Mechanism of the Photodegradation of A-D-A Acceptors for Organic Photovoltaics
In this work, we elucidate the photodegradation pathway of IT-4F, a benchmark A-D-A type semiconductor for organic photovoltaics. The photoproducts were isolated and shown to be isomers of IT-4F formed via a 6-e electrocyclic reaction between the dicyanomethylene unit and the thiophene ring, followed by a 1,5-sigmatropic hydride shift. Importantly, this photoisomerization is accelerated under inert conditions which is explained by DFT calculations that predict the reaction to occur via the excited triplet state (quenchable by oxygen). Adding controlled amounts of the photocyclized product P1 to PM6:IT-4F bulk heterojunction cells shows a progressive decrease of photocurrent and fill factor attributed to its poorer absorption and charge transport properties. The power conversion efficiency drops from 12% for pure IT-4F to 3% for pure P1 acceptor. This cyclization is a general photodegradation pathway for a series of analogous A-D-A molecules with 1,1-dicyanomethylene-3-indanone termini. However, the rate of the reaction varies widely with the nature of the donor moiety.
<!>Scheme 1. Mechanism of the photodegradation of IT-4F.
<p>The past decade has seen a remarkable progress in the field of organic photovoltaics (OPVs) holding the promise of clean, low-cost and renewable energy sources. 1,2,3,4 The greatly improved device performance is mainly attributed to the design of new semiconducting materials. In particular, the use of nonfullerene acceptors (NFAs) enhances light absorption and expands spectral coverage, resulting in significantly higher photocurrent compared to conventional fullerene cells. 5,6 Furthermore, the structural diversity of NFAs offers excellent synthetic flexibility and tunability, realizing a wide range of structures with highly optimized electronic characteristics. 7,8,9,10 Among all NFAs, the acceptor-donor-acceptor (A-D-A) architecture has proven to be the most successful, owing to high absorptivity in the visible and near-infrared (NIR) region and a special solid state packing with 3D π-π interactions that lead to efficient charge transport. 11,12,13,14 The emergence of new A-D-A structures including IT-series 15,16,17 and Y-series 18,19,20 has pushed the power conversion efficiency (PCE) beyond 18% in single-junction organic solar cells, closing the gap with inorganic photovoltaic technologies. Despite these breakthrough performance of NFA-based OPVs, their stability is typically lower than that of fullerene-based devices, and remains a major hurdle for commercialization. 21,22 Among many factors (such as morphological change), photodegradation of the active layer material plays a crucial role in the deterioration of device performance, yet relatively little effort was devoted to understanding its mechanism. 23,24 Several degradation pathways were proposed in the past. Most of these assume photooxidation as the key mechanism, 25,26 although photocatalytic degradations of the vinylene bridge at the interface (with ZnO electron extraction layer) have also been proposed. 27,28,29 However, no unambiguous identifications of degradation products have been reported, and this limited understanding of the molecular origin of degradation remains a key challenge for improving the stability of OPVs. 30,31 Here, we uncover a new mechanism of the photodegradation of A-D-A NFAs which is based on intramolecular cyclization involving the 1,1-dicyanomethylene-3-indanone (INCN) moiety. As an isomerization reaction, it cannot be suppressed by eliminating oxygen (with antioxidants or encapsulations) but can be effectively mitigated by the structural design of the A-D-A molecule.</p><p>We have chosen IT-4F as one of the current benchmarks in OPVs: it's (fluorinated) INCN termini have recently become the most popular acceptor group in the community (Fig. S1). The material was subjected to intense illumination in preparative scale to obtain sufficient amounts of photoproducts for detailed structural analysis, optical characterizations and device studies. During this process, two major higher-polarity products were formed sequentially (Fig. 1a) and were isolated via column chromatography. MALDI-MS shows the molecular weight of both species identical to IT-4F, confirming their isomeric nature (Fig. 1b). Their molecular structures were determined with 1 H and 13 C NMR (including 2D correlation) spectroscopy (Fig. S9~S16), and the proposed mechanism of reaction involving cis-trans isomerization (1  2), electrocyclic reaction (23) and sigmatropic shift (34) is shown in Scheme 1.</p><!><p>The first degradation product (P1) is an asymmetric isomer of IT-4F resulting from an electrocyclic ring closure involving one terminal INCN moiety, and a subsequent 1,5-hydride shift which restores the conjugation across the backbone. A similar process on the second INCN moiety leads to the fully fused P2. Formed by reversible aldol condensation, the vinylene bond bridging the donor and acceptor moieties was often speculated to be the origin of the photoinduced decomposition of IT-series and many other A-D-A type NFAs. 26,27,29 However, our results unequivocally show that the photodegradation involves the dicyanomethylene moiety, while the vinylene bridge is not the immediate reaction center. The 1 H NMR spectrum of both degradation products features an sp 3 singlet at ~4.7 ppm (Fig. S2), which was also observed in a recent photostability study and attributed to the formation of epoxides upon photooxidation of the vinylene bridge. 26 However, the corresponding 13 C chemical shift of the degradation product we obtained (~45 ppm, Fig. S14) is not consistent with that for epoxides, which is expected to be >55 ppm. Being isomers of IT-4F, P1 and P2 are undetectable via MS which is commonly applied in mechanistic studies of OPV photodegradation. Importantly, this process does not involve oxygen or water and even proceeds at a slightly faster rate under an inert atmosphere (Fig. 3).</p><p>Compared to IT-4F, the absorption maxima of P1 and P2 are blue-shifted by 45 nm (0.13 eV) and 164 nm (0.57 eV), respectively (Fig. 1c and Table S1). This shift of the intramolecular charge transfer band is explained by the decoupling of the strongest acceptor moiety (dicyanomethylene) from the donor core. A similar trend is also found in the emission spectra. However, while IT-4F To support the proposed mechanistic hypothesis, we performed DFT calculations to find the transition states along the proposed reaction path (Fig. 2). First, the initial structure (1) must undergo a cis-trans isomerization of the vinylene bridge to adopt proper orientation of the terminal group (2). This isomerization was already suggested to be the initial step of the photodegradation in related INCN derivatives although its exact role was not fully understood. 26 The activation energy for the electrocyclic ring closure (2-3) in the singlet state (S0, 35 kcal mol -1 ) is too high for a thermally driven reaction at this temperature. However, a much lower barrier of 19 kcal mol -1 was calculated in the triplet state (T1), as expected for a photo-driven transition. Although a large activation barrier (41 kcal mol -1 ) is predicted for the subsequent 1,5-hydride shift (3-4) along the adiabatic (T1T1) path, an intersystem crossing from intermediate 3 may achieve the transition to the ground state product at a cost of <10 kcal mol -1 (dashed curve). The electrocyclic reaction being the rate-limiting step is in line with the fact that intermediate 3 was not experimentally observed. Predictably, restricted molecular motions in the solid state suppress the photocyclization of IT-4F under similar illumination conditions: >50% degradation was observed only after ~5 days (as compared to <1 h in solution). Further deceleration of the reaction in PM6:IT-4F blends (35% conversion after 8 days, Fig. S3) could be explained by the photoinduced electron transfer depopulating the reactive excited state. In both cases the formation of P1 in thin films was confirmed spectroscopically (characteristic absorption at ~650 nm) and chromatographically (Fig. S4). This indicates that the photocyclization reaction is of prime importance to OPV devices, where oxygen concentration is typically low.</p><p>Bulk heterojunction OPV cells with PM6 (Scheme S1) as the donor polymer were fabricated to examine the implication of the formation of P1 on device performance (Table 1 and Fig. S5).</p><p>Already at 1:1 IT-4F:P1 ratio, the PCE drops from 12% to ~9%, and it further deteriorates to ~3%</p><p>for pure PM6:P1 blends. This drop of performance largely originates from the drop of short-circuit current density (JSC) and fill factor (FF). The lower NIR absorption of P1, as well as the steric hindrance from the protruding cyano groups leading to weaker intermolecular π-π interactions and lower electron mobility, are the two possible reasons for the observed changes.</p><p>Table 1. Effect of P1 on the performance of Glass/ITO/PEDOT:PSS/Blend/PDINN/Ag cells. PM6 : IT-4F : P1 VOC / V JSC / mA cm -2 FF / % PCE / % 0.5 : 0.5 : 0 0.91 ± 0.01 19.3 ± 0.3 69 ± 1 12.0 ± 0.3 0.5 : 0.25 : 0.25 0.92 ± 0.01 16.3 ± 0.4 59 ± 1 8.9 ± 0.3 0.5 : 0 : 0.5 0.89 ± 0.02 9.4 ± 0.6 36 ± 2 3.1 ± 0.2</p><p>We tested the photostability of a similar NFA, previously reported IDTT-4F with non-fused backbone, 32 as shown in Scheme 2. IDTT-4F showed similar degradation pathway (Fig. S6) but much higher photostability (Fig. 3 and Fig. S7). The predicted activation energy for its electrocyclic reaction (~21 kcal mol -1 ) is 2 kcal mol -1 higher than that of IT-4F, as shown in Table 2. The</p>