The Optical Constants of Solution-Processed Semiconductors-New Challenges with Perovskites and Non-Fullerene Acceptors

Accurate determination of the optical constants of thin film solids has been an ongoing endeavor in optoelectronics and related fields for decades. These constants, namely the refractive index and extinction (or attenuation) coefficient, are the fundamental material properties that dictate electromagnetic field propagation in any medium. They form the inputs to well-established models that allow for design and optimization of multilayer stack structures such as thin film solar cells, light-emitting diodes, and photodetectors. These determinations are particularly challenging for materials that are scattering and highly absorbing. In this work, a new and resource-efficient approach for optical constant determination based upon transmission spectrophotometry in combination with an iterative, reverse transfer matrix model and the Kramers-Kronig relation is reported. The approach is validated using more conventional ellipsometry for a number of functionally important semiconductors, including the recently emergent organic non-fullerene electron acceptors (NFAs) and perovskites for which the optical constants in the UV-vis-near IR region are provided. Notably, the NFAs are found to present anomalously high refractive indices and extinction coefficients that are predicted to have a profound influence on the cavity electro-optics of the new record efficiency organic solar cells of which they are key components.