CZTSSe Thin Films Research Articles

Electrochemically Deposited CZTSSe Thin Films for Monolithic Perovskite Tandem Solar Cells with Efficiencies Over 17%

In spite of the high potential economic feasibility of the tandem solar cells consisting of the halide perovskite and the kesterite Cu2ZnSn(S,Se)4 (CZTSSe), they have rarely been demonstrated due to the difficulty in implementing solution‐processed perovskite top cell on the rough surface of the bottom cells. Here, we firstly demonstrate an efficient monolithic two‐terminal perovskite/CZTSSe tandem solar cell by significantly reducing the surface roughness of the electrochemically deposited CZTSSe bottom cell. The surface roughness (Rrms) of the CZTSSe thin film could be reduced from 424 to 86 nm by using the potentiostatic mode rather than using the conventional galvanostatic mode, which can be further reduced to 22 nm after the subsequent ion‐milling process. The perovskite top cell with a bandgap of 1.65 eV could be prepared using a solution process on the flattened CZTSSe bottom cell, resulting in the efficient perovskite/CZTSSe tandem solar cells. After the current matching between two subcells involving the thickness control of the perovskite layer, the best performing tandem device exhibited a high conversion efficiency of 17.5% without the hysteresis effect.

High photovoltaic efficiency CZTSSe thin film solar cells obtained by Li doping in air

Alkali metal doping is considered as an effective strategy in the pathway of improving the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) photovoltaic materials. In this paper, Li is successfully doped into CZTSSe films in air and high efficiency devices are obtained. By X-ray diffraction and scanning electron microscopy, it can be found that the crystalline quality of CZTSSe films after Li doping is effectively improved and full and dense absorber layer crystals are obtained. In addition, TRPL also shows that the carrier lifetime of CZTSSe by doping with Li is significantly enhanced. Lithium-doped CZTSSe thin-film solar cells finally achieve a high efficiency of 9.84%. This Li doping route with simple process and superior device performance obtained provides a new idea for further development of element-rich and non-toxic CZTSSe thin film materials.

Crystallization mechanism and defect passivation of Cu2ZnSn(S,Se)4 thin film solar cells via in situ potassium doping

Alkali metal doping has achieved prominent results in Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells. In this paper, in situ K-doped CZTSSe thin film solar cells were prepared by magnetron sputtering with a K-containing CZTS target.

Photo-excited carrier transport and secondary phases of Na-engineered kesterite flexible thin films

Kesterite [Cu 2 ZnSn(S,Se) 4 (CZTSSe)], which is one of the most promising solar cell materials, can be used as a light absorber owing to its suitable optical and electrical properties. In this study, the optimization process of NaF layers was performed in the deposition of the precursor stacks of Sn/Cu/Zn/Mo to form CZTSSe thin films. The distribution of the secondary phases on the CZTSSe surfaces strongly depends on the insertion of the NaF layer. In particular, the Cu 2-x S binary phase increases with the closeness of the insertion location of the NaF layer to the front side. Moreover, the grain boundary properties can be altered by varying the deposition conditions since Na segregation and defect formation affect the surface electrical properties and carrier transport mechanism. Photo-assisted conductive-atomic force microscopy was used to measure the local variation in the surface photocurrent flow into the CZTSSe grains, and the incorporation of sodium increased the surface current formation. This study exhibits the effects of Na on the optimal growth of kesterite formation and carrier transport of the relevant photovoltaic devices. • Na incorporation was engineered by controlling the insertion location of NaF doping layer. • Formation of binary and ternary phases was affected by the location of NaF doping layer. • Distribution of Cu 2-x S binary phase increased as the insertion location of NaF close to the surface. • Generation of photocurrent was enhanced in Na doped CZTSSe.

Synthesis and investigation of solution-processed Bi-doped Cu2ZnSn(S, Se)4 thin-film solar cells

Surface morphology plays a crucial role in influencing the film quality and photoelectric conversion efficiency (PCE) of a Kesterite Cu 2 ZnSn(S, Se) 4 (CZTSSe) solar cell. Herein, the bismuth (Bi) doping strategy to improve the surface morphology of the solution-processed CZTSSe film for high solar cell efficiency is reported. With a two-step selenization strategy to facilitate the Bi-doped CZTSSe (CZTBiSSe) phase formation, improving the crystalline quality of the absorber, increasing the carrier mobility, and reducing the band tailing, a cell efficiency of 6.94% has been achieved. These results demonstrate the significance of Bi doping into CZTSSe to improve the device's performance. This work offers new insights into the effects of Bi doping on CZTSSe via a solution-processed approach and provides a deeper understanding of the origin of the efficiency improvement of Bi-doped CZTSSe thin film solar cells. • A new method to improve Voc and PCE of CZTSSe solar cell by Bi doping is put forward. • This is the first report on solution-processed CZTBiSSe using two-step selenization. • This study shows hopeful character of Bi in increasing the Voc of CZTSSe solar cell. • Bi doping can improve crystal quality and decrease Sn Zn antisite defect states. • It is proved that Bi-doped CZTSSe is a promising band-gap tunable absorption layers.

Rear interface engineering via a facile oxidation process of Mo back contact for highly efficient CZTSSe thin film solar cells

The kesterite-based Cu2ZnSn(S,Se)4 (CZTSSe) thin-film solar cells (TFSCs) are promising absorber material for next-generation TFSCs technology because they exhibit earth-abundant, non-toxic elements and favorable optoelectronic properties. However, the progress and device performance development in CZTSSe TFSCs were limited because of unstable rear interface characteristics near the absorber/molybdenum (Mo) interface, which causes large open-circuit voltage loss. In this study, we report the rear interface engineering of CZTSSe TFSCs to improve device performance by introducing an amorphous thin MoOx intermediate layer by a facile oxidation process. We observed that (i) an amorphous thin MoOx intermediate layer can effectively control the rear interface properties (i.e., inhibiting the formation of Mo(S,Se)2 interfacial layers and voids at CZTSSe/Mo interface) and (ii) the improved device performance in CZTSSe TFSCs with oxidation (w-oxidation) is attributed primarily to the dramatically decreased shunt conductance, formation of large-sized grains within CZTSSe absorber layer, and improved rear interface characteristics. This study presents a simple and low-temperature oxidation process for rear interface engineering of highly efficient kesterite CZTSSe-based TFSCs.

Thermoelectric properties of solution-prepared n-type and p-type CZTSSe thin films

This research investigates the transport and thermoelectric behaviors of the CZTSSe thin films. Both p-type and n-type films were successfully synthesized, allowing us to comprehensively study the characteristics of the full-composition CZTS. Cu contents in CZTS precursors were controlled from Cu/Zn + Sn = 0.7 to 0.9 with a constant Zn/Sn = 1. After salinization, the Cu/Zn + Sn ratio in CZTS thin films shows positive deviation from the precursor, ranging from 0.78 to 1.1 that indicated a Zn or Sn loss with the increase of Cu contents. A decline of Zn/Sn ratio in the films from 1.53 to 1.2 was obtained when Cu/Zn + Sn in precursors increased from 0.7 to 0.9, indicating a Zn loss. Hall measurement showed that the increase of Cu contents yielded an increase in the conductivity and carrier concentration. Conducting type of the CZTS changed from p-type to n-type when the Cu/Zn + Sn exceeded 1. Absorption spectroscopy revealed a decrease of band gap of CZTS and Raman spectroscopy indicated an increase in the full width at half maximum when the Cu content was increased. Photoluminescence spectroscopy analyses indicated a defect related peak with 47 meV lower than the band gap. Accordingly, CuZn + SnZn was proposed to be the dominated defects of the Cu-rich n-type CZTS thin films, different from the CuZn dominated p-type films. As a result, we proposed that the n-type CZTS with Cu/Zn + Sñ1.01 and Zn/Sñ1.1 CZTS had the best power factor than other composition. The result could be reference for CZTS thermoelectric research in the future.

Effect of pre-annealing of Mo foil substrate on CZTSSe thin films and Mo(S,Se)2 interface layer

Cu2ZnSn(S,Se)4 (CZTSSe) thin film deposited on flexible Mo foil substrate has advantage of high mass specific power and good ductility. However, a thick Mo(S,Se)2 interface layer is easily to be formed between CZTSSe and Mo foil substrate. The ohmic contact property of CZTSSe/Mo is deteriorated by the formation of Mo(S,Se)2. In this work, the Mo foil substrate was pre-annealed to inhibit the growth of Mo(S,Se)2 interface layer. CZTSSe thin films were prepared on the pre-annealed Mo foil substrate by sol-gel and selenization methods. The pre-annealing treatment of Mo foil substrate leads to the oxidation of Mo. During the high temperature selenization process, the MoOx acts as a buffer layer to suppress the formation of the Mo(S,Se)2 interface layer. With the increase of the pre-annealing temperature of the Mo foil substrate, the thickness of the Mo(S,Se)2 interface layer decreases, and the resistance of CZTSSe/Mo(S,Se)2/Mo structure decreases. The ohmic contact properties of CZTSSe/Mo can be improved by the pre-annealing treatment of metal Mo foil substrates.

Simulation of heat generation from defects at different energy levels in CZTSSe thin film solar cells

Simulation of heat generation from defects at different energy levels in CZTSSe thin film solar cells

Effect of pre-deposition annealing on physical properties of CZTSSe thin films deposed by RF-sputtering based on nanoparticles synthesized by solvothermal technique

Effect of pre-deposition annealing on physical properties of CZTSSe thin films deposed by RF-sputtering based on nanoparticles synthesized by solvothermal technique

The Degradation of Cu2ZnSn(S,Se)4 Kesterite Thin Film Solar Cells Induced by Proton Radiation

AbstractAiming at the current bottleneck of Cu2ZnSn(S,Se)4 (CZTSSe) thin film solar cells, an experimental analysis of the performance and loss mechanism of CZTSSe thin film solar cells upon proton radiation exposure is presented. In order to deduce the important cell parameters, the environment of the cells is simulated by subjecting the devices to 200 keV proton irradiation with fluences from 1 × 1010 to 1 × 1017 cm‐2. It is found that the damage constants are sensitive to the elemental diffusion at heterojunction and Na redistribution in device. The incidence of the proton can lead to bandgap, tails fluctuations, and Urbach energy changes in the cells those directly impact defects and structural disorder in CZTSSe material thereby the device performance. The time‐resolved photoluminescence results show that the carrier lifetime is also very sensitive to the distribution of interface elements, especially Na, Cd, and S. These findings may provide new ideas for solving the VOC deficit of CZTSSe solar cells. From this work, it can be also suggested that the CZTSSe thin film solar cells are robust to proton irradiation and have the potential for future space applications.

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 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.

Spectroscopy and Theoretical Modeling of Phonon Vibration Modes and Band Gap Energy of Cu2ZnSn(S x Se1-x )4 Bulk Crystals and Thin Films.

Semiconductor Cu2ZnSn(SxSe1–x)4 (CZTSSe) solid solution is considered as a perspective absorber material for solar cells. However, during its synthesis or deposition, any modification in the resulting optical properties is hardly predicted. In this study, experimental and theoretical analyses of CZTSSe bulk crystals and thin films are presented based on Raman scattering and absorption spectroscopies together with compositional and morphological characterizations. CZTSSe bulk and thin films are studied upon a change in the x = S/(S + Se) aspect ratio. The morphological study is focused on surface visualization of the solid solutions, depending on x variation. It has been discovered for the first time that the surface of the bulk CZTSSe crystal with x = 0.35 has pyramid-like structures. The information obtained from the elemental analysis helps to consider the formation of a set of possible intrinsic lattice defects, including vacancies, self-interstitials, antisites, and defect complexes. Due to these results and the experimentally obtained values of the band gap within 1.0–1.37 eV, a deviation from the calculated band gap values is estimated in the range of 1.0–1.5 eV. It is suggested which defects can have an influence on such a band gap change. Also, on comparing the experimental Raman spectra of CZTSSe with the theoretical modeling results, an excellent agreement is obtained for the main Raman bands. The proposed theoretical approach allows to estimate the values of concentration of atoms (S or Se) for CZTSSe solid solution directly from the experimental Raman spectra. Thus, the visualization of morphology and the proposed theoretical approach at various x values will help for a deeper understanding of the CZTSSe structure to develop next-generation solar cells.

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.

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

As a low-cost substitute that uses no expensive rare-earth elements for the high-efficiency Cu(In,Ga)(S,Se)2 solar cell, the Cu2 ZnSn(S,Se)4 (CZTSSe) solar cell has borrowed optimization strategies used for its predecessor to improve its device performance, including a profiled band gap and surface inversion. Indeed, there have been few reports of constructing CZTSSe absorber layers with surface inversion to improve efficiency. Here, a strategy that designs the CZTSSe absorber to attain surface modification by using n-type Ag2 ZnSnS4 is demonstrated. It has been discovered that Ag plays two major roles in the kesterite thin film devices: surface inversion and front gradient distribution. It has not only an excellent carrier transport effect and reduced probability of electron-hole recombination but also results in increased carrier separation by increasing the width of the depletion region, leading to much improved VOC and JSC . Finally, a champion CZTSSe solar cell renders efficiency as high as 12.55%, one of the highest for its type, with the open-circuit voltage deficit reduced to as low as 0.306V (63.2% Shockley-Queisser limit). The band engineering for surface modification of the absorber and high efficiency achieved here shine a new light on the future of the CZTSSe solar cell.