CZTSe Solar Cells Research Articles

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.

Fabricating Cu2Zn(SnxGe1-x)Se4 thin-film solar cells with back surface Ge grading by magnetron sputtering

In this paper, an approach of forming a band gap gradient by depositing a layer of Ge between the Mo layers by magnetron sputtering is proposed. The diffusion depth of the Ge into Cu2ZnSnSe4 (CZTSe) thin film can be effectively controlled by the Mo transition layer, resulting in the formation of Cu2Zn(SnxGe1-x)Se4 (CZTGSe) thin film with a band gap grading along the back surface. To validate the formation of the Ge gradient in the absorber, energy dispersive spectrometer line scanning and grazing incidence X-ray diffraction were performed. Furthermore, the impact of Ge layer on the morphologies of CZTSe was assessed using scanning electron microscopy. As a result, the introduced Ge layer formed a Ge grading along the back surface and improved the crystal quality of the absorber. Compared to CZTSe solar cells, the power conversion efficiency (PCE) of CZTGSe solar cells with a Ge gradient improved from 5.79 % to 9.28 %.

Improving device performance of sputtered CZTSe based solar cells by Manganese doping

Despite many efforts, Cu2ZnSnSe4 (CZTSe) based solar cells have remained relatively low efficiency compared to other thin film-based devices. One of the prevalent strategies used to improve cell performance in CZTSe is cation substitution. In this contribution, the effects of substituting Mn for Zn on CZTSe solar cell performance were systematically examined. Samples with various Mn content were grown by a two-stage method involving high temperature annealing of precursor layers comprising Cu, Zn, Sn, Mn and Se. All of the samples were found to have Cu-poor and (Zn + Mn)-rich or nearly stoichometric compositions. They crystalized in kesterite phase irrespective of the Mn content as confirmed by X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) measurements. The highest efficiency solar cell (5.56%) was obtained employing the 5% Mn doped CZTSe absorber. This film was found to have improved microstructure and reduced defect density. This is the first report on over 5% efficient kesterite thin film solar cell fabricated on a Cu2Mn0.05Zn0.95SnSe4absorber layer.

Optimizing CZTSe Solar Cell Architecture: Comparative Study of ZnO, TiO₂, and MoO₃ as Electron Transport Layers

Optimizing CZTSe Solar Cell Architecture: Comparative Study of ZnO, TiO₂, and MoO₃ as Electron Transport Layers

Improved CZTSe solar cell efficiency via silver and germanium alloying

In this study, we report systematic investigation of the effects of Ag and Ge alloying on properties of CZTSe layers, as well as, on the performance of solar cells fabricated using these films. In this context, Ag-Ge doped CZTSe layers were produced by selenization of Cu/Sn/Zn/Cu/(Ag,Ge)/Se precursor stack structures using rapid thermal processing. All precursor stacks and the Ag-Ge doped CZTSe films obtained after selenization exhibited (Cu + Ag)-poor and Zn-rich chemical composition. XRD studies demonstrated pure kesterite phase for all reacted films. Raman spectra confirmed this finding. Cross-sectional SEMs showed large grain structure, which resulted from Ag-Se and Ge-Se liquid phase formation that assisted crystal growth during high temperature annealing. While a slight Ag-front-gradient was achieved in Ag-doped CZTSe film, the Ag gradient disappeared with incorporation of Ge into the lattice. Addition of Ge formed a gradient within the material such that near-contact region was more Ge-rich. Solar cells fabricated using films with various compositions demonstrated that double doping CZTSe with both Ag and Ge improved the device efficiency from about 5 % to over 8 %.

Defect Engineering in Earth‐Abundant Cu2ZnSnSe4 Absorber Using Efficient Alkali Doping for Flexible and Tandem Solar Cell Applications

To demonstrate flexible and tandem device applications, a low‐temperature Cu2ZnSnSe4 (CZTSe) deposition process, combined with efficient alkali doping, was developed. First, high‐quality CZTSe films were grown at 480 °C by a single co‐evaporation, which is applicable to polyimide (PI) substrate. Because of the alkali‐free substrate, Na and K alkali doping were systematically studied and optimized to precisely control the alkali distribution in CZTSe. The bulk defect density was significantly reduced by suppression of deep acceptor states after the (NaF + KF) PDTs. Through the low‐temperature deposition with (NaF + KF) PDTs, the CZTSe device on glass yields the best efficiency of 8.1% with an improved VOC deficit of 646 mV. The developed deposition technologies have been applied to PI. For the first time, we report the highest efficiency of 6.92% for flexible CZTSe solar cells on PI. Additionally, CZTSe devices were utilized as bottom cells to fabricate four‐terminal CZTSe/perovskite tandem cells because of a low bandgap of CZTSe (~1.0 eV) so that the tandem cell yielded an efficiency of 20%. The obtained results show that CZTSe solar cells prepared by a low‐temperature process with in‐situ alkali doping can be utilized for flexible thin‐film solar cells as well as tandem device applications.

All-Vacuum-Deposited Bifacial Cu2ZnSnSe4 Photovoltaic Cells with Sputtered Cd-Free Buffer Layer

By depositing metal precursors on fluorine-doped tin oxide substrates using evaporation and postselenisation and modifying the number of stacked metallic precursor layers, this study systematically analysed the effect of cation changes on the absorber layer and the solar cell properties of Cu2ZnSnSe4 (CZTSe). Furthermore, in this study, an all-vacuum method was adopted to prepare a cadmium-free bifacial CZTSe solar cell and conducted damp heat tests on the device. The findings indicate that the increase in the number of stacked metallic precursor layers suppresses secondary phase generation, thereby enhancing the film performance and bifacial solar cell characteristics. The cadmium-free bifacial CZTSe solar cell prepared in this study exhibited the efficiency of 4.13% under bifacial incident light. Moreover, the degradation of the device was effectively mitigated in the damp heat tests compared with previously reported devices with metal substrates. These findings can provide new directions for research and lead to novel applications for CZTSe solar cells in the future.

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%.

Investigation into complex defect properties of near-stoichiometric Cu2ZnSnSe4 thin film

In this work, a systematic investigation of the effects of point defects on the efficiency of a Cu2ZnSnSe4 (CZTSe) thin film solar cell was conducted using temperature-/power-dependent photoluminescence (PL) and time-resolved photo-luminescence (TRPL). The studied stoichiometric CZTSe absorber layer for a CZTSe solar cell with an efficiency of 4.4 % was prepared using the low-toxicity selenization process. The fitting to the Arrhenius plot and the stretched tail at the long-wavelength side of PL spectrum obtained at 10 K indicated a high concentration of zinc on copper (CuZn) point defects. The dispersive-photon-energy-dependent TRPL at 10 K exhibited non-exponential stretched PL decay at all photon energies and, in particular, revealed the highly fluctuating potential of CZTSe that caused carrier localization. This effect was observed to be caused by [2CuZn− + SnZn2+] defect clusters, which produce a significant conduction/valence band-edge shift in their vicinity in CZTSe and can cause shunt leakage and an increase in series resistance. This systematic investigation presents the experimental characterization of [2CuZn− + SnZn2+] defect clusters and provides perspective to help clarify the performance deterioration of CZTSe solar cells.

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.

Efficiency enhancement of CZTSe solar cells based on in situ K-doped precursor

High FF: in situ K-doped precursors make large-grain and void-free absorbers. The doping-enhanced Sn diffusion eliminates the local content vibration, suppresses deep level defects, and to higher cell efficiency with greatly improved FF.

Study effect of window and BSF layers on the properties of the CZTS / CZTSe solar cell by SCAPS–1D

The solar cell CZTS / CZTSe was studied using SCAPS-1D * computer simulator. It was noted that increasing the thickness of the absorber layer p-CZTSe from 250nm to 5μm leads to increase the IV curve. thus increasing the values of Voc, Jsc, FF

Insights into the impact of defect states and temperature on the performance of kesterite-based thin-film solar cells

In this study, we explore the influence of defect states density and operating temperatures on the performance of kesterite-based (CZT(S,Se)) solar cells such as CZTS, CZTSe, and CZTSSe. SCAPS codes were used as a constructive technique in modeling all structures. The CZT(S,Se) structures were modified by incorporating a p-Mo(S,Se)2 interfacial layer between the back contact and the absorber. Interestingly, only a 100 nm thick layer of p-Mo(S,Se)2 can establish a quasi-ohmic configuration at a CZT(S,Se)/Mo heterojunction, which was demonstrated by an increase in the slope of the J-V characteristics. In the active layer region, a series of systematic simulations were conducted depicting the response of CZTSSe/MoSSe structures to defect density states ranging from 1013 cm-3 to 1017 cm-3. The recombination behavior of CZTSSe/MoSSe solar cells is significantly superior to those made of CZTS/MoS2 or CZTSe/MoSe2, with a minimum increase in recombination current of 2.03 mA/cm2. The increased recombination rates are attributed to a shorter minority carrier lifetime at the buffer-absorber interface due to a reduced diffusion length between electrons and holes. Additionally, the photovoltaic parameters of the CZT(S,Se) structures were examined over a wide temperature range, from 275 K to 475 K, including open-circuit voltage (Voc), short circuit current (Jsc), fill factor (FF), and power conversion efficiency (PCE). A CZTS/MoS2-based structure exhibited the most stable performance at elevated temperatures up to 395 K with minimum degradation in PCE of approximately 2% compared to other solar cells. As a result, the voltage variation to temperature coefficients obtained for the CZTS and CZTSe solar cells were − 0.75 mV/K and − 1.2 mV/K, respectively. These findings support the use of CZTS/MoS2 structures in the development of photovoltaic modules that are capable of operating at high temperatures.

CZTSe-Based Solar Cell Performance Improvement Using the CSLO Technique

Here we investigated a novel layer-based optimization technique to improve the performance of a CZTSe solar cell. By using this technique, the optical behavior and electrical properties of the proposed solar cell improved significantly as a result of the changes in the layer specifications and the layer materials. The structure of the cell consisted of an absorber laid on a conducting layer and covered by Indium Tin Oxide (ITO), with ZnO on its top surface. Due to the employment of the CSLO technique, a p+pn junction was formed between the absorber and window layers, which provided a lower recombination rate by transmitting more electrons and holes to the contacts. In addition, the main important parameters affecting the solar cell’s performance such as layer thickness, carrier lifetime, and total effect density were investigated. According to the results, the proposed CZTSe solar cell achieved a 32.6% and 79.5% efficiency and fill factor, respectively—which in comparison to a conventional solar cell is remarkable. Moreover, hybrid structures made by utilizing CZTS-based, Ge-based Cu2ZnGeSe4, and Si-based Cu2ZnSiSe4 with the proposed CZTSe-based solar cell were implemented and better results were achieved, yielding an efficiency of about 42, 50, and 34% and a fill factor of 66, 55, and 42%, respectively, due to the materials’ properties.

Effect of Self‐Seed Inducing on the Growth Mechanism and Photovoltaic Performance of Cu2ZnSnSe4 Thin Films

The self‐seed inducing grain growth mechanism for a Cu2ZnSnSe4 (CZTSe) thin film is explored via the existence of the fine‐grain CZTSe synthesized spontaneously at a relatively low soft‐selenization temperature of 300°C. Fine control over the elementary proportion and distribution of metal precursors is an important approach for obtaining the self‐seed inducing effect after soft selenization. Herein, a multiple‐cycle metal stack precursor, prepared by a weighting method, is developed to control the elementary proportion and distribution. By shortening the diffusion distance to produce a self‐seed inducing effect at soft selenization, the formation of detrimental intrinsic defects and secondary phases is effectively suppressed, contributing to the optimization of the CZTSe absorber back contact and surface. Consequently, the conductivity of the grain interiors and boundaries of the absorber layer is enhanced, the electric potential and roughness of the surface are reduced considerably, and the minority carrier lifetime of the layer is increased. When the number of stacked cycles increases from 1 to 7, the champion efficiency of CZTSe solar cell improves from 2.97% to 8.05% with a significant decrease in the open circuit voltage deficit from 0.831 to 0.617 V. These results may prove useful for developing a wider range of multinary compound semiconductor devices.

Growth and reaction mechanism of solution-processed Cu2ZnSnSe4 thin films for realising efficient photovoltaic applications

This article reports on Cu2ZnSnSe4 (CZTSe) thin film preparation via a nonhydrazine, nonpyridine and environmentally friendly low-cost solution process method. CZTSe fabrication through a solution-based process has not yet been suitably demonstrated given the impediments to addressing the presence of selenium in solutions. In this study, we introduced a two-step CZTSe fabrication method that used monoethanolamine as the chelating agent/co-solvent and ethanol as the main solvent. Selenization was then conducted. In this process, we successfully avoided the use of hydrazine to synthesise CZTSe thin films. Material characterisations (e.g. UV–VIS–NIR, scanning electron microscopy, electron dispersive spectroscopy, X-ray diffractometry and Fourier transform infrared spectroscopy) confirmed the high quality of the deposited thin films. The deposited CZTSe thin film showed high crystallinity without carbon residues, indicating its potential application as a photovoltaic absorber. Hence, we investigated the photovoltaic parameters of the CZTSe-based solar cells on the basis of the deposited thin film’s optoelectronic properties. We utilised Solar Cell Capacitance Simulator to examine the electrical effects of CZTSe solar cells and used three-dimensional finite-difference time-domain optical simulations to investigate the optics of the solar cells. We estimated that the realistic power conversion efficiency of the CZTSe solar cells could reach 18.5% with a short-circuit current density of 30 mA/cm2.

Back Shallow Ge Gradient Enhanced Carrier Separation for CZTSe Solar Cells through a Coselenization Process.

Given the prominent success of the Ga gradient in CuIn1-xGaxSe2 (CIGSe) solar cells, Ge gradient implementation is a promising way to boost Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. However, Ge-graded CZTSSe solar cells only possess a low efficiency of 9.2%, far from that of Ge-incorporated CZTSSe without a gradient (12.3%). Herein, we demonstrated a shallow Ge gradient CZTSe solar cell with an improved efficiency over 10%. The Ge gradient was achieved through a GeSe2-Se coselenization process, where GeSe2 acts as a low-temperature fluxing agent to assist crystallization and induce Ge transport toward the back interface. The relieved band tails and improved junction quality, leading to a better carrier separation, were found to take a primary responsibility for device improvement. These results highlight a remarkable breakthrough for Ge-graded CZTSe solar cells and offer a promising way to develop Ge-involved solar cells.

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.

Location-Optoelectronic Property Correlation in ZnO:Al Thin Film by RF Magnetron Sputtering and Its Photovoltaic Application.

In this study, a radio-frequency magnetron sputter system was used to deposit Al2O3 doped ZnO (AZO) thin films at room temperature, and the soda lime glass (SLG) substrates were placed at different zones relative to the center of the sample holder under the target. The samples were then analyzed using an X-ray diffractometer, Hall-effect measurement system, UV-visible spectrophotometer, and X-ray photoelectron spectroscopy. It was found that the electrical, structural, and optical properties of AZO films strongly depend on the target racetrack. The AZO thin film grown at a location outside the racetrack not only has the most suitable figure of merit for transparent conductive films, but also retains the least residual stress, which makes it the most suitable candidate for use as a CZTSe transparent conductive layer. When applied to CZTSe solar cells, the photoelectric efficiency is 3.56%.

Investigation on the Structure and Morphology of CZTSe Solar Cells by Adjusting Cu–Ge Buffer Layers

Investigation on the Structure and Morphology of CZTSe Solar Cells by Adjusting Cu–Ge Buffer Layers