3HT:PCBM
0
Y (nm)
λ =400nm
Glass
100
P3HT:PCBM
0
Y (nm)
Y (nm)
(a)
CME
CME (TE)
dAct=100nm
Y (nm)
P3HT:PCBM
0
Glass
100
CME
Y (nm)
Glass
100
CME (TM)
dAct=100nm
CME (TM)
dAct=100nm
Y (nm)
Y (nm)
CME (TM)
dAct=100nm
(d)
λ =600nm
CME (TE)
dAct=200nm
Glass
200
CME
0.5
0
2.2
2.0
1.5
1.0
Y (nm)
Y (nm)
EME (TM)
dAct=200nm
Glass
200
EME (TM)
dAct=200nm
Glass
(i)
-150
(g)
λ =600nm
EME (TM)
dAct=200nm
Glass
200
200
(h)
λ =600nm
EME (TE)
dAct=200nm
Glass
SiO2
SiO2
SiO2
EME
EME
EME
EME
λ =400nm
0
X (nm)
(j)
λ =500nm
P3HT:PCBM
P3HT:PCBM
P3HT:PCBM
0
0
(k)
λ =600nm
0
(l)
0.5
0
2.2
2.0
1.5
200
SiO2
P3HT:PCBM
0
λ =500nm
(f)
0
Y (nm)
λ =400nm
0
Y (nm)
(e)
0
P3HT:PCBM
P3HT:PCBM
P3HT:PCBM
P3HT:PCBM
0
λ =600nm
1.0
0.5
0
150
Fig. 3. Normalized electric field amplitudes: Upper, middle, and bottom rows are for optimized
CME with dAct = 100nm, CME with dAct = 200nm, and optimized EME with dAct = 200nm
respectively. First, second, and third columns are for TM polarization at λ = 400nm, 500nm,
and 600nm, and fourth column is for TE polarization at λ = 600nm respectively.
In detail, the optimized CME exhibits strong absorption enhancement (compared to the
ITO reference with dAct = 100nm – see black dashed line) at longer wavelengths (λ>570nm),
and a distinct absorption peak at λ~600nm. The corresponding normalized electric field
amplitude distributions at λ = 600nm are plotted in Fig. 3(c) for TM, and Fig. 3(d) for TE
polarization. The characteristic coupled plasmonic mode between electrode and metallic back
plane is evident for TM polarization [10, 11], while a photonic mode (termed 'waveguide
effect' in Ref [10]) is evident between neighboring electrodes for TE. Increasing dAct from
100nm to 200nm strikingly weakens the TM resonance, and transforms the TE resonance, as
shown in Fig. 3(g) and 3(h). While the CME with dAct = 200nm is more highly absorbing in
the shorter wavelength range for both polarizations (see blue curves in Fig. 2(e) and 2(f)),
#210672 - $15.00 USD Received 22 Apr 2014; revised 24 May 2014; accepted 27 May 2014; published 10 Jun 2014
(C) 2014 OSA
30 June 2014 | Vol. 22, No. S4 | DOI:10.1364/OE.22.0A1145 | OPTICS EXPRESS A1149
which is simply due to its larger active layer volume (compared to the optimized CME or
EME structures) as shown in the field plots at λ = 400nm (Fig. 3(a), 3(e), and 3(i)), the severe
degradation of the plasmonic coupled mode for TM polarization results in a poor FOM
compared to the EME with same dAct.
Both optimized CME and EME structures resemble each other in the shape of their
absorption spectra, which shows that EME still maintains the cavity resonances of the
optimized CME, while additional absorption is accrued due to an increased optical path length
of propagating waves in the thicker active layer - compare Fig. 3(k) and 3(l) with Figs. 3(c)
and 3(d). Indeed, the electric field profiles show that the EME preserves conditions for
optimal absorption enhancement associated with the plasmonic mode coupling – dCav, the
main coupling parameter, taking the same value as in the optimized CME structure – while
the increased active layer thickness provides additional absorption by increasing the path
length of propagating waves in the active layer. Comparing field plots at λ = 500nm (Fig.
3(b), 3(f), and 3(j)) shows that the CME with dAct = 200nm exhibits poor plasmonic
enhancement of absorption since the field of the localized plasmonic mode of the electrode is
mostly confined to the Glass substrate. On the whole, for thicker electrodes of dAg = 30m, the
superiority of EME is also preserved although the FOM decreases due to an increase in ohmic
loss and light reflection. The effect of refractive index (nDie) of the dielectric element on the
optical absorption for the optimized EMEs is shown in Fig. 2(d). The nDie range of 1.0~3.0
was chosen considering that MoO3, P3HT:PCBM, and Glass have indices within the range
1.5~2.2. Interestingly, it can be seen that the optimal case is nDie ≈1.5, which suggests that
index matching of the dielectric element with the Glass substrate is preferable.
From Fig. 2 (c), we see that the optimized EME with dAg = 20nm has a maximum FOM
of 0.5085 at dAct = 200nm, which constitutes a ~15% FOM improvement compared to the
optimized CME (FOM = 0.4437). Even at large dAct, the EME structure maintains
comparable optical performance to the ITO-counterpart with dAct = 200nm. However, it is
important to note that the ITO reference with dAct = 200nm would exhibit very low charge
collection efficiency, as does the CME, when dAct>100nm [1]. In this regard, the EME makes
the best use of a thick active layer without sacrificing the electrical performance. It is
noteworthy that the hole mobility (3.0 x 10−8 m2/V/s) is ten times smaller than the electron
mobility (3.0 x 10−7 m2/V/s) in active layer [1]. Therefore, it is the hole mobility which limits
the thickness of the active layer. But more generally, it is not the active layer thickness that is
the limiting factor, but rather the distance between outlying electron-hole production sites and
the metal electrodes (i.e., the anodes which function as hole collectors). A CME (or ITO
reference) structure with dAct>100nm is not desirable in terms of electrical performance [9,
10]; increased carrier recombination along the hole diffusion path would destroy the solar cell
efficiency. Conversely, embedding of the electrodes into the active layer in the EME structure
can preserve the same effective hole diffusion path length as the optimal CME while enabling
a doubling of the active layer thickness. That is, the EME with dAct = 200nm preserves this
distance at roughly 100nm since the electrodes are embedded roughly 100nm into the 200nm
thick active layer, thus maintaining the same electrical performance as the CME with dAct =
100nm.
3. Conclusion
We proposed a new type of plasmonic organic solar cell based on a periodic array of metal
nanowire electrodes embedded into an active layer. A large (~15%) enhancement of optical
absorption in the EME structure over both conventional plasmonic solar cells with top-loaded
metallic gratings and ITO based structures is demonstrated by effectively accessing the
regime of large active layer thickness where its CME and ITO counterparts are known to
suffer from poor plasmonic coupling and low hole collection efficiency. The performance
improvements are shown to originate from the increased optical path length inherent to a thick
active layer, while at the same time preserving the optimal conditions of plasmonic coupling
#210672 - $15.00 USD Received 22 Apr 2014; revised 24 May 2014; accepted 27 May 2014; published 10 Jun 2014
(C) 2014 OSA
30 June 2014 | Vol. 22, No. S4 | DOI:10.1364/OE.22.0A1145 | OPTICS EXPRESS A1150
between the metal nanowire and metallic back-plane. Our suggested EME scheme, providing
improved optical performance, a similar charge collection efficiency to CME and ITO
structures despite its twofold increase in the active layer thickness, and with a lower sheet
resistance than a 100nm thick ITO transparent electrode, stands as an excellent candidate for
ITO replacement in organic solar cells, and motivates further study of plasmonic organic solar
cells with electrodes embedded into thick active layers.
Appendix: Simulation details
Under TM / TE normally incident plane wave illumination (magnetic / electric field parallel
to the z-axis) from Glass substrate, 2D FEM calculations are conducted on a single unit cell
uniform in the z-direction, with periodic boundary conditions along the x-axis, and perfectly
matched layers at the top and bottom of the unit cell.
Absorption properties of the solar cells are investigated in the wavelength range of 350nm
to 750nm, using the relative illumination intensity obtained from the standard AM1.5G solar
radiation spectrum. To quantify the absorption performance over the entire considered
spectral range under averaged polarization of (TM+TE)/2, we also defined a figure of merit
(FOM) based on the ratio of number of photons absorbed in the active layer to the total
number of incident photons,
 λmax λ
  λmax λ

FOM =  
⋅ I (λ ) ⋅ A(λ ) ⋅ d λ  ×  
⋅ I (λ ) ⋅ d λ 
λmin hc
λmin hc

 

−1
(A1)
where h, c, and I(λ) are the Plank constant, speed of light in free space, and solar irradiance
spectrum (AM1.5G), respectively, and the optical absorption efficiency A(λ) is defined as the
fraction of incident power absorbed in the active layer [26]:
1 2π ⋅ c
2
) ⋅ ε 2 (λ ) ⋅ E ( x, y, λ ) dV
A(λ ) = Pin−1  Qav dV = Pin−1  (
2 λ
(A2)
where Qav and Pin are the time-averaged power loss per unit volume and the incident power.
The integral is evaluated over the active layer, using the volume integration of Qav during
post-processing in COMSOL [27]; λ and E are the free-space wavelength and electric field
vector, and ε2 is the imaginary part of the dielectric constant of the active layer.
We determine the DC sheet resistance (Rs) of a periodic silver nanowire array by applying
the ‘FS:MS fitting model’ to include quantum effects as follows [6, 28]:
 ρ + ρ MS − ρ Bulk   Period 
Rs =  FS
×

dAg

  wAg 
(A3)
where ρFS, ρMS, and ρBulk are the resistivities (in units of Ωm) of a thin metal film from the FS
model, MS model, and bulk metal, respectively.
The optimization of the CME structure is presented in Fig. 4, which shows color maps of
FOMs for different Period, as a function of wAg/Period and dAct when dAg = 20nm.
#210672 - $15.00 USD Received 22 Apr 2014; revised 24 May 2014; accepted 27 May 2014; published 10 Jun 2014
(C) 2014 OSA
30 June 2014 | Vol. 22, No. S4 | DOI:10.1364/OE.22.0A1145 | OPTICS EXPRESS A1151
0.8
(a)
0.7
wAg / Period
Period
0.6 = 200nm
0.5
0.4
50 60 70
Period
0.6 = 300nm
0.42
0.5
0.38
0.4
0.34
80
50 60 70
90 100 110 120 130 140 150
dAct (nm)
0.8
(c)
0.7
wAg / Period
Period
0.6 = 400nm
0.5
0.4
80
90 100 110 120 130 140 150
0.50
(d)
0.46
0.7
Period
0.6 = 500nm
0.42
0.5
0.38
0.4
0.34
0.30
0.3
0.3
50 60 70
80
90 100 110 120 130 140 150
dAct (nm)
0.30
dAct (nm)
FOM
wAg / Period
0.46
0.7
0.3
0.3
0.8
0.50
(b)
FOM
wAg / Period
0.8
50 60 70
80
90 100 110 120 130 140 150
dAct (nm)
Fig. 4. Color maps of FOMs for different Period, as a function of dAct and wAg/Period when
dAg = 20nm; (a) Period = 200nm, (b) Period = 300nm, (c) Period = 400nm, (d) Period =
500nm.
Acknowledgment
This work was supported by the National Research Foundation under the Ministry of Science,
the Global Research Laboratory (GRL) Program K20815000003 (2008-00580), the Global
Frontier Centre for Multiscale Energy Systems 2011-0031561, and the Centre for
Subwavelength Optics, SRC 2008-0062256, all funded by the South Korean government
(GRL, Frontier, SRC).
#210672 - $15.00 USD Received 22 Apr 2014; revised 24 May 2014; accepted 27 May 2014; published 10 Jun 2014
(C) 2014 OSA
30 June 2014 | Vol. 22, No. S4 | DOI:10.1364/OE.22.0A1145 | OPTICS EXPRESS A1152