Energetic Offset in Organic Solar Cells- Importance, Confusion and Outlook.

The energetic offset between donor (D) and acceptor (A) is one of the most important parameters which influences the charge generation and voltage loss, and hence overall performance of the organic solar cells (OSCs). However, the different methodologies/techniques used to determine this offset provides a wide range of values, even for the same D:A system. This creates confusion in the community and makes it difficult to make comparisons between different labs. This discrepancy arises because each technique probes fundamentally different physical processes and because of the limited understanding of interfacial energy level alignment, particularly at buried interfaces. In this perspective, we discuss the pros and cons of the frequently used methods/techniques that are used to determine the offset, and their underlying principles. We identify temperature-dependent electro-optical techniques as a reasonable method which can help to achieve a consistent comparison between different material systems. In order to minimize the voltage losses of organic photovoltaics below 0.5 V, we suggest that the focus should be on designing low bandgap molecules with inherently high photoluminescence quantum yield and ensuring that the donor:acceptor blends exhibit strong luminescence efficiency.