Ligand-Exchanged CZTS Nanocrystal Thin Films: Does Nanocrystal Surface Passivation Effectively Improve Photovoltaic Performance?

Nanocrystal (NC) Cu 2 ZnSnS 4 (CZTS) solar cells, composed of a nontoxic and earth abundant absorber material, have great potential in low-cost solar energy harvesting. However, CZTS NC films typically must be thermally annealed at elevated temperatures and under harsh environments to produce high-efficiency devices. The efficiencies of unannealed CZTS NC solar cells have been hampered by low open circuit potentials ( V oc, <325 mV) and low short circuit current densities ( J sc, <2 mA), primarily because of the incomplete passivation of the crystal surface. Although great progress has been made in understanding the surface chemistry of II–VI and IV–VI semiconductor NCs, the surface chemistry of complex quaternary CZTS NCs is largely unexplored. Here, for the first time, we report a comprehensive study of the surface chemistry of CZTS NCs focusing on depositing ligand-passivated, uniform NC thin films to address the issue of large V oc deficit and low current collection efficiency typically observed for CZTS NC solar cells. The ligand exchange reactions were rationally designed to target each metal ion on the surface [using both organic L-type ligands such as ethylenediamine and inorganic X-type ligands (I – and S 2– )] and to passivate anionic chalcogen sites with inorganic Z-type ligands (ZnCl 2 ). Herein, we show that CZTS/CdS heterojunction NC solar cells made of uniformly passivated CZTS NCs demonstrate a >180 mV improvement in V oc . Furthermore, the influence of device configuration on the collection efficiency of photogenerated carriers in the CZTS NC absorber layer is presented, and the implications of both surface and internal defects in CZTS NCs for photovoltaic performance are discussed.