Elemental Precursor Solution Processed (Cu1-xAgx)2ZnSn(S,Se)4 Photovoltaic Devices with over 10% Efficiency

Solvent Engineering-Enabled Surface Defect Passivation in Cu2ZnSn(S,Se)4 Solar Cells with Low Open-Circuit Voltage Losses and Improved Carrier Lifetime

The efficiency of earth-abundant kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells has been lagging behind the Shockley-Queisser limit primarily due to the presence of deep-level defects. These deep-level defects cause critical issues such as short carrier diffusion length, significant band tailing, and a large open-circuit voltage (Voc) deficit, ultimately leading to low device efficiency. To address these issues, we propose a post-fabrication defect healing strategy by dip-coating the CZTSSe film in dimethylformamide (DMF) solvent. Immersing the absorber layer in DMF (a polar solvent), neutralizes CuSn antisite defects through chemical bonding and facilitates the formation of a dense, smooth CZTSSe film with larger grain size. Deep-level transient spectroscopy revealed a remarkable increase in carrier diffusion length from 93 nm (control device) to 142 nm (champion device), confirming the beneficial effect of solvent-assisted post-treatment on mitigating CuSn antisite defects. The reduction in defect densities led to a decrease in Voc deficit by up to 289 mV, accompanied by an increased champion device efficiency of 11.4 %. This work highlights the huge potential of the DMF post-treatment strategy for defect healing in CZTSSe solar cells.

Multi-Layer Kesterite-Based Photocathodes for NH3 Photosynthesis from N2 Reduction Reaction

The necessity of new methods to substitute the Haber-Bosch process in the NH3 synthesis, generating fewer greenhouse gases, and dispensing less energy, drove the investigation of the photoelectrocatalytic approach in the N2 reduction reaction (N2RR). For that, this work presents the synthesis and characterization of the layered CZTSSe/CdS/TiO2 photocathode decorated with Pt nanoparticles for application in NH3 production using the photoelectrocatalysis technique. The CZTSSe/CdS/TiO2-Pt characterization showed a well-designed and stable photocatalyst synthesized layer by layer with an important contribution of the Pt nanoparticles for the catalyst performance, improving the photocurrent density and the charge transfer. The N2RR in a two-compartment photochemical cell with 0.1 mol L-1 Na2SO3 and 0.05 mol L-1 H2SO4 in the cathodic and anodic chamber, respectively, using CZTSSe/CdS/TiO2-Pt and under 1 sun of light incidence and applied potential of -0.4 VAg/AgCl reached 0.22 mmol L-1 cm-2 NH3, a value 28 folds higher than using the catalyst without Pt modification. The superiority of N2RR under the photoelectrocatalysis technique was demonstrated compared to photocatalytic and electrocatalytic techniques, together with the investigation of the supporting electrolyte influence in the cathodic compartment. Additionally, that is the first time a kesterite-based photocathode has been applied to NH3 photosynthesis, showing excellent photoconversion capability.

Over 10% Efficiency Pure Sulfide Kesterite Solar Cells on Transparent Electrode with Cd-Ag Co-Alloying

The environmentally friendly elements composed of high bandgap pure sulfide Cu2ZnSnS4 (CZTS) semiconductor has broad prospects for building integrated photovoltaic, double-sided, and semi-transparent solar cells when fabricated on transparent substrates. The key issues limiting the performance of CZTS solar cells are poor absorber quality and unfavorable band energy alignment causing serious charge carrier recombination. Here, thefabrication of CZTS solar cells are reported on fluorine-doped tin oxide (FTO) substrates from dimethyl sulfoxide solution and the effects of the Cd and Ag alloying on device performance. Characterizations show that Cd alloying greatly decreases defect concentration and converts Cliff-type band alignment to favorable Spike-type, leading to greatly improved current density. Further, Ag alloying eliminates near-horizontal grain boundaries and passivates defects in both bulk and heterojunction interface, resulting in a champion device with a power conversion efficiency of 10.3%, the highest efficiency pure sulfide CZTS solar cell on FTO substrate. The results demonstrate the great application potential of pure sulfide kesterite solar cells.

Insight into the Role of Rb Doping for Highly Efficient Kesterite Cu2ZnSn(S,Se)4 Solar Cells

Various copper-related defects in the absorption layer have been a key factor impeding the enhancement of the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Alkali metal doping is considered to be a good strategy to ameliorate this problem. In this article, Rb-doped CZTSSe (RCZTSSe) thin films were synthesized using the sol-gel technique. The results show that the Rb atom could successfully enter into the CZTSSe lattice and replace the Cu atom. According to SEM results, a moderate amount of Rb doping aided in enhancing the growth of grains in CZTSSe thin films. It was proven that the RCZTSSe thin film had the densest surface morphology and the fewest holes when the doping content of Rb was 2%. In addition, Rb doping successfully inhibited the formation of CuZn defects and correlative defect clusters and promoted the electrical properties of RCZTSSe thin films. Finally, a remarkable power conversion efficiency of 7.32% was attained by the champion RCZTSSe device with a Rb content of 2%. Compared with that of un-doped CZTSSe, the efficiency improved by over 30%. This study offers new insights into the influence of alkali metal doping on suppressing copper-related defects and also presents a viable approach for improving the efficiency of CZTSSe devices.

Progress and prospectives of solution-processed kesterite absorbers for photovoltaic applications

Solar cells based on emerging kesterite Cu₂ZnSn(S,Se)₄ (CZTSSe) materials have reached certified power conversion efficiency (PCE) as high as 13.6%, showing great potential in the next generation of photovoltaic technologies because of their earth-abundant, tunable direct bandgap, high optical absorption coefficient, environment-friendly, and low-cost properties. The predecessor of CZTSSe is Cu(In,Ga) Se₂ (CIGS), and the highest PCE of CIGS fabricated by the vacuum method is 23.35%. However, the recorded PCE of CZTSSe devices are fabricated by a low-cost solution method. The characteristics of the solvent play a key role in determining the crystallization kinetics, crystal growth quality, and optoelectronic properties of the CZTSSe thin films in the solution method. It is still challenging to improve the efficiency of CZTSSe solar cells for future commercialization and applications. This review describes the current status of CZTSSe solar cell absorbers fabricated by protic solvents with NH (hydrazine), protic solvents with SH (amine-thiol), aprotic solvents (DMSO and DMF), ethylene glycol methyl ether-based precursor solution method (EGME), and thioglycolic acid (TGA)-ammonia solution (NH₃H₂O) deposition methods. Furthermore, the performances of vacuum-deposited devices and solution-based processed devices are compared. Finally, the challenges and outlooks of CZTSSe solar cells are discussed for further performance improvement.

Ex situ Ge-doping of CZTS nanocrystals and CZTSSe solar absorber films

Cu₂ZnSn(S,Se)₄ (CZTSSe) is a promising material for thin-film photovoltaics, however, the open-circuit voltage (V_OC) deficit of CZTSSe prevents the device performance from exceeding 13% conversion efficiency. CZTSSe is a heavily compensated material that is rich in point defects and prone to the formation of secondary phases. The landscape of these defects is complex and some mitigation is possible by employing non-stoichiometric conditions. Another route used to reduce the effects of undesirable defects is the doping and alloying of the material to suppress certain defects and improve crystallization, such as with germanium. The majority of works deposit Ge adjacent to a stacked metallic precursor deposited by physical vapour deposition before annealing in a selenium rich atmosphere. Here, we use an established hot-injection process to synthesise Cu₂ZnSnS₄ nanocrystals of a pre-determined composition, which are subsequently doped with Ge during selenisation to aid recrystallisation and reduce the effects of Sn species. Through Ge incorporation, we demonstrate structural changes with a negligible change in the energy bandgap but substantial increases in the crystallinity and grain morphology, which are associated with a Ge-Se growth mechanism, and gains in both the V_OC and conversion efficiency. We use surface energy-filtered photoelectron emission microscopy (EF-PEEM) to map the surface work function terrains and show an improved electronic landscape, which we attribute to a reduction in the segregation of low local effective work function (LEWF) Sn(II) chalcogenide phases.

Effects of incorporation of Ag into a kesterite Cu2ZnSnS4 thin film on its photoelectrochemical properties for water reduction

Kesterite Cu₂ZnSnS₄ (CZTS) thin films in which the Cu site was partially replaced with Ag were prepared by spray deposition on an Mo-coated glass substrate. Successful replacement of Cu components in the CZTS lattice with Ag up to an Ag/(Cu + Ag) ratio of 0.20 was achieved. Samples with relatively low contents of Ag (Ag/(Cu + Ag) ratios of 0.05 and 0.10) showed obvious grain growth compared to that of bare CZTS, whereas samples with higher Ag contents showed an appreciable decrease in grain sizes. Photoelectrochemical properties for water reduction (H₂ production), which was examined after surface modifications with an In₂S₃/CdS double layer and Pt catalyst for H₂ evolution, depended strongly on such morphological differences; a maximum conversion efficiency, i.e., half-cell solar to hydrogen efficiency, of 2.4% was achieved by the photocathode based on the film with an Ag/(Cu + Ag) ratio of 0.10. Minority carrier dynamics examined by photoluminescence measurements indicated that such an active sample of PEC H₂ production had a relatively long carrier lifetime, suggesting that the suppression of carrier recombination at grain boundaries in the bulk of these kesterite films is one of the important factors for enhancing PEC functions.

Emerging Chalcogenide Thin Films for Solar Energy Harvesting Devices

Chalcogenide semiconductors offer excellent optoelectronic properties for their use in solar cells, exemplified by the commercialization of Cu(In,Ga)Se₂- and CdTe-based photovoltaic technologies. Recently, several other chalcogenides have emerged as promising photoabsorbers for energy harvesting through the conversion of solar energy to electricity and fuels. The goal of this review is to summarize the development of emerging binary (Sb₂X₃, GeX, SnX), ternary (Cu₂SnX₃, Cu₂GeX₃, CuSbX₂, AgBiX₂), and quaternary (Cu₂ZnSnX₄, Ag₂ZnSnX₄, Cu₂CdSnX₄, Cu₂ZnGeX₄, Cu₂BaSnX₄) chalcogenides (X denotes S/Se), focusing especially on the comparative analysis of their optoelectronic performance metrics, electronic band structure, and point defect characteristics. The performance limiting factors of these photoabsorbers are discussed, together with suggestions for further improvement. Several relatively unexplored classes of chalcogenide compounds (such as chalcogenide perovskites, bichalcogenides, etc.) are highlighted, based on promising early reports on their optoelectronic properties. Finally, pathways for practical applications of emerging chalcogenides in solar energy harvesting are discussed against the backdrop of a market dominated by Si-based solar cells.

N-Type Surface Design for p-Type CZTSSe Thin Film to Attain High Efficiency

As a low-cost substitute that uses no expensive rare-earth elements for the high-efficiency Cu(In,Ga)(S,Se)₂ solar cell, the Cu₂ ZnSn(S,Se)₄ (CZTSSe) solar cell has borrowed optimization strategies used for its predecessor to improve its device performance, including a profiled band gap and surface inversion. Indeed, there have been few reports of constructing CZTSSe absorber layers with surface inversion to improve efficiency. Here, a strategy that designs the CZTSSe absorber to attain surface modification by using n-type Ag₂ ZnSnS₄ is demonstrated. It has been discovered that Ag plays two major roles in the kesterite thin film devices: surface inversion and front gradient distribution. It has not only an excellent carrier transport effect and reduced probability of electron-hole recombination but also results in increased carrier separation by increasing the width of the depletion region, leading to much improved V_OC and J_SC . Finally, a champion CZTSSe solar cell renders efficiency as high as 12.55%, one of the highest for its type, with the open-circuit voltage deficit reduced to as low as 0.306 V (63.2% Shockley-Queisser limit). The band engineering for surface modification of the absorber and high efficiency achieved here shine a new light on the future of the CZTSSe solar cell.

Modification of Ag8SnS6 Photoanodes with Incorporation of Zn Ions for Photo-Driven Hydrogen Production

In this study, Zn ions were incorporated into Ag8SnS6 thin films on glass and indium–tin–oxide-coated glass substrates using chemical bath deposition. Detailed procedures for the growth of Ag–Zn–Sn–S semiconductor films and their optical, physical and photoelectrochemical performances were investigated. X-ray diffraction patterns of samples revealed that kesterite Ag2ZnSnS4 phase with a certain amount of Ag8SnS6 phase can be obtained using ethylenediaminetetraacetic acid disodium salt and trisodium citrate as the chelating agent couples. Images of field-emission scanning electron microscope showed that plate-like microstructures with some spherical aggregates were observed for the sample at low Zn content. It changed to irregular spherical grains with the [Zn]/[Sn] ratios being higher than 0.95 in samples. The energy band gaps of the samples were in the range of 1.57–2.61 eV, depending on the [Zn]/[Sn] molar ratio in sample. From the Hall measurements, the carrier concentrations and mobilities of samples were in the ranges of 6.57 × 1012–1.76 × 1014 cm−3 and 7.14–39.22 cm2/V·s, respectively. All samples were n-type semiconductors. The maximum photoelectrochemical performance of sample was 1.38 mA/cm2 in aqueous 0.25 M K2SO3 and 0.35 M Na2S solutions.

A Facile Process for Partial Ag Substitution in Kesterite Cu2ZnSn(S,Se)4 Solar Cells Enabling a Device Efficiency of over 12

A cation substitution in Cu₂ZnSn(S,Se)₄ (CZTSSe) offers a viable strategy to reduce the open-circuit voltage (V_oc)-deficit by altering the characteristics of band-tail states, antisite defects, and related defect clusters. Herein, we report a facile single process, i.e., simply introducing a thin Ag layer on a metallic precursor, to effectively improve the device characteristics and performances in kesterite (Ag_x,Cu_1-x)₂ZnSn(S_y,Se_1-y)₄ (ACZTSSe) solar cells. Probing into the relationship between the external quantum efficiency derivative (dEQE/dλ) and device performances revealed the V_oc-deficit characteristics in the ACZTSSe solar cells as a function of Cu and Ag contents. The fabricated champion ACZTSSe solar cell device showed an efficiency of 12.07% and a record low V_oc-deficit of 561 mV. Thorough investigations into the mechanism underpinning the improved performance in the ACZTSSe device further revealed the improved band-tailing characteristic, effective minority carrier lifetime, and diode factors as well as reduced antisite defects and related defect clusters as compared to the CZTSSe device. This study demonstrates the feasibility of effectively suppressing antisite defects, related defect clusters, and band-tailing characteristics by simply introducing a thin Ag layer on a metallic precursor in the kesterite solar cells, which in turn effectively reduces the V_oc-deficit.

Significantly Improving the Crystal Growth of a Cu2ZnSn(S,Se)4 Absorber Layer by Air-Annealing a Cu2ZnSnS4 Precursor Thin Film

The crystal quality of a Cu₂ZnSn(S,Se)₄ (CZTSSe) thin film is crucially important to a high-performance CZTSSe solar cell. After selenization, a bilayer CZTSSe thin film consisting of a large-grain top layer and a small-particle bottom layer is usually observed according to the literature. In this work, a facile air-annealing pretreatment is conducted for a Cu₂ZnSnS₄ precursor thin film prior to selenization, which can lead to sodium diffusion into the CZTS precursor thin film and surface oxidization of the CZTS thin film. Our experimental results revealed that the Na prediffusion and the surface oxidation of the CZTS precursor thin film can significantly promote the crystal growth of the CZTSSe thin film, which can completely remove the small-particle bottom layer and form a large-grain-spanned CZTSSe thin film. As a result, a photoelectric conversion efficiency of 9.80% was achieved by this method.

Enhancing Grain Growth for Efficient Solution-Processed (Cu,Ag)2ZnSn(S,Se)4 Solar Cells Based on Acetate Precursor

Material crystallinity is the overriding factor in the determination of the photoelectric properties of absorber materials and the overall performance of the photovoltaic device. Nevertheless, in the Cu₂ZnSn(S,Se)₄ (CZTSSe) photovoltaic device, the bilayer or trilayer structure for the absorber has been broadly observed, which is generally harmful to the cell performance because the probability of photogenerated carrier recombination at grain boundaries significantly increased. Herein, our experiment reveals that the application of anions to a new family of (Cu,Ag)₂ZnSn(S,Se)₄ (CAZTSSe) materials leads to an increase in grain size and crystallinity. It is inspiring that using acetate starting materials in the precursor solution, a uniform, compact, and pinhole-free CAZTS precursor film was obtained, and the smoothness of the films surpassed that of films fabricated from the oxide route. More importantly, the crystallization of the CAZTSSe film has been considerably enhanced after selenization, and large grains going through the entire absorber layer was successfully obtained. Additionally, it is observed that the V_oc accompanied by excellent crystallinity improved significantly due to the pronouncedly reduced carrier recombination loss at grain boundaries. As a consequence, the power conversion efficiency (PCE) of the CAZTSSe photovoltaic device is successfully increased from 10.35% (oxide route) to 11.32% (acetate route). Importantly, our work attests to the feasibility of tuning the crystallization of the CZTSSe film by simple chemistry.

Improvement of Cu2 ZnSn(S,Se)4 Solar Cells by Adding N,N-Dimethylformamide to the Dimethyl Sulfoxide-Based Precursor Ink

Cu₂ ZnSn(S,Se)₄ (CZTSSe) solar cells based on dimethyl sulfoxide (DMSO) Cu-Zn-Sn-S precursor ink have seen tremendous progress in recent years. However, the wettability between the ink and Mo substrate is poor, owing to the high viscosity of the highly concentrated Cu-Zn-Sn-S ink. Herein, a solvent engineering process is proposed in which N,N-dimethylformamide (DMF) is added into the DMSO-based Cu-Zn-Sn-S ink for the deposition of CZTSSe thin-film absorbers in air. The addition of DMF significantly improves the wettability between the precursor ink and Mo substrate. The DMF/(DMF+DMSO) ratio also plays a critical role in determining the crystal quality of the resulting CZTSSe absorber and the device performance. The grain size of CZTSSe thin films increases with increasing DMF/(DMF+DMSO) ratio. Particularly, large grains through the whole cross section can be achieved with 20 % DMF addition. Accordingly, the power conversion efficiency of the device increases from 6.5 % to 8.6 % under AM 1.5G illumination. However, the efficiency decreases to 5.4 % when the DMF content is further increased to 30 %. Interface recombination and back contact barrier are found to be the main limitations of these devices.

Progress and Perspectives of Thin Film Kesterite Photovoltaic Technology: A Critical Review

The latest progress and future perspectives of thin film photovoltaic kesterite technology are reviewed herein. Kesterite is currently the most promising emerging fully inorganic thin film photovoltaic technology based on critical raw-material-free and sustainable solutions. The positioning of kesterites in the frame of the emerging inorganic solar cells is first addressed, and the recent history of this family of materials briefly described. A review of the fast progress achieved earlier this decade is presented, toward the relative slowdown in the recent years partly explained by the large open-circuit voltage (V_OC ) deficit recurrently observed even in the best solar cell devices in the literature. Then, through a comparison with the close cousin Cu(In,Ga)Se₂ technology, doping and alloying strategies are proposed as critical for enhancing the conversion efficiency of kesterite. In the second section herein, intrinsic and extrinsic doping, as well as alloying strategies are reviewed, presenting the most relevant and recent results, and proposing possible pathways for future implementation. In the last section, a review on technological applications of kesterite is presented, going beyond conventional photovoltaic devices, and demonstrating their suitability as potential candidates in advanced tandem concepts, photocatalysis, thermoelectric, gas sensing, etc.

Phase-Separation-Induced Crystal Growth for Large-Grained Cu2ZnSn(S,Se)4 Thin Film

Large-grained Cu₂ZnSn(S,Se)₄ (CZTSSe) absorber layers are highly desirable for high-performance CZTSSe thin film solar cells. However, solution-deposited CZTSSe thin films usually consist of a large-grained top layer and a smaller-grained bottom layer. In this work, we adopt a phase-separation strategy to promote the crystal growth of the CZTSSe thin film. By choosing ZnCl₂, SnCl₂, CuCl (instead of CuCl₂), and thiourea as the starting materials, a Cu₂S/SnS/ZnS hybrid precursor thin film can be prepared, the composition of which has been testified by X-ray diffraction, X-ray photoelectron spectroscopy, Raman, and scanning electron microscopy-energy-dispersive spectrometry characterization. Owing to the volume expansion caused by Se incorporation and the high migration abilities of Cu(I) and Sn(II) ions during selenization, large-grained and compact CZTSSe films with a thickness up to 5 μm can be obtained. The corresponding thin film solar cell devices have achieved active power conversion efficiencies above 10% (8.78% for total area), much higher than those of CuCl₂-based CZTSSe devices in our lab.

Solar light harvesting with multinary metal chalcogenide nanocrystals

The paper reviews the state of the art in the synthesis of multinary (ternary, quaternary and more complex) metal chalcogenide nanocrystals (NCs) and their applications as a light absorbing or an auxiliary component of light-harvesting systems. This includes solid-state and liquid-junction solar cells and photocatalytic/photoelectrochemical systems designed for the conversion of solar light into the electric current or the accumulation of solar energy in the form of products of various chemical reactions. The review discusses general aspects of the light absorption and photophysical properties of multinary metal chalcogenide NCs, the modern state of the synthetic strategies applied to produce the multinary metal chalcogenide NCs and related nanoheterostructures, and recent achievements in the metal chalcogenide NC-based solar cells and the photocatalytic/photoelectrochemical systems. The review is concluded by an outlook with a critical discussion of the most promising ways and challenging aspects of further progress in the metal chalcogenide NC-based solar photovoltaics and photochemistry.

Influences of Silver and Zinc Contents in the Stannite Ag2ZnSnS4 Photoelectrodes on Their Photoelectrochemical Performances in the Saltwater Solution

Multicomponent metal sulfide (stannite Ag₂ZnSnS₄) samples were grown onto the conductive metal oxide-coated glass substrates by using the sulfurization of cosputtering silver-zinc-tin precursors. Several [Ag]/[Zn + Sn] and [Zn]/[Sn] ratios were set in the metal precursors to investigate their influences on the crystal phases, microstructures, and physical properties of the stannite Ag₂ZnSnS₄ samples. The results of the crystal phases and the compositions of the samples showed that the stannite Ag₂ZnSnS₄ phase can be obtained using the two-step sulfurization process, which maintained the silver-zinc-tin precursors at 160 °C for 1 h and then kept them at 450 °C for 30 min under a sulfur/nitrogen atmosphere. N-type stannite Ag₂ZnSnS₄ samples with the carrier concentrations of 5.54 × 10¹² to 9.11 × 10¹² cm^-3 can be obtained. High resistivities of Ag₂ZnSnS₄ samples were observed because of the low values of carrier concentration. Increasing the silver content in the sample can improve its photoelectrochemical (PEC) performance because of the decrease in the sample resistivity. The ratio of [Ag]/[Zn + Sn] at 0.8 and the ratio of [Zn]/[Sn] set at 0.90 in the stannite Ag₂ZnSnS₄ sample had the highest PEC performance of 0.31 mA·cm^-2, with the potential set at 1.23 V versus the relative hydrogen electrode applied on the sample because of it having the lowest charge-transfer resistance in the electrolyte.

Cation Substitution in Earth-Abundant Kesterite Photovoltaic Materials

As a promising candidate for low-cost and environmentally friendly thin-film photovoltaics, the emerging kesterite-based Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have experienced rapid advances over the past decade. However, the record efficiency of CZTSSe solar cells (12.6%) is still significantly lower than those of its predecessors Cu(In,Ga)Se2 (CIGS) and CdTe thin-film solar cells. This record has remained for several years. The main obstacle for this stagnation is unanimously attributed to the large open-circuit voltage (VOC) deficit. In addition to cation disordering and the associated band tailing, unpassivated interface defects and undesirable energy band alignment are two other culprits that account for the large VOC deficit in kesterite solar cells. To capture the great potential of kesterite solar cells as prospective earth-abundant photovoltaic technology, current research focuses on cation substitution for CZTSSe-based materials. The aim here is to examine recent efforts to overcome the VOC limit of kesterite solar cells by cation substitution and to further illuminate several emerging prospective strategies, including: i) suppressing the cation disordering by distant isoelectronic cation substitution, ii) optimizing the junction band alignment and constructing a graded bandgap in absorber, and iii) engineering the interface defects and enhancing the junction band bending.

Efficient (Cu1−xAgx)2ZnSn(S,Se)4 solar cells on flexible Mo foils

Cation substitution plays a crucial role in improving the efficiency of Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells. In this work, we report a significant efficiency enhancement of flexible CZTSSe solar cells on Mo foils by partial substitution of Cu⁺ with Ag⁺. It is found that the band gap (E_g) of (Cu_1−xAg_x)₂ZnSn(S,Se)₄ (CAZTSSe) thin films can be adjusted by doping with Ag with x from 0 to 6%, and the minimum E_g is achieved with x = 5%. We also found that Ag doping can obviously increase the average grain size of the CAZTSSe absorber from 0.4 to 1.1 μm. Additionally, the depletion width (W_d) at the heterojunction interface of CAZTSSe/CdS is found to be improved. As a result, the open-circuit voltage deficit (V_oc,def) is gradually decreased, and the band tailing is suppressed. Benefiting from the enhanced open-circuit voltage (V_oc), the power conversion efficiency (PCE) is successfully enhanced from 4.34% (x = 0) to 6.24% (x = 4%), and the V_oc,def decreases from 915 to 848 mV.

Cation substitution plays a crucial role in improving the efficiency of Cu₂ZnSn(S,Se)₄ (CZTSSe) solar cells.