Interplay Between Electrodes and Active Materials: The Open-Circuit Voltage and S-shaped J-V Curves

Abstract

This chapter contains a detailed study on the effects of energy barriers at the electrode. To vary these barriers in a controlled and systematic way, the interface between a hole-transport layer and the donor is selected. This interface serves as a model system for a contact between active layer and electrode in general. Different values for the HOMO levels of both organic materials give rise to barriers for injection or extraction of charge carriers. We discuss through experiment and simulation how these barriers influence the current-voltage characteristics, in particular the open-circuit voltage. One main observation are S-shaped current-voltage curves, which severely limit the fill factor. Such “S-kinks” are frequently observed when characterizing organic solar cells. That is why we develop methods to identify the reasons for S-kinks. Amongst those methods are measurements as a function of temperature or light intensity. Another method is based on transient photocurrent measurements, which reveal redistributions of the electric field or a pile-up of charge carriers. The following questions are of particular relevance: (a) What are the definitions of injection and extraction barriers for charges? What causes such barriers? (b) Why, and under what conditions does an injection barrier change the open-circuit voltage? (c) Why are injection barriers detrimental for efficient extraction of charge carriers? (d) What experimental approaches exist to distinguish S-kinks introduced by extraction barriers from those caused by injection barriers? (e) In which case does the illumination intensity influence the S-kink and why? (f) What can transient photocurrent measurements close to open-circuit voltage tell about the driving forces for charge carrier extraction?