Ultrafast Dynamics\nin Electronically Excited States\nof Side Chain-Modified Diketopyrrolopyrrole Quaterthiophene Thin Films

The self-assembly of a conjugated donor–acceptor\npolymer\ncan be improved by controlling the intrachain hydrogen bonding via\nthe incorporation of urea groups in the side chains. This also enhances\nthe power conversion efficiency of organic photovoltaic devices. Side\nchain engineering with urea groups directly influences the electronic\nassembly, the interchain packing, and the backbone configuration of\nthe polymers. The charge generation in this next-generation donor–acceptor\nconjugated polymer is directly compared to that of its predecessor\nmolecule pDPP4T without urea groups. The ultrafast charge dynamics\nwas studied with femtosecond laser sources applying static and dynamic\ntwo-photon photoemission spectroscopy. Ultrafast spectroscopy gives\ninsights into the charge generation process in the conjugated polymer\npDPP4T-1 (diketopyrrolopyrrole (DPP) quaterthiophene) thin films on\nsilicon. While annealing organic semiconductor thin films leads to\nsignificant improvements in charge carrier mobility and photovoltaic\nefficiency, the ultrafast charge generation seems to be close to unaffected\non the picosecond timescale. In addition, ultrafast charge generation\nin blended polymer/fullerene thin films was studied, as used in prospective\norganic photovoltaic devices. We suppose that long-lived transient\nstates, which are observed in the time-resolved 2PPE measurement and\npersist beyond the picosecond range, are associated with the generation\nof charge carriers. This long-living component is increased for the\nblended thin film. Flash-photolysis time-resolved microwave conductivity\nprovides deeper insight into the photoconductivity and the charge\ncarrier lifetimes on long timescales. Matching to the photocarrier\ngeneration kinetics in time-resolved 2PPE, a one order higher photoconductivity\ntransient is measured for the PDPP4T-1:PCBM blend compared to the\npristine polymer.