Letter pubs.acs.org/JPCL
Charge Generation Dynamics in CdS:P3HT Blends for Hybrid Solar Cells Ute B. Cappel,* Simon A. Dowland, Luke X. Reynolds, Stoichko Dimitrov, and Saif A. Haque* Department of Chemistry, Imperial College London, South Kensington Campus, Exhibition Road, South Kensington, United Kingdom S Supporting Information *
ABSTRACT: Development of design rules for hybrid inorganic−organic solar cells through understanding charge generation and recombination dynamics is an important pathway for the improvement of solar cell conversion efficiencies. In this Letter, we study the dynamics of charge generation in CdS:polymer blends by transient absorption spectroscopy. We show that charge generation following excitation of the inorganic component is highly efficient and can occur up to a few nanoseconds after excitation, allowing for diffusion of charges within the inorganic component to an interface. In contrast, charge generation following excitation of the organic component occurs on subpicosecond time scales but suffers from two loss processes, incomplete exciton dissociation and geminate recombination. SECTION: Energy Conversion and Storage; Energy and Charge Transport
H
smaller band gap than the organic component, exciton transfer from the polymer to the inorganic semiconductor can occur at the interface followed by hole transfer back to the polymer. Finally, light can be absorbed directly in the inorganic material, and charges can be generated through hole transfer from the inorganic nanocrystals to the polymer. Especially the kinetics of this latter process have not been widely studied, though it has been shown that hole transfer can play an important role in hybrid solar cells as well as in semiconductor sensitized solar cells.25,26 In order to develop design rules and therefore a more directed approach for future materials development in hybrid solar cells, gaining a better understanding of the charge generation and recombination processes is highly important. In this Letter, we aim to address some of these issues by studying the dynamics of charge generation in CdS:poly-3-hexylthiophene (P3HT) blends through either hole or electron transfer. The blends were fabricated by an in situ fabrication method demonstrated in our group.7 A xanthate precursor is blended with the polymer in a spin-coating solution, and sulfide nanocrystals are formed within the polymer by thermal annealing of the as-spun film. Solar cell efficiencies of more than 2% have been achieved by this method,8 and it has been shown that charge generation when exciting P3HT is improved compared to the use of quantum dots with capping ligands.19 We have recently shown that charge generation in these CdS:P3HT blends is highly composition (and morphology) dependent when exciting the polymer, while charge generation from CdS is relatively independent of morphology and
ybrid solar cells combining organic conductive polymers and inorganic nanocrystals have been developed in recent years as an alternative to all-organic solar cells.1,2 They offer the possibility of replacing fullerene derivatives, which are commonly used as acceptors in organic solar cells, with nanostructured inorganic semiconductors. Materials that have been used include wide band gap semiconductors such as ZnO3 and TiO24 and absorbing materials such as PbS,5 CdS,6−9 CdSe,10−12 and Sb2S3.13 These materials offer the advantage of higher morphological stability and higher electron mobilities compared to fullerenes and, in the case of absorbers, improved light harvesting due to higher extinction coefficients. Many studies of these systems have focused on the development of new processing methods for active layer formation, with the aim of achieving intimately mixed films of the organic and inorganic components, with good interconnection of each phase and good charge generation. Among these methods, the use of short ligands on the inorganic component,14 alternative particle shapes such as nanorods and tetrapods,11,12 and nanowires of the polymer9 has been particularly successful. Requirements for the morphology are taken from bulk heterojunction organic solar cells, where charge generation has been extensively studied15,16 and is limited by the exciton diffusion lengths of organic polymers (typically about 10 nm)17 and by geminate recombination due to charge-transfer states.18 In contrast to this, only a few studies have addressed charge generation in polymer blends with inorganic absorbing acceptors.19−23 In such systems, charge generation can occur via three different mechanisms.21,24 First, light absorption by the polymer can be followed by exciton diffusion to an interface and then electron transfer to the acceptor, that is, a very similar mechanism of charge generation to organic solar cells. Second, if the inorganic material has a © XXXX American Chemical Society
Received: November 5, 2013 Accepted: November 27, 2013
4253
dx.doi.org/10.1021/jz402382e | J. Phys. Chem. Lett. 2013, 4, 4253−4257
The Journal of Physical Chemistry Letters
Letter
therefore often more efficient.27 Here, we aim to rationalize these results by studying the charge generation dynamics in blends that yield the highest device performances though not the highest charge generation yields (1:1 volume ratio of CdS and P3HT) with femtosecond transient absorption spectroscopy (fsTAS). Figure 1a shows the absorption spectrum of the 1:1 volume ratio P3HT:CdS blend as well as the absorption spectrum of a
Charge generation from P3HT therefore has to be associated with one or more loss processes, which we will investigate in the following section. We studied the kinetics of P3HT excitons and polarons with fsTAS with a NIR probe (see the Supporting Information for details of the setup) as both species are expected to show distinctive peaks in this region.28−30 TA spectra of the CdS:P3HT blend following excitation at 550 nm are shown in Figure 2a (and in Figure S2, Supporting Information) and compared to the TA spectrum of the pristine P3HT film obtained 1 ps after excitation. The pristine P3HT spectrum shows a broad absorption peak at 1250−1300 nm, which we attribute to P3HT excitons.28,29 The 1 ps TA spectrum of the blend looks similar to this spectrum, with an additional absorbance between 900 and 1100 nm where the P3HT polaron absorbs. This indicates that polarons already form within the first picosecond after excitation. At 6 ns, the TA spectrum shows only features due to polaron absorbance, and the ΔA at wavelengths larger than 1200 nm is very close to zero. We therefore use the kinetics at 1280 nm to monitor the exciton decay in the blend and in pristine P3HT (Figure 2b). The exciton lifetime is decreased in the blend compared to pristine P3HT. Average time constants of multiexponential fits to these decays were determined to be 90 ps for pristine P3HT and 16 ps for the blend (see the Supporting Information for details). Figure 2c shows decay kinetics for CdS:P3HT blends obtained at 960 nm, both for the standard blend ratio and for a blend with a higher CdS content (2.3:1 CdS:P3HT by volume ratio) excited with the same laser intensity at 550 nm. We have previously shown that blends with higher CdS content have smaller, less crystalline P3HT domains, leading to better charge generation but worse photocurrent extraction.27 At 960 nm, both P3HT excitons and polarons are observed. In the 1:1 blend sample, the signal at early times is dominated by excitons. In contrast to this, much fewer excitons are observed in the sample with higher CdS content. At longer delay times, when all excitons have decayed, both samples show similar magnitudes of signal, suggesting that similar amounts of P3HT polarons are formed. As the ground-state absorbance of P3HT decreases with P3HT content (see the Supporting Information), this means that the polaron yield relative to the number of absorbed photons is higher in the sample with lower P3HT content. Furthermore, this suggests that the exciton decay observed in the standard blend is not associated with any additional polaron formation beyond that occurring within 1 ps of excitation (Figure 2a). Whether this is due to excitons not reaching the interface or geminate recombination competing with polaron formation is unclear from these data. We also studied the recombination of polarons as a function of laser intensity (Figure 2d). The signals at 960 nm between 200 ps and 6 ns, normalized to the laser intensity, were fitted with a single-exponential function, yielding the following time constants: 1.8 ± 0.8 ns at 3.8 μJ cm−2, 1.9 ± 0.4 ns at 11 μJ cm−2, and 1.4 ± 0.2 ns at 27 μJ cm−2. These decay constants are very similar, therefore suggesting a polaron recombination process, which is independent of charge concentration. Similar effects have been observed for blends of P3HT and PCBM and were assigned to geminate recombination of charge-transfer states.28,30 Furthermore, it has been shown that charge-transfer states also exist in ZnO/polymer and CdS:polymer-based hybrid solar cells.31,32 In particular, we have recently shown that such charge-transfer states can limit the efficiency of long-lived
Figure 1. (a) Absorption spectra of films used for TAS. Absorbance of the CdS:PS film scaled by 2. TAS excitation wavelengths are indicated by vertical lines. (b) TA spectra of the CdS:P3HT blend 1 μs after excitation with a nanosecond laser pulse at either 355 or 550 nm normalized to 1 × 1013 absorbed photons cm−2.
pristine P3HT film and a film of CdS formed through decomposition of Cd-xanthate in polystyrene (PS). It can be seen that CdS contributes significantly to the absorption of the blends below wavelengths of 500 nm. We used microsecond TAS to determine relative charge separation yields in the blend sample from excitation of either CdS or P3HT. In order to excite mostly the CdS, we chose an excitation wavelength of 355 nm (as indicated by the blue vertical line in Figure 1a). To solely excite P3HT, we chose an excitation wavelength of 550 nm (red vertical line in Figure 1a). Transient absorption spectra of the blend film obtained at 1 μs after excitation with a nanosecond laser pulse are shown in Figure 1b. The spectra are normalized to the number of photons in a laser pulse and to the fraction of absorbed photons at the excitation wavelength. The characteristic P3HT polaron absorption spectrum7 is observed for both excitation wavelengths, indicating that charge separation occurs. The signal is significantly higher when exciting at 355 nm. As the spectral intensity is directly related to the charge generation yield through the extinction coefficient of the P3HT polaron, this indicates that charge generation via hole transfer from CdS to P3HT is more efficient than charge generation through electron transfer from P3HT to CdS. 4254
dx.doi.org/10.1021/jz402382e | J. Phys. Chem. Lett. 2013, 4, 4253−4257
The Journal of Physical Chemistry Letters
Letter
Figure 2. (a) TA spectra of the CdS:P3HT blend at 1 ps and 6 ns excited at 550 nm with a laser intensity of 3.8 μJ cm−2 compared to the 1 ps TA spectrum of a pristine P3HT film obtained with a laser intensity of 4.5 μJ cm−2 scaled by 0.59. (b) Kinetics of the CdS:P3HT blend and of pristine P3HT at 1280 nm, with the same intensity and normalization as those in (a). (c) Kinetics at 960 nm of a standard CdS:P3HT blend and of a blend with a higher CdS content. (d) Kinetics at 960 nm of a standard CdS:P3HT blend at different laser intensities with single-exponential fits (dashed lines). Traces are normalized to 1 × 1013 incident photons cm−2 (3.6 μJ cm−2).
charge generation in CdS:polymer films prepared by the xanthate method.32 As such, our data can therefore be interpreted as further evidence of geminate recombination at earlier times decreasing polaron yields observed on a microsecond time scale. We now turn to charge generation from CdS. TA spectra of the 1:1 volume ratio CdS:P3HT blends excited at 355 nm are shown in a surface plot in Figure 3a. At most delay times, the largest TA signal is observed at shorter wavelengths than 1000 nm and is assigned to P3HT polarons. In the spectra up to 10 ps, an additional positive TA signal between 1200 and 1300 nm is present, which we assign to P3HT excitons, indicating that excitation of P3HT could not be fully excluded at this wavelength. However, this signal is very small compared to the magnitude of the exciton signal observed when exciting at 550 nm (Figure 2a), and the polaron signal is dominant already within the instrument response time. Therefore, there is subpicosecond hole transfer from CdS to P3HT that results in polaron formation. No signals due to charges or excitons in CdS were observed in the wavelength range probed here. This was confirmed by measuring the blend of CdS:PS (data not shown). Kinetics at 960 nm measured with different laser intensities are shown in Figure 3b. In the first few picoseconds, similar magnitudes of intensity-corrected signals are observed. These signals are due to both P3HT excitons and polarons, as described above. At low excitation intensities, the P3HT polaron signal increases further within 2 ns by up to 40% of the initial amplitude. Upon increasing the excitation intensity, the rise in polaron absorbance diminishes, and instead, the polaron signal decreases. To determine whether this decrease is due to intensitydependent polaron recombination or to intensity-dependent recombination within CdS, we assessed how the lifetime of
excitations in CdS depends on the intensity by measuring fsTAS spectra of CdS:PS blends with a visible probe. No charge transfer should occur from CdS to PS, and we were therefore only probing transient species in CdS. A negative TA signal was observed at wavelengths below 500 nm (Figure S4, Supporting Information), matching the steady-state absorption of CdS. No emission in this wavelength region was observed from the samples, and we therefore attribute this signal to a ground-state bleach of CdS absorption. This confirms that photogenerated electron and hole pairs in CdS prepared by the xanthate method separate into free charges within CdS, as we have previously suggested.27 The observed bleach is a direct measure of the depopulation of the ground state of CdS and as such of the photogenerated charges present in CdS. The kinetics of the bleach at different laser intensities monitored at 470 nm are shown in Figure 4. Average decay times decrease from 1500 to 760 ps with increasing power (see the Supporting Information for details), with some signal remaining at 6300 ps. The lifetimes of charges in CdS are much longer than the lifetimes of excitons in P3HT. Charge generation from CdS in CdS:P3HT blends can therefore occur within a few nanoseconds of excitation, even at higher laser intensities. We suggest that the ultrafast component of the hole transfer observed in Figure 3 comes from excitations occurring close to the CdS:P3HT interface, while the slower charge generation observed at low light intensities follows diffusion of holes in CdS to the interface. This slow process is no longer observed at higher light intensities as intensity-dependent polaron recombination across the CdS:P3HT interface leads to an overall decrease in P3HT polaron concentration. It is therefore important to carry out measurements at low excitation intensities in order to minimize the influence of nongeminate recombination on the measurements. 4255
dx.doi.org/10.1021/jz402382e | J. Phys. Chem. Lett. 2013, 4, 4253−4257
The Journal of Physical Chemistry Letters
Letter
transport materials only, as a design pathway for future development of hybrid solar cells. Alternatively, shortcomings of the polymer in terms of charge generation will have to be addressed in order to achieve highly efficient hybrid solar cells. Geminate recombination might be avoided through the use of highly crystalline inorganic acceptors,32 while the use of a more ordered morphology such as polymer nanowires might help to overcome the trade-off between charge generation and good charge transport present in randomly oriented morphologies.
■
ASSOCIATED CONTENT
S Supporting Information *
Details of sample preparation and of experimental setups. Model for fitting kinetic data and individual time constants. Absorption spectrum of the 2.3:1 CdS:P3HT blend. TA spectra of CdS:P3HT blends and the CdS:PS blend. This material is available free of charge via the Internet at http://pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (U.B.C.). *E-mail:
[email protected] (S.A.H.). Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This research was financially supported by a Marie Curie IntraEuropean Fellowship within the seventh European Community Framework Programme, by the Engineering and Physical Science Research Council (EPSRC) through the Supergen (EP/G031088/1) and UK−India (EP/H040218/2) programmes, and by the Royal Society.
Figure 3. (a) Surface plot of TA spectra of a CdS:P3HT blend excited at 355 nm with a laser intensity of 5.0 μJ cm−2. (b) Kinetics at 960 nm at different laser intensities. Traces are normalized to 1 × 1013 incident photons cm−2 (5.6 μJ cm−2).
■
REFERENCES
(1) Moulé, A. J.; Chang, L.; Thambidurai, C.; Vidu, R.; Stroeve, P. Hybrid Solar Cells: Basic Principles and the Role of Ligands. J. Mater. Chem. 2012, 22, 2351−2368. (2) Gao, F.; Ren, S.; Wang, J. The Renaissance of Hybrid Solar Cells: Progresses, Challenges, and Perspectives. Energy Environ. Sci. 2013, 6, 2020−2040. (3) Oosterhout, S. D.; Koster, L. J. A.; Van Bavel, S. S.; Loos, J.; Stenzel, O.; Thiedmann, R.; Schmidt, V.; Campo, B.; Cleij, T. J.; Lutzen, L.; et al. Controlling the Morphology and Efficiency of Hybrid ZnO:Polythiophene Solar Cells via Side Chain Functionalization. Adv. Energy Mater. 2011, 1, 90−96. (4) Van Hal, P. A.; Wienk, M. M.; Kroon, J. M.; Verhees, W. J. H.; Slooff, L. H.; Van Gennip, W. J. H.; Jonkheijm, P.; Janssen, R. A. J. Photoinduced Electron Transfer and Photovoltaic Response of a MDMO-PPV:TiO2 Bulk-Heterojunction. Adv. Mater. 2003, 15, 118− 121. (5) Strein, E.; Colbert, A.; Subramaniyan, S.; Nagaoka, H.; Schlenker, C. W.; Janke, E.; Jenekhe, S. A.; Ginger, D. S. Charge Generation and Energy Transfer in Hybrid Polymer/infrared Quantum Dot Solar Cells. Energy Environ. Sci. 2013, 6, 769−775. (6) Wang, L.; Liu, Y.; Jiang, X.; Qin, D.; Cao, Y. Enhancement of Photovoltaic Characteristics Using a Suitable Solvent in Hybrid Polymer/Multiarmed CdS Nanorods Solar Cells. J. Phys. Chem. C 2007, 111, 9538−9542. (7) Leventis, H. C.; King, S. P.; Sudlow, A.; Hill, M. S.; Molloy, K. C.; Haque, S. A. Nanostructured Hybrid Polymer−Inorganic Solar Cell Active Layers Formed by Controllable In Situ Growth of Semiconducting Sulfide Networks. Nano Lett. 2010, 10, 1253−1258. (8) Dowland, S.; Lutz, T.; Ward, A.; King, S. P.; Sudlow, A.; Hill, M. S.; Molloy, K. C.; Haque, S. A. Direct Growth of Metal Sulfide
Figure 4. Normalized kinetics of the CdS:PS blend excited at 355 nm and probed at 470 nm at different laser intensities.
To summarize, we have demonstrated why charge generation following excitation of the inorganic component is more efficient than charge generation from the organic polymer. While electron transfer shows an ultrafast component, it is limited by excitons not reaching the interface (or being split at the interface) and by geminate recombination. In contrast to this, charge generation via hole transfer from the inorganic component can occur on an ultrafast time scale but also up to at least 1 ns after excitation. Using the inorganic material as an absorber therefore relaxes morphology constraints observed when light absorption is carried out by the polymer.27 We therefore suggest the use of panchromatic inorganic absorbers in conjunction with transparent polymers, which act as hole4256
dx.doi.org/10.1021/jz402382e | J. Phys. Chem. Lett. 2013, 4, 4253−4257
The Journal of Physical Chemistry Letters
Letter
Nanoparticle Networks in Solid-State Polymer Films for Hybrid Inorganic−Organic Solar Cells. Adv. Mater. 2011, 23, 2739−2744. (9) Ren, S.; Chang, L.-Y.; Lim, S.-K.; Zhao, J.; Smith, M.; Zhao, N.; Bulović, V.; Bawendi, M.; Gradecak, S. Inorganic−Organic Hybrid Solar Cell: Bridging Quantum Dots to Conjugated Polymer Nanowires. Nano Lett. 2011, 11, 3998−4002. (10) Sun, B.; Greenham, N. C. Improved Efficiency of Photovoltaics Based on CdSe Nanorods and Poly(3-hexylthiophene) Nanofibers. Phys. Chem. Chem. Phys. 2006, 8, 3557−3560. (11) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Hybrid Nanorod− Polymer Solar Cells. Science 2002, 295, 2425−2427. (12) Dayal, S.; Kopidakis, N.; Olson, D. C.; Ginley, D. S.; Rumbles, G. Photovoltaic Devices with a Low Band Gap Polymer and CdSe Nanostructures Exceeding 3% Efficiency. Nano Lett. 2010, 10, 239− 242. (13) Bansal, N.; O’Mahony, F. T. F.; Lutz, T.; Haque, S. A. Solution Processed Polymer−Inorganic Semiconductor Solar Cells Employing Sb2S3 as a Light Harvesting and Electron Transporting Material. Adv. Energy Mater. 2013, 3, 986−990. (14) Krebs, F. C.; Olson, J. D.; Gray, G. P.; Carter, S. A. Optimizing Hybrid Photovoltaics through Annealing and Ligand Choice. Sol. Energy Mater. Sol. Cells 2009, 93, 519−523. (15) Clarke, T. M.; Jamieson, F. C.; Durrant, J. R. Transient Absorption Studies of Bimolecular Recombination Dynamics in Polythiophene/Fullerene Blend Films. J. Phys. Chem. C 2009, 113, 20934−20941. (16) Dimitrov, S. D.; Durrant, J. R. Materials Design Considerations for Charge Generation in Organic Solar Cells. Chem. Mater. 2013, DOI: 10.1021/cm402403z. (17) Kroeze, J. E.; Savenije, T. J.; Vermeulen, M. J. W.; Warman, J. M. Contactless Determination of the Photoconductivity Action Spectrum, Exciton Diffusion Length, and Charge Separation Efficiency in Polythiophene-Sensitized TiO2 Bilayers. J. Phys. Chem. B 2003, 107, 7696−7705. (18) 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. (19) Reynolds, L. X.; Lutz, T.; Dowland, S.; MacLachlan, A.; King, S.; Haque, S. A. Charge Photogeneration in Hybrid Solar Cells: A Comparison Between Quantum Dots and In Situ Grown CdS. Nanoscale 2012, 4, 1561−1564. (20) Noone, K. M.; Subramaniyan, S.; Zhang, Q.; Cao, G.; Jenekhe, S. A.; Ginger, D. S. Photoinduced Charge Transfer and Polaron Dynamics in Polymer and Hybrid Photovoltaic Thin Films: Organic vs Inorganic Acceptors. J. Phys. Chem. C 2011, 115, 24403−24410. (21) Colbert, A. E.; Janke, E. M.; Hsieh, S. T.; Subramaniyan, S.; Schlenker, C. W.; Jenekhe, S. A.; Ginger, D. S. Hole Transfer from Low Band Gap Quantum Dots to Conjugated Polymers in Organic/ Inorganic Hybrid Photovoltaics. J. Phys. Chem. Lett. 2013, 4, 280−284. (22) Ten Cate, S.; Schins, J. M.; Siebbeles, L. D. A. Origin of Low Sensitizing Efficiency of Quantum Dots in Organic Solar Cells. ACS Nano 2012, 6, 8983−8988. (23) Xu, Z.; Hine, C. R.; Maye, M. M.; Meng, Q.; Cotlet, M. Shell Thickness Dependent Photoinduced Hole Transfer in Hybrid Conjugated Polymer/quantum Dot Nanocomposites: From Ensemble to Single Hybrid Level. ACS Nano 2012, 6, 4984−4992. (24) Greenham, N.; Peng, X.; Alivisatos, A. Charge Separation and Transport in Conjugated-Polymer/Semiconductor-Nanocrystal Composites Studied by Photoluminescence Quenching and Photoconductivity. Phys. Rev. B 1996, 54, 17628−17637. (25) O’Mahony, F. T. F.; Lutz, T.; Guijarro, N.; Gómez, R.; Haque, S. A. Electron and Hole Transfer at Metal Oxide/Sb2S3/spiroOMeTAD Heterojunctions. Energy Environ. Sci. 2012, 5, 9760−9764. (26) Christians, J. A.; Kamat, P. V. Trap and Transfer. Two-Step Hole Injection Across the Sb2S3/CuSCN Interface in Solid-State Solar Cells. ACS Nano 2013, 7, 7967−7974. (27) Dowland, S. A.; Reynolds, L. X.; MacLachlan, A.; Cappel, U. B.; Haque, S. A. Photoinduced Electron and Hole Transfer in CdS:P3HT
Nanocomposite Films: Effect of Nanomorphology on Charge Separation Yield and Solar Cell Performance. J. Mater. Chem. A 2013, 1, 13896−13901. (28) Guo, J.; Ohkita, H.; Benten, H.; Ito, S. Charge Generation and Recombination Dynamics in Poly(3-hexylthiophene)/Fullerene Blend Films with Different Regioregularities and Morphologies. J. Am. Chem. Soc. 2010, 132, 6154−6164. (29) Guo, J.; Ohkita, H.; Benten, H.; Ito, S. Near-IR Femtosecond Transient Absorption Spectroscopy of Ultrafast Polaron and Triplet Exciton Formation in Polythiophene Films with Different Regioregularities. J. Am. Chem. Soc. 2009, 131, 16869−16880. (30) Howard, I. A.; Mauer, R.; Meister, M.; Laquai, F. Effect of Morphology on Ultrafast Free Carrier Generation in Polythiophene:Fullerene Organic Solar Cells. J. Am. Chem. Soc. 2010, 132, 14866− 14876. (31) Vaynzof, Y.; Bakulin, A. A.; Gélinas, S.; Friend, R. H. Direct Observation of Photoinduced Bound Charge-Pair States at an Organic−Inorganic Semiconductor Interface. Phys. Rev. Lett. 2012, 108, 246605. (32) Bansal, N.; Reynolds, L. X.; MacLachlan, A.; Lutz, T.; Ashraf, R. S.; Zhang, W.; Nielsen, C. B.; McCulloch, I.; Rebois, D. G.; Kirchartz, T.; et al. Influence of Crystallinity and Energetics on Charge Separation in Polymer−Inorganic Nanocomposite Films for Solar Cells. Sci. Rep. 2013, 3, 1531.
4257
dx.doi.org/10.1021/jz402382e | J. Phys. Chem. Lett. 2013, 4, 4253−4257
