Improving the efficiency of organic solar cells requires atomic insight into the interface electronic band alignment of the donor and acceptor moieties composing the device. In this article, we use ab initio calculations, with inclusion of long-range (van der Waals) interactions, to address the solid-state properties of a bulk heterojunction heterointerface between a single ordered layer of PCBM molecules adsorbed on a clean P3HT crystalline polymer. The studied interface model allowed us to focus on the basic mechanisms responsible for charge polarization and migration at the interface and to refer the energies of both moieties to the same origin. After the accurate evaluation of the relative energy positions of the near-gap electronic levels in the PCBM/P3HT complex and of the optical spectra useful for determining the nature of the electronic states, we analyzed the effect of uniaxial stress on the band alignment, and we found that both the polymer band gap and the offset between the LUMO levels of the donor and the acceptor materials decrease for compressive stress. This suggests that the donor band gap can be reduced, thus increasing solar energy harvest, and that the open-circuit voltage of the system can be tuned to improve the efficiency of PCBM/P3HT-based solar cell devices.
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