Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)
Publication date: 5th February 2015
Two important unresolved questions in organic photovoltaics are how photogenerated charge pairs dissociate into free carriers despite low dielectric constant of the materials and how carriers can be extracted without non-geminate recombination losses. Recent studies showed that quantum efficiencies of solar cells are independent of excess energy even when interfacial charge transfer states are directly excited which indicates that vibrational energy gained from donor-acceptor energy offset may not be essential to achieve high performance. Charge delocalisation and high carrier mobility have been suggested as possible answers to both these questions but no conclusive explanation has been reached yet.
Here we combine transient absorption spectroscopy with optical probing of carrier drift using time-resolved electric-field-induced second harmonic generation (TREFISH)1 to understand free carrier generation and recombination in efficient PTB7:PC71BM blends. We observe strong dependence of the charge pair dissociation efficiency on blend ratio. It is high in the blends with 20 and 40 wt% of PTB7 but lower in polymer-rich and fullerene-rich blends despite fast and efficient charge generation. This correlates with a fast decrease of carrier mobility in polymer-rich blends in the first 100 ps which is attributed to carrier trapping at low energy sites. Blends prepared with the additive 1,8-diodooctane show two times higher pair dissociation efficiency which can be explained by finer intermixing of polymer and fullerene.2 The results show the advantages of mixed-phase morphology for charge pair dissociation.
Fig.1. EQE vs blend ratio and the sum of time dependent electron and hole mobility measured by TREFISH
[1] Vithanage, D. A.; Devižis, A.; Abramavičius, V.; Infahsaeng, Y.; Abramavičius, D.; MacKenzie, R.C.I.; Keivanidis, P.E.; Yartsev, A.; Hertel, D.; Nelson, J.; Sundström, V.; Gulbinas, V. Visualizing Charge Separation in Bulk Heterojunction Organic Solar Cells, Nature Comm. 2013, 4, 2334. [2] Hedley, G. J.; Ward, A. J.; Alekseev, A.; Howells, C.T.; Martins, E.R.; Serrano, L.A.; Cooke, G.; Ruseckas, A.; Samuel, I.D.W. Determining the optimum morphology in high-performance polymer-fullerene organic photovoltaic cells, Nature Comm. 2013, 4, 2867.
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