PNNL

Abstract

Molecular inks based on dimethyl sulfoxide, thiourea, and metal salts may be used to form high optoelectronic quality semiconductors and have led to high power conversion efficiencies for solution-processed photovoltaic devices for Cu 2 ZnSn(S,Se) 4 (CZTS), Cu 2 Zn(Ge,Sn)(S,Se) 4 (CZGTS), CuIn(S,Se) 2 (CIS) and Cu(In,Ga)(S,Se)2 (CIGS). However, several metal species of interest including Ag(I), In(III), Ge(II), and Ge(IV) have either low solubility (requiring dilute inks) or lead to precipitation or gelation. Here, we demonstrate that the combination of N , N -Dimethylformamide (DMF) and thiourea (TU) has the remarkable ability to form intermediate-stability acid-base complexes with a wide number of metal chloride Lewis acids (CuCl, AgCl, ZnCl 2 , InCl 3 , GaCl 3 , SnCl 4 , GeCl 4 , and SeCl 4 ) to form high-concentration stable molecular inks. Using calorimetry, Raman spectroscopy, and solubility experiments, we reveal the important role of chloride transfer and thiourea to stabilize metal cations in DMF. Methylation of thiourea is used to vary the strength of the Lewis basicity and demonstrate that the strength of the thiourea:metal chloride complex formed after DMF evaporation is critical to prevent volatilization of metal containing species. Further, we formulated a sulfur-free molecular ink which was used to deposit photoluminescent, crystalline CuInSe 2 without selenization . Finally, we demonstrate the ability of the DMF-TU molecular ink chemistry to lead to high photovoltaic power conversion efficiencies and high open-circuit voltages for solution-processed CIS and CZGTS with PCE’s of 13.4% and 11.0% and V oc /V oc,SQ of 67% and 63%, respectively.