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

Approaching 24% Efficiency in Four-Terminal Perovskite/CZTSSe Tandem Solar Cells Using Diphenylammonium Chloride Additive-Based Wide-Bandgap Perovskite Absorber

In the quest for high-efficiency photovoltaics, tandem solar cells combining perovskite and CZTSSe (copper zinc tin sulfide selenide) hold significant promise. This study explores the integration of diphenylammonium chloride (DPACl) as an additive within a wide-bandgap (WBG) perovskite layer to enhance the performance of a four-terminal (4-T) hybrid tandem solar cells (HTSCs) device. The DPACl additive has been systematically optimized and utilized for WBG perovskite solar cells (PSCs). Here, an optimum amount of DPACl additive effectively enhances the quality of perovskite films, and improves charge carrier dynamics thereby reducing non-radiative recombination losses. An optimized 2 mg mL-1 DPACl-based PSC achieved a power conversion efficiency (PCE) of 19.66% with thehighest open circuit voltage (VOC) of 1.172 V. Further, the WBG-based PSCs integrate into a 4-T mechanically stacked with narrow bandgap (NBG-1.05 eV)-based CZTSSe for HTSCs, which demonstrates a high PCE of 23.96%. This research contributes essential insights into the development of efficient photovoltaic systems based on perovskite and CZTSSe tandem architectures.

Tailoring selenization dynamics: How heating rate manipulates nucleation and growth boosts efficiency in kesterite solar cells

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) has emerged as a promising photovoltaic material due to its low cost and high stability. The CZTSSe film for high-performance solar cells can be obtained by annealing the deposited CZTS precursor films with selenium (a process known as selenization). The design of the selenization process significantly affects the quality of the absorber layer. In this work, we systematically investigate the impact of heating rate on the selenization kinetics and the microstructural characteristics of the films using a two-step selenization method. The results indicate that a slow heating rate promotes surface crystallization, resulting in a thick and dense layer of large grains at the film surface that impedes the diffusion of Se vapor. Conversely, a rapid heating rate enhances the diffusion of Se into the interior of the film, synthesizing more low-melting-point intermediate compounds that facilitate grain growth and reduce the thickness of fine grains at the film bottom. Ultimately, a CZTSSe solar cell with an efficiency of 10.17% was fabricated at a heating rate of 200 °C/min. This research deepens the understanding of thin film growth mechanisms and advances the development of high-performance solar cells.

11.88% Efficient Flexible Ag-Free CZTSSe Solar Cell: Spontaneously Tailoring the Alkali Metal Level

Alkali metal is the requirement for highly efficient Cu2ZnSn(S, Se)4 (CZTSSe) solar cells, thus it is crucial to additionally incorporate alkali metal into the absorber layer for flexible solar cells. However, the efficiency of flexible CZTSSe devices reported to date, based on the conventional alkali incorporation strategies, still lags behind those made on rigid substrates. One of the main issues is the inability to control the alkali content and distribution in the absorber layer. Here, a facile alkaline incorporation approach is proposed, effectively regulating the content and distribution of alkali metals in the film. Such a method can spontaneously tailor the alkali metal content to a proper level, thus leading to the suppression of non-radiative recombination and a better carrier transport through the enhanced film quality and the optimized band binding structure. Finally, a champion flexible CZTSSe solar cell with an efficiency of 11.88% is achieved, the highest reported efficiency for a CZTSSe solar cell without noble Ag doping. This study affords an innovative spontaneous alkali-doping design for the preparation of high-performance flexible CZTSSe solar cells and provides a deeper insight into the extent of alkali metal doping.

Formation of Alloyed Cu2CoxZn1-xSn(S,Se)4 Absorption Layer and Its Application in Solar Cells

Partial substitution of cations is crucial for suppressing harmful defects in Cu2ZnSn(S,Se)4 thin-film solar cells. In this study, based on the mixed n-butylammonium and butyrate solution system, the alloyed Cu2CoxZn1-xSn(S,Se)4 phase can be prepared by substituting Zn2+ with Co2+, which can suppress harmful defects and optimize the crystallinity of the Cu2ZnSn(S,Se)4 absorption layer, and improve the photoelectric conversion efficiency (PCE) of devices. By systematic investigation of the impact of Co content on the performance of devices, the optimal substitution amount of Zn2+ with Co2+ is 0.05. At this time, PCE, the open-circuit voltage (VOC), current density (JSC), and fill factor (FF) of the devices can reach 9.0%, 416 mV, 33.87 mA/cm2, and 64%, respectively. It is the first time that the replacement of Zn2+ with Co2+ is applied to optimize PCE of CZTSSe solar cells. The excellent results also demonstrate that the substitution of Zn2+ with Co2+ can become a new approach for further performance optimization of Cu2ZnSn(S,Se)4 solar cells.

Tailoring Li assisted CZTSe film growth under controllable selenium partial pressure and solar cells

It is still critical to prepare a high-quality absorber layer for high-performance Cu2ZnSnSe4 (CZTSe) multi-component thin film solar cell. The gas pressure during the selenization process is commonly referred to as the pressure of inert gas in the tube furnace, while the exact selenium partial pressure is difficult to be controlled. Therefore, the grain growth under different selenium partial pressures cannot be made clear, and the film quality cannot be controlled as well. In this work, we use a sealed quartz tube as the selenization vessel, which can provide a relatively high and controllable selenium partial pressure during the selenization process. To further tailor the grain growth, lithium doping is also utilized. We find that lithium can greatly promote the growth of CZTSe films as the selenium partial pressure is controlled near the selenium saturation vapor pressure. Combined with ALD-Al2O3, the crystallization quality of CZTSe absorber films is significantly enhanced and the efficiency of CZTSe solar cells achieved a significant improvement. This work clarifies the effect of controllable Se pressure on CZTSe film growth and can lead to better results in CZTSe and other multi-compound thin film solar cells.

In Situ Vanadium Modification Induced a Back Interfacial Field Passivation Effect toward Efficient Kesterite Solar Cells beyond 11% Efficiency

Realization of a high-quality back electrode interface (BEI) with suppressed recombination is crucial for Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. To achieve this goal, the construction of a traditional chemical passivation effect has been widely adopted and investigated. However, there is currently a lack of reports concerning the construction of a field passivation effect (FPE) for the BEI. Herein, considering the characteristic of the negligible difference in ionic radius between Mo (0.65 Å) and V (0.64 Å) as well as the presence of one less valence electron compared to Mo, vanadium (V) was employed and in situ incorporated into the MoSe2 interfacial layer during the deposition of the Mo:V electrode and selenization process. This allowed for the establishment of a desirable in situ VI-FPE interface with p-MoSe2:V/p-CZTSSe at the BEI. The p-type characteristic in MoSe2:V is attributed to the presence of the VMo acceptor; notably, the Fermi energy level of MoSe2:V has shifted downward by 0.62 eV compared to MoSe2, thereby facilitating the formation of an optimized band alignment between MoSe2:V and the absorber. Consequently, the photovoltaic parameters of the cell-FPE have experienced a significant increase due to the enhanced carrier transportation efficiency compared to cell-ref, resulting in a remarkable improvement in efficiency from 8.28 to 11.11%.

Improvement of Performance of CZTSSe Solar Cells by the Synergistic Effect of Back Contact Modification and Ag Doping

To improve the performance of Cu2ZnSn(S,Se)4 solar cells, a strategy is proposed to improve the quality of absorber and back interface simultaneously by substituting V-doped Mo (Mo:V) for a conventional Mo back electrode and incorporating Ag into the Cu2ZnSn(S,Se)4 (ACZTSSe) absorber in this work. Since p+-type V-doped MoSe2 (MoSe2:V) is formed in the site between the absorber and Mo:V during selenization, the conventional Mo/n-MoSe2 back contact is modified to Mo:V/p+-MoSe2:V, a back surface passivation field (BSPF) is established at the back interface, the band bending of MoSe2:V is downward and that of bottom of the absorber is upward. Further investigation reveals that the back contact modification and Ag doping have a synergistic effect on inhibiting carrier recombination, decreasing series resistance and increasing shunt resistance, thereby leading to the PCE of device without antireflection coating increased from 8.61 to 10.98%, which is larger than the sum of increase in PCE induced by Ag doping alone (8.61 to 9.66%) and back contact modification alone (8.61 to 9.63%). It is demonstrated that the synergistic effect stems mainly from the strengthened BSPF and the further reduced back contact barrier height. The former is due to the increased difference in work function (WF) between MoSe2:V and absorber induced by the reduced WF of the absorber after Ag doping and the raised WF of MoSe2:V after V doping. The latter is due to the downshifted valence band maximum of absorber after Ag doping. This work highlights the synergistic effect of back contact modification and Ag doping on improving the performance of CZTSSe solar cells and also provides an effective way to suppress carrier recombination.

Regulating Hetero-Nucleation Enabling Over 14% Efficient Kesterite Solar Cells

Developing well-crystallized light-absorbing layers remains a formidable challenge in the progression of kesterite Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. A critical aspect of optimizing CZTSSe lies in accurately governing the high-temperature selenization reaction. This process is intricate and demanding, with underlying mechanisms requiring further comprehension. This study introduces a precursor microstructure-guided hetero-nucleation regulation strategy for high-quality CZTSSe absorbers and well-performing solar cells. The alcoholysis of 2-methoxyethanol (MOE) and the generation of high gas-producing micelles by adding hydrogen chloride (HCl) as a proton additive into the precursor solution are successfully suppressed. This tailored modification of solution components reduces the emission of volatiles during baking, yielding a compact and dense precursor microstructure. The reduced-roughness surface nurtures the formation of larger CZTSSe nuclei, accelerating the ensuing Ostwald ripening process. Ultimately, CZTSSe absorbers with enhanced crystallinity and diminished defects are fabricated, attaining an impressive 14.01% active-area power conversion efficiency. The findings elucidate the influence of precursor microstructure on the selenization reaction process, paving a route for fabricating high-quality kesterite CZTSSe films and high-efficiency solar cells.

Cs-treatments in Kesterite Thin-Film Solar Cells for Efficient Perovskite Tandems

Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells are an attractive choice for a bottom cell of the low-cost and environmental tandem solar cells with perovskite. However, the progress in developing efficient perovskite/CZTSSe tandem solar cells has been hindered by the lack of high performance of the CZTSSe bottom cell. Here, an efficient CZTSSe bottom cell is demonstrated by adopting a facile and effective CsF treatment process. It is found that the CsF treatment not only facilitates grain growth and improves phase homogeneity by suppressing the detrimental deep-level defects and secondary phases, but also induces larger band bending and stronger drift force at the P-N junction. As a result, the carrier extraction/transport can be effectively accelerated, while reducing the interfacial recombination. These combined effects eventually result in a significant performance enhancement from 8.38% to 10.20%. The CsF-treated CZTSSe solar cell is finally applied to the mechanically-stacked perovskite/CZTSSe 4-terminal tandem cell by coupling a semi-transparent perovskite top cell, which exhibits the highest reported tandem efficiency of 23.01%.

Improving the Device Performance of CZTSSe Thin-Film Solar Cells via Indium Doping

Cation incorporation emerges as a promising approach for improving the performance of the kesterite Cu2ZnSn(S,Se)4 (CZTSSe) device. Herein, we report indium (In) doping using the chemical bath deposition (CBD) technique to enhance the optoelectronic properties of CZTSSe thin-film solar cells (TFSCs). To incorporate a small amount of the In element into the CZTSSe absorber thin films, an ultrathin (<10 nm) layer of In2S3 is deposited on soft-annealed precursor (Zn-Sn-Cu) thin films prior to the sulfo-selenization process. The successful doping of In improved crystal growth and promoted the formation of larger grains. Furthermore, the CZTSSe TFSCs fabricated with In doping exhibited improved device performance. In particular, the In-CZTSSe-2-based device showed an improved power conversion efficiency (PCE) of 9.53%, open-circuit voltage (Voc) of 486 mV, and fill factor (FF) of 61% compared to the undoped device. Moreover, the small amount of In incorporated into the CZTSSe absorber demonstrated reduced nonradiative recombination, improved carrier separation, and enhanced carrier transport properties. This study suggests a simple and effective way to incorporate In to achieve high efficiency and low Voc loss.