Exciton Diffusion Length and Lifetime in Subphthalocyanine Films

Organic photovoltaic cells with chloro[subphthalocyaninato]boron(III) (SubPc) as donor in combination with buckminster fullerene (C-60) as acceptor in a planar heterojunction configuration have shown high power conversion efficiencies. The parameter that determines the critical distance to the heterojunction over which light-generated excitons can be harvested is the exciton diffusion length. Here, we characterize this exciton diffusion length in SubPc thin films by photoluminescence (PL) quenching experiments in a planar bilayer system. As the layer thickness is much smaller than the optical penetration depth, we apply the transfer matrix method (TMM) to calculate the internal optical field inside the organic structures and use this to derive the intrinsic PL spectrum. An exciton diffusion length of 28 nm is determined, and good agreement is found with the diffusion lengths obtained from modeling the external quantum efficiency. In addition, time-resolved PL measurements are performed to determine the exciton lifetime to be 0.3 ns.