Effect of structural variation on photovoltaic characteristics of phenyl substituted diketopyrrolopyrroles

• Published in: RSC advances Vol. 4; no. 27; pp. 14101 - 14108
• Main Authors: Lin, Jason D. A., Liu, Jianhua, Kim, Chunki, Tamayo, Arnold B., Proctor, Christopher M., Nguyen, Thuc-Quyen
• Format: Journal Article
• Language: English
• Published: 2014
• Subjects: Blends; Charge; Conjugation; Devices; Order disorder; Phenyls; Segregations; Solar cells
• ISSN: 2046-2069; 2046-2069
• DOI: 10.1039/C3RA45662E

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 (PC sub(71)BM) 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.