Cu2ZnSnSe4 Solar Cells Research Articles

Achieving Over 10% Efficiency in Kesterite Solar Cells via Selenium-Free Annealing

The unsatisfactory crystalline quality of the absorber layer has seriously impeded the development of kesterite photovoltaic devices, including a small grain size, a typical multi-layer structure, and inherent defects and defect clusters. Herein, we simultaneously modify the nature of the absorber bulk and the heterojunction interface by sputtering quaternary alloy targets and subsequent selenium-free annealing to improve the photovoltaic performance of Cu2ZnSnSe4 (CZTSe) solar cells. By adjusting the sputtering power, the elemental content and distribution in the precursor film are effectively controlled, which helps to obtain a single-layer large grain CZTSe absorber layer with better homogenization and compactness. The harmful inherent defects are significantly suppressed by such a modification. Simultaneously, the widened depletion width and enhanced built-in electric field, as well as improved band alignment at the heterojunction, enhance carrier collection. As a consequence, the efficiency of CZTSe solar cells has improved greatly from 6.91% to 10.12%. The single-target preparation strategy without selenization provides a simple and novel perspective for simultaneously adjusting the morphology and defects of kesterite photovoltaic materials, paving the way for promoting innovative industrial applications.

Design and Numerical Analysis of Thin Film Solar Cells Based on Cu₂ZnSn (SₓSe₁₋ₓ)₄ with SnO₂ TCO and ZnS Buffer

Due to their earth-abundance, direct and adjustable bandgap in the range of visible light, and lower fabrication cost on large areas, Cu2ZnSn(SxSe1-x)4 semiconductors, commonly known as kesterite, are gaining recognition as potential materials for affordable, environment‐friendly, and high‐efficiency thin‐film photovoltaics. In this work, we numerically investigated the performance parameters of a single-heterojunction solar cell using Cu2ZnSnS4 (CZTS) / Cu2ZnSnSe4 (CZTSe) absorber layer and ZnS buffer layer, with SnO2 transparent conducting oxide (TCO) under a range of operating conditions and cell parameters. Maximum efficiency of 20.68% was obtained for a 5.5 µm absorber layer with 0.01 µm ZnS buffer layer for Cu2ZnSnSe4 solar cell, and 22.86% was obtained for a 6 µm thick Cu2ZnSnS4 absorber layer with 0.03 µm ZnS layer at room temperature and standard irradiance. The increase of irradiance to thrice the standard value led to 1% and 5% increase in efficiency for CZTS and CZTSe cells, respectively. Efficiency increased by 39% and 6.6% when the front surface was textured at an angle of 90° for CZTS and CZTSe cells, respectively. The CZTS and CZTSe based solar cells with optimized thickness and 90° textured front surface with three times the standard irradiation, showed 25.59 % and 21.52 % efficiency with open circuit voltage 1045 mV and 579.4 mV and short circuit current 85.84 mA/cm2 and 140.7 mA/cm2, respectively. The proposed architecture in this study will help produce next-generation solar cells that are both economical and energy-efficient.

Impacts of selenization time on Cu2ZnSnSe4 thin films prepared from an ethanolamine-dimethyl formamide based precursor solution

Most precursor solutions for preparing Cu2ZnSn(S,Se)4 thin films use sulfur containing groups to complex metal ions. However, solutions with sulfur containing groups are always toxic and harmful gases, such as H2S and SOx, will be released when drying precursor films. Based on the excellent chelating ability of ethanolamine, a stable molecular precursor solution without using sulfur based complexing agent was successfully developed. Cu-Sn alloy phase is formed in the precursor film, which makes the phase evolution of the absorber during selenization similar to that of Cu2ZnSnSe4 film grown from metallic precursor. Impacts of selenization time on compositional, morphological and photovoltaic properties of the Cu2ZnSnSe4 film were studied. Finally, a Cu2ZnSnSe4 solar cell with conversion efficiency of 7.53% was obtained by using the ethanolamine-dimethyl formamide based solution.

Performance Enhancement in Powder-Fabricated Cu2(ZnSn)Se4 Solar Cell by Roll Compression.

Despite the improved conversion efficiency of Cu2(ZnSn)Se4 (CZTSe) solar cells, their roll-to-roll fabrication nonetheless leads to low performance. The selenization time and temperature are typically considered major parameters for a powder-based CZTSe film; meanwhile, the importance of the densification during the roll-to-roll process is often overlooked. The densification process is related to the porosity of the light-absorbing layer, where high porosity lowers cell performance. In this study, we fabricated a dense CZTSe absorber layer as a method of controlling the compression of a powder precursor (Cu1.7(Zn1.2Sn1.0)S4.0 (CZTS)) during the roll-press process. The increased particle packing density of the CZTS layer was crucial in sintering the powder layer into a dense film and preventing severe selenization of the Mo back electrode. The pressed absorber layer of the CZTSe solar cell exhibited a more uniform chemical composition determined using dynamic secondary ion mass spectrometry (SIMS). Under the AM 1.5G illumination condition, the power conversion efficiency of the pressed solar cell was 6.82%, while the unpressed one was 4.90%.

Interface-suppressed high-quality symmetrical bifacial flexible CZTSe solar cells through a green electrodeposition process

A symmetrical bifacial flexible Cu2ZnSnSe4 (CZTSe) solar cells are fabricated through a green electrodeposition process with championed efficiencies for both sides.

Unveiling microscopic carrier loss mechanisms in 12% efficient Cu2ZnSnSe4 solar cells

Understanding carrier loss mechanisms at microscopic regions is imperative for the development of high-performance polycrystalline inorganic thin-film solar cells. Despite the progress achieved for kesterite, a promising environmentally benign and earth-abundant thin-film photovoltaic material, the microscopic carrier loss mechanisms and their impact on device performance remain largely unknown. Herein, we unveil these mechanisms in state-of-the-art Cu2ZnSnSe4 (CZTSe) solar cells using a framework that integrates multiple microscopic and macroscopic characterizations with three-dimensional device simulations. The results indicate the CZTSe films have a relatively long intragrain electron lifetime of 10–30 ns and small recombination losses through bandgap and/or electrostatic potential fluctuations. We identify that the effective minority carrier lifetime of CZTSe is dominated by a large grain boundary recombination velocity (~104 cm s−1), which is the major limiting factor of present device performance. These findings and the framework can greatly advance the research of kesterite and other emerging photovoltaic materials.

Effects of composition and thermal treatment on VOC‐limiting defects in single‐crystalline Cu2ZnSnSe4 solar cells

AbstractSingle‐crystalline Cu2ZnSnSe4 (CZTSe) solar cells with open circuit voltages reaching 500 mV are achieved through a combination of composition control and a low‐temperature thermal ordering treatment. A comparison of the device results for Cu‐poor CZTSe with Cu/Zn + Sn ratios of 0.77 and 0.86 is presented with and without the implementation of a 130°C absorber annealing treatment. An increase in bandgap energy is observed via external quantum efficiency measurements with both the decrease in Cu content and the implementation of the order anneal, the latter of which also leads to a decrease in Urbach energy. Defect characterization performed with admittance spectroscopy on devices is demonstrated as insufficient because of low‐temperature current barriers. Photoluminescence (PL) on crystal surfaces however enables a qualitative comparison of the defect landscape between crystal compositions and annealing treatments. Both a highly compensated and a lightly doped defect model are used to fit PL as a function of laser fluence to identify defects contributing to each observed recombination channel. The PL signatures attributed to the ZnSn defect become unresolvable with a decrease in Cu/Zn + Sn ratio from 0.86 to 0.77. Furthermore, a decrease in Cu–Zn disorder is observed upon the implementation of the annealing treatment through both a comparison of potential fluctuation depths and of both PL models.

Optimization of Zn1–xSnxO Buffer Layer for Application in CZTSe Solar Cells with H2‐Assisted Reactive Sputtering

CdS thin films are commonly used as buffer layer in Cu2ZnSnSe4 (CZTSe) solar cells inherited from CuInGaSe2 (CIGS) solar cells. However, in addition to the toxicity problems with Cd in conventional CdS buffer layers, the unfavorable band alignment at the CZTSe/CdS heterojunction confines the further improvement of CZTSe solar cells. ZnSnO (ZTO) is studied in this work as promising buffer layers for CZTSe solar cells. The stoichiometric composition and thickness of the ZTO layer are first optimized by sputtering. Subsequently, the improvement of the performance is demonstrated by sputtering ZTO in Ar gas mixed with H2. The effect of H2 on sputtering ZTO is investigated. The presence of O vacancies in ZTO buffer layer and its effect on carrier transport and device performance are presented. Through optimization, comparable VOC and higher fill factor (FF) are obtained for the CZTSe/ZTO solar cells compared with CZTSe/CdS reference.

Insights into High-Efficiency Ag-Alloyed CZTSSe Solar Cells Fabricated through Aqueous Spray Deposition

Kesterite Cu2ZnSnSe4 (CZTSe), Cu2ZnSn(S,Se)4 (CZTSSe), and Cu2ZnSnS4 (CZTS) solar cells show considerably lower open-circuit voltages than their theoretical values. The large open-circuit voltage deficiency (Vocdef) hinders the improvement of the power conversion efficiency (PCE) and the development of the pathway to mass production of kesterite solar cells. The main reason behind the Vocdef is considered to be the low formation energy of Cu/Zn disorders and their highly distributed defect complexes. To diminish the Cu/Zn disorder, we substituted Ag with a relatively large atomic radius into the host CZTSSe as (AgxCu1-x)2ZnSn(S,Se)4 (ACZTSSe) and investigated its beneficial effect in a systematic way. The ACZTSSe absorbers were all fabricated using aqueous spray pyrolysis in ambient air. The device performance was found to increase up to the optimum Ag substitution and decrease after the optimum Ag substitution. Admittance spectroscopy revealed that the optimal substitution of Ag reduced the Cu-/Zn-related defects, that is, charge recombination centers, which further mitigates the band tailing issue and enhances the PCE of the solar cell, and higher Ag substitution induced the generation of deeper defects, which decreases the PCE back. At the optimum Ag content of Ag/(Ag + Cu) = ∼9%, the ACZTSSe solar cell with the highest PCE of 11.83% was obtained, where both the interface recombination and bulk recombination were found to be minimized.

In Situ Electrochemical Treatment Evoked Superior Grain Growth for Green Electrodeposition-Processed Flexible CZTSe Solar Cells.

Flexible Cu2ZnSnSe4 (CZTSe) solar cells gradually attract much attention due to their low-cost, lightweight, and environmentally friendly features. However, the efficiency of flexible CZTSe solar cells obtained through the nonvacuum green electrodeposition process remains sluggish (3.82%), far away from that obtained from other methods (∼10% by magnetron sputtering). Herein, a championed 6.33% efficiency of flexible CZTSe solar cells prepared by the electrodeposition process is achieved through an in situ electrochemical treatment (ET) process. It is found that the ET process drives the formation of a thin MoOx layer, evoking a series of beneficial results, thus accounting for an enhancement in photovoltaic performance. With the ET process, the MoSe2 thickness is compressed and Cu- and Sn-related undesirable defects/secondary phases are inhibited, leading to improved film quality. Additionally, it prolongs the depletion region width and minority lifetime, accelerates the charge separation and collection, relieves the band tails, and favorably reverses the band bending from downward to upward at/near grain boundaries. With these effects, the efficiency increases from 4.21% to 6.33%, far beyond the highest reports on electrodeposited flexible CZTSe solar cells. Our findings offer a promising way to improving Mo foil-based flexible devices and mark a significant breakthrough for the development of electrodeposition-processed flexible CZTSSe-based solar cells.

Efficiency enhancement of CZTSe solar cell using CdS(n)/(AgxCu1–x)2ZnSnSe4 (p) /Cu2ZnSnSe4 (p+) structure

Direct bandgap Cu2ZnSnSe4 (CZTSe) material is a promising absorber for thin film photovoltaics due to its affordable production cost and ecofriendly constituents. However, bulk defects, small grain size, short minority carrier lifetime and band bending at CZTSe/Mo (back contact) interface limit the performance of CZTSe solar cells. To overcome these issues, we design a single junction CdS(n)/ (AgxCu1–x)2ZnSnSe4 (p) /Cu2ZnSnSe4 (p + ) solar cell where we use (AgxCu1–x)2ZnSnSe4 (ACZTSe) as the main absorber layer. We have chosen the carrier density of each layer in a such way that ACZTSe becomes depleted and generated minority are collected efficiently. In our design, we utilize CZTSe as the back surface field layer to prevent surface recombination and indium doped tin oxide (ITO) as the back electrode to avoid band bending and increase the open circuit voltage. Additionally, we optimize the thicknesses of different layers and achieve a maximum efficiency of 18.63% including Shockley-Read-Hall, radiative and surface recombination mechanisms. The photo conversion efficiency (PCE) of our designed solar cell is 6% higher (12.6% vs 18.63%) than that of the best reported selenium rich CZTSe solar cell. The design concept proposed in this work would help in harvesting solar energy efficiently from next generation CZTSe solar cells.

Li2S doping into CZTSe drives the large improvement of VOC of solar cell

Alkali metal doping or sulfurization are commonly applied in Cu2ZnSnSe4 (CZTSe) solar cell to improve the open-circuit voltage (VOC). However, alkali metal sulfide affording both alkali metal and sulfur is seldom to be studied, which restrains the development of kesterite solar cells. In this study, we evaporate Li2S during selenization process and hope to provide both alkali metal and sulfur to CZTSe film. The result indicates that Li shows a gradient distribution near the surface of CZTSe film and the content of S is slight. The film quality is improved and the recombination at grain boundaries is decreased after Li2S treatment. Besides, the bandgap of the absorber gets wider. Under the synergy of sulfur and lithium (mainly from lithium), the work function of the treated absorber gets higher and the conduction band offset (CBO) is in the ideal range. Combined with these contributions, the VOC of the champion device treated by Li2S dramatically increase by 120 mV. This study discloses that alkali metal brings the main effect on the performance of the kesterite solar cell even an alkali metal sulfide is evaporated, which deepens the understanding of sulfurization of CZTSe and also promote the progress of kesterite solar cells.

Cover Image

The cover image is based on the Research Article Rear interface engineering of kesterite Cu2ZnSnSe4 solar cells by adding CuGaSe2 thin layers by Sergio Giraldo et al., https://doi.org/10.1002/pip.3366.

Facile Sb2Se3 and Se co-selenization process improves the performance of Cu2ZnSnSe4 solar cells

Selenization is effective to fabricate high-efficiency kesterite Cu2ZnSnSe4 (CZTSe) solar cells that facilitates the composition modification, crystallization enhancement, and bandgap adjustment. The incorporation of Sb into CZTSe absorbers can further boost the device efficiency mainly due to promoting the grain growth of CZTSe absorber. Here we report a Sb2Se3 and Se co-selenization process that significantly improves the quality of CZTSe absorber layers. This strategy enables the increase of CZTSe grain size and elimination of pinholes. We find that Sb is uniformly distributed in the absorber layer. When the Sb/(Cu + Zn + Sn) ratio is tuned to 6.67‰, the CZTSe solar cell exhibits the best device performance with an efficiency up to 9.64%, over 40% higher than the traditional selenization process. Our work provides a new route for enhancing the device efficiency of CZTSe solar cells.

Preparing Cu2ZnSnSe4 thin film solar cells based on selenium containing precursors

Abstract Selenium containing precursors were prepared by magnetron sputtering SnSe, ZnSe and Cu targets. Effects of different annealing temperature on performances of Cu2ZnSnSe4(CZTSe) thin film and solar cells were investigated. CZTSe absorber layers based on selenium containing precursors were obtained using different selenization temperatures. The crystal structure, phase purity, surface and cross section morphologies and the element compositions of CZTSe thin films were characterized by XRD, Raman, SEM and EDS, respectively. It was found that the crystallization quality of the thin film gradually improved with increasing temperature, and CZTSe thin films annealed at 530°Chad the best crystallization quality. The results showed that the surfaces were flat and dense without obvious secondary phase. Complete CZTSe thin films solar cells were prepared by common structure. The electrical properties of solar cells were tested by Hall, EQE and J-V. The results showed that the maximum mobility was 4.33 cm2V-1S−1, and the maximum power conversion efficiency was 3.65%, with an open circuit voltage of 346 mV, a short circuit current density of 31.36 mA/cm2 and a fill factor of 33.57%.

Tuning the Work Function of the Metal Back Contact toward Efficient Cu2ZnSnSe4 Solar Cells

Carrier collection is crucial for thin film photoelectronic devices. As photoelectronic devices, the undesirable energy band structure between the metal contact and p‐type absorber is a serious limitation to boost the open‐circuit voltage (VOC) of thin film solar cells. Herein, the carrier collection near the back contact of a kesterite solar cell is promoted by tailoring the work function of the back contact. By diffusing P into Mo back contact of the kesterite solar cell, the work function of the Mo back contact is increased from 4.68 to 5.62 eV and the diffused P bonds with Mo as anions, thus resulting in the energy band bending upward at the back interface and enabling the fluent transmission of holes. The efficiency of Cu2ZnSnSe4 solar cells increases from 6.8% to 11.28%. The results expand the path to improving carrier collection in solar cells and electronic devices utilizing metal film contacts.

Rear interface engineering of kesterite Cu2ZnSnSe4 solar cells by adding CuGaSe2 thin layers

Rear interface engineering of kesterite Cu2ZnSnSe4 solar cells by adding CuGaSe2 thin layers

Sustainable Cu2ZnSnSe4 photovoltaic cells fabricated with a sputtered CdS buffer layer

AbstractIn this study, the effect of the Cu concentration on the characteristics of the Cu2ZnSnSe4 (CZTSe) absorber layers and solar cells was systematically analyzed by postselenization of the evaporated precursor deposits. The results demonstrated that an increase in the copper concentration of the precursor linearly increased the Cu content in the final CZTSe thin‐film formation. This significantly increased the particle size and improved the re‐evaporation loss of the SnSe, thus improving the solar cell characteristics. However, a further increase in the Cu concentration reduced the quality of the absorber layer and formed either CuxSe or CTSe secondary phases, which was detrimental to the solar cell characteristics. Here, we introduce a sputtered CdS buffer layer for the CZTSe solar cells. These findings offer new research directions for solving the persistent challenges in the chemical bath deposition (CBD) of the CdS in the CZTSe solar cells.

Nanoscale Surface Electrical Properties Tailored by Room‐Temperature Sulfurization for High‐Efficient CZTSe Solar Cells

AbstractSurface sulfurization can tailor the energy band alignment between p–n heterojunction for the kesterite‐structured Cu2ZnSnSe4 (CZTSe) solar cells. However, the traditional high‐temperature sulfurization process result in the decomposition of kesterite film and thus introducing deep defects. Room‐temperature surface sulfurization, which do not introduce new deep defects during the sulfurization process, is a good method for sulfurization. Combined by different experimental techniques, in this study, it is found that CZTSe film etched by concentrated ammonia can not only tailor the surface nanoscale composition, but also make CZTSe film to expose more crystallographic sites for the successful surface sulfurization. The conductivity of grains and grain boundaries for the absorber layer is improved, the minority carrier lifetime is increased, and the interface band alignment is modified by reducing the conduction band offset successfully after following (NH4)2S vapor treatments, which drive the improvement of the open‐circuit voltage (VOC) of the solar cell from 376 to 430 mV. This work can direct the surface bandgap engineering with nontraditional sulfurization to further enhance the performance of kesterite‐based thin film solar cells as well as other Se‐ or S‐containing solar cells.

On the Germanium Incorporation in Cu2ZnSnSe4 Kesterite Solar Cells Boosting Their Efficiency

The presence of Ge during the synthesis of thin film kesterite Cu2ZnSnSe4 (CZTSe) solar cell absorbers boosts their power conversion efficiency, especially due to an improved open circuit voltage. The mechanism underlying this beneficial effect of Ge is still under debate. We gained deep insights into the role of Ge by applying advanced synchrotron nanoprobe-based X-ray fluorescence spectroscopy and X-ray absorption near-edge structure spectroscopy to cross-sectional lamellas taken from high efficiency devices. We observe that Ge remains in the CZTSe absorber layer after the synthesis process with a specific heterogeneous distribution. Different grains contain different Ge concentrations. Moreover, Ge depletion exists at random grain boundaries but not at symmetric Σ3-boundaries, leading to different band alignments. The incorporated Ge occupies Sn lattice sites in the CZTSe crystal structure; however, the concentration is only 0.1 to 0.5 at %. Also Ge aggregates in nanoscale inclusions, which we could id...