The Quantitative Effect of Surface Wetting Layers on the Performance

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The Quantitative Effect of Surface Wetting Layers on the Performance of Organic Bulk Heterojunction Photovoltaic Devices B. P. Lyons,† N. Clarke,*,‡ and C. Groves*,† † ‡

School of Engineering and Computing Sciences, Durham University, South Road, DH1 3LE, United Kingdom Department of Physics and Astronomy, Sheffield University, Hicks Building, Hounslow Road, S3 7RH, United Kingdom

bS Supporting Information ABSTRACT: We use realistic morphological and charge transport modeling to unambiguously quantify the effect of wetting layers on organic photovoltaic device (OPV) performance. Direct measurements of the blend surface of OPVs commonly reveal surface wetting layers that may hinder charge extraction. However, quantifying the effect of these layers on OPV performance experimentally is extremely challenging due to the conjoined nature of the bulk and surface morphology. We show that wetting layers of both the “wrong” and the “correct” material can adversely affect performance, the latter effect being due to subsurface features. The results are generalized to show that it is the peak composition of “wrong” material en route to an electrode, not the surface composition, which determines the effect of wetting layers. The predictions are in good agreement with available data, suggesting that our model can be used to assess when seemingly ubiquitous wetting layers will adversely affect OPV performance.

’ INTRODUCTION Organic photovoltaic devices (OPVs) have emerged as a promising technology to harvest solar energy. One of the key advantages of this technology is the ability to make devices from solutions of active materials using cheap, scalable printing techniques.1 However, in order to realize this potential, the current rapid improvement in OPV power conversion efficiency must continue until the $/W offered by OPVs competes with their inorganic counterparts. A continuing theme in OPV optimization is that the morphology, that is, the arrangement of the donor and acceptor materials “frozen in” at the end of solvent removal or annealing, is key to determining performance.2
5 In particular, the donor
acceptor morphology should provide a continuous path for electrons and holes all the way to their respective electrodes. The extent to which this can be achieved is dependent to a large extent on the chemistry of the materials used. Typically, the surface energies of the donor and acceptor are different, which, in turn, leads to an excess of one material at one or both interfaces.6 Unfortunately, it is often the case that the “wrong” material accumulates at the surface, which is expected to hinder extraction of photocurrent. Indeed, it appears that excesses of the “wrong” material at the film surface are the rule rather than the exception, since there are many examples in OPV blends of interest, including polythiophene/fullerene,7
16 polyfluorene/fullerene,17,18 alkoxy poly(phenylene vinylene)/fullerene,19 and various all-polymer blends.20
22 This common underlying feature has driven research into OPVs with inverted electrodes3,23 as well as techniques to control the surface morphology, such as self-assembled monolayers (SAMs),9,23
25 interlayers,25
28 modification of fullerene surface energy,29 prespin drying,16 and solvent-vapor annealing.24,30 r 2011 American Chemical Society

Whether these methods are used or not, the end result is almost always that one or both surfaces will have an excess of “wrong” or “correct” material, which we term as blocking or favorable surfaces, respectively. Intuitively, one would expect these blocking or favorable surfaces to affect OPV performance in some way, but it is extremely challenging to quantify this experimentally due to the conjoined nature of the bulk and surface morphology.31,32 In this paper, we use a joint morphological and charge transport modeling approach, which allows us to unambiguously quantify the effect that surface wetting layers have upon OPV performance. We find, surprisingly, that both blocking and favorable surfaces can lead to substantial loss in OPV performance, the latter being due to subsurface layers of blocking material. We are able to quantify, again surprisingly, that surface or subsurface layers with up to 85% of the “wrong” material will minimally affect charge extraction. These general results agree with available data and define a range of wetting layer composition over which OPV performance will be affected.

’ CAHN
HILLIARD MORPHOLOGIES Bulk heterojunction morphologies are generated using a modified version of Cahn
Hilliard theory, assuming that the donor and acceptor materials have equal molecular weight, similar to all-polymer OPVs33 and OLEDs.33,34 The technique Received: August 16, 2011 Revised: October 5, 2011 Published: October 05, 2011 22572

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Figure 1. Evolution of bulk and surface morphology predicted by Cahn
Hilliard theory. (a, b) Three-dimensional representations of the donor
acceptor morphology with τanneal = 40: (a) with surface effects included at the cathode only; (b) with surface effects not included. (c, d) Composition profiles moving into the film from the cathode surface at z = 127 nm: (c) with τanneal = 40 and varying τstart = 10, 20, 30, and 38; (d) with τstart = 0 and varying τ anneal = 10, 20, 30, 40, and 50.

used to simulate the blend formation process is described in detail elsewhere35,36 and in the Supporting Information. Briefly, morphologies are described by a regular 3D Cartesian lattice, each cell of which has an associated fraction of donor material, ϕ. Initially, it is assumed that the morphology is homogeneous and comprises equal parts donor and acceptor (ϕ = 0.5). This featureless morphology evolves into a phase-separated structure over a characteristic dimensionless “annealing time”, τanneal, which relates nonlinearly to the spin speed during deposition or the time for which a device is thermally annealed. We assume that the blend components have different surface energies such that a blocking wetting layer forms at the cathode surface. The anode surface is assumed neutral such that the anode surface composition matches the bulk. Morphologies with favorable wetting layers at the cathode are created by inverting the donor and acceptor materials. Control over wetting layer formation is afforded by selecting the time at which surface effects are “turned on”, τstart (hence, the period of time over which a wetting layer will develop is τanneal
τstart). For purposes of comparison, we additionally generate morphologies in which both surfaces are assumed to be neutral. Three-dimensional representations of morphologies with and without surface wetting layers are shown in Figure 1a,b, respectively. In Figure 1c, we show the evolution of the composition profile adjacent to the cathode as a function of τstart for blocking morphologies with τanneal = 40, where τanneal = 40 corresponds to optimum annealing of the bulk film morphology (as discussed later). Figure 1c, therefore, considers morphologies that have largely the same bulk morphology but differ in surface morphology. In all cases, a blocking wetting layer with a surface composition of ϕ > 0.95 forms, in broad agreement with direct spectroscopic measurements on polymer/ fullerene9,12
15 and all-polymer20,21 blends. The donor composition reduces moving into the bulk, reaching ϕ = 0.5 when ∼4 nm below the surface. For experimental verification of this feature, we can compare to Auger electron yield (AEY) and total electron yield (TEY) near-edge X-ray absorption fine structure (NEXAFS)

measurements, which, when combined, indicate the composition profile of the top ∼3 nm of the film.10,14 In this respect also, the current data agree well with available data; for example, poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM) OPVs have a ϕ within the top 2 and 3 nm of 0.96 and 0.84, respectively,14 in exact agreement with the τstart = 38 morphology. Figure 1c shows that, beneath the surface wetting layer, there are subsurface oscillations in composition, the amplitude and period of which increase with reducing τstart. These subsurface spinodal waves have been reported in a variety of all-polymer6,37 and polymer/ fullerene13,17 blends. Note that, when the blocking morphologies of Figure 1c are used in a favorable configuration (i.e., where ϕ maps to 1
ϕ), the subsurface features may present barriers to charge extraction. Finally, we note that the influence of the surface wetting layer does not extend to the opposite side of the device for the morphologies considered, in agreement with experiment.9,14 Figure 1d shows a range of morphologies that have τstart = 0 but vary in the total anneal time, τanneal. While the general features in Figure 1d are similar to those of Figure 1c, namely, donor-rich surface wetting layer and underlying subsurface oscillations in composition, the subsurface features have a more complex dependence on τanneal. The complexity of changes in composition profile when τanneal is varied (Figure 1d) compared to when τstart is varied (Figure 1c) underlines the difficulty in isolating the effects of surface wetting layers on OPV performance experimentally, since one generally has better control over τanneal than τstart.

’ MONTE CARLO CHARGE TRANSPORT SIMULATION The OPV performance corresponding to these Cahn
Hilliard morphologies was then predicted using a Monte Carlo charge transport simulation. Again, the techniques used are described in detail elsewhere38,39 and in the Supporting Information. Briefly, each donor and acceptor site is assigned a random, Gaussian distributed, value of energetic disorder. Excitons are created at 22573

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Figure 2. Effect of bulk and surface morphology evolution upon OPV photocurrent at fields of 2.2  107 (black), 1.6  107 (red), and 1  107 V/m (green). (a) Variation in J for neutral morphologies with anneal time, τanneal. (b, c) Variation in J for morphologies with τanneal = 40 and varying τstart for a blocking surface (b) and a favorable surface (c). Note that τstart = 40 in (b) and (c) corresponds to the neutral surface data shown in (a). (d) Variation in J for blocking morphologies with τstart = τanneal
2. Error bars represent the standard deviations in the average values of at least 10 repetitions of the simulation for each τanneal. In some cases, the error bar is obscured by the symbol.

random locations and move by F€orster transfer with a rate depending on site separation and difference in site energies. The rate of exciton creation is chosen to be similar to that experienced in an all-polymer device at AM 1.5 illumination.40 Excitons may dissociate into an electron
hole pair if they reach a boundary between donor and acceptor sites before decaying. Charges may recombine at a constant rate if adjacent or hop to a nearestneighbor site of the appropriate type (e.g., electron to acceptor) at a rate determined by a Marcus expression. The hopping rate calculation includes the internal electric field, all Coulombic interactions between charges and image charges, and the polaronic reorganization energy. Carriers that avoid recombination and exit the device via the selective contacts contribute to photocurrent. Because we are primarily interested in the photocurrent here, we do not include dark injection at the contacts. We use parameters similar to those reported previously for all-polymer OPVs,39 which are listed in Table S2 of the Supporting Information. For reference to upcoming data discussing the effects of surface wetting layers, Figure 2a shows the variation in photocurrent with τanneal for morphologies with neutral surfaces at field values corresponding approximately to voltages between short circuit and reverse bias. The optimum domain size that balances the competing needs of charge generation and charge separation41 occurs when τanneal is approximately 40. We first consider morphologies with τanneal = 40 and varying τstart with both blocking and favorable surfaces, as shown in Figure 2b,c, respectively. Blocking surfaces for τstart < 38 were completely blocking (ϕ = 1) and so photocurrent is nonzero only when 38 e τstart e 39, as shown in Figure 2b. As expected, the photocurrent in the presence of a blocking surface is substantially lower than when the surface is neutral (Figure 2a, τanneal = 40). Analysis of the loss mechanisms for the data in Figure 2b reveals that the reduction in photocurrent is accompanied by increased bimolecular recombination, because carriers take longer to exit the device, as well as increased geminate recombination, due to a buildup of electrons (not shown for brevity) that screens the

field.42 The exciton dissociation efficiency is largely unaffected by the presence of a surface blocking layer. Full details of the loss mechanisms for the morphologies relating to Figure 2 are shown in Figure S4 of the Supporting Information. Figure 2c shows corresponding data for morphologies with a favorable surface. Photocurrent is nonzero over the whole range considered since no surface or subsurface layers are completely blocking (ϕ = 1). Perhaps most significantly, Figure 2c shows that favorable surface layers can lead to substantially lower photocurrent than the corresponding neutral interface (here, when τstart < 10). This is because, as shown in Figure 1c, favorable surfaces imply subsurface blocking layers formed by the exclusion of the blocking material from the surface. Techniques to encourage the formation of favorable surfaces may, therefore, reduce performance. The reduction in photocurrent is found to be due to an increase in bimolecular recombination due to the presence of a (subsurface) blocking layer. Geminate recombination is found to be largely unaffected by the favorable surface, which is different to the case of the blocking surface. This is because both holes and electrons accumulate on either side of the subsurface blocking layer (not shown for brevity), and therefore, the internal electric field is affected to a lesser extent. We note a slight ( 0.95 that can be inferred from TEY NEXAFS contrast. Commonly studied P3HT:PCBM OPVs, depending upon the preparation conditions, have 0.75 < ϕB < 0.95,9,10,13,14 which Figure 3 suggests may reduce the photocurrent by up to 30%. However, recent experiments suggest that P3HT does not block electrons effectively,15 and consequently, the effect of the P3HT wetting layer will be less than predicted here (as will also be the case for other materials that do not block the opposite carrier type effectively). Consequently, one might expect that optimized standard devices46 would perform similarly well as their optimized inverted counterparts,3,23,47 which seems to be the case. ’ EFFECT OF WETTING LAYERS ON CURRENT HETEROGENEITY While our data suggest that photocurrent may be little affected up to ϕB = 0.85, we would additionally expect an increasingly blocking surface layer to “funnel” current through narrower sections of the OPV, which, in turn, may lead to device failure via heating.48,49 Although Monte Carlo cannot model device failure, it can quantify the heterogeneity of photocurrent extracted from the cathode surface, which may lead to device failure. Figure 4a shows the current extracted from a neutral surface, with light regions showing the well-defined acceptor domains. The probability distribution function (pdf) of the 22575

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Figure 4. Heterogeneity in photocurrent extraction at the cathode surface. (a
c) Current maps showing the number of carriers extracted per site, normalized to the mean number of extractions per site, for morphologies with τanneal = 40 and a neutral surface (a), a blocking surface (b), and a favorable surface (c). (d) Probability distribution functions corresponding to (a) (black symbols), (b) (blue filled symbols), and (c) (blue open symbols), together with the Poisson distribution for the corresponding mean number of carriers extracted per site (dashed line).

number of charges extracted per site (Figure 4d) shows that energetic disorder results in preferential current flow along filaments,4,50 when compared to a Poisson distribution that represents random charge extraction. Corresponding data for the blocking surface in Figure 4b unsurprisingly show that current flow is funnelled through the few regions where the blocking layer has gaps. The quantitative effect on the carrier extraction pdf is, however, surprisingly large, with some sites carrying over 1000 times the mean amount of current (note that these simulations have a resolution corresponding to the 1 nm grid spacing, while AFM measurements would be blurred by the ∼10 nm diameter of the tip4). The favorable surface represented in Figure 4c yields current where the underlying subsurface blocking layer has gaps. Although a subsurface blocking layer is present, which implies concentration of the current as in Figure 4b, the charges have spread laterally en route to the electrode, and consequently, the pdf is similar to that for the neutral surface. In summary, the blocking surfaces examined here increase the current flow through parts of the OPV by 3 orders of magnitude compared with the mean value.

’ CONCLUSIONS In conclusion, we have used a combination of modeling approaches to quantify the effects of ubiquitous surface wetting layers on OPV performance. Surprisingly, we have shown that both favorable and blocking surfaces can hamper charge extraction, the former being due to subsurface features. These data can, however, be generalized to describe the effect on OPV performance in terms of the peak concentration of “wrong” material en route to the electrode, ϕB. These general data show that compositions below the

classical percolation threshold can be tolerated, due to the small thickness of the layers involved. However, when ϕB is in excess of 0.85, substantial reductions in OPV performance are predicted. Indeed, for wetting layers having ϕB > 0.95, as has been reported for a range of OPVs,20,21 our data predict losses in photocurrent by up to a half. This suggests that OPV interfaces should be engineered to avoid surface wetting of predominantly one component where possible. Achieving an optimal neutral interface is of course challenging but may be accessible through the use of interlayers25,28 (which have additional benefits51), modification of the surface energy of the blend components29 or buried interface,24,31 or by drying the film in a supersaturated atmosphere of a common solvent.24 However, it should be noted that changing the surface energy can alter the evolution of the bulk morphology above,31,32,52 meaning that OPV recipes need to be reoptimized if the substrate and/or surface energy is altered. In cases where a neutral interface is not possible, our data suggest that OPV performance will be minimally affected provided surface concentration does not exceed 85% of either material. These data additionally highlight the increasing need for surface characterization techniques53 both to understand significant aspects of device performance and to decide when remedial measures are necessary.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information is available regarding the processing of Cahn
Hilliard morphologies, Monte Carlo technique, convergence and parameters, and loss

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The Journal of Physical Chemistry C mechanisms. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: n.clarke@sheffield.ac.uk (N.C.), [email protected] (C.G.).

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