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

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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 ECTqVOC ≈ 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.

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

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Publication details

The article was received on 05 Feb 2018, accepted on 11 Jun 2018 and first published on 13 Jun 2018


Article type: Paper
DOI: 10.1039/C8TA01195H
Citation: J. Mater. Chem. A, 2018, Advance Article
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    Relating open-circuit voltage losses to the active layer morphology and contact selectivity in organic solar cells

    Z. Tang, J. Wang, A. Melianas, Y. Wu, R. Kroon, W. Li, W. Ma, M. R. Andersson, Z. Ma, W. Cai, W. Tress and O. Inganäs, J. Mater. Chem. A, 2018, Advance Article , DOI: 10.1039/C8TA01195H

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