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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% and 13.12% being realized in rigid and flexible 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.

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% (certified at 14.9%) and 13.5% efficiency (certified at 13.3%) in 0.27 cm2 and 1.1 cm2-area CZTSSe 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% for 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% (active-area efficiency: 14.1%) in the cell and in particular a high open-circuit voltage of >800 mV. We have also achieved a certified total-area cell efficiency of 13.16%, realizing a substantial step forward for the pure sulfide Kesterite solar cell.

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.

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.

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.

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.

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.