Efficient Solar Cells from Semi-random P3HT Analogues Incorporating Diketopyrrolopyrrole

Three novel semi-random poly(3-hexylthiophene) (P3HT) based donor–acceptor copolymers containing 5–15% of the acceptor diketopyrrolopyrrole (DPP) were synthesized by Stille copolymerization and their optical, electrochemical, charge transport and photovoltaic properties were investigated. Poly(3-hexylthiophene-thiophene-diketopyrrolopyrrole) polymers with various percentages of DPP, represented as P3HTT-DPP-5%, P3HTT-DPP-10%, and P3HTT-DPP-15%, had high molecular weights (M n 17 500–24 500 g/mol) and broad, intense absorption spectra, covering the spectral range from 350 nm up to 850 nm with absorption maxima at 685 and 703 nm for P3HTT-DPP-10% and P3HTT-DPP-15%. The low content of acceptor units allowed preservation of many important properties of P3HT, among which are semicrystallinity (as verified by GIXRD) and high hole mobilities (μ h = (1–2.3) × 10–4 cm2/(V s)). Photovoltaic devices fabricated from P3HTT-DPP polymers blended with PC61BM showed higher short-circuit current densities (J sc ) of 9.57 – 13.87 mA/cm2 compared to P3HT (9.49 mA/cm2), leading to average power conversion efficiencies of 3.6–4.9%, which exceed the average value of 3.4% for P3HT under AM 1.5G illumination (100 mW/cm2). External quantum efficiency (EQE) measurements revealed a strong photoresponse from the semirandom polymers up to 850 nm, with EQE values above 40% across the visible and into the near-infrared for P3HTT-DPP-10% and P3HTT-DPP-15%. These results indicate that semi-random P3HT analogues provide a simple and effective route toward polymers with a broad photocurrent response in bulk heterojunction solar cells.