Reduced voltage losses yield 10% efficient fullerene free organic solar cells with >1 V open circuit voltages
D. Baran, T. Kirchartz, S. Wheeler, S. Dimitrov, M. Abdelsamie, J. Gorman, R.S. Ashraf, S. Holliday, A. Wadsworth, N. Gasparini, P. Kaienburg, H. Yan, A. Amassian, C.J. Brabec, J.R. Durrant, I. McCulloch
Energy and Environmental Science, 9 (12), 3783-3793, (2016)
Voltage losses, Efficient fullerene, Free organic solar cells, Circuit voltages
Optimization of the energy levels at the donor–acceptor
interface of organic solar cells has driven their efficiencies to above
10%. However, further improvements towards efficiencies comparable with
inorganic solar cells remain challenging because of high recombination
losses, which empirically limit the open-circuit voltage (Voc)
to typically less than 1 V. Here we show that this empirical limit can
be overcome using non-fullerene acceptors blended with the low band gap
polymer PffBT4T-2DT leading to efficiencies approaching 10% (9.95%). We
achieve Voc up to 1.12 V, which corresponds to a loss of only Eg/q − Voc = 0.5 ± 0.01 V between the optical bandgap Eg of the polymer and Voc. This high Voc
is shown to be associated with the achievement of remarkably low
non-geminate and non-radiative recombination losses in these devices.
Suppression of non-radiative recombination implies high external
electroluminescence quantum efficiencies which are orders of magnitude
higher than those of equivalent devices employing fullerene acceptors.
Using the balance between reduced recombination losses and good
photocurrent generation efficiencies achieved experimentally as a
baseline for simulations of the efficiency potential of organic solar
cells, we estimate that efficiencies of up to 20% are achievable if band
gaps and fill factors are further optimized.
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