Investigation of Hole-Transfer Dynamics through Simple EL De-Convolution in Non-Fullerene Organic Solar Cells

In conventional fullerene-based organic photovoltaics (OPVs), in which the excited electrons from the donor are transferred to the acceptor, the electron charge transfer state (_CT) that electrons pass through has a great influence on the device's performance. In a bulk-heterojunction (BHJ) system based on a low bandgap non-fullerene acceptor (NFA), however, a hole charge transfer state (_CT) from the acceptor to the donor has a greater influence on the device's performance. The accurate determination of _CT is essential for achieving further enhancement in the performance of non-fullerene organic solar cells. However, the discovery of a method to determine the exact _CT remains an open challenge. Here, we suggest a simple method to determine the exact _CT level via deconvolution of the EL spectrum of the BHJ blend (EL_B). To generalize, we have applied our EL_B deconvolution method to nine different BHJ systems consisting of the combination of three donor polymers (PM6, PBDTTPD-HT, PTB7-Th) and three NFAs (Y6, IDIC, IEICO-4F). Under the conditions that (i) absorption of the donor and acceptor are separated sufficiently, and (ii) the onset part of the external quantum efficiency (EQE) is formed solely by the contribution of the acceptor only, EL_B can be deconvoluted into the contribution of the singlet recombination of the acceptor and the radiative recombination via _CT. Through the deconvolution of EL_B, we have clearly decided which part of the broad EL_B spectrum should be used to apply the Marcus theory. Accurate determination of _CT is expected to be of great help in fine-tuning the energy level of donor polymers and NFAs by understanding the charge transfer mechanism clearly.