Passive grain boundaries (GBs) are essential for polycrystalline solar cells to reach high efficiency. However, the GBs in Cu2ZnSn(S,Se)4 have less favorable defect chemistry compared to CuInGaSe2. Here, using scanning probe microscopy we show that lithium doping of Cu2ZnSn(S,Se)4 changes the polarity of the electric field at the GB such that minority carrier electrons are repelled from the GB. Solar cells with lithium-doping show improved performance and yield a new efficiency record of 11.8% for hydrazine-free solution-processed Cu2ZnSn(S,Se)4. We propose that lithium competes for copper vacancies (forming benign isoelectronic LiCu defects) decreasing the concentration of ZnCu donors and competes for zinc vacancies (forming a LiZn acceptor that is likely shallower than CuZn). Both phenomena may explain the order of magnitude increase in conductivity. Further, the effects of lithium doping reported here establish that extrinsic species are able to alter the nanoscale electric fields near the GBs in Cu2ZnSn(S,Se)4. This will be essential for this low-cost Earth abundant element semiconductor to achieve efficiencies that compete with CuInGaSe2 and CdTe.

It has previously been shown that substitution of germanium for tin in Cu2ZnSn­(S,Se)4 provides a means to change the band gap. Here, we show that Ge substitution can also be used to improve carrier collection and decrease the open-circuit voltage deficit that has hindered kesterites. Using a simple molecular ink, we spray coat continuous composition gradients of Cu2Zn­(Sn,Ge)­(S,Se)4 spanning 0–90% Ge/(Ge + Sn). By mapping the absolute intensity photoluminescence and making devices from these gradients, we are able to clearly resolve changes in material properties and device performance with composition. We find that Ge can be used to increase the band gap as high as 1.3 eV (50% Ge/(Ge + Sn)) without any loss in optoelectronic material quality, but beyond 50% Ge/(Ge + Sn), the device efficiency decreases rapidly. We show evidence that the degradation results from both a deep defect located about 0.8 eV above the valence band and unfavorable band alignment. As the band gap increases, the defect moves toward midgap and becomes a stronger recombination site. After the completed devices were aged for 1 month in the ambient laboratory environment, the deep defect heals, and the V oc improves by as much as 227 mV. We demonstrate an 11.0% efficient spray-coated CZTGSSe device, without antireflective coating, that achieves 63% of the theoretical V oc as compared to the 58% for the current record device.

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.