(34a) Inorganic Ligand-Stabilized CuInSe2 Nanocrystals Used in Photovoltaic Devices

Nanocrystal solar inks show promise for developing high efficiency photovoltaic (PV) devices at low cost. Currently, the best nanocrystal-based PV devices require high temperature sintering, which limits substrate flexibility and increases cost. One of the greatest obstacles to higher device efficiencies without high temperature processing relates to the poor electrical transport and charge extraction from nanocrystal films. In particular, the organic capping ligands used during synthesis to stabilize nanocrystals in solution create an electrically insulating barrier which hinders particle to particle charge transport and limits PV device performance. Replacing the organic molecules with inorganic capping ligands could greatly improve charge transport through nanocrystal films in PVs as well as device performance.

We report the use of inorganic metal chalcogenide-hydrazinium complexes (MCCs) as well as S^2-, HS^-, and OH^- ions as capping ligands in CuInSe₂ (CIS) nanocrystal-based photovoltaic devices. A ligand exchange is used to replace the organic capping ligand, leaving the MCC ligand-capped nanocrystals dispersed in hydrazine and the S^2- ion-capped nanocrystals dispersed in water. PV devices made from MCC ligand-capped CIS nanocrystals exhibited power conversion efficiency comparable to oleylamine-capped CIS nanocrystal PVs. S^2- ion-capped CIS devices exhibit lower efficiency, but show that nanocrystal PVs with inorganic ligands can be deposited using aqueous dispersions.