Observation of Triplet Exciton Formation in a ... - ACS Publications

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Observation of Triplet Exciton Formation in a Platinum-Sensitized Organic Photovoltaic Device Sean T. Roberts, Cody W. Schlenker, Vincent Barlier, R. Eric McAnally, Yuyuan Zhang, Joseph N. Mastron,† Mark E. Thompson, and Stephen E. Bradforth* Department of Chemistry, University of Southern California, Los Angeles, California 90089, United States

ABSTRACT Organic photovoltaics (OPVs) constitute a promising new technology due to their low production costs. However, OPV efficiencies remain low because excitons typically diffuse only ∼5-20 nm during their lifetime, limiting the effective thickness of the light-absorbing layer. One strategy to improve OPVs is to increase exciton lifetimes by converting them into triplet states, which typically persist 103-105 times longer than singlet excitons. We present femtosecond transient absorption and steady-state photovoltaic measurements of a model OPV system consisting of diphenyltetracene (DPT) films doped with platinum tetraphenylbenzoporphyrin (Pt(TPBP)). Photoexcitation of Pt(TPBP) creates a singlet excitation that rapidly intersystem crosses to a triplet state before transferring to the DPT host matrix. This transfer is rapid and efficient, occurring in 35 ps with an 85% conversion ratio of porphyrin singlets to DPT triplets. These triplet excitons lead to enhanced photocurrent response that increases with device thickness. SECTION Energy Conversion and Storage rganic photovoltaics (OPVs) represent a promising new technology due to their low manufacturing costs, high absorptivities, and ability to be fabricated on flexible substrates.1-3 The active layers of these devices typically consist of a mixture of electron-donating (D) and electron-accepting materials (A). Light absorption leads to the formation of a singlet exciton that must diffuse to a D/A interface to undergo charge separation. Unfortunately, OPV active layers are often disordered, resulting in small exciton diffusion lengths (LD) on the order of 5-20 nm.4,5 In contrast, active layer thicknesses of 100-200 nm are often needed to ensure full capture of the incident solar flux,6 and small values of LD are commonly identified as a bottleneck that limits OPV power conversion efficiencies. One strategy for improved OPV performance is to increase exciton lifetimes, allowing them more time to diffuse to a D/A interface before they deactivate. This can be achieved through the exchange of singlet excitons, which typically have excitedstate lifetimes in the range of hundreds of picoseconds to tens of nanoseconds, for triplet excitons that have much longer lifetimes (μs-ms).7 However, while singlet excitons diffuse via a long-ranged F€ orster mechanism, triplet excited states often diffuse via Dexter energy transfer, which has an exponential distance scaling.8 This can result in lower diffusivities for triplet excitons compared to singlets, potentially negating enhancements in LD that result from the increased triplet exciton lifetime. While experiments probing triplet diffusion in crystalline anthracene9-11 and rubrene12 have suggested LD values of a few to tens of micrometers, how triplets behave in disordered systems is less well characterized. Likewise, given the lower energy of triplet excitons compared

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to that of singlets, it is not clear whether triplets will have enough energy to charge separate at a D/A interface. Given these observations, experiments that uniquely probe the behavior of triplet excitons are needed. Unfortunately, few model systems exist to exclusively probe triplet excitons. Triplet formation via direct intersystem crossing (ISC) has been observed in OPV materials, but with variable yields. While some commonly employed OPV materials display relatively high ISC yields, notably, C60, which forms triplets because its size allows for a reduction in electron-electron repulsion,13,14 and tetracene, which undergoes ISC due to the near-isoenergetic nature of its S1 and T2 states,15 triplet formation via ISC is often slow in many hydrocarbon materials (ns-μs).16 This allows other relaxation mechanisms to compete with ISC, lowering triplet yields. Triplet exciton formation can also occur through singlet fission, a process wherein a singlet exciton splits into two separate triplet excitons located on neighboring molecules.17-19 Singlet fission has been observed in conjugated acenes,20-23 carotenoids,24 and other conjugated materials.19 However, the efficiency of singlet fission is often morphology-dependent, occurring with a higher efficiency in single crystals with respect to amorphous films of the same material.20 Also, because the two triplet excitons created by singlet fission are close to one another spatially, the role of annihilation in lowering the overall triplet yield is unclear.25 Received Date: November 15, 2010 Accepted Date: December 15, 2010 Published on Web Date: December 21, 2010

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DOI: 10.1021/jz101554m |J. Phys. Chem. Lett. 2011, 2, 48–54

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In contrast to many organic materials, transition-metal complexes, such as metal-coordinating phthalocyanines and porphyrins, often exhibit high ISC yields26,27 due to strong spin-orbit coupling.16 This opens the possibility for the exclusive creation of triplet excitons in complementary hostguest systems where singlet excitations can pass to a guest transition-metal complex that rapidly intersystem crosses, forming triplet excitons that transfer back to the host. Such a system has the added benefit that a guest molecule whose absorption spectrum complements that of the host can be selected, enabling the collection of photons in spectral regions where absorption by the host is lacking.28 We have recently demonstrated a similar intramolecular energy-transfer scheme, but here, we show that this approach is generalizable to films of blended materials.29 In this Letter, we present the results of ultrafast transient absorption (TA) and steady-state photocurrent response measurements for a model host-guest architecture consisting of platinum tetraphenylbenzoporphyrin (Pt(TPBP)) blended into a film of diphenyltetracene (DPT). While previous reports have demonstrated improved OPV performance following the introduction of platinum30,31 or iridium32 based guest materials, these systems exhibit either limited solar photon capture or overlapping spectral features that complicate analysis of triplet exciton contributions to the device performance. The purpose of the present study is to define a model material system for which singlet and triplet contributions to the system's optoelectronic response can be selectively probed. The structures and absorption spectra of DPTand Pt(TPBP) appear in Figure 1 with an illustration of possible energytransfer pathways in blended films of these materials. DPT was selected as a host matrix because, like tetracene, it exhibits a strong absorption band between 400 and 500 nm. However, in contrast to tetracene, frustrated DPT crystal growth renders vapor-deposited films relatively insensitive to processing conditions, such as impurity (guest) concentration, deposition rate, and substrate passivation. Additionally, triplet formation via ISC is suppressed in DPT due to an increased T2-S1 energy gap relative to tetracene.15,33 By suppressing native triplet formation in DPT, we can cleanly monitor triplets that result from sensitization by Pt(TPBP). Pt(TPBP) was chosen as a sensitizer because Pt porphyrins are known to undergo rapid (picosecond) ISC.34 Excitation of Pt(TPBP) in solution results in an intense phosphoresce at 769 nm with little fluorescence emission.35,36 Due to the larger T1-S0 energy gap of Pt(TPBP) (E(T1) - E(S0)=1.6 eV)37 relative to that for DPT (E(T1) - E(S0) = 1.2 eV),15 triplet energy transfer from Pt(TPBP) to DPT is energetically favorable. Additionally, the Q band of Pt(TPBP) exhibits an intense absorption at 618 nm, where DPT is transparent. The presence of a unique Pt(TPBP) absorption feature both extends the spectral range over which solar photons are collected and allows us to selectively excite Pt(TPBP) to study energy transfer between it and DPT. Figure 2 plots TA spectra of a vapor-deposited film of 20% Pt(TPBP) in a matrix of tetra(9,90 -dimethylfluoren-2-yl)silane (TFS). Because TFS's T1 state (-2.9 eV) is higher in energy than that of Pt(TPBP), this experiment allows us to characterize the Pt(TPBP) excited-state absorption and the kinetics


Figure 1. (Top and Middle) Structures of platinum tetraphenylbenzoporphyrin (Pt(TPBP)) and diphenyltetracene (DPT) are displayed above a cartoon illustrating each of the absorption and energy-transfer processes that can lead to DPT triplet formation. (Bottom) Absorption spectra of a neat DPT film (red dotted), a film of Pt(TPBP) doped into a wide-band-gap matrix (green dashdotted), and a DPT film containing 5% Pt(TPBP) (black).

of its triplet-state formation. Pumping Pt(TPBP) at 618 nm leads to an immediate photobleach of its Q (618 nm) and Soret (435 nm) bands, indicating a depopulation of the Pt(TPBP) ground state (Figure 2A). In the spectral range between these two transitions, an induced absorption is observed whose shape evolves with time. Figure 2B highlights the spectral evolution during the first 10 ps after photoexcitation and reveals the presence of an isosbestic point at 515 nm. Beyond 10 ps, little evolution of the Pt(TPBP) signal occurs within our experimental time window of 750 ps. Given the TA signal's lack of change after a 10 ps delay and the similarity of the signal at this delay to the reported triplet spectrum of Pt(TPBP) measured using nanosecond TA experiments,38 we conclude that the isosbestic point observed at early delays is due to ISC between the S1 and T1 states of Pt(TPBP). Fitting of the decay traces using a two-state kinetic model allows us to determine the ISC rate, 1/kISC = 400 fs.

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quantified its cross section.15 This state absorbs strongly between 450 and 500 nm, with a peak at 490 nm. This suggests that porphyrin ground-state repopulation occurs via triplet energy transfer to DPT. To confirm this hypothesis, in Figure 3C, we plot the TA signal obtained for 5% Pt(TPBP) in DPT averaged over time delays between 600 and 750 ps. By 600 ps, the transient spectrum has stopped evolving, suggesting that energy transfer between Pt(TPBP) and DPT is largely complete. Contributions due to the Pt(TPBP) T1 state TA are removed from this integrated signal through scaled subtraction of the Pt(TPBP) transient signal in TFS. The residual TA spectrum is overlaid with the DPT T1 line shape39 expected based on the extinction spectra of ref 15 but red-shifted by 2 nm to account for thinfilm stabilization relative to solution. The agreement between the predicted line shape and the residual TA spectra is excellent, matching well the relative scaling between the peaks at 490 and 450 nm, and supports our assignment of this new TA feature to the formation of DPT triplet excitons. With this spectral assignment in hand, we have fit the TA spectra in Figure 3 using a three-state kinetic model kISC

kTT

S1, PtðTPBPÞ sf T1, PtðTPBPÞ sf T1, DPT

where kISC is the ISC rate of Pt(TPBP) and kTT is the rate of triplet energy transfer from Pt(TPBP) to DPT. Similar to the data in Figure 2, we find that the optimum value of 1/kISC is 400 fs, while the best fit for 1/kTT is 35 ps. This triplet energytransfer rate is rapid compared to that observed in hydrogenbonded and covalently linked porphyrin systems, wherein triplet energy transfer occurs over hundreds of nanoseconds to microseconds.40-42 Our measured value of kTT suggests that the π-systems of Pt(TPBP) and DPTare in close, intimate contact. In addition to allowing the identification of DPT triplet exciton formation, the ratio between the DPT T1 extinction spectra of ref 15 and the TA residual in Figure 3C provides an estimate of the concentration of triplet excitons formed via Pt(TPBP) photoexcitation. Similarly, the initial concentration of excited Pt(TPBP) molecules can be determined by comparison of the initial photobleach of the porphyrin's Q band to Pt(TPBP) extinction spectra.35 Comparing these two values, we find that triplet energy transfer from Pt(TPBP) to DPT is highly efficient, with 85 ( 6% of the excitations created on Pt(TPBP) leading to the formation of DPT triplet excitons. Other platinum porphyrins absorbing in different spectral regions were also observed to serve as effective triplet sensitizers. In the Supporting Information, we describe experiments performed on DPT films doped with platinum octaethyl porphyrin (Pt(OEP)), a molecule that has previously been investigated as a potential triplet sensitizer.30,31 Photoexcitation of the Q band of Pt(OEP) first leads to rapid ISC (1/kISC = 165 fs) followed by triplet energy transfer to DPT (1/kTT = 62 ps), wherein 60 ( 3% of the singlets formed on Pt(OEP) are converted to DPT triplet excitons. To determine if triplet excitons resulting from Pt(TPBP) absorption contribute to the photocurrent in OPV devices, we analyzed the photocurrent response for glass/ITO/donor/C60 (17.5 nm)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline

Figure 2. TA spectra of a film of 20% Pt(TPBP) in TFS. (A) Spectral slices for time delays out to 10 ps. (B) A close-up of the induced absorption band between 460 and 570 nm, highlighting the presence of an isosbestic point at 515 nm. (C) Spectral slices taken between 10 and 600 ps, showing that the data display little evolution over this time period.

The results obtained for Pt(TPBP) in a noninteracting TFS film are now compared to those obtained for a DPT film doped with 5% Pt(TPBP) (Figure 3A). Following photoexcitation, a TA spectrum similar to that of Pt(TPBP) in TFS is observed. However, over the course of 200 ps, the photobleaches of both the Soret and Q bands of Pt(TPBP) decay, indicating repopulation of the porphyrin's ground state (Figure 3B). Concomitant with the photobleach relaxation, the induced absorption band between 440 and 600 nm evolves from a shape indicative of Pt(TPBP)'s T1 state (Figure 2C) into a narrower band with peaks located at 450 and 490 nm. A previous nanosecond TA study in benzene identified the absorption spectrum of DPT's T1 state and


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Figure 3. (A) TA spectra of 5% Pt(TPBP) in DPT measured following photoexcitation at 618 nm. With increasing time delay, new spectral bands appear at 450 and 490 nm. (B) Time dependence of the ground-state bleach of Pt(TPBP)'s Q band at 618 nm. When Pt(TPBP) is included in a DPT matrix, we see a refilling of the porphyrin's ground state over the course of ∼200 ps. (C) Comparison between the TA residual calculated as described in the text and the expected line shape for DPT triplet excitons.

(BCP, 10 nm)/Al (100 nm) devices, where donor is either neat DPT or DPT doped with 5% Pt(TPBP). In Figure 4A, we plot external quantum efficiency (EQE) spectra of these devices. The addition of Pt(TPBP) leads to photocurrent response at 625 nm that resembles the porphyrin's Q band absorption, demonstrating that excitation of Pt(TPBP) leads to charge generation. The DPT photoresponse (λ = 400-500 nm) also increases markedly with donor thickness for the host-guest devices, up to a thickness of 25 nm. Given that the DPT response in the 25 nm neat device is significantly lower than that in the doped device, this increase does not appear to be due to direct dissociation of DPT singlet excitons. More likely, the enhanced DPT response comes from the process highlighted in Figure 1, wherein a DPT singlet passes its energy to a nearby Pt(TPBP) molecule, leading to the formation of a DPT triplet exciton. Mechanistic analysis of this process is beyond the scope of the present study. To more clearly characterize the thickness dependence for direct excitation of Pt(TPBP), in Figure 4B, we compare the current-voltage characteristics of devices under Xe arc illumination filtered through a 600 nm long-pass filter, allowing preferential excitation of the guest material. While the neat DPT donor device exhibits nonzero photoresponse due to absorption by C60, upon addition of 5% Pt(TPBP) to a DPT layer of the same thickness, we find that the short-circuit current (JSC) more than doubles as a result of Pt(TPBP) absorption and sensitized DPT triplet diffusion to the D/A interface (see Table 1). As the host-guest layer thickness is


increased from 15 to 25 nm, we observe a corresponding increase in JSC, with virtually no change in the open-circuit voltage (VOC). Thus, triplet excitons generated throughout the device appear to generate photocurrent, regardless of their distance from the D/A interface. This result can be contrasted with the triplet exciton diffusion length of 5.7 nm measured for a neat Pt(TPBP) film36 and suggests that in our Pt(TPBP)doped devices, triplet excitons are migrating to the C60 interface via triplet energy transfer to DPT. The DPT devices exhibit photovoltages greater than 0.7 V, slightly higher than those measured for similar devices based on planar conjugated acenes, such as tetracene (0.58 V)43 and pentacene (0.35 V).44 This can be attributed in part to DPT's relatively deep HOMO energy (estimated to be -5.4 eV from electrochemistry,45 compared to -4.8 to -5.1 eV for pentacene46,47) and the steric inaccessibility of DPT's π-system, which aids in limiting back electron transfer following exciton dissociation.37,48 Interestingly, the VOC values measured for the doped and undoped DPT devices are comparable, 0.77 and 0.73 V, respectively. This suggests that dissociation of singlet and triplet DPT excitons leads to a mutual chargetransfer state, from which charge recombination or charge separation occurs. This is an encouraging result as it demonstrates that triplet excitons may be used in OPVs with no loss in photovoltage. For a host-guest layer thickness of 25 nm, a small decrease in the device's fill factor occurs. This effect likely results from hole trapping by Pt(TPBP)'s HOMO (-4.8 eV),37

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demonstrate the viability for intermolecular host-guest systems to enhance exciton collection in OPV devices. Future tuning of the energy levels of the guest to remove hole trapping effects may allow the use of thicker absorbing layers to obtain larger photocurrents. In conclusion, we have demonstrated that the addition of a triplet sensitizer to an OPV can result in the formation of longlived triplet excitons located on the host medium in high yield. In the DPT/Pt(TPBP) model system that we have investigated, this process is extremely rapid, taking place within a few tens of picoseconds, and occurs with high efficiency (85%). EQE spectra measured for devices demonstrate that absorption by Pt(TPBP) contributes to the device photoresponse. Photocurrent measurements show that these current contributions increase with device thickness, implying that triplet excitons are harvested from throughout the entire absorbing layer. Future measurements will seek to quantify both the efficiency of singlet energy transfer from the host to the sensitizer and the triplet exciton diffusion length in this system.

EXPERIMENTAL METHODS Synthesis and Photovoltaic Measurements. Pt(TPBP)35 and DPT were synthesized according to literature procedures and purified by vacuum thermal gradient sublimation (2) prior to use. C60 (99þ%) was purchased from MER Corporation, and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (bathocuproine, BCP, 96%) was purchased from Aldrich Chem. Co. Both materials were purified by sublimation prior to use (C60 1; BCP 2). The aluminum cathode metal was used as received from Aldrich Chem. Co. (99.999% pure). Photovoltaic cells were grown on 1.25 cm2 solvent-cleaned, UV-ozone-treated, 150 nm ITO-coated glass substrates with a sheet resistance of 20 ( 5 Ω/square (Thin Film Devices Inc.). All materials were deposited in a high-vacuum chamber (34 10-6 Torr) in the order Pt(TPBP)/DPT (5%)(0.2:4 Å/s), C60(1 Å/s), BCP(1 Å/s), Al(3 Å/s). The Pt(TPBP)/DPT doping ratio was controlled in situ using a calibrated dual quartz crystal microbalance assembly. Aluminum cathodes were evaporated through a shadow mask with 1 mm diameter openings. Current-voltage measurements were made using a Keithley 2420 SourceMeter and were carried out in the dark and under illumination from an Oriel Instruments Xe Arc Lamp, 600 nm long-pass filtered for Pt(TPBP) Q band illumination. Chopped illumination (250 Hz, 10 nm fwhm) from a Cornerstone 260 1/4 M monochromator (Newport 74125) coupled to a 300 W Xe arc lamp was used in conjunction with an EG&G 7220 DSP lock-in amplifier and calibrated (NREL) silicon photodiode (Hamamatsu S1787-04; 8RA Filtered) to make all spectral responsivity measurements. UV-Visible Absorption Measurements. Absorption spectra were measured using a Cary 50 Conc UV-vis spectrometer in a transmission geometry. To correct for spurious signal contributions from scattered light, the baseline of each spectrum was determined by fitting the data to a quadratic or cubic function between 750 and 1000 nm, where no absorption features are observed. Femtosecond Transient Absorption. Pump and probe excitation pulses were derived from the output of a 1 kHz Ti: sapphire regenerative amplifier (Coherent Legend, 4 mJ,

Figure 4. (A) The EQE spectrum measured for a device whose electron-donating layer consists of neat DPTcompared to that for devices whose donor layer is doped with 5% Pt(TPBP). The addition of Pt(TPBP) leads to the appearance of a photoresponse at 625 nm that grows with donor layer thickness. (B) The currentvoltage characteristics of the same devices described in (A) measured under conditions designed to preferentially excite Pt(TPBP). Dark curves are shown as dashed lines. As the thickness of the Pt(TPBP) doped film is increased, we observe a gain in JSC with little drop in VOC. Table 1. Current-Voltage Parameters Measured for Glass/ITO/ Donor/C60 (17.5 nm)/BCP (10 nm)/Al (100 nm) Devices Measured under Xe Arc Illumination Filtered through a 600 nm Long-Pass Filtera donor

thickness (nm) JSC (mA/cm2) VOC (V) fill factor

DPT

25

0.030

0.83

5% Pt(TPBP)/DPT

10

0.043

0.73

0.53

5% Pt(TPBP)/DPT

15

0.047

0.76

0.53

5% Pt(TPBP)/DPT

20

0.053

0.77

0.53

5% Pt(TPBP)/DPT

25

0.066

0.77

0.50

a

0.40

Donor refers to either neat DPT or DPT doped with 5% Pt(TPBP).

which is higher in energy than that of DPT and related tetracene derivatives (-5.4 eV).49,50 Nevertheless, our results


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35 fs). Excitation pulses peaked at 618 nm were generated by using 340 μJ to pump a visible OPA (Spectra Physics OPA800C), while white light supercontinuum probe pulses (320-950 nm) were created by focusing a small amount of the amplifier output into a CaF2 plate that was slowly rotated to prevent photodamage. The supercontinuum probe was collimated and focused into the sample using a pair of off-axis aluminum parabolic mirrors, while the pump was focused to a point behind the sample using a 50 cm CaF2 lens. The crosscorrelation between the pump and probe in a 1 mm glass substrate had a fwhm of 180 fs. All presented measurements were performed with perpendicular pump and probe polarizations. This allowed excess scatter from the pump to be suppressed by passing the probe through an analyzing polarizer after the sample prior to detection. A spectrograph (Oriel MS127I) dispersed the probe onto a 256 pixel silicon diode array (Hamamatsu), allowing multiplex detection of the probe as a function of wavelength. The samples consisted of vapor-deposited films sandwiched between two 1-2 mm thick quartz plates. The outside edges of these plates were sealed with epoxy within the confines of a N2 glovebox to prevent exposure of the samples to oxygen. The thickness of each sample was chosen such that the peak optical density of the Pt(TPBP) Q band was near 0.1. The spot size of the pump at the sample had a fwhm of 370 μm, and data were measured for a variety of pump energies between 100 and 970 nJ. Over this range, the signal was found to scale linearly with pump power. The data in Figure 2 and 3 were measured using pump energies of 500 and 800 nJ, respectively, and have been adjusted to account for the dispersion of the supercontinuum probe.

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SUPPORTING INFORMATION AVAILABLE A detailed description of our kinetic fitting model and a comparison between TA spectra of TFS films containing 5 and 20% Pt(TPBP) are presented. Also included are absorption and TA spectra of TFS and DPT films doped with Pt(OEP). This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION (16)

Corresponding Author: *To whom correspondence should be addressed.

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Present Addresses: †

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Cerritos College, 11110 Alondra Blvd., Norwalk, CA 90650.

ACKNOWLEDGMENT This material is based on work supported (19)

as part of the Center for Energy Nanoscience, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (DE-SC0001011). S.T.R. would also like to thank the National Science Foundation for support from an ACC-F fellowship (CHE-0937015). J.N.M. is supported as part of a summer undergraduate research program sponsored in part by the National Science Foundation.

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Unfortunately, both the T1 and ground state of DPT absorb strongly in the range of 400-500 nm. The red trace in Figure 3C represents the sum of two contributions, an induced absorption due to DPT's T1 state, and a photobleach of DPT's ground-state absorption. These two contributions are scaled in a one-to-one ratio based on the extinction spectra of ref 15, which are reproduced in the Supporting Information. Sessler, J. L.; Wang, B.; Harriman, A. Photoinduced Energy Transfer in Associated by Noncovalently Linked Photosynthetic Model Systems. J. Am. Chem. Soc. 1995, 117, 704–714. Andr easson, J.; Kajanus, J.; Mårtensson, J.; Albinsson, B. Triplet Energy Transfer in Porphyrin Dimers: Comparison between π- and σ-Chromophore Bridged Systems. J. Am. Chem. Soc. 2000, 122, 9844–9845. Albinsson, B.; Mårtensson, J. Excitation Energy Transfer in Donor-Bridge-Acceptor Systems. Phys. Chem. Chem. Phys. 2009, 12, 7338–7351. Chu, C.-W.; Shao, Y.; Shrotriya, V.; Yang, Y. Efficient Photovoltaic Energy Conversion in Tetracene-C60 Based Heterojunctions. Appl. Phys. Lett. 2005, 86, 243506. Potscavage, W. J., Jr.; Yoo, S.; Kippelen, B. Origin of the OpenCircuit Voltage in Multilayer Heterojunction Organic Solar Cells. Appl. Phys. Lett. 2008, 93, 193308. D'Andrade, B. W.; Datta, S.; Forrest, S. R.; Djurovich, P.; Polikarpov, E.; Thompson, M. E. Relationship between the Ionization and Oxidation Potentials of Molecular Organic Semiconductors. Org. Electron. 2005, 6, 11–20. Hwang, J.; Wan, A.; Kahn, A. Energetics of Metal-Organic Interfaces: New Experiments and Assessment of the Field. Mater. Sci. Eng., R 2009, 64, 1–31. Armstrong, N. R.; Wang, W.; Alloway, D. M.; Placencia, D.; Ratcliff, E.; Brumbach, M. Organic/Organic' Heterojunctions: Organic Light Emitting Diodes and Organic Photovoltaic Devices. Macromol. Rapid Commun. 2009, 30, 717–731. Yang, L.; Zhou, H.; You, W. Quantitatively Analyzing the Influence of Side Chains on Photovoltaic Properties of Polymer-Fullerene Solar Cells. J. Phys. Chem. C 2010, 114, 16793–16800. Sato, N.; Seki, K.; Inokuchi, H. Polarization Energies of Organic Solids Determined by Ultraviolet Photoelectron Spectroscopy. J. Chem. Soc., Faraday Trans. 2 1981, 77, 1621–1633. Sato, N.; Inokuchi, H.; Silinsh, E. A. Reevaluation of Electronic Polarization Energies in Organic Molecular Crystals. Chem. Phys. 1987, 115, 269–277.


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