Ambient Air Processing Causes Light Soaking Effects in Inverted

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Ambient Air Processing Causes Light Soaking Effects in Inverted Organic Solar Cells Employing Conjugated Polyelectrolyte Electron Transfer Layer Chang-Yong Nam* Center for Functional Nanomaterials, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *

ABSTRACT: Inverted polymer:fullerene bulk heterojunction solar cells employing a conjugated polyelectrolyte electron transfer layer display light soaking effects as the oxygen adsorbed on indium tin oxide (ITO) during an ambient air device processing induces interface charge trap states in the conjugated polyelectrolyte layer and reduces its interface dipole. The light soaking populates the trap states with photoexcited electrons and reinstates the electric dipole, leading to a recovery of efficient charge extraction and normal illuminated current−voltage characteristics consequently. The identified effect of adsorbed oxygen not only enables a remedy of the light soaking issue of the inverted solar cells via hydrogen plasma treatment of ITO but also suggests the importance of properly handling adsorbed oxygen species on ITO for achieving high performance organic devices based on ITO substrates in general.

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INTRODUCTION With the champion efficiency approaching over 12%1,2 and continuing commercialization efforts in the field, the attention to the organic solar cell continues. The structure of organic solar cells has been largely based on an active organic layer sandwiched between a transparent metal oxide bottom anode (typically indium tin oxide (ITO) coated with a hole transfer layer) and a metal cathode on top. Reversing the order, the inverted organic solar cell uses the bottom ITO as a cathode after applying a certain electron transfer layer (ETL) before depositing the active organic layer. The inverted structure is more attractive for a large-scale manufacturing,2,3 displays a good ambient stability,3,4 and in the case of polymer:fullerene bulk heterojunction (BHJ) active layer utilizes the natural composition gradient of BHJ, where the concentration of fullerene electron acceptor tends to increase toward the bottom.5 For the ETL, the material choice has been mostly metal oxides (e.g., TiOx and ZnO),2,6−9 but recently organic and organic−inorganic hybrid composite materials were also used.6,10,11 Particularly, 9.2% power conversion efficiency (PCE) has been demonstrated by applying a conjugated organic polyelectrolyte PFN (poly[(9,9-bis(3′-(N,Ndimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctylfluorene)) as an ETL, in conjunction with a low band gap polymer donor PTB7 (poly[[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl]]).11 Most of inverted organic solar cells adopting metal oxide ETLs display so-called light soaking effects,2,7−9 wherein the device PCE increases with increasing duration of solar illumination due to increasing open circuit voltage (VOC) and fill factor (FF). This gradual, light-sensitive change in © 2014 American Chemical Society

photovoltaic (PV) characteristics is driven by a disappearance of the S-shaped current−voltage curve near the open circuit point, whose potential origins have been correlated to trap filling in the metal oxide layer and subsequent increase in photoconductivity;7 energy barrier between ITO and the metal oxide ETL;8 chemisorbed oxygen on the metal oxide surface that creates carrier depletion;9 or energy barrier at the metal oxide/organic active layer interface.2 Interestingly, similar light soaking effects have been rarely documented when organic or hybrid ETLs were used. Only recently, the Riedl group reported light soaking phenomenon involving organic conjugated polyelectrolyte poly[3-(6-bromohexyl)thiophene].12 Meanwhile, ambient air processing of organic solar cells promises compatibility with low-cost manufacturing methods such as the roll-to-roll coating, but concerns over the effects of oxygen and humidity on device performance and stability have been forcing virtually all of the high-performance organic solar cells reported in the literature to be fabricated in an air-free environment such as glovebox.11,13,14 Previously, we studied the influences of air processing on charge transport properties and PV performance in polythiophene and polythiophene:fullerene organic BHJ blend, demonstrating benefits of postfabrication vacuum thermal annealing in controlling charge carrier density in polythiophene and removing oxygen traps in the active blend layer.15,16 In this work, we report the light soaking effects in inverted organic BHJ solar cells having PFN as an organic conjugated polyelectrolyte ETL and explain the phenomenon through the Received: July 31, 2014 Revised: October 27, 2014 Published: October 31, 2014 27219

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substrates, which were cleaned by detergent by the manufacturer after sputter deposition. No extra treatment is applied during the following device fabrication procedure. The fabricated inverted solar cell features S-shape current density− voltage (J−V) characteristics near the open circuit point upon the initial solar illumination (air mass 1.5 global (AM1.5G), 100 mW/cm2 (1 sun)) (Figure 1a), a typical sign of significant charge recombination caused by inefficient extraction of photogenerated free charge carriers from the active blend layer. Within 240 s of light soaking, the S-shape gradually vanishes along with increasing diode forward current, and the corresponding PCE increases from 2.8% (0 s) to 3.8% (240 s) (Figure 1b). The short circuit current density (JSC) remains nearly constant at ∼11 mA/cm2 over the course of 240 s light soaking, but both VOC and FF gradually increase up to 0.59 V and 0.58 (from 0.55 V and 0.46), respectively, driving the observed increase in PCE (Figure 1c). A few prior works report the fabrication of polymer:fullerene BHJ solar cells adopting PFN as an ETL layer, but light soaking effects have not been reported so far, in both inverted and noninverted solar cell structures.11,17−19 The active donor in those studies includes P3HT and low band gap polymers such as PTB7, and these devices were fabricated in an inert air-free environment. Meanwhile, we find that the improved VOC and FF by the initial light soaking degrade over time under dark condition but improve again during the next light soaking (i.e., reversible degradation, Figure S1, Supporting Information), similarly to previous reports.8 Investigation of control unipolar devices confirms that the light soaking effect is originating from the bottom ITO/PFN cathode stack. The J−V characteristics of hole-only device consisting of ITO/MoO3/P3HT:PCBM/MoO3/Au display negligible difference after a 300 s soaking under AM1.5G, 1 sun solar illumination (Figure 2a), ruling out the possibility that MoO3 or its interfaces with ITO or active blend layer is responsible for the soaking effect. This is consistent with a previous study by Trost et al. in which they confirmed MoO3 in contact with organic active layer was not causing light soaking effects.2 On the other hand, the electron-only device having ITO/PFN/P3HT:PCBM/Al structure shows an increased forward-bias conductance (ITO electrode is grounded) after the same 300 s light soaking (Figure 2b). Since the Al contact on the P3HT:PCBM blend does not cause light soaking issues,16 a light-sensitive electron extraction barrier must exist at one of the interfaces of ITO/PFN and PFN/P3HT:PCBM blend layer or in PFN itself. Among these possibilities, we find that the ITO/PFN interface is the one responsible for the soaking phenomenon as we observe no light soaking behavior in the air-processed noninverted solar cells having a structure of ITO/MoO3/P3HT:PCBM/PFN/Al (Figure S2, Supporting Information). We find that the observed light soaking effect is primarily mediated by the excitation of electronic states in ITO/PFN cathode stack with an activation energy between 2.7 and 3.1 eV. The P3HT:PCBM inverted solar cells were subjected to a 300 s illumination whose spectral range was controlled by a series of long-pass optical filters (which block all wavelengths below a specified cutoff wavelength), used in place of AM1.5G filter. The cutoff wavelength of long-pass filter was varied from 610 to 400 nm with ∼50 nm interval, and we find that the onset wavelength for appreciable light soaking effects (i.e., increase in VOC and FF with lapsing illumination duration) is present between 400 and 455 nm (thus, between 2.7 and 3.1 eV)

influences of oxygen on ITO originating from the ambient air device processing. We examine model devices based on P3HT (poly(3-hexylthiophene-2,5-diyl)):PCBM ([6,6]-phenyl C61 butyric acid methyl ester) BHJ active layer employing PFN as a conjugated polyelectrolyte ETL on ITO substrates. By combining control unipolar device studies, wavelength-dependent light soaking experiments, and a variation of plasma treatment condition of ITO surface, we suggest the light soaking effect originates from oxygen adsorbed on ITO, which creates charge trap states within PFN in contact and subsequently increases the effective work function and electron extraction/injection barrier height of ITO/PFN cathode. On the basis of this understanding, we demonstrate the suppression of the light soaking issue by using hydrogen plasma treatment of ITO surface.

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RESULTS AND DISCUSSION Organic solar cells in our study are fabricated in an ambient air condition unless indicated otherwise (see Methods section). The structure of inverted solar cells consists of, from the bottom to the top, ITO (150 nm)/PFN (∼2 nm)/ P3HT:PCBM (150−200 nm)/MoO3 (19 nm)/Au (100 nm) (Figure 1a, inset). We use as-received ITO-coated glass

Figure 1. (a) Representative J−V characteristics of an air-processed P3HT:PCBM inverted BHJ solar cell having PFN as an ETL during a 240 s light soaking under AM1.5G, 1 sun solar illumination. The arrow indicates, from left to right, the time lapse from 0 to 240 s during the light soaking. The inset depicts the structure of the inverted solar cell. (b) Soaking-time-dependent variation of PCE (left axis, solid blue square) and JSC (right axis, solid red circle). (c) Soaking-timedependent variation of VOC (left axis, solid blue square) and FF (right axis, solid red circle). 27220

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used, but for the cutoff wavelengths from 455 to 610 nm, VOC and FF increases only by ∼2% and ∼7%, respectively, with no difference in the magnitude of increase with respect to chosen cutoff wavelengths. The reported band gap of PFN is ∼3.47 eV,19 and thus the observed range of onset wavelength for the light soaking effect suggests that midgap electronic states at the PFN/ITO interface are the most likely sources that mediate the light soaking process. We note that the universal 2−7% increases in VOC and FF for the cutoff wavelengths >455 nm is expected to be caused by a gradual heating of the active blend layer over room temperature and accompanying increases in charge mobility and phonon-assisted delocalization therein that are known to enhance VOC and FF of P3HT:PCBM solar cells.20 We propose that the observed light soaking effects result from oxygen species adsorbed on ITO surface during ambient processing, which oxidize the PFN in contact and create the interface midgap charge trap states. Important consequences of these trap states are the reduction of the electric dipole within PFN layer and concomitant increase in the effective ITO work function against the electron extraction from PCBM of the active blend layer. Because of its molecular arrangement, the conjugated polyelectrolyte PFN is known to form a permanent interface dipole at the ITO surface and to reduce the work function of ITO down to 4.1 eV (Figure 4a), enabling a fabrication of inverted organic solar cells boasting 9.2% PCE based on the PTB7:fullerene BHJ as Cao and co-workers demonstrated.11,21 Meanwhile, the adsorption of oxygen on ITO surface is commonly observed during oxygen plasma or ultraviolet (UV)-ozone treatments,22,23 and molecular oxygen and water in the ambient air are also suggested to generate adsorbed oxygen on ITO.24 Significantly, these adsorbed oxygen species can not only increase the work function of ITO23,25 but also create midgap charge trap states on the organic semiconductor deposited on top by removing electrons from the organic layer (i.e., oxidation), even resulting in an irreversible degradation of the organic semiconductor as Lo et al. reported recently.23 Under these circumstances, we anticipate a presence of adsorbed oxygen on ITO during the ambient air processing and oxidation of the PFN coming into contact, ultimately inducing the formation of midgap charge trap states in the PFN interfaced with ITO. Considering the observed onset wavelength of light soaking effect between 400 and 455 nm, the corresponding energy level of the trap states is expected to lie between 2.7 and 3.1 eV above the highest occupied molecular orbital (HOMO) of PFN. Such a formation of positively charged trap states in PFN near the ITO interface will decrease the strength of dipole within PFN and lead to an increase in the effective ITO work function as well as its electron injection/extraction barrier height to/from PCBM (Figure 4b). Resulting inefficiency in charge extraction in turn should create a significant charge accumulation and induce carrier recombination in the active blend layer, eventually causing the observed S-shape J−V curve near the VOC point during the initial period of solar illumination. With continuing illumination, photoexcited electrons populate these trap states (i.e., neutralization of trap states), leading to the reinstatement of the dipoles in PFN and subsequent recovery of normal J−V characteristics near the VOC point as the ITO work function and barrier height against electron extraction from PCBM become reduced again (Figure 4c). We confirm the suggested effects of adsorbed oxygen by controlling the extent of light soaking phenomenon via oxygen

Figure 2. Dark J−V characteristics of air-processed control P3HT:PCBM BHJ unipolar devices before and after a light soaking under AM1.5G, 1 sun solar illumination. Solid blue square marks without light soaking (0 s) and solid red circle after a 300 s soaking. (a) Hole-only device. (b) Electron-only device. Insets show the structures of respective unipolar devices.

(Figure 3)there were ∼14% increases in both VOC and FF after the 300 s illumination when a 400 nm long-pass filter was

Figure 3. Light soaking-time-dependent variation of (a) VOC and (b) FF of an air-processed P3HT:PCBM inverted BHJ solar cell having PFN as an ETL under different input light spectrum controlled by a series of long-pass optical filters, which block lights below specified cutoff wavelengths. The cutoff wavelength is varied from 400 nm (solid square), 455 nm (open circle), 495 nm (solid triangle), 550 nm (open square), to 610 nm (solid circle). Data are normalized with respect to the values for soaking time = 0 s. 27221

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Figure 4. Schematic flat-band energy diagram of an air-processed P3HT:PCBM inverted BHJ solar cell depicting the work function of ITO and its corresponding electron injection/extraction barrier heights (marked by vertical red arrows) from/to PCBM, which are affected by the dipole strength within PFN. (a) On ITO without adsorbed oxygen species, PFN exerts its maximum dipole strength (marked by four − and + signs), resulting in the most reduced work function and barrier height. (b) Adsorbed oxygen (Oδ−) on ITO takes electrons from PFN (oxidation, δ+), creating mid gap charge trap states (marked by blue lines), reducing the dipole strength in PFN and thus increasing the work function as well as the barrier height of ITO. (c) Under a light soaking, photoexcited electrons (solid circle) neutralize the charge traps, reinstating dipole strength in PFN along with the reduced work function and barrier height of ITO.

and hydrogen plasma treatments of ITO surface. The oxygen plasma treatment has been widely used to remove carbonaceous species on ITO substrates and increase their work function (to ∼4.7−5.2 eV) by actively generating adsorbed oxygen species on the ITO surface.22,23,25,26 In the present case, we expect these actively generated adsorbed oxygen can induce a formation of higher charge trap density in PFN compared with the PFN coated on the as-received ITO, causing a larger extent of changes in VOC and FF during the light soaking by rendering the recovery of interfacial dipole in PFN more difficult. Indeed, when the ITO surface is treated with oxygen plasma (20 W, 100 mTorr, 5 min), the P3HT:PCBM inverted solar cell with a PFN ETL displays exacerbated light soaking effectsduring the initial period of solar illumination, the Sshaped J−V curve features a drastically suppressed diode forward current with very low VOC and FF (0.16 V and 0.11) and near zero JSC (Figure 5a). This much difficulty in the charge extraction most likely results from the compounded effects of a higher charge trap density in PFN caused by the plasma-generated oxygen species on ITO as well as intrinsically increased work function of ITO due to the plasma treatment. As the light soaking continues for 300 s, VOC and FF increase up to 0.4 V and 0.18, respectively (i.e., increase by 160% and 57%, respectively), but without being able to fully recover a normal J−V curve shape (Figure 5a). Despite the low absolute values of final VOC and FF, the larger extent of changes in these parameters during light soaking, compared with what were observed in the as-received ITO counterpart, supports the suggested role of adsorbed oxygen in the light soaking process. The electron-only unipolar device (i.e., ITO/PFN/ P3HT:PCBM/Al) with the same ITO oxygen plasma treatment consistently exhibits a suppressed forward-bias current even after the 300 s light soaking (Figure 5c, left), confirming an insufficient decrease of the heightened work function at the ITO/PFN cathode by the light soaking. In contrast, if a hydrogen plasma treatment (4% hydrogen with argon balance, 20 W, 100 mTorr, 5 min) is applied to the

ITO surface before spin-casting PFN, the light soaking effect disappears almost completely in the inverted P3HT:PCBM solar cells, as evidenced by the normal J−V characteristics of the solar cell from the start of solar illumination with an absence of S-shaped curve near the VOC point, along with negligible changes in the J−V characteristics and their VOC and FF over the course of 300 s light soaking (Figure 5b). The corresponding unipolar device with hydrogen-plasma-treated ITO consistently displays a high forward-bias current comparable to that of the reverse bias with no change in magnitude after a 300 s light soaking (Figure 5c, right). Opposite to the oxygen plasma, the hydrogen plasma treatment is known to reduce the ITO surface by removing adsorbed oxygen species while decreasing its work function (to ∼4.5−4.7 eV).27−29 Thus, the disappearance of the light soaking effects after the hydrogen plasma treatment of ITO provides another evidence that the oxygen adsorbed on ITO surface during the ambient air device processing contributes to the observed light soaking effects in the inverted solar cells adopting PFN as an ETL. It is worthy to note that oxygen desorption from ITO surface by illumination can decrease the intrinsic ITO work function, separately from the effects of PFN. Zhou et al. explained the light soaking behavior of inverted organic solar cells having no cathode modifier on ITO via such effects.30 However, the contribution of these effects to our case appears unlikely because the lowered ITO work function by PFN (∼4.0−4.5 eV)11,31 is comparable to the electron affinity of PCBM (∼4.3 eV) and is expected to pin its Fermi level, rather suppressing any light soaking behavior from the start of illumination if only the change in intrinsic ITO work function is considered.30 Our additional light soaking experiments on control P3HT:PCBM inverted solar cells having no PFN on ITO show negligible light soaking effects regardless of types of ITO surface treatments (as-received, oxygen-plasma-treated, and hydrogen-plasmatreated; i.e., different amounts of adsorbed oxygen on ITO surface; Figure S3, Supporting Information). These further 27222

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study, but it is plausible to expect that a varying degree of oxidation of PEDOT:PSS by the oxygen species adsorbed on ITO during air processing could be one of the contributing factors to such a variation in device performances.

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CONCLUSIONS

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METHODS

In summary, we find that adsorbed oxygen on ITO surface during the ambient air device processing causes the light soaking effects in inverted P3HT:PCBM solar cells adopting a conjugated electrolyte PFN as an ETL. The adsorbed oxygen on ITO creates midgap charge trap states in PFN and in turn increases the effective work function of ITO by reducing the interface dipole within PFN. The consequently increased electron extraction barrier height causes the observed S-shaped J−V curve near the open circuit point during the initial period of solar illumination. The continuing light soaking populates the trap states with photoexcited electrons and reinstates the electric dipole within PFN as well as the lowered ITO work function, finally enabling the recovery of efficient charge extraction and normal illuminated J−V characteristics. We show that the light soaking issue can be remedied by removing the adsorbed oxygen on ITO via hydrogen plasma treatment. The results not only elucidate the role of adsorbed oxygen species on ITO in causing the light soaking phenomenon in airprocessed inverted organic solar cells having organic ETLs but also suggest the importance of handling these adsorbed oxygen species on ITO for achieving high performance organic devices based on ITO substrates in general. Figure 5. Representative illuminated J−V characteristics of airprocessed P3HT:PCBM inverted BHJ solar cells with (a) oxygenplasma-treated ITO and (b) hydrogen-plasma-treated ITO before PFN coating, during a light soaking under AM1.5G, 1 sun solar illumination. The arrows indicate the time-lapse from 0 to 300 s during the light soaking. (c) Dark J−V characteristics of corresponding electron-only unipolar devices (ITO/PFN/P3HT:PCBM/Al) with oxygen-plasma-treated ITO (left) and hydrogen-plasma-treated ITO (right), before and after the 300 s light soaking under AM1.5G, 1 sun solar illumination. Solid blue square marks without light soaking (0 s) and solid red circle after the 300 s soaking.

The structure of inverted solar cells consists of ITO (150 nm)/ PFN (∼2 nm)/P3HT:PCBM (150−200 nm)/MoO3 (19 nm)/ Au (100 nm). All the fabrication procedures are performed under an ambient air condition unless indicated otherwise. PFN solution is prepared in methanol (1 mg/mL) with addition of acetic acid (1 μL) for a good dissolution of PFN. The PFN layer is spin-cast on as-received ITO-coated glass substrates at 3000 rpm, yielding ∼2 nm thickness (determined by ellipsometer). The as-received ITO-coated glass substrate is rinsed by detergent by the manufacturer (Thin Film Devices Inc.) after sputter deposition, and no extra surface treatment is applied except for the control device studies wherein plasma treatments are applied (oxygen or 4% hydrogen/balance argon, 20 W, 100 mTorr, 5 min). 3 wt % P3HT:PCBM blend (1:1 ratio) solution (in monochlorobenzene) is then spin-cast atop at 700 rpm, followed by deposition of MoO3/Au anode stack via thermal evaporation (base pressure 10−6 Torr) through a shadow mask (active device area 1.23 mm2). Finally, a postfabrication vacuum thermal annealing is applied to the devices at 150 °C for 10 min (∼100 mTorr) for the phase separation in the active blend layer. We use a similar fabrication procedure and component layer thicknesses to prepare control unipolar devices and noninverted solar cells. Device PV characteristics are measured under AM1.5G, 1 sun bottom solar illumination (calibrated by a KG5 filtered reference Si solar cell (Newport), nominal spectral mismatch factor of 1.01)32 by using a modified electrical probe station equipped with a 150 W solar simulator (Newport) and semiconductor parameter analyzer (Agilent).

support the suggested role of PFN in the light soaking phenomenon we observe. One of the implications the current study provides is that any organic interface modifier applied to ITO in principle can be negatively affected by the adsorbed oxygen of ITO surface, making a rational control of the ITO surface treatment condition important for achieving high-performing organic devices such as solar cells. This is a notion in line with a recent report by Lee and co-workers in which the adsorbed oxygen on UV-ozone-treated ITO substrates posed degradation issues in the organic semiconductors deposited on top as their photoluminescence, photoemission spectroscopy, and PV device studies confirmed.23 As another example, in noninverted organic solar cells, ITO is typically treated by oxygen plasma or UV-ozone before applying an organic hole transfer layer (e.g., PEDOT:PSS (poly(3,4-ethylenedioxythiophene):polystyrenesulfonate)), and we often observe a rather large variation of device series resistance and overall PV performances across different batches of fabricated devices despite an identical device structure/dimension and fabrication condition (data not shown). A concrete conclusion warrants a more systematic 27223

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(11) He, Z.; Zhong, C.; Su, S.; Xu, M.; Wu, H.; Cao, Y. Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nat. Photonics 2012, 6, 591−595. (12) Zilberberg, K.; Behrendt, A.; Kraft, M.; Scherf, U.; Riedl, T. Ultrathin interlayers of a conjugated polyelectrolyte for low workfunction cathodes in efficient inverted organic solar cells. Org. Electron. 2013, 14, 951−957. (13) Liu, Y.; Chen, C.-C.; Hong, Z.; Gao, J.; Yang, Y.; Zhou, H.; Dou, L.; Li, G.; Yang, Y. Solution-processed small-molecule solar cells: breaking the 10% power conversion efficiency. Sci. Rep. 2013, 3, 3356. (14) You, J.; Dou, L.; Yoshimura, K.; Kato, T.; Ohya, K.; Moriarty, T.; Emery, K.; Chen, C.-C.; Gao, J.; Li, G.; et al. A polymer tandem solar cell with 10.6% power conversion efficiency. Nat. Commun. 2012, 4, 1446. (15) Nam, C.-Y. Facile determination of bulk charge carrier concentration in organic semiconductors: Out-of-plane orientation hopping conduction characteristics in semicrystalline polythiophene. J. Phys. Chem. C 2012, 116, 23951−23956. (16) Nam, C.-Y.; Su, D.; Black, C. T. High-performance air-processed polymer−fullerene bulk heterojunction solar cells. Adv. Funct. Mater. 2009, 19, 3552−3559. (17) Voroshazi, E.; Cardinaletti, I.; Uytterhoeven, G.; Shan, L.; Empl, M.; Aernouts, T.; Rand, B. P. Role of electron- and hole-collecting buffer layers on the stability of inverted polymer: fullerene photovoltaic devices. IEEE J. Photovoltaics 2014, 4, 265−270. (18) He, Z.; Zhang, C.; Xu, X.; Zhang, L.; Huang, L.; Chen, J.; Wu, H.; Cao, Y. Largely enhanced efficiency with a PFN/Al bilayer cathode in high efficiency bulk heterojunction photovoltaic cells with a low bandgap polycarbazole donor. Adv. Mater. 2011, 23, 3086−3089. (19) Zhang, L.; He, C.; Chen, J.; Yuan, P.; Huang, L.; Zhang, C.; Cai, W.; Liu, Z.; Cao, Y. Bulk-heterojunction solar cells with benzotriazolebased copolymers as electron donors: largely improved photovoltaic parameters by using PFN/Al bilayer cathode. Macromolecules 2010, 43, 9771−9778. (20) Chirvase, D.; Chiguvare, Z.; Knipper, M.; Parisi, J.; Dyakonov, V.; Hummelen, J. C. Temperature dependent characteristics of poly(3 hexylthiophene)-fullerene based heterojunction organic solar cells. J. Appl. Phys. 2003, 93, 3376−3383. (21) Wu, H.; Huang, F.; Peng, J.; Cao, Y. High-efficiency electron injection cathode of Au for polymer light-emitting devices. Org. Electron. 2005, 6, 118−128. (22) Milliron, D. J.; Hill, I. G.; Shen, C.; Kahn, A.; Schwartz, J. Surface oxidation activates indium tin oxide for hole injection. J. Appl. Phys. 2000, 87, 572−576. (23) Lo, M.-F.; Ng, T.-W.; Mo, H.-W.; Lee, C.-S. Direct threat of a UV-ozone treated indium-tin-oxide substrate to the stabilities of common organic semiconductors. Adv. Funct. Mater. 2013, 23, 1718− 1723. (24) Zhou, C.; Li, J.; Chen, S.; Wu, J.; Heier, K. R.; Cheng, H. Firstprinciples study on water and oxygen adsorption on surfaces of indium oxide and indium tin oxide nanoparticles. J. Phys. Chem. C 2008, 112, 14015−14020. (25) Ding, X. M.; Hung, L. M.; Cheng, L. F.; Deng, Z. B.; Hou, X. Y.; Lee, C. S.; Lee, S. T. Modification of the hole injection barrier in organic light-emitting devices studied by ultraviolet photoelectron spectroscopy. Appl. Phys. Lett. 2000, 76, 2704−2706. (26) Kim, J. S.; Granstrom, M.; Friend, R. H.; Johansson, N.; Salaneck, W. R.; Daik, R.; Feast, W. J.; Cacialli, F. Indium-tin oxide treatments for single- and double-layer polymeric light-emitting diodes: The relation between the anode physical, chemical, and morphological properties and the device performance. J. Appl. Phys. 1998, 84, 6859−6870. (27) Major, S.; Kumar, S.; Bhatnagar, M.; Chopra, K. L. Effect of hydrogen plasma treatment on transparent conducting oxides. Appl. Phys. Lett. 1986, 49, 394−396. (28) Furukawa, K.; Terasaka, Y.; Ueda, H.; Matsumura, M. Effect of a plasma treatment of ITO on the performance of organic electroluminescent devices. Synth. Met. 1997, 91, 99−101.

ASSOCIATED CONTENT

S Supporting Information *

Representative J−V characteristics of an inverted P3HT:PCBM BHJ solar cell (ITO/PFN/P3HT:PCBM/MoO3/Au) showing reversible degradation after light soaking; J−V characteristics of an air-processed noninverted P3HT:PCBM BHJ solar cell having a structure of ITO/MoO3/P3HT:PCBM/PFN/Al during a 300 s light soaking; J−V characteristics of inverted P3HT:PCBM BHJ solar cells having no PFN on ITO (ITO/ P3HT:PCBM/MoO3/Au) with different ITO surface treatment conditions during a 300 s light soaking. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected], phone 1-631-344-7066. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This research was carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory (BNL), which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract DE-AC02-98CH10886.

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