Molecular
inks based on dimethyl sulfoxide, thiourea (TU), and
metal salts have been used to form high optoelectronic quality semiconductors
and have led to high power conversion efficiencies for solution-processed
photovoltaic devices for Cu2ZnSn(S,Se)4 (CZTS),
Cu2Zn(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),
either have low solubility (requiring dilute inks) or lead to precipitation
or gelation. Here, we demonstrate that the combination of N,N-dimethylformamide (DMF) and TU has the remarkable
ability to form intermediate-stability acid–base complexes
with a wide number of metal chloride Lewis acids (CuCl, AgCl, ZnCl2, InCl3, GaCl3, SnCl4, GeCl4, and SeCl4), to give high-concentration stable
molecular inks. Using calorimetry, Raman spectroscopy, and solubility
experiments, we reveal the important role of chloride transfer and
TU to stabilize metal cations in DMF. Methylation of TU is used to
vary the strength of the Lewis basicity and demonstrate that the strength
of the TU-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 crystalline
CuInSe2 without selenization that sustains high quasi-Fermi
level splitting under constant illumination. 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 power conversion efficiencies of 13.4% and 11.0%
and V
oc/V
oc,SQ of 67% and 63%, respectively.