Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV18)
Publication date: 21st February 2018
The triplet exciton energy in low bandgap donor materials is lower than the energy of charge transfer (CT) states which makes recombination to the triplet state energetically feasible and opens a new loss channel in OPVs. Triplet excitons can also lead to chemical degradation of OPVs, hence it is important to understand how they form in order to seek ways to minimise triplet formation.
In this work we combine broadband transient absorption spectroscopy with time-resolved fluorescence to determine recombination pathways of free charge carriers in PTB7:PC71BM blends. We use time-dependent carrier mobility measured by integral-mode photocurrent to describe the encounter rate of non-geminate charge pairs and to model the observed recombination dynamics [1]. We find that recombination rate of the triplet CT state to the polymer triplet state is about eight times higher than recombination of the singlet CT state. We observe that trap-assisted recombination dominates at typical charge densities in solar cells. Triplet excitons are short lived in these blends which can be explained by energy transfer to trapped holes.
[1] Pranculis, V.; Ruseckas, A.; Vithanage, D. A.; Hedley, G. J.; Samuel, I. D. W.; Gulbinas, V. Influence of Blend Ratio and Processing Additive on Free Carrier Yield and Mobility in PTB7:PC71BM Photovoltaic Solar Cells. The Journal of Physical Chemistry C 2016, 120, 9588-9594.
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