A bis(diketopyrrolopyrrole) dimer-containing ligand in platinum(ii) polyyne oligomers exhibiting ultrafast photoinduced electron transfer with PCBM and solar cell properties†
Two polyyne oligomers using the thiophene–(diketopyrrolopyrrole)–thiophene unit, T–DPP–T, and the organometallic synthon trans-Pt(PBu3)2(CC)2, [Pt], ([L1]–[Pt])m (P1), here used for comparison purposes, and ([L4]–[Pt])m (P4), where L1 and L4 are T–DPP–T and (T–DPP–T)2, respectively, were synthesized by CuI-catalyzed dehydrohalogenation and fully characterized. The optical and electrochemical properties were investigated by UV-visible absorption and cyclic voltammetry, and P4 was found to exhibit a very low band gap. DFT (density functional theory) and TD-DFT (time-dependent DFT) computations were undertaken to address the polymers’ geometry, their electronic structures and calculated HOMO and LUMO energies. The S1 level is best described as a ππ* excited state. P1 and P4 exhibit fluorescence lifetime in ps timescale and the fs-TAS data indicate an ultrafast electron transfer to PC61BM ((1Px* + PC61BM → Px+˙ + (PC61BM)−˙; time scale < 113–128 fs) evidenced by an increased formation of the triplet state upon back electron transfer (Px+˙ + (PC61BM)−˙) → 1,3Px* + PC61BM; x = 1, 4). The timescale of recombination for P4 (P4+˙ + (PC61BM)−˙ → 3P4* + PCBM) is found to be ∼3 ps. The photovoltaic performance of Px was investigated by fabricating the photovoltaic solar cells (PSCs) with conventional device structure ITO/PEDOT:PSS/(P1 or P4):PC71BM/PFN/Al. After solvent vapour annealing with THF, the power conversion efficiencies, PCEs, are 7.36% (P1; Jsc = 12.94 mA cm−2, Voc = 0.92 V, FF = 0.62) and 9.54% (P4; Jsc = 16.24 mA cm−2, Voc = 0.89 V, FF = 0.66). The higher PCE of P4 may be related to the faster extraction and longer charge carrier lifetime for P4 based PSCs as compared to P1.