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

Synergistic Crystallization Modulation and Defects Passivation in Kesterite via Anion-Coordinate Precursor Engineering for Efficient Solar Cells

It has been validated that enhancing crystallinity and passivating the deep-level defect are critical for improving the device performance of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Coordination chemistry interactions within the Cu-Zn-Sn-S precursor solution play a crucial role in the management of structural defects and the crystallization kinetics of CZTSSe thin films. Therefore, regulating the coordination environment of anion and cation in the precursor solution to control the formation process of precursor films is a major challenge at present. Herein, a synergetic crystallization modulation and defect passivation method is developed using P2S5 as an additive in the CZTS precursor solution to optimize the coordination structure and improve the crystallization process. The alignment of theoretical assessments with experimental observations confirms the ability of the P2S5 molecule to coordinate with the metal cation sites of CZTS precursor films, especially more liable to the Zn2+, effectively passivating the Zn-related defects, thereby significantly reducing the defect density in CZTSSe absorbers. As a result, the device with a power conversion efficiency of 14.36% has been achieved. This work provides an unprecedented strategy for fabricating high-quality thin films by anion-coordinate regulation and a novel route for realizing efficient CZTSSe solar cells.

Suppressing Element Inhomogeneity Enables 14.9% Efficiency CZTSSe Solar Cells

Kesterites, Cu2ZnSn(SxSe1- x)4 (CZTSSe), solar cells suffer from severe open-circuit voltage (VOC) loss due to the numerous secondary phases and defects. The prevailing notion attributes this issue to Sn-loss during the selenization. However, this work unveils that, instead of Sn-loss, elemental inhomogeneity caused by Cu-directional diffusion toward Mo(S,Se)2 layer is the critical factor in the formation of secondary phases and defects. This diffusion decreases the Cu/(Zn+Sn) ratio to 53% at the bottom fine-grain layer, increasing the Sn-/Zn-related bulk defects. By suppressing the Cu-directional diffusion with a blocking layer, the crystal quality is effectively improved and the defect density is reduced, leading to a remarkable photovoltaic coversion efficiency (PCE) of 14.9% with a VOC of 576 mV and a certified efficiency of 14.6%. The findings provide insights into element inhomogeneity, holding significant potential to advance the development of CZTSSe solar cells.

Segmented Control of Selenization Environment for High-Quality Cu2ZnSn(S,Se)4 Films Toward Efficient Kesterite Solar Cells

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.

Revealing the reason for enhanced CZTSSe device performance after Ag heavily doped into absorber surface

Ag-doping treatment is a popular method for enhancing the performance of kesterite-structured Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. Among the various methods, incorporating a high concentration of Ag+ into an absorber surface has proven to be particularly effective. However, the exact mechanisms behind this improvement are still unclear. This study aims to investigate the key factors that improve device performance through simulation. Specifically, the influence of the change in the carrier density, CuZn antisite defects, interface defect density, and formation of an n-type AZTSSe surface after heavy surface Ag doping have been examined. The simulation results indicate that the formation of an n-type AZTSSe layer on an absorber surface can significantly improve the open circuit voltage (VOC) and overcome the efficiency saturation problem induced by severe interface recombination for CZTSSe devices with a negative conduction band offset (CBO), compared to other affecting factors. This is because the modified conduction band alignment and the realization of interface-type inversion reduce interface recombination and retard the Fermi level pinning. However, the formation of interface-type inversion does not significantly improve CZTSSe devices with a positive CBO, as these devices already have weaker interface recombination. This work implies that the formation of an n-type AZTSSe layer is crucial for further improving the performance of CZTSSe devices with a negative CBO and can pave the way for improving the performance of thin film solar cells with severe interface recombination.

Passivating Grain Boundaries via Graphene Additive for Efficient Kesterite Solar Cells

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.

Toward High Efficient Cu2 ZnSn(Sx ,Se1- x )4 Solar Cells: Break the Limitations of VOC and FF

Increasing the fill factor (FF) and the open-circuit voltage (V_OC ) simultaneously together with non-decreased short-circuit current density (J_SC ) are a challenge for highly efficient Cu₂ ZnSn(S,Se)₄ (CZTSSe) solar cells. Aimed at such target in CZTSSe solar cells, a synergistic strategy to tailor the recombination in the bulk and at the heterojunction interface has been developed, consisting of atomic-layer deposited aluminum oxide (ALD-Al₂ O₃ ) and (NH₄ )₂ S treatment. With this strategy, deep-level Cu_Zn defects are converted into shallower V_Cu defects and improved crystallinity, while the surface of the absorber is optimized by removing Zn- and Sn-related impurities and incorporating S. Consequently, the defects responsible for recombination in the bulk and at the heterojunction interface are effectively passivated, thereby prolonging the minority carrier lifetime and increasing the depletion region width, which promote carrier collection and reduce charge loss. As a consequence, the V_OC deficit decreases from 0.607 to 0.547 V, and the average FF increases from 64.2% to 69.7%, especially, J_SC does not decrease. Thus, the CZTSSe solar cell with the remarkable efficiency of 13.0% is fabricated. This study highlights the increased FF together with V_OC simultaneously to promote the efficiency of CZTSSe solar cells, which could also be applied to other photoelectronic devices.

One-Step Gas-Solid-Phase Diffusion-Induced Elemental Reaction for Bandgap-Tunable CuaAgm1Bim2In/CuI Thin Film Solar Cells

Lead-free inorganic copper-silver-bismuth-halide materials have attracted more and more attention due to their environmental friendliness, high element abundance, and low cost. Here, we developed a strategy of one-step gas-solid-phase diffusion-induced reaction to fabricate a series of bandgap-tunable CuaAgm1Bim2In/CuI bilayer films due to the atomic diffusion effect for the first time. By designing and regulating the sputtered Cu/Ag/Bi metal film thickness, the bandgap of CuaAgm1Bim2In could be reduced from 2.06 to 1.78 eV. Solar cells with the structure of FTO/TiO2/CuaAgm1Bim2In/CuI/carbon were constructed, yielding a champion power conversion efficiency of 2.76%, which is the highest reported for this class of materials owing to the bandgap reduction and the peculiar bilayer structure. The current work provides a practical path for developing the next generation of efficient, stable, and environmentally friendly photovoltaic materials.

Environment-friendly copper-based chalcogenide thin film solar cells: status and perspectives

Copper chalcogenides (CuCh) have attracted considerable attention due to their promising potential as environmental-friendly photoactive material for lightweight and flexible thin film solar cells. Further, CuCh can be fabricated from simple to complex chemical compositions and offer a remarkable charge carrier mobility and excellent absorption coefficient with a desirable bandgap (up to ∼1.0 eV). Currently, they have demonstrated maximum power conversion efficiencies of over 23% for single-junction, around 25% and 28% for monolithic 2-Terminal (2T) and mechanically-stacked 4-Terminal (4T) perovskite/CuCh tandem solar cells, respectively. This article presents an overview of CuCh-based materials, from binary- to quaternary-CuCh compounds for single- and multi-junction solar cells. Then, we discuss the development of fabrication methods and the approaches taken to improve the performance of CuCh-based thin film itself, including chemical doping, the development of complement layers, and their potential application in flexible and lightweight devices. Finally, these technologies' stability, scalability, and toxicity aspects are discussed to enhance their current marketability.

Optimization of the Selenization Temperature on the Mn-Substituted Cu2ZnSn(S,Se)4 Thin Films and Its Impact on the Performance of Solar Cells

Cu₂ZnSn(S,Se)₄ (CZTSSe) films are considered to be promising materials in the advancement of thin-film solar cells. In such films, the amounts of S and Se control the bandgap. Therefore, it is crucial to control the concentration of S/Se to improve efficiency. In this study, Cu₂Mn_xZn_1-xSnS₄ (CMZTS) films were fabricated using the sol-gel method and treated in a Se environment. The films were post-annealed in a Se atmosphere at various temperature ranges from 300 °C to 550 °C at intervals of 200 °C for 15 min to obtain Cu₂Mn_xZn_1-xSn(S,Se)₄ (CMZTSSe). The elemental properties, surface morphology, and electro-optical properties of the CMZTSSe films were investigated in detail. The bandgap of the CMZTSSe films was adjustable in the scope of 1.11-1.22 eV. The structural propeties and phase purity of the CMZTSSe films were analyzed by X-ray diffraction and Raman analysis. High-quality CMZTSSe films with large grains could be acquired by suitably changing the selenization temperature. Under the optimized selenization conditions, the efficiency of the fabricated CMZTSSe device reached 3.08%.

Microenvironment Created by SnSe2 Vapor and Pre-Selenization to Stabilize the Surface and Back Contact in Kesterite Solar Cells

The ambient air-processed preparation of kesterite Cu₂ ZnSn(S,Se)₄ (CZTSSe) thin film is highly promising for the fabrication of low-cost and eco-friendly solar cells. However, the Sn volatilization loss and formation of a thick Mo(S,Se)₂ interfacial layer during the traditional selenization process pose challenges for fabricating high-efficiency CZTSSe solar cells. Here, CZTS precursors prepared by a sol-gel process in ambient air are selenized and assisted with SnSe₂ vapor via one- and two-step selenization to prepare a CZTSSe absorber on a Mo film and, subsequently, solar cells. For one-step selenization, the thickness of the fine grain and Mo(S,Se)₂ layers near the back contact can be significantly reduced with increasing SnSe₂ vapor partial pressure in the mixed selenization atmosphere, while the device efficiency is only 7.97% due to the severe interface recombination. For two-step selenization, the desired morphology and stoichiometry of the absorber can be achieved through the assistance of Sn-poor precursors selenized with high SnSe₂ vapor partial pressure to regulate the Sn content in CZTSSe, yielding the highest efficiency of 10.85%. This study improves the understanding of the key role of the microenvironment during film growth towards the production of high-efficiency thin film solar cells and other photoelectronic devices.

Insight into the Effect of Selenization Temperature for Highly Efficient Ni-Doped Cu2ZnSn(S,Se)4 Solar Cells

Cu₂Ni₀·₀₅Zn₀·₉₅Sn(S,Se)₄ (CNZTSSe) films were synthesized on Mo-coated glass substrates by the simple sol-gel means combined with the selenization process, and CNZTSSe-based solar cells were successfully prepared. The effects of selenization temperature on the performance and the photoelectric conversion efficiency (PCE) of the solar cells were systematically studied. The results show that the crystallinity of films increases as the selenization temperature raises based on nickel (Ni) doping. When the selenization temperature reached 540 °C, CNZTSSe films with a large grain size and a smooth surface can be obtained. The Se doping level gradually increased, and Se occupied the S position in the lattice as the selenization temperature was increased so that the optical band gap (Eg) of the CNZTSSe film could be adjusted in the range of 1.14 to 1.06 eV. In addition, the Ni doping can inhibit the deep level defect of Sn_Zn and the defect cluster [2Cu_Zn + Sn_Zn]. It reduces the carrier recombination path. Finally, at the optimal selenization temperature of 540 °C, the open circuit voltage (V_oc) of the prepared device reached 344 mV and the PCE reached 5.16%.

Doping of Sb into Cu2ZnSn(S,Se)4 absorber layer via Se&Sb2Se3 co-selenization strategy for enhancing open-circuit voltage of kesterite solar cells

Kesterite-structured Cu2ZnSn(S,Se)4 (CZTSSe) thin film photovoltaics have attracted considerable attention in recent years because of its low-cost and eco-friendly raw material, as well as high theoretical conversion efficiency. However, its photovoltaic performance is hindered by large open-circuit voltage (VOC) deficiency due to the presence of intrinsic defects and defect clusters in the bulk of CZTSSe absorber films. The doping of extrinsic cation to the CZTSSe matrix was adopted as an effective strategy to ameliorate defect properties of the solar cell absorbers. Herein, a novel Se&Sb2Se3 co-selenization process was employed to introduce Sb into CZTSSe crystal lattice. The results reveal that Sb-doping plays an active role in the crystallization and grain growth of CZTSSe absorber layer. More importantly, one of the most seriously detrimental SnZn deep defect is effectively passivated, resulting in significantly reduced deep-level traps and band-tail states compared to Sb free devices. As a result, the power conversion efficiency of CZTSSe solar cell is increased significantly from 9.17% to 11.75%, with a VOC especially enlarged to 505 mV from 449 mV. This insight provides a deeper understanding for engineering the harmful Sn-related deep defects for future high-efficiency CZTSSe photovoltaic devices.

Synergetic Effects of Zn Alloying and Defect Engineering on Improving the CdS Buffer Layer of Cu2ZnSnS4 Solar Cells

The inferior electrical properties at the interface of the Cu₂ZnSnS₄/CdS (CZTS/CdS) heterojunction resulting in the severe loss of open-circuit voltage (V_oc) highly restrict the photovoltaic efficiency of CZTS solar cell devices. Here, first-principles calculations show that the Zn-alloyed CdS buffer layer reverses the unfavorable cliff-like conduction band offset (CBO) of CZTS/CdS to the desirable spike-like CBO of CZTS/Zn_0.25Cd_0.75S, which suppresses carrier nonradiative recombination and blocks electron backflow. In addition, the weakened n-type conductivity of Zn_0.25Cd_0.75S can be enhanced by In, Ga, and Cl doping without the introduction of detrimental deep-level defects and severe band-tail states, which improves the V_oc of CZTS solar cells by promoting strong band bending and large quasi-Fermi-level splitting at the absorber side of the CZTS/Zn_0.25Cd_0.75S heterojunction. This study finds that the synergetic effects of Zn alloying and defect engineering on the CdS buffer layer are promising for overcoming the long-standing issue of the V_oc deficit in CZTS solar cells, and understanding the optimized interfacial electrical properties provides theoretical guidance for improving the efficiency of semiconductor devices.

Ge Bidirectional Diffusion to Simultaneously Engineer Back Interface and Bulk Defects in the Absorber for Efficient CZTSSe Solar Cells

Aiming at a large open-circuit voltage (V_OC ) 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 V_OC 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.