Implications of Multichromophoric Arrays in Organic Photovoltaics

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Chemistry of Materials

Implications of Multi-chromophoric arrays in OPVs Patrick Erwin, Sarah M. Conron, Jessica H. Golden, Kathryn Allen, Mark E. Thompson Department of Chemistry, University of Southern California, Los Angeles, California 90089

Abstract A covalently linked multi-chromophoric array (BDP-Por) was used as the donor layer in planar hetero-junction organic photovoltaic (OPV) cells with the structure (ITO/BDPPor/C60/BCP//Al, BCP = bathocuproine). BDP-Por is a platinum tetrabenzoporphyrin core (Pt(TPBP)) bonded through the phenyl groups to the meso position of four 4,4-difluoro-3,5dimethyl-4-bora-3a,4a-diaza-s-indacene (BODIPY) moieties. The OPV outperforms analogous OPVs which use only the Pt(TPBP) as the donor, predominantly by producing a higher short circuit current (JSC of 2.5 vs. 4.5 mA/cm2), due to the enhanced absorptivity of the multi-chromophoric array. The open circuit voltages (VOC) for Pt(TPBP) and BDP-Por based OPVs are both 0.65 volts. The system was further investigated by preparing and testing OPVs made from an analogous system with porphyrin and BODIPY components blended into a single homogeneous donor layer that absorbs similarly to the array. It is then found that the mixed system generates the same photocurrent as the array with a similar responsivity, but gives an unstable open circuit voltage that quickly degrades to 0.34 V. It is determined that the donor layer components undergo phase segregation at the interface upon illumination, resulting in the decreased voltage and highlighting an advantage of the multi-chromophoric array.

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Introduction Organic photovoltaics (OPVs) have received a great deal of attention in the last decade due to their promise of being a low-cost, lightweight, and environmentally friendly alternative source of energy.1, 2 These organic dye materials have a natural advantage over most inorganic semiconductors in that their absorbtivities are usually orders of magnitude greater, being as high as 105 - 106 M-1cm-1. In many organic dyes, including the ones used here, the primary absorption in the visible region is due to π to π* or n to π* transitions in aromatic systems.3 While silicon owes its absorption to classically forbidden transitions, these π to π* excitations are fully allowed and as consequence are quite intense.4 However, a significant limitation of this method of photon absorption is that the bands tend to be narrow compared to inorganic systems which absorb continuously above the band gap. When organic dyes, which rely on these discrete stateto-state transitions with limited spectral coverage, are made into films, they typically will have near unity absorption in some parts of the solar spectrum while being effectively transparent in others. It is therefore difficult to have a single chromophore cover the entire spectrum of harvestable light, making it necessary to find innovative ways to broaden the absorption spectra of OPVs to further increase efficiencies. There are two basic approaches that have been reported for achieving broadband coverage of the solar spectrum with an OPV. Tandem OPVs that use complementary donor and/or acceptor materials in the separate devices of the tandem device, contribute to provide broad spectral coverage.5 The tandem approach, however, involves a somewhat complex device architecture and current-matching between the cells to achieve high efficiency. A simpler approach is to build OPVs with multiple complimentary donors or acceptors in a single junction OPV. Ternary OPVs have been demonstrated to give good absorption from the UV into the


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NIR.6 Ternary devices have been reported in which polymeric,7-9 molecular10-13 or nanoparticle14-16 sensitizers were incorporated into the donor or acceptor regions of an OPV, leading to enhanced light absorption. In these ternary devices, a sensitizer typically transfers energy to the donor or acceptor for ultimate charge transfer and separation at the D/A interface, so the active wavelength range for the ternary device is the sum of those for the sensitizer, donor and acceptor. In this report we investigate the use of two intense chromophores, a porphyrin and a borondipyrromethene (BODIPY), covalently coupled in a multi-chromophoric array, giving broadband absorption. We have shown that the exciton in this array is rapidly equilibrated between the porphyrin and pendent BODIPY moieties, with roughly equal population on both.17 Here we contrast the performance of the array with a physical mixture of the same two chromophores. Porphyrins and closely related materials are ubiquitous in nature for light absorption and have been incorporated into modern small molecule OPVs.18-20 These compounds have two intense absorption peaks, a strong but narrow absorption in the blue called the Soret band and a second absorption in the red called the Q band (figure 1(c)). Devices using PtTPBP have been made and characteristic J-V and EQE spectra of these devices are shown in figure 1(b).21 The devices perform well, but the gap in absorptivity negatively impacts the JSC. The spectral responsivity of this device illustrates the problem of using a porphyrin, which results in low conversion efficiencies between 500 and 600 nm, figure 1(c). Were it not for the strong C60 absorbance between 400-500 nm, the spectral response would be even worse than shown here.



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BODIPYs are promising dyes that have recently been incorporated into OPVs with much success due to their desirable electronic properties and a high molar absorptivity.22-24 The standard BODIPY absorption falls directly between the Soret and Q-bands of the PtTPBP (figure 1(c)). These complementary absorption bands provide an opportunity to make a multi-chromophoric array with a broad absorption profile and increase the photocurrent and power conversion efficiency relative to the PtTPBP based OPV. To this end, these moieties were covalently bound together making BODIPY-PtTPBP (BDP-Por), shown in figure 1(a).25 The absorption spectrum of the array is nearly identical to the arithmetic sum of the absorptions of the separate PtTPBP and BDP chromophores, indicating that there is poor ground Figure 1 (a) The structures of the porphyrins used here are shown. (b) The absorption spectra of neat PtTPBP and C60 films are plotted against the EQE of the device made from these materials. (c) The absorbance spectra of PtTPBP, BDP1, and the arithmetic sum of these two plotted against the absorption spectrum of BDP-Por. The composite spectra overlays nicely with the spectrum of the BDP-Por, showing that there is little ground state coupling in the multi-chromophoric array.

state coupling between the chromophores. This new molecule has a broadened absorption spectrum, with an AM1.5G photon capture percentage in solution 60% greater than that of PtTPBP alone.25 The absorption spectra of the BDP-Por film changes little from that in solution,


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with the film spectrum showing only a small red-shift of both the BODIPY absorption (23 nm) peak and the Q-band of the porphyrin (13 nm). Additionally, the film spectral features are slightly broadened relative to those in the solution spectrum, which is attributed to increased π-orbital overlap in the film. In many multi-chromophoric arrays, energy is collected by peripheral (shell) chromophores and funneled to a central core in a unidirectional manner,26-31 thereby isolating the exciton from its nearest neighbors. BDP-Por has a structure similar to other core-shell arrays, but the distribution of the exciton is very different. After excitation, intersystem crossing at the platinum porphryin forms a triplet exciton on the picosecond time scale. The triplet then equilibrates between the porphyrin (T1 = 1.62 eV) and the nearly degenerate BODIPY triplet (T1 = 1.64 eV) with a Keq of 0.61.25 Thus the exciton is evenly distributed over the both the core and shell chromophores. In the present report we discuss the use of the BDP-Por array as a donor layer in OPVs and contrast the properties of those devices with analogous OPVs prepared with donor layers consisting of a physical mixture of PtTPBP and BDP. The array and mixed donor based OPVs give strong response from both the porphyrin and BDP, but the mixed donor layer based device is unstable to illumination. Experimental Materials Synthesis and Characterization. All chemicals were purchased from SigmaAldrich unless otherwise noted and used without further purification. Dry solvents were purified using a Glass Contour Solvent System, and all reactions were performed under inert nitrogen atmosphere. Thin-film absorption spectra were obtained from a spin-cast film of the respective donor material on quartz. UV−vis spectra were recorded on a Hewlett-Packard 4853 Diode Array Spectrometer. Cyclic voltammetry (CV) measurements were performed using an



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EG&G Potentiostat/Galvanostat model 283. Samples were run in 0.1 M tetra-n-butyl-ammonium hexafluorophosphate solution in acetonitrile and purged with nitrogen. The counter, reference, and working electrodes were platinum, silver, and glassy carbon, respectively. Scans were performed at 100 mV/s, and oxidation/reduction values were calibrated to ferrocene/ferrocenium internal references. Both BDP2 and BDP-Por give fully reversible cyclovoltametric oxidation and reduction waves. All NMR spectra were recorded on a Varian 400 MHz spectrometer and processed using MestReNova software. CHN analyses were performed using 2-3 mg samples in tin foil on a Thermo Scientific Flash 2000 CHNS Analyzer equipped with an 18 mm OD Empty Quartz Reactor. Samples were analyzed against sulfanilamide and 2,5-Bis (5-tert-butylbenzoxazol-2-yl) thiophene standards. 10,10'-(1,3-phenylene)bis(5,5-difluoro-5H-dipyrrolo[1,2-c:2',1'-f][1,3,2]diazaborinin-4-ium-5uide) (BDP2). Isophthaloyl dichloride (1 g, 4.95 mmol) was dissolved in dry dichloromethane (80 ml) under N2. Four equivalents of 2,4-Dimethyl-3-ethylpyrrole (2.43 g, 19.7 mmol) were added and the flask was fitted with a condenser and refluxed for 3 to 4 hrs. N,NDiisopropylethylamine (6.87 ml, 39.4 mmol) was added at reflux. After 15 minutes, the mixture was cooled to room temperature and boron trifluoride etherate (5.59g, 39.4 mmol) was added in one portion. After one hour, the reaction was quenched with saturated Na2S2O3 (50 mL), washed with saturated NaHCO3 (2 × 50 mL) and water (2 × 50 mL). The organic layer was removed, dried over MgSO4, filtered and concentrated. The product was purified by re-crystallizing from DCM and MeOH to give 1.73 g (yield = 68%). Final purification was accomplished by and sublimation in a three zone oven with temperatures of 300°C, 275°C and 250°C. 1H NMR (400 MHz, CDCl3): δ 7.61 (t, 1H), 7.54 (d, 1H), 7.40 (s, 1H), 2.53 (s, 12H), 2.31 (q, 8H), 1.51 (s, 12H), 1.00 (t, 12H). 13C NMR (400 MHz, CDCl3): δ 154.74, 139.65, 138.14, 136.36, 133.76,


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131.97, 131.28, 128.90, 53.89, 17.63, 14.93, 13.10. Excitation λmax (CH2Cl2) 529 nm Emission λmax (CH2Cl2) 553 nm. MALDI m/z for C40H48B2F2N4 Calculated 682.4 Found 682.7. CHN analysis: 69.88% C 7.28% H 8.32% N, theoretical: 70.4% C 7.09% H 8.21% N Tetra(4,4-difluoro-3,5-dimethyl-4-bora-3a,4a-diaza-s-indacene) ethylene-bridged hexenoporphyrin The starting ethylene bridged pyrrole, 4,7-dihydro-4,7-ethano-2H-isoindole, was synthesized according to literature procedure32. The ethylene bridged hexenoporphyrin was made by following literature procedures,33 yielding a mixture of four diastereomers after recrystallization with dichloromethane/diethyl ether. The diastereomers were not separated and the mixture was used in the next step. 1H-NMR (400 MHz, CDCl3), all four diastereomers, δ 8.83– 8.70 (m, 8H), 8.21 (d, 8H, J = 8.00 Hz), 7.11–7.02 (m, 8H), 6.92–6.80 (m, 8H), 6.48 (d, 8H, J = 8 Hz), 3.74–3.51 (m, 8H), 2.78 (s, 24H), 2.73–2.57 (m, 8H), 1.44 (d, 4H, J = 8 Hz), 0.85 (d, 4H, J = 8 Hz). Platinum tetra-BODIPY Beznoporphyrin (BDP-Por) Platinum (II) chloride (90 mg, 0.338 mmol) was added to dry, degassed benzonitrile (100 mL), and the mixture was heated while stirring under N2 at 100 ˚C for 20 minutes, until the platinum salts dissolved, turning the solution yellow. The BODIPY ethylene bridged hexenoporphyrin (90 mg, 0.047 mmol) was added as a solid and the solution was heated to reflux with stirring for 3h 20 min, until product peaks ceased to increase in intensity as monitored via UV-Vis. The reaction mixture was cooled to 0 ˚C, and the solvent was removed by vacuum distillation at 70 ˚C. The solid residues were dissolved in CH2Cl2 and filtered to remove solids and the filtrate was dried via rotavap to yield a bright green solid, which was filtered and washed with MeOH (3x 10 mL). The solids were purified by flash column chromatography on silica, using a gradient from 1:1 Hexanes/CH2Cl2 to 97.5:2.5 CH2Cl2 /acetone, and the pure product was recovered as an olive green solution, which was further



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purified by recrystallization from CH2Cl2/diethyl ether (36 mg, 41%). The 1H and 13C NMR spectra matched literature spectra: 1H-NMR (400 MHz, CDCl3)25 δ 8.55 (d, 8H, J = 8 Hz), 8.05 (d, 8H, J = 8 Hz), 7.44–7.31 (m, 8H), 7.24-7.26 (m, 8H), 7.17 (d, 8H, J = 4 Hz), 6.5 (d, 8H, J = 4 Hz), 2.79 (s, 24H).

13

C NMR (400 MHz, CDCl3) δ 158.29, 143.33, 141.89, 137.46, 134.94,

134.70, 134.43, 131.11, 130.09, 126.19, 124.30, 119.91, 115.07, 15.07. OPV Preparation and Testing. BDP1 was synthesized according to literature procedure.34 C60 (MTR Unlimited), 2,9-dimethyl-1-4,7-diphenyl-1,10-phenanthroline (BCP) (Aldrich), were purified by thermal gradient sublimation in vacuum prior to use. Aluminum (99.999% pure, Alfa Aesar) was used as received and evaporated through a shadow mask to form 2 mm width striped cathodes. Photovoltaic cells were fabricated on patterned indium tin oxide (ITO)-coated glass substrates that were solvent cleaned and baked with UV ozone for 10 minutes. Films of BDP-Por, Por+BDP1, and Por+BDP2, were made using a spin-coater operated at 4000 rpm for 40 seconds. The remaining materials were grown by vacuum thermal evaporation at the following rates: C60 (2 Ås-1), BCP (1 Ås-1), and Al (2 Ås-1). Current-voltage characteristics of the cells were measured in the dark and under simulated AM1.5G solar illumination conditions (Oriel Instruments) using a Keithley 2420 3A Source Meter. Incident power was adjusted using a calibrated Si photodiode to match 1 sun intensity (100 mWcm-2), and spectral response was measured using a Newport-Oriel monochromatic light source. Spectral mismatch was calculated and used to correct the measured efficiencies following standard procedures.35 Results and Discussion We have reported the synthesis of BDP-Por previously, but have prepared it by a different route in the present study. The 4,5,6,7-tetrahydroisoindole starting material utilized in the reported method was replaced with a bicyclic “masked” derivative, 4,7-dihydro-4,7-ethano-


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2H-isoindole, according to the simplified procedure for the synthesis of benzoporphyrins proposed by Ono.36 This synthetic route presents two advantages; the first is that the masked porphyrin has increased solubility in organic solvents and is more readily purified by column chromatography. The second is that oxidation of the benzene rings occurs readily via a retroDiels-Alder mechanism at 200°C, which is the temperature of reflux during the platination step. The platination and oxidation steps, previously separate, are now combined as shown in Scheme 1 into a single synthetic step, the overall yield of which is four-fold higher than as obtained by the earlier two-step method.

Scheme 1. BDP-Por is not stable to vacuum thermal evaporation, so devices of composition ITO/BDP-Por(X nm)/C60(40 nm)/BCP (10nm)/Al were made with the BDP-Por layer deposited by spin coating from a chloroform solution in concentrations of 1, 2, and 3 mg/mL. Films were annealed for 10 minutes at 80°C under nitrogen to drive off residual solvent. These three concentrations produced films of thicknesses 8.8 nm, 12 nm and 23 nm. The C60, BCP, and Al layers were deposited by vacuum thermal deposition. The J-V curves from these devices are shown on figure 2(a). The device with the thinnest donor layer (8.8 nm) was the most efficient,



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with a power conversion efficiency of 1.42%. Increasing the thickness does not increase the short circuit current (JSC) despite elevated light absorption, suggesting that the exciton diffusion length (LD) of BDP-Por is on the order of 8 nm, similar to that of PtTPBP.21 The BDP-Por OPV compares favorably to the PtTPBP device (figure 2(a)), achieving the primary objective by increasing absorption of BDP-Por, consequently raising the photocurrent (JSC of 3.84±0.81 vs. 2.47±0.05 mA/cm2). Error bars are calculated from the averaging over several devices. The BDP-Por devices have higher error bars than Por devices due to the greater thickness variation caused by the spin coating process, as opposed to VTE deposition. The EQE for these devices shows that the enhanced current density is indeed due to an improved response in the spectral region covered by the BODIPY unit which absorbs to 550 nm (figure 2(b)). The fill factor is also comparable between BDP-Por and PtTPBP, though it decreases as the BDP-Por thickness increases. The VOC is unchanged in the BDP-Por device (VOC of 0.66±0.03 vs. 0.64±0.01 V), indicating that the porphyrin unit still acts as the electron donor in the CT process and that its energy levels have not been shifted by the substitution.37, 38


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Since the BDP and porphyrin units are not electronically coupled, an important question is if a simple mixture of BODIPY and PtTPBP would give similar properties to BDP-Por in an OPV. To answer this question, blended films of varying ratios of PtTPBP and the BODIPY itself (BDP1 figure 3(a))39 were prepared. Unfortunately, BDP1 is too volatile and is lost from the codeposited film under vacuum, which is required for deposition of the C60, BCP and Al layers of the device. Roughly half of the BDP1 is lost from the film in the deposition chamber. Figure 2 (a) J-V curves for BDP-Por at several thicknesses against that of a solution processed 15 nm PtTPBP device. It is obvious that the increased absorption has led to an increase in JSC. This can be seen in the EQE plot (b) where there is enhanced spectral response past 550 nm due to the BODIPY absorption.

To address the volatility problem of BDP1, we prepared a phenyl linked BODIPY compound 10,10'-(1,3phenylene)bis(2,8-diethyl-5,5-difluoro1,3,7,9-tetramethyl-5H-dipyrrolo[1,2-c:2',1'-

f][1,3,2]diazaborinin-4-ium-5-uide) (BDP2, figure 3(a)), that has nearly double the molecular weight and thus has a much lower volatility. The two BODIPY units of BDP2 show no interaction in their ground states, leading to absorption spectra that are unchanged from BDP1. In addition, the fluorescence spectra and redox properties are unchanged between BDP1 and BDP2. Blended films of PtTPBP and BDP2 in ratios 1:1, 1:2, and 1:3 were prepared by spin



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coating chloroform solutions. As BDP2 contains two BODIPY units, its molar absorptivity is doubled. The absorption spectra of the Por+BDP2 films were indistinguishable from the Por+BDP1 films, given the corresponding chromophore ratios. BDP2 has sufficiently high molecular weight such that there was no detectable loss of BDP2 from the Por+BDP2 film after extensive exposure to high vacuum. Having made a stable blended film of similar composition to the covalently bound chromophore array, we turned to make OPVs with the same structure as the BDP-Por OPVs. Devices with the three different blended donor layers and a reference device with a BDP2 donor layer were prepared. The J-V curves for these devices are shown in figure 3(b). OPVs with the 1:2 Por+BDP2 blend, i.e. the same chromophore ratio of BODIPY to Figure 3 (a) The structures of BDP1 and BDP2. (b) J-V curves of devices made with donor layers of Por+BDP2 show that the 1:2 ratio generates the best performance. (c) The EQE spectra of the same devices shows a shoulder grow in with increasing BDP2 concentration.

porphyrin as in BDP-Por, gave the best device performance (PCE of 1.33%). The performances of the devices with the different ratios do not differ significantly, as the JSC and VOC remain

fairly constant with the increasing BDP2 content. The primary difference between devices made


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from different Por:BDP2 ratios is seen in the FF of the devices, which maximize at the 1:2 Por to BDP2 ratio. The effect of the increased BDP2 content on the quantum efficiency is seen in a proportional enhancement in the response from the 500 to 550 nm. When comparing these blended donor films to the devices made with the multi-chromophoric array, the JSC, FF, and the general shape of the EQE curve are comparable as expected given their very similar absorption patterns. However the VOC values measured in the devices with a blended donor layer were markedly lower than those with the array (0.57±0.03 V vs. 0.66±0.03 , respectively) The BDP2 donor OPV gives a lower JSC and VOC than either the blended or BDP-Por donor layer. While a lower JSC was expected for the BDP2 based OPV, due to the lack of the porphyrin absorber, the lower VOC was not. The oxidation potential for BDP2 is 0.68 V (vs. Fc/Fc+) while that of BDP-Por is 0.45 V (vs. Fc/Fc+), similar to the oxidation potential reported for PtTPBP of 0.4 V (vs. Fc/Fc+)40. Based on the 230 mV larger energy difference between the donor-HOMO and C60-LUMO (∆EDA) for the BDP2 OPV, we would have expected a higher VOC for that device. A similar situation was reported when comparing copper-phthalocyanine and PtTPBP based devices, which was attributed to steric interactions in PtTPBP hindering close approach of the donor and C60.37 BDP2 can associate directly with the C60 through the face of the dipyrrin moiety, while the structure of BDP-Por allows only an edge-on interaction of the BODIPY moiety with C60. We expect the different D/A structures to influence the VOC as observed here. J-V curves for a set of Por+BDP2 devices are shown in figure 4(a). For this set of data, the dark curve was measured before illumination. The first light scan (0 sec) was illuminated under 1 sun intensity for ca. 2 seconds, the time it takes to complete the voltage sweep. When this scan was repeated after further illumination, the VOC of the devices decreased. The dark



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curve remains stable for multiple voltage scans as long as no light is cast on the device. It is typically the case that the light and dark curves will converge at high forward biases, assuming that there isn’t substantial photoconductivity.41-43 Upon initial illumination, the Por+BDP2 device produced a J-V curve, with a VOC of 0.58 V, which clearly does not converge with the dark curve

Figure 4 The J-V curves of an ITO/Por+BDP2/C60/BCP//Al device after varying periods of illumination shows how the VOC starts around 0.64V and eventually stabilizes at 0.36 V.

at high potentials (figure 4(a) red curves), indicating that some change has already occurred in the 2 seconds it takes to measure the J-V curve. Measuring a new dark scan after illumination produces a curve that is consistent with the preceding light curve, converging at high forward bias (compare blue dark and red light curves in figure 4(a)). Further illumination of the device produces J-V curves with progressively lower VOC’s, but leaves the JSC and FF parameters unchanged. These devices ultimately stabilize after about 1 minute of illumination at a VOC of 0.36 V. The instability of the Por+BDP2 devices under illumination occurs when the devices are tested under an inert atmosphere, ruling out any oxidation pathway as an explanation for the gradual loss in VOC. The identity of the donor and acceptor,37, 38, 44-47 as well as the structure at the D/A interface,20, 48-52 have been shown to be the primary factors determining the VOC values observed for OPVs. These two factors are manifested in a clear correlation between the VOC and the energy of the charge transfer state (ECT) between the donor and acceptor materials in the OPV.46, 51, 52

As the VOC is characteristic of a particular material system, one can use the VOC of a device


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to give insight into the molecular D/A pair at the interface of the device when more than one donor or acceptor is present.53-56 The BDP-Por devices and the initial scans of the Por+BDP2 devices give VOC values very close to the one observed for an OPV with a neat PtTPBP donor layer, suggesting that the devices have a similar CT state, involving a PtTPBP-C60 pair at the interface. After illumination the VOC values for the mixed donor Por+BDP2 devices stabilize at 0.36 V, very close to that of the BDP2/C60 OPV. The similarity of the photochemically aged Por+BDP2 and BDP2 only OPVs suggests that they both have the same ECT, namely one based on a BDP2-C60 CT state. Illumination of the Por+BDP2/C60 device appears to have led to a morphological change in the device in which BDP2 concentrates at the D/A interface, and thus dominates the CT process. The films of Por+BDP2 were investigated before and after illumination by GIXRD but show no crystallization and no discernible difference between the array and the blended film, indicating that both films are amorphous, and any domains are too small to be detected by diffraction. Summarizing these results, we have determined that when a film composed of both Por and BDP is intimately intermixed, either as an array or blended film, the principal D/A pair that contributes to the CT process at the D/A interface is Por/C60. This is not surprising as it mirrors the emission of these films, where energy is dispersed over both chromophores but only a porphyrin signal is detected due to the competition of emission rates.25 However, after the morphological changes that are induced during illumination, the blended films appear to phase segregate in such a way that BDP2 concentrates at the D/A interface, giving rise to a VOC shift based on the BDP2/C60 CT state. However, despite the phorphyrin moiety being moved from the interface, light is collected by both moieties, because the exciton is still in a dynamic equilibrium



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between PtTPBP and BDP2. This leads to the reduced VOC but leaves the other device parameters unchanged. Conclusions Broadening the spectral response of OPVs is a primary challenge that needs to be overcome. It has been shown here that the inclusion of multiple chromophores with complementary absorptions in one or more of the photoactive layers can help extend the spectral response of devices, provided that the energetics are correctly designed as to not introduce charge or exciton traps. This works by both creating a multi-chromophoric array or by simple blending of two separate chromophores. It was found here that both methods can enhance the performance over the neat device; however, the multi-chromophoric array here produces a more stable device. The covalently bonded chromophores in the array form a stable, intimately intermixed film, while the blended donor layer films are only intimately intermixed at casting and appear to reorganize under illumination to give a high concentration of BDP at the D/A interface, leading to a diminished VOC. The light induced phase segregation results in a VOC similar to that produced by a device with a neat layer of BDP2 as the donor layer, indicating that it has phase segregated to the interface. In this way the multi-chromophoric array can hold an advantage to blended films by producing an inherently more morphologically stable film, though it is more difficult to synthesize. Acknowledgement: The authors would like to thank the Nanoflex Power Corporation for funding this work.


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Implications of Multichromophoric Arrays in Organic Photovoltaics

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