Photophysical study of DPPTT-T/PC70BM blends and solar devices as a function of fullerene loading: an insight into EQE limitations of DPP-based polymers

摘要

Diketopyrrolopyrrole (DPP) - based polymers have been consistently used for the fabrication of solar cell devi ces and transistors, due to the existence of intermolecular short contacts, resulting in high electron and hole mobilities. However, they also often show limit ed external quantum efficiencies (EQEs). In this contribution we analyze the limitations on EQE by a combined study of exciton dissociation efficiency, charge separation and recombination kinetics in thin films and solar devices of a DPP - based donor polyme r, DPPTT - T ( thieno[3,2 - b]thiophene - diketopyrrolopyrrole copolymer ) blended with varying weight fractions of the fullerene acceptor PC 70 BM. From the correlations between photoluminescence quenching (PLQ), transient absorption studies an d EQE measurements, w e conclude that the main limitation of photon - to - charge conversion in DPPTT - T/PC 70 BM devices is poor exciton dissociation. This exciton quenching limit is related to the low affinity/miscibility of the materials, as confirmed by WAXD diffraction and t ransmission electron microscopy data, but also to the relatively short DPPTT - T singlet exciton lifetime , possibly associated with high non - radiative losses . A further strategy to improve EQE in this class of polymers without sacrificing the good extraction properties in optimized blends is therefore to limit those non - radiative decay processes.