Charge-generating mid-gap trap states define the thermodynamic limit of organic photovoltaic devices

Detailed balance is a cornerstone of our understanding of artificial light-harvesting systems. For next generation organic solar cells, this involves intermolecular charge-transfer (CT) states whose energies set the maximum open circuit voltage V-OC. We have directly observed sub-gap states significantly lower in energy than the CT states in the external quantum efficiency spectra of a significant number of organic semiconductor blends. Taking these states into account and using the principle of reciprocity between emission and absorption results in non-physical radiative limits for the V-OC. We propose and provide compelling evidence for these states being non-equilibrium mid-gap traps which contribute to photocurrent by a non-linear process of optical release, upconverting them to the CT state. This motivates the implementation of a two-diode model which is often used in emissive inorganic semiconductors. The model accurately describes the dark current, V-OC and the long-debated ideality factor in organic solar cells. Additionally, the charge-generating mid-gap traps have important consequences for our current understanding of both solar cells and photodiodes - in the latter case defining a detectivity limit several orders of magnitude lower than previously thought. The inability to accurately measure the charge-generating energy states in organic solar cells makes elucidating the photovoltaic effect in these devices difficult. Here, the authors report charge-generating mid-gap trap states in organic solar cells via ultra-sensitive photovoltaic measurements.