Elucidating the Role of Heterojunction Interface in Exciton Harvest and Charge Collection of Organic Solar Cells through Planar Heterojunction Structure

The rapid progress in the development of non-fullerene electron acceptors has led to key breakthroughs in the power conversion efficiency of organic solar cells (OSCs). Nevertheless, the development of non-fullerene OSCs are still hindered by the short exciton diffusion length in both donor and acceptor domains, which is paramount in determining the charge generation properties in OSCs. Reducing the domain size to match the diffusion length via bulk heterojunction (BHJ) structure has been an essential strategy to overcome this limitation, but will inevitably compromise charge collection due to the intermixing of donor and acceptor. Here, we use a solution-based contact film transfer method to fabricate planar heterojunction (PHJ) device, which serves as an excellent platform to investigate exciton diffusion and charge collection owing to the clearly defined interface. By reconstructing external quantum efficiency spectra and device modeling in PHJ cells based on a large bandgap polymer J52 and a low bandgap small molecule IEICO-4F, exciton diffusion lengths of 9.2 nm and 7.5 nm are extracted for J52 and IEICO-4F, respectively. The experimentally obtained values for exciton diffusion length confirm that BHJ structure is an important prerequisite for efficient organic solar cells. Nevertheless, as revealed by the carrier dynamic studies, BHJ devices could suffer from severe recombination losses as a result of energetic disorder and traps. By considering the recombination of free and trapped carriers and intermediates in the photocurrent generation processes, we show that the stronger recombination of BHJ compared to PHJ is also related to its shorter CT states lifetime and lower CT states density based on the reduced Langevin recombination analysis. The inherent limitation in BHJ devices is thus a major disadvantage that need to be overcome for further improvement. On the basis of the results of exciton diffusion length measurement and carrier dynamic study, we propose that finely tuning domain size in non-fullerene OSCs still play a crucial role in controlling the device performance.