Proceedings of International Conference on Hybrid and Organic Photovoltaics (HOPV16)
Publication date: 28th March 2016
The recombination of mobile electrons and holes forms a serious factor hindering the photoconversion in organic solar cells. It is currently regarded as the primary obstacle to overcome in order to realize commercially feasible cells based on bulk heterojunctions of polymers and fullerene derivatives. Originally, charge recombination was considered in the framework of Langevin theory; a macroscopic description based on electron and hole diffusion constants which was known to provide reliable recombination rates for single organic materials. However, for a variety of polymer:fullerene heterojunctions, experimentally determined recombination rates were found to be significantly lower than estimates based on Langevin theory. Particularly for annealed blends of poly-3-hexylthiophene (P3HT) and the fullerene derivative PCBM, recombination turns out to be suppressed by up to 4 orders of magnitude. An understanding of the mechanism responsible for this suppression opens an avenue to deliberately engineer photovoltaics with low recombination, and hence optimized photoconversion efficiency. However, it has recently become clear that to this end macroscopic descriptions such as Langevin theory are inherently insufficient, and that instead a molecular representation is necessary.
In this contribution, we present a principle based on molecular charge excitations that possibly accounts for the observed low recombination rates [1]. Through calculations, we show that charge recombination is sensitively dependent on the degree of coherent delocalization of electrons and holes along the heterojunction interface. Depending on the relative signs of the electron and hole transfer integrals, this delocalization results in a dramatic suppression of the recombination rate through destructive quantum interference. Besides forming a possible explanation of recombination in bulk heterojunctions, this finding opens up a design strategy for photovoltaic devices with enhanced efficiencies through coherently suppressed charge recombination. For example, we anticipate that through molecular crystal engineering, microscopic heterojunction structures with negligible recombination can be realized.
[1] R. Tempelaar, L.J.A. Koster, R.W.A. Havenith, J. Knoester, & T.L.C. Jansen. J. Phys. Chem. Lett. 7, 198 (2016).
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