Size-Controllable Metal Chelates as Both Light Scattering Centers and Electron Collection Layer for High-Performance Polymer Solar Cells

An electron collection layer (ECL) between a photo-active overlay and an electrode plays a crucial role in optimizing the light field and charge extraction in bulk-heterojunction (BHJ) polymer solar cells (PSCs). However, the typical thickness of the photo-active layer is thinner than its optical path lengths, limiting further improvement of light absorption and device performance. Herein, we modulated the conjugated length of acetylacetonate-based ligands to synthesize a series of metal chelates [Hf(Acac)(4), Hf (ACB1)(4), and Hf(ACB)(4)] and used them as light-scattering functionalized ECLs for PSCs. Benefitting from the strong and different self-assembly effects of these chelates, size-controllable nanoparticles were formed and well distributed on the entire surface of the photoactive layer, which acted as scattering centers to modulate the light field of the active layers effectively. Among these chelates, due to the most suitable aggregation size of Hf(ACB1)(4) nanoparticles, a preferable light-harvesting framework was achieved in PSCs using Hf(ACB1)(4) as ECL that eventually yielded a high PCE of >17%. Thus, our work demonstrates a flexible design strategy for obtaining a series of size-controllable metal chelates as efficient ECL for facilitating light trapping in PSCs.