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Indium tin oxide (ITO) and intrinsic ZnO (i-ZnO) are usually used as the window layer of Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells within a typical structure. The optoelectronic properties of the window layer play a crucial role in determining the efficiency of solar cells. In this work, two types of CZTSSe solar cells with and without i-ZnO layers were fabricated using dimethyl sulfoxide (DMSO) and dimethylformamide (DMF) as solvents, respectively. The impact of the window layer on the performance of CZTSSe solar cells was investigated. The results suggest that the device without i-ZnO still exhibits a high power conversion efficiency (PCE) compared to conventional bilayer i-ZnO/ITO structures. By removing i-ZnO and optimizing the thickness of ITO, the highest power conversion efficiency (PCE) of DMSO and DMF-based CZTSSe solar cells has been up to 10.17% and 10.27%, respectively, with an increase in open-circuit voltage (V OC) of 15 mV (in DMSO system) and 10 mV (in DMF system), as well as an enhancement in fill factor (FF) by ∼5 percentage points. These performance improvements are mainly attributed to the reduction of series resistance (R s) of the window layer due to the removal of i-ZnO layer. Further analysis shows that the generation and collection of photon-generated carriers in CZTSSe solar cells can be affected by the optimized window layer due to optical interference effects and its resistance. Our research may provide insights for the preparation of high efficient CZTSSe solar cells with new structures of the i-ZnO free window layer. • The inclusion of i-ZnO in the CZTSSe solar cell window layer is not essential. • A more streamlined, single-layer window layer of CZTSSe solar cells was developed. • Optimizing the thickness of ITO can enhance the performance of CZTSSe solar cells. [ABSTRACT FROM AUTHOR]

Compared with the traditional structure, the new Cu 2 ZnSn(S, Se) 4 (CZTSSe) solar cell without ZnO window layer has a larger short-circuit current reduction. Due to the poor band matching, serious interface recombination exists at the CZTSSe/CdS heterojunction interface. We have designed a strategy for the preparation of CdS(In) buffer layers by In ion doping to increase the depletion layer width, enhance the carrier concentration in the CdS(In) buffer layer, and improve the energy band alignment problem at the interface of the CZTSSe/CdS heterojunction, which in turn reduces the J SC loss caused by the removal of the ZnO layer. The energy band alignment of the CZTSSe/CdS heterojunction was modulated by finely controlling the doping amount of In ions, which decreased the conduction band offset (CBO) from 0.32 eV to 0.28 eV. Good band alignment is more conducive to carrier separation and transport, and reducing the nonradiative charge recombination at the CZTSSe/CdS heterojunction interface can effectively improve the J SC. Based on the contribution of device electrical parameters to photoelectric conversion efficiency (PCE), the contributions of J SC , open-circuit voltage, and fill factor were calculated to be 107.56%, -6.89%, and -0.67%, respectively, which indicates that the method significantly improves short-circuit currents and reduces the loss of J SC due to the absence of ZnO layer. This study provides a method to achieve high-efficiency CZTSSe solar cells by optimizing the energy band matching of CZTSSe/CdS heterojunctions from 7.07% PCE in conventional cells to 9.01% PCE in novel cells. [Display omitted] • New Cu 2 ZnSn(S, Se) 4 (CZTSSe) cell structure with ZnO removal based on DMF system. • The width of depletion layer is increased by adjusting the amount of In doping in CdS. • The CdS band gap is increased, the band arrangement is adjusted, and the conduction band offset is reduced. • The short circuit current density is effectively increased by increasing the CdS carrier concentration. [ABSTRACT FROM AUTHOR]

The CZTSSe absorber layer prepared with DMSO exhibits extensive cavities at its bottom, and a prominent double-layer structure is frequently observed within the CZTSSe absorber synthesized using DMF, both of which have a detrimental impact on the device performance. In this paper, an alternate spin-coating process using DMSO and DMF solvent systems is proposed, which eliminates the double-layer structure of CZTSSe and reduces the number of cavities at the bottom of the absorber layer, thereby enhancing its overall quality. This method outperforms the traditional single-solution spin-coating technique by combining the advantages of two solvent systems, allowing for the preparation of an ideal CZTSSe film without disturbing the elemental ratio, ultimately resulting in a single-layer absorber composed of large grains. After three repetitions of an identical alternate spin-coating process, the average grain size of the absorber layer increased from 0.83 to 1.21 μm. This innovative process leads to a reduction in carrier recombination and an improvement in the short-circuit current density, ultimately raising the photoelectric conversion efficiency from 7.40% to 8.88%. • Modified the conventional spin-coating process in the preparation of CZTSSe absorber layers. • Presented a method combining the benefits of a dual-solution system approach. • The alternate spin-coating process using DMSO and DMF solvent systems improved the morphology of absorber. [ABSTRACT FROM AUTHOR]

Interfaces quality have always been considered the key factors in the fabrication of efficient Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells, and harmful defects at the interface can significantly reduce the performance of the device. Here, we propose a dual passivation strategy by adding Na to the surface of the absorber layer (CZTSSe) to passivate the CZTSSe/CdS interface and treating the buffer layer (CdS) using ultraviolet (UV) irradiation to passivate the CdS/ZnO interface. Due to the Na doping passivation, the absorber surface conductivity increases while the roughness decreases and detrimental defects that can lead to nonradiative recombination are reduced. The power conversion efficiency (PCE) of the cell is improved by about 1 percentage point by Na passivation. The contributions of open-circuit voltage (V oc), short-circuit current density (J sc), and fill factor (FF) to the improvement of PCE have been quantified as 50.64%, −5.58%, and 54.94%, indicating that Na doping can significantly boost V oc and FF. UV-ozone passivation reduces residual carbon organics in the CdS buffer layer, weakens carrier recombination, allows for more light absorption, and widens the depletion region, which favors efficient carrier collection. The estimated contribution of V oc , J sc , and FF to the PCE of the device is 8.78%, 119.59%, and −28.37%, respectively, demonstrating that UV-ozone passivation can significantly improve the J sc of the device. The two passivation strategies complement each other's advantages and achieve a synergistic effect. Our prepared champion device exhibits a PCE of 10.69% and remarkable long-term stability. [Display omitted] • Innovative dual passivation strategy proposed for CZTSSe/CdS and CdS/ZnO interfaces. • Sodium passivation of the absorber surface can significantly boost the V oc and FF. • UV-ozone passivation can effectively reduce residual carbon organics in the CdS buffer layer. • Under the dual passivation strategy, the PCE of CZTSSe device is improved from 8.07% to 10.69%. [ABSTRACT FROM AUTHOR]

The absorbers of kesterite Cu 2 ZnSn(S,Se) 4 (CZTSSe) that have undergone rapid thermal process (RTP) selenization typically exhibit a double-layer structure, with a fine crystal layer at the bottom, which degrades the performance of devices. We propose an efficient strategy for actively constructing the double-layer absorber to improve the quality of the small grains in the bottom layer. Compared to traditional methods, this method ensures uniform element distribution, particularly for Se element, and enables a more desirable Cu-poor and Zn-rich composition with a Zn/Sn ratio closer to the standard stoichiometric ratio, resulting in a reduction in harmful internal defects. This strategy eliminates small grains at the bottom and decreases grain boundary formation, resulting in a dense double-layer structure. The high-quality CZTSSe absorber not only reduces carrier recombination during back transmission, further reducing reverse saturation current (J 0) and increasing optical current density (J L), but also enhances shunt resistance (R sh) and decreases series resistance (R S). The estimated contribution percent for PCE improvement due to changes in J L , J 0 , R sh , R S , and A are 17.20 %, 120.01 %, 1.15 %, 6.50 %, and -44.86 %, respectively. Ultimately, the highest photoelectric conversion efficiency (PCE) increased from 8.73 % to 10.49 %, and device uniformity improved considerably. This study highlights the significance of optimizing the microstructure of CZTSSe absorber layer in enhancing the performance of CZTSSe solar cells, and will provide guidance for producing high-quality CZTSSe absorber layers. • Actively construct the double-layer CZTSSe absorber. • The uniform distribution of elements with the better ratio of Cu-poor and Zn-rich. • Eliminate the small bottom grains forming the dense double-layer structure. • Decrease the carrier recombination in the back transmission process. • Optimize the J L J 0 , R s , and R sh for improving the performance of devices. [ABSTRACT FROM AUTHOR]