Ideal diode equation for organic heterojunctions. II. The role of polaron pair recombination

In paper I [N. C. Giebink, G. P. Wiederrecht, M. R. Wasielewski, and S. R. Forrest, Phys. Rev. B 82, 155305 (2010)], we proposed that current transport in a donor-acceptor heterojunction (HJ) depends on the balance of polaron pair (PP) dissociation and recombination. Here, we directly investigate these processes in archetype planar copper phthalocyanine (CuPc)/C₆₀ and boron subpthalocyanine chloride (SubPc)/C₆₀ HJs. Using intensity-modulated photocurrent spectroscopy (IMPS) along with emission from interfacial Pc/C₆₀ exciplex states, we monitor the geminate PP density at the HJ as a function of bias and illumination intensity. We find that the SubPc/C₆₀ PP density is limited by the dynamics of dissociation, where it increases from short circuit, and peaks at open circuit. In contrast, that of CuPc/C₆₀ is dominated by faster recombination kinetics and declines monotonically over the same voltage domain. We conclude that the PP recombination rate depends on electric field, and propose a simple expression that qualitatively explains the observed exciplex luminescence and IMPS behavior for these HJs. Our results provide insight into polaron pair recombination, which governs the current-voltage characteristics of organic heterojunctions in the dark and under illumination.