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Impact of mesoscale order on open-circuit voltage in organic solar cells

Abstract

Structural order in organic solar cells is paramount: it reduces energetic disorder, boosts charge and exciton mobilities, and assists exciton splitting. Owing to spatial localization of electronic states, microscopic descriptions of photovoltaic processes tend to overlook the influence of structural features at the mesoscale. Long-range electrostatic interactions nevertheless probe this ordering, making local properties depend on the mesoscopic order. Using a technique developed to address spatially aperiodic excitations in thin films and in bulk, we show how inclusion of mesoscale order resolves the controversy between experimental and theoretical results for the energy-level profile and alignment in a variety of photovoltaic systems, with direct experimental validation. Optimal use of long-range ordering also rationalizes the acceptor–donor–acceptor paradigm for molecular design of donor dyes. We predict open-circuit voltages of planar heterojunction solar cells in excellent agreement with experimental data, based only on crystal structures and interfacial orientation.

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Figure 1: Electrostatic potential of donor materials.
Figure 2: Convergence of energy levels.
Figure 3: Hole and electron energy profiles with and without long-range correction.
Figure 4: Evolution of energy profiles and offsets during growth of the donor layer.
Figure 5: Convergence of energy levels for pentacene, EL86 and D5M.
Figure 6: Ionization energies and open-circuit voltage across different donor/acceptor interfaces.

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Acknowledgements

This work has been supported by the BMBF programme MEDOS (FKZ 03EK3503B). We are grateful to T. Bereau for critical reading of the manuscript and P. Bäuerle and C. Körner for fruitful collaborations.

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C.P., B.B. and D.A. conceived and performed simulations. M.T., C.E. and K.L. contributed experimental results on DCVnTs. S.O., D.H., F.W. and K.M. contributed experimental results on merocyanines. C.P. and D.A. wrote the paper.

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Correspondence to Carl Poelking or Denis Andrienko.

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Poelking, C., Tietze, M., Elschner, C. et al. Impact of mesoscale order on open-circuit voltage in organic solar cells. Nature Mater 14, 434–439 (2015). https://doi.org/10.1038/nmat4167

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