Field-Assisted Exciton Dissociation in Highly Efficient PffBT4T- 2OD

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Field-Assisted Exciton Dissociation in Highly Efficient PffBT4T-2OD:Fullerene Organic Solar Cells Andreas Weu, Thomas R. Hopper, Vincent Lami, Joshua A. Kress, Artem A. Bakulin, and Yana Vaynzof Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00094 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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Chemistry of Materials

Field-Assisted Exciton Dissociation in Highly Efficient PffBT4T2OD:Fullerene Organic Solar Cells Andreas Weu†‡, Thomas R. Hopper§, Vincent Lami†‡, Joshua A. Kress†‡, Artem A. Bakulin§*, and Yana Vaynzof†‡

†

Kirchhoff Institute for Physics, Im Neuenheimer Feld 227, 69120 Heidelberg, Germany Centre for Advanced Materials, Im Neuenheimer Feld 225, 69120 Heidelberg, Germany § Department of Chemistry, Imperial College London, London SW7 2AZ, United Kingdom ‡

*E-mail: [email protected]

Abstract: Understanding the photophysics of charge generation in organic semiconductors is a critical step towards the further optimization of organic solar cells. The separation of electron-hole pairs in systems with large energy offsets is relatively well understood; however, the photophysics in blends with low driving energy remains unclear. Herein, we use the material system PffBT4T-2OD:PC71BM as an example to show that the built-in electric field plays a critical role towards long-range charge separation in high-performance devices. By using steady state and time-resolved spectroscopic techniques we show that in neat films an energetic barrier impedes polymer exciton dissociation, preventing charge transfer to the fullerene acceptor. In complete devices, this barrier is diminished due to the built-in electric field provided by the interlayers/contacts and accompanying space-charge distribution. The observed behavior could also be relevant to other systems with low driving energy and emphasizes the importance of using complete devices, rather than solely films, for photophysical studies.

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Main text: Polymer-based organic solar cells (OSCs) provide a promising alternative to conventional photovoltaics. The cheap and easy production process of the devices along with the flexibility and semi-transparent material properties are key advantages compared to their inorganic counterparts. Since the first reported synthesis in 2014, poly[(5,6-difluoro-2,1,3benzothiadiazol-4,7-diyl)-alt-(3,3′′′-di(2-octyldodecyl)-2,2′;5′,2′′;5′′,2′′′-quaterthiophen-5,5′′′diyl)]

(PffBT4T-2OD) has gained wide-spread attention as a high-performance donor

material for OSCs, owing to the superb aggregation properties and crystallinity of the polymer when blended with the common fullerene acceptor [6,6]-Phenyl-C71-butyric acid methyl ester (PC71BM) in a bulk heterojunction device.[1] While much effort has been dedicated to optimizing the processing of PffBT4T2OD:PC71BM blends and fabrication of devices,[1–6] relatively little attention has been paid to the essential process of charge photogeneration in such systems. This process is generally well understood for polymer:fullerene systems,[7–9] and tends to display a strong dependence on materials properties; particularly donor:acceptor (D:A) phase segregation. Nevertheless, for a broad range of previously studied polymer:fullerene systems, charge photogeneration appears to be insensitive to modest electric fields when an external bias (comparable to the potential difference between the electrodes) is applied to the devices. Specifically, early photophysics within the active layers remains unchanged, and exciton quenching occurs with near-unity efficiency as soon as donor and acceptor are in contact.[10] These observations are typically rationalized by considering the effect of delocalized states on the rapid motion of charges, and by taking into account the large energy offsets between donor and acceptor molecular orbitals, which provides a substantial driving force for exciton dissociation.[11–15] Indeed, most polymer:fullerene systems with small energy offsets exhibit expectedly low charge generation yields and power conversion efficiencies (PCEs).[16–18] Exceptions to this trend exist,[19,20] with PffBT4T-2OD:PC71BM acting as a benchmark system for high2 ACS Paragon Plus Environment

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performance OSCs with low driving force. Such behaviour, and the advent of highly-efficient non-fullerene OSCs with similarly small energy offsets,[21–24] calls for a possible re-evaluation of the photophysics underlying the performance of such systems, with particular emphasis on elucidating the phenomena which influence the dynamics of charges within the devices. The role of external electric fields towards charge photogeneration in polymer-based devices has appeared in numerous works spanning over four decades. Early studies on poly(phenylenevinylene) devices showed that a very high external bias (1-2 orders above solar cell operation) was required to quench the photoluminescence (PL) of the polymer excited state by facilitating the dissociation of excitons into bound electron-hole (geminate) pairs.[25,26] Similarly, transient absorption (TA) experiments on ladder-type methyl-substituted poly(paraphenylene) demonstrated increased charge separation yields with large applied biases.[27] In the last decade, the research has mostly shifted to the field-dependent separation of charge transfer (CT) states in bulk heterojunction materials,[28–34] where the yield of singlet exciton dissociation is close to unity. For example, Gerhard et al. recently reported on fieldinduced exciton dissociation in PTB7:PC71BM solar cells, showing that small electric fields are able to separate relaxed interfacial electron-hole pairs.[35] In contrast to field-dependent CT state splitting, the effect of fields on the dissociation of singlet excitons in D:A blends has not been regarded as a subject of interest, and was generally neglected. The obvious reason for this, as mentioned previously, is the observation of near-unity PL quenching, without any field assistance, in many traditional polymer:fullerene materials. Currently, the development of low-offset systems like PffBT4T2OD:PC71BM is shifting the photophysics of OSCs to a new performance regime, where external and internal electric fields may play a substantial role in exciton dissociation and photocurrent generation. So far, in the case of PffBT4T-2OD:PC71BM, recent photophysical studies by Cha et al. have suggested that this high efficiency is attributed to low geminate recombination (GR) losses within the blend.[36] Still, the ultrafast photophysics occurring in 3 ACS Paragon Plus Environment


complete PffBT4T-2OD:PC71BM devices, and therefore the precise effect of electric fields on the charge dynamics of this system, is hitherto unreported. In this study, we choose a ‘practical’ approach and directly compare charge dynamics in material films with that in the complete devices. Using a combination of steady-state and time-resolved PL experiments along with ultrafast pump-probe and pump-push-photocurrent (PPPC) techniques, we are able to show the influence of the internal field on initial exciton dissociation and generation of charge carriers in highly efficient (10.75%) PffBT4T2OD:PC71BM devices. This internal field is present in complete OSCs, due to the built-in potential generated by the work function differences of the extracting layers/contacts, but absent in blend films. We suggest that the presence of this field reduces the energy of CT states in PffBT4T-2OD:PC71BM blends which mediate the conversion from excitons to free carriers. This effect explains why we observe efficient charge formation in the devices, but not in the isolated blends outside of a device structure.

Figure 1. (a) Chemical structures of PffBT4T-2OD and PC71BM. (b) UV-Vis absorption spectra of pristine PffBT4T-2OD and PC71BM films used to determine the optical bandgap of each component (1.65 eV and 1.85 eV, respectively). (c) UPS data for PffBT4T-2OD and PC71BM used to estimate the HOMO energy level of each component (5.35 eV and 5.90 eV with respect to the vacuum level). (d) Band diagram of the PffBT4T-2OD:PC71BM blend. (e) Standard and (f) inverted architecture devices. (g) J-V characteristic of the best-performing (inverted architecture) solar cell with efficiency of 10.75%.

To obtain an accurate value for the energetic offset between the PffBT4T-2OD donor and the PC71BM acceptor, we performed UV-Vis absorption and ultraviolet photoemission spectroscopy (UPS) measurements. The chemical structures of the materials are provided in Figure 1a, alongside the UV-Vis absorption spectra of the neat materials in Figure 1b. By fitting the onset of the band edge we obtain optical bandgaps of 1.65 eV and 1.85 eV, 4 ACS Paragon Plus Environment

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respectively, which are in agreement with previously reported results.[1,6,36] UPS was employed to determine the ionization potential, and thus energy of the highest occupied molecular orbital (HOMO) of each material. As depicted in Figure 1c, fitting the low energy offset reveals HOMO energy levels of 5.35 eV for PffBT4T-2OD, and 5.90 eV for PC71BM. Assuming that the optical and transport gaps are similar, the results can be used to construct the band diagram in Figure 1d, which outlines the offset between the lowest unoccupied molecular orbitals (LUMO) of the materials (~0.35 eV). As mentioned previously, this driving force is quite small in comparison to most other OSC material systems; the photophysical implications of this are discussed in more detail later. Two possible architectures can be used for OSC fabrication; standard (Figure 1e) and inverted (Figure 1f). In the former, holes are extracted through a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) layer and electrons through a Ca/Al electrode. In the latter, thin layers of ZnO and MoO3 are used for electron and hole extraction, respectively. In agreement with previous reports,[1,5,6,36,37,4,38,39] the performance of OSCs that employ PffBT4T2OD:PC71BM as an active layer is very high, resulting in a power conversion efficiency (PCE) of 10.75% (Figure 1g).

Figure 2. Steady-state PL spectra (a) and time-resolved PL decay dynamics at 735 nm (b) for the pure polymer film, optimized blend film, and blend devices (standard and inverted architecture). IRF: instrument response function.

Steady-state and time-resolved photoluminescence (PL) experiments were conducted to investigate electron-transfer from the photoexcited polymer donor to the fullerene acceptor in the PffBT4T-2OD:PC71BM blend. The samples were excited at 600 nm and 635 nm in the former and latter cases. Since PC71BM does not exhibit strong absorption at these wavelengths (Figure 1a), the polymer donor is mainly excited. Figure 2a shows the PL 5 ACS Paragon Plus Environment


emission of a pristine PffBT4T-2OD film. The emission maxima at ~730 nm and overall spectral shape resembles that found in the literature.[1,2,36] Limited quenching of the polymer PL is observed in the PffBT4T-2OD:PC71BM film, as marked by a mere ~20% reduction in PL intensity. This reduction is accompanied by a modest decrease in the decay lifetime from 1.3 ± 0.1 ns for the polymer to 1.1 ± 0.1 ns for the blend (Figure 2b), which may simply be the result of subtle morphological differences to the polymer upon blending with PC71BM. These lifetimes correspond to a PL quenching efficiency of ~16%, in agreement with the aforementioned ~20% reduction in the steady-state PL intensity. Interestingly, the time-resolved measurements show that the PL decay of both the polymer and blend films is seemingly monoexponential, with no additional decay features characteristic of excitonic quenching by CT processes that are commonly present in highperformance material systems.[40] To investigate the disparity between high device performance and weak excitonic quenching, the PL of the complete OSCs was measured in both the standard and inverted architectures. The quenching effect in functional OSCs is significantly stronger than that observed in the films. In Figure 2a, a ~90% reduction in PL intensity is observed in the inverted OSC with respect to the polymer film. The decay kinetics in Figure 2b also exhibit an additional quenching feature at early times which, in agreement with the steady-state results, engenders a ~93% reduction in the relative PL intensity. Biexponential fitting of the decay curve reveals that this additional component has a lifetime of 0.13 ± 0.07 ns. This initial process dominates the dynamics up to ~1 ns, and is superimposed with a decay at later times which has a lifetime of ~1 ns, akin to the decay curve of the blend film. Similar results are obtained for the standard OSCs. These findings suggest that no additional quenching mechanism is present in the blend film, and that the observed emission is purely due to radiative recombination of the PffBT4T-2OD exciton. The initial decay component observed for the complete devices is therefore attributed to CT to the PC71BM acceptor, as one would expect to find in the D:A blend system. 6 ACS Paragon Plus Environment

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The fact that strong quenching is observed in the complete blend devices, but not in the blend films is a notable result which suggests that the thermodynamic driving force provided by the LUMO-LUMO offset in the blend is insufficient to afford substantial CT. One possible explanation for the quenching being present solely in complete OSCs is the effect of interfaces with the charge extracting layers/contacts.[41–46] Alternatively, the internal electric field generated by the work function difference between the charge extracting layers/contacts in the device may play a role in determining the quenching efficiency. To discount the role of interfacial effects towards the strong excitonic quenching in devices, control experiments were performed with PffBT4T-2OD in contact with the single charge extraction layers (top or bottom electrodes outside of a device structure). Figure S1 shows that only minor quenching is observed by steady-state and time-resolved measurements, even when the polymer active layer is integrated into a complete OSC device with both electrodes present. We speculate that this relatively weak quenching arises from morphological factors.[1,6] To support the latter argument based on the electric field, a “symmetric device” with two equal electrodes was approximated with PffBT4T-2OD:PC71BM sandwiched between a ZnO bottom electrode and a ZnO-nanoparticle top electrode. The quenching in the “symmetric device” is much weaker than in the complete OSC, and in fact matches very closely with the isolated blend (Figure S2). This convincingly demonstrates that the PL quenching in the blend active layer is driven by the electric field derived from the mismatch of electrode energy levels. To further examine the dynamics of the excited states in films and devices, we employed ultrafast TA spectroscopy, which is sensitive to both exciton dissociation and charge formation. PffBT4T-2OD excitons were generated by a 700 nm pump, and the photoinduced absorption (PIA) of the broadband near-infrared (NIR) probe was mapped out as a function of the pump-probe delay time. The pump fluence was set to 2.5 µJ cm-2; at higher fluences, high-order exciton-exciton annihilation kinetics become increasingly 7 ACS Paragon Plus Environment


prominent (Figure S3). Figure 3a compares the PIA decay for the polymer film, blend film and inverted architecture blend device at the same pump fluence (2.5 µJ cm-2) and probe wavelength (1100 ± 10 nm, where the TA signal is strong in all three cases; the full TA maps and time-resolved spectral information are provided in Figure S4). All three transients in Figure 3a possess roughly the same decay profile in the first 50 ps; a rapid build-up within the time-resolution of the setup (~100 fs) attributed to exciton formation, followed by a much slower decay associated with exciton recombination. Beyond 50 ps, both the polymer and blend film transients continue to decay with a lifetime of ~200 ps, which is comparable to the PffBT4T-2OD exciton lifetime reported in the literature at the same pump fluence.[47] The fact that this time constant is shared by both the polymer and blend transients suggests that, in the blend system reported herein, the dissociation of the PffBT4T-2OD excitons is not accelerated by the presence of the PC71BM acceptor. While these results are consistent with the steadystate and time-resolved PL findings presented above, the apparent lack of CT in the blend film is in stark constant with previous studies on PffBT4T-2OD:PC71BM.[1,36] The only explanation we can offer is a difference in the preparation conditions of the blend (sample concentration, glovebox conditions, annealing temperature, spin-coating rates etc.).[6] In any case, the devices prepared from our blends appear to perform efficiently.

Figure 3. (a) Normalized TA kinetics for the polymer film, blend film and inverted architecture blend device. Pump: 700 nm, 2.5 µJ cm-2; Probe: 1100 ± 10 nm. Also shown are the normalized SVD results for the blend film (b) and inverted architecture blend device (c). Multiexponential fits are shown as a guide to the eye.

The most striking result in Figure 3a is the long-lived component observed in the complete device, which is absent from the polymer and blend film transients. It should be noted that similar findings were obtained from the standard architecture OSCs (Figure S5), which reasserts that exciton dissociation in both device architectures is governed by the same 8 ACS Paragon Plus Environment

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photophysics. To understand the origin of the additional PIA feature, singular-value decomposition (SVD) was applied to the data to mathematically deconvolute the TA signal into excitonic (SVD-1) and polaronic (SVD-2) contributions.[48] The full results are presented in Figure S6. For the blend device (Figure 3c), the normalized SVD-2 kinetics emerge on the ~20 ps time scale, commensurate with the initial exciton decay reflected by the SVD-1 kinetics. After ~100 ps, the decay of the SVD-2 signal continues beyond the time limit of the set-up (>6 ns). The growth and decay of SVD-2 are interpreted as the formation and subsequent recombination of free charges in the device. The long lifetime of the final decay component suggests that the nature of charge recombination is mainly bimolecular, in consensus with recently reported TA experiments on this material system.[36] The SVD-2 component for the blend film in Figure 3b follows the same pattern as for the device, although the amplitude of the signal is markedly lower in accordance with a smaller population of free charges. Slight differences in the observed kinetics may be explained by the effect of accumulated charges in the device electrodes, which according to similar in situ TA studies on complete OSCs, can influence charge transport and recombination rates.[49] It is apparent that exciton dissociation in the PffBT4T-2OD:PC71BM (and pure PffBT4T-2OD) films is a limiting step towards the generation of free charges. The behaviour of excitons in the polymer and blend devices was probed further by PPPC spectroscopy. Following excitation of the device active layer by a visible (650 nm) “pump”, the resultant bound excitons are separated by a time-displaced NIR “push” pulse, and the additional photocurrent associated with this process (dJ/J, where dJ is the push-induced photocurrent, and J is the pump-induced photocurrent) is plotted as a function of the pump-push delay time.[12,41–44] A 1300 nm push was selected, where the TA response is mainly associated with PffBT4T-2OD excitons (Figure S4). The PPPC transient in Figure 4a shows that for the pure polymer device, early (

Field-Assisted Exciton Dissociation in Highly Efficient PffBT4T- 2OD

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