Essential relation of spin states, trap states and photo-induced polarization for efficient charge dissociation in a polymer-nonfullerene based organic photovoltaic system

Organic nonfullerene acceptors (NFA) have received tremendous research interests due to the efficient interfacial charge transfer (CT) when they combine with polymeric donors. It is of critical importance to unravel, to which extent, spin-dependent excited states, energetic disorder and photo-induced polarization are essentially correlated, and eventually contribute for the photovoltaic action. Herein, we explore them in a polymer-NFA ITTC based organic bulk heterojunction (BHJ) solar cell. With assist of magneto-photocurrent measurements at the short-circuit current density (Jsc) condition and steady state impedance spectroscopic measurements, the dominant mechanism for the Jsc generation is governed by the triplet-charge reaction. We postulate that long lived triplets are more likely to interact with trapped charges at excited states, giving rise to Jsc. It turns out that the trap states may not always act as passive role for the energy loss in the organic photovoltaic system. More importantly, both applied bias voltage and light can effectively manipulate the magnitude and sign of the magneto-photocurrent. By quenching the photo-generated build-in field, the bias-dependent magneto-photocurrent evidently proves the efficient dissociation of polaron pairs at CT states. In particular, the photo-excitation wavelength has strong impact on the hyperfine field. Lastly, we demonstrate that the double beam photo-excitation generated dipole-dipole interaction assists for the electron-hole dissociation. Our work may pave the way for understanding the inside spin- and photo-physical phenomena in the NFA based organic photovoltaic system.