Complexation Chemistry in N,N-Dimethylformamide-Based Molecular Inks for Chalcogenide Semiconductors and Photovoltaic Devices

Abstract

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.

Defect Control for 12.5% Efficiency Cu2ZnSnSe4 Kesterite Thin‐Film Solar Cells by Engineering of Local Chemical Environment

“…Notably, 12.5% efficiency is a new record for pure selenide‐CZTSe solar cells, and the low V OC deficit of 0.546 V also represents the best performance for kesterite solar cells in the literature. In comparison, the previous record CZTSe device shows V OC of 423 mV and V OC deficit of 0.577 V, [ 35 ] and the recently reported Cu 2 Zn(Ge,Sn)(S,Se) 4 solar cell with notably low V OC deficit shows V OC of 583 mV and V OC deficit of 0.567 V. [ 38 ] In comparison with other state‐of‐the‐art kesterite solar cells with total area efficiency beyond 11% ( Table 2 ), the 12.5% efficiency in this work is very close to the 12.6% record efficiency even though the bandgap is relatively low. Using V OC / V OC SQ to describe the V OC deficit for the solar cells with different bandgaps, [ 32 ] this device also demonstrates the lowest V OC deficit among the cutting‐edge kesterite devices as listed in Table 2.…”

Present Status of Solution‐Processing Routes for Cu(In,Ga)(S,Se)2 Solar Cell Absorbers

“…[ 73 ] The solvent system DMSO–thiourea is highly suitable for chalcogenide film formation offering i) different Lewis base sites (O in dimethyl sulfoxide, S and N in thiourea) to accommodate metal cations of different size and hardness and ii) a high dipole moment (DMSO: 3.97 D; see Table 1) allowing for the solvation of a wide range of metal salts precursors. [ 168 ] Furthermore, the hygroscopic DMSO effectively binds to water molecules (if present) and prevents hydrolysis reactions. [ 169 ] As a result, it precludes the precipitation of hydroxides and premature decomposition of TU.…”

“…In contrast to the DMSO–TU mixtures, the DMF–TU solvent system offers four Lewis characteristic base sites (O and N donor sites in DMF and S and N donor sites in TU), which stabilize many different metal cations at higher concentrations. [ 168 ] Also, DMF–TU solvent systems can solvate a wide spectrum of chloride salts including hard cations like Cu(II), Zn(II), Ge(IV) (useful for kesterite PV more broadly), and softer cations like Ag(I) and Cu(I), unlike DMSO. [ 72 ] However, cuprous chloride has poor solubility in DMF alone (0.01 m [ 168 ] ), and undergoes a disproportionation reaction, according to 2Cu + →Cu 0 + Cu 2+ .…”

Spin-coated $$\hbox {Cu}_2\hbox {ZnSnS}_{4}$$ solar cells: A study on the transformation from ink to film

“…Complex formation. Metal complex formation was investigated by Raman spectroscopy for one metal at a time, using a similar methodology as Clark and coworkers 22 . Raman spectroscopy has proven a valuable tool for the characterization of inks and solids, and it can be used to identify broken covalent bonds in molecules, i.e.…”