Strongly field-dependent dissociation of charge transfer states for high performance bulk heterojunction organic photodetectors
Journal
ACS Nano
Date
2020.12.17
Abstract
We report the photophysics and molecular morphology of high-performance organic photodetectors (OPDs) and how these differ from analogous organic photovoltaics (OPVs). We employ OPDs based on a bulk heterojunction photoactive layer, co-evaporated blend of a naphthalene-based donor and C60 acceptor, with thermal annealing at 160 °C yielding optimum performance. Ultrafast transient absorption spectroscopy resolves exciton separation in <1 ps, revealing a highly intermixed blend morphology. This exciton separation is found to yield a bound, highly emissive charge transfer (CT) state which is readily dissociated both thermally and by applied electric field, with a binding energy of ~135 meV. Thermal annealing aids CT state dissociation, accelerates response time and suppresses dark current of OPDs. Raman and Photoluminescence spectroscopy indicate that these improvements originate from molecular morphology changes via twisting and aggregation of the donor in the blend. Our results hence highlight not only parallels but also differences between OPDs and analogous OPVs, with important molecular processes determining efficient device function.