Visualising the Vertical Energetic Landscape in Organic Photovoltaics

TL;DR

This paper introduces a new visualization method for the vertical energetic landscape in organic photovoltaics, improving understanding of device physics and degradation effects.

Contribution

A novel method combining spectroscopy and etching to directly visualize the energetic landscape, aligning photovoltaic gaps with device measurements.

Findings

Photovoltaic gaps closely match measured CT energies and open-circuit voltages.

Method enables study of energetic landscape evolution during degradation.

Improves interpretation of device physics in organic solar cells.

Abstract

Energy level diagrams in organic electronic devices play a crucial role in device performance and interpretation of device physics. In the case of organic solar cells, it has become routine to estimate the photovoltaic gap of the donor:acceptor blend using the energy values measured on the individual blend components, resulting in a poor agreement with the corresponding open-circuit voltage of the device. To address this issue, we developed a method that allows a direct visualisation of the vertical energetic landscape in the blend, obtained by combining ultraviolet photoemission spectroscopy and argon cluster etching. We investigate both model and high-performance photovoltaic systems and demonstrate that the resulting photovoltaic gaps are in close agreement with the measured CT energies and open-circuit voltages. Furthermore, we show that this method allows us to study the evolution of the energetic landscape upon environmental degradation, critically important for understanding degradation mechanisms and development of mitigation strategies.