Infrared Colloidal Quantum Dot Photovoltaics via Coupling Enhancement and Agglomeration Suppression

Materials optimized for single-junction solar spectral harvesting, such as silicon, perovskites, and large-band-gap colloidal quantum dot solids, fail to absorb the considerable infrared spectral energy that lies below their respective band gap. Here we explore through modeling and experiment the potential for colloidal quantum dots (CQDs) to augment the performance of solar cells by harnessing transmitted light in the infrared. Through detailed balance modeling, we identify the CQD band gap that is best able to augment wafer-based, thin-film, and also solution-processed photovoltaic (PV) materials. The required quantum dots, with an excitonic peak at 1.3 mu m, have not previously been studied in depth for solar performance. Using computational studies we find that a new ligand scheme distinct from that employed in better-explored 0.95 mu m band gap Pb5 CQDs is necessary; only via the solution-phase application of a short bromothiol can we prevent dot fusion during ensuing solid-state film treatments and simultaneously offer a high valence band-edge density of states to enhance hole transport. Photoluminescence spectra and transient studies confirm the desired narrowed emission peaks and reduced surface-trap-associated decay. Electronic characterization reveals that only through the use of the bromothiol ligands is strong hole transport retained. The films, when used to make PV devices, achieve the highest AM1.5 power conversion efficiency yet reported in a solution-processed material having a sub-1 eV band gap.