A novel molecular‐ink deposition route based on thiourea and N,N‐dimethylformamide (DMF) that results in a certified solar cell efficiency world record for non‐vacuum deposited CuIn(S,Se)2 (CIS) absorbers and non‐vacuum deposited absorbers with a bandgap of 1.0 eV, is presented. It is found that by substituting the widely employed solvent dimethyl sulfoxide with DMF, the coordination chemistry of InCl3 could be altered, dramatically improving ink stability, enabling up to tenfold increased concentrations, omitting the necessity for elevated ink temperatures, and radically accelerating the deposition process. Furthermore, it is shown that by introducing compositionally graded precursor films, film porosity, compositional gradients, and the surface roughness of the absorbers are effectively reduced and device conversion efficiencies are increased up to 13.8% (13.1% certified, active area). The reduced roughness is also seen as crucial to realize monolithically interconnected CIS‐perovskite tandem devices, where semitransparent MAPbI3 devices are directly deposited on the CIS bottom cell. Confirming the feasibility of this approach, monolithic devices with near perfect voltage addition between subcells of up to 1.40 V are presented.

The limiting factor preventing kesterite (CZTSSe) thin film solar cell performance further improvement is the large open-circuit voltage deficit (Voc,def) issue, which is 0.345V for the current world record device with an efficiency of 12.6%. In this work, SnCl4 and SnCl2•2H2O are respectively used as tin precursor to investigate the Voc,def issue of dimethyl sulfoxide (DMSO) solution processed CZTSSe solar cells. Different complexations of tin compounds with thiourea and DMSO lead to different reaction pathways from solution to absorber material and thus dramatic difference in photovoltaic performance. The coordination of Sn 2+ with Tu leads to the formation of SnS and ZnS and Cu2S in the precursor film, which converted to selenides first and then fused to CZTSSe, resulting in poor film quality and device performance. The highest efficiency obtained from this film is 8.84% with a Voc,def of 0.391V. The coordination of Sn 4+with DMSO facilitates direct formation ofkesterite CZTS phase in the precursor film which directed converted to CZTSSe during selenization, resulting in compositional uniform absorber and high device performance. A device with active area efficiency 12.2% and a Voc,def of 0.344 V was achieved from Sn 4+ solution processed absorber. Furthermore, CZTSSe/CdS heterojunction heat treatment (JHT) significantly improved Sn 4+ device performance but had slightly negative effect on Sn 2+ device. A champion CZTSSe solar cell with a total area efficiency of 12.4% (active are efficiency 13.6%) and low Voc,def of 0.297 V was achieved from Sn 4+ solution. Our results demonstrate the preformed uniform kesterite phase enabled by Sn 4+ precursor is the key in achieving highly efficient kesterite absorber material. The lowest Voc-def and high efficiency achieved here shines new light on the future of kesterite solar cell.

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.

Cu2ZnSn­(S,Se)4 absorbers deposited from a nontoxic, DMSO-based molecular ink have yielded the most efficient, hydrazine-free Cu2ZnSn­(S,Se)4 photovoltaic (PV) device made through solution-processing. Although this chemistry has been widely adopted, absorber morphologies with a device-limiting fine-grained bottom layer are often reported. Here we demonstrate how the annealing profile of coatings from this ink critically affect absorber morphologies. Calibrated glow discharge optical emission spectroscopy (GDOES) is used to determine depth-dependent elemental ratios and atomic concentrations of impurities before and after selenization. An annealing temperature of 400 °C is shown to exhibit the most pronounced fine-grained bottom layer, which contains 10 atom % carbon (C) and 5 atom % nitrogen (N). Raman analysis of the mechanically exfoliated, fine-grained layer reveals that it contains amorphous carbon nitride, which is attributed to the polymerization of thiourea decomposition products during annealing. An annealing temperature of 300 °C avoids this polymerization and allows C and N to escape during selenization, while an annealing temperature of 500 °C vaporizes C and N compounds before selenization. The annealing profile of 500 C 1.5 min/layer removes nearly all but ∼0.25 atom % of C and N, which impedes the formation of a fine-grained layer and allows for a PV device efficiency of 10.7%. It is also shown how lithium-doping enhances sodium transport from the soda-lime glass substrate.