In Situ Measurement of Energy Level Shifts and Recombination Rates

Sep 3, 2014 - Growth of thermally stable crystalline C 60 films on flat-lying copper phthalocyanine. Terry McAfee , Aubrey Apperson , Harald Ade , Dan...
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In Situ Measurement of Energy Level Shifts and Recombination Rates in Subphthalocyanine/C60 Bilayer Solar Cells Dan Credgington,*,†,∥ Shun-Wei Liu,§ Jenny Nelson,‡ and James R. Durrant† †

Department of Chemistry and Centre for Plastic Electronics, and ‡Department of Physics and Centre for Plastic Electronics, Imperial College London, London SW7 2AZ, United Kingdom § Department of Electronic Engineering, Ming Chi University of Technology, New Taipei City 24301, Taiwan, Republic of China S Supporting Information *

ABSTRACT: Understanding the nature and impact of internal interfaces is critical to understanding the operation of nanostructured organic devices, such as organic photovoltaics. Here, we use transient optoelectronic analyses to quantify in situ the HOMO level shifts and changes in interfacial recombination rate that occur within thermally evaporated subphthalocyanine (SubPc)/C60 bilayer solar cells as the SubPc evaporation source is varied. We show how such measurements can complement ex situ optical and physical techniques to access the functional impact of device modification, particularly with respect to the resulting device open-circuit voltage (VOC). We are able to explain how subtle changes in SubPc deposition conditions lead to significant modification of interfacial energetics and recombination dynamics, which in turn cause substantial changes in VOC.

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INTRODUCTION Within the field of organic photovoltaics, evaporated bilayer (or “planar”) heterojunction solar cells, comprising well-defined layers of electron-donating and electron-accepting materials, are capable of achieving power conversion efficiencies (PCEs) close to those reported for their bulk heterojunction (BHJ) counterparts.1,2 Moreover, they are of interest for studies of the fundamental physics of device function, due their well-defined interface structure,3−6 and the prospect of controllably altering device performance through the insertion of functional interlayers.7 However, linking changes in device structure and fabrication to their functional impact on photovoltaic performance is not always straightforward. In this Article, we focus on the voltage output of such bilayer devices, quantifying the impact of changes in molecular energetics and charge recombination rate upon device open-circuit voltage (VOC) and showing how this can provide a sensitive in situ probe of the functional effect of device modification. We illustrate why such measurements have been difficult to conduct on efficient planar heterojunctions, and describe the analysis necessary to apply such measurements to probe bilayer devices. For polymer/fullerene BHJ solar cells, there is increasing evidence that the factors that limit VOC under normal operation are (1) the relative positions of donor and acceptor densities of states (DoS), typically characterized by the offset of their respective HOMO and LUMO levels,8 and (2) the rate of recombination of dissociated carriers (nongeminate recombination) across the donor/acceptor interface or at the device electrodes.9−14 The importance of nongeminate recombination in BHJ devices can be attributed, at least in part, to their deliberately high heterointerface area. By contrast, the bilayer architecture provides a relatively small interface area, which recent experimental15,16 and theoretical17 studies have shown © 2014 American Chemical Society

can give rise to a reduction in recombination rate constant. A similar reduction in rate constant has been observed where BHJ domain size is increased using thermal treatments,18 thus reducing heterointerface area and facilitating the escape of charge carriers from the heterojunction. The bilayer architecture should also significantly reduce the rate of surface recombination at the device electrodes, because carriers are unable to reach the “wrong” electrode without the assistance of physical defects bridging the donor or acceptor layers. We would therefore expect VOC for a bilayer solar cell to exceed that of an equivalent BHJ, if nongeminate recombination is the primary loss mechanism within the device. Previous analyses have suggested a number of factors other than nongeminate recombination that limit the voltage output of bilayer solar cells, including the influence of geminate pair separation,19 band-bending at the electrode interfaces,20 and the contribution of thermal excitation of carriers across the heterointerface (as characterized by the reverse saturation current).21 However, a key limitation of such studies is that the nongeminate recombination rate is not quantified under operating conditions, meaning its impact can be included only indirectly. We address this issue by applying a set of transient optoelectronic measurements to quantify nongeminate recombination losses in the boron subphthalocyanine chloride22 (SubPc)/C60 system, which in a bilayer architecture has previously demonstrated high PCE and relatively large VOC,1,2,23−25 due in part to the large offset between the SubPc HOMO and fullerene LUMO. We choose a molecular, rather than polymeric, system due to the clean heterointerfaces Received: May 29, 2014 Revised: September 2, 2014 Published: September 3, 2014 22858

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Figure 1. (a) Boat and crucible SubPc evaporation sources. (b) Cell structure. (c) Dark (dashed) and 1-sun equivalent (solid) J(V) curves for the two representative SubPc/C60 bilayer solar cells, with insets showing chemical structures. (d) Absolute dark current density plotted on a logarithmic scale. Inset shows dark ideality factor ηDARK, which has a minimum for B (C) of 2.2 (1.8). Figures of merit for B (C): VOC 0.861 (0.739) V, JSC 5.4 (5.1) mA cm−2, FF 0.65 (0.58), PCE 3.0 (2.2)%.

Figure 1c and d along with values for short-circuit current density (JSC), fill-factor (FF), VOC, and PCE. ΔVOC is 122 mV. To quantify nongeminate recombination flux across the heterojunction, we require both the average lifetime of charge carriers within the bilayer (τ) and their number density (n), here defined as the average 3D number density to allow straightforward comparison between data sets. Variations in n(V) between devices are also indicative of changes in the underlying DoS.28,29 As previously, we define n to be zero at dark, short-circuit conditions, and make no distinction between free and trapped charge carriers. We estimate n using two methods, charge extraction (CE)30 and differential charging,31 and estimate τ using transient photovoltage (TPV). For CE, the cell is allowed to equilibrate at a particular bias/ illumination condition, then rapidly switched to short-circuit in the dark. The discharge current transient I(t) is recorded by monitoring the voltage dropped across a small measurement resistor in series with the cell. We have previously shown that applying CE to bilayer cells can be complicated by the significant contribution of the cell’s geometric capacitance to the discharge current, which may preclude measurement of the internal recombination rate constant.32 To quantify the geometric capacitance as accurately as possible, we conduct CE on devices that are equilibrated both at open-circuit, as a function of light intensity, and at applied bias in the dark. Particular attention is paid to dark reverse bias, where little charge is injected into the cell but where the change in charge on the electrodes (as the device is shorted) is significant. This allows us to estimate the contribution of the geometric capacitance of the cell (C0) to the total extractable charge. The measured value of C0 is 11 nF for both B and C, corresponding to an average permittivity ε ≈ 4.5ε0 if we take the dielectric thickness to be the sum of the SubPc, C60, and BCP layers (63 nm). We make the assumption that this capacitance does not vary significantly as a function of activelayer carrier density, and the capacitive charge may therefore be subtracted from the total extractable charge to obtain an estimate of the additional “chemical” capacitance due to carriers within the bilayer.33 This in turn yields an estimate for the average bilayer carrier density n:

achievable using thermal evaporation, and to reduce unwanted interdiffusion of materials.26 We show that, rather than being relatively slow, nongeminate recombination rate constants in efficient SubPc/C60 bilayers are remarkably high as compared to many polymer/fullerene BHJ systems, leading to significant loss of carriers across the donor/ acceptor interface despite the low carrier concentration present under operation. This interfacial recombination is shown to dominate at open-circuit, as is typically observed for BHJ cells. We also compare SubPc/C60 bilayer solar cells fabricated by two different deposition processes: using either a small quartz crucible or a large tungsten boat as the SubPc evaporation source. These yield cells with markedly different open-circuit voltages, a phenomenon that has been observed by several previous authors to arise from both intentional and unintentional variations in substrate temperature during deposition.1,27 This comparison allows us to show that our measurements may be used to distinguish and quantify the functional differences between subtly different cell types and predict their impact on cell voltage.

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EXPERIMENTAL SECTION AND RESULTS SubPc/C60 bilayers were thermally evaporated on to precleaned ITO glass with sheet resistance of 15 Ω/□ and with structure ITO/SubPc (11 nm; 0.03 nm/s)/C60 (45 nm; 0.1 nm/s)/ bathocuproine (BCP) (7 nm; 0.1 nm/s)/Al (100 nm), which was found to be close to optimal in our facilities. Each pixel, defined by the crossover of the patterned ITO and Al electrodes, was 0.16 cm2. The two types of evaporation source are shown in Figure 1a. Changing the SubPc source led to a change in VOC (ΔVOC) in the range 50−300 mV, depending on other fabrication parameters, which is far larger than the typical variation between devices fabricated under nominally identical conditions.1 A schematic of the cell structure is given in Figure 1b. Devices were encapsulated under glass using optically transparent epoxy resin and shipped from Taipei to London with minor degradation. ITO glass substrates were coated with uncapped SubPc layers under the same conditions to allow for surface characterization. AFM images of these exposed SubPc layers suggest a smooth SubPc/C60 interface, with surface roughness of approximately 0.35 nm (see the Supporting Information). Two representative cells, referred to below as B (boat) and C (crucible), were chosen for more detailed study. Their J(V) curves, measured during this analysis, are shown in

nCE = 22859

1 [ Ade

∫0

∞

I(t ) dt − C0VOC]

(1)

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Figure 2. (a) Comparison of CE and differential charging data giving active-layer carrier density after subtraction of the geometric capacitance, assuming holes are confined within the 11 nm SubPc thickness. Dashed lines are monoexponentials with characteristic temperature T0. (b) Activelayer carrier lifetime versus active-layer carrier density (from CE). Dashed lines are power-laws with “reaction order” (δ = 1 − power) shown. (c) CE data corrected for losses but not the contribution of electrode capacitance; dashed line shows 11 nF electrode capacitance estimated from dark CE data (data from device C shown, both sets are identical). (d) Raw differential capacitance data, with offset-exponential fits forced through C0 = 11.5 nF. (e) VOC versus light intensity (LI) plotted as a percentage of 100 mW cm−2 AM1.5G, with light ideality factors ηid highlighted (note a secondary loss process dominates B for LI < 5%, consistent with the ideality of the dark “shunt”).

where A is the cell area, e is the elementary charge, and d is the thickness of the appropriate layer. nCE is more sensitive to recombination rate than nDC obtained using differential charging (vide infra) because the carriers must escape the device to make a net contribution to I(t). As a result, uncorrected nCE is likely to underestimate the total charge, meaning that a measure of the number of carriers lost to recombination during extraction must be obtained. We calculate this iteratively using the recombination lifetimes obtained TPV measurements, described below.29 We find that, at open-circuit, the extraction losses for cell C are ∼15% under one-sun illumination, while for cell B the losses are ∼30% under one-sun illumination. At lower optical or electrical bias, the losses rapidly become negligible. All nCE data herein have been corrected for these loss estimates. Figure 2a shows the active-layer carrier density determined from eq 1 as a function of cell voltage for the two devices, while Figure 2c shows the total charge extracted from the device (QCE) without subtraction of the electrode charge. To obtain n(V) using differential charging, we conduct shortcircuit transient photocurrent (TPC) measurements under constant white-light bias using a low-intensity, weakly absorbed nanosecond optical perturbation identical to that used for the TPV measurements below.31 Integrating the TPC transient provides an estimate of the yield of excess photogenerated charge ΔQ resulting from the excitation pulse, as a function of bias light intensity. In practice, we find ΔQ to be independent of bias light intensity, because extraction at short-circuit is highly efficient: JSC is found to be linear with light intensity for both cells to >3 suns equivalent illumination (Figure 3). ΔQ

Figure 3. Comparison of JLOSS at open-circuit calculated using eq 5 with short-circuit current density under equivalent illumination conditions. Light intensity is plotted as a percentage of 100 mW cm−2 AM1.5G. Beyond 20% intensity, the two are equivalent to within 10−20%. JSC scales linearly with light intensity to over 3 suns in both cases.

may be combined with the initial voltage rise ΔV0, obtained from TPV at each bias light intensity, to calculate the differential capacitance of the cell, CDC, as a function of VOC: C DC(VOC) =

ΔQ ΔV0(VOC)

Because the differential capacitance includes contributions from all available charge reservoirs close to the quasi-Fermi levels (the positions of which are set by the choice of light 22860

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we find the relationship to be a simple power law, giving τT = τΔn(1 − power), where the power is negative. To calculate τ, we must recognize that the τT will be influenced by the equilibration of charge carriers between the bilayer and the device electrodes. In our previous studies of polymer/fullerene BHJ devices, the chemical capacitance of the active-layer was much greater than the electrode capacitance, such that this equilibration was relatively unimportant. However, in the devices studied herein, the active-layer capacitance CAL is less than the electrode capacitance C0, at least for cell voltages up to one-sun VOC. In this case, if we assume rapid equilibration of the electrode and active-layer charge carriers during the TPV experiment, the effective decay lifetime of the active-layer charge carriers can be determined from36

bias), integrating CDC with respect to V yields the cumulative carrier density (nDC) at a given open-circuit voltage. This method does not require the carriers present at open-circuit to be extracted, only that they equilibrate with the cell electrodes more quickly than they are lost to recombination. However, this measurement is compromised by any underlying variation in photogeneration efficiency between open-circuit and shortcircuit conditions, which will affect ΔQ. In addition, due to the finite rise-time of the TPV signal, ΔV0 is likely to be a slight underestimate of the initial quasi Fermi level splitting, and therefore in contrast to charge extraction measurements, nDC is likely to slightly overestimate the active-layer carrier density. CDC approaches a limiting value C0 of 11.5 nF at low VOC (Figure 2d), which we again interpret as the contribution of the geometric capacitance of the cell electrodes. While both methods yield very similar values for C0, we assume the value obtained from CE (11 nF) to be the more accurate. We again assume that this geometric contribution does not vary significantly as a function of carrier density, and so may be subtracted from the total capacitance to estimate the average active-layer carrier density, as before: nDC =

1 Ade

∫0

VOC

[C DC(VOC) − C0] dV

⎛ CAL ⎞ 1 ⎛ C0 ⎞1 1 =⎜ ⎟ +⎜ ⎟ τT ⎝ CAL + C0 ⎠ τ ⎝ CAL + C0 ⎠ τ0

and so: ⎡1 ⎛ C0 ⎞ 1 ⎤ ⎛ CAL + C0 ⎞ 1 =⎢ −⎜ ⎟ ⎥×⎜ ⎟ τ ⎝ CAL + C0 ⎠ τ0 ⎥⎦ ⎝ CAL ⎠ ⎣⎢ τT

(2)

(3)

where τ0 is the self-discharge (RC) time of the cell’s electrodes, either through the external circuit or through a parallel shunt resistance. Accurate determination of this resistance under illumination is difficult, because it may be masked (i.e., reduced) by competing recombination pathways. Taking the full J−V gradient at short-circuit under illumination to be due only to shunt pathways, we obtain a worst-case estimate of ∼10 kΩ. For C0 = 11 nF, τ0 will therefore exceed 100 μs and so only impact the calculation of τ at intensities below ∼5%. The true shunt resistance is likely to be substantially higher, and thus have even less effect. At higher light intensities, eq 3 therefore simplifies to

which is plotted in Figure 2a. For both of these methods, we have taken d to be the thickness of the SubPc layer, meaning these values represent average hole densities if we assume that holes are primarily confined within the donor material. Average electron density in the C60 will be approximately 4 times lower due to its greater thickness. Within error, the CE and differential charging estimates of n are the same. Both depend exponentially on bias voltage through the approximate form n ∝ eeV/kT0, where k is Boltzmann’s constant and T0 is a characteristic temperature of approximately 800 K. This weak dependence on voltage is consistent with the presence of an exponential tail of states for the HOMO or LUMO levels caused by energetic disorder.34 If we may use this active-layer charge as a proxy for the filled DoS, then we deduce from the voltage offset between the data sets presented in Figure 2a that the electronic bandgap for cell B is ∼180 meV larger than that for cell C. This offset is consistent with UPS measurements on exposed SubPc layers, which show that the ionization potential (IP) for a SubPc layer of type B is 220 ± 50 meV larger than that of a SubPc layer of type C (see the Supporting Information). To measure recombination lifetimes within the bilayer, we employ TPV. We briefly note that the recombination lifetime must be defined carefully. In the following, we refer to (i) the small-perturbation lifetime τΔn, which is measured directly by TPV, (ii) the “total” lifetime τT of all carriers, averaged over those within the active-layer and those on the electrodes, and (iii) the lifetime of carriers within the active-layer alone, τ. For the TPV measurement, we apply a small, weakly absorbed, nanosecond optical perturbation to a cell held at open-circuit under white-light bias, then record the photovoltage transient ΔV(t) induced.31 For small perturbations (ΔV ≪ VOC), and where the excitation wavelength is not strongly absorbed within the cell (620 nm was chosen in this case), these transients are well-approximated by monoexponential decays, from which we obtain τΔn and ΔV0 (i.e., ΔV = ΔV0 e−t/τΔn). τΔn may be used to calculate the total carrier lifetime τT if a functional relationship between τT and n is known.35 Here,

⎛ CAL ⎞ τ=⎜ ⎟τT ⎝ CAL + C0 ⎠

(4)

For C0 we use the geometric capacitance of 11 nF. To estimate CAL, we characterize the residual, bilayer carrier population by a single exponential, as shown in Figure 2a, and take its derivative. Figure 2b shows the active-layer charge carrier decay time τ, as calculated from eq 3 and including the corrections described above,35 versus the donor layer carrier density n for the two devices studied herein. The derived recombination lifetimes close to one-sun open-circuit conditions are similar to those of many polymer/fullerene BHJs,29 and significantly shorter than observed for many polythiophene/fullerene blends.9 To quantify the impact of nongeminate recombination, we calculate the associated lost current density at VOC (JLOSS) as a function of illumination intensity: JLOSS(VOC) = ed

n(VOC) τ(n)

(5)

Figure 3 compares this rate of carrier loss with the shortcircuit current density measured under the same illumination conditions, which provides an upper limit on the rate of carrier generation at VOC. For both cells, the difference between JSC and JLOSS is only 10−20%; that is, at least 80−90% of the carriers available for collection under short-circuit conditions 22861

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model, which assumes that at VOC the interfacial recombination current density JLOSS exactly balances the photogenerated current, which we estimate as equal to JSC (thereby neglecting variations in geminate recombination rate with applied voltage).29 Using eq 5, we identify the value of VOC at which:

are lost to nongeminate recombination. This implies that at open-circuit, the losses within these bilayer cells are dominated by carrier density-dependent nongeminate recombination. Moreover, the values of the ideality factor for these cells under illumination (1.3−1.6, Figure 2e) combined with the relatively weak dependence of τ on n (“reaction order” of 2− 2.3, Figure 2b) and steep tail-state slopes (Figure 2a) suggest that this nongeminate process is dominated by Shockley− Read−Hall recombination via relatively deep interfacial traps.34,37 The dark ideality factor (Figure 1d) is likewise consistent with this mechanism, although it is unlikely to reach a true minimum given the relatively small voltage range over which shunt and series resistances do not dominate. We utilize these measurements of recombination flux as a tool to understand the origin of the change in VOC between these cells. For the pair of devices studied herein, ΔVOC is 122 mV. Because the donor, acceptor, and electrode materials are identical, this shift in VOC cannot be attributed to a change in the intrinsic energy levels of the donor, different electrode work functions, or a shift in the acceptor LUMO. ΔVOC must therefore arise from a change in molecular packing of the donor and/or an electrostatic interaction at the interface. The reversebias (and EQE, see the Supporting Information) behavior of both devices is also sufficiently similar that differences in carrier generation efficiency can be largely neglected. There is slight evidence for an increase in exciton diffusion length in cell B, leading to improved collection efficiency from excitons formed in the SubPc, but this effect does not significantly affect the photocurrent. Where acceptor energetics and photogeneration rate are unchanged, the two factors primarily affecting VOC are the relative positions of the SubPc and C60 DoS and the rate of recombination across the SubPc/C60 interface. The latter will determine the equilibrium positions of the charge carriers’ quasi Fermi levels, while the former will set the energetic range over which these vary during operation. Both will therefore affect the achievable Fermi-level splitting under illumination, and thus ΔVOC.29 Measurements of IP obtained from UPS, C(V) obtained from differential charging, and n(V) obtained from CE are all consistent with a shift in HOMO level of 180 meV between B and C, which might be expected to lead to a concomitant change in VOC.8 Shifts of this magnitude may arise either from an increase in SubPc crystallinity, leading to an upshift in the HOMO energy, or from variations in interfacial dipoles due to a change in molecular alignment.38,39 Because we can discern no significant difference between EQE or absorption spectra, which would indicate a change in crystallinity, and given the previous work of Chou et al.27 demonstrating the change in SubPc orientation with increasing substrate temperature, we consider the latter most likely. We can also largely rule out the possibility that such changes are the result of physical variation in the SubPc top surface. Water contact-angle and AFM measurements (see the Supporting Information) show only very slight changes in surface energy and roughness. While our in situ measurements confirm the energetic shift persists within complete devices, it is insufficient to explain the more modest shift in VOC. To understand this, we compare recombination lifetimes for the two deposition regimes. Figure 2d shows that while both cells have similar power−law relationships between recombination flux and carrier density, cell B has measurably faster recombination dynamics at a given average carrier density than cell C. To assess the impact of this change in recombination rate on VOC, we use a simple kinetic

JLOSS = ed

n(VOC) = JSC τ(n)

(6)

From eq 6, we find the effect of increased recombination rate reduces the expected ΔVOC between B and C to 127 ± 5 mV, rather than ∼180 mV. The absolute open-circuit voltages predicted by this model are 0.873 and 0.746 V, respectively (while 0.861 and 0.739 V are measured), confirming the importance of including kinetic effects as well as energy levels when considering contributions to VOC. The systematic overestimation of VOC by approximately 10 mV, and the behavior of both cells under reverse bias, are both consistent with the efficiency of geminate-pair dissociation depending weakly on internal field such that JSC is an overestimate of the photogeneration rate at open-circuit.36 In our analysis, a 10 mV overestimate of VOC would arise from a ∼20% overestimate of photogeneration. We may therefore also rule out ΔVOC arising from a significant difference in how dissociation efficiency varies with applied bias between B and C.

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DISCUSSION The short nongeminate recombination lifetimes and high interfacial recombination flux we observe are surprising: in general it might be expected for the bilayer architecture to reduce the role of recombination across the donor/acceptor interface. However, this result may be understood in part by considering the low donor volume and the relatively high mobilities typical of thermally evaporated small molecule films.40,41 The necessarily thin SubPc films used herein result in an interface to volume ratio not significantly different from that of a phase-separated bulk heterojunction. We also find areal carrier densities of 1014−1015 m−2 in the SubPc layer, meaning that the average separation between carriers is of the same order (or larger) as the donor thickness, and there is little distinction between bulk and interfacial carriers. The net effect of high mobility may therefore be to allow carriers to diffuse laterally within the device, increasing the likelihood of Coulomb capture. This will occur only where extraction rate is not equally enhanced,14,40 for instance, where extraction is also limited by the choice of contacts. It has previously been shown that the contact between ITO and SubPc is not ideal,42 likely due to the large (around 0.8 eV) injection barrier between the ITO work function and the SubPc HOMO,43 which would provide this asymmetry. The increase in recombination rate for cells B may therefore arise from an increase in SubPc mobility due to the change in deposition conditions, specifically the increase in radiative heating caused by using a large metal boat, rather than a smaller quartz crucible. Considering other possible causes, we rule out differing levels of chemical contamination by using the same purified source of SubPc for all devices, and because SubPc will not deposit on substrates held above 130 °C,27 we likewise rule out changes in thermal decomposition, and so infer a physical origin. This is in agreement with Chou et al.,27 who observed that while modest differences in substrate temperature during deposition were able to significantly affect VOC, very little change in the SubPc surface was discernible, as we also find. Instead, the primary 22862

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ACKNOWLEDGMENTS We thank Thomas Kirchartz for advice on interpreting ideality and recombination mechanism using device models. We also gratefully acknowledge the EPSRC Supergen and Apex programmes and Solvay SA for funding

physical change was determined to be a difference in molecular orientation, which correlated with both an increase in VOC and SubPc hole mobility.27 Such changes in interfacial structure have been linked to significant changes in VOC (and other figures of merit) in a number of small-molecule systems.44,45 We suggest the same effect is responsible for the phenomena described herein, and are thus able to link such physical changes to their functional impact. This implies that even the small increase in radiative heating from the change in source is sufficient to induce a change in organization of the deposited films. Because this will also be a function of deposition rate, we explain the changes in VOC that have previously been reported when this is varied.1

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CONCLUSIONS

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ASSOCIATED CONTENT

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REFERENCES

(1) Lin, C.-F.; Liu, S.-W.; Lee, C.-C.; Hunag, J.-C.; Su, W.-C.; Chiu, T.-L.; Chen, C.-T.; Lee, J.-H. Open-Circuit Voltage and Efficiency Improvement of Subphthalocyanine-Based Organic Photovoltaic Device through Deposition Rate Control. Sol. Energy Mater. Sol. Cells 2012, 103, 69−75. (2) Pandey, R.; Zou, Y.; Holmes, R. J. Efficient, Bulk Heterojunction Organic Photovoltaic Cells Based on Boron Subphthalocyanine Chloride-C70. Appl. Phys. Lett. 2012, 101, 033308−4. (3) Barker, J. A.; Ramsdale, C. M.; Greenham, N. C. Modeling the Current-Voltage Characteristics of Bilayer Polymer Photovoltaic Devices. Phys. Rev. B 2003, 67, 075205. (4) Peumans, P.; Forrest, S. R. Separation of Geminate Charge-Pairs at Donor−Acceptor Interfaces in Disordered Solids. Chem. Phys. Lett. 2004, 398, 27−31. (5) Terao, Y.; Sasabe, H.; Adachi, C. Correlation of Hole Mobility, Exciton Diffusion Length, and Solar Cell Characteristics in Phthalocyanine/Fullerene Organic Solar Cells. Appl. Phys. Lett. 2007, 90, 103515−3. (6) Rim, S.-B.; Fink, R. F.; Schoneboom, J. C.; Erk, P.; Peumans, P. Effect of Molecular Packing on the Exciton Diffusion Length in Organic Solar Cells. Appl. Phys. Lett. 2007, 91, 173504−3. (7) Tada, A.; Geng, Y.; Wei, Q.; Hashimoto, K.; Tajima, K. Tailoring Organic Heterojunction Interfaces in Bilayer Polymer Photovoltaic Devices. Nat. Mater. 2011, 10, 450−455. (8) Scharber, M. C.; Mühlbacher, D.; Koppe, M.; Denk, P.; Waldauf, C.; Heeger, A. J.; Brabec, C. J. Design Rules for Donors in BulkHeterojunction Solar Cells - towards 10% Energy-Conversion Efficiency. Adv. Mater. 2006, 18, 789−794. (9) Credgington, D.; Hamilton, R.; Atienzar, P.; Nelson, J.; Durrant, J. R. Non-Geminate Recombination as the Primary Determinant of Open-Circuit Voltage in Polythiophene:Fullerene Blend Solar Cells: An Analysis of the Influence of Device Processing Conditions. Adv. Funct. Mater. 2011, 21, 2744−2753. (10) Dibb, G. F. A.; Kirchartz, T.; Credgington, D.; Durrant, J. R.; Nelson, J. Analysis of the Relationship between Linearity of Corrected Photocurrent and the Order of Recombination in Organic Solar Cells. J. Phys. Chem. Lett. 2011, 2, 2407−2411. (11) Maurano, A.; et al. Recombination Dynamics as a Key Determinant of Open Circuit Voltage in Organic Bulk Heterojunction Solar Cells: A Comparison of Four Different Donor Polymers. Adv. Mater. 2010, 22, 4987−4992. (12) Shuttle, C. G.; Hamilton, R.; O’Regan, B. C.; Nelson, J.; Durrant, J. R. Charge-Density-Based Analysis of the Current-Voltage Response of Polythiophene/Fullerene Photovoltaic Devices. Proc. Natl. Acad. Sci. U.S.A. 2010, 107, 16448−16452. (13) Kniepert, J.; Schubert, M.; Blakesley, J. C.; Neher, D. Photogeneration and Recombination in P3HT/PCBM Solar Cells Probed by Time-Delayed Collection Field Experiments. J. Phys. Chem. Lett. 2011, 2, 700−705. (14) Boix, P. P.; Guerrero, A.; Marchesi, L. F.; Garcia-Belmonte, G.; Bisquert, J. Current-Voltage Characteristics of Bulk Heterojunction Organic Solar Cells: Connection between Light and Dark Curves. Adv. Energy Mater. 2011, 1, 1073−1078. (15) Foertig, A.; Wagenpfahl, A.; Gerbich, T.; Cheyns, D.; Dyakonov, V.; Deibel, C. Nongeminate Recombination in Planar and Bulk Heterojunction Organic Solar Cells. Adv. Energy Mater. 2012, 2, 1483−1489. (16) Sánchez-Díaz, A.; Burtone, L.; Riede, M.; Palomares, E. Measurements of Efficiency Losses in Blend and Bilayer-Type Zinc Phthalocyanine/C60 High-Vacuum-Processed Organic Solar Cells. J. Phys. Chem. C 2012, 116, 16384−16390.

We have shown that measurements of the internal recombination dynamics for a working SubPc/C60 bilayer heterojunction solar cell are achievable using transient electrical techniques. While the majority of carriers are held on the device electrodes and appear to be broadly inert, by carefully accounting for the geometric capacitance we are able to estimate the average carrier density within the bilayer by two complementary methods. This enables us to estimate the carrier-density dependence of the nongeminate recombination rate at the bilayer interface. We have found that charge recombines within these bilayers at higher rates than previously reported for many polymer/fullerene bulk heterojunctions. We have also shown that in situ measurements of electronic bandgap can complement ex situ measurements such as UPS for accurately determining energetic shifts. By tracking changes in both energetics and recombination dynamics between devices, we achieve a more thorough understanding of how each leads to measurable changes in device J(V) behavior, and particularly VOC. We demonstrate that the fabrication-dependent shift in VOC reported for SubPc/C60 bilayers by several authors may be understood in terms of both a deepening of the SubPc HOMO and an increase in the rate coefficient for recombination across the donor/acceptor interface. These two effects act in opposition, such that the beneficial energetic shift is partly canceled by a loss of achievable voltage due to the increase in recombination rate, which quantitatively accounts for the observed ΔVOC. Such an approach may also be used to more fully understand the impact of deliberate interfacial modification7 on device performance, beyond monitoring simple figures-of-merit.

S Supporting Information *

Full fabrication and experimental details, AFM, UPS, contactangle, absorption, and EQE measurements. This material is available free of charge via the Internet at http://pubs.acs.org.

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Article

AUTHOR INFORMATION

Corresponding Author

*Tel.: +44 (0)1223 337284. E-mail: [email protected]. Present Address ∥

D.C.: Cavendish Laboratory, University of Cambridge, Cambridge CB3 0HE, United Kingdom. Notes

The authors declare no competing financial interest. 22863

dx.doi.org/10.1021/jp505297u | J. Phys. Chem. C 2014, 118, 22858−22864

The Journal of Physical Chemistry C

Article

Quantification of Recombination Losses at Short Circuit. J. Phys. Chem. C 2007, 111, 14001−14010. (36) Credgington, D.; Jamieson, F. C.; Walker, B.; Nguyen, T.-Q.; Durrant, J. R. Quantification of Geminate and Non-Geminate Recombination Losses within a Solution-Processed Small-Molecule Bulk Heterojunction Solar Cell. Adv. Mater. 2012, 24, 2135−2141. (37) Wetzelaer, G. A. H.; Kuik, M.; Lenes, M.; Blom, P. W. M. Origin of the Dark-Current Ideality Factor in Polymer:Fullerene Bulk Heterojunction Solar Cells. Appl. Phys. Lett. 2011, 99, 153506. (38) Linares, M.; et al. On the Interface Dipole at the Pentacene− Fullerene Heterojunction: A Theoretical Study. J. Phys. Chem. C 2010, 114, 3215−3224. (39) Vázquez, H.; Gao, W.; Flores, F.; Kahn, A. Energy Level Alignment at Organic Heterojunctions: Role of the Charge Neutrality Level. Phys. Rev. B 2005, 71. (40) Singh, T. B.; Sariciftci, N. S.; Yang, H.; Yang, L.; Plochberger, B.; Sitter, H. Correlation of Crystalline and Structural Properties of C60 Thin Films Grown at Various Temperature with Charge Carrier Mobility. Appl. Phys. Lett. 2007, 90, 213512. (41) Zhang, X. H.; Domercq, B.; Kippelen, B. High-Performance and Electrically Stable C60 Organic Field-Effect Transistors. Appl. Phys. Lett. 2007, 91, 092114. (42) 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. (43) Tress, W.; Leo, K.; Riede, M. Influence of Hole-Transport Layers and Donor Materials on Open-Circuit Voltage and Shape of I-V Curves of Organic Solar Cells. Adv. Funct. Mater. 2011, 21, 2140− 2149. (44) Verlaak, S.; Beljonne, D.; Cheyns, D.; Rolin, C.; Linares, M.; Castet, F.; Cornil, J.; Heremans, P. Electronic Structure and Geminate Pair Energetics at Organic−Organic Interfaces: The Case of Pentacene/C60 Heterojunctions. Adv. Funct. Mater. 2009, 19, 3809− 3814. (45) Rand, B. P.; et al. The Impact of Molecular Orientation on the Photovoltaic Properties of a Phthalocyanine/Fullerene Heterojunction. Adv. Funct. Mater. 2012, 22, 2987−2995.

(17) Ray, B.; Lundstrom, M. S.; Alam, M. A. Can Morphology Tailoring Improve the Open Circuit Voltage of Organic Solar Cells? Appl. Phys. Lett. 2012, 100, 013307. (18) 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. (19) Rand, B.; Burk, D.; Forrest, S. Offset Energies at Organic Semiconductor Heterojunctions and Their Influence on the OpenCircuit Voltage of Thin-Film Solar Cells. Phys. Rev. B 2007, 75. (20) Cheyns, D.; Poortmans, J.; Heremans, P.; Deibel, C.; Verlaak, S.; Rand, B. P.; Genoe, J. Analytical Model for the Open-Circuit Voltage and Its Associated Resistance in Organic Planar Heterojunction Solar Cells. Phys. Rev. B 2008, 77, 165332. (21) 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. (22) Claessens, C. G.; González-Rodríguez, D.; Torres, T. Subphthalocyanines: Singular Nonplanar Aromatic Compounds Synthesis, Reactivity, and Physical Properties. Chem. Rev. 2002, 102, 835− 854. (23) Mutolo, K. L.; Mayo, E. I.; Rand, B. P.; Forrest, S. R.; Thompson, M. E. Enhanced Open-Circuit Voltage in Subphthalocyanine/C60 Organic Photovoltaic Cells. J. Am. Chem. Soc. 2006, 128, 8108−8109. (24) 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. (25) Heremans, P.; Cheyns, D.; Rand, B. P. Strategies for Increasing the Efficiency of Heterojunction Organic Solar Cells: Material Selection and Device Architecture. Acc. Chem. Res. 2009, 42, 1740− 1747. (26) Treat, N. D.; Brady, M. A.; Smith, G.; Toney, M. F.; Kramer, E. J.; Hawker, C. J.; Chabinyc, M. L. Interdiffusion of PCBM and P3HT Reveals Miscibility in a Photovoltaically Active Blend. Adv. Energy Mater. 2011, 1, 82−89. (27) Chou, C.-T.; Tang, W.-L.; Tai, Y.; Lin, C.-H.; Liu, C.-H. J.; Chen, L.-C.; Chen, K.-H. Effect of Substrate Temperature on Orientation of Subphthalocyanine Molecule in Organic Photovoltaic Cells. Thin Solid Films 2012, 520, 2289−2292. (28) MacKenzie, R. C. I.; Kirchartz, T.; Dibb, G. F. A.; Nelson, J. Modeling Nongeminate Recombination in P3HT:PCBM Solar Cells. J. Phys. Chem. C 2011, 115, 9806−9813. (29) Credgington, D.; Durrant, J. R. Insights from Transient Optoelectronic Analyses on the Open-Circuit Voltage of Organic Solar Cells. J. Phys. Chem. Lett. 2012, 3, 1465−1478. (30) Shuttle, C. G.; Maurano, A.; Hamilton, R.; O’Regan, B.; de Mello, J. C.; Durrant, J. R. Charge Extraction Analysis of Charge Carrier Densities in a Polythiophene/Fullerene Solar Cell: Analysis of the Origin of the Device Dark Current. Appl. Phys. Lett. 2008, 93, 183501−3. (31) Shuttle, C. G.; O’Regan, B.; Ballantyne, A. M.; Nelson, J.; Bradley, D. D. C.; de Mello, J. C.; Durrant, J. R. Experimental Determination of the Rate Law for Charge Carrier Decay in a Polythiophene: Fullerene Solar Cell. Appl. Phys. Lett. 2008, 92, 093311. (32) Credgington, D.; Kim, Y.; Labram, J.; Anthopoulos, T. D.; Durrant, J. R. Analysis of Recombination Losses in a Pentacene/C60 Organic Bilayer Solar Cell. J. Phys. Chem. Lett. 2011, 2759−2763. (33) Bisquert, J. Chemical Capacitance of Nanostructured Semiconductors: Its Origin and Significance for Nanocomposite Solar Cells. Phys. Chem. Chem. Phys. 2003, 5, 5360−5364. (34) Kirchartz, T.; Pieters, B. E.; Kirkpatrick, J.; Rau, U.; Nelson, J. Recombination Via Tail States in Polythiophene:Fullerene Solar Cells. Phys. Rev. B 2011, 83, 115209. (35) O’Regan, B. C.; Durrant, J. R.; Sommeling, P. M.; Bakker, N. J. Influence of the Ticl4 Treatment on Nanocrystalline Tio2 Films in Dye-Sensitized Solar Cells. 2. Charge Density, Band Edge Shifts, and 22864

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