Relating open-circuit voltage losses to the active layer morphology and contact selectivity in organic solar cells

Zheng Tang, Jing Wang, Armantas Melianas, Yang Wu, Renee Kroon, Weiwei Li, Wei Ma, Mats R. Andersson, Zaifei Ma, Wanqing Cai, Wolfgang Richard Tress, Olle W. Inganäs

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22 Citations (Scopus)

Abstract

We demonstrate that voltage losses due to both radiative and non-radiative recombination of charge carriers are strongly dependent on D/A phase separation. By processing the active layer with various solvent additives, we create distinct morphologies that lead to significantly different device open-circuit voltages (VOC), even though the charge transfer state energy (ECT) of the D/A blend remains rather constant. We find that radiative recombination losses are significantly increased for a finely intermixed morphology, due to the large D/A interface area. This leads to a total recombination loss of ECT - qVOC ≈ 0.7 eV. However, considerably smaller losses (0.5 eV), due to suppressed non-radiative recombination, are possible in solar cells where the D/A materials are organized to only allow for selective charge carrier extraction. Using a drift diffusion model, we show that the origin of the reduced non-radiative recombination losses is related to an effect which has not been considered for 'optimized' solar cells-the suppression of minority carrier diffusion to the 'wrong' contact. Our results suggest that the built-in field is not sufficiently strong even in 'optimized' organic solar cells and that selective carrier extraction is critical for further improvements in VOC.

Original languageEnglish
Pages (from-to)12574-12581
Number of pages8
JournalJournal of Materials Chemistry A
Volume6
Issue number26
DOIs
Publication statusPublished - 2018

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    Tang, Z., Wang, J., Melianas, A., Wu, Y., Kroon, R., Li, W., Ma, W., Andersson, M. R., Ma, Z., Cai, W., Tress, W. R., & Inganäs, O. W. (2018). Relating open-circuit voltage losses to the active layer morphology and contact selectivity in organic solar cells. Journal of Materials Chemistry A, 6(26), 12574-12581. https://doi.org/10.1039/c8ta01195h