Ultrafast Long-Range Charge Separation in Non-Fullerene Organic Solar Cells

Transcription

1 SUPPORTING INFORMATION Ultrafast Long-Range Charge Separation in Non-Fullerene Organic Solar Cells Yasunari Tamai, 1 Yeli Fan, 2 Vincent O. Kim, 1 Kostiantyn Ziabrev, 2 Akshay Rao, 1 Stephen Barlow, 2 Seth R. Marder, 2 Richard H. Friend, 1, * S. Matthew Menke 1, * 1 Cavendish Laboratory, Department of Physics, University of Cambridge, Cambridge CB3 0HE, United Kingdom 2 Center for Organic Photonics and Electronics and School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, GA , United States Table of Contents 1. Chemical structure, steady-state absorption, and TA spectra of PTB7-Th:ortho-di-PDI blend 2. Absorption spectra of pristine PTB7-Th, bay-di-pdi, and ortho-di-pdi 3. Photoluminescence spectra of PTB7-Th, PTB7-Th:bay-di-PDI blend, and bay-di-pdi 4. TA spectra of a PTB7-Th pristine film 5. Singlet exciton kinetics in PTB7-Th pristine and blend films 6. Normalized TA spectra in visible region 7. Electroabsorption spectrum of PTB7-Th 8. Genetic algorithm 9. Effect of dimer structure on charge separation 10. Excitation intensity dependence of polaron kinetics for PTB7-Th:bay-di-PDI and PTB7-Th:PC 71 BM 11. TA spectra of the PTB7-Th:bay-di PDI blend after PDI selective excitation 12. Downhill relaxation of polarons in PTB7-Th:bay-di-PDI blends 13. Assignment of PDI anion spectrum 14. Charge dynamics of P3HT:PDI blends 15. Intensity dependence of long-time TA spectra for PTB7-Th:bay-di-PDI and PTB7-Th:PC 71 BM blends 16. Estimation of absorption cross-sections 17. Estimation of charge carrier lifetime 18. Triplet exciton generation via intersystem crossing in the CT state

2 1. Chemical structure, steady-state absorption, and TA spectra of PTB7-Th:ortho-di-PDI blend Figure S1. (a) Chemical structure of ortho-di-pdi. 1 (b) Steady-state absorption spectrum of a PTB7-Th:ortho-di-PDI blend with a blend ratio of 1:1.5 by weight. (c) TA spectra of the PTB7-Th:ortho-di-PDI blend excited at 650 nm with a fluence of 1.6 µj cm 2. The spectra in the NIR region ( nm) were rescaled to the IR region ( nm) owing to small changes in the pump-probe overlap. 2. Absorption spectra of pristine PTB7-Th, bay-di-pdi, and ortho-di-pdi Figure S2. Absorption spectra of a pristine PTB7-Th, bay-di-pdi doped in polystyrene (50 wt%), and ortho-di-pdi doped in polystyrene (60 wt%) films. S2

3 3. Photoluminescence spectra of PTB7-Th, PTB7-Th:bay-di-PDI blend, and bay-di-pdi Figure S3. (a) Photoluminescence (PL) spectra of the PTB7-Th:bay-di-PDI blend (red solid) and pristine PTB7-Th (black broken) excited at 650 nm (polymer-selective excitation). (b) PL spectra of the PTB7-Th:bay-di-PDI blend (red solid) and pristine bay-di-pdi doped in polystyrene (black broken) excited at 500 nm (PDI-selective excitation). The PL intensities were corrected for the difference in absorbance for each excitation wavelength. These spectra were also normalized to the peak intensity of each pristine film. 4. TA spectra of a PTB7-Th pristine film Figure S4 shows the TA spectra of a pristine PTB7-Th film. Immediately after the photoexcitation at 650 nm, a broad PIA tail is observed at around 1400 nm, which decays with a lifetime of ~220 ps. We attribute this band to singlet excitons of PTB7-Th, which is consistent with previous reports. 2-5 Figure S4. TA spectra of a pristine PTB7-Th film. The excitation wavelength was set at 650 nm with a fluence of 1.3 µj cm 2. S3

4 5. Singlet exciton kinetics in PTB7-Th pristine and blend films Figure S5 shows the kinetics of singlet excitons in the pristine PTB7-Th compared with fullerene and non-fullerene blends employed in this study. The singlet decay in the PTB7-Th pristine film is fitted with two exponential functions with an average lifetime of ~220 ps. The rapid singlet exciton decay observed in the PDI blends are well fitted with an exponential function and a constant fraction with a lifetime of 1.3 ps (red) and 3.2 ps (blue), respectively, indicating efficient charge transfer between PTB7-Th and PDI. The singlet decay in the PC 71 BM blend was faster than the PDI blends, suggesting that the PC 71 BM blend forms a more fine-scale morphology. Note that the constant, residual signals observed in the blend films are due to a contribution from polaron absorption tail. Figure S5. Singlet exciton kinetics in the pristine PTB7-Th compared with fullerene and non-fullerene blends (averaged over nm). The excitation wavelength was set at 650 nm with a fluence of µj cm 2. S4

5 6. Normalized TA spectra in visible region Figure S6 shows normalized TA spectra of the PTB7-Th:ortho-di-PDI blend and pristine PTB7-Th. In the blend film, the peak position and absorption onset both blue-shift. This blue-shift is also observed in the PTB7-Th:bay-di-PDI blend (data is not shown), but not in the pristine PTB7-Th. This indicates that the blue-shift is caused by the presence of charges. Figure S7 shows the peak position of the main PIA band. The time evolutions can be fitted with an exponential function and a constant fraction with a lifetime of 2.4 ps (red) and 9.1 ps (blue), respectively. This slightly slower rise, as compared with singlet decay (Figure S5), indicates that there is at least one additional species in this region with dynamics different from either singlet excitons or charges. As discussed in the main text, we attribute the third component to electroabsorption (EA) of PTB7-Th. Figure S6. Normalized TA spectra of (a) PTB7-Th:ortho-di-PDI blend and (b) pristine PTB7-Th films. The excitation wavelength was set at 650 nm with a fluence of 1.6 and 1.3 µj cm 2, respectively. Figure S7. Peak energy of (a) PTB7-Th:bay-di-PDI and (b) PTB7-Th:ortho-di-PDI blends. The peak energies were obtained by fitting the main band with a Gaussian function. Solid lines represent the best fitting curves with an exponential function and a constant fraction. S5

6 7. Electroabsorption spectrum of PTB7-Th Figure S8 shows the electroabsorption (EA) spectrum of a pristine PTB7-Th device with the layer structure ITO/ZnO/PTB7-Th/MoO x /Ag. The EA spectrum was measured as a function of applied AC bias using a quasi-steady-state electroabsorption setup and detected by a lock-in-amplifier where the 2ω component was measured. The DC bias was set at 1.5 V to avoid charge injection from electrodes. The EA amplitude is quadratic against electric field E in V/cm with the prefactor of ~ Figure S8. EA spectrum of the PTB7-Th device. S6

7 8. Genetic algorithm In order to decompose the overlapping spectra of individual transients, we used a numerical method based on a genetic algorithm (GA). 6-8 Figure S9a shows decomposed spectra obtained by the GA analysis and their kinetic traces. The TA spectra can be decomposed into three species, which we attribute to GSB associated with singlets (black), GSB associated with charges (blue), and the EA (red). The green line represents a superposition of these spectra according to their amplitude (panel (b)). Figure S9. (a) TA spectrum of the PTB7-Th:bay-di-PDI blend (open circles). Here, the spectrum at 1 ps is shown as an example. Solid lines represent decomposed spectra obtained by the GA analysis. Open triangles show the device EA spectrum of the pristine PTB7-Th. (b) Kinetic traces of singlets (black), charges (blue), and the EA (red). S7

8 9. Effect of dimer structure on charge separation Figure S10 shows time evolutions of the EA amplitude per unit charge obtained by the same methods as mentioned in the main text. Interestingly, the bay-di-pdi blend stores more energy during the first 200 fs, indicating that charges separate faster in the bay-di-pdi blend. This is probably due to the difference in dimer linkage position. As shown in Figures S1 and S2, absorption spectrum of ortho-di-pdi shows sharp vibronic bands similar to monomeric PDI, suggesting that inter-pdi interaction in the ortho-di-pdi would be weaker than the bay-di-pdi. 1 Since initial charge separation undergoes coherently through delocalized states as discussed in the main text, stronger inter-pdi interaction in the bay-di-pdi should be beneficial for long-range charge separation. Note that the EA amplitudes increase with time, and reach nearly the same value after several tens of picoseconds. This indicates that charges subsequently separate via hopping, and hence both bayand ortho-di-pdi dimers can generate free carriers efficiently. Figure S10. Time evolutions of the EA amplitude per unit charge population: (a) PTB7-Th:bay-di-PDI blend and (b) PTB7-Th:ortho-di-PDI blend. S8

9 10. Excitation intensity dependence of polaron kinetics for PTB7-Th:bay-di-PDI and PTB7-Th:PC 71 BM Figure S11. (a,b) Excitation intensity dependence of the TA kinetics of the PTB7-Th:bay-di-PDI blend averaged over nm (GSB, panel a) and nm (polaron, panel b). (c) Excitation intensity dependence of the TA kinetics of the PTB7-Th:PC 71 BM averaged over nm (GSB). Excitation intensity was varied between 0.5 (black), 0.8 (blue), 1.6 (green), and 3.2 (red) µj cm TA spectra of the PTB7-Th:bay-di-PDI blend after PDI selective excitation Figure S12. TA spectra of the PTB7-Th:bay-di-PDI blend. The excitation wavelength was set at 500 nm with a fluence of 1.3 µj cm 2. S9

10 12. Downhill relaxation of polarons in PTB7-Th:bay-di-PDI blends Figure S13 shows the TA spectra for the PTB7-Th:bay-di-PDI blend measured after laser excitation at 650 nm. The polaron band was observed at 1130 nm immediately after the laser excitation which then red-shifted to 1160 nm on a time scale of pico- to sub-nanoseconds. Such spectral shift is indicative of the downhill energy relaxation towards lower energy sites in the polaron density of states (DOS). 9,10 Figure S13. TA spectra of the PTB7-Th:bay-di-PDI blend in IR region. The excitation wavelength was set at 650 nm with a fluence of 1.6 µj cm 2. S10

11 13. Assignment of PDI anion spectrum Figure S14a shows the TA spectra of a poly(3-hexylthiophene) (P3HT):bay-di-PDI blend film excited at 532 nm. The spectra of the P3HT/PDI blend film show a clear GSB at nm and a PIA above 630 nm, which can be ascribed to charged species. As reported previously, P3HT has a flat polaron absorption band at nm. 10 On the other hand, we observed a clear absorption peak at around 700 nm, meaning that PDI anion has an absorption band around 700 nm. This assignment is consistent with previous reports. 2,11,12 Note that Figure S14a suggests that absorption cross-section of bay-di-pdi anion is comparable to that of polymer polaron ( cm 2 ), and hence the anion absorption is masked by the large GSB band as shown in Figure 1. Figure S14. (a) TA spectra of a P3HT:bay-di-PDI blend film. The excitation wavelength was set at 532 nm with a fluence of 2.0 µj cm 2. (b) TA kinetics of the P3HT:bay-di-PDI blend averaged over nm. The excitation fluence was varied over 2.0, 4.1, and 8.0 µj cm 2 from bottom to top. The blue broken line represents the TA decay of a P3HT:ortho-di-PDI blend excited at 532 nm with a fluenece of 8.0 µj cm 2. (c) Steady-state absorption spectra of the P3HT:bay-di-PDI (red) and P3HT:ortho-di-PDI (blue) blends. S11

12 14. Charge dynamics of P3HT:PDI blends As shown in Figure S14b, charges generated in the P3HT:bay-di-PDI blend decayed to ~20% of the initial value in 2 ns independent of the excitation intensity, suggesting that ~80% of charges decayed geminately within the first 2 ns. This is much lower than the charge dissociation efficiency in P3HT:PCBM blend films, in which negligible charge recombination was observed in this time domain. 10 On the other hand, ~75% of charges survived at 2 ns in the P3HT:ortho-di-PDI blend film. This trend is different from Figure S10 where the bay-di-pdi blend shows larger EA amplitude when blended with PTB7-Th. This is most probably due to the difference in blend morphology. Charge dissociation efficiency increases with increasing P3HT crystallinity in the P3HT:PCBM blend as reported previously. 10,13 Judging from absorption shoulder at 600 nm, which is attributed to absorption by P3HT crystalline domain, crystallinity of P3HT in the ortho-di-pdi blend is higher than that in the bay-di-pdi blend. These findings suggest that not only acceptor but also donor crystallinity is an important factor for efficient charge separation delocalization of the hole wavefunction would also lead efficient and fast charge separation. 13 Note that the film fabrication condition employed in this section was not optimised, and hence charge dynamics observed here may not be consistent with previously reported device performances. 15. Intensity dependence of long-time TA spectra for PTB7-Th:bay-di-PDI and PTB7-Th:PC 71 BM blends Figures S15 and S16 show normalized TA spectra and excitation intensity dependence of the polaron decay of the PTB7-Th:bay-di-PDI and PTB7-Th:PC 71 BM blends. An additional PIA band was observed around 1300 nm in panel (b), and the new band became more pronounced with increasing excitation intensity. We attribute this band to polymer triplets generated through non-geminate charge recombination. The triplet absorption is not clear in the PDI blend, suggesting that triplet decay rate is comparable or faster than triplet generation rate in this blend. Such rapid triplet deactivation can be rationalized by considering triplet polaron annihilation (TPA) or triplet triplet annihilation (TTA). Since triplet decay occurs within polymer phase, triplet decay rate is expected to be independent of the acceptor material employed. This means that charge S12

13 recombination to triplet states is more likely to occur in the PC 71 BM blend as discussed in the main text. Figure S15. TA spectra of (a) PTB7-Th:bay-di-PDI and (b) PTB7-Th:PC 71 BM blends after the excitation at 532 nm (averaged over 5 10 ns after the excitation). The intensity of the excitation intensity was varied as indicated. Figure S16. TA kinetics of (a) PTB7-Th:bay-di-PDI and (b) PTB7-Th:PC 71 BM blends after the excitation at 532 nm (averaged over nm). Colour legends are the same as Figure S Estimation of absorption cross-sections For a quantitative description of TA data, the T/T measured should be converted into the number density for corresponding transients, n(t). Here we describe detailed methods for the estimation of polaron and triplet absorption cross-sections σ P and σ T. Because excitons are completely quenched with a time constant of ~1.3 ps in the PTB7-Th:bay-di-PDI blend, the negative signal at 100 ps corresponds to the polaron absorption. In this time domain, both geminate and non-geminate charge recombination can be ignored. We therefore assumed that the polaron population at 100 ps was equal to the number of photons absorbed by the film. Using this approach, we obtain σ P = cm 2. S13

14 Estimation of σ T is much more difficult, and hence we decline a quantitative estimation on the σ T. However, we can safely conclude that the σ T is much larger than the σ P since we observed clear triplet rise despite of small charge loss. Larger triplet absorption cross-section can be also seen for similar donors, PTB1 and PBDTTT-C. 5, Estimation of charge carrier lifetime As shown in Figure S17a, the polaron decay dynamics in nano- to microsecond time domain are well fitted with a power-law equation Eq. S1: = 1+ (S1) where n 0 is the initial charge carrier density at t = 0. Here, the T/T is converted into the charge carrier density n(t) using polaron absorption cross-section obtained above. This power-law equation has been theoretically derived for bimolecular recombination in energetically disordered materials, and has been applied for charge dynamics in OSCs. 15,16 On the other hand, the aim of this section is to estimate charge carrier lifetime (τ n ) as a function of charge carrier density n (cm 3 ). Rate equation for the charge density can be written as: = (S2) where k n is charge density dependent quasi first order rate coefficient. 17 The right hand side of Equation S2 can be alternatively expressed as follows: = = (S3) where γ and k are charge density dependent and independent rate coefficient, respectively, and λ+1 represents the reaction order of charges where λ = 1/α as reported previously. 17 From Eqs. S1 and S2: = 1+ = 1+ (S4) (S5) Alternatively, the rate coefficient k n can be also expressed as a function of the carrier density n by substituting Eq. S1 to S5: S14

15 = (S6) Thus, charge carrier lifetime can be obtained by using fitting parameters in Eq. S1: = 1 = 1 (S7) We can also estimate bimolecular recombination rate coefficient γ as: = 1 (S8) Figure S17b shows charge density dependence of the bimolecular recombination rate coefficient γ. The Langevin reduction factor ζ, which is defined as the ratio of the bimolecular recombination rate coefficient γ to the Langevin recombination rate constant γ L = qµ/εε r (ζ = γ/γ L ), is an important parameter to discuss how much the bimolecular recombination is suppressed. Assuming carrier mobility µ of the bay-di-pdi and PC 71 BM blends to be 10 4 and 10 3 cm 2 V 1 s 1, 1,18-20 the reduction factor ζ is roughly estimated to be ~0.1 and ~0.01, respectively, indicating that the CT states in the PDI blends are more likely to recombine. This finding is well consistent with the poor FF of these devices. Figure S17. (a) Polaron decay dynamics of the PTB7-Th:bay-di-PDI (red), PTB7-Th:ortho-di-PDI (blue) and PTB7-Th:PC 71 BM (black) after the excitation at 532 nm with a fluence of 3.2 µj cm 2. Broken lines represent the best fitting curves with Eq. S1. (b) Bimolecular recombination rate coefficient γ plotted against charge carrier density n calculated by Eq. S8. S15

16 18. Triplet exciton generation via intersystem crossing in the CT state. We should discuss possibility of triplet generation via geminate charge recombination. In Figure 1e, an additional PIA band was observed at around 1300 nm at later time stage, which we attributed to polymer triplets as discussed above. Triplet formation can occur via both geminate and bimolecular charge recombination. In order to reveal the mechanism of triplet formation observed in this time stage, excitation intensity dependence of triplet generation was measured. As shown in Figure S18a, the formation dynamics is independent of the excitation intensity and wavelength. Moreover, the triplet signal increased linearly with increasing charge density (Figure S18b). These findings suggest that triplets observed in this time stage are generated through geminate recombination of 3 CT states following after intersystem crossing (ISC) from 1 CT states. The apparent difference in triplet formation efficiency between PC 71 BM and PDI blends is most probably because of larger spin-orbit coupling of fullerenes. It s important to emphasize that, however, triplet formation via geminate recombination in the PTB7-Th:PC 71 BM blend is very minor loss process since most of CT states can be dissociated into free carrier with an ultrafast time scale. The clear triplet rise observed here is because of its large absorption cross-section as mentioned above. Figure S18. (a) Excitation intensity and wavelength dependence of triplet formation dynamics (averaged over nm). Excitation wavelength was 650 nm (black, red, green) and 500 nm (blue). (b) Excitation intensity and wavelength dependence of triplet yield obtained by the GA analysis. The solid line represents the best fitting curve with a power-law equation: y = ax m. In panel (b), we plot the triplet PIA amplitude at 500 ps to minimise the influence of bimolecular processes such as triplet formation via non-geminate charge recombination as well as triplet decay through TPA and TTA. Polaron amplitude at 100 ps was used as an internal reference of photon S16

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