Sulfide kesterite Cu2ZnSnS4 (CZTS)—a non-toxic and low-cost photovoltaic material—has always faced severe charge recombination and poor carrier transport, resulting in its cell efficiency record stagnating at around 11

Charge loss at grain boundaries of kesterite Cu2ZnSn(S, Se)4 polycrystalline absorbers is an important cause limiting the performance of this emerging thin-film solar cell. Herein, we report a Pd element assisted reaction strategy to suppress atomic vacancy defects in GB regions. The Pd, on one hand in the form of PdSe x compounds, can heterogeneously cover the GBs of the absorber film, suppressing Sn and Se volatilization loss and the formation of their vacancy defects (i.e. VSn and VSe), and on the other hand, in the form of Pd(II)/Pd(IV) redox shuttle, can assist the capture and exchange of Se atoms, thus contributing to eliminating the already-existing VSe defects within GBs. These collective effects have effectively reduced charge recombination loss and enhanced p-type characteristics of the kesterite absorber. As a result, high-performance kesterite solar cells with a total-area efficiency of 14.5% (certified at 14.3%) have been achieved. Charge loss at grain boundaries of kesterite polycrystalline absorbers limits the performance of thin film solar cells. Here, authors report the use of Pd(II)/Pd(IV) redox shuttle to assist the capture and exchange of Se atoms and eliminate vacancy defects, realizing certified efficiency of 14.3%.

Kesterite Cu2ZnSn(S, Se)4 (CZTSSe) solar cells are highly promising low-cost thin-film photovoltaics. However, the efficiency of these solar cells is challenged by severe charge losses and complex defects. Here we reveal through a data-driven correlation analysis that the dominant deep defect in CZTSSe exhibits a donor character. We further propose that incomplete cation exchange in the multi-step crystallization reactions of CZTSSe is the kinetic mechanism responsible for the defect formation. To facilitate the cation exchange, we introduce a multi-elemental alloying approach aimed at weakening the metal-chalcogen bond strength and the stability of intermediate phases. This strategy leads to a significant reduction in charge losses within the CZTSSe absorber and to a total-area cell efficiency of 14.6% (certified at 14.2%). Overall, these results not only present a significant advancement for kesterite solar cells but could also help identify and regulate defects in photovoltaic materials. A detailed understanding of defects in kesterite solar cells is still lacking. Shi et al. identify a key defect and its formation mechanism and use alloying to suppress it, achieving solar cells with 14.2% certified efficiency.

Kesterite Cu2ZnSn(S, Se)4 (CZTSSe) solar cell has emerged as one of the most promising candidates for thin-film photovoltaics. However, severe charge loss occurring at the grain boundaries (GBs) of Kesterite polycrystalline absorbers has hindered the improvement of cell performance. Herein, we report a redox reaction strategy involving palladium (Pd) to eliminate atomic vacancy defects such as VSn and VSe in GBs of the Kesterite absorbers. We demonstrate that PdSex compounds could form during the selenization process and distribute at the GBs and the absorber surfaces, thereby aiding in the suppression of Sn and Se volatilization loss and inhibiting the formation of VSn and VSe defects. Furthermore, Pd(II)/Pd(IV) serves as a redox shuttle, i.e., on one hand, Pd(II) captures Se vapor from the reaction environment to produce PdSe2, on the other hand, PdSe2 provides Se atoms to the Kesterite absorber by being reduced to PdSe, thus contributing to the elimination of pre-existing VSe defects within GBs. These effects collectively reduce defects and enhance the p-type characteristics of the Kesterite absorber, leading to a significant reduction in charge recombination loss within the cell. As a result, high-performance Kesterite solar cells with a total-area efficiency of 14.5% have been achieved. This remarkable efficiency increase benefited from the redox reaction strategy offers a promising avenue for the precise regulation of defects in Kesterite solar cells and holds generally significant implications for the exploration of various other photovoltaic devices.

Atomic disorder, a widespread problem in compound crystalline materials, is a imperative affecting the performance of multi-chalcogenide Cu2ZnSn(S, Se)4 (CZTSSe) photovoltaic device known for its low cost and environmental friendliness. Cu-Zn disorder is particularly abundantly present in CZTSSe due to its extraordinarily low formation energy, having induced high-concentration deep defects and severe charge loss, while its regulation remains challenging due to the contradiction between disorder-order phase transition thermodynamics and atom-interchange kinetics. Herein, through introducing more vacancies in the CZTSSe surface, we explored a vacancy-assisted strategy to reduce the atom-interchange barrier limit to facilitate the Cu-Zn ordering kinetic process. The improvement in the Cu-Zn order degree has significantly reduced the charge loss in the device and helped us realize 15.4 and 13.5 solar cells, respectively, thus bringing substantial advancement for emerging inorganic thin-film photovoltaics.

Sulfide Kesterite Cu2ZnSnS4 (CZTS), a nontoxic and low-cost photovoltaic material, has always being facing severe charge recombination and poor carrier transport, resulting in the cell efficiency record stagnating around 11 years. Gradient bandgap is a promising approach to relieve these issues, however, has not been effectively realized in Kesterite solar cells due to the challenges in controlling the gradient distribution of alloying elements at high temperatures. Herein, targeting at the Cd alloyed CZTS, we propose a pre-crystallization strategy to reduce the intense vertical mass transport and Cd rapid diffusion in the film growth process, thereby realizing front Cd-gradient CZTS absorber. The Cd-gradient CZTS absorber, exhibiting downward bending conduction band structure, has significantly enhanced the minority carrier transport and additionally improved band alignment and interface property of CZTS/CdS heterojunction. Ultimately, we have achieved a champion total-area efficiency of 13.5 in particular a high open-circuit voltage of >800 mV. We have also achieved a certified total-area cell efficiency of 13.16 forward for the pure sulfide Kesterite solar cell.

In addition to open-circuit voltage (V-OC) loss, fill factor (FF) loss is considered another major factor restricting the further optimization of Cu2ZnSn(S,Se)(4) (CZTSSe) device efficiency. In this work, a comprehensive investigation into the loss mechanisms of FF has been conducted, and implemented a Li&Ag co-doping approach to enhance FF. The results indicate that the FF loss caused by insufficient carrier extraction is higher than that caused by non-radiative recombination. The carrier extraction capability is significantly influenced by the band alignment of the CdS/CZTSSe interface and has little relationship with the carrier concentration of the absorber. Therefore, although Ag doping reduces the hole concentration and conductivity, it reduces the FF loss caused by carrier extraction due to the improvement of band alignment. Ag doping is also superior to Li in passivating harmful defects, which helps reduce FF losses caused by non-radiative recombination. Correspondingly, Li performs better than Ag in increasing the hole carrier concentration and optimizing band alignment, greatly reducing FF losses caused by insufficient carrier transport. Finally, the Li and Ag co-doping strategy enables a 14.91% efficient kesterite solar cell with the highest reported FF to date of 74.30% through collaborative optimization of carrier extraction and suppression of non-radiative recombination.

With the rapid development of Kesterite Cu2ZnSn(S, Se)4 solar cells in the past few years, how to achieve higher cost-performance ratio has become an important topic in the future development and industrialization of this technology. Herein, we demonstrate an all-solution route for the cell fabrication, in particular targeting at the solution processed window layer comprised of ZnO nanoparticles/Ag nanowires. A multi-interface engineering strategy assisted by organic polymers and molecules is explored to synergistically improve the film deposition, passivate the surface defects and facilitate the charge transfer. These efforts help us achieve high-performance and robust Kesterite solar cells at extremely low time and energy costs, with efficiency records of 14.37 Kesterite solar cells, respectively. Our strategy here is also promising to be transplanted into other solar cells with similar geometric and energy band structures, helping reduce production costs and shorten the production cycle (i.e. increasing production capacity) of these photovoltaic industries.

Kesterite Cu2ZnSn(S, Se)4 (CZTSSe) solar cell has emerged as one of the most promising candidates for thin-film photovoltaics. However, severe charge losses occurring at the grain boundaries (GBs) of Kesterite polycrystalline absorbers has hindered the improvement of cell performance. Herein, we report a redox reaction strategy involving palladium (Pd) to eliminate atomic vacancy defects such as VSn and VSe in GBs of the Kesterite absorbers. We demonstrate that PdSex compounds could form during the selenization process and distribute at the GBs and the absorber surfaces; thereby aid in the suppression of Sn and Se volatilization loss and inhibiting the formation of VSn and VSe defects. Furthermore, Pd(II)/Pd(IV) serves as a redox shuttle, i.e., on one hand, Pd(II) captures Se vapor from the reaction environment to produce PdSe2, on the other hand, PdSe2 provides Se atoms to the Kesterite absorber by being reduced to PdSe, thus contributing to the elimination of pre-existing VSe defects within GBs. These effects collectively reduce defects and enhance the p-type characteristics of the Kesterite absorber, leading to a significant reduction in charge recombination loss within the cell. As a result, high-performance Kesterite solar cells with a total-area efficiency of 14.5% have been achieved. This remarkable efficiency increase benefited from the redox reaction strategy offers a promising avenue for the precise regulation of defects in Kesterite solar cells and holds generally significant implications for the exploration of various other photovoltaic devices.

Kesterite Cu 2 ZnSn(S, Se) 4 is considered one of the most competitive photovoltaic materials due to its earth-abundant and nontoxic constituent elements, environmental friendliness, and high stability. However, the preparation of high-quality Kesterite absorbers for photovoltaics is still challenging for the uncontrollability and complexity of selenization reactions between metal element precursors and selenium. In this study, we propose a solid-liquid/solid-gas (solid precursor and liquid/vapor Se) synergistic reaction strategy to precisely control the selenization process. By pre-depositing excess liquid selenium, we provide the high chemical potential of selenium to facilitate the direct and rapid formation of the Kesterite phase. The further optimization of selenium condensation and subsequent volatilization enables the efficient removal of organic compounds and thus improves charge transport in the absorber film. As a result, we achieve high-performance Kesterite solar cells with total-area efficiency of 13.6% (certified at 13.44%) and 1.09 cm 2 -area efficiency of 12.0% (certified at 12.1%).

The nature of interfaces between each functional layerof Cu2ZnSnS x Se4-x (CZTSSe) solar cells is the key issue impactingcell performance, typically its influence on carrier recombinationand charge transportation. The routine magnetron sputtering methodfor the ZnO/ITO layers can destroy ordered lattices due to the continuousbombardment of the CZTSSe/CdS heterojunction region. In this work,a sol-gel solution-processed ZnO layer (ZnO-SG) is incorporatedto construct a nondestructive heterojunction interface, and 18-crown-6-ether(18C6) is used to further modify this ZnO-SG window layer for betterperformance. Our investigation reveals that noncovalent interactionbetween 18C6 and ethanolamine from the ZnO-SG not only reduces defectsfrom the ZnO layer itself but also modulates the energy band positionfor better band alignment to facilitate carrier transportation. Finally,based on our 18C6-modified ZnO-SG, a total-area 14.06% efficiencyand 13.77% certified efficiency have been achieved.

Flexible Cu2ZnSn(S, Se)4 (CZTSSe) solar cells take the advantages of environmental friendliness, low cost, and multi-scenario applications, and have drawn extensive attention in recent years. Compared with rigid devices, the lack of alkali metal elements in the flexible substrate is the main factor limiting the performance of flexible CZTSSe solar cells. This work proposes a Rb ion additive strategy to simultaneously regulate the CZTSSe film surface properties and the CdS chemical bath deposition (CBD) processes. Material and chemical characterization reveals that Rb ions can passivate the detrimental Se0 cluster defect and additionally provide a more active surface for the CdS epitaxial growth. Furthermore, Rb can also coordinate with thiourea (TU) in the CBD solution and improve the ion-by-ion deposition of the CdS layer. Finally, the flexible CZTSSe cell fabricated by this strategy has reached a high total-area efficiency of 12.63% (active-area efficiency of 13.2%), with its VOC and FF reaching 538 mV and 0.70, respectively. This work enriches the alkali metal passivation strategies and provides new ideas for further improving flexible CZTSSe solar cells in the future.

We introduced La to realize a synergetic doping of the CdS/kesterite heterojunction to overcome the carrier concentration limit of kesterite solar cells and achieved a high efficiency of 13.9%.

The control of the phase evolution during the selenization of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is crucial for efficient solar cells. Here, we regulate the phase-evolution kinetics of Ag-alloyed CZTSSe by applying a positive pressure in the reaction chamber at the initial stage of the annealing process. The partial pressure of Se decreases, reducing the collision probability between selenium molecules and the kesterite precursor during the initial formation of the crystals. This results in the precursor transforming into CZTSSe in a single step, without the formation of secondary phases. CZTSSe forms at relatively higher temperature than conventional methods, leading to high-crystallinity kesterite films with fewer defects. We demonstrate solar cells with a total area efficiency of 14.1% and a certified total area efficiency of 13.8%. This work provides insights into the selenization mechanism and phase evolution of kesterite absorbers, enabling efficient solar cells. Secondary phases or multi-step phase formation lead to poorly crystallized and defective kesterite films. Now Zhou et al. convert precursors into kesterite in a single step, using low partial pressure of selenium, and achieve solar cells with 13.8% certified efficiency.

Cu2ZnSn(S, Se)4 (CZTSSe) is one of most competitive photovoltaic materials for its earth-abundant reserves, environmental friendliness, and high stability.The quality of CZTSSe absorber determines the power-conversion efficiency (PCE) of CZTSSe solar cells. The absorber's quality lies on post-selenization process, which is the reaction of Cu-Zn-Sn precursor and selenium vapor. And the post-selenization is dependent on various factors (e.g. temperature, precursor composition, reaction atmosphere, etc).However, synergistic regulation of these factors cannot be realized under a widely-used single-temperature zone selenization condition.Here, in our dual-temperature zone selenization scheme, a solid-liquid and solid-gas (solid precursor and liquid/gas phase Se) synergistic reaction strategy has been developed to precisely regulate the selenization. Pre-deposited excess liquid Se provides high Se chemical potential to drive a direct and fast formation of the CZTSSe phase, significantly reducing the amount of binary and ternary compounds within phase evolution. And organics removal can be accomplished via a synergistic optimization of Se condensation and subsequent volatilization. We achieve a high-performance CZTSSe solar cell with a remarkable PCE of 13.6%, and the highest large-area PCE of 12.0% (over 1cm2). Our strategy will provide a new idea for further improving efficiency of CZTSSe solar cells via phase evolution regulation, and also for other complicated multi-compound synthesis.

The Cu2ZnSn(S, Se)4 (CZTSSe) emerging inorganic solar cell is highly promising for accelerating the large-scale and low-cost applications of thin-film photovoltaics. It possesses distinct advantages such as abundant and non-toxic constituent elements, high material stability, and excellent compatibility with industrial processes. However, CZTSSe solar cells still face challenges related to complex defects and charge losses. To overcome these limitations and improve the efficiency of CZTSSe solar cells, it is crucial to experimentally identify and mitigate deep defects. In this study, we reveal that the dominant deep defect in CZTSSe materials exhibits donor characteristics. We propose that incomplete cation exchange during the multi-step crystallization reactions of CZTSSe is the kinetics mechanism responsible for the defect formation. To address this issue, we introduce an elemental synergistic alloying approach aimed at weakening the metal-chalcogen bond strength and the stability of intermediate phases. This alloying strategy has facilitated the kinetics of cation exchange, leading to a significant reduction in charge losses within the CZTSSe absorber. As a result, we have achieved a cell efficiency of over 14.5%. These results represent a significant advancement for emerging inorganic solar cells and additionally bring more opportunities for the precise identification and regulation of defects in a wider range of multinary inorganic compounds.

Due to their environmental friendliness, low cost, and versatility in applications, flexible Cu2ZnSn(S, Se)(4) (CZTSSe) solar cells have garnered significant attention in recent years. However, the lack of alkali metal elements in the flexible substrate has posed a challenge to the advancement of flexible CZTSSe solar cells. In this study, a post-treatment strategy is proposed involving Rb ions to simultaneously regulate the surface properties of the CZTSSe film and the reactions during the CdS chemical bath deposition (CBD) process. Material and chemical characterization results confirm that Rb ions effectively passivate the detrimental Se-0 defect, leading to a more active surface for the CdS epitaxial growth. Additionally, the coordination between Rb ions and thiourea in the CBD solution improves the ion-by-ion deposition of the CdS layer. These beneficial effects significantly improve the heterojunction interface and consequently enable the flexible CZTSSe solar cell to achieve a record total-area efficiency of 12.84% and excellent bending performance. Overall, the heterojunction interface regulation strategy employed in this work, along with the obtained remarkable results, offer promising prospects for the development of flexible CZTSSe solar cells.

Aiming at a large open-circuit voltage (VOC ) deficit in Cu2 ZnSn(S,Se)4 (CZTSSe) solar cells, a new and effective strategy to simultaneously regulate the back interface and restrain bulk defects of CZTSSe absorbers is developed by directly introducing a thin GeO2 layer on Mo substrates. Power conversion efficiency (power-to-efficiency) as high as 13.14% with a VOC of 547 mV is achieved for the champion device, which presents a certified efficiency of 12.8% (aperture area: 0.25667 cm2 ). Further investigation reveals that Ge bidirectional diffusion simultaneously occurs toward the CZTSSe absorber and MoSe2 layer at the back interface while being selenized. That is, some Ge element from the GeO2 diffuses into the CZTSSe absorber layer to afford Ge-doped absorbers, which can significantly reduce the defect density and band tailing, and facilitate quasi-Fermi level split by relatively higher hole concentration. Meanwhile, a small amount of Ge element also participates in the formation of MoSe2 at the back interface, thus enhancing the work function of MoSe2 and effectively separating photoinduced carriers. This work highlights the synergistic effect of Ge element toward the bulk absorber and the back interface and also provides an easy-handling way to achieve high-performance CZTSSe solar cells.

Photo-generated carrier recombination loss at the CZTSSe/Cd S front interface is a key issue to the opencircuit voltage(VOC) deficit of Cu 2 ZnSnS x Se 4-x (CZTSSe) solar cells. Here, by the aid of an easy-handling spin-coating method, a thin PCBM([6,6]-phenyl-C61-butyric acid methyl ester) layer as an electron extraction layer has been introduced on the top of CdS buffer layer to modify CZTSSe/CdS/ZnO-ITO(In 2 O 3 :Sn) interfacial properties. Based on Sn 4+ /DMSO(dimethyl sulfoxide) solution system, a totalarea efficiency of 12.87% with a VOC of 529 m V has been achieved. A comprehensive investigation on the influence of PCBM layer on carrier extraction, transportation and recombination processes has been carried out. It is found that the PCBM layer can smooth over the Cd S film roughness, thus beneficial for a dense and flat window layer. Furthermore, this CZTSSe/Cd S/PCBM heterostructure can accelerate carrier separation and extraction and block holes from the front interface as well, which is mainly ascribed to the downward band bending of the absorber and a widened space charge region. Our work provides a feasible way to improve the front interfacial property and the cell performance of CZTSSe solar cells by the aid of organic interfacial materials.

Phase evolution during the selenization is crucial for high-quality kesterite Cu2ZnSn(S, Se)4 (CZTSSe) absorbers and efficient solar cells. Herein, we regulate kinetic process of phase evolution from Cu+-Sn4+-MOE (MOE: 2-methoxyethanol) system by precisely controlling positive chamber pressure. We found that, at the heating-up stage, Se vapor concentration is intentionally suppressed in low-temperature region, which effectively reduces collision probability between the CZTS and Se atoms, thus remarkably inhibiting formation of secondary phases on the surface and multiple-step phase evolution processes. This strategy enables the phase evolution to start at relatively higher temperature and thereby leading to high crystalline quality CZTSSe absorber with fewer defects, and corresponding CZTSSe solar cell can present 14.1% efficiency (total area), which is the highest result so far. This work provides important insights into selenization mechanism of CZTSSe absorbers and explores a new way of kinetic regulation strategy to simplify the phase evolution path to efficient CZTSSe solar cells.