CZTSSe Thin Film Solar Cells Research Articles

Analyses of p–n heterojunction in 9.4%-efficiency CZTSSe thin-film solar cells: Effect of Cu content

Successful formation of the absorber layer is regarded as the most important step when fabricating Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells. A high-quality CZTSSe absorber layer can enhance the parameters of thin-film solar cells. We concentrated on analyzing the CZTSSe surface, which is directly related to forming the p-n junction. We made changes to the surface of the absorber layer by controlling the Cu content via the sputtering time. When the Cu content was insufficient, secondary phases and agglomerated metallic precursors appeared on top of the CZTSSe absorber layer. We analyzed the defects and secondary phases that could adversely affect the formation of the p-n junction by using energy-dispersive X-ray spectroscopy, X-ray diffractometer, and Raman spectroscopy. We also measured and calculated the electric parameters of CZTSSe thin-film solar cells to investigate the CZTSSe/CdS interface. The performance of a CZTSSe thin-film solar cell was enhanced and stabilized by optimizing the concentration of Cu in the absorber layer. Our results indicate the importance of the Cu ratio on the surface of the CZTSSe absorber layer as it affects the p-n junction in the CZTSSe thin-film solar cell. The highest photoconversion efficiency was 9.4% when the Cu component was optimized.

Optimizing the properties of Cu2ZnSn(S,Se)4 solar cells via cationic substitution with trace Ca

Cation substitutions have the significant impact on suppressing the detrimental defects and associated clusters of CZTSSe thin film solar cells. In our work, trace Ca is used to replace Zn to control the cationic disorder in CZTSSe absorber, improving the performance of CZTSSe solar cells. By tuning the amount of Ca doping, the alloyed Cu2CaxZn1-xSn(S,Se)4 phase can be formed, and the effects of tiny Ca doping on the properties of CZTSSe absorber are studied in detail. After 2% Ca is doped into the precursor solution, the highest efficiency of CZTSSe solar cells can be optimized to 8.73% from 7.06%, and the improvement of VOC is observed from 382 to 417 mV, which both benefit from the reduced formation of the detrimental defects. Our findings also provide the new insights for further improvement of CZTSSe solar cells based on earth abundant elements.

COMSOL simulation of non-radiative recombination heat and joule heat in CZTSSe thin film solar cells

A detailed coupled simulation model has been developed to analyze the impact of defect, voltage, and temperature on heat generation rate in CZTSSe thin film solar cells. This has been simulated by mapping the non-radiative recombination heat and Joule heat due to increased defect and voltage across the bulk of this solar cell. Both non-radiative recombination heat and Joule heat depend on defect density and voltage. The non-radiative recombination in solar cells includes the Auger recombination and the Shockley-Read-Hall (SRH) recombination due to mid-gap defects. Increasing the defect density from 1012 cm−3 to 1018 cm−3 increased the heat generation from 105 W/m3 to 109 W/m3. Also, voltage is an effective parameter in heat generation in solar cells. Increasing the voltage from 0 to 0.6 V (near the open-circuit voltage of the cell) has flattened out the profile of non-radiative recombination heat across the cell and the spike of heat generation at the junction has significantly reduced. In any case, the heat generation (due to defect or voltage increment) shows a maximum peak at CZTSSe/CdS junction and a smaller increase at (Mo(S,Se)2/CZTSSe) interface. Finally, the non-radiative recombination heat was calculated at both room temperature (initial run) and at secondary temperature distribution (coupled run).

Effect of Metal-Precursor Stacking Order on Volume-Defect Formation in CZTSSe Thin Film: Formation Mechanism of Blisters and Nanopores.

In this study, we investigated the effect of the stacking order of metal precursors on the formation of volume defects, such as blisters and nanopores, in CZTSSe thin-film solar cells. We fabricated CZTSSe thin films using three types of metal-precursor combinations, namely, Zn/Cu/Sn/Mo, Cu/Zn/Sn/Mo, and Sn/Cu/Zn/Mo, and studied the blister formation. The blister-formation mechanism was based on the delamination model, taking into consideration the compressive stress and adhesion properties. A compressive stress could be induced during the preferential formation of a ZnSSe shell. Under this stress, the adhesion between the ZnSSe film and the Mo substrate could be maintained by the surface tension of a metallic liquid phase with good wettability, or by the functioning of ZnSSe pillars as anchors, depending on the type of metal precursor used. Additionally, the nanopore formation near the back-contact side was found to be induced by the columnar microstructure of the metal precursor with the Cu/Zn/Mo stacking order and its dezincification. Based on the two volume-defect-formation mechanisms proposed herein, further development of volume-defect-formation suppression technology is expected to be made.

Positive Role of Inhibiting CZTSSe Decomposition on Intrinsic Defects and Interface Recombination of 12.03% Efficient Kesterite Solar Cells

Cu2ZnSn(S,Se)4 (CZTSSe) solar cells, which are emerging as promising photovoltaic devices, are currently suffering serious issues of large open‐circuit voltage deficit and low fill factor. Decomposition of CZTSSe is one of many factors limiting the efficiency improvement of CZTSSe solar cells, which can lead to the deviation of the chemical environment during the synthesis of CZTSSe. Herein, the Sn‐vapor is provided during the synthesis of CZTSSe to inhibit the decomposition, and the effect of decomposition on the intrinsic defects and interface recombination are systematically investigated. The high‐quality CZTSSe without the secondary phase and with the low ZnSn and CuZn acceptor defects density is obtained. By inhibiting the decomposition, the recombination activation energy at depletion region is improved and the interface defect density is dramatically decreased, indicating the interface recombination is effectively reduced. Consequently, the performance of CZTSSe thin film solar cells, especially the open‐circuit voltage and fill factor, has been significantly improved. Finally, based on the excellent CZTSSe film, a photovoltaic device with 12.03% efficiency (active area efficiency is 12.96%) is prepared. These encouraging results provide a new route to controlling the defects and interface recombination of CZTSSe solar cells.

The Role of the Graphene Oxide (GO) and Reduced Graphene Oxide (RGO) Intermediate Layer in CZTSSe Thin-Film Solar Cells.

Cu2ZnSn(S,Se)4 (CZTSSe) solar cells with low cost and eco-friendly characteristics are attractive as future sources of electricity generation, but low conversion efficiency remains an issue. To improve conversion efficiency, a method of inserting intermediate layers between the CZTSSe absorber film and the Mo back contact is used to suppress the formation of MoSe2 and decomposition of CZTSSe. Among the candidates for the intermediate layer, graphene oxide (GO) and reduced GO have excellent properties, including high-charge mobility and low processing cost. Depending on the type of GO, the solar cell parameters, such as fill factor (FF), were enhanced. Thus, the conversion efficiency of 6.3% was achieved using the chemically reduced GO intermediate layer with significantly improved FF.

Influence of Mo-pretreating on microstructure evolution of solution-processed absorbers for high efficient CZTSSe solar cells

For Mo-based Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells, high quality CZTSSe absorbers together with a moderately thick MoSe2 layer at CZTSSe/Mo interfaces are crucial for boosting the cell efficiency. Herein, a simple Mo pretreating method is proposed and its influence on microstructure evolution of solution-processed CZTSSe absorbers under different selenization temperatures is detailedly investigated. It is found that Mo pretreatment can not only significantly affect the microstructure formation of CZTSSe film but also suppress the over-selenization of Mo electrode, and effectively promote the cell efficiency from 2.48% to 6.67% by combing an optimized selenization process. The results are of great significance for the optimization of absorber layer structures in high-efficient CZTSSe thin-film solar cells.

11.39% efficiency Cu2ZnSn(S,Se)4 solar cells from scrap brass

Kesterite Cu2ZnSn(S,Se)4 is one of the most promising next-generation thin-film photovoltaic materials. Along with the efficiency advancement of kesterite solar cells, a cost-effective fabrication process with low carbon footprint plays an increasingly important role considering the near-future industrialization of CZTSSe solar cells. In this study, we proposed an environmental friendly process to recover copper and zinc from the scrap brass, which can be directly used as the precursor salts to fabricate efficient CZTSSe thin-film solar cells. The best power conversion efficiency (PCE) of the device using the proposed eco-friendly process is 11.39% (certified at the National PV Industry Measurement and Testing Center) for CZTSSe solar cells.

A Thin In2 S3 Interfacial Layer for Reducing Defects and Roughness of Cu2 ZnSn(S,Se)4 Thin-Film Solar Cells.

Cu2 ZnSn(S,Se)4 (CZTSSe) has generated considerable research interest owing to its composition of abundant elements and excellent light-absorption properties. However, CZTSSe thin-film solar cells suffer from a considerable deficit in the open-circuit voltage (VOC ), which is mainly due to the severe interfacial recombination induced by the rough surface of CZTSSe and numerous physical defects. In this study, to improve the morphology and reduce the interfacial recombination, an In2 S3 passivation layer was introduced between the CZTSSe and CdS layers via a chemical bath deposition process, and the effects of the In2 S3 layer on the device performance were systematically examined by performing various electrodynamic analyses. The CZTSSe solar cells with thin In2 S3 layers exhibited impressive increases in VOC and conversion efficiency (from 7.33 to 9.24 %), due to the suppression of physical defects and the refined surface morphology resulting from filling the voids and pinholes. In addition, the nanoscale roughness of the In2 S3 /CZTSSe surface increased the number of nucleation sites for the CdS nuclei, which may reduce the activation energy of the heterogeneous nucleation. The presence of In2 S3 layer resulted in uniform growth of CdS without macroscopic CdS agglomerates (i. e., reduced roughness of full devices), which improved the quality of the interface. These findings confirmed that the reduction of physical defects and the improved deposition of the CdS layer enabled by the added In2 S3 passivation layer improved the device performance.

Enhanced efficiency of graded-bandgap thin-film solar cells due to concentrated sunlight.

A systematic study was performed with a coupled optoelectronic model to examine the effect of the concentration of sunlight on the efficiencies of CIGS, CZTSSe and AlGaAs thin-film solar cells with a graded-bandgap absorber layer. Efficiencies of 34.6% for CIGS thin-film solar cells and 29.9% for CZTSSe thin-film solar cells are predicted with a concentration of 100 suns, the respective one-sun efficiencies being 27.7% and 21.7%. An efficiency of 36.7% is predicted for AlGaAs thin-film solar cells with a concentration of 60 suns, in comparison to 34.5% one-sun efficiency. Sunlight concentration does not affect the per-sun electron-hole-pair (EHP) generation rate but reduces the per-sun EHP recombination rate either near the front and back faces or in the graded-bandgap regions of the absorber layer, depending upon the semiconductor used for that layer, and this is the primary reason for the improvement in efficiency. Other effects include the enhancement of open-circuit voltage, which can be positively correlated to the higher short-circuit current density. Sunlight concentration can therefore play a significant role in enhancing the efficiency of thin-film solar cells.

Influence mechanism of Cd ion soaking on performance of flexible CZTSSe thin film solar cells

Realizing flexible Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cell with high efficiency and excellent mechanical durability has always been a challenging task in green demanded era. In this work, Cu2CdxZn1-xSn(S,Se)4 (CCZTSSe) absorber was synthesized by selenizing Cd2+ treated CZTS precursor. Due to the powerful ability of Cd2+ soaking in boosting grain growth, the grain size of CCZTSSe was remarkably increased from nano to micron level. Cd atom actually act entering into the lattice and replaces Zn location in the post-selenization process. The band gap of CCZTSSe exhibits a decreasing trend (1.37 eV–1.29 eV) as x increases from 0 to 0.13. In comparison to CZTSSe absorber without Cd2+ soaking treatment, Cu2CdxZn1-xSn(S,Se)4 (x = 0.03, 0.07 and 0.13) can establish the heterojunction with better matching effect with CdS buffer layer, demonstrating an enhanced conduction band offset (CBO). The improved heterojunction characteristic substantially prevents carrier recombination caused by interface state defects, which can be the crucial reason of the enhanced device performance. Finally, for x = 0.07, the device efficiency was optimized to 4.38%. This systematic work offers a novel solution for vital problems encountered in flexible CZTSSe thin film solar cells, which can lead to prospective applications in wearable devices.

Influence of the oxidation state of Sn in the precursor and selenization temperature on Cu2ZnSn(S,Se)4 thin film solar cells

Kesterite Cu2ZnSn(S,Se)4 (CZTSSe) is emerging as one of the most promising materials owing to its low-cost, non-toxicity, similar electronic properties with Cu2(In,Ga)Se2 (CIGS). However, the current highest power conversion efficiency (PCE) for CZTSSe device is far below the CIGS because of the high open-circuit voltage (Voc) deficit and low fill factor (FF). Here, we fabricated CZTSSe thin film from 2-methoxyethanol solutions and investigated the effects of different Sn (Sn2+ vs Sn4+) oxidation states of precursor solution and selenization temperature on the crystal quality of the CZTSSe thin film and device performances. By characterizations and analyses, we found that incomplete redox reaction caused by the off-stoichiometric component in Sn2+ precursor solution led to the formation of detrimental secondary phases such as SnSe, Cu2Se, and SnSe2 in the CZTSSe thin film. The existence of volatile SnSe could cause the formation of voids and thus affected the FF and Voc. The reaction from Sn4+ precursor solution to absorber material avoided the formation of SnSe. Ultimately, the Sn4+ thin film showed better features in terms of improving crystal quality and reducing Voc deficit that limited the efficiency of kesterite CZTSSe thin film solar cells. Besides, we demonstrated that an appropriate selenization temperature could further reduce the number of secondary phases and improve the surface morphology and crystallinity of the CZTSSe thin film. As a result, we obtained CZTSSe thin film solar cells with 8.27% PCE from the Sn4+ precursor solution under the selenization temperature of 580 °C.

Evolution of structural and optoelectronic properties in fluorine–aluminum co-doped zinc oxide (FAZO) thin films and their application in CZTSSe thin-film solar cells

The window layer is an important layer that transmits sunlight and generates electric current as the carriers move. Aluminum (Al) -doped zinc oxide (AZO) has been widely applied as the window layer in thin-film solar cells (TFSCs) owing to its favorable optoelectronic properties, as its properties can be controlled and further improved with suitable dopants. In this study, we strategically co-dope Al and fluorine (F) in ZnO thin films (FAZO) to improve the transmittance with controlled carrier concentration. To study the influence of F dopant on the structural, optical, and electrical properties of FAZO thin films, the F doping concentration is varied (0–1.5%) while keeping the Al content fixed (2%). With increasing F doping concentration, the carrier concentration, transmittance, and bandgap values increase, whereas, the sheet resistance and resistivity of the single-layer FAZO thin films decrease, compared to the reference AZO film. The addition of a small amount of F alters the surface roughness with increasing concentration. The properties of the single-layer FAZO thin films show deteriorated properties at the highest doping concentration (1.5%). Under the optimum condition of 1% F, FAZO thin films exhibit the lowest sheet resistance of 5.84 Ω/sq, in addition to the highest transmittance of 89.3% in the visible light region. Furthermore, the FAZO thin films applied in Cu2ZnSn(S,Se)4 (CZTSSe) device shows significant enhancements in the short-circuit current density (Jsc) and fill factor (FF), resulting in improved device performance from ~8 to 9.57%.

Preparing Cu2ZnSn(SxSe1−x)4 thin-film solar cells with front band gap grading by plasma sulfurization

In this paper, Cu2ZnSn(SxSe1−x)4(CZTSSe) thin-films with a front-graded band gap were prepared by plasma sulfurization. The method of selenization followed by low-temperature plasma sulfurization was adopted to effectively control the uneven diffusion of S in the absorption layer. By investigating and analyzing the grazing incidence X-ray diffraction, Raman spectroscopy, and PL spectra, the CZTSSe absorption layer with an S-rich top and S-poor bottom layer could be obtained by plasma sulfurization. The inhomogeneous distribution of S led to the formation of band gap gradients. Based on that, the band gap near the top of the absorption layer was determined to be 1.13 eV and the bottom band gap was 1.03 eV. Approximately 300 nm thick sulfur-rich CZTSSe films were found near the surface favoring an increase in the band gap at the interface, which could be attributed to a high open circuit voltage (Voc). The establishment of the band gap gradient formed a barrier towards the motion of holes towards the interface and increased the effective interfacial recombination band gap, both of which significantly reduced the interfacial recombination. Finally, the CZTSSe thin-film solar cells with a band gap gradient were prepared by plasma sulfurization exhibited a power conversion efficiency (PCE) of 8.11%. Compared with CZTSSe thin-film solar cells without post plasma sulfurization, which had a PCE of 6.98%, the Voc of the CZTSSe solar cell improved considerably from 471.37 mV to 513.93 mV; however, it had little effect on short-circuit current density (Jsc).

Using Cu-Zn-Sn-O Precursor to Optimize CZTSSe Thin Films Fabricated by Se Doping With CZTS Thin Films.

The copper–zinc–tin oxide (CZTO) precursor was synthesized to avoid sudden volume expansion from CZTO precursor to Cu2ZnSnS4 (CZTS) thin films and smooth CZTSSe thin-film surfaces without pinholes. The CZTO precursor was prepared by coprecipitation and ball milling to form nanoink of CZTO. Based on the CZTO precursor, the CZTS thin film was fabricated and then selenized to make pinhole-free and flat Cu2ZnSn(S,Se)4(CZTSSe) thin films. The results show that the CZTO precursor greatly contributed to elevating the homologous surface characteristics and crystallinity of CZTSSe thin films by controlling selenium temperature, selenium time, and selenium source temperature. Finally, the conversion efficiency of the CZTSSe thin-film solar cell fabricated from the CZTO precursor was 4.11%, with an open-circuit voltage (Voc) of 623 mV, a short circuit current density (Jsc) of 16.02 mA cm−2, and a fill factor (FF) of 41.2%.

Effect of evaporated CdS layer on formation and performance enhancement of flexible Cu2ZnSn(S,Se)4 solar cells

Antisite defects induced undesirable open-circuit voltage (Voc) and fill factor (FF) have always been considerable factors that plagued the efficiency enhancement of Cu2ZnSn(S,Se)4 (CZTSSe)-based photovoltaic devices. Accordingly, sputtered Cu–Zn–Sn–S precursor with e-beam evaporated CdS layer on its top and bottom followed by selenization process was firstly proposed to obtain Cd-alloyed flexible CZTSSe thin film solar cells. The introduced CdS significantly improved crystalline quality and promoted grain growth of CZTSSe absorber. Besides, captured conduction band offset (CBO) exhibited a “cliff-like” structure with the maximum value of −0.15 eV when CdS layer was evaporated on top of Cu–Zn–Sn–S precursor, which largely relieved the band alignment issue in CZTSSe/CdS heterojunction. Interestingly, for CZTSSe prepared with CdS on top, Cd element shows a better uniform distribution in absorber than that with CdS on bottom. Finally, thanks to the evaporated CdS layer on precursor top, the efficiency of flexible CZTSSe thin film solar cell was improved from 2.31% to 4.52%, featuring a Voc of 343.53 mV, Jsc of 27.94 mA/cm2 and FF of 47.70%.

Effect of Ge nanolayer stacking order on performance of CZTSSe thin film solar cells

In the present work, the effect of germanium (Ge) stacking order on optoelectronic properties of Cu2ZnSn(S, Se)4 (CZTSSe) thin films are investigated. It was found that the addition of the Ge nanolayer significantly improved the CZTSSe absorber quality, elemental distribution, and reduced secondary phases. The Ge nanolayer deposited on top and bottom of the precursor improved open-circuit voltage (Voc) and short-circuit current density (Jsc) in the device respectively. The device with optimum position and thickness of the Ge nanolayer improved device efficiency from 6.91% to 7.87%. Further, these experimental results suggest that the optimum amount of Ge nanolayer and their position can further improve optoelectronic properties of kesterite CZTSSe absorbers which can be used to achieve high-efficiency in kesterite based solar cells.

Na-doping-induced modification of the Cu2ZnSn(S,Se)4/CdS heterojunction towards efficient solar cells

Na-doping-induced modification of the Cu 2 ZnSn(S,Se) 4 /CdS heterojunction enables photovoltaic device with a remarkable conversion efficiency. The physical reasons for the improvement of V OC and J SC are analyzed by theoretical and experimental studies. • Na-doping can induce the heterojunction modification of Cu 2 ZnSn(S,Se) 4 /CdS • Optical losses in CdS are mitigate as its thinner thickness by Na doping treatment. • Proper Na-doping treatment enables device with a remarkable conversion efficiency. • Physical reasons are analyzed by theoretical and experimental studies. • This work provides a deeper understanding of Na doped CZTSSe thin film solar cells. It is very important to understand why a small amount of alkali metal doping in Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells can improve the conversion efficiency. In this work, Na-doped CZTSSe is prepared by a simple solution method, and then the effects on the surface properties of the absorber layer, the buffer layer growth, and the modifications of the solar cell performance induced by the Na doping are studied. The surface of the absorber layer is more Cu-depletion and less roughness due to the Na doping. In addition, the contact angle of the surface increases because of Na doping. As a consequence, the thickness of the CdS buffer layer is significantly reduced and the optical losses in the CdS buffer layer are decreased. The difference of quasi-Fermi levels ( E Fn – E Fp ) increases with a small amount of Na doping in the CZTSSe solar cell, so that open circuit voltage ( V OC ) increased significantly. This work offers new insights into the effects of Na doping on CZTSSe via a solution-based approach and provides a deeper understanding of the origin of the efficiency improvement of Na-doped CZTSSe thin film solar cells.

Improving the efficiency of CZTSSe solar cells by engineering the lattice defects in the absorber layer

One of the major challenges in increasing the efficiency of the CZTSSe solar cells is to control the lattice defects formation and secondary phases in the absorber layer of the cells. Moreover, by controlling and decreasing lattice defects and thus improving the efficiency, larger-scale applications would be financially acceptable. In this paper, several CZTSSe thin-film solar cells have been prepared and one device with champion efficiency of 10.33% has been chosen for the modeling. Afterward, numerical simulations based on the Finite Element Method (FEM) and Finite Difference (FD) were conducted on these cells. The effect of defects density and defect types, which are located at different energy levels in the absorber layer bandgap, was evaluated. The results indicated that by decreasing the defects which are located close to the electron Fermi level and the middle of the band gap (Eg/2), the maximum efficiency of 18.47% for these solar cells can be achieved.

Effect of selenium partial pressure on the performance of Cu2ZnSn(S, Se)4 solar cells

Sputtering followed by selenization is one of the most common methods for preparing CZTSSe thin films. However, the influence of selenium partial pressure on the crystallinity of the CZTSSe film has been rarely reported. In this study, CZTSSe thin films were prepared by selenization using quartz tubes of different lengths. The influence of Se saturated vapor pressure and temperature on the structure, composition, optical, and electrical properties of CZTSSe films and solar cells was analyzed and these results were used to optimize the performance of the CZTSSe film. It was found that the maximum partial pressure of selenium was 22,542 Pa when the selenization process was carried out in a quartz tube with a length of 30 cm, which largely improved the structural and electrical properties of CZTSSe. However, quartz tube with an over-short length would bring strong partial pressure during selenization, which resulted in a generation of secondary phases. Finally, CZTSSe thin-film solar cell with a maximum efficiency of 3.27% was obtained at an optimal selenium partial pressure of 22542 Pa.