Charge Generation Dynamics in Efficient All-Polymer Solar Cells

Letter www.acsami.org

Charge Generation Dynamics in Efficient All-Polymer Solar Cells: Influence of Polymer Packing and Morphology Bhoj R. Gautam,† Changyeon Lee,‡ Robert Younts,† Wonho Lee,‡ Evgeny Danilov,§ Bumjoon J. Kim,‡ and Kenan Gundogdu*,† †

Department of Physics and Organic and Carbon Electronics Laboratory (ORaCEL) and §Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, United States ‡ Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea S Supporting Information *

ABSTRACT: All-polymer solar cells exhibit rapid progress in power conversion efficiency (PCE) from 2 to 7.7% over the past few years. While this improvement is primarily attributed to efficient charge transport and balanced mobility between the carriers, not much is known about the charge generation dynamics in these systems. Here we measured exciton relaxation and charge separation dynamics using ultrafast spectroscopy in polymer/ polymer blends with different molecular packing and morphology. These measurements indicate that preferential face-on configuration with intermixed nanomorphology increases the charge generation efficiency. In fact, there is a direct quantitative correlation between the free charge population in the ultrafast time scales and the external quantum efficiency, suggesting not only the transport but also charge generation is key for the design of high performance all polymer solar cells. KEYWORDS: all polymer solar cells, charge generation/recombination, nanomorphology, polymer packing, ultrafast spectroscopy

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generation efficiency.26 Also similar to the fullerene-based PSCs, the domain purity and domain size strongly affect the charge generation and recombination in these systems.3,20,27 Enhanced charge generation was observed with increase in domain purity and decrease in domain size. Charge collection is another important issue in all-PSCs. AllPSCs have typically lower short-circuit current (Jsc) and fill factors (FF) than their counterpart with phenyl-C61-butyric acid methyl ester (PCBM), which are partly due to (1) undesired blend morphology in the active layer and (2) much lower electron mobility of polymer acceptors than that of fullerenes.8,15 In particular, polymer packing geometry in thin films (e.g., face-on or edge-on orientation) significantly affects the charge transport ability of polymer acceptors that have strong anisotropic electrical properties, which is very different from the fullerene-type acceptors (i.e., PCBM).28 Also, the orientation of the polymer acceptors at the donor/acceptor interface should be carefully considered to improve charge transfer at the interface. Recently, some of our coauthors showed that side chain engineering of the acceptor polymers can be used to tune the packing and the domain size in all polymer BHJs. When electronic coupling along the π−π stacking direction is improved in the acceptor domains, charges

olar cells based on conjugated polymers exhibit a steady performance improvement over the last two decades, reaching a power conversion efficiency (PCE) up to ∼12%.1 This improvement is mainly due to intensive research on polymer/fullerene bulk heterojunctions (BHJ) structures, in which a conjugated polymer is the primary light-harvesting unit and acts as an electron donor at the interface with fullerene. Recently with the synthesis of new acceptor polymers, all polymer solar cells (all-PSCs)2−14 are emerging as an alternative to the polymer/fullerene structures. Chemical tunability of acceptors,7,8,15 broader coverage across the solar emission spectrum,2,10 higher open circuit voltage,16−19 and improved morphological and chemical stability4,10,20,21 make all-PSCs promising candidates for organic photovoltaic (OPV) applications. Although performance of the state of the art allPSCs are not yet at the level of polymer/fullerene devices,22,23 the progress in efficiency is significant from 1.8% to 7.7% in the last 5 years.8,15,17,24 Detailed understanding of fundamental optical and electronic processes such as charge separation and transport in these systems can lead to further improvement in device performance as they provide guidance for material design. However, in comparison to polymer/fullerene systems, much less is known about all-PSCs in terms of charge generation and separation processes. Earlier work showed that charge separation is adversely affected by geminate recombination in subnanosecond time scales in all-PSCs.5,25 Following studies showed that correlated orientation of the donor and acceptor domains suppress geminate recombination and lead to higher charge © 2015 American Chemical Society

Received: September 10, 2015 Accepted: December 2, 2015 Published: December 2, 2015 27586

DOI: 10.1021/acsami.5b08531 ACS Appl. Mater. Interfaces 2015, 7, 27586−27591

Letter

ACS Applied Materials & Interfaces

evidenced from photoluminescence quenching (Figure S1), soft resonant X-ray scattering (RSoXS), and atomic force microscopy (Figure S2). Accordingly, a remarkable increase in the power conversion efficiency (PCE) from 3.25% (PTB7Th:PNDIT-DT: Voc = 0.81 V; Jsc = 7.81 mA cm−2; FF = 0.52) to 5.96% (PTB7-Th:PNDIT-HD: Voc = 0.79 V; Jsc = 13.46 mA cm−2; FF = 0.56) was observed (Figure S3). Thus, a major contribution to the PCE increase in PTB7-Th:PNDIT-HD is the dramatic enhancement of the Jsc value.

are collected efficiently, leading to a significant increase in the Jsc and the device performance. Here, we report that the same side chain modification not only increases the charge collection efficiency but also reduces the geminate recombination and leads to efficient charge separation. We measured exciton relaxation and charge generation dynamics in blends of poly[[4,8-bis[5-(2ethylhexyl)thiophene-2-yl]benzo[1,2-b:4,5-b′]dithiophene-2,6 diyl][3-fluoro-2-[(2ethylhexyl)carbonyl] thieno[3,4-b]thiophenediyl]] (PTB7-Th) and naphthalene diimide-thiophene copolymers (PNDIT-R) using ultrafast transient absorption spectroscopy (TAS). The chemical structures of donor and acceptor polymers used in this study are shown in Figure 1. The acceptor copolymers PNDIT-HD and PNDIT-

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EXPERIMENTAL SECTION

Blend of polymer acceptor (PNDIT-R) and polymer donor (PTB7Th) was prepared at a weight ratio of 1.3:1 (w/w) in chloroform solution with total concentration of 12.5 mg/mL including 1.0 vol % of diiodoctane and was heated at 45 °C for 24 h. Blend films with resulting thickness of 90−100 nm were prepared by spin-casting the solution on ultrasonically cleaned glass substrates. Neat PTB7-Th films were prepared using same procedure from 15 mg/mL solution. The thin film samples were encapsulated using UV curable glue before measurement. In the transient absorption experiments, the output of an amplified Ti:sapphire laser provides 100 fs pulses centered at 800 nm at 1 kHz repetition rate. These pulses are then split into two different paths to generate pump and probe pulses. Pump pulses are generated using an optical parametric amplifier (OPA) to excite the sample. Probe pulses are generated through a white light continuum in sapphire and in flint glass for the experiments in the visible (∼1.6−2.6 eV) and the infrared (∼0.8−1.6 eV) spectral ranges, respectively. In these experiments, pump pulses with 1.75 eV photon energy from an optical parametric amplifier excite the donor polymer (PTB7-Th) in the blend. The excitation fluence is purposely kept low to have an initial photoexcitation density of ∼1 × 1016 /cm3. Then a CCD spectrometer combination spectrally resolves and measures the differential transmission of probe pulses at different time delays.

Figure 1. Chemical structures of polymer donor and polymer acceptors. The notations (PTB7-Th, NDI-polymers) are included inside of Figure 1.

DT are used in two different blends, where HD and DT refer to 2-hexyldecyl and 2-decyltetradecyl groups, respectively. The acceptor PNDIT-HD has much shorter alkyl side chains as compared to PNDIT-DT. These two acceptor polymers have very similar optical and electrochemical properties. The energy levels of both the highest occupied molecular orbitals (HOMO) and the lowest unoccupied molecular orbitals (LUMO) are very similar in these acceptors (see Table 1). They also have similar number-average molecular weights (Mn) and polydispersity (PDI),8 which indicate the minimum molecular weight effect on their properties. However, because of the variation in the side chain length and its impact on the molecular organization, the two PNDIT-HD and PNDIT-DT acceptor polymers exhibit very different thermal properties; namely the melting temperature (Tm), crystallization temperature (Tc) and crystallization enthalpy (ΔHc) are much higher for the shorter side chain, i.e., PNDIT-HD (Table 1). This pronounced difference in the crystalline characteristics of the polymers affects their crystalline microstructures in thin films and the blend morphology with the polymer donors. The PNDIT-HD polymer with shorter side chain has a more tightly packed structure with very strong face-on geometry (as gathered from X-ray diffraction data).8 Also, when blended with the PTB7-Th donor polymer, a better degree of intermixing was observed in PTB7-Th:PNDIT-HD blend as

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RESULTS AND DISCUSSION Figure 2 shows the absorption spectra of neat PTB7-Th, neat PNDIT-DT, neat PNDIT-HD, PTB7-Th:PNDIT-DT, and

Figure 2. Absorption spectra of neat PTB7-Th, neat PNDIT-DT, neat PNDIT-HD, PTB7-Th:PNDIT-DT, and PTB7 Th:PNDIT-HD films.

Table 1. Basic Properties of NDI-Thiophene Copolymers polymer information

Mn (kg/mol)a

PDI (Mw/Mn)

HOMO (eV)b

LUMO (eV)c

Egopt (eV)

Tc (°C)d

Tm (°C)d

ΔHc (J/g)d

PNDIT-HD PNDIT-DT

40.0 34.1

2.0 1.8

−5.64 −5.64

−3.79 −3.81

1.85 1.83

260.0 229.6

277.9 236.6

11.5 3.7

Determined by gel permeation chromatography. bDetermined by HOMO = LUMO − Egopt. cDetermined by cyclic voltammetry. dDetermined by differential scanning calorimetry. a

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Figure 3. Room-temperature transient absorption spectra for (a) neat PTB7-Th, (b) PTB7-Th:PNDIT-DT, and (c) PTB7-Th:PNDIT-HD, with a pump excitation of 1.75 eV in spectral range 0.8−2.6 eV. Exciton peak (EX) is due to excited state absorption of donor polymer excitons, polaron peak (P) is due to absorption by polarons and SE peak is due to stimulated emission from donor polymer. GSB in the visible range refers to groundstate bleaching.

Figure 4. Normalized dynamics of (a) singlet exciton PIA, (b) polaron, and (c) GSB acquired at room temperature for neat PTB7-Th, PTB7Th:PNDIT-DT and PTB7-Th:PNDIT-HD with a pump excitation of 1.75 eV.

PTB7-Th:PNDIT-DT blend films. Vibronic features above 1.6 eV are reflected in the absorption spectra of neat donor and blended films. In the blended films, the 0−1 transition is higher than 0−0, whereas the neat film shows stronger 0−0 absorption transition. This is due to the overlap of the acceptor absorption peak with the 0−1 transition of the neat donor. Interestingly, the ratio of the intensity of 0−0 to 0−1 vibronic peaks for both polymer blends is very similar indicating that side chain modification of acceptor does not influence the donor aggregation despite the differences in the domain sizes in blended films. Figure 3 displays the TAS of the two blends and the neat PTB7-Th donor polymer. In the IR region there are three spectral features apparent in the TAS. The strong negative band centered at 0.85 eV is due to excited state absorption of excitons. The weaker absorption band at 1.1 eV is due to polarons and the positive peak at 1.45 eV is due to stimulated emission (SE) of excitons. This assignment of the exciton and polaron peaks is consistent with the earlier results of TAS in polymer blends using low band gap donor polymers.29,30 It is also consistent with the evolution of the features in the presented data itself. For instance in the neat donor polymer thin film (Figure 3a), as expected, the excitonic peak is dominant. The polaron feature is just a tail that extends toward higher energies. In the later delays (5 ns) the signal decayed almost completely. On the other hand, in blends excitons quench faster and a clear polaron peak at 1.1 eV lasts at longer delays. Similarly, the stimulated emission feature is stronger in the neat polymer compared to the blends. In the visible range (>1.5 eV) of the spectrum, the increased transmission of the probe pulses is due to ground state bleaching (GSB).

The spectra presented in Figure 3 clearly show that the polaron peak is significantly stronger in PTB7-Th:PNDIT-HD blends compared to PTB7-Th:PNDIT-DT blends. Without further analysis, this difference suggests that side chain modification in the acceptor polymer leads to differences in charge yield in these structures. Given that these samples are BHJ and not connected to external fields, this difference outlines the intrinsic charge separation dynamics in these two different blends. To compare the time evolution of excitons and polarons in different blends, we isolated individual contributions of these species by deconvolution of spectra in the IR region using Gaussian fitting.30 Figure 4a shows the singlet exciton decay dynamics (band at 0.85 eV) in these three samples. The singlet excitons quench much faster in the PTB7-Th:PNDIT-HD relative to PTB7-Th:PNDIT-DT. The half-life time of photoinduced electron transfer from PTB7-Th exciton to acceptor is 2.5 and 8 ps in PTB7-Th:PNDIT-HD and PTB7Th:PNDIT-DT, respectively. In contrast, the half lifetime of singlet exciton in the neat PTB7-Th is 17 ps. This indicates that the dissociation of singlet donor polymer excitons is more favored than recombination in PTB7-Th:PNDIT-HD relative to PTB7-Th:PNDIT-DT and is consistent with the photoluminescence quenching (Figure S1) and stimulated emission data. In agreement with this argument, the polaron decay kinetics in PTB7-Th:PNDIT-HD shows stark contrast to those in neat PTB7-Th and PTB7-Th:PNDIT-DT. Figure 4b shows that approximately 50% of the polaron population has decayed within 50 ps for neat PTB7-Th and PTB7-Th:PNDIT-DT. In contrast in PTB7-Th:PNDIT-HD the polaron population first increases for the first 50 ps and then decays with a half-life time of 5 ns. The exponential fits to the polaron peak for the 27588


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that 64% of the excitons generate charges in the blend with the PNDIT-HD acceptor, as opposed to 30% in the PNDIT-DT blend. In addition, we determined from our model that the photoinduced electron transfer time is 2.9 and 7.8 ps for the PTB7-Th:PNDIT-HD and PTB7-Th:PNDIT-DT, respectively, consistent with our estimations from the exciton half lifetime of the blend films. External quantum efficiency (EQE) values at the excitation energy 1.75 eV (∼65% for PTB7-Th:PNDIT-HD and ∼35% for PTB7-Th:PNDIT-DT) match very closely with charge generation efficiency that we observed using TAS (Figure S6). Therefore, the PNDIT-HD blend is two times more efficient in charge generation, which is well correlated with both EQE ratio (1.85) and Jsc ratio (1.72) in these two samples. Furthermore, we studied the TAS of the previously reported8 PTB7-Th:PNDIT-OD polmer blend, which shows intermediate phase segregation as compared to the PNDIT-HD and PNDIT-DT blend films, and demonstrates a slightly decreased PCE (5.05%) and Jsc (11.97 mA cm−2) as compared to the PNDIT-HD blend. Using the same analysis of the IR TA spectra (Figure S7), we determined the charge generation efficiency to be ∼64% which correlates well with the EQE reported at the excitation energy of 1.75 eV. This further evidence that with optimized donor−acceptor phase segregation, the geminate recombination pathway is reduced increasing the overall long-lived charge generation process. We attribute the difference in charge generation yield in these two blends to the differences in the morphology. In the previous work, we have shown that shorter π−π stacking distance, strong crystalline characteristics and better face-on alignment of the polymers in PNDIT-HD as compared to PNDIT-DT,8 leads to improved vertical transport of carriers across the device. Here we observe that in addition to the charge collection, intrinsic charge separation efficiency is also dramatically improved in PTB7-Th:PNDIT-HD interfaces. This is due to several potential reasons. First of all, the earlier RSoXS measurements shows that the PTB7-Th:PNDIT-HD blend has a much finer BHJ morphology with greatly increased interfacial area and reduced formation of the polymer aggregates, whereas the PTB7-Th:PNDIT-DT blend has a much larger scale phase separation (i.e., 95 nm). In addition, Figure S2 shows the AFM images of these blends, which clearly exhibit smaller crystallites and better intermixing for the acceptor and donor domains in the PTB7-Th:PNDIT-HD blend. This is further supported by the PL quenching studies shown in Figure S1. Better intermixing leads to higher quenching in the PTB7-Th:PNDIT-HD. To study the polymer packing in these blends, we performed grazing incidence X-ray scattering (GIXS) measurement (Figure S5). It can be seen from GIXS results that PTB7-Th:PNDIT-HD has a slightly more face-on geometry and shorter π-stacking distances (3.99 Å for PTB7-Th:PNDIT-DT blend and 3.97 Å for PTB7Th:PNDIT-HD blend). Therefore, the higher charge generation efficiency observed in PTB7-Th:PNDIT-HD blend is mostly due to intermixing of the donor−acceptor domains but not to shorter π-stacking distances. As a result of smaller domains, excitons may diffuse and split at the interfaces more efficiently in the PTB7-Th:PNDIT-HD blend. Whereas excitons in PTB7-Th:PNDIT-DT are more localized in the donor and prone to geminate recombination. In addition to these arguments, previous studies showed that in all-PSCs, correlated orientation of acceptor and donor domains reduces geminate recombination.26 Both neat PTB7-Th and PNDIT-

PNDIT-HD blend resulted in 2.9 ps time constant for the rise and a biphasic decay with 270 ps (15%) and 12 ns (85%) time constants. On the other hand for the PNDIT-DT blend the polaron peak shows only decaying transient behavior with 53.2 ps (61%) and 10.6 ns (39%) characteristic times. This is very similar to the dynamics observed for the neat polymer for the same spectral region, i.e 44 ps (73%) and 13.2 ns (27%) decay times. The similarity in the decay characteristics between the neat polymer and the PNDIT-DT blend suggests the charge generation in PTB7-Th:PNDIT-DT blend is hindered by geminate recombination. We further studied this by performing excitation fluence dependence of the polaron population dynamics. The results shown in Figure S4 exhibit clear differences between the blends. Although PTB7-Th:PNDITDT does not show any excitation fluence dependence, the later delay decay rate of the PTB7-Th:PNDIT-HD polarons increases significantly with increasing excitation intensity. This indicates that geminate recombination is the dominant depopulation process in PTB7-Th:PNDIT-DT, hindering the separation of free charges, leading to reduced density of free charges surviving at later delays. In contrast, because in PTB7Th:PNDIT-HD geminate recombination is significantly suppressed, the effect of non-geminate recombination is clearly observable at later delays. These observations are consistent with the improved fill factor and superior Jsc value observed in the PTB7-Th:PNDIT-HD device compared to that of the PTB7-Th:PNDIT-DT device (Figure S3). The efficient charge generation in the PNDIT-HD blend is further evident from the time evolution of the transient absorption spectra in visible region. Figure 4c shows the decay of the GSB peak in all three samples. Unlike the exciton and polaron peaks, the magnitude of the GSB reflects the total population of all excitations and its decay reflects the recovery of the ground state. In Figure 3, spectral line shapes for the GSB region in all samples are similar, but a higher population of excitations survive on the longest delay (5 ns, time window of our experiment) in the PTB7-Th:PNDIT-HD blend. This again indicates efficient conversion of excitons to charges and suppressed exciton recombination in this blend. Furthermore, the GSB evolution in Figure 4c (probed at 1.95 eV) shows that early dynamics within 2 ps is same for both blends. The difference in the longer delay dynamics is due to the different branching ratio of charge generation and exciton recombination in these blends. The half-life time of GSB in the PTB7Th:PNDIT-HD is same as that of the polaron peak (5 ns), indicating that this long time contribution of GSB is from polarons. The decay of GSB provides further information about the interspecies kinetics. The decay time constants are 6.8 ps (37%), 105 ps (54%), and 12.3 ns (9%) for neat PTB7-Th; 8.1 ps (38%), 108 ps (40%), and 12.8 ns (22%) for PNDIT-DT blend; and 13.4 ps (20%), 410 ps (18%), and 27.4 ns (62%) for PNDIT-HD blend (Figure 4). The fast component of GSB is due to the exciton, whereas other two slow time constants are related to the polaron species and follow the trend that we observed in the polaron dynamics. To quantify the charge generation efficiency, we used a simple model in which we assumed that the recombination in both blends followed the same dynamics as in the neat polymer. In this simple assumption, the decay of the exciton peak has two components; one is the exciton recombination pathway, extracted from the neat polymer data, and the other one is the charge generation pathway in blended films. By fitting the exciton decay with this model (Figure S6), we found 27589


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(5) Hodgkiss, J. M.; Campbell, A. R.; Marsh, R. A.; Rao, A.; AlbertSeifried, S.; Friend, R. H. Subnanosecond Geminate Charge Recombination in Polymer-Polymer Photovoltaic Devices. Phys. Rev. Lett. 2010, 104, 177701. (6) Earmme, T.; Hwang, Y. J.; Subramaniyan, S.; Jenekhe, S. A. AllPolymer Bulk Heterojuction Solar Cells with 4.8% Efficiency Achieved by Solution Processing from a Co-Solvent. Adv. Mater. 2014, 26, 6080−6085. (7) Earmme, T.; Hwang, Y.-J.; Murari, N. M.; Subramaniyan, S.; Jenekhe, S. A. All-Polymer Solar Cells with 3.3% Efficiency Based on Naphthalene Diimide-Selenophene Copolymer Acceptor. J. Am. Chem. Soc. 2013, 135, 14960−14963. (8) Lee, C.; Kang, H.; Lee, W.; Kim, T.; Kim, K. H.; Woo, H. Y.; Wang, C.; Kim, B. J. High-Performance All-Polymer Solar Cells Via Side-Chain Engineering of the Polymer Acceptor: The Importance of the Polymer Packing Structure and the Nanoscale Blend Morphology. Adv. Mater. 2015, 27, 2466−2471. (9) Holcombe, T. W.; Norton, J. E.; Rivnay, J.; Woo, C. H.; Goris, L.; Piliego, C.; Griffini, G.; Sellinger, A.; Bredas, J.-L.; Salleo, A. Steric Control of the Donor/Acceptor Interface: Implications in Organic Photovoltaic Charge Generation. J. Am. Chem. Soc. 2011, 133, 12106− 12114. (10) Kang, H.; Kim, K.-H.; Choi, J.; Lee, C.; Kim, B. J. HighPerformance All-Polymer Solar Cells Based on Face-On Stacked Polymer Blends with Low Interfacial Tension. ACS Macro Lett. 2014, 3, 1009−1014. (11) Mori, D.; Benten, H.; Okada, I.; Ohkita, H.; Ito, S. LowBandgap Donor/Acceptor Polymer Blend Solar Cells with Efficiency Exceeding 4%. Adv. Energy. Mater. 2014, 4, 1301006. (12) Mu, C.; Liu, P.; Ma, W.; Jiang, K.; Zhao, J.; Zhang, K.; Chen, Z.; Wei, Z.; Yi, Y.; Wang, J. High-Efficiency All-Polymer Solar Cells Based on a Pair of Crystalline Low-Bandgap Polymers. Adv. Mater. 2014, 26, 7224−7230. (13) Mori, D.; Benten, H.; Okada, I.; Ohkita, H.; Ito, S. Highly Efficient Charge-Carrier Generation and Collection in Polymer/ Polymer Blend Solar Cells with a Power Conversion Efficiency of 5.7%. Energy Environ. Sci. 2014, 7, 2939−2943. (14) Kang, H.; Uddin, M. A.; Lee, C.; Kim, K.-H.; Nguyen, T. L.; Lee, W.; Li, Y.; Wang, C.; Woo, H. Y.; Kim, B. J. Determining the Role of Polymer Molecular Weight for High-Performance All-Polymer Solar Cells: Its Effect on Polymer Aggregation and Phase Separation. J. Am. Chem. Soc. 2015, 137, 2359−2365. (15) Jung, J. W.; Jo, J. W.; Chueh, C. C.; Liu, F.; Jo, W. H.; Russell, T. P.; Jen, A. K. Y. Fluoro-Substituted n-Type Conjugated Polymers for Additive-Free All-Polymer Bulk Heterojunction Solar Cells with High Power Conversion Efficiency of 6.71%. Adv. Mater. 2015, 27, 3310− 3317. (16) Gélinas, S.; Paré-Labrosse, O.; Brosseau, C.-N.; Albert-Seifried, S.; McNeill, C. R.; Kirov, K. R.; Howard, I. A.; Leonelli, R.; Friend, R. H.; Silva, C. The Binding Energy of Charge-Transfer Excitons Localized at Polymeric Semiconductor Heterojunctions. J. Phys. Chem. C 2011, 115, 7114−7119. (17) McNeill, C. R.; Abrusci, A.; Hwang, I.; Ruderer, M. A.; MüllerBuschbaum, P.; Greenham, N. C. Photophysics and Photocurrent Generation in Polythiophene/Polyfluorene Copolymer Blends. Adv. Funct. Mater. 2009, 19, 3103−3111. (18) Kietzke, T.; Hörhold, H.-H.; Neher, D. Efficient Polymer Solar Cells Based on M3EH-PPV. Chem. Mater. 2005, 17, 6532−6537. (19) Granström, M.; Petritsch, K.; Arias, A.; Lux, A.; Andersson, M.; Friend, R. Laminated Fabrication of Polymeric Photovoltaic Diodes. Nature 1998, 395, 257−260. (20) Zhou, Y.; Kurosawa, T.; Ma, W.; Guo, Y.; Fang, L.; Vandewal, K.; Diao, Y.; Wang, C.; Yan, Q.; Reinspach, J.; Mei, J.; Appleton, A. L.; Koleilat, G. I.; Gao, Y.; Mannsfeld, S. C. B.; Salleo, A.; Ade, H.; Zhao, D.; Bao, Z. High Performance All-Polymer Solar Cell via Polymer Side-Chain Engineering. Adv. Mater. 2014, 26, 3767−3772. (21) Jørgensen, M.; Norrman, K.; Gevorgyan, S. A.; Tromholt, T.; Andreasen, B.; Krebs, F. C. Stability of Polymer Solar Cells. Adv. Mater. 2012, 24, 580−612.

HD have preferential face-on orientation with respect to the substrate.10 Hence, BHJ of these two polymers may form correlated domains, as all domains would prefer to align with respect to the substrate. As a result, geminate recombination would be suppressed because of interfacial alignment favoring charge separation. Therefore, the difference in the charge separation dynamics we observed can be well explained by the difference in morphology and packing between the two different blends.8 In conclusion, we examined the photophysics of all-PSCs based on PTB7-Th as donor and PNDIT-R as acceptor to understand the effect of nanomorphology and polymer packing on the exciton and charge dynamics. We found that poor morphology of the PTB7-Th:PNDIT-DT blend causes rapid geminate recombination, thereby preventing charge separation. On the other hand, the PTB7-Th:PNDIT-HD blend, which has well-intermixed morphology and strong preferential face-on configuration, shows efficient charge generation with reduced geminate recombination. Importantly, our analysis in charge generation efficiency provides a quantitative explanation for the difference in the EQE and the photovoltaic performances in allPSCs. Therefore, our finding provides an improved understanding of the factors that control not only charge transport but also charge generation and recombination, both of which are important for the design of highly efficiency all-PSCs.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b08531. Additional experimental results (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by Office of Naval Research (ONR) grant N000141310526 P00002 (B.R.G., R.Y., and K.G). This research was also supported by the National Research Foundation Grant (2012M3A6A7055540, 2013R1A2A1A03069803), funded by the Korean Government (C.L., W.L., and B.J.K).

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REFERENCES

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Charge Generation Dynamics in Efficient All-Polymer Solar Cells

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