Incoherent Pathways of Charge Separation in Organic & Hybrid Solar

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Incoherent Pathways of Charge Separation in Organic & Hybrid Solar Cells Alexander Grupp, Philipp Ehrenreich, Julian Kalb, Arne Budweg, Lukas Schmidt-Mende, and Daniele Brida J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b01873 • Publication Date (Web): 19 Sep 2017 Downloaded from http://pubs.acs.org on September 20, 2017

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The Journal of Physical Chemistry Letters

Incoherent Pathways of Charge Separation in Organic & Hybrid Solar Cells

Alexander Grupp*, Philipp Ehrenreich*, Julian Kalb, Arne Budweg, Lukas Schmidt-Mende†, Daniele Brida†. Department of Physics and Center for Applied Photonics, University of Konstanz, D-78457 Konstanz, Germany AUTHOR INFORMATION *These authors contributed equally to this work. †e-mail: [email protected], [email protected].

KEYWORDS organic electronics, charge generation, coherence, delocalization, charge transfer states

ACS Paragon Plus Environment

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Abstract In this work, we investigate the exciton dissociation dynamics occurring at the donoracceptor interface in organic and hybrid blends employed in the realization of photovoltaic cells. Fundamental differences in the charge separation process are studied with the organic semiconductor polymer poly(3-hexylthiophene) (P3HT) and either [6,6]-phenylC61-butyric acid methyl ester (PCBM) or titanium dioxide (TiO2) acting as acceptor. By using ultrafast broadband transient absorption spectroscopy with few-fs temporal resolution, we observe that in both cases the incoherent formation of free charges is dominating the charge generation process. From the optical response of the polymer and by tracking the excited state absorption, we extract pivotal similarities in the incoherent energy pathways that follow the impulsive excitation. On timescales shorter than 200 fs we observe that the two acceptors display similar dynamics in the exciton delocalization. Significant differences arise only on longer timescales with only an impact on the overall photocarrier generation efficiency. TOC

The fundamental mechanism that leads to charge generation after photoexcitation in organic and hybrid solar cells is still object of extensive investigations. In general, organic semiconductors exhibit a rather low electric permittivity resulting in localized Frenkel excitons with binding energies that are more than 10 times larger than kBT at room temperature1. This intrinsic property requires the introduction of donor and acceptor moieties, which offer an energetic driving force for the generation of free polarons. Upon successful exciton splitting, opposing charge carriers can still be coulombically bound to each other leading to the formation of intermediate charge


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transfer (CT) states across such a heterointerface2-6. This step is a precursor to the formation of free charges while it has been observed that coherent electron dynamics can concur to the overall process7. The polymer poly(3-hexylthiophene) (P3HT) is a prototypical system for the investigation of the charge separation process in organic solar cells. In combination with silicon, it is found that free polaron formation is a rather delayed process and occurs via dissociation of localized states8. On the other hand, there is compelling evidence for instantaneous free charge generation upon light absorption prior to carrier cooling7,

9-12

. In this regard, femtosecond stimulated Raman

spectroscopy results show no polaronic vibrational signature within the first 100 fs after photoexcitation, while hot free carriers can be generated without an intermediate occupation of CT states9. In general, coupling of electronic and nuclear degrees of freedom can trigger charge delocalization and charge separation7. Coherent charge transfer takes place if the LUMOs and the vibrational modes of donor and acceptor are in resonance at the same time. Therefore, the wavepacket that describes the excited state in the donor transfers “ballistically” to the acceptor with the vibronic wavefunction preserving its phase7. In any case, in order to achieve charge separation out of hot carrier conditions or fully thermalized states, it is widely shown that wavefunction delocalization promotes efficient charge pair splitting 5, 13-16. However, untill now it is not clear whether coherent charge separation can contribute significantly to the overall yield of realistic solar cells designs or if incoherent models are sufficient to describe the generation process of free charge carriers 17-20. In this work, we compare a pristine P3HT film to blends consisting of P3HT and either [6,6]phenyl-C61-butyric acid methyl ester (PCBM) or titanium dioxide (TiO2). The fullerene derivative PCBM is a widely used organic acceptor material

21-22

which permits to achieve

beneficial solar cell morphologies required for charge delocalization. TiO2 employed as acceptor allows for the delocalization of the carrier wavefunctions owing to the low effective mass for electrons

21-22

and the favorable band alignment

23

. However, the optimization of complicated

nanostructures based on metal oxides is experimentally challenging24. Different to fullerenes, vibrational modes in TiO2 are off resonant to molecular vibrations in P3HT and coherent coupling of orbitals between donor and acceptor is perturbed25-26. Consequently, the quantum mechanical description of electron eigenfunctions on molecular orbitals of the fullerene is


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fundamentally different to energetic bands occurring in TiO2. Such fundamental differences make these two acceptor materials to excellent model systems, which allow for the evaluation of the role of hot carrier separation versus non-coherent charge generation. In terms of coherent charge separation, our model systems differ significantly since coherent coupling in the inorganic blend is strongly disturbed. If coherent effects dominate charge separation, it is expected to observe significant exciton separation within less than 300 fs time duration, i.e. before dephasing occurs

27-28

. Based on our observations, we can demonstrate compelling

evidence that charges are predominantly separated out of localized conditions on a ps time-scale. In our comparison we do not see meaningful differences between a pristine polymer film and a donor:acceptor blend on a sub 200 fs timescale. Instead of free polarons, hot carrier dissociation delivers a large portion of polaron pairs, excimers or carriers in excited charge transfer states29-30, i.e. two Coulombically bound polarons of opposite charge that are separated in a rate limited, incoherent process. In our experiments, ultrafast carrier dynamics are investigated with ultrabroadband transient absorption spectroscopy in order to track the evolution of excitons created by impulsive illumination with a 10 fs temporal resolution. Spectra of pump and probe pulses are shown in Figure 1a and compared to the absorbance of pristine P3HT (dotted blue). A large spectral overlap with the polymer absorbance leads to excitation of the π-π* transition in the aggregated polymer while the absorption of fullerenes and TiO2 is negligible within the excitation spectrum. Two markers sign the transition from the 0th-phonon level of the electronic ground state S0 of the polymer into the 0th and 1st-phonon replica of the excited state S1 at 605 nm and 555 nm, respectively. The three samples under investigation are films spin-coated on thin fused silica substrates. In order to ensure comparable polymer aggregation among all films, we have used a 10 mg/ml polymer/chlorobenzene solution.


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In Figure 1b we present transient absorption spectra of the three films at different time delays between pump and probe pulses. The instantaneous response at 25 fs shows a common spectral signature in all samples. For wavelengths shorter than 620 nm, there are two distinct positive peaks which can be assigned to the photo-bleaching (PB) of the exciton absorption S0-S1 (0–0) and the first phonon replica (0–1). Although a constant excitation fluence of 30 µJ/cm² in the focal plane has been applied, the generation yield varies by a factor of two among these films and hinders the analysis of absolute signals (for more details see31 or the Supplementary Information). However, a comparison between samples is possible by normalizing all transient spectra to the photo-bleach (PB) at 555 nm after 25 fs. As the differential signals increase linearly with excitation density (see Supplementary Information) this normalization approach is

Figure 1. a, Normalized absorbance spectrum of pristine P3HT (dotted blue), spectral intensity of pump (purple line) and probe pulse used in this study (dashed black line). Two markers indicate the transition from the 0th-phonon level of the electronic ground state S0 of the polymer into the 0th and 1st-phonon replica of the excited state S1. The pump spectrum is resonant with both transitions of P3HT and it does not excite electronic transitions in PCBM or TiO2. b, Transient absorption spectra measured at different time delays τ between pump and probe for pristine P3HT (blue), TiO2:P3HT (black) and PCBM:P3HT (red). All spectra are normalized to the signal at 555 nm and = 25 fs. The absolute differential signals / which we use for the normalization are: 2.7 ⋅ 10 (pristine P3HT), 3.5 ⋅ 10 (TiO2:P3HT) and 1.5 ⋅ 10 (PCBM:P3HT). At early time delays, TAS spectra are similar, while at later delays PCBM:P3HT differs strongly from pristine P3HT and TiO2:P3HT which evolve similarly.

well justified and allows a better comparison between the samples.


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A common feature in all examined samples is a superimposed oscillatory component in the time delay scans, which affects all probe wavelengths in this study (e.g. seen in Figure 2 d and e at ultrafast time scales). We extract an oscillation at the frequency of 1450 cm-1 and assign it to coherent excitation of the C=C stretching mode in P3HT25. Although we observe a superposition of vibrational coherence of the ground and the excited state28, dephasing does not deviate significantly between samples and the damping is identical (see Supplementary Information). For this reason, we conclude that interaction with the phonon bath is comparable due to similar

Figure 2. a, Dynamics of the exciton excited state absorption probed at 1200 nm, normalized to unity. PIA in blends decays faster than in pristine P3HT. b, Comparison between differential transmission signals of blends and pristine P3HT. This difference forms within 500 fs and shows a long lifetime. c, Dynamics of the 0–1 transition probed at 555 nm and of the excited state absorption probed at 1200 nm plotted separately for all samples. All signals are normalized to unity for comparison of the decay. In blends we find a faster decay of PIA (dashed lines) compatible with the transfer of the excitation towards another state / formation of free charges. d, Dynamics of the PIA from polaron pairs probed at 650 nm and e, dynamics of the PIA from polarons / CT state probed at 720 nm. Signals in both panels are normalized to the bleaching amplitude measured at 555 nm (cf. Figure 1b). All samples display a comparable instantaneous build-up. f, and g, depict the same data normalized to unity for comparison of decay dynamics. PIA from PCBM:P3HT decays notably slower or even rises when probed at 720 nm g).

polymer morphologies and aggregation. The negative signal above 620 nm corresponds to photo-induced absorption (PIA) in P3HT, with a local broad maximum around 650 nm. This signal has been attributed to absorption by polaron


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pairs located on the polymer29-30,

32-35

. The lower curves in Figure 1b depict measurements at

time delays of 1 ps and 10 ps. In these spectra, we observe that the amplitude of induced transmission, i.e. the PB at 555 nm, sustains much longer in the PCBM:P3HT blend compared to pristine P3HT or TiO2:P3HT blends. This is consistent with the fact that recombination is prevented in the PCBM blend and excited carriers are efficiently transferred to different states since the excitation of the pristine polymer decays on a faster time scale. Between pristine P3HT and TiO2:P3HT we only observe small deviations. However, the bleaching of the direct transition cannot reveal final states to which the excitations are transferred. Therefore, it is necessary to investigate more precisely the evolution of the excited state in each individual species. At 1200 nm probe wavelength (Figure 2a), we observe PIA in all samples and attribute this to induced absorption into a higher electronic state S1-Sn32, 36

. In order to display the decay of a common excited state population we normalize signals to

unity. A description of the dynamics requires at least three exponential decay processes. First, a fast component describes the thermalization within S1 that follows the initial hot carrier distribution created by the impulsive photoexcitation. Upon cooling, excitons evolve into different states on timescales of hundreds of fs to ps. In contrast, the third long timescale is consistent with the recombination of excitons that remain trapped within the P3HT moiety as reported in photoluminescence experiments37. Here, we focus the discussion on the intermediate timescales since those decay dynamics can serve as a direct measure for transitions of excitons into free charges via incoherent processes. To visualize the impact of these mechanisms, we subtract the signal of the pristine P3HT (Figure 2b) from the dynamics of the two blends. Since the signal represent the dynamics occurring in an ensemble, and since the temporal evolution of this system is a linear problem for which rate equations apply, our procedure is justified for the extraction of qualitative differences. With this approach, we can ignore the phenomena occurring only in the donor moieties such as the formations of polarons localized on one or more P3HT chains. This difference in dynamics clearly indicates the presence of decay channels being possible at the donor:acceptor interface: upon photoexcitation the signal builds up within 500 fs and remains almost constant within our observation window while the amplitude of the dynamics is larger in case of TiO2.


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Further insight comes from a direct comparison of the excited state dynamics (as dashed lines) with the PB (as solid lines) of the S0-S1 transition. Panels in Figure 2c depict these curves separately for all samples, each normalized to unity. In pristine P3HT, we observe only minor deviations between the two curves indicating that the recombination pathways are limited and that the excited state evolution leads to the recovery of the initial unperturbed state. Instead, in TiO2:P3HT and PCBM:P3HT, the excited state absorption decays much faster than the PB. In addition, the spectral weight which is lost in the near-IR PIA is not recovered as a reduction of the PB. This observation indicates that the energy acquired by the P3HT after photoinduced absorption is exploited for the formation of CT states that lead to free charges without recombination resulting in a relaxation to the ground state of the donor molecule. This analysis is in full agreement with the comparison of the near-IR PIA between the blends and the pristine P3HT. It is also interesting to note that the PB decays slower in PCBM:P3HT than in TiO2:P3HT and that its spectral weight is larger at longer time delays. This can also be observed in Figure 1b at a pump-probe delay of 10 ps - the PB amplitude is larger for the red curve than for the black one. Such differences point towards a higher efficiency of free charge carrier generation through CT states in the organic blend. Furthermore, it has been noted that PIA from TiO2 can overlap with the PB of P3HT38. This observation is consistent with the reduction of the PB signal in pumpprobe experiments. The analysis becomes more delicate when dealing with the photoinduced absorption that is measured in all the samples above 620 nm as described in Figure 1b. In Figure 2d and e, we plot the dynamics at two selected wavelengths normalized to the PB signal at 555 nm. In both graphs, we find instantaneous PIA after excitation, which is increasing in all films during the first 200 fs. Induced absorption at 650 nm is assigned to polaron pairs formed on the polymer. Since all samples have similar amplitudes and rise-times, we conclude a comparable yield for the formation of polarons on the fast time scale independent of the sample. In panel f, signals at 650 nm are normalized to unity and thus, a comparison of dynamics is improved. Surprisingly, photoinduced absorption in pristine P3HT and TiO2:P3HT decays almost identically, while in PCBM:P3HT PIA persists. At 720 nm probe wavelength (panels e and g), we observe exponential decay only in pristine P3HT and TiO2:P3HT. In contrast, the organic blend shows


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increasing PIA on a ps time scale. This rise can be attributed to the formation of delocalized polarons or CT states that have a photoinduced absorption in the same spectral range35. The ps build-up points towards a significant contribution of this incoherent transfer of polarons via states that lead to the formation of free charges. This constitutes an important difference between TiO2 and PCBM blends, as is also observed in devices. In order to precisely exclude any influence on our observations from variations of the donor:acceptor interface or morphology, we compared results obtained in different sample geometries. In contrast to blends with largely enhanced interface and a variation of phase purity, we have also studied a P3HT:TiO2 bilayer structure (see Supplementary Information) with just one distinct interface and two rather pure material phases. Despite this modification of sample geometry, the dynamic evolution of spectra confirms our findings on donor:acceptor blends.

Figure 3. Scheme of charge generation at organic and hybrid donor-acceptor interfaces. (Left) After excitation, excitons form and decay subsequently via three different pathways. While in pristine P3HT only recombination to the ground state and excitation of polaron pairs occurs, donor:acceptor blends offer an additional channel via occupation of a charge transfer state (CT) prior to the formation of free charge carriers (CS). (Right top) Energetic landscape describing the path of free carrier formation in PCBM:P3HT blends. (Right bottom) In TiO2:P3HT blends electrostatic forces are too strong for carrier separation out of CT states and only few free charges are observable. Photoinduced absoprtion into CT1 (probed at 720 nm) reveals CT state and recombination dynamics.


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Figure 3 schematically displays the pathways that follow the impulsive photoexcitation in our samples resulting from the experimental observation of the polymer excited state absorption at 1200 nm and the polaron absorption measured between 650 nm and 720 nm. Charge carriers in the excited state form Frenkel excitons that are located on individual or coupled polymer chromophores39. In pristine P3HT, recombination to the ground state occurs as well as excitation of polaron pairs in the polymer. Thereby, dynamics of the PB and excited states match on the ps timescale (c.f. Figure 2c). Hence, only few carriers remain excited and are available for charge separation. In contrast, donor:acceptor blends offer an additional relaxation channel via CT states. The relaxation of the excited state population is enhanced (c.f. Figure 2c) while polaron absorption is nearly constant (c.f. Figure 2d) or increases (c.f. Figure 2e) in the PCBM:P3HT blend even further on a ps-timescale. This is a general behavior that can be observed for manifold fully organic donor:acceptor combinations and is related to the formation of delocalized CT states8, 13, 40-46. Additionally, bleaching of the direct transition is prolonged with respect to the pristine P3HT since efficient separation of charges does not allow for the recovering of the initial ground state on the ultrafast timescale. When TiO2 is used as acceptor, we observe a strong relaxation of the excited state population (c.f. Figure 2c) which is significantly faster than the decay of the bleaching signal. Surprisingly, polaron signatures are less prominent than in pristine P3HT films (c.f. Figure 2d and 2e). Consequently, energy is transferred to states that are not fully covered by our experimental bandwidth. Injection of free charges in TiO2 is possibly observed at 550 nm where the PB dynamics of P3HT is too similar to the pristine sample and thus cannot be explained without an additional contribution (c.f. Figure 1b). In fact, the positive signal should be higher if recombination is prevented but the differential transmission can be reduced by the overlap with a PIA absorption in the charged oxide as hinted by continuous wave PIA investigations

38

. In addition, many works account trap states in the

metal oxide as localization centers for strongly bound CT states

47-48

that display stronger

attractive Coulomb forces than in the case of PCBM:P3HT (schematically shown in Figure 3). Finally, it is worth noticing that the polaron signals between 620 and 720 nm are almost identical on the few fs time scale independently from the acceptor. This observation indicates that coherent polaron formation cannot be a dominant mechanism for the formation of free charges. Also the near-IR PIA corroborates this finding and assesses that the simple architecture of heterojunction solar cells is not fully exploiting vibrational coherence.


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In summary, we observed the evolution of the energy deposited in photovoltaic blends towards the generation of free charges. In particular, the access to the dynamics of the excited state allows us to assess that organic and inorganic blends behave in a very similar way with the main difference being the intermediate state formed when the acceptor is a fullerene. In this case, we can track the formation of intermediate states at energies that overlap with the polarons in the pristine P3HT polymer. These states drive the wavefunction delocalization and overcome the formation of bound charge pairs. In TiO2, the presence of intermediate steps, that was elusive so far, is now observed in the existence of delocalized wavefunctions. These states are required since the interface interactions provides stronger Coulomb forces for direct charge separation. From these findings, it becomes also clear why in hybrid solar cells, polymer contributions to the photocurrent are only efficient if localized excitons transfer via a Förster resonant energy transfer process to a covalently bound dye molecule which shows successful charge injection for both, hot and cold carrier conditions23, 49. Lastly, our investigation shows that coherence is not yet a key force in the generation of free charges and that new strategies must be envisaged for fully exploiting the potential efficiency of this mechanism.


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Experimental Methods SAMPLE PREPARATION Substrates from fused silica (size: 10x20 mm2, thickness: 275µm) were cleaned in acetone and isopropanol for 10 min in an ultrasonic bath, followed by an oxygen plasma cleaning for 7 min. Flat 40 nm TiO2 films were sputtered at room temperature employing a TiO2 target (99.99% purity, Testburne Ltd). These films were post-annealed in oxygen at 450°C to form a polycrystalline anatase crystal structure. Poly(3-hexylthiophene) (Mw = 51 kDa, PDI = 2.1, regioregularity: 96%, Rieke Metals) was cast from a 10 mg/ml chlorobenzene solution at 2000 rpm. The same polymer was used for 1:1 wt% blends, containing either [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) or P25 TiO2 nanoparticles in a 10 mg/ml chloroform solution. The evaporation rate was chosen to 1 Å/s (Pressure < 5*10-6mbar) with a final thickness of 20 nm on top of the polymer layer. An evaporated 20 nm thick LiF layer (evaporation rate 1 Å/s; pressure < 5*10-6mbar) is covering all samples for surface passivation. EXPERIMENTAL SETUP The ultrafast transient absorption setup is based on a commercial Yb:KGW regenerative amplifier system operating at 50 kHz repetition rate. The output pulses drive a home-built noncollinear optical parametric amplifier (NOPA) which delivers an ultra-broadband spectrum centered at 570 nm (2.18 eV). The spectrum spans 0.6 eV and, after compression, pulse durations of 8 fs are achieved. We probe the change induced in the transmission by an ultra-broadband supercontinuum spectrum generated in a YAG crystal. Similarly to the pump pulses, we compress this visible spectrum with chirped dielectric mirrors to ensure a high overall temporal resolution in the experiment. In the near-IR experiments, we compress the long-wavelength tail of the supercontinuum to probe dynamics at wavelengths from 1100 nm to 1400 nm. Pump and probe pulses are focused to beam diameters of 100 µm and overlapped at a small angle on the samples. After filtering the pump radiation, we record the induced change in transmission with a fast spectrometer. This device is triggered at the repetition rate of the laser and allows us to acquire the full spectrum of each laser shot individually. By modulating the pump pulse train at half the repetition rate (25 kHz) with a pockels cell it is possible to calculate the differential transmission change

of the probe pulse as a function of wavelength and time delay between


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pump and probe. The detection of the differential signal in the near-IR is based on lock-in readout of a diode coupled to a monochromator. UV/Vis absorbance measurements have been performed with an Agilent Cary 5000 UV-Vis-NIR spectrometer equipped with an integrating sphere. ASSOCIATED CONTENT Supplementary Information. In the Supplementary Information UV/Vis, AFM and additional TAS spectra are shown. Furthermore, we have included transfer matrix calculations that reveal differences in the generation yield of excitons in the films, while using the same laser fluence. AUTHOR CONTRIBUTION A.G. and P.E. have contributed equally to this work. P.E. fabricated the samples, measured UV/Vis linear absorption performed simulations and has contributed to the data analysis. J.K. prepared TiO2 films of various crystallinity. A.G. and A.B. developed the TAS systems. A.G. has performed TAS measurements and the data analysis. L.S.-M. and D.B supervised the experimental work. All authors have contributed to the discussion and writing of this manuscript. ACKNOWLEDGMENT We acknowledge funding from the Deutsche Forschungsgemeinschaft through the Emmy Noether program (BR 5030/1-1), the German Federal Ministry of Education and Research (BMBF, MesoPIN project) and the Baden-Württemberg foundation (SuperSol project).

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Incoherent Pathways of Charge Separation in Organic & Hybrid Solar

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