Origin of Photocurrent in Fullerene-Based Solar Cells

Fullerene-based organic solar cells with only a minute amount of donor show a substantial photocurrent while maintaining a large open-circuit voltage. At low concentrations the donor is fully dispersed within the fullerene and no percolation pathways of holes toward the anode exist; this morphology is in contrast to bulk-heterojunction donor:acceptor blends where percolation pathways for both electrons and holes are present within their respective transport phases. Therefore, the question of how holes contribute to the photocurrent arises. Here we demonstrate that the photocurrent is readily explained by photogenerated holes transferring back to the fullerene matrix due to Coulomb repulsion and the fullerene acting as an ambipolar conductor for both electrons and holes. The two critical parameters controlling this process are the values of the highest occupied molecular orbital level difference between the donor and the acceptor and of the recombination strength; both are found to agree between experimental measurements and kinetic Monte Carlo simulations. We provide evidence that the highest occupied molecular orbital level difference between donor and acceptor is smaller in a dilute donor configuration. Successive percolation pathways toward the contacts - the reason for introducing the bulk-heterojunction configuration - are not an absolute requirement to obtain substantial photocurrents in organic solar cells.