The Role of Acceptor Properties on the Hole Transport Mechanism in Low-Donor-Content Organic Solar Cells

Organic solar cells offer several advantages over conventional photovoltaics, such as flexibility, low cost, and abundant materials, and are thus seen as a promising choice for energy harvesting. A rather particular case refers to diluted systems, where a small number of electron-donating molecules are dispersed within an acceptor matrix, hampering hole transport in the absence of percolation paths. Nonetheless, diluted systems present a reasonable performance, thereby raising discussions on how this is achieved. Here, we investigate the hole transport by measuring the hole mobility in several systems with varying donor concentrations. We found an unexpected correlation between the hole mobility and the energy offset between the donor HOMO and the acceptor LUMO (instead of the acceptor HOMO), with the mobility increasing as the offset decreases. Such a correlation is supported by molecular modeling suggesting a contribution from the acceptor LUMO-mediated superexchange mechanism. These results corroborate our experimental observation, hinting at a previously neglected mechanism of transport, which depends on the coupling between the donor HOMO and the acceptor LUMO.