Role of the donor material and the donor–acceptor mixing ratio in increasing the efficiency of Schottky junction organic solar cells

• Fullerene aggregation is prevalent at high concentrations.
• These aggregates display enhanced absorption from λ = 400 nm to 600 nm.
• Low donor concentration enables photocurrent extraction from the aggregates.
• The HOMOC60–HOMOdonor offset drives fullerene aggregate exciton dissociation.
• Choice of donor affects open circuit voltage but does not impact hole extraction.

Schottky junction organic solar cells (OSCs) employ a high work-function anode and an active layer comprised of fullerene and low concentrations of donor. In this study, the roles of the donor material and the donor–acceptor mixing ratio in Schottky junction OSCs are explored. The results show that the high short circuit current (Jsc) seen in Schottky junction OSCs at low donor concentrations arises primarily from photocurrent contributions from charge-transfer intermolecular states in C60 aggregates. These aggregates absorb light at 400–600 nm and are thus well matched to the solar spectrum. The presence of the donor molecules is shown to be necessary for the dissociation of the C60 aggregate excitons, which ultimately allows for enhanced photocurrents. The exciton dissociation process is governed primarily by the highest occupied molecular orbial (HOMO) energy level difference between the donor and C60, and is only efficient when this difference is large enough for the energetically favorable transfer of holes from C60 to the donor material. Increasing the donor concentration beyond a certain threshold hinders C60 aggregate formation and thus removes its contribution to photocurrent completely. Furthermore, the Voc is shown to be strongly influenced by the choice of donor material, indicating that it is not set by the Schottky junction barrier height as previously thought. In spite of this influence on Voc, the choice of donor in the active layer does not appear to play a significant role in the extraction of holes from the Schottky junction organic solar cells. Optimized chlorine indium phthalocyanine (ClInPc) doped C70 Schottky cells were fabricated to demonstrate a peak power conversion efficiency of 3.6%.

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