Effect of structural variation on photovoltaic characteristics of phenyl substituted diketopyrrolopyrroles

Abstract

A comprehensive study has been performed on a series of solution processable phenyl substituted diketopyrrolopyrroles blended with [6,6]-phenyl-C71-butyric acid methyl ester (PC₇₁BM) in order to investigate how systematic chemical modifications such as solubilizing groups and conjugation length impact solar cell performance. We find that replacement of linear alkyl chains with bulky ethyl-hexyl groups or the removal of linear alkyl chains on the terminal thiophene units leads to micron scale phase segregation at high donor:acceptor blend ratios. It is found that the conjugation length can be used to simultaneously tune energy levels, solubility, and molecular ordering. We show that over-extending the conjugation length can reduce solubility making film fabrication difficult while decreasing the conjugation length past a critical limit can significantly enhance molecular ordering thereby inducing micron scale phase segregation in blend films. This work shows that a material's potential device performance can be limited by slight chemical modifications which prevent device optimization at high donor:acceptor blend ratios and elevated annealing temperatures where charge mobility is balanced and charge collection is enhanced in the donor and acceptor phase.