The substantial open-circuit voltage (V OC) deficit in kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells remains a major challenge, primarily attributed to interfacial recombination resulting from non-ideal band alignment, low carrier concentration, and secondary phase formation within the CdS buffer layer. Conventional cationic doping strategies for CdS often improve carrier density but exacerbate the conduction band offset (CBO), limiting V OC enhancement. Herein, we propose a novel cyclic-immersion technique for trifunctional yttrium (Y3+) doping of the CdS layer. The incorporated Y3 + ions substitute Cd sites and introduce donor defects (YCd and VS), significantly increasing electron concentration. Simultaneously, Y doping induces a downward shift of the conduction band minimum, thereby reducing the CBO and facilitating better electron extraction. Moreover, Y incorporation effectively passivates interfacial defects (e.g., VCd) by suppressing the formation of impurity phases like CdO and Cd(OH)2, thus mitigating non-radiative recombination. Consequently, the optimized device demonstrates a power conversion efficiency of 15.19% with a V OC of 572.1 mV, ranking among the most efficient CZTSSe solar cells. This work demonstrates a synergistic approach that integrates carrier concentration enhancement, band alignment engineering, and defect passivation through rare-earth doping of CdS, providing a validated route to high-performance kesterite photovoltaics.

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells suffer from significant open-circuit voltage (VOC) deficits due to severe interfacial and bulk recombination, restricting their power conversion efficiency (PCE) far below the Shockley-Queisser limit. This work proposes a low-temperature annealing strategy during ITO sputtering (SA) to synergistically address these challenges. The temperature applied during ITO sputtering not only improves the crystallinity, carrier concentration, and optical transmittance of the ITO layer but also promotes the diffusion of In from ITO into both CdS and CZTSSe layers. Consequently, lattice matching at the CZTSSe/CdS interface is optimized, enabling epitaxial growth. And a favorable ITO/In:CdS/In Cd:CZTSSe structure with optimal band alignment is obtained. As a result, a champion device with a PCE of 14.29

The large open-circuit voltage (VOC) deficit remains a central bottleneck in Cu2ZnSn(S,Se)4 (CZTSSe) solar cells, originating from the coupled effects of uncontrolled MoSe2 growth at the rear contact and defect-mediated non-radiative recombination in the absorber. Here, we report a defect-selective back-contact engineering strategy via a thermally oxidized MnS interlayer that simultaneously regulates interfacial reaction kinetics and defect energetics. The MnS interlayer suppresses excessive MoSe2 formation and reduces the valence-band offset from 0.32 to 0.10 eV, thereby promoting hole-selective transport. Meanwhile, the junction quality is substantially improved, as evidenced by an expanded depletion width (236 to 286 nm), a reduced interfacial defect density (1.31 × 1015 to 4.60 × 1014 cm-3), and prolonged carrier lifetimes (1.20 to 2.48 and 99 to 208 µs, respectively). First-principles calculations further reveal that Mn incorporation reconstructs defect formation energetics by suppressing deep SnZn antisites while favoring shallow acceptor-type defects, thus mitigating Shockley-Read-Hall recombination and strengthening p-type transport. Consequently, a VOC of 550.7 mV and an efficiency of 14.35% are achieved, representing the highest performance reported to date for Mn-modified CZTSSe solar cells.

Cu2ZnSn(S,Se)4 kesterite solar cells, while promising for sustainable photovoltaics, are constrained by undefined crystallization kinetics during selenization, which introduces bilayer crystallization with detrimental horizontal grain boundaries, voids, and secondary phases. This work traces this issue to early-stage Se-driven reaction at the back interface, which forms a low-melting-point Cu(S,Se) phase and triggers uncontrolled reverse crystallization. To address this, we propose a thermal-decoupled selenization strategy that creates a vertical Se concentration gradient in the initial stage. This approach decouples Se supply from the Cu(S,Se) formation temperature range, thereby suppressing bilayer crystallization. Consequently, it enables the growth of top-down columnar grains, which enhance carrier transport and suppress recombination, achieving a champion power conversion efficiency of 15.7% (certified 15.3%) in the resultant devices. This approach offers critical insights into crystallization kinetics and is also applicable to solution-processed Cu(In,Ga)Se2 solar cells, highlighting its great significance for diverse copper-based chalcogenides.

The large open-circuit voltage deficit (VOC,def) is currently recognized as the primary obstacle hindering the development of kesterite solar cells. Surface electrostatic potential fluctuations and bandgap fluctuations resulting from Cu/Zn antisite defects hinder energy level alignment and lead to VOC-limiting recombination at the CZTSSe/CdS interface. Herein, we develop a new strategy to reconstruct the absorber surface by spin-coating a high-concentration Li-containing solution onto the CZTS precursor films. This treatment facilitates the formation of a more stable weak n-type phase and n-type ZnLi defects on the surface of the absorber layer, thereby enhancing effective p-n conversion at the heterojunction. Moreover, the emergence of LiZn shallow-level defects contributes to suppressing CuZn defects on the surface of the absorber layer, which in turn increases band bending and reduces non-radiative recombination at the interface. As a consequence, after undergoing the Li-containing interface treatment, CZTSSe solar cells with a champion efficiency of 14.92% (certified 14.52%) have been realized, mainly benefitting from remarkably high VOC approaching 570 mV and greatly reduced VOC,def of 0.266 V. This study presents a feasible strategy to modulate kesterite solar cell surfaces for minimizing interfacial losses and enhancing device efficiency.

Copper indium sulfide (CuInS2) is a promising absorber among emerging chalcogenide photovoltaics. However, solution-processed CuInS2 solar cells at low temperature (< 400 degrees C) still lag far behind the requirement of at least 10% efficiency for practical application since 1993. Here, a low-temperature strategy, featuring a solution-processed In-rich CuInS2 film interdigitated with electron-extracting interfaces and a sulfur anion-induced gradient phase transformation (S2--GPT) process at < 200 degrees C, is conceptually developed for preparing the CuInS2 solar cells with a breakthrough efficiency up to 12.28%. The interdigitated structure has unique advantages over the simple bilayer structure conventionally used; moreover, the S2--GPT process leads to the transformation from CuAu to CuIn5S8 phase to reduce the defect density and charge recombination inside CuInS2 film for boosting solar cell performance. The detailed principles behind material and device are elucidated. Our findings offer a promising way to chalcogenide solar cells and the application of CuInS2 devices.

The selenization reaction process is the key step in determining the quality of Cu2ZnSn(S,Se)4 thin films. Imbalanced migration kinetics of metal ions during selenization led to high concentrations of deep-level defects, resulting in dramatic open-circuit voltage loss. In this work, we reported a Li2SnS3 interphase strategy to modify cation migration paths and balance Zn2+/Sn4+ migration differences. The Li2SnS3 interphase selectively encapsulates the Cu2Sn(S,Se)3 intermediate grains, serving as the rate-determining layer for ion migration. The Zn2+/Sn4+ migration barrier difference in the interphase decreases from 0.41 eV in Cu2Sn(S,Se)3 to 0.21 eV in Li2SnS3, which promotes the formation of larger, uniform, high-crystallinity grains. As a result, device efficiency improves from 13.86% to 15.45% (certified at 15.04%), and open-circuit voltage reaches 602 mV at a bandgap of 1.10 eV.

The presence of secondary phases and a high concentration of deep-level defects led to a large open-circuit voltage deficit (Voc,deficit) for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Here we regulate the phase evolution from CZTS to CZTSSe in the initial selenization stage to obtain high-quality absorber with minimal defects and secondary phases. By adding the bidentate chelation structured mercaptopropionic acid (MPA) into the air-prepared 2-methoxyethanol (MOE) precursor solution, large CZTS colloidal particles and dense precursor films are prepared. During the initial selenization stage, the reduced nucleation sites can decrease selenium-molecule interactions and extend the phase evolution process. This extension makes the heterogeneous nucleation more controllable, fostering uniform element distribution and enhanced growth of permeating the large-grain layer. These benefits demonstrate a substantial increase in device efficiency up to 14.99% (certified at 14.38%) with a reduced Voc,deficit of 281.18 mV. The findings are of great significance for further efficiency leaps of kesterite solar cells.

Solution-processed Cu(In,Ga)Se2 (CIGS) solar cells suffer from serious carrier recombination and power conversion efficiency (PCE) loss because of the poor film properties and easy formation of defects. Herein, we propose Ag&Se co-selenization strategy to enhance the crystallization and passivate harmful defects of the CIGS films. The formation of Ag-Se phase during the selenization process enables the formation of large grains and suppresses the deep level defects. It is found that Ag doping can enlarge the depletion region width, lower the Urbach energy and prolong the carrier lifetime. As a result, a champion solution-processed CIGS solar cell presents a high efficiency of 16.48% with the highly improved open-circuit voltage (VOC) of 662 mV and fill factor (FF) of 75.8%. This work provides an efficient strategy to prepare high quality solution-processed CIGS films for high-performance CIGS solar cells.

Although it is widely recognized by the CZTSSe community that the CdS buffer layer have a profound effect on the carrier transport to reach the Shockley-Queisser limit and thus the resulting device performance, such interfacial engineering on the CZTSSe/CdS heterojunction is still lagging. Herein, a novel strategy in CZTSSe family via hydrogen-plasma treatment of CdS layer to regulate the quality of the CZTSSe/CdS heterojunction, and thus, to boost the open-circuit voltage (VOC) and power conversion efficiency (PCE), is reported. Simultaneously, a detailed mechanism for enhancing the device performance is also presented. Systematic studies reveal that the hydrogen modification in CdS layer induces strong chemical interactions and effective surface passivation, which is beneficial for improving the electrical properties of CdS film and elevating the quality of heterojunction. Importantly, the hydrogen modification strategy provides a reliable approach for interfacial defect management and energy level alignment optimization, thereby reducing defect-assisted recombination and interfacial energy losses. Consequently, the omnibearing modification regarding the heterojunction interface enables the CZTSSe device achieving an outstanding PCE of 14.96 % with a smaller VOC-deficit of 287 mV, accompanied by a substantial enhancement in stability. The as-proposed hydrogen modification strategy provides valuable insights into interfacial engineering for pushing CZTSSe devices toward theoretical efficiency limit.

The deep defects and secondary phases arising from complex phase evolution pathways pose a significant challenge that hinders the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Here, we regulate the phase-evolution pathways of CZTSSe by implementing a novel ZnO blocking layer on the kesterite precursor surface. The ZnO layer, which remains stable at low temperatures, effectively prevents contact and reactions between the precursor and low-concentration selenium (Se), thereby suppressing complex phase evolution pathways. As the temperature rises, the gradual diffusion and eventual disappearance of the ZnO layer facilitate a one-step phase transition between the precursor and high-concentration Se at elevated temperatures, leading to the direct formation of CZTSSe. This method yields high-quality CZTSSe films characterized by high crystallinity, absence of secondary phases, and fewer defects. Consequently, band tail states and non-radiative recombination are significantly suppressed, enhancing charge transportation and extraction. Ultimately, we achieve a state-of-the-art CZTSSe device with an efficiency of 14.45% and a VOC of 572.6 mV, featuring the lowest VOC deficit reported in kesterite solar cells (VOC/VSQOC = 69.7%). This study offers valuable insights into the regulation of phase evolution and selenization process of CZTSSe absorbers, paving the way for more efficient solar cell designs.

Poor crystallinity is a common problem of kesterite absorbers based on non‐hydrazine solution method, which obstructs charge transfer and affects photovoltaic performance of the thin‐film devices, especially the open‐circuit voltage (VOC). Se diffusion is often insufficient during the crystal growth of kesterite absorber, resulting in uneven selenization reaction. Herein, Se molecule is introduced into kesterite precursor film to promote the absorber crystallinity while preventing the formation of a thick Mo(Se,S)2 layer. It is found that after Se‐introduction treatment, Se element distributes more uniformly in the absorber film after high‐temperature annealing. During selenization, the lower part of the precursor film can easily obtain Se and experience crystallization, thus promoting the crystallization of the whole absorber. As a result, the absorber defects are passivated. According to charge carrier characterization, the carrier lifetime of the device is prolonged due to the reduced carrier recombination centers. Finally, a champion device with the VOC increases by 23 mV, and an efficiency of 12.39% (active area efficiency of 13.57%) is achieved.

As an emerging thin-film photovoltaic device, kesterite solar cells are affected by severe non-radiative recombination at the heterojunction and within the absorber bulk, which is a major cause of their low open-circuit voltage (VOC). Herein, an effective strategy is proposed by spin-coating Na2S solution onto the surface of the precursor film to reduce the recombination by simultaneously passivating the defective grain boundaries and tailoring the heterojunction band alignment. The obtained absorber shows improved crystallinity and passivated defects. A significantly reduced interface defect is observed. Besides, the increased S/Se ratio on the absorber surface elevates the conduction band minimum (CBM), making the electron transport barrier at the heterojunction smaller. As a result, the interface carrier recombination decreases, and the carrier lifetime increases. The devices treated with Na2S show obvious improvement in VOC and fill factor (FF), with a champion efficiency of 14.26%. This provides a new aspect for the application of Na doping method.

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells have attracted considerable attention in recent years, and considered to be potential alternative solutions to existing commercialized solar cells for future energy demands. However, poor-quality p-n junction causes severe carrier recombination at the front interface, resulting high open-circuit voltage deficit and unsatisfactory efficiency of CZTSSe solar cells. In this work, Ag,Cd double gradient doping was introduced to front interface for interface carrier recombination suppression. Benefited from the formed Ag,Cd double gradient, defect and/or defect cluster at front interface significantly reduced, thus evidently suppressing the carrier recombination at front interface. As a result, the power conversion efficiency (PCE) of champion device increased from 10.04 % (Reference) to 12.66 % (With Ag,Cd). This Ag,Cd double gradient architecture offers new ideas for improvement of high efficiency CZTSSe as well as other solar cells.

Solution-processed CIGS solar cells present a cost-effective alternative for photovoltaic production, yet they exhibit significant power conversion efficiency (PCE) losses compared to vacuum-deposited counterparts. The performance limitations originate from inferior film quality, which stems from fundamentally different growth mechanisms. Here, we propose a novel preannealing strategy for the CIGS precursor film, which can regulate the nucleation and crystallization pathway. Characterization reveals that preannealing enlarges nucleation sizes while reducing nucleation sites, thereby promoting grain growth and yielding high-quality absorbers with compact morphology, enhanced crystallinity, and fewer defects. Ultimately, the optimized CIGS solar cells exhibit an impressive PCE of 17.51%, which is among the top values for solution-processed CIGS solar cells. This nucleation and crystallization regulation strategy provides a framework for growth optimization to achieve high-performance, solution-processed chalcopyrite photovoltaics.

Grain boundaries (GBs)-triggered severe non-radiative recombination is recently recognized as the main culprits for carrier loss in polycrystalline kesterite photovoltaic devices. Accordingly, further optimization of kesterite-based thin film solar cells critically depends on passivating the grain interfaces of polycrystalline Cu2 ZnSn(S,Se)4 (CZTSSe) thin films. Herein, 2D material of graphene is first chosen as a passivator to improve the detrimental GBs. By adding graphene dispersion to the CZTSSe precursor solution, single-layer graphene is successfully introduced into the GBs of CZTSSe absorber. Due to the high carrier mobility and electrical conductivity of graphene, GBs in the CZTSSe films are transforming into electrically benign and do not act as high recombination sites for carrier. Consequently, benefitting from the significant passivation effect of GBs, the use of 0.05 wt% graphene additives increases the efficiency of CZTSSe solar cells from 10.40% to 12.90%, one of the highest for this type of cells. These results demonstrate a new route to further increase kesterite-based solar cell efficiency by additive engineering.

High-crystalline-quality absorbers with fewer defects are crucial for further improvement of open-circuit voltage (VOC) and efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. However, the preparation of high-quality CZTSSe absorbers remains challenging due to the uncontrollability of the selenization reaction and the complexity of the required selenization environment for film growth. Herein, a novel segmented control strategy for the selenization environment, specifically targeting the evaporation area of Se, to regulate the selenization reactions and improve the absorber quality is proposed. The large evaporation area of Se in the initial stage of the selenization provides a great evaporation and diffusion flux for Se, which facilitates rapid phase transition reactions and enables the attainment of a single-layer thin film. The reduced evaporation area of Se in the later stage creates a soft-selenization environment for grain growth, effectively suppressing the loss of Sn and promoting element homogenization. Consequently, the mitigation of Sn-related deep-level defects on the surface and in the bulk induced by element imbalance is simultaneously achieved. This leads to a significant improvement in nonradiative recombination suppression and carrier collection enhancement, thereby enhancing the VOC. As a result, the CZTSSe device delivers an impressive efficiency of 13.77% with a low VOC deficit.

The persistent challenge in kesterite solar cells is the low open-circuit voltage (V-oc) and fill factor (FF) due to nonradiative recombination at the CdS/Cu2ZnSn(S,Se)(4) (CZTSSe) interface. Here we demonstrate a convenient combination of low-temperature annealing and In doping within the buffer layer to establish an electrically benign high-quality CdS:In/CZTSSe heterojunction. The low-temperature annealing facilitates the migration of Cu, Zn, and Sn impurity elements from the buffer layer to the absorber side, improving lattice match and reducing detrimental defects and the conduction band offset (CBO) barrier involving large recombination losses. The In doping boosts the donor concentration and crystallinity of the buffer layer, thereby improving the electron transport and extraction processes. Consequently, the CdS:In device achieves the highest efficiency of 14.5% with the V-oc,V-deficit decreasing from 348 mV to 287 mV and the FF increasing from 66.6% to 70.3%, promising a significant efficiency leap for CZTSSe solar cells

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells suffer from severe carrier recombination, limiting the photovoltaic performance. Unfavorable energy band alignment at the p-n junction and defective front interface are two main causes. Herein, oxygen incorporation in CZTSSe via absorber air-annealing was developed as a strategy to optimize its surface photoelectric property and reduce the defects. With optimized oxygen incorporation conditions, the carrier separation and collection behavior at the front interface of the device is improved. In particular, it is found that oxygen incorporated absorber exhibits increased band bending, larger depletion region width, and suppressed absorber defects. These indicate the dynamic factors for carrier separation become stronger. Meanwhile, the increased potential difference between grain boundaries and intra grains combined with the decreased concentration of interface deep level defect in the absorber provide a better path for carrier transport. As a consequence, the champion efficiency of CZTSSe solar cells has been improved from 9.74% to 12.04% with significantly improved open-circuit voltage after optimized air-annealing condition. This work provides a new insight for interface engineering to improve the photoelectric conversion efficiency of CZTSSe devices.

Insufficient selenization and uneven distribution of elements caused by the poor diffusion and reaction activity of selenium clusters is one of the main issues limiting the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Here, this work designs a simple and feasible strategy to improve the activity of selenium (Se) by implementing high-temperature treatment on graphite boxes loaded with Se pellets. The rapid adsorption/desorption characteristics of graphite on active gaseous small-molecule selenium have successfully introduced hyperactive Se4, Se3, and Se2 into the selenization process. The results indicate that the adsorbed non-toxic gaseous active Se3 and Se4 can quickly and uniformly diffuse into the precursor film at low temperatures, thereby inducing nucleation and grain growth at both surface and back interface simultaneously, which inhibits the upward migration and aggregation of cations, especially Cu, and promotes the homogenization of elements. The overall relatively Cu-poor chemical environment suppresses the formation of CuZn defects and [2CuZn+SnZn] defect clusters, and also promotes the generation of favorable VCu. The band tail states and non-radiative recombination are then optimized. Finally, the CZTSSe solar cells achieve a power conversion efficiency (PCE) of 14.5%, with VOC/VOCSQ of 67% being one of the highest in the literature. The adsorption and desorption process of selenium (Se) clusters, especially small molecule Se3 and Se4, on graphite enhance the diffusion and reaction activity of traditional Se particles, thereby improving the chemical environment for grain growth, optimizing the film crystallinity, and passivating defects, and finally making the device efficiency up to 14.5% with a VOC of 553.1 mV. image