Benzodithiophene unit copolymerization to improve the stability of thiophene-based organic solar cells

With the increasing performance of organic solar cells (OSCs), the design of stable donor materials is of paramount importance. In this work, we synthesised a simple alternating copolymer with thiophene and BDT (P3T-BDT) units to understand the impact of incorporation of BDT on the thermal stability of the commonly used thiophene-based OSCs. OSCs fabricated with P3T-BDT:PC71BM and P3HT:PC71BM with 1:4 ratios displayed a different degradation kinetics despite their similar initial power conversion efficiency (PCE). The PCE of P3HT-based device degraded by half while the P3T-BDT-based device lost only 15% of its initial PCE after thermally being degraded at 85 ∘C for 48 h mainly due to the decrease in J s c . Thermally-induced degradation of P3HT caused chain breakage while the P3T-BDT remained intact. The surface morphology of P3HT:PC71BM film exhibited a large increase in roughness in the first few hours leading to a loss of more 40% of its initial PCE while the P3T-BDT based OSC had a similar roughness throughout the accelerated ageing. This is also confirmed by the growth of the PL intensity of P3HT:PC71BM while P3T-BDT:PC71BM remained the same confirming a suppression of demixing of the donor and acceptor in the latter. In summary, the addition a BDT unit in the thiophene-based homopolymer was found to improve the thermal stability thiophene-based OSCs.