Efficient Organic Solar Cells with ... - Wiley Online Library

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Dong-Seok Leem, Angharad Edwards, Mark Faist, Jenny Nelson, Donal D. C. Bradley, and John C. de Mello*

Dr. D.-S. Leem,[+] A. Edwards, M. Faist, Prof. J. Nelson, Prof. D. D. C. Bradley, Prof. J. C. de Mello Centre for Plastic Electronics Imperial College London Exhibition Road, London SW7 2AZ, UK E-mail: [email protected] [+] Present address: Display Devices Lab., Samsung Advanced Institute of Technology (SAIT), Republic of Korea

DOI: 10.1002/adma.201100871 Adv. Mater. 2011, 23, 4371–4375

a production environment. Here we investigate an alternative method for applying AgNWs as the lower electrode using a transparent buffer layer to ameliorate the AgNW surface. Figure 1a shows transmission spectra of spin-coated AgNWs (Cambrios ClearOhm) deposited on glass between 1000 and 5000 rpm, with higher spin speeds leading to lower surface coverage and higher transparency. All films exhibited high transmittances, greater than 88%, between 500 and 800 nm, with a drop in transmittance occurring at shorter wavelengths due to absorption by surface plasmons.[19] The inset shows the

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The demand for printed electronic devices on plastic substrates has generated a need for solution-processable flexible electrodes with high transparency and low sheet resistance.[1] Conducting polymers,[2–4] metal inks,[5] nanoparticulate metal oxides,[6] carbon nanotubes,[7–9] and graphene[10–12] have been investigated as potential alternatives to brittle indium tin oxide (ITO), but none can yet compete in terms of transparency and sheet resistance. Thin meshes of silver nanowires (AgNWs)[13–18] have recently emerged as promising electrodes due to their ability to provide transmittances greater than 85% at sheet resistances less than 20 Ω sq−1.[13,14] Their application to printed electronics, however, is challenging due to a highly non-uniform topography, which can cause shorting through other layers. This is especially problematic for devices using AgNWs as the lower (substrate-facing) electrode since the mesh presents an extremely rough base layer on which to build the device, leading to significant interelectrode shorting. This in turn leads to low shunt resistances, high dark currents, and poor device efficiencies. For instance, Peumanns and co-workers reported vacuum-deposited organic solar cells (OSCs) on AgNW-coated glass with low shunt resistances of less than 1 kΩ cm2 and power conversion efficiencies (PCEs) less than 0.5%.[13] They subsequently reported polythiophene/ fullerene OSCs with PCEs of 2.5%,[16,17] using the AgNWs in a top-electrode configuration where their rough morphology is less detrimental to other layers. However to achieve these efficiencies they first had to pulse the devices at 10 V to burn-out localized shorts with potentially adverse implications for device lifetimes, suggesting a need for alternative device architectures that better suppress shunt formation. In practice most organic devices utilize a transparent substrate through which light passes, and hence they require a transparent lower electrode. To address this need Zeng et al. embedded AgNWs in a thick film of polyvinyl alcohol, ensuring the exposed nanowires sat flush with the top of the film and so provided a planar surface on which to deposit further layers.[18] Their method however involved transfer of the composite film from one substrate to another and may be difficult to adapt to

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Efficient Organic Solar Cells with Solution-Processed Silver Nanowire Electrodes

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Figure 1. a) Transmittance of spin-coated AgNWs on glass deposited at different spin speeds; the bold line indicates device-grade ITO on glass. The inset shows transmittance versus sheet-resistance characteristics for the same AgNW and ITO samples. b) SEM (main) and AFM (inset) images of ≈29 Ω sq−1 AgNWs on glass.

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transmittance (T) at 550 nm versus sheet resistance (R), and as expected T and R decrease progressively with decreasing spinspeed from T = 98% and R = 34 ± 2 Ω sq−1 at 5000 rpm to T = 89% and R = 9.7 ± 0.3 Ω sq−1 at 1000 rpm. The open marker indicates typical device-grade ITO on glass, and it is clear that the AgNW samples match well in terms of their T−R balance. The main image in Figure 1b shows a scanning electron microscopy (SEM) image of ≈29 Ω sq−1 AgNWs on glass, while the inset shows an atomic force microscopy (AFM) image of the same. Typical tube lengths were a few tens of micrometers and typical diameters were a few tens of nanometers. The AgNW morphology is characterized by void regions where the

substrate is fully exposed and narrow ridges corresponding to the nanowires. High peaks exist where wires overlap and, even for relatively low surface coverage, feature heights can exceed 100 nm. Hence, unless the AgNWs are adequately buffered by a suitable coating, they are likely to short through subsequent layers, thus hindering device operation.[13,15] To begin, non-inverted devices were fabricated on glass by sequentially coating ITO (reference) or ≈29 Ω sq−1 AgNWs with a thin 30 nm layer of poly(3,4-ethylendioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), a 200 nm composite layer of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), a 30 nm Ca cathode, and a 100 nm Al capping layer (see Figure 2a and Experimental Section).

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7.4 Figure 2. Current density versus voltage characteristics for a) non-inverted devices with 30 or 150 nm of PEDOT:PSS and b) inverted devices with 0 or 200 nm of TiOx. Data is shown for devices on AgNWs (circles) and ITO (squares). Energy level diagrams are shown adjacent.

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the dark current of increasing the PEDOT:PSS thickness. For AgNWs it causes a lowering of Jdark(V) and hence an increase in the magnitude of Jlight(V), but for ITO it causes an increase in Jdark(V) and therefore a reduction in the magnitude of Jlight(V). Consequently, Voc and FF increase for the AgNWs but decrease for ITO. Overall, the effect on PCE of the increased PEDOT:PSS thickness is an increase from 1.3 to 2.0% for the AgNWs but a decrease from 3.6 to 2.2% for ITO. Hence, although increasing the PEDOT:PSS thickness brings the performance of the AgNW devices up to near parity with equivalent ITO devices, this is partly due to a reduction in the efficiency of the latter. Switching to a more transparent buffer layer that can yield high short-circuit currents and low dark currents even at high layer thicknesses might therefore be expected to further improve the efficiencies of AgNW devices and so narrow their performance gap with optimized ITO devices. Nanostructured metal oxides such as titania and zinc oxide are an attractive choice of buffer material due to their high transparency in the visible and good charge transport properties.[23] However, since they are electron transporters, they must be used in an inverted configuration with the transparent electrode acting as the cathode. Inverted devices were fabricated by depositing onto ITO or ≈17 Ω sq−1 AgNWs: 200 nm of nanoparticulate titania (TiOx), 200 nm of P3HT:PCBM, 5 nm of (hole-transporting) MoO3, and 100 nm of Ag (see Figure 2b and Experimental Section). Otherwise-identical control devices were fabricated with the TiOx layer omitted. Figure 2b shows the J–V characteristics of the four devices. The TiOx-free AgNW device had a (predictably) low RSH of 1.1 kΩ cm2, a low Jsc of 3.5 mA cm−2, a low Voc of 0.36 V, and a low FF of 32.1%, resulting in a poor PCE of just 0.4%. The TiOx-free ITO device had a much higher RSH of 397 kΩ cm2 (consistent with the smoother electrode), a relatively high Jsc of 9.4 mA cm−2, a slightly higher Voc of 0.49 V, and a FF of 41.4%, resulting in a modest PCE of 1.9%. Including a 200 nm TiOx layer in the AgNW device structure led to a much higher RSH of 148 kΩ cm2, a high Jsc of 10.1 mA cm−2, a high Voc of 0.56 V, and a high FF of 61.1%, corresponding to a PCE of 3.45%. To our knowledge this is the highest value reported for OSCs using AgNW electrodes. In fact, the device characteristics compare favorably with those of an ITO reference device with the same thickness of TiOx, which showed an RSH of 610 kΩ cm2, a Jsc of 10.9 mA cm−2, a Voc of 0.55 V, a FF of 58.7%, and a PCE of 3.52%. Importantly, in contrast to PEDOT:PSS, the thick TiOx buffer layer caused a beneficial reduction in Jdark(V) and a significant improvement in JSC for both ITO and AgNW based devices, indicating effective buffering of the underlying electrode with minimal absorptive losses in the TiOx. Insight into the improved device characteristics (reduced dark current) can be obtained using AFM. Figure 3ai shows a 4 μm × 4 μm microscopy image of uncoated AgNWs on glass: sharp knife-edge ridges due to individual nanowires are clearly evident, with the highest features occurring at crossing points with other wires. After coating with 200 nm TiOx (Figure 3aii), the ridged features are completely obscured and a rolling hill morphology is obtained. While the resulting surface is clearly nonplanar, it has none of the sharp asperities associated with the uncoated AgNWs and represents a superior surface on



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Current density versus voltage (J–V) characteristics are shown in Figure 2a for the ITO and AgNW devices (open markers). The dark characteristics of the ITO device were similar to those reported elsewhere, with a small strongly rectifying dark current and a relatively high shunt resistance (RSH) of 3000 kΩ cm2. Note, all RSH values refer to dark conditions. Those of the AgNW device however were quite different, with an RSH of just 0.069 kΩ cm2 and negligible rectification between forward and reverse bias. These are typical characteristics for devices with high shunt densities.[13] Since the rate of injection into the donor and acceptor materials is very low in reverse bias (due to large energy barriers), the high reverse bias current is entirely attributable to shunt conduction and signifies ineffective buffering of the rough AgNWs by the thin PEDOT:PSS coating. The detrimental influence of shunts on device performance is clear from the J–V characteristics under AM1.5 illumination. The ITO device had a short-circuit current density (Jsc) of 10.3 mA cm−2, an open-circuit voltage (Voc) of 0.58 V, and a fill factor (FF) of 59.4%, resulting in a PCE of 3.6%, which is typical for optimized P3HT:PCBM devices on ITO.[20] The AgNW device by contrast had a reasonably high Jsc of 8.9 mA cm−2 but a lower Voc of 0.45 V and a much lower FF of 31%, resulting in an overall PCE of just 1.3%. The low Voc and FF of the AgNW device are attributable to the high dark current. For a given photovoltage V, the measured photocurrent Jlight(V) is equal to the sum of two opposing parts: i) a negative current Jph(V) due to extraction of photogenerated carriers and ii) a positive current Jinj(V) due to injection of charges under the influence of the positive photovoltage. Jinj(V) is numerically equal to the dark current Jdark(V).[21] Hence, if Jdark(V) increases sharply with voltage, Jlight(V) will decrease rapidly to zero, resulting in a reduced FF and Voc. To maximize the PCE, the dark current must therefore be suppressed by minimizing the density of conductive shunts. Since the peak heights in the AgNW mesh are typically a small multiple of the nanowire diameter, coating the electrode with a thick (>100 nm) PEDOT:PSS layer can be expected to provide a smoother top surface on which to deposit subsequent layers, leading to reduced shorting. New AgNW devices were therefore fabricated using five times the thickness of PEDOT:PSS (150 nm). This resulted in a greater than 700-fold increase in RSH to 51 kΩ cm2 and the appearance of strong rectification (closed circles, Figure 2a), consistent with a much lower density of conductive shunts. Indeed the shunt resistance of the new AgNW device was only slightly lower than that of an ITO reference device with the same thickness of PEDOT:PSS (110 kΩ cm2), indicating effective buffering of the underlying AgNW asperities. Note, in agreement with findings by Friedel et al., the ITO device with 150 nm PEDOT:PSS showed a much (27-fold) lower shunt-resistance than the one with 30 nm PEDOT:PSS, attributable to slower drying of the film and higher consequent surface roughness.[22] For both ITO and AgNWs, a significant decrease in Jsc was observed on increasing the PEDOT:PSS layer from 30 to 150 nm due to greater absorptive losses in the PEDOT:PSS layer. The FF and Voc on the other hand showed a slight decrease for ITO but a considerable increase for AgNWs. The contrary behavior with respect to Voc and FF is attributable to the contrasting effect on

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Experimental Section AgNW meshes of varying surface coverage were spin-coated onto oxygen-plasma-treated glass from as-received Cambrios ClearOhm AgNW dispersion at speeds in the range 1000–5000 rpm for 120 s. The AgNWs were annealed at 140 °C for 5 min, after which their thickness, sheet resistance, and transmittance were measured using a Tencor Instruments Alpha Step 200 surface profilometer, a four-point probe system, and a UV-vis spectrophotometer (Shimadzu, UV-2550). Films were characterized by SEM (LEO Gemini 1525) and AFM (Pacific Nanotechnology). AgNW work function ϕ was measured as 4.82 eV, using the Kelvin probe technique with highly oriented pyrolytic graphite (ϕ = 4.48 eV) as a reference. Non-inverted devices with 30 or 150 nm of PEDOT:PSS (Clevios P AI4083, 70 °C) were fabricated by spin-coating onto ITO or ≈29 Ω sq−1 AgNWs, and then annealing at 150 °C for 30 min. A 200 nm layer of P3HT (Merck Chemicals Ltd.) and PCBM (Nano-C) was spin-coated on the PEDOT:PSS from a 1:0.8 blend by weight in chlorobenzene and then annealed at 150 °C for 20 min in dry nitrogen. A Ca (30 nm)/Al (100 nm) cathode was deposited by thermal evaporation at 10−6 mbar. Inverted devices were fabricated on ITO or 17 Ω sq−1 AgNWs by spin-coating titania paste (Solaronix Ti-Nanoxide HT-L/SC) mixed with Figure 3. a) 4 μm × 4 μm AFM images for AgNWs on glass coated with i) 0 nm and ii) 200 nm 0.1 wt% Zonyl surfactant for improved wetting TiOx. b) ADFs for i) uncoated and ii) TiOx-coated AgNWs. Vertical dotted lines enclose 95% and then annealing at 150 °C for 30 min to yield confidence intervals (CIs). a 200 nm film of TiOx. 200 nm P3HT:PCBM (composition as above) was spin-coated onto the TiOx and then annealed at 150 °C for 20 min in dry which to deposit subsequent layers. Prior to coating with TiOx, N2. A 5 nm MoO3 hole transport layer was evaporated at 10−6 mbar, followed by 100 nm Ag. The pixel size, defined by overlap of the two the amplitude density function (ADF) is highly non-Gaussian electrodes, was 0.045 cm2 for both cell configurations. Devices were with a skewness of +1.7 and a kurtosis of 8.2 (Figure 3bi), indictested under simulated AM1.5 conditions (Oriel Instruments).

ative of a spiked morphology that is liable to induce shorting through subsequent layers.[24] After coating, however, the ADF adopts a Gaussian profile and becomes significantly broader, signifying an increase in the average peak-to-valley height Rtm. Note, ADF broadening was also seen for ITO/TiOx with the span of the 95% CI increasing from 16 to 32 nm. The increase in Rtm, while in itself undesirable, is more than offset by a large reduction in the steepness of individual peaks and a three fold increase in the 1/e coherence length[24] from 137 nm to 390 nm, indicating a much smoother surface onto which subsequent layers may be deposited conformally without suffering pinholes or other defects that could cause shorting and high consequent dark currents. In conclusion we have fabricated P3HT:PCBM OSCs using AgNWs as the lower electrode and PEDOT:PSS or TiOx as a buffer layer. In both cases, use of a thick buffer improved fill factors and open-circuit voltages relative to unbuffered devices. Non-inverted devices with 150 nm PEDOT:PSS exhibited low short-circuit currents due to absorptive losses inside the PEDOT:PSS, leading to PCEs of 2.0%. Inverted devices using 200 nm TiOx experienced minimal absorptive losses in the buffer layer, leading to high PCEs of up to 3.5% (equal to equivalent ITO-based devices). It is likely that the use of metaloxide buffered AgNWs will prove effective for a variety of applications requiring solution-processable transparent lower electrodes.

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Acknowledgements The authors thank Cambrios Technologies Corporation for the ClearOhm AgNW dispersion and EPSRC for support under EP/F061757/1. Received: March 7, 2011 Revised: May 26, 2011 Published online: August 22, 2011

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[9] S. Kim, J. Yim, X. Wang, D. D. C. Bradley, S. Lee, J. C. de Mello, Adv. Funct. Mater. 2010, 20, 2310. [10] K. S. Kim, Y. Zhao, H. Jang, S. Y. Lee, J. M. Kim, J. H. Ahn, P. Kim, J. Y. Choi, B. H. Hong, Nature 2009, 457, 706. [11] G. Eda, M. Chhowalla, Adv. Mater. 2010, 22, 2392. [12] P. H. Wöbkenberg, G. Eda, D. S. Leem, J. C. de Mello, D. D. C. Bradley, M. Chhowalla, T. D. Anthopoulos, Adv. Mater. 2011, 23, 1558. [13] J. Y. Lee, S. T. Connor, Y. Cui, P. Peumans, Nano Lett. 2008, 8, 689. [14] S. De, T. M. Higgins, P. E. Lyons, E. M. Doherty, P. N. Nirmalraj, W. J. Blau, J. J. Boland, J. N. Coleman, ACS Nano 2009, 3, 1767. [15] L. B. Hu, H. S. Kim, J. Y. Lee, P. Peumans, Y. Cui, ACS Nano 2010, 4, 2955. [16] W. Gaynor, J. Y. Lee, P. Peumans, ACS Nano 2010, 4, 30. [17] J. Y. Lee, S. T. Connor, Y. Cui, P. Peumans, Nano Lett. 2010, 10, 1276.

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