Selective Interlayers and Contacts in Organic Photovoltaic Cells

PERSPECTIVE pubs.acs.org/JPCL

Selective Interlayers and Contacts in Organic Photovoltaic Cells Erin L. Ratcliff, Brian Zacher, and Neal R. Armstrong* Department of Chemistry & Biochemistry and The Center for Interface Science: Solar Electric Materials, University of Arizona, Tucson, Arizona 85721, United States

bS Supporting Information ABSTRACT: Organic photovoltaic cells (OPVs) are promising solar electric energy conversion systems with impressive recent optimization of active layers. OPV optimization must now be accompanied by the development of new charge-selective contacts and interlayers. This Perspective considers the role of interface science in energy harvesting using OPVs, looking back at early photoelectrochemical (photogalvanic) energy conversion platforms, which suffered from a lack of charge carrier selectivity. We then examine recent platforms and the fundamental aspects of selective harvesting of holes and electrons at opposite contacts. For blended heterojunction OPVs, contact/interlayer design is especially critical because charge harvesting competes with recombination at these same contacts. New interlayer materials can modify contacts to both control work function and introduce selectivity and chemical compatibility with nonpolar active layers and add thermodynamic and kinetic selectivity to charge harvesting. We briefly discuss the surface and interface science required for the development of new interlayer materials and take a look ahead at the challenges yet to be faced in their optimization.

T

hin-film photovoltaic (PV) energy conversion platforms coupling high efficiencies and low cost, ease of scalability to large areas, and acceptable long-term stability are likely to be important components of a portfolio of energy conversion systems that meets the U.S. Department of Energy goal to create electricity from sunlight at less than $1/Wp installed.1
4 Organic photovoltaic cells (OPVs) are a promising entrant to this portfolio and have recently shown impressive increases in efficiency (η), and in some recent reports, η g 8%.5
15 OPVs are created from inexpensive, earth-abundant, but complex material sets including (i) active layers based on either combinations of solution-processed polymers and small molecules or vacuum deposited small molecules, (ii) metal, metal oxide, or conductive polymer electrical contacts, (iii) oxide or polymer/ small molecule interlayers, and (iv) a variety of glass or plastic substrates and barrier layers. The most efficient OPV architectures are created from intermixed donor (D) and acceptor (A) phases to form bulk heterojunction (BHJ) platforms with enhanced D/A interfacial contact areas, thus ensuring high probabilities for photoinduced electron transfer (PIET).5,9,10,15
19 Three separate J. Phys. Chem. Lett. by Mauer et al., Jamieson et al., and Inal, et al.20
22 have recently reviewed some of the issues surrounding photocurrent generation and recombination in the active layers of OPVs. Interlayer films (1
50 nm thick) are a critical component of most OPV platforms and are often added to one or both contacts to (i) control wettability and compatibility of the (typically polar) contact with (typically nonpolar) organic active layers, (ii) control energy barriers to charge harvesting, and (iii) increase the degree of “selectivity” of the contact, so that the contact/interlayer combination demonstrates a higher harvesting rate for one of the charge carriers (holes or electrons).7,8,23
25 Some r 2011 American Chemical Society

of the concepts underpinning the development of these materials have a strong parallel to those that have driven the development of selective interlayers in other inorganic thin-film PV technologies.26

Interlayer films are a critical component of most organic solar cell platforms and are added to one or both contacts to control wettability and compatibility of the contact with organic active layers, control energy barriers to charge harvesting, and increase the degree of “selectivity” of the contact, so that the contact/interlayer combination demonstrates a higher harvesting rate for one of the charge carriers. Received: February 20, 2011 Accepted: May 10, 2011 Published: May 23, 2011 1337

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The Journal of Physical Chemistry Letters How these interlayer materials and contacts function at nanometer length scales, the extent to which they are truly chargeselective, and what design criteria are needed to optimize charge harvesting are ongoing challenges to be faced in optimizing OPV platforms.7,8,15,17,27
37 Compositional changes undergone by these interlayers over time are also likely to be a key determinant of device lifetime. The Center for Interface Science: Solar Electric Materials, a recently formed Energy Frontier Research Center for the U.S. Department of Energy (www.solarinterface.org). has made this area one of its central research themes. This Perspective provides a brief overview of OPV development and our emerging understanding of charge-selective electrodes and interlayers.7,8,17,24,25,31,38 The development of new contact materials, especially alternatives to the transparent conducting oxide indium
tin oxide (ITO), has been discussed elsewhere.39,40 OPV Devices: An Overview of Contact Limitations. For all solar electric energy conversion platforms, the objective is to obtain the highest possible power conversion efficiency, η = JSC 3 VOC 3 FF/PSOLAR, where JSC is the short-circuit current, VOC is the open-circuit photopotential, FF is the fill factor, and PSOLAR is the incident solar power density (AM1.5G = ∼1000 W/m2). Figure S1 in the Supporting Information shows a PV cell with nearly ideal J/V response. Diode equivalent circuit models used to describe OPVs routinely include parasitic series (RS) and shunt resistance (RP) terms, which become simple modifiers of the Shockley equation. Optimized contact and interlayer materials should provide for RS e 1 Ω 3 cm2 at the module level41 and promote wetting and conformal film formation, leading to RP g 104 Ω 3 cm2 and high charge collection efficiencies over large areas (leading to both low RS and FF g 0.65). In the simplest equivalent circuit models, it has been useful to assume that OPV contacts are uniform and ohmic regardless of area, implying that only the bulk electrical properties of the active layers and the intrinsic conductivity of the contacts contribute to RS.42 As the sizes of the OPV and the contacts increase, this assumption may not be valid.11,13,41,43,44 Compositional and energetic heterogeneity in the contacts leads to enhanced recombination in nanometer- to micrometer-sized regions at contact/active layer interfaces, compromising overall OPV performance.8,36,45
50 When charge mobilities in active layers are low, regardless of the PV platform, these problems become more pronounced.46,47 As discussed further below, one of the key roles of interlayer materials now being developed may be to provide the compositional and energetic uniformity otherwise missing at the contact/ active layer interfaces. Figure S2 (Supporting Information) shows the dark and illuminated J/V behavior of an OPV platform (described further below) with varying collection efficiencies between the holecollection electrode (ITO) and the donor layer.33,34 Lowering the rate of charge collection of holes at the oxide/donor interface by increasing the energy barrier to charge harvesting at the ITO surface perturbs the J/V behavior. At the extreme, the J/V curve takes on an S-shape associated with enhanced recombination within the active layer or at one of the contacts.28,46,47,50
52 Increases in the recombination probability can arise from numerous sources, including poor local electrical activity of the contacts, for example, partially blocked electrodes with localized “hot spots”, or decreased charge collection efficiency rates stemming from other bulk or contact limiting factors.8,48 As with any thin-film PV platform, contact/interlayer combinations must match rates of hole (or electron) collection to the

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opposing contact and should be compositionally and energetically uniform from nanometer to macroscopic length scales. Contact/interlayer films must simultaneously provide high rates of charge collection for the desired charge carrier (holes or electrons) and low rates of recombination at the interlayer/organic interface (i.e., recombination arising from charge collection of the undesired carrier or annihilation of the desired charge carrier). Determining what compositional factors have to be controlled to achieve these properties is one of the important interface science questions to be addressed in the optimization of all thin-film PV technologies. Photogalvanic and Dye-Sensitized Solar Cells. The architectures of modern BHJ OPVs are functionally similar to solution-based photogalvanic (PG) cells proposed over 30 years ago53
55 and take advantage of some of the principles underlying the evolution of dye-sensitized solar cells (DSSCs).23,56
59 Both of these technologies are summarized in Figure 1A,B. PG cells consist of a thin solution layer sandwiched between two electrodes (one of them transparent) containing carefully selected concentrations of a donor (D) (e.g., Ru(bipy)3þ2) and an electron acceptor (A) (e.g., Fe3þ) (eqs 1
5). Dark equilibrium D þ A h Dþ þ A


ð1Þ

D þ hν f D

ð2Þ

D þ A h Dþ þ A


ð3Þ

PIET

Hole harvesting kET;D

Dþ þ e
h D

ð4Þ

Electron harvesting kET;A

A
h A þ e


ð5Þ

Excitation of D to create D* and subsequent ET leads to Dþ and A
(eqs 2 and 3), which are in equilibrium with D and A through eq 1 (recombination). If the back reaction to regenerate D and A is slower than the forward reaction, excess Dþ and A
can diffuse toward each of the collection electrodes. By adjusting the concentration of D and the optical density of the solution, PIET occurs near the illuminated electrode, and neutralization of Dþ (hole harvesting) may be dominant at that contact. High values of VOC ≈ ΔEcell are possible (defined by the D/Dþ and A/A
half-cells, which would ideally dominate the potential at the illuminate and dark electrodes, respectively), easily exceeding 1 V for many D/A combinations. Optimized PG platforms were initially predicted to show AM1.5 power conversion efficiencies of up to 18%; however, maximum efficiencies of PG platforms rarely approached 1%. These low efficiencies arise in part from the absence of sufficient compositional and energetic asymmetry in the PG cell; both charge collection electrodes may show comparable rates of electron transfer for both of the redox couples (Dþ/D and A
/ A). Under illumination, such nonselective electrodes equilibrate 1338

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Figure 1. Schematic views of thin-film solar cells. (A) An idealized view of a photogalvanic cell; the illumination gradient is indicated from left to right.53
55 (B) DSSCs use nanoporous TiO2 (or ZnO) films with monolayers of chemisorbed dyes (D) to create a high optical density platform, which collects electrons selectively; solution transport of Dþ (e.g., I3
) to the dark electrode and regeneration of D (e.g., I
) completes the circuit.56
60 (C) Ideal planar heterojunction (PHJ) OPVs are comprised of separate D and A films; the energy offset in the center of the device drives PIET and provides an energy barrier that ensures that holes and electrons are harvested “selectively” at opposite contacts.63,64 (D) BHJ devices (normal, left or inverted, right) fully mix condensed-phase donor and acceptor small molecules, polymers, and/or semiconductor or oxide nanocrystalline materials in a final configuration that is reminiscent of solution PG cells.5
15 Hole- and electron-selective contacts are essential in BHJ OPVs because recombination events, similar to those which plague PG cells, compete with charge harvesting and lower device efficiency.

at nearly equivalent “mixed” Nernstian potentials, limiting the photopotential (VOC) to less than 0.2 V and generating little net power.53 Parasitic electrochemical processes in PG cells appear to have counterparts in OPVs if both donor and acceptor phases are allowed to electronically equilibrate with either electrical contact (see below). Interestingly, even at their introduction, it was recognized that the performance of PG cells would be greatly improved by introducing kinetic selectivity at one or both collection electrodes,53 one electrode demonstrating higher rates of electron transfer (kET,D) for neutralization of Dþ and small

kET,A for neutralization of A
. The other electrode would need to show complementary kinetic selectivity. Energy conversion systems exhibiting selective harvesting of charges began to emerge with development of the DSSCs, where nanoporous metal oxide films (such as TiO2 and ZnO) support high surface coverages of light-absorbing dyes in contact with solutions of an electron donor such as I
, and demonstrated some of the first indications of charge selectivity in harvesting electrons from the excited state of the donor dye.57,58,60 This selectivity arises from the favorable alignment of the conduction 1339

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Figure 2. (A) Schematic view of the energy level alignment in an OPV with interlayers providing ohmic contacts and hole selectivity and electron selectivity (see also refs 7, 24, and 25). At open circuit, holes and electrons (at EF,h and EF,e) are ideally in electronic equilibrium with the contacting electrodes, and VOC is determined by the differences in these quasi-Fermi levels. (B) Schematic views of redox couples in equilibrium with semiconductor electrodes, using the density of states model to describe electron transfer to/from these molecular species (see also refs 91
94). We show the two semiconductor electrodes (interlayers) after achieving equilibrium with these redox couples. Extrapolation of this model to condensedphase systems helps to rationalize the need for careful matching of the frontier orbital energies of the molecular system in an OPV with the band-edge energies and Fermi levels of electron- or hole-selective contacts.

band energy (ECB) of the oxide with the excited-state energies for many adsorbed dyes combined with the high ionization potentials of these oxides (IP ≈ EVB), making hole extraction from adsorbed dyes and from the solution mediator (e.g., I3
/I
) energetically unfavorable for light excitation below the band gap energy of the oxide. Solution species such as I
are oxidized only by Dyeox, the capture of a “hole” from I3
(reducing it back to I
) is slower than the forward electron-transfer process, and the opposite dark electrode equilibrates only with I
/I3
at a potential dictated by the ratio of their interfacial concentrations. VOC is therefore determined by the difference between the quasiFermi potential at the illuminated electrode (approximately set by ECB of the metal oxide) and the Fermi level of the dark electrode (set by the half-cell potential for I
/I3
). The architecture of these devices, coupled with the hole-blocking/electron collecting nature of TiO2 and ZnO, provides the compositional and energetic asymmetry missing in PG cells.23 The relative selectivity discussed here only applies to dyes with IPs lower than the oxide itself, and the degree of selectivity is truly defined by the oxide/organic dye combination. Improvements to this architecture are emerging, and several polymeric hole-transport/donor layers have been recently introduced as replacements for problematic liquid electrolytes and redox mediators, providing promising efficiencies in more robust platforms.57,61,62 In these asymmetric thin-film devices, concentration gradients drive diffusion of holes in the polymer (via hopping) to the dark electrode, and concentration gradients for electrons in the nanoporous oxide drive their diffusion toward the illuminated collection electrode.57 Planar and Bulk Heterojunction OPVs. Planar heterojunction (PHJ) platforms, introduced by Tang and co-workers as OPVs and as organic light-emitting diodes,63,64 solved the problem of contact selectivity by arranging donor and acceptor dye layers in planar Type II heterojunctions (Figure 2C). This innovation also moved OPV architectures away from far less efficient diodes

using active layers with only one organic dye, bracketed by metal and metal oxide contacts.65 The energetic offsets in the ionization potentials (IPs) and electron affinities (EAs) at the D/A interface in these heterojunctions lead to efficient PIET.12,66,67 For OPV bias near VOC (near the maximum power point, Pmax), diffusion is chiefly responsible for transport of holes and electrons to the collection electrodes.7,68 If “bulk-limited” electrical contacts are made to both D and A films, the VOC is independent of the work function of the contacting electrodes and is determined by differences in quasi-Fermi potentials in the donor and acceptor phases.7,24,25 In device configurations where collection efficiencies are limited by rate of extraction at the contact(s), VOC becomes dependent on the work function difference between the contacting electrodes.8,42,69 PHJ OPVs are limited in efficiency by the small values of R 3 LD (where R = absorptivity (cm
1) and LD = exciton diffusion length);66 however, “texturing” of the D/A layers, on 10
100 nm length scales,70
74 or intimate mixing of the donor and acceptor layers increases the interfacial contact area between D and A to form BHJ OPVs and provides for much higher photocurrents.5,10,15,70,71,75 Mixtures of solution- or vacuumprocessed donors and acceptors can be annealed after processing into a thin film to create bicontinuous donor/acceptor phases with appropriate aggregate length scales.5,10,27 These active layers are now deposited on a variety of semitransparent oxide, metal grid, or nanoparticles arrays22,76
78 in single junction or tandem junction cells. “Normal” BHJ configurations start with a transparent bottom electrode, adding a conducting polymer (CP) or oxide thin film as a hole-selective interlayer, followed by the active layer, and an electron-selective interlayer, and the top electrode. “Inverted” BHJ configurations use an electron-selective interlayer over the bottom contact (e.g., ZnO), followed by the active layer, a hole-selective interlayer, and the top electrode.15,32,70
73,75,79 Normal configurations have given the best efficiencies to date in 1340

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The Journal of Physical Chemistry Letters polymer/small-molecule OPVs. Inverted configurations are preferred because they use more air stable top electrodes (e.g., Ag). Thoroughly blending donor and acceptor polymers and small molecules to form BHJ OPVs (Figure 2D), recently reviewed in J. Phys. Chem. Lett. by Venkataraman and co-workers19 and by Neher and co-workers,22 can reintroduce the problems associated with nonselective contacts seen in PG cells,80 and in PVs with poor contact selectivity, recombination may dominate over charge harvesting at either electrode.29,46,47,51,81,82 Charge-Selective Interlayers: Overview. Interlayer films, with thicknesses of 1
50 nm, have been used in OPVs and OLEDs to enhance their efficiencies. These interlayer films have been used to (i) make one of the contacts ohmic/bulk-limited toward either charge to improve charge injection (OLED) or charge collection (OPV), without enhancement of charge selectivity, (ii) make the electrical conductivity of the contact high and uniform over long length scales while enhancing wettability toward nonpolar organic active layers, (iii) add thermodynamic selectivity (tuning of work function or EVB, ECB of the interlayer), or (iv) add kinetic selectivity (tuning kET for Dþ or A
) so that the contact preferentially harvests holes or electrons. These latter modifications introduce real rectification to the J/V response for that contact/organic heterojunction.13,29,33,36,49,83,84 Carrying such modifications out for both contacts, to balance the rates of hole and electron harvesting, leads to the highest overall OPV efficiencies.85
87 W€urfel likened the role of selective contacts or interlayers to semipermeable membranes,24,25 where real charge selectivity is introduced by ensuring that one of the charge carriers is harvested more rapidly than the other, either because of energetic barriers or because of the (tunable) electrical properties of the interlayer material that controls hole or electron mobilities. A review of photovoltaic activity in photoelectrochemical energy conversion systems by Bisquert et al. adopted a similarly useful approach to describing the need for selective contacts.23 The use of charge-selective interlayer films based on doped small molecules, conductive polymers, or metal oxides is, for many OPVs, conceptually analogous to the formation of p
i
n architectures in amorphous silicon (a-Si) PV platforms. In such platforms, the undoped active layer is surrounded by heavily doped p- or n-layers; p-doped thin film/active layer/n-doped thin film7,15,24,25,88 and lifetimes and diffusion lengths of photogenerated carriers are too short (relative to crystalline Si) for diffusion alone to effectively transport carriers to the contacts.89 Bracketing the a-Si region with highly doped p- and n-type interlayers provides for ohmic contacts to the i layer, ensuring that the electric field drops exclusively across this active layer (Figure 2A). This comparison is most relevant for vacuumdeposited p
i
n platforms created from doped small-molecule interlayers that bracket a codeposited, fully mixed photoactive donor/acceptor I layer (Figure 2A).7,14,15,32 Careful examination of these technologies indicates that the p-type and n-type interlayer regions may be designed to show thermodynamic and kinetic selectivity for holes or electrons, respectively, with valence or conduction band energies that impede transport of the unwanted charge carrier.24,25,88 Similar issues arise in the design of such membrane interlayers for thin-film inorganic PV systems.26 Figure 2A summarizes an idealized energetic configuration of interlayer and contact materials that have been proposed to optimize OPV performance, borrowing from earlier proposed models.7,24,25,88 The band-edge energies are displayed both with

PERSPECTIVE

respect to vacuum and on an electrochemical scale (using the potential of the NHE ≈
4.4 eV versus vacuum).90 Both holeselective (yellow) and electron-selective (gray) interlayers are highly doped and have large band gap energies EBG,interlayer g EBG,active layer, along with appropriate alignment of either conduction or valence band energies to block flow of the undesired charge carrier and to be barrier free for collection of the desired charges.91 The large band gap of the interlayer may also help confine excitons in the active layer enhancing exciton dissociation.32,66 At VOC and high light intensities, the quasi-Fermi levels for holes and electrons in the active layer are assumed to be in electronic equilibrium with the interlayers and contacts and are separated by energy differences up to EHOMOD
ELUMOA. At a device bias near VOC (near the maximum power point in the OPV), the high conductivities of the doped interlayers ensure that the internal field drops across the active layer, and a sufficient field is present to suppress undesired charge harvesting between the p-type (n-type) interlayer and reduced acceptor (oxidized donor) (Figure 2A). Density of states (DOS) models (Figure 2B) used to describe electron transfer at metal/solution or semiconductor/solution interfaces (Gerischer model) may be useful to rationalize the additional properties needed for effective interlayer materials.92
95 Extrapolating models for solution electron transfer to processes associated with interfacial charge transfer in condensed molecular systems is challenging. These models provide guidance, however, in selection and optimization of interlayers and in understanding their response to the presence of charged smallmolecule or polymeric species that represent the products of PIET in OPVs. We show a highly doped, large band-gap p-type semiconductor interlayer (yellow) in equilibrium with a redox couple, whose filled (reduced) and empty (oxidized) distributions are represented by Dred and Dox respectively, arranged symmetrically around an equilibrium potential, aligned with the Fermi potential of the interlayer. A similar equilibrium situation is shown for the n-type semiconductor (gray), and Dred0 /Dox0 . Dred and Dox for outer sphere electron transfer are depicted as Gaussian functions, separated by 2λ, where λ is the total reorganization energy associated with ET.92
95 Rates of ET upon contact of the redox couple (active layer) with the interlayer are controlled by these reorganization energies as well as the integrals that describe the overlap of the filled state distributions (Dred) or unfilled state distributions (Dox) with the unfilled or filled state distributions in the p-type or n-type interlayers, respectively. These models suggest that optimized hole-selective interlayers in contact with organic layers should have valence band energies, EVB, and work functions, j, closely matched to the EVB or IP of the donor, while electron-selective interlayers should have conduction band energies, ECB, and j close to the ECB or EA of the acceptor, so that electronic equilibrium can be easily established and charge injection barriers minimized. Efficiencies in charge harvesting are predicted to be highest for those systems with minimized reorganization energies during charge transfer and minimized energetic disorder at the interface. Chemical composition of interlayer materials will control EVB and ECB, while dopant concentration controls j (EF) of the interlayer and will ensure adequate conductivity.8,23,93 These models do not account for the presence of midgap states in the interlayer material, which typically arise in the nearsurface regions of many oxides and polymer films where they 1341

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Figure 3. Schematic view of the energy gaps and frontier orbital energies of some of the components of recent OPVs. Estimated frontier energy levels are shown for the donor polymers P3HT and PCDTBT, for copper phthalocyanine (CuPc), and for acceptors such as C60 and PCBM.8 The colored areas for the metal oxides and many of the interlayer materials represent their approximate valence and conduction bands, using estimates for EVB (IP) and ECB (EA) and, where available, estimates for EF (dashed lines). TiO2 and ZnO represent common electron-selective interlayer materials.98,99,117
120 NiOx represents a wide-band-gap metal oxide proposed as a hole-selective interlayer.29,121 WO3 and MoO3 tin films show high work functions, high IP, and n-type electronic character.112,113 The polymeric interlayers, PEDOT/PSS, and e-P3HT possess large band gaps and low EAs in their neutral forms but are used in their doped forms as interlayers. Polaronic and bipolaronic states introduced into the band gap provide for tunability of the effective work function.32,33,103
106 Ranges of effective work functions are shown for ITO and Ag; however, these levels are sensitive to surface activation and chemical modification with dipolar small molecules.31,108
111 For simplicity, we ignore shifts in the local vacuum level that occur at each of the interfaces in an OPV.

Matching of the frontier orbital energies of the interlayer to those for the organic layer is required for electronic equilibrium to be easily established and for minimization of charge injection barriers. depart from the bulk stoichiometry and possess much different frontier orbital energies.96
98 For example, conductivities in oxides are controlled by species added as substitutional or interstitial dopants and/or by removal of oxygen from the lattice. Energetic heterogeneity in the near-surface region may result from doping, especially for oxides processed at low temperatures, which creates midgap states through which charges are nonselectively transported.99
103 In the case of CPs and smallmolecule films, where conductivity and work function are controlled by oxidative or reductive doping accompanied by incorporation of counterions, the location and density of counter charges can control electrical properties, the electrochemical potential range over which the material is in its conductive state, and rates of charge transfer (see below).33,34,49,83,104
107 The added effect of local vacuum level shifts and interface dipoles associated with dipolar or charged molecules at the

organic/interlayer interface may be important even when all other parameters are optimized.37,108
110 No variations in local vacuum levels are shown in Figure 2A; however, organic/oxide and organic/metal interfaces often demonstrate vacuum level shifts up to 0.8 eV over length scales of 1
5 molecules, which can be important additional barriers to charge injection and can greatly affect the “ohmicity” of a contact.35,37,42,109,111,112 Intentional chemical modification to add dipolar molecules to both interfaces has been the most common approach to date to lowering injection barriers while simultaneously controlling surface free energy and wettability.35,37,109,110,112 Metal Oxide Interlayers. Figure 3 summarizes some of the frontier orbital energy levels for active materials in both BHJ and PHJ heterojunction OPVs, and several recently proposed interlayer materials in OPVs, including metal oxides and CP thin films.8,30,36,113
115 Shaded regions define the band gap of each material, bracketed by estimates of EVB and ECB. Brabec and coworkers have recently reviewed a smaller range of interlayer materials.8 Estimated ECB and EVB energies in Figure 3 can vary depending on deposition conditions, doping levels, and the presence of interface dipoles formed with both contacts and organic layers.37,108 Some of the interlayer materials, however, appear to satisfy at least the thermodynamic requirements for charge-selective interlayers set in Figure 2, while other interlayers may make the contact ohmic but not necessarily charge selective. This continues to represent one of the biggest challenges facing this area — understanding the correlation between the electronic and 1342

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The Journal of Physical Chemistry Letters chemical properties of the interlayer and its actual effect on OPV performance. n-Type oxides like TiO2 and ZnO are good candidates for electron collection interlayers because the conduction band energies and work functions are positioned close to the electrontransport energies for common molecular acceptors in OPVs (e.g., the fullerenes C60 and PCBM), and simultaneously, these oxides have higher electron versus hole mobilities.8,82,116,117 The use of ZnO films as interlayers in OPVs has been recently reviewed in J. Phys. Chem. Lett. by Bisquert and co-workers.118 These oxides have high IPs, which further inhibits hole harvesting (in the dark) from conventional donors (poly(thiophenes) and related active layer polymers and small molecules),23,96
98 properties that have prompted the use of nanostructured versions of these oxides as the acceptor layer in BHJ OPVs.61 p-Type hole-selective interlayers, such as nickel oxide (NiOx), have recently been introduced for use in poly(3-alkylthiophene)based BHJ OPVs.30,119 EVB and j are close to the hole-transport energies for several donor OPV materials, and ECB and the electron affinity for NiOx films appear to be thermodynamically suited to block electron injection from fullerenes such as C60 and its soluble derivative PCBM; however, the degree to which real rectification is achieved by these interlayers is still under investigation. For donor polymer layers with a higher IP than that of the conventional poly(thiophenes), a higher work function binary or ternary oxide interlayer may be required as the holeselective interlayer. Controlled doping of thin n-type or p-type oxide interlayers can be challenging, especially those deposited from solution precursors. Substoichiometric regions and excessive donor concentrations in interlayer materials, especially oxides like TiO2 and ZnO and NiOx, can lead to states of intermediate energy in the band gap that lower charge selectivity and may act as recombination centers. The roles played by defects in oxide films, controlling both electrical properties and providing for specific interfacial interactions with polymers or small molecules, and the development of new oxide films that afford greater control of these properties will continue to be important areas of investigation, using many of the tools developed by the surface science community in understanding more conventional, well-ordered oxide materials.99,100 Oxides with high concentrations of midgap states may, however, be essential as the bridging interlayers in tandem cell OPVs, where two complementary PV platforms are series connected by an interlayer film, providing a site where holes from one OPV and electrons from its partner are annihilated.14,116,120 Control of the work functions, conductivities, concentrations of defects and midgap states, and charge selectivities in these thin-film oxides is essential.121,122 Energy levels for high work function oxides such as MoO3 and WO3, recently established by Kahn and co-workers, are also shown in Figure 3.113,114 These vacuum-deposited interlayer materials have been used by several research groups to improve the performance of both OLEDs and OPVs.123
126 Recent UPS/ IPES studies show that their high IP and large work functions are not consistent with real hole selectivity. They appear instead to improve the electrical properties of the organic/interlayer interface by virtue of their high effective work function and their tendency toward oxidative doping of organic layers located at the oxide/organic interface, which is similar to the role played by strong electron-acceptor molecular dopants used to make p-type interlayer regions in both OLEDs and OPVs.15,75

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Figure 4. (A) J/V (i) and log|J|/V (ii) behavior of C60 thin films with ITO/PEDOT/PSS (black) and ITO/e-P3HT bottom contacts and BCP/Al top contacts. For optimally doped e-P3HT, excellent rectification is observed. At forward bias, the e-P3HT/C60 heterojunction is the site for recombination of holes and electrons. At reverse bias, electron injection across the ITO/e-P3HT heterojunction is blocked (shown in schematically in B), consistent with previous reports of photovoltaic activity for these heterojunctions.33 (C) Energy level diagrams showing how the Fermi levels of PEDOT/PSS and e-P3HT can be adjusted to provide for conductivity in potential regions capable of supporting redox activity for several different redox couples. (D) Voltammetric characterization of three redox couples in acetonitrile using bare ITO (i), ITO/ PEDOT/PSS (ii), and ITO/e-P3HT (iii). Each redox couple behaves normally on the bare ITO electrode and on ITO/PEDOT/PSS (with enhanced rates of ET). On ITO/e-P3HT, the two-electron oxidation/ reduction of TPD proceeds with rates of ET comparable to those of bare Au electrodes. The oxidation of Fc to Fcþ is inhibited until the e-P3HT film reaches its optimally doped state, and the reduction of Fcþ to Fc is completely blocked. The oxidation of Me10Fc is almost completely blocked, and only oxidation through pinholes in the e-P3HT film provides for a small, distorted voltammetric response. The reduction of Me10Fcþ is completely blocked by the e-P3HT film. 1343

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The Journal of Physical Chemistry Letters Doped Small-Molecule and Conducting Polymer Interlayers. OPVs using both small-molecule and CP interlayers are now common in many OPV platforms.7,32,127 Both p-type and n-type small-molecule interlayers have been formed via vacuum codeposition of a host small molecule (hole- or electron-transporting) accompanied by a small-molecule oxidant like F4-TCNQ or molybdenum dithiolene for p-doping (shown in Figure 3) or a small molecule reductant (e.g., acridine orange) for n-doping.75,91,128,129 The conductivity and apparent Fermi level of these doped small-molecule interlayers is tunable to achieve an ohmic contact while minimizing resistive losses and optimizing VOC. OPVs fabricated using such vacuum deposition protocols provide good control of film purity, reproducibility, and the ability to stack current-matched tandem cells.27,52 More widely used CP films have been mainly available to date as p-type materials. They are deposited by spin-casting or printing of emulsions of oxidatively doped polymers with polymeric counteranions or can be electrochemically deposited on the contact of choice.27,33,34,49,83 It has been hypothesized that p-type CP and small-molecule interlayers for hole-harvesting contacts are electron-blocking toward many of the common OPV acceptor materials such as C60 and PCBM. This is now in question and remains to be fully explored for a wider range of CPs. In some cases, the role of the polymer interlayer may simply be to enhance charge-harvesting rates over what they would be for an unmodified oxide (e.g., ITO) and to provide more uniformity in electrical response. The left side of Figure 3 shows estimated energy levels for two polymer interlayers, one created from the popular dispersion of (poly(ethyleneoxythiophene/poly(styrenesulfonate) (PEDOT/PSS) and the other from electrodeposited poly(3hexylthiophene) (e-P3HT).33,34 The use of PEDOT/PSS as an interlayer material, its effectiveness in optimizing both OPVs and OLEDs, and some of the challenges in the use of such CP/ionic polymer dispersions have been recently reviewed by Ginger and co-workers.49 The effective work function of PEDOT/PSS films has been estimated to be ∼5.1 eV but is strongly dependent on the segregation of PSS chains to the film surface after annealing.107 Electrodeposited ultrathin poly(thiophene) films such as e-PEDOT and e-P3HT, if grown as compact films, provides tunability of both the work function and conductivity as a result of the variability in doping of these polymeric materials. Our studies have shown that conductivity can reach a maximum over a narrow potential range, and the effective work function can be varied from ∼3.9 to 5.1 eV, with a variable DOS near the IP of several common OPV donor layers.33,34,105 The low electron affinity of undoped P3HT or PEDOT may help block electron injection from acceptor phases that might reside near the hole-harvesting contact, but as shown below, this needs to be rigorously tested in real device platforms. Rectification in Device Platforms and in Simple Electrochemical Reactions. The limitations to charge selectivity in CP films like PEDOT/PSS and the improvements that can be expected from interlayers like e-P3HT are summarized in Figure 4. We show the J/V behavior for heterojunctions based on ITO/PEDOT/PSS/ C60 and ITO/e-P3HT/C60 thin-film combinations and a schematic of the offsets in frontier orbital energies for C60/e-P3HT (Figure 4B). The potential ranges over which both PEDOT/PSS and e-P3HT films have high conductivity and support facile solution redox reactions are shown in Figure 4C, and the voltammetric responses of several solution probe molecules on ITO, ITO/PEDOT/PSS, and ITO/e-P3HT electrodes are

PERSPECTIVE

summarized in Figure 4D. For the electrical property measurements, the completed devices use a thin bathocuproine (BCP) layer between the C60 film and the evaporated Al top electrode, which mitigates damage to the fullerene.130,131 When an optimally doped e-P3HT layer is interposed between ITO and C60, the J/V response exhibits rectification.34 The high forward recombination current indicates that rates of injection of holes from the ITO/e-P3HT contact and electrons through the Al contact are well matched, reaching space-chargelimited behavior at ∼0.2 V and satisfying the first assumption for a selective contact. Under reverse bias, electrons injected from the ITO/e-P3HT contact appear to be completely blocked at the e-P3HT/C60 interface, as indicated by the low current density, even at high bias (
1 V), as expected from the proposed energy level diagram in Figure 4B. Photovoltaic activity has been documented by this type of heterojunction.34 On the basis of its effective work function and frontier orbital energy levels, we would expect the PEDOT/PSS layer to act similarly and block electron injection to the ITO contact. However, in the forward bias, recombination currents are low and suggest that the rate of hole injection from the PEDOT/PSS is not well matched to the rate of electron injection at the top contact. The J/V response is only partially rectified, with a much higher current density in the reverse bias direction; the PEDOT/ PSS layer is not as blocking toward electron injection at the PEDOT/PSS/C60 interface as expected. Marks and co-workers have recently confirmed that PEDOT/PSS exhibits poor electron blocking in certain BHJ OPVs, requiring an additional electron-blocking layer in combination with this polymer film at the ITO contact.29 Kippelen and co-workers have further shown that PEDOT/PSS layers can be used as both bottom (hole-harvesting) and top (electron-harvesting) contacts in OPVs that use no ITO at all, provided that the electron-harvesting PEDOT/PSS interlayer is accompanied by a thin electronselective ZnO layer.117 Thus, it is expected that PEDOT/PSS has a distribution of states within its band gap (midgap states), with densities sufficient to support charge transfer in both bias directions over a wide potential window. In order to further evaluate the distribution of midgap states for PEDOT/PSS relative to e-P3HT, electrochemical studies shown in Figure 4D were performed, inspired by the studies of Bard and co-workers, who explored the electrochemical selectivity of several n-type and p-type semiconductor electrodes correlated with their band-edge energies.96
98 For n-type oxides such as TiO2 and ZnO, this group demonstrated that electron injection to solution probe molecules, such as the multielectron reduction of Ru(bipy)3 in acetonitrile, was blocked until potentials negative of the flat-band potential for the oxide were reached, driving the semiconductor into an accumulation mode. For redox couples whose formal potentials lie energetically within the band gap of the n-type oxide, or overlapping the valence band region, electron transfer at the oxide/solution interface was blocked and voltammetric activity suppressed. Similar predictions arise from the use of these oxides as electron-selective interlayers in OPVs, that is, electron flow is allowed, in either direction, at energies above the conduction band energy of the oxide. These electrochemical studies were also useful in demonstrating the presence of midgap states in oxides that were substoichiometric in the near-surface region. These states were capable of mediating electron transfer for redox couples in potential regions where their electroactivity should have been blocked, a condition predicted to be likely for 1344

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The Journal of Physical Chemistry Letters thin polycrystalline oxide interlayers formed at low processing temperatures.100 Figure 4D shows the variability in electron blocking/hole selectivity for the thin polymer films in Figure 4A, extrapolated to solution electrochemical studies of the oxidation/reduction of simple redox couples, including N,N0 -diphenyl-N,N0 -bis(3methylphenyl)-(1,10 -biphenyl)-4,40 -diamine (TPD, undergoing two successive one-electron oxidations),132 ferrocene (Fc), and decamethylferrocene (CH3)10-Fc. The bare ITO electrode shows nearly reversible behavior for all three redox couples; the PEDOT/PSS-coated ITO electrode shows enhanced rates of ET for oxidation/reduction of all three redox couples at scan rates of 10 mV/s, consistent with the fact that this polymer remains in its conductive state over a wide potential range.133,134 In contrast, e-P3HT films on ITO, while enhancing the rates for TPD oxidation/reduction, show partial blocking of the redox behavior for Fc/Fcþ and complete blocking of the redox reactions for (CH3)10-Fc (small oxidation currents are observable through the small number of pinholes in these films at high overpotentials). As implied in Figure 2B, oxidation of Fc is achieved because there are unoccupied states when the polymer is held at a sufficiently positive potential; however, the reduction of Fcþ is blocked because in this potential range, all states in the e-P3HT film are occupied. Charge transfer between the e-P3HT film and (CH3)10-Fc is completely blocked because at these potentials, all states of the e-P3HT are filled. This behavior is ideal for a true hole-selective contact in that charge transfer only takes place when the density of states is at or below the HOMO of the hole-selective contact. The flow of charge in these cases is in the opposite direction to that expected in an OPV, but the electrochemical experiment nevertheless establishes a threshold energy where charges cannot flow to the probe molecule, determined by the potential where conductivity is significantly decreased. The assumption is made that these threshold energies for the CP/solution interface are directly related to the energy threshold required for electron blocking in condensed-phase environments. Figure 4C summarizes the potential regions (and energies) over which these two poly(thiophenes) are in their conductive states. PEDOT/PSS films in general appear to show greater energetic heterogeneity than other poly(thiophenes) that we have explored, providing a wider range over which they are conductive,69,133 accompanied by a lower degree of hole selectivity versus the electrochemically grown polymer films. It is likely that for doped small-molecule interlayers and certain oxide interlayer films, similar electrical and electrochemical properties presented above will be observed. Therefore, careful control of doping and band-edge energies with minimal midgap states will be needed in order to optimize selective charge collection. Electrochemical studies of probe molecules will be useful in screening these new interlayers and mapping out the regions over which they are conductive and the regions where they will reject transport of charge carriers. Future Issues and Challenges. At the time of the writing of this Perspective, cell efficiencies have risen in some single junction OPVs to over 8%. The challenge for researchers in the OPV community will be to bring research cell efficiencies to much higher levels and module efficiencies to near 10% in low-cost, lightweight formats with lifetimes adequate to make them commercially viable.5,27,52,135 For both vacuum-processed and solution-processed OPVs, multijunction (tandem) cells will be essential, each with a VOC as close to 1 V as possible, providing

PERSPECTIVE

Careful control of doping and bandedge energies with minimal midgap states will be needed in order to optimize selective charge collection. Electrochemical studies of probe molecules will be useful in screening new interlayers, mapping out the regions over which the interlayers are conductive, and distinguishing regions where they will reject transport of charge carriers. adequate coverage of the AM1.5G solar spectrum from ∼400 to 1100 nm.13,14,116 This will add to the number of critical interfaces requiring control of composition and energetics at the nanometer scale; (i) charge selectivity will be required in at least four interlayers, (ii) charge recombination between OPV subcells will need to occur in specially designed, thin p
n junctions (or their metal nanoparticle equivalents) to ensure adequate current matching, and (iii) ohmic top and bottom contacts will be required, which may both need to be transparent if the objective is to integrate OPVs into architectural glass.136

Development of selective contacts for organic solar cells will require an unprecedented level of understanding and control of interface composition, modifications to chemical and physical properties at these interfaces, and development of earth-abundant, low-cost materials sets whose properties are tailored from inception to ensure compatibility with new polymer or small-molecule active layers. Development of selective contacts for organic solar cells will require an unprecedented level of understanding and control of interface composition, modifications to chemical and physical properties at these interfaces, and development of earth-abundant, low-cost materials sets whose properties are tailored from inception to ensure compatibility with new polymer or smallmolecule active layers. These materials sets must be compatible 1345

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The Journal of Physical Chemistry Letters with the processing conditions needed to take full advantage of the technological opportunities afforded by OPVs, that is, the ability to print or coat active layers and produce hundreds of square meters of PV platforms per day. The basic questions to be addressed center on understanding the relationship between the chemical and physical composition of interlayer materials with their energetic and electrical properties on nanometer length scales. The challenge facing us is to quantify such compositional changes on nanometer length scales, often in environments with the complexity of the working solar cell platform. Several of the recently funded Energy Frontier Research Centers, and related small and large DOE basic research efforts, coupled with funding from other federal agencies and industrial sponsors, has given us the needed start on these problems and an enthusiastic base of young scientists to ensure that these technical challenges are met.

The basic questions to be addressed center on the relationships between the chemical and physical composition of interlayer materials and their energetic and electrical properties, on nanometer length scales, often in environments with the complexity of the working solar cell platform.

PERSPECTIVE

Dr. Erin L. Ratcliff is a Research Scientist in the EFRC CIS: SEM, at the University of Arizona. She received her Ph.D. in 2007 from Iowa State University in Physical Chemistry and then was a postdoctoral fellow at the University of Arizona from 2007 to 2010. Her research is currently focused on the characterization and role of surface and interface states on the performance of organic and inorganic materials with respect to photoinduced charge-transfer rates. Brian Zacher is a Ph.D. chemistry graduate student in the Armstrong group at the University of Arizona, with a background in electrical engineering. His research interests are focused on modeling and characterizing the interfacial layers proposed for use in the optimization of organic solar cells and related energy conversion technologies.

’ ACKNOWLEDGMENT Preparation of this manuscript was supported as part of the Center for Interface Science: Solar-Electric Materials (CIS: SEM), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Basic Energy Sciences under Award Number DE-SC0001084. The authors also gratefully acknowledge critical discussions with Antoine Kahn (Princeton University), Bernard Kippelen (Georgia Institute of Technology), David Ginley, Dana Olson, Joseph Berry, and Xerxes Steirer (National Renewable Energy Laboratories), Fredrick Krebs (RISØ, Denmark), Rene Kogler and Gilles Dennler (Konarka), and Martin Pfeiffer (Heliatek). We also gratefully acknowledge the use of an electron micrograph of an interlayer/ ITO interface from Delvin Tadytin and Kai-Lin Ou, used in the Table of Contents figure. ’ REFERENCES

’ ASSOCIATED CONTENT

bS

Supporting Information. A schematic view of an OPV and it current
voltage behavior, an equivalent circuit model of the OPV along with modified versions of the equations used to describe J/V response, VOC, and the power conversion efficiency, and the dark and illumination J/V response for a generic OPV with an activated and partially blocked bottom contact. This material is available free of charge via the Internet at http://pubs. acs.org

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ BIOGRAPHIES Neal R. Armstrong is Professor of Chemistry & Biochemistry and Optical Sciences at the University of Arizona and is the Director for the Center for Interface Science: Solar Electric Materials (CIS:SEM), a Department of Energy, Basic Energy Sciences, Energy Frontier Research Center. His research interests are focused on the interface science of molecular electronic materials and the development of new chemical sensor platforms. See (http://www.cbc.arizona.edu/facultyprofile?fid_call=Arms) or (www.solarinterface.org).

(1) Lewis, N.; Crabtree, G.; Nozik, A. J.; Wasielewski, M. R.; Alivisatos, A. P. Basic Research Needs for Solar Energy Utilization, Report on the Basic Energy Sciences Workshop on Solar Energy Utilization; 2005, http://www.sc.doe.gov/bes/reports/files/SEU_rpt.pdf. (2) $1/W Photovoltaic Systems: White Paper to Explore A Grand Challenge for Electricity from Solar. Department of Energy Advanced Research Projects Agency Energy Energy Efficiency and Renewable Energy. http://www1.eere.energy.gov/solar/pdfs/dpw_white_paper. pdf (2010). (3) Kazmerski, L. L. Solar photovoltaics R&D at the tipping point: A 2005 technology overview. J. Electron Spectrosc. Relat. Phenom. 2006, 150, 105–135. (4) Ginley, D.; Green, M. A.; Collins, R. Solar Energy Conversion toward 1 Terawatt. MRS Bull. 2008, 33, 355–364. (5) Dennler, G.; Scharber, M. C.; Brabec, C. J. Polymer
Fullerene Bulk-Heterojunction Solar Cells. Adv. Mater. 2009, 21, 1323–1338. (6) Dennler, G.; Scharber, M. C.; Ameri, T.; Denk, P.; Forberich, K.; Waldauf, C.; Brabec, C. J. Design Rules for Donors in Bulk-Heterojunction Tandem Solar Cells — Towards 15% Energy-Conversion Efficiency. Adv. Mater. 2008, 20, 579–583. (7) Riede, M.; Mueller, T.; Tress, W.; Schueppel, R.; Leo, K. SmallMolecule Solar Cells — Status and Perspectives. Nanotechnology 2008, 19, 1–12. (8) Steim, R.; Kogler, F. R.; Brabec, C. J. Interface Materials for Organic Solar Cells. J. Mater. Chem. 2010, 20, 2499–2512. (9) Chen, H. Y.; Hou, J. H.; Zhang, S. Q.; Liang, Y. Y.; Yang, G. W.; Yang, Y.; Yu, L. P.; Wu, Y.; Li, G. Polymer Solar Cells with Enhanced Open-Circuit Voltage and Efficiency. Nat. Photonics 2009, 3, 649–653. (10) Park, S. H.; Roy, A.; Beaupre, S.; Cho, S.; Coates, N.; Moon, J. S.; Moses, D.; Leclerc, M.; Lee, K.; Heeger, A. J. Bulk Heterojunction 1346

dx.doi.org/10.1021/jz2002259 |J. Phys. Chem. Lett. 2011, 2, 1337–1350

The Journal of Physical Chemistry Letters Solar Cells with Internal Quantum Efficiency Approaching 100%. Nat. Photonics 2009, 3, 297–U295. (11) Perez, M. D.; Borek, C.; Forrest, S. R.; Thompson, M. E. Molecular and Morphological Influences on the Open Circuit Voltages of Organic Photovoltaic Devices. J. Am. Chem. Soc. 2009, 131, 9281–9286. (12) Rand, B. P.; Burk, D. P.; Forrest, S. R. Offset Energies at Organic Semiconductor Heterojunctions and Their Influence on the Open-Circuit Voltage of Thin-Film Solar Cells. Phys. Rev. B 2007, 75, 115327. (13) Potscavage, W. J.; Sharma, A.; Kippelen, B. Critical Interfaces in Organic Solar Cells and Their Influence on the Open-Circuit Voltage. Acc. Chem. Res. 2009, 42, 1758–1767. (14) Timmreck, R.; Olthof, S.; Leo, K.; Riede, M. K. Highly Doped Layers As Efficient Electron-Hole Recombination Contacts for Tandem Organic Solar Cells. J. Appl. Phys. 2010, 108, 033108. (15) Wynands, D.; Mannig, B.; Riede, M.; Leo, K.; Brier, E.; Reinold, E.; Bauerle, P. Organic Thin Film Photovoltaic Cells Based on Planar and Mixed Heterojunctions between Fullerene and a Low Bandgap Oligothiophene. J. Appl. Phys. 2009, 106, 054509. (16) Yi, Y. P.; Coropceanu, V.; Bredas, J. L. Exciton-Dissociation and Charge-Recombination Processes in Pentacene/C-60 Solar Cells: Theoretical Insight into the Impact of Interface Geometry. J. Am. Chem. Soc. 2009, 131, 15777–15783. (17) Chen, L. M.; Xu, Z.; Hong, Z. R.; Yang, Y. Interface Investigation and Engineering — Achieving High Performance Polymer Photovoltaic Devices. J. Mater. Chem. 2010, 20, 2575–2598. (18) Bredas, J. L.; Norton, J. E.; Cornil, J.; Coropceanu, V. Molecular Understanding of Organic Solar Cells: The Challenges. Acc. Chem. Res. 2009, 42, 1691–1699. (19) Venkataraman, D.; Yurt, S.; Venkatraman, B. H.; Gavvalapalli, N. Role of Molecular Architecture in Organic Photovoltaic Cells. J. Phys. Chem. Lett. 2010, 1, 947–958. (20) Mauer, R.; Howard, I. A.; Laquai, F. Effect of Nongeminate Recombination on Fill Factor in Polythiophene/Methanofullerene Organic Solar Cells. J. Phys. Chem. Lett. 2010, 1, 3500–3505. (21) Jamieson, F. C.; Agostinelli, T.; Azimi, H.; Nelson, J.; Durrant, J. R. Field-Independent Charge Photogeneration in PCPDTBT/ PC70BM Solar Cells. J. Phys. Chem. Lett. 2010, 1, 3306–3310. (22) Inal, S.; Schubert, M.; Sellinger, A.; Neher, D. The Relationship between the Electric Field-Induced Dissociation of Charge Transfer Excitons and the Photocurrent in Small Molecular/Polymeric Solar Cells. J. Phys. Chem. Lett. 2010, 1, 982–986. (23) Bisquert, J.; Cahen, D.; Hodes, G.; Ruhle, S.; Zaban, A. Physical chemical principles of photovoltaic conversion with nanoparticulate, mesoporous dye-sensitized solar cells. J. Phys. Chem. B 2004, 108, 8106–8118. (24) Wurfel, P. Photovoltaic Principles and Organic Solar Cells. Chimia 2007, 61, 770–774. (25) Wurfel, P. Physics of Solar Cells. Wiley-VCH: Weinheim, Germany, 2005. (26) Jaegermann, W.; Klein, A.; Mayer, T. Interface Engineering of Inorganic Thin-Film Solar Cells — Materials-Science Challenges for Advanced Physical Concepts. Adv. Mater. 2009, 21, 4196–4206. (27) Krebs, F. C. All Solution Roll-to-Roll Processed Polymer Solar Cells Free from Indium-Tin-Oxide and Vacuum Coating Steps. Org. Electron. 2009, 10, 761–768. (28) Kumar, A.; Sista, S.; Yang, Y. Dipole Induced Anomalous S-Shape I
V Curves in Polymer Solar Cells. J. Appl. Phys. 2009, 105, 094512. (29) Hains, A. W.; Liu, J.; Martinson, A. B. F.; Irwin, M. D.; Marks, T. J. Anode Interfacial Tuning via Electron-Blocking/Hole-Transport Layers and Indium Tin Oxide Surface Treatment in Bulk-Heterojunction Organic Photovoltaic Cells. Adv. Funct. Mater. 2010, 20, 595–606. (30) Irwin, M. D.; Buchholz, B.; Hains, A. W.; Chang, R. P. H.; Marks, T. J. p-Type Semiconducting Nickel Oxide As an EfficiencyEnhancing Anode Interfacial Layer in Polymer Bulk-Heterojunction Solar Cells. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 2783–2787.

PERSPECTIVE

(31) Motiei, L.; Yao, Y.; Choudhury, J.; Yan, H.; Marks, T. J.; van der Boom, M. E.; Facchetti, A. Self-Propagating Molecular Assemblies as Interlayers for Efficient Inverted Bulk-Heterojunction Solar Cells. J. Am. Chem. Soc. 2010, 132, 12528–12530. (32) Schueppel, R.; Schmidt, K.; Uhrich, C.; Schulze, K.; Wynands, D.; Bredas, J. L.; Brier, E.; Reinold, E.; Bu, H. B.; Baeuerle, P.; Maennig, B.; Pfeiffer, M.; Leo, K. Optimizing Organic Photovoltaics Using Tailored Heterojunctions: A Photoinduced Absorption Study of Oligothiophenes with Low Band Gaps. Phys. Rev. B 2008, 77, 14. (33) Ratcliff, E. L.; Lee, P. A.; Armstrong, N. R. Work Function Control of Hole-Selective Polymer/ITO Anode Contacts: An Electrochemical Doping Study. J. Mater. Chem. 2010, 20, 2672–2679. (34) Ratcliff, E. L.; Jenkins, J. L.; Nebesny, K.; Armstrong, N. R. Electrodeposited “Nano-Textured” Poly(3-hexyl-thiophene) (e-P3HT) Films for Photovoltaic Applications. Chem. Mater. 2008, 20, 5796–5806. (35) Paniagua, S. A.; Hotchkiss, P. J.; Jones, S. C.; Marder, S. R.; Mudalige, A.; Marrikar, F. S.; Pemberton, J. E.; Armstrong, N. R. Phosphonic Acid Modification of Indium
Tin Oxide Electrodes: Combined XPS/UPS/Contact Angle Studies. J. Phys. Chem. C 2008, 112, 7809–7817. (36) Armstrong, N. R.; Veneman, P. A.; Ratcliff, E.; Placencia, D.; Brumbach, M. Oxide Contacts in Organic Photovoltaics: Characterization and Control of Near-Surface Composition in Indium
Tin Oxide (ITO) Electrodes. Acc. Chem. Res. 2009, 42, 1748–1757. (37) Hwang, J.; Wan, A.; Kahn, A. Energetics of Metal
Organic Interfaces: New Experiments and Assessment of the Field. Mater. Sci. Eng. Rep. 2009, 64, 1–31. (38) Ma, H.; Yip, H. L.; Huang, F.; Jen, A. K. Y. Interface Engineering for Organic Electronics. Adv. Funct. Mater. 2010, 20, 1371–1388. (39) Fortunato, E.; Ginley, D.; Hosono, H.; Paine, D. C. Transparent Conducting Oxides for Photovoltaics. MRS Bull. 2007, 32, 242–247. (40) Hosono, H., Paine, D. C., Ginley, D. S.; Eds. Handbook of Treansparent Conductors; Springer: New York, 2010; pp 1
27. (41) Choi, S.; Potscavage, J.; William, J.; Kippelen, B. Area-Scaling of Organic Solar Cells. J. Appl. Phys. 2009, 106, 054507. (42) Shen, Y. L.; Hosseini, A. R.; Wong, M. H.; Malliaras, G. G. How to Make Ohmic Contacts to Organic Semiconductors. ChemPhysChem 2004, 5, 16–25. (43) Vandewal, K.; Tvingstedt, K.; Gadisa, A.; Inganas, O.; Manca, J. V. On the Origin of the Open-Circuit Voltage of Polymer
Fullerene Solar Cells. Nat. Mater. 2009, 8, 904–909. (44) Giebink, N. C.; Wiederrecht, G. P.; Wasielewski, M. R.; Forrest, S. R. Ideal Diode Equation for Organic Heterojunctions. I. Derivation and Application. Phys. Rev. B 2010, 82, 155305. (45) Strobel, T.; Deibel, C.; Dyakonov, V. Role of Polaron Pair Diffusion and Surface Losses in Organic Semiconductor Devices. Phys. Rev. Lett. 2010, 105, 266602. (46) Wagenpfahl, A.; Rauh, D.; Binder, M.; Deibel, C.; Dyakonov, V. S-Shaped Current-Voltage Characteristics of Organic Solar Devices. Phys. Rev. B 2010, 82, 115306. (47) Wagenpfahl, A.; Deibel, C.; Dyakonov, V. Organic Solar Cell Efficiencies Under the Aspect of Reduced Surface Recombination Velocities. IEEE J. Sel. Top. Quantum Electron. 2010, 16, 1759–1763. (48) Steim, R.; Choulis, S. A.; Schilinsky, P.; Lemmer, U.; Brabec, C. J. Formation and Impact of Hot Spots on the Performance of Organic Photovoltaic Cells. Appl. Phys. Lett. 2009, 94, 043304. (49) Pingree, L. S. C.; MacLeod, B. A.; Ginger, D. S. The Changing Face of PEDOT:PSS Films: Substrate, Bias, And Processing Effects on Vertical Charge Transport. J. Phys. Chem. C 2008, 112, 7922–7927. (50) Lilliedal, M. R.; Medford, A. J.; Madsen, M. V.; Norrman, K.; Krebs, F. C. The Effect of Post-Processing Treatments on Inflection Points in Current-Voltage Curves of Roll-to-Roll Processed Polymer Photovoltaics. Sol. Energy Mater. Sol. Cells 2010, 94, 2018–2031. (51) Datta, A.; Rahmouni, M.; Nath, M.; Boubekri, R.; Cabarrocas, P. R. I.; Chatterjee, P. Insights Gained from Computer Modeling of Heterojunction with Instrinsic Thin Layer “HIT” Solar Cells. Sol. Energy Mater. Sol. Cells 2010, 94, 1457–1462. 1347

dx.doi.org/10.1021/jz2002259 |J. Phys. Chem. Lett. 2011, 2, 1337–1350

The Journal of Physical Chemistry Letters (52) Alstrup, J.; Jorgensen, M.; Medford, A. J.; Krebs, F. C. Ultra Fast and Parsimonious Materials Screening for Polymer Solar Cells Using Differentially Pumped Slot-Die Coating. ACS Appl. Mater. Interfaces 2010, 2, 2819–2827. (53) Albery, W. J. Development of Photogalvanic Cells for SolarEnergy Conversion. Acc. Chem. Res. 1982, 15, 142–148. (54) Albery, W. J.; Archer, M. D. Optimum Efficiency of Photogalvanic Cells for Solar-Energy Conversion. Nature 1977, 270, 399–402. (55) Albery, W. J.; Archer, M. D. Photogalvanic Cells. 4. Maximum Power from a Thin-Layer Cell with Differential Electrode-Kinetics. J. Electroanal. Chem. 1978, 86, 19–34. (56) O’Regan, B.; Gratzel, M. A Low-Cost, High-Efficiency SolarCell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737–740. (57) Graetzel, M. Recent Advances in Sensitized Mesoscopic Solar Cells. Acc. Chem. Res. 2009, 42, 1788–1798. (58) Bisquert, J. Physical Electrochemistry of Nanostructured Devices. Phys. Chem. Chem. Phys. 2008, 10, 49–72. (59) Spitler, M. T.; Parkinson, B. A. Dye Sensitization of Single Crystal Semiconductor Electrodes. Acc. Chem. Res. 2009, 42, 2017–2029. (60) O’Regan, B. C.; Durrant, J. R. Kinetic and Energetic Paradigms for Dye-Sensitized Solar Cells: Moving from the Ideal to the Real. Acc. Chem. Res. 2009, 42, 1799–1808. (61) Oosterhout, S. D.; Koster, L. J. A.; van Bavel, S. S.; Loos, J.; Stenzel, O.; Thiedmann, R.; Schmidt, V.; Campo, B.; Cleij, T. J.; Lutzen, L.; Vanderzande, D.; Wienk, M. M.; Janssen, R. A. J. Controlling the Morphology and Efficiency of Hybrid ZnO:Polythiophene Solar Cells Via Side Chain Functionalization. Adv. Energy Mater. 2010, 1, 1–7. (62) Bai, Y.; Cao, Y. M.; Zhang, J.; Wang, M.; Li, R. Z.; Wang, P.; Zakeeruddin, S. M.; Gratzel, M. High-Performance Dye-Sensitized Solar Cells Based on Solvent-Free Electrolytes Produced from Eutectic Melts. Nat. Mater. 2008, 7, 626–630. (63) Tang, C. W. 2-Layer Organic Photovoltaic Cell. Appl. Phys. Lett. 1986, 48, 183–185. (64) Tang, C. W.; Vanslyke, S. A. Organic Electroluminescent Diodes. Appl. Phys. Lett. 1987, 51, 913–915. (65) Chamberlain, G. A. Organic Solar-Cells — A Review. Sol. Cells 1983, 8, 47–83. (66) Forrest, S. R. The Limits to Organic Photovoltaic Cell Efficiency. MRS Bull. 2005, 30, 28–32. (67) Zhu, X. Y.; Kahn, A. Electronic Structure and Dynamics at Organic Donor/Acceptor Interfaces. MRS Bull. 2010, 35, 443–448. (68) Gregg, B. A.; Hanna, M. C. Comparing Organic to Inorganic Photovoltaic Cells: Theory, Experiment, and Simulation. J. Appl. Phys. 2003, 93, 3605–3614. (69) Frohne, H.; Shaheen, S. E.; Brabec, C. J.; Muller, D. C.; Sariciftci, N. S.; Meerholz, K. Influence of the Anodic Work Function on the Performance of Organic Solar Cells. ChemPhysChem 2002, 3, 795–799. (70) Hamilton, R.; Shuttle, C. G.; O’Regan, B.; Hammant, T. C.; Nelson, J.; Durrant, J. R. Recombination in Annealed and Nonannealed Polythiophene/Fullerene Solar Cells: Transient Photovoltage Studies versus Numerical Modeling. J. Phys. Chem. Lett. 2010, 1, 1432–1436. (71) Barrau, S.; Andersson, V.; Zhang, F. L.; Masich, S.; Bijleveld, J.; Andersson, M. R.; Inganas, O. Nanomorphology of Bulk Heterojunction Organic Solar Cells in 2D and 3D Correlated to Photovoltaic Performance. Macromolecules 2009, 42, 4646–4650. (72) Placencia, D.; Wang, W. N.; Shallcross, R. C.; Nebesny, K. W.; Brumbach, M.; Armstrong, N. R. Organic Photovoltaic Cells Based On Solvent-Annealed, Textured Titanyl Phthalocyanine/C-60 Heterojunctions. Adv. Funct. Mater. 2009, 19, 1913–1921. (73) Matsuo, Y.; Sato, Y.; Niinomi, T.; Soga, I.; Tanaka, H.; Nakamura, E. Columnar Structure in Bulk Heterojunction in SolutionProcessable Three-Layered p
i
n Organic Photovoltaic Devices Using Tetrabenzoporphyrin Precursor and Silylmethyl[60]fullerene. J. Am. Chem. Soc. 2009, 131, 16048–16050.

PERSPECTIVE

(74) Yang, F.; Forrest, S. R. Photocurrent Generation in Nanostructured Organic Solar Cells. ACS Nano 2008, 2, 1022–1032. (75) Walzer, K.; Maennig, B.; Pfeiffer, M.; Leo, K. Highly Efficient Organic Devices Based on Electrically Doped Transport Materials. Chem. Rev. 2007, 107, 1233–1271. (76) Tvingstedt, K.; Inganas, O. Electrode Grids for ITO-Free Organic Photovoltaic Devices. Adv. Mater. 2007, 19, 2893–2897. (77) Zou, J. Y.; Yip, H. L.; Hau, S. K.; Jen, A. K. Y. Metal grid/ conducting polymer hybrid transparent electrode for inverted polymer solar cells. Appl. Phys. Lett. 2010, 96, 203301. (78) Lee, J. Y.; Connor, S. T.; Cui, Y.; Peumans, P. Semitransparent Organic Photovoltaic Cells with Laminated Top Electrode. Nano Lett. 2010, 10, 1276–1279. (79) Ajuria, J.; Etxebarria, I.; Cambarau, W.; Munecas, U.; Tena-Zaera, R.; Jimeno, J. C.; Pacios, R. Inverted ITO-Free Organic Solar Cells Based on P and N Semiconducting Oxides. New Designs for Integration in Tandem Cells, Top or Bottom Detecting Devices, And Photovoltaic Windows. Energy Environ. Sci. 2011, 4, 453–458. (80) Meiss, J.; Riede, M. K.; Leo, K. Towards Efficient Tin-Doped Indium Oxide (ITO)-Free Inverted Organic Solar Cells Using Metal Cathodes. Appl. Phys. Lett. 2009, 94, 3. (81) Gruber, M.; Stickler, B. A.; Trimmel, G.; Schurrer, F.; Zojer, K. Impact of Energy Alignment and Morphology on the Efficiency in Inorganic
Organic Hybrid Solar Cells. Org. Electron. 2010, 11, 1999–2011. (82) Zhou, Y. H.; Cheun, H.; Potscavage, W. J.; Fuentes-Hernandez, C.; Kim, S. J.; Kippelen, B. Inverted Organic Solar Cells with ITO Electrodes Modified with an Ultrathin Al2O3 Buffer Layer Deposited by Atomic Layer Deposition. J. Mater. Chem. 2010, 20, 6189–6194. (83) Friedel, B.; Keivanidis, P. E.; Brenner, T. J. K.; Abrusci, A.; McNeill, C. R.; Friend, R. H.; Greenham, N. C. Effects of Layer Thickness and Annealing of PEDOT:PSS Layers in Organic Photodetectors. Macromolecules 2009, 42, 6741–6747. (84) Peisert, H.; Knupfer, M.; Zhang, F.; Petr, A.; Dunsch, L.; Fink, J. The Interface between Phthalocyanines and PEDOT:PSS: Evidence for Charge Transfer and Doping. Surf. Sci. 2004, 566, 554–559. (85) Blom, P. W. M.; Mihailetchi, V. D.; Koster, L. J. A.; Markov, D. E. Device Physics of Polymer: Fullerene Bulk Heterojunction Solar Cells. Adv. Mater. 2007, 19, 1551–1566. (86) Brenner, T. J. K.; Hwang, I.; Greenham, N. C.; McNeill, C. R. Device Physics of Inverted All-Polymer Solar Cells. J. Appl. Phys. 2010, 107, 114501. (87) Mandoc, M. M.; Koster, L. J. A.; Blom, P. W. M. Optimum Charge Carrier Mobility in Organic Solar Cells. Appl. Phys. Lett. 2007, 90, 133504. (88) Schulze, K.; Uhrich, C.; Schuppel, R.; Leo, K.; Pfeiffer, M.; Brier, E.; Reinold, E.; Bauerle, P. Efficient Vacuum-Deposited Organic Solar Cells Based on a New Low-Bandgap Oligothiophene and Fullerene C-60. Adv. Mater. 2006, 18, 2872–2875. (89) Shah, A.; Torres, P.; Tscharner, R.; Wyrsch, N.; Keppner, H. Photovoltaic Technology: The Case for Thin-Film Solar Cells. Science 1999, 285, 692–698. (90) Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. Single-Ion Solvation Free Energies and the Normal Hydrogen Electrode Potential in Methanol, Acetonitrile, And Dimethyl Sulfoxide. J. Phys. Chem. B 2007, 111, 408–422. (91) Maennig, B.; Drechsel, J.; Gebeyehu, D.; Simon, P.; Kozlowski, F.; Werner, A.; Li, F.; Grundmann, S.; Sonntag, S.; Koch, M.; Leo, K.; Pfeiffer, M.; Hoppe, H.; Meissner, D.; Sariciftci, N. S.; Riedel, I.; Dyakonov, V.; Parisi, J. Organic p
i
n Solar Cells. Appl. Phys. A 2004, 79, 1–14. (92) Bard, A. J.; Faulkner, L. R. Electrochemical Methods — Fundamentals and Applications, 2nd ed.; John Wiley & Sons, Inc.: New York, 2001. pp 87
136. (93) Memming, R. Semiconductor Electrochemistry; Wiley-VCH: Weinheim, Germany, 2001; pp 122
128. (94) Creutz, C.; Brunschwig, B. S.; Sutin, N. Interfacial ChargeTransfer Absorption: 3. Application to Semiconductor
Molecule Assemblies. J. Phys. Chem. B 2006, 110, 25181–25190. 1348

dx.doi.org/10.1021/jz2002259 |J. Phys. Chem. Lett. 2011, 2, 1337–1350

The Journal of Physical Chemistry Letters (95) Creutz, C.; Brunschwig, B. S.; Sutin, N. Interfacial ChargeTransfer Absorption: Semiclassical Treatment. J. Phys. Chem. B 2005, 109, 10251–10260. (96) Kohl, P. A.; Frank, S. N.; Bard, A. J. Semiconductor Electrodes. 11. Behavior of n-Type and p-Type Single-Crystal Semiconductors Covered with Thin Normal-TiO2 Films. J. Electrochem. Soc. 1977, 124, 225–229. (97) Kohl, P. A.; Bard, A. J. Semiconductor Electrodes. 13. Characterization and Behavior of n-Type ZNO, CDS, and Gap Electrodes in Acetonitrile Solutions. J. Am. Chem. Soc. 1977, 99, 7531–7539. (98) Frank, S. N.; Bard, A. J. Semiconductor Electrodes. 2. Electrochemistry at n-Type TiO2 Electrodes in Acetonitrile Solutions. J. Am. Chem. Soc. 1975, 97, 7427–7433. (99) Diebold, U.; Li, S. C.; Schmid, M. Oxide Surface Science. Annu. Rev. Phys. Chem. 2010, 61, 129–148. (100) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53–229. (101) Morfa, A. J.; Beane, G.; Mashford, B.; Singh, B.; Della Gaspera, E.; Martucci, A.; Mulvaney, P. Fabrication of ZnO Thin Films from Nanocrystal Inks. J. Phys. Chem. C 2010, 114, 19815–19821. (102) Brumbach, M.; Veneman, P. A.; Marrikar, F. S.; Schulmeyer, T.; Simmonds, A.; Xia, W.; Lee, P.; Armstrong, N. R. Surface Composition and Electrical and Electrochemical Properties of Freshly Deposited and Acid-Etched Indium Tin Oxide Electrodes. Langmuir 2007, 23, 11089–11099. (103) Gassenbauer, Y.; Schafranek, R.; Klein, A.; Zafeiratos, S.; Havecker, M.; Knop-Gericke, A.; Schlogl, R. Surface States, Surface Potentials, and Segregation at Surfaces of Tin-Doped In2O3. Phys. Rev. B 2006, 73, 245312. (104) Inganas, O. Hybrid Electronics and Electrochemistry with Conjugated Polymers. Chem. Soc. Rev. 2010, 39, 2633–2642. (105) Marrikar, F. S.; Brumbach, M.; Evans, D. H.; Lebron-Paler, A.; Pemberton, J. E.; Wysocki, R. J.; Armstrong, N. R. Modification of Indium
Tin Oxide Electrodes with Thiophene Copolymer Thin Films: Optimizing Electron Transfer to Solution Probe Molecules. Langmuir 2007, 23, 1530–1542. (106) Snaith, H. J.; Kenrick, H.; Chiesa, M.; Friend, R. H. Morphological and Electronic Consequences of Modifications to the Polymer Anode 'PEDOT:PSS'. Polymer 2005, 46, 2573–2578. (107) Hwang, J.; Amy, F.; Kahn, A. Spectroscopic Study on Sputtered PEDOT 3 PSS: Role of Surface PSS Layer. Org. Electron. 2006, 7, 387–396. (108) Cahen, D.; Kahn, A. Electron Energetics at Surfaces and Interfaces: Concepts and Experiments. Adv. Mater. 2003, 15, 271–277. (109) Alloway, D. M.; Graham, A. L.; Yang, X.; Mudalige, A.; Colorado, R.; Wysocki, V. H.; Pemberton, J. E.; Lee, T. R.; Wysocki, R. J.; Armstrong, N. R. Tuning the Effective Work Function of Gold and Silver Using omega-Functionalized Alkanethiols: Varying Surface Composition through Dilution and Choice of Terminal Groups. J. Phys. Chem. C 2009, 113, 20328–20334. (110) Heimel, G.; Romaner, L.; Zojer, E.; Bredas, J. L. The Interface Energetics of Self-Assembled Monolayers on Metals. Acc. Chem. Res. 2008, 41, 721–729. (111) Duhm, S.; Heimel, G.; Salzmann, I.; Glowatzkl, H.; Johnson, R. L.; Vollmer, A.; Rabe, J. P.; Koch, N. Orientation-Dependent Ionization Energies and Interface Dipoles in Ordered Molecular Assemblies. Nat. Mater. 2008, 7, 326–332. (112) Hotchkiss, P. J.; Li, H.; Paramonov, P. B.; Paniagua, S. A.; Jones, S. C.; Armstrong, N. R.; Bredas, J. L.; Marder, S. R. Modification of the Surface Properties of Indium Tin Oxide with Benzylphosphonic Acids: A Joint Experimental and Theoretical Study. Adv. Mater. 2009, 21, 4496–4501. (113) Meyer, J.; Kroger, M.; Hamwi, S.; Gnam, F.; Riedl, T.; Kowalsky, W.; Kahn, A. Charge Generation Layers Comprising Transition Metal-Oxide/Organic Interfaces: Electronic Structure and Charge Generation Mechanism. Appl. Phys. Lett. 2010, 96, 193302. (114) Kroger, M.; Hamwi, S.; Meyer, J.; Riedl, T.; Kowalsky, W.; Kahn, A. Role of the Deep-Lying Electronic States of MoO3 in the

PERSPECTIVE

Enhancement of Hole-Injection in Organic Thin Films. Appl. Phys. Lett. 2009, 95, 123301. (115) Gutmann, S.; Wolak, M. A.; Conrad, M.; Beerbom, M. M.; Schlaf, R. Effect of Ultraviolet and X-ray Radiation on the Work Function of TiO2 Surfaces. J. Appl. Phys. 2010, 107, 8. (116) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T. Q.; Dante, M.; Heeger, A. J. Efficient Tandem Polymer Solar Cells Fabricated by All-Solution Processing. Science 2007, 317, 222–225. (117) Zhou, Y.; Cheun, H.; Choi, S.; William J. Potscavage, J.; Fuentes-Hernandez, C.; Kippelen, B. Indium Tin Oxide-Free and Metal-Free Semitransparent Organic Solar Cells. Appl. Phys. Lett. 2010, 97, 153304. (118) Boix, P. P.; Ajuria, J.; Etxebarria, I.; Pacios, R.; GarciaBelmonte, G.; Bisquert, J. Role of ZnO Electron-Selective Layers in Regular and Inverted Bulk Heterojunction Solar Cells. J. Phys. Chem. Lett. 2011, 2, 407–411. (119) Steirer, K. X.; Chesin, J. P.; Widjonarko, N. E.; Berry, J. J.; Miedaner, A.; Ginley, D. S.; Olson, D. C. Solution Deposited NiO ThinFilms As Hole Transport Layers in Organic Photovoltaics. Org. Electron. 2010, 11, 1414–1418. (120) Gilot, J.; Wienk, M. M.; Janssen, R. A. J. Optimizing Polymer Tandem Solar Cells. Adv. Mater. 2010, 22, E67–E71. (121) Kuwabara, T.; Kawahara, Y.; Yamaguchi, T.; Takahashi, K. Characterization of Inverted-Type Organic Solar Cells with a ZnO Layer as the Electron Collection Electrode by ac Impedance Spectroscopy. ACS Appl. Mater. Interfaces 2009, 1, 2107–2110. (122) Exarhos, G. J.; Windisch, C. F.; Ferris, K. F.; Owings, R. R. Cation Defects and Conductivity in Transparent Oxides. Appl. Phys. A 2007, 89, 9–18. (123) Zhang, M. L.; Irfan; Ding, H. J.; Gao, Y. L.; Tang, C. W. Organic Schottky Barrier Photovoltaic Cells Based on MoOx/C-60. Appl. Phys. Lett. 2010, 96, 183301. (124) Irfan; Ding, H. J.; Gao, Y. L.; Kim, D. Y.; Subbiah, J.; So, F. Energy Level Evolution of Molybdenum Trioxide Interlayer between Indium Tin Oxide and Organic Semiconductor. Appl. Phys. Lett. 2010, 96, 183301. (125) Hancox, I.; Sullivan, P.; Chauhan, K. V.; Beaumont, N.; Rochford, L. A.; Hatton, R. A.; Jones, T. S. The Effect of a MoOx Hole-Extracting Layer on the Performance of Organic Photovoltaic Cells Based on Small Molecule Planar Heterojunctions. Org. Electron. 2010, 11, 2019–2025. (126) Cho, S. W.; Piper, L. F. J.; DeMasi, A.; Preston, A. R. H.; Smith, K. E.; Chauhan, K. V.; Hatton, R. A.; Jones, T. S. Soft X-ray Spectroscopy of C-60/Copper Phthalocyanine/MoO3 Interfaces: Role of Reduced MoO3 on Energetic Band Alignment and Improved Performance. J. Phys. Chem. C 2010, 114, 18252–18257. (127) Gao, J.; Yu, G.; Heeger, A. J. Polymer p
i
n Junction Photovoltaic Cells. Adv. Mater. 1998, 10, 692–695. (128) Meiss, J.; Leo, K.; Riede, M. K.; Uhrich, C.; Gnehr, W. M.; Sonntag, S.; Pfeiffer, M. Efficient Semitransparent Small-Molecule Organic Solar Cells. Appl. Phys. Lett. 2009, 95, 213306. (129) Li, F. H.; Pfeiffer, M.; Werner, A.; Harada, K.; Leo, K.; Hayashi, N.; Seki, K.; Liu, X. J.; Dang, X. D. Acridine Orange Base As a Dopant for N Doping of C-60 Thin Films. J. Appl. Phys. 2006, 100. (130) Gommans, H.; Verreet, B.; Rand, B. P.; Muller, R.; Poortmans, J.; Heremans, P.; Genoe, J. On the Role of Bathocuproine in Organic Photovoltaic Cells. Adv. Funct. Mater. 2008, 18, 3686–3691. (131) Vogel, M.; Doka, S.; Breyer, C.; Lux-Steiner, M. C.; Fostiropoulosa, K. On the Function of a Bathocuproine Buffer Layer in Organic Photovoltaic Cells. Appl. Phys. Lett. 2006, 89, 163501. (132) Anderson, J. D.; McDonald, E. M.; Lee, P. A.; Anderson, M. L.; Ritchie, E. L.; Hall, H. K.; Hopkins, T.; Mash, E. A.; Wang, J.; Padias, A.; Thayumanavan, S.; Barlow, S.; Marder, S. R.; Jabbour, G. E.; Shaheen, S.; Kippelen, B.; Peyghambarian, N.; Wightman, R. M.; Armstrong, N. R. Electrochemistry and Electrogenerated Chemiluminescence Processes of the Components of Aluminum Quinolate/Triarylamine, and Related Organic Light-Emitting Diodes. J. Am. Chem. Soc. 1998, 120, 9646– 9655. 1349

dx.doi.org/10.1021/jz2002259 |J. Phys. Chem. Lett. 2011, 2, 1337–1350

The Journal of Physical Chemistry Letters

PERSPECTIVE

(133) Frohne, H.; Muller, D. C.; Meerholz, K. Continuously Variable Hole Injection in Organic Light Emitting Diodes. ChemPhysChem 2002, 3, 707–711. (134) Shallcross, R. C.; D’Ambruoso, G.; Pyun, J.; Armstrong, N. R. Photoelectrochemical Processes in Polymer-Tethered CdSe Nanocrystals. J. Am. Chem. Soc. 2010, 132, 2622–2632. (135) Hoth, C. N.; Schilinsky, P.; Choulis, S. A.; Brabec, C. J. Printing Highly Efficient Organic Solar Cells. Nano Lett. 2008, 8, 2806–2813. (136) Ameri, T.; Dennler, G.; Waldauf, C.; Azimi, H.; Seemann, A.; Forberich, K.; Hauch, J.; Scharber, M.; Hingerl, K.; Brabec, C. J. Fabrication, Optical Modeling, and Color Characterization of Semitransparent Bulk-Heterojunction Organic Solar Cells in an Inverted Structure. Adv. Funct. Mater. 2010, 20, 1592–1598.

’ NOTE ADDED AFTER ASAP PUBLICATION This paper was published on the Web on May 23, 2011. Various changes were made to the text, and ref 23 was changed. The corrected version was reposted on June 2, 2011.

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