Effect of Mesogenic Side Groups on the Redox, Photophysical, and Solar Cell Properties of Diketopyrrolopyrrole-trans-bis(diphosphine)diethynylplatinum(II) Polymers

Three ([Pt]-DPP)n polymers PA, PB, and PC bearing cyanobiphenyl, alkyloxyphenyl, and triphenylene self-assembling groups were synthesized. The optical, electrochemical, photophysical, and solar cell properties were investigated and compared to analogue polymer P1 deprived of any organizing group. Significant differences lie in the more stabilized lowest unoccupied molecular orbital (LUMO) level for PA and the highest occupied molecular orbital (HOMO) level for PC. X-ray diffraction analyses on thin films evidenced π-aggregation of molecules for PA and PB. Nanosecond timescale fluorescent lifetimes were measured for PA, PB, and PC. An ultrafast photoinduced electron transfer between these polymers and phenyl-C61-butyric acid methyl ester (PC61BM) was brought out from femtosecond transient absorption spectroscopy (fs-TAS). It is faster for PC than for PA and PB. The recombination leads to singlet- and triplet-state formation (([Pt]-DPP)n)•+ + PC61BM•– → 3([Pt]-DPP)n* + ([Pt]-DPP)n + PC61BM. Solar cells were fabricated using polymers blended with an acceptor (PC71BM or IT-4F) as an active layer in conventional device structure indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)/active layer/poly[(9,9-dioctyl-2,7-fluorene)-alt-(9,9-bis(3′-(N,N-dimethylamino)propyl)-2,7-fluorene)] (PFN)/Al. Unlike PA, polymers PB and PC exhibit better photovoltaic performances than P1. Higher power conversion efficiencies (PCEs) were reached with IT-4F, PB:IT-4F (PCE = 10.83%, Jsc = 17.83 mA cm–2, Voc = 0.98 V), and PC:IT-4F (PCE = 13.26%, Jsc = 18.95 mA cm–2, Voc = 1.06 V). This work highlights the beneficial impact of triphenylene self-assembling groups on photovoltaic properties. PC exhibits a record-high PCE (13.26%) for [Pt]-containing polyyne oligomers and small molecules.