Comparing random and regular diketopyrrolopyrrole-bithiophene-thienopyrrolodione terpolymers for organic photovoltaics

Rene AJ Janssen^a, Martijn M Wienk^a, Gaël HL Heintges^a, Koen H Hendriks^a

^aEindhoven University of Technology (TU/e), PO Box 513, Eindhoven, 5600, Netherlands

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Roma, Italy, 2015 May 11th - 13th

Organizer: Filippo De Angelis

Poster, Gaël HL Heintges, 388
Publication date: 5th February 2015

Two isomeric terpolymers were synthesized and applied in photovoltaic devices to study the impact of the distribution of repeating units on the semiconductor properties. A terpolymer consisting of diketopyrrolopyrrole (DPP), thienopyrrolodione (TPD) and bithiophene (2T) was therefore synthesized in a random and alternating fashion. Both the optical and semiconducting properties were found to be significantly different, reflecting in a dramatic difference in solar cell performance. It was found that the random polymer exhibited a deepened LUMO level and heightened HOMO level with regard to the regular polymer. It was inferred that this shift was caused by DPP2T-rich segments, as both the optical and electrical properties of the random polymer approach those of the PDPP2T copolymer. This explanation was further supported by DFT modeling of oligomeric systems containing these units. The shift in LUMO level in the random polymer hindered charge separation in solar cell devices, resulting in a low efficiency of 1% compared to the 5.3% obtained with the regular polymer. In addition to the insufficient LUMO-LUMO offset of the random polymer, a difference in active layer morphology was also observed, with a smooth morphology in the case of the regular material and a more demixed morphology in the case of the random polymer. This can either be explained by a difference in molecular weight between both materials, or by the different distribution of solubilizing alkyl chains. We conclusively demonstrated that it is important to consider the electronic nature of all possible segments that can be formed when designing new polymers.

Hendriks, K.H.; Heintges, G.H.L.; Wienk, M.M.; Janssen, R.A.J. Comparing random and regular diketopyrrolopyrrole–bithiophene–thienopyrrolodione terpolymers for organic photovoltaics. J. Mater. Chem. A 2014, 2, 17899-17905

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