Maximising performance of organic solar cells by designing ohmic contacts
15/8/2018. Achieving ohmic transition at both contacts is key to maximizing fill factor, compromising neither the open-circuit voltage nor the short-circuit current of a solar cell. Jun Kai TAN and co-workers demonstrated in a Nat. Commun paper that the maximisation of the open circuit voltage of an organic solar cell is achieved not at the Fermi pinning condition as commonly believed, but when the work function of the hole-contact electrode exceeds said condition by few tenths of an eV. This arises as a consequence of the charge-transfer contact resistance at the semiconductor/ electrode interface continuing to decline strongly with work function even in the Fermi-level pinned regime due to the increasing accumulation of carrier density at the semiconductor interface. The observed ohmic transition, characterised by decreased series resistance and improved fill factor, was achieved beyond the Fermi-level pinning transition; this provides a design rule for high-performance organic solar cells.