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

Nasim Zarrabi,Drew B Riley,Ardalan Armin,Stefan Zeiske,Wei Li,Oskar J Sandberg,Paul Meredith

doi:10.1038/s41467-020-19434-0

Abstract

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 VOC. 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 VOC. 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, VOC 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.

Highlights

Detailed balance is a cornerstone of our understanding of artificial light-harvesting systems
Photovoltaics as a field has relied upon detailed balance and reciprocity since its inception in the early 1960s1—irrespective of the semiconductor in question be it crystalline Silicon (c-Si), GaAs or more latterly organohalide perovskites and organics
These non-linear processes explain the apparent violation from the equilibrium detailed balance but demands a modified picture for organic solar cells to incorporate the non-equilibrium mid-gap trap states

Summary

Introduction

Detailed balance is a cornerstone of our understanding of artificial light-harvesting systems. We observe distinct sub-gap features in a large variety of organic semiconductor blends at energies well below the CT state Including these additional low energy states in the calculation of VORaCd from EQEPV (as one would using the principle of reciprocity) results in considerably lower apparent non-radiative losses than determined from EQELED. Based on the two separate charge generation processes (direct photogeneration and photogeneration via traps) we implement a standard two-diode model which includes radiative transitions via mid-gap states and provides a unified description of the dark current-voltage characteristics (J–V) of organic photovoltaic devices Such a model has often been used to explain solar cells which are highly emissive and its use in organic semiconductor systems has never been justified. Based on these results, revised thermodynamic limits for the detectivity of organic photodiodes operating in reverse bias are defined and the open-circuit voltage and ideality factor of organic solar cells explained

Methods

Results

Conclusion