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Dan Credgington , * Fiona C. Jamieson , Bright Walker , Thuc-Quyen Nguyen , and James R. Durrant *

Quantifi cation of Geminate and Non-Geminate Recombination Losses within a Solution-Processed Small-Molecule Bulk Heterojunction Solar Cell

The development of organic photovoltaics (OPV) has pro-gressed at an impressive pace over the last decade, as a result of tremendous efforts in optimising molecular design, mate-rials processing, and device engineering. [ 1–5 ] Power conversion effi ciencies for the best laboratory-scale devices now exceed 9%, however among the many materials systems reported in the literature there are relatively few that yield this high per-formance. It is therefore crucial to understand the fundamental processes that limit device performance and develop routes to minimising them. One route to effi cient devices which has received considerable attention recently is the development of solution-processable small-molecule organic donors, which have reached effi ciencies equivalent to that of their polymer or evaporation-deposited counterparts. [ 6–10 ] Such solution-proc-essed small-molecule based bulk heterojunction (BHJ) solar cells have the potential to combine the advantages of chemical purifi cation and material reproducibility with the low cost solu-tion processing associated with conjugated polymers. [ 11–13 ] While both solution-processed and thermally-evaporated small-molecule cells have demonstrated impressive PCEs, to date they have often suffered from comparatively low fi ll factors ( FF ). [ 14–19 ] In this contribution, we employ the charge extraction (CE), transient photovoltage (TPV) and transient absorption spectroscopy (TAS) techniques to investigate the dependence of both geminate and non-geminate recombination losses upon device voltage, and thereby their impact on fi ll factor, for a solu-tion-processed, small-molecule solar cell.

The losses within any heterojunction solar cell may be grouped into three categories: 1) relaxation of excitons which fail to diffuse to, and separate at, a suitable molecular inter-face, 2) recombination of geminate pairs formed at that inter-face which fail to fully dissociate (geminate recombination) and 3) recombination of dissociated carriers generated by different absorption events (non-geminate recombination). The latter

© 2012 WILEY-VCH Verlag GAdv. Mater. 2012, 24, 2135–2141

Dr. D. Credgington , F. C. Jamieson , Prof. J. R. Durrant Centre for Plastic Electronics Department of Chemistry Imperial College London, London, SW7 2AZ, UK E-mail: d.credgington@imperial.ac.uk; j.durrant@imperial.ac.uk Dr. B. Walker , Prof. T.-Q. Nguyen Center for Polymers and Organic Solids Department of Chemistry and Biochemistry University of California Santa Barbara, Santa Barbara, CA 93106-9510, USA

DOI: 10.1002/adma.201104738

encompasses processes involving carriers recombining at the “wrong” electrode (surface recombination, or leakage) and proc-esses involving the recombination of oppositely charged car-riers within the bulk, often referred to as bimolecular recombi-nation. We emphasise here that we use the term “bimolecular” in the chemical sense, i.e. to refer to a reaction involving two distinct species. We do not refer to the “order” of the reaction, and note additionally that such recombination may occur via both the Langevin and Shockley-Read-Hall mechanisms. [ 20 ] In most effi cient bulk heterojunction solar cells, the donor exciton quenching effi ciency is close to unity, implying that exciton diffusion is not usually a strongly limiting process. [ 21 , 22 ] In addition, under short-circuit conditions, non-geminate losses are generally observed to be very low. [ 23 ] Sub-optimal external quantum effi ciency therefore arises primarily from geminate losses within the cells. [ 24 ] However, away from short-circuit con-ditions, and particularly close to open-circuit voltage ( V OC ), this need not be the case, and a combination of geminate and non-geminate losses are expected to shape a cell’s J ( V ) behaviour. In this regard, we note that as the operating condition is moved from short-circuit to open-circuit, the macroscopic electric fi elds present in the device due to electrode charge tend towards zero (‘fl at band’ conditions). This reduction in internal electric fi eld results in a retardation of the charge carrier sweep-out time, which leads to an increase in the steady state charge car-rier density in the photoactive layer towards open-circuit. This increase in charge carrier density can result in an increase in non-geminate recombination losses as the cell voltage is increased. We have previously shown that these non-geminate losses may be quantifi ed as a function of bias and light inten-sity and used to understand the light-intensity dependence of FF both for poly(3-hexylthiophene)/[6,6]-phenyl-C 61 -butyric acid methyl ester (P3HT/PCBM) bulk heterojunctions [ 25 ] and Pentacene/C 60 bilayers. [ 26 ] A second consideration of the reduc-tion of the internal electric fi eld towards open-circuit is that the dissociation effi ciency of photogenerated charge carrier pairs, and therefore the magnitude of geminate recombina-tion, may be directly dependent upon this fi eld, and therefore upon cell voltage. Investigation of this second consideration is the primary aim of this paper, specifi cally a quantifi cation of the importance of such voltage-dependent geminate recombi-nation, in addition to charge density dependent non-geminate recombination, in limiting the V OC and FF of a representative solution processed small-molecule BHJ solar cell.

The extent to which the dissociation of geminate pairs is affected by macroscopic electric fi elds within the active layer is

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Figure 1 . (a) Chemical structure of the donor small molecule used for these analyses. (b) Lines: measured J(V) curves as a function of illumi-nation intensity for the cell analysed herein. 100%-sun is equivalent to 100 mWcm − 2 AM1.5 illumination. Triangles: Calculated J(V) behaviour assuming only non-geminate recombination contributes to cell losses, with generation fi xed equal to J SC . Both V OC and FF are signifi cantly over-predicted. A parallel shunt resistance, measured from the dark curve at V = 0, of 4.5 kΩ has been included for all curves.

currently the subject of signifi cant controversy. Device models which include fi eld-dependent geminate recombination losses are widely employed to analyse the J ( V ) performance of OPV cells, and a range of studies based upon analyses of device J ( V ) behaviour have provided support for the existence of fi eld-dependent geminate recombination losses. [ 27–31 ] Direct meas-urements of such fi eld-dependent losses have often observed them to be important only under strong reverse bias - important for devices used as photodetectors, but of less signifi cance for the operation of photovoltaic cells. [ 32–35 ] Indirect investigations of fi eld-dependent geminate recombination within the power-generating quadrant of OPV devices have often concluded that the dependence of geminate recombination upon device elec-tric fi eld is too weak to impact signifi cantly upon device FF and V OC . [ 25 , 33 , 36–41 ]

Herein we use TPV and CE measurements to measure the non-geminate recombination losses within small-molecule blend cells as a function of light intensity and bias. We note we have previously employed these techniques to analyse a range of polymer/ fullerene blend solar cells, and concluded that device FF and V OC are primarily determined by non-geminate rather than geminate recombination losses for these devices. [ 25 , 37 , 38 , 41 ] These techniques are described in more detail elsewhere. [ 42 ] Such measurements of non-geminate recombination loss are complemented by low-intensity TAS on the same devices under applied bias to measure the polaron photoinduced absorption on ns-ms timescales, and thereby quantify geminate recombi-nation losses.

The primary material system we examine here is that of 5,5 ″′ -bis(3-(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione-6-yl)-3 ″ -(bis(2-ethylhexyl)silanolyl)-2,2 ′ :5 ′ ,2 ″ :5 ″ ,2 ″′ -quaterthiophene [ 43 ] (SDT(TDPPTBFu) 2 , Figure 1 a) blended with PCBM. An iden-tical analysis was conducted using the related small-molecule DPP–OT–3,6-bis(5-(benzofuran-2-yl)thiophen-2-yl)-2,5-bis(2-ethylhexyl)pyrrolo[3,4-c]pyrrole-1,4-dione (DPP(TBFu) 2 ), reaching similar conclusions, and is given in the Sup-porting Information (SI). The device structure used was ITO/PEDOT:PSS/SDT(TDPPTBFu) 2 :PCBM/Al. Details of materials and device fabrication follow those reported previously and are summarised in the SI. Figure 1 b shows J(V) curves for a typical device studied herein. It is apparent that this device exhibits impressive J SC and V OC but a relatively low FF . We have previ-ously suggested that such low FF may be attributable, at least in part, to fast non-geminate recombination, identifi ed using Impedance Spectroscopy. [ 44 ]

Values for the total extractable charge determined from CE data both at open-circuit as a function of light intensity, and at a range of light intensities as a function of device voltage, are shown in Figure 2 a, converted to an effective carrier density n and plotted relative to the charge carrier density present in the dark at short-circuit conditions. The main panel shows the total extractable charge corrected for extraction losses, whilst the inset shows an estimate of the active layer carrier density at open-circuit, after subtraction of the electrode charge. The elec-trode capacitance was estimated at approximately ∼ 5 nF from the slope of the charge/voltage data under reverse bias, where little carrier injection into the active layer is expected. Assuming the cell behaves as a parallel plate capacitor with 95nm plate

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separations, this would correspond to a relative permittivity of ∼ 3. After subtraction of the electrode charge, active layer carrier density as a function of voltage under open-circuit conditions can be well-approximated by a single exponential, as has been observed previously. [ 37 , 38 , 45 ] We note that the charge carrier den-sity in the active layer of the devices studied herein is signifi -cantly lower than that we have reported for a range of polymer/ fullerene OPV devices, [ 37 , 38 ] such that the electrode capacitance is greater than the active layer (or ‘chemical’ [ 46 ] ) capacitance over much of the measured voltage range. We also note that short-circuit photocurrent transients were in all cases at least fi ve times faster than the corresponding photovoltage decay transients (Figure 2 c), indicative of relatively effi cient charge extraction from the device. The number of lost carriers during extraction has been estimated and accounted for for all data presented herein. Figure 2 b shows a plot of small perturbation carrier decay time τ Δ n versus active layer charge carrier density, with both measured under open-circuit conditions, showing the characteristic power-law behaviour we have previously observed for other OPV devices. [ 35 , 37 , 38 ]

Under one-sun illumination, the measured small-perturba-tion carrier lifetime of 800 ns is signifi cantly faster than that we have previously reported for P3HT/PCBM devices. Given the

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Figure 2 . (a) Main: total extracted charge (plotted as effective carrier density relative to dark short-circuit conditions) as a function of applied bias and light intensity, corrected for esti-mated losses during extraction. Dashed line indicates the estimated contribution from the cell’s geometric capacitance. Inset: open-circuit carrier density after subtraction of the electrode charge, dashed line is a mono-exponential characterization of the data. (b) Small-perturbation carrier lifetime as a function of active layer carrier density, dashed line is a power-law charac-terization of the data. (c) Comparison of TPC (right axis) and TPV (left axis) transients at 1-sun background light intensity using the same perturbing laser pulse. The discharge time of the additional carriers ( ∼ 150 ns, fi tted after initial rise) is ∼ 5 times faster than the recombination time under steady-state open-circuit conditions ( ∼ 800 ns, fi tted after initial rise).

relatively low carrier densities present in these small molecular devices, this is indicative of a substantially faster non-geminate recombination coeffi cient (by 2-3 orders of magnitude). This faster recombination is unlikely to be due to enhanced carrier mobility, as the hole mobility for SDT(TDPPTBFu) 2 is reported to be similar to that of P3HT. [ 9 ] Rather, this faster recombination is assigned to a much lower degree of phase segregation within the active layer, [ 47 , 48 ] consistent with structural analyses of equiv-alent fi lms which shows them to be topographically featureless and lacking in any detectable X-ray diffraction (see SI).

© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, WeinAdv. Mater. 2012, 24, 2135–2141

From these measurements, we are able to quantify the non-geminate recombina-tion fl ux as a function of voltage and light intensity ( LI ). This calculation includes con-sideration of the impact of discharging of the electrode charge carrier density through the photoactive layer, with this being of par-ticular importance for these cells due to the relatively low active-layer capacitance rela-tive to the electrode capacitance (detailed in the SI). Following this approach, we are able to employ our measurements of n ( V , LI ) and τ Δ n ( n ) to determine the non-geminate recom-bination fl ux J NGR . This allows us to predict the cell’s current-voltage response using a simple device model, as expressed by:

J (V ) = JGE N − JNGR(V ) (1) Where the overall device current J results

from two opposing terms: J GEN representing the generation fl ux of dissociated charges, and J NGR representing the non-geminate recombination fl ux of these dissociated charges. Following our previous analyses of other OPV devices, [ 25 , 26 , 41 , 42 ] we will assume that J GEN is dependent only upon light inten-sity, and independent of cell voltage. We therefore make the approximation J GEN = J SC as previously.

The results of this analysis, including only the charge-density dependence, and therefore voltage dependence, of J NGR are shown by the triangle points in Figure 1 b. While the non-geminate recombination lifetime is extremely short (compared to materials analysed in pre-vious studies [ 37 ] ) the amount of charge sup-ported by the active layer is rather small. As a result, and in contrast to our previous anal-yses of a range of OPV devices, [ 25 , 26 , 37 , 38 ] this simple recreation of the device J(V) curve does not reproduce the directly measured behaviour, substantially overestimating device FF and V OC . This implies that an additional voltage-dependent loss pathway is signifi cant for these small molecule blend devices.

We now consider the possibility that charge dissociation may itself be fi eld-dependent, thereby resulting in fi eld-dependent gemi-

nate recombination losses. For this purpose, we conducted TAS measurements of polaron dynamics in complete devices as a function of applied bias. Devices were excited at 600 nm, and the resulting photoinduced absorption monitored in the near-infrared on ns-ms timescales. Transient spectra (see SI) showed a broad, long lived absorption peaking at ∼ 980 nm, which we assign to polaron absorption by SDT(TDPPTBFu) 2 . Excitation densities of 6 μ Jcm − 2 were used to minimise non-linear excita-tion processes, as confi rmed by the linearity of initial transient signal intensity with excitation density (see SI). Figure 3 a,b

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Figure 3 . (a) Raw TAS decays at applied voltages corresponding to open-circuit, short-circuit and reverse bias with biphasic fi ts to the data superimposed. t = 0 ns corresponds to the trigger of the pump pulse. (b) Time slices through the complete set of raw TAS decays, normalised to the amplitude at –4V to highlight the evolving shape of the profi le. t = 20 ns corresponds to the instrument response of the TAS system. t = 0 ns points are estimated from an exponential extrapolation of the fast phase back to the trigger time of the pump pulse. The dashed grey line is a quadratic characterization of the data at 400ns.

shows the dependence of the transient absorption signal moni-tored at 980 nm as a function of applied bias and time. A sig-nifi cant acceleration of the signal decay dynamics is apparent as the bias voltage approaches V OC . This acceleration cannot be assigned to sweep-out of carriers to the device electrodes, as we would expect this process to become slower towards open-circuit. Rather, this acceleration is assigned to enhanced recombination losses. The initial signal amplitude, extrapolated to t = 0 ns, was not strongly bias dependent (see Figure 3 b), indicative of only minor fi eld-dependent losses prior to our instrument-limited time resolution ( ∼ 20 ns). The transient signals could be well-fi tted by a biphasic decay (see solid lines in Figure 3 a), with a fast ( ∼ 100 ns), mono-exponential decay phase followed by a slower power-law decay. As previously, we

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assign the power-law decay to the non-geminate recombination of dissociated charges. The amplitude of the fast decay phase was observed to be strongly bias dependent, and is assigned to an early-time fi eld-dependent recombination process. Due to this fi eld-dependent recombination, we observe the amplitude of the power-law decay phase, and therefore the yield of dissoci-ated charge carriers, to increase substantially between V OC and reverse bias.

Our observation that the yield of dissociated charge carriers increases under reverse bias conditions is strongly indicative of fi eld-assisted splitting of geminate charge pairs. The frac-tion of geminate pairs lost to recombination increases over the fi rst ∼ 100 ns, beyond which time the bias-dependence of the transient signal converges to a single profi le, as illustrated in Figure 3 b. We assume that this convergence represents the situation whereby the majority of fi eld-dependent geminate recombination events have occurred, and the transient signal therefore correlates with the yield of dissociated polarons which are available for collection. The fi eld-dependence of this dis-sociated polaron yield can be accurately characterised (close to the power-generating quadrant) by a simple quadratic function, illustrated by the fi t to the signal at 400 ns shown in Figure 3 b. We note that while the Onsager-Braun model of fi eld-assisted dissociation suggests a specifi c functional relationship between dissociation probability and fi eld strength, in the absence of other measurements and without detailed knowledge of the internal fi eld distribution, we do not attempt herein to under-take such a detailed analysis. It is apparent that this fi eld-de-pendent geminate recombination results in an approximate halving of the yield of dissociated charges between –4 and + 1 V bias, and ∼ 30% reduction between biases corresponding to short- and open-circuit conditions.

We now have separate quantifi cations of the bias depend-ence of both geminate and non-geminate losses in the device studied herein. Using these measured dependencies, we are able to modify Equation 1 to incorporate fi eld-dependent gemi-nate recombination losses J GR such that:

J (V ) = JGE N(V ) − JNGR(V ) (2)

with J GEN ( V ) = J MAX − J GR ( V ) where J MAX represents the max-imum possible current density as set by the number of exci-tons reaching the donor-acceptor interface. In order to remove the need for an absolute measure of J MAX , and to allow us to convert directly between Δ OD and current density, we reference J GEN relative to the generation at V = 0 at 50%-sun illumination and scale this generation profi le linearly with light intensity. Using Equation 2 , we again attempt to calculate the device J ( V ) curves as a function of light intensity, shown in Figure 4 a,b. It is apparent that these re-creations, which are the same as those used in Figure 1 b but now include our measured bias-dependence of carrier generation, are in excellent agreement with the measured device J(V) curves. We emphasise that these re-creations do not use any fi tting parameters (except the refer-encing to a single value of J at V = 0, 50%-sun), all parameters are experimentally derived and the measured fi eld-dependent generation profi le is simply scaled linearly with light intensity. We conclude that our simple device model, as expressed by Equation 2 , accurately reproduces the shape of the J ( V ) curves

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Figure 4 . (a) one-sun (100 mWcm − 2 ) reconstruction (diamonds) illus-trating the relative contributions of geminate recombination fl ux J GR (rela-tive to fl ux at V = 0) and non-geminate recombination fl ux J NGR compared to the measured J(V) curve (solid line) (b) simultaneous 0–2 sun recon-structions highlighting the fi eld-dependent generation profi le for each. Scaling factors for the generation profi le after compensation for series resistance effects are 0, 4.9, 9.8 and 19.6 mAcm − 2 . A parallel shunt resist-ance of 4.5 kΩ, measured from the dark curve at V = 0, has been included for all curves.

throughout the power-generating quadrant as a function of light intensity, except under dark conditions - see SI for discus-sion of this limitation. We recreate not only the FF and V OC but also the apparent “photoshunt” (the slope 1A

dVd J

|V=0 in excess of the dark shunt) which at normal intensity is the result of the fi eld-dependence of geminate pair separation, and at higher intensity includes a small additional contribution from non-geminate losses.

The low FF for these cells (and DPP(TBFu) 2 /PCBM cells, see SI) therefore results from two voltage-dependent limitations

© 2012 WILEY-VCH Verlag GAdv. Mater. 2012, 24, 2135–2141

on cell performance. First, these cells show fast non-geminate recombination within the active layer, suggestive of poor phase segregation in the blend [ 47 , 48 ] which leads to signifi cant non-geminate losses even away from open-circuit. In addition, gem-inate pair dissociation is relatively ineffi cient in these blends, and requires an additional macroscopic fi eld to assist the sepa-ration process. Our analyses allow us to quantify that ∼ 70% of the loss of photocurrent going from short-circuit to open-circuit results from charge-density dependent non-geminate recombi-nation losses, and 30% from additional geminate recombina-tion losses. These observations are in contrast to similar studies of both bulk heterojunction and bilayer solar cells, where non-geminate losses in the active layer were typically found to be negligible towards short-circuit, and no signifi cant fi eld-dependence of the geminate pair dissociation effi ciency was found within the power-generating quadrant.

The geminate recombination phase we report herein exhibits a reasonably long decay time ( ∼ 100 ns). This implies that the fi eld-assisted separation we report may not be of tightly bound geminate pairs (often called ‘charge transfer states’ or ‘charge transfer excitons’) which are typically observed to decay on faster timescales. [ 28 ] Instead, it has been suggested that gemi-nate recombination on the nanoseconds to hundreds of nano-second timescale may occur for more loosely bound charge pairs [ 49 ] which are unable to separate without assistance. This situation is consistent with a picture in which phase separa-tion is relatively poor, leading to a lack of disorder-assisted [ 50 ] or crystallisation-assisted [ 22 , 51 ] separation of the initially formed charge transfer state.

The physical reason as to why the small-molecule cells studied herein appear to show a stronger fi eld-dependence of charge dissociation than the polymer/fullerene cells we have studied previously is unclear. We note that higher charge car-rier densities were typically present such polymer/fullerene cells, which might result in screening of device electric fi elds in those devices. However, this cannot account for the difference in behaviour persisting into reverse bias, where charge densi-ties are low. We suggest instead that the difference may be an effect of the low crystallinity and lack of well-defi ned phase-sep-arated microstructure (see SI). The absence of such microstruc-tural features may give rise to loosely bound charge pairs which require an additional driving force to escape to different regions of the fi lm (as well as increasing the non-geminate recombina-tion rate). This interpretation is supported in part by the fact that SDT(TDPPTBFu) 2 appears more amorphous than many other effi cient OPV materials, potentially limiting the available crystallinity-assisted driving force for charge dissociation. [ 22 , 51 ] In addition, this suggests that methods which increase the ten-dency one or both blend components to phase-segregate may be successfully applied to improve the performance, and par-ticularly the FF , of such small molecule cells, consistent with a recent report of the use of diiodooctane as a crystallisation-inducing solvent additive. [ 10 ]

Notwithstanding their origin, we conclude that geminate recombination losses on the ∼ 100 ns timescale are a signifi cant factor limiting the performance of these small-molecule solar cells. A key challenge for enhancing the performance of such devices is therefore to enhance the effi ciency of charge carrier dissociation, and in particular to enable this charge dissociation

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Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements B.W. and T.Q.N. thank the Offi ce of Naval Research and the Camille Dreyfus Teacher Scholar for fi nancial support. T.Q.N. is the Alfred P. Sloan Research Fellow. The materials synthesis is supported by the NSF-SOLAR. D.C., F.C.J., and J.R.D. gratefully acknowledge fi nancial support from the EPSRC Apex and Supergen programmes, and Thomas Kirchartz for helpful comments.

Received: December 12, 2011 Revised: January 12, 2012

Published online: March 19, 2012

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