[Link Foundation Fellowship Final Report]
Efficient conversion of solar radiation into electricity is a compelling scientific and economic goal. As the world’s energy demands continue to grow it has become clear that burning fossil fuels for energy is unsustainable and detrimental to global health and geopolitics. More than any other renewable resource, solar energy conversion has the potential to meet human energy needs with a total yearly insolation of 120,000 TW/year.1 While organic photovoltaics (PVs) are currently not as efficient as their inorganic counterparts, their efficiencies have been steadily increasing over the past decade. The most significant advances are attributable to greater control over the donor-acceptor interface where photogenerated excitons are dissociated into electrons and holes.2-4 We hypothesized that by shrinking the heterojunction dimensions to the molecular scale such that every donor is in contact with acceptor molecules (and vice versa), the issue of exciton diffusion could be fully mitigated. By depositing each phase independently we believed formation of isolated heterojunctions could be avoided. Slow charge transport dynamics in our organic heterojunctions shifted our focus to the mixed organic/inorganic heterojunction of dye-sensitized solar cells (DSSCs). These photoelectrochemical cells use molecular dyes to sensitize high area, wide band gap semiconductor oxide anodes built on conductive glass. Since the electron collection efficiency is a measure of the competition between transport and recombination we hypothesized that faster transport would make the cells tolerant to faster recombination dynamics. In principle, faster redox shuttles could then be employed. As dye regeneration by inherently faster shuttles need not be accelerated by large overpotentials, this change could directly address the problem of low photovoltage that has plagued DSSCs since their inception.