On the Effect of Mid-Gap Trap States on the Thermodynamic Limit of OPV 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 $$V_\mathrm{OC}$$ . In the study presented in this chapter, sub-gap states significantly lower in energy than the CT states in the external quantum efficiency spectra of a substantial number of organic semiconductor blends have directly been observed. Taking these states into account and using the principle of reciprocity between emission and absorption results in non-physical radiative limits for the $$V_\mathrm{OC}$$ . Compelling evidence has been provided 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, $$V_\mathrm{OC}$$ 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. This chapter is written based on collaborative work published by the author in Nature Communications in 2020 [1].