Solution Processed Organic Solar Cells Based on Fullerene and Non-Fullerene Acceptors

Abstract

Organic solar cells (OSCs) can lead to innovative niche photovoltaic applications due to their unique, distinguishing properties compared to traditional solar cell technologies such as low weight, flexibility, esthetic possibilities and large area production through solution processing. In this dissertation the focus is on solution-processed OSCs based on fullerene and non-fullerene acceptors, by investigating a broad variety of novel conjugated polymers. First of all, three random terpolymers (P1, P2, and P3) with donor–acceptor–donor–acceptor molecular configuration, comprising fluorinated benzotriazole (FTAZ) and thienothiophene-capped diketopyrrolopyrrole (TTDPP) as the first and second electronaccepting moieties and thienyl-substituted benzodithiophene (BDTT) as the electron-donating unit, were designed and synthesized. By tuning the ratio of TTDPP and FTAZ, the optoelectronic properties of the terpolymers were systematically varied. All materials exhibited a broad absorption window spanning from 300 to 900 nm, illustrating the success of the terpolymer approach. Fullerene-based OSCs fabricated from the terpolymer with the highest content of TTDPP (P1) afforded a power conversion efficiency (PCEs) of 5.7%, with a short-circuit current density (Jsc) of 15.2 mA cm–2 . On the other hand, solar cell devices composed of the terpolymer with the lowest content of TTDPP (P3) and the narrow gap non-fullerene acceptor IEICO-4F exhibited a higher efficiency of 6.3%, with an enhanced Jsc of 17.6 mA cm–2 , as a result of a better complementarity in the absorption of the donor and acceptor materials and well-balanced charge carrier mobilities. This efficiency represented the best value for non-fullerene OSCs based on DPP-containing polymers at the time of publication of this work. In the next phase, donor copolymers P4, P5, and P6, based on 2H-benzo[d][1,2,3]triazole-5,6- dicarboxylic imide, were studied. Both fullerene and non-fullerene OSCs were prepared and characterized. The fullerene-based OSCs fabricated using 3% 1-chloronaphthalene (CN) as a solvent additive afforded PCEs of 7.6, 7.2, and 7.2% for P4, P5, and P6, respectively, with enhanced hole and electron mobilities. Furthermore, non-fullerene devices with an inverted device architecture were also fabricated in combination with ITIC, affording PCEs of 8.4, 6.4, and 7.6%, respectively, by controlling the morphology of the active layer with 1,8-diiodooctane (DIO) as a solvent additive and thermal annealing at 120 °C. Inspired by the results obtained, we further extended our investigation toward intrinsically stable OSCs through studying the in-situ intrinsic thermal window of operation (from 30 to 180 °C) for both conventional and inverted type devices. It was found that both the fullerene and non-fullerene OSCs functioned well over a large thermal window with excellent thermal stability. In another part of the dissertation, the effect of side chain variations on the photophysical, morphological and photovoltaic properties of two novel high gap donor polymers P7 and P8 in combination with both fullerene and non-fullerene acceptors were investigated. P7 showed a deeper highest occupied molecular orbital (HOMO) energy level, which resulted in higher open-circuit voltages (Voc). Nevertheless, the polymer solar cells fabricated using P8 in combination with PC71BM afforded a higher PCE of 7.3% (vs 4.0% for P7:PC71BM). By using the non-fullerene acceptor ITIC, better device efficiencies of 6.9% and 9.6% for P7:ITIC and P8:ITIC, respectively, were achieved. The better performance of P8:ITIC is attributed to the broad absorption, balanced charge transport and favorable blend morphology, and is among the best TzBI-containing non-fullerene solar cells. Furthermore, two push-pull type high band gap conjugated polymers P9 and P10, based on the ladder-type donor unit indacenodithieno[3,2-b]thiophene (IDTT) and bithiazole (BTz) as the acceptor component were studied. The polymers exhibited relatively high optical gaps of ~2.0 eV with strong absorption in the range of 400 to 600 nm. Electrochemical investigations indicated a lower HOMO energy level (–5.58 eV) for P10 as compared to P9 (–5.46 eV), enabling to achieve a higher Voc. The PCE of 5.1% achieved from solar cells based on P9:PC71BM blend was higher than those obtained from P10:PC71BM (4.6%) blend. The photovoltaic performances of five benzotriazole (BTZ)-based conjugated copolymers P11–P15 were also investigated. The polymers showed high thermal stability and HOMO energy levels from –5.33 to –5.40 eV. Due to the deep HOMO level of P13, the Voc of the device improved to 0.88 V. The PCEs of the devices were all in the range from 1.8 to 2.1%. Finally, a series of low-HOMO level conjugated copolymers P16–P22, based on pyrrolobenzothiadiazoledione, pyrrolobenzotriazoledione, and benzothiadiazole were studied for organic photovoltaic applications. The OSCs fabricated from P16–P22 afforded a PCE from 1.0% to 3.8% with a high Voc up to 1.06 V. Furthermore, some low band gap copolymers P23–P30 with extended absorption ≥ 1000 nm were also investigated in OSCs. They afforded very low PCEs from 0.02% to 0.5%. However, such low band gap polymers with broad spectral response from 300 to 1500 nm and different bands in the ultraviolet and near-infrared (NIR) range may be interesting for NIR photodetector applications. Non-fullerene OSCs based on indacenodithienothiophene-altbenzothiadiazole copolymers P31 and P32 were then fabricated as well. The best PCE obtained was 1.0%, with a relatively high Voc of 0.96 V for P32:ITIC. Furthermore, all-polymer solar cells were also fabricated from a 2H-benzo[d][1,2,3]triazole-5,6-dicarboxylic imide donor polymer and naphthalene diimide-based acceptor polymers P33 and P34. The best PCE achieved was 1.7% for P4:P34-based devices.