CZTS Solar Cells Research Articles

Manipulating the Distributions of Na and Cd by Moisture‐Assisted Postdeposition Annealing for Efficient Kesterite Cu2ZnSnS4 Solar Cells

Effectively incorporating alkali metals or alternative isovalent cations into Cu2ZnSnS4 (CZTS) is considered one of the most promising strategies for realizing a step‐change improvement in the photovoltaic device performance. Herein, the local distribution of Na and Cd by a moisture‐assisted postdeposition annealing (MAPDA) treatment combined with a subsequent heterojunction heat treatment is manipulated. The MAPDA treatment facilitates the controllable reduction of the Na concentration, thus promoting the spontaneous diffusion of Cd into the heterojunction region. A subsequent 150 °C low‐temperature heterojunction heat treatment after MAPDA treatment enables further modification of Cd and Na distributions, leading to significantly enhanced optoelectronic properties at the CZTS/CdS heterojunction and greatly improved device performance with a peak conversion efficiency of 9.40%. The modified heterojunction significantly improves quasi‐Fermi‐level splitting under low‐photon injection, making CZTS solar cells more feasible in low‐light applications. This work provides an effective approach to simultaneously manipulate the distribution of Na and Cd, enabling pronounced modification of the heterojunction quality of CZTS solar cells and boost of conversion efficiencies. Insights gleaned herein may also be applicable to manipulating other critical trace elements in chalcogenide materials in general.

Synergetic Effects of Zn Alloying and Defect Engineering on Improving the CdS Buffer Layer of Cu2ZnSnS4 Solar Cells.

The inferior electrical properties at the interface of the Cu2ZnSnS4/CdS (CZTS/CdS) heterojunction resulting in the severe loss of open-circuit voltage (Voc) highly restrict the photovoltaic efficiency of CZTS solar cell devices. Here, first-principles calculations show that the Zn-alloyed CdS buffer layer reverses the unfavorable cliff-like conduction band offset (CBO) of CZTS/CdS to the desirable spike-like CBO of CZTS/Zn0.25Cd0.75S, which suppresses carrier nonradiative recombination and blocks electron backflow. In addition, the weakened n-type conductivity of Zn0.25Cd0.75S can be enhanced by In, Ga, and Cl doping without the introduction of detrimental deep-level defects and severe band-tail states, which improves the Voc of CZTS solar cells by promoting strong band bending and large quasi-Fermi-level splitting at the absorber side of the CZTS/Zn0.25Cd0.75S heterojunction. This study finds that the synergetic effects of Zn alloying and defect engineering on the CdS buffer layer are promising for overcoming the long-standing issue of the Voc deficit in CZTS solar cells, and understanding the optimized interfacial electrical properties provides theoretical guidance for improving the efficiency of semiconductor devices.

Defect Engineering for Efficient Cu2ZnSnS4 Solar Cells via Moisture‐Assisted Post‐Deposition Annealing

AbstractSulfide kesterite Cu2ZnSnS4 (CZTS) solar cells, containing earth‐abundant and environmentally benign constituents, are regarded as promising candidates for thin‐film photovoltaic technologies. CZTS device performance, however, is currently limited by severe nonradiative recombination caused by abundant deep‐level defects. Herein, an effective defect engineering approach for high bandgap CZTS solar cells using a newly introduced moisture‐assisted post‐deposition annealing treatment is reported. This treatment modifies the local chemical composition within the heterojunction and CZTS grain boundaries and enhances the incorporation of Cd within the CZTS layer during CdS deposition. Cd not only accumulates at the grain boundaries, but it also presents in grain interiors where it occupies Cu lattice sites. The overall modification of the local chemical environment suppresses deep level defects and activates relatively shallow acceptor CuZn antisites and Cu vacancies, giving rise to remarkably improved device performance. This work opens a new direction for defect engineering of kesterite materials, which may also be applicable to other thin film semiconductors.

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.

Simulation of a New CZTS Solar Cell Model with ZnO/CdS Core-Shell Nanowires for High Efficiency

The numerical modeling of Cu2ZnSnS4 solar cells with ZnO/CdS core-shell nanowires of optimal dimensions with and without graphene is described in detail in this study. The COMSOL Simulation was used to determine the optimal values of core diameter and shell thickness by comparing their optical performance and to evaluate the optical and electrical properties of the different models. The deposition of a nanolayer of graphene on the layer of MoS2 made it possible to obtain a maximum absorption of 97.8% against 96.5% without the deposition of graphene.The difference between generation rates and between recombination rates of electron–hole pairs of models with and without graphene is explored.The electrical parameters obtained, such as the filling factor (FF), the short-circuit current density (Jsc), the open-circuit voltage (Voc), and the efficiency (EFF) are, respectively, 81.7%, 6.2 mA/cm2, 0.63 V, and 16.6% in the presence of graphene against 79.2%, 6.1 mA/cm2, 0.6 V, and 15.07% in the absence of graphene. The suggested results will be useful for future research work in the field of CZTS-based solar cells with ZnO/CdS core-shell nanowires with broadband light absorption rates.

Computer-aided measurement technology for Cu2ZnSnS4 thin-film solar cell characteristics

Abstract Currently, there are more perfect theoretical basis and operation methods for the research of the Cu2ZnSnS4 (CZTS) solar cell characteristics, but the experimental process is tedious. In this article, to measure the characteristics of the CZTS solar cells more accurately and quickly, the computer-aided measurement method was used. During testing the characteristics of solar cells, the results were collected and analyzed by using LabCoder and Origin software. By designing experiments and using software to record data and complete solar cell characterization tests in one step, the number of artificial changes in resistance during testing is reduced. In the experiments, the data can be obtained in real time by using experimental results of computer-aided measurement technology, which can significantly improve the experimental efficiency. Through computer real-time monitoring, the maximum output power of as-prepared solar cells is P m = 1.62 W, and the calculated filling factor remains at 86%. It means that computer-aided measurement technology is suitable for the experiment of CZTS solar cell characteristics.

Enhancing Hole Density and Suppressing Recombination Centers through Illumination in Kesterite Thin Film Solar Cells.

Enhancing carrier density and increasing carrier lifetime are critical for the good performance of thin film solar cells. We apply illumination during the growth of kesterite Cu2ZnSnS4 (CZTS) to enhance hole density and suppress defects of nonradiative electron-hole recombination centers simultaneously. To examine the effect of the injected carriers generated by illumination, we first extend the scheme of detailed balance equations relating free carriers and defects beyond thermal equilibrium conditions by developing an extended Fermi level (EF') to characterize a homogeneous semiconductor with non-equilibrium carriers. On the basis of this scheme, we find that illumination can promote the formation of carrier-providing defects and suppress the formation of carrier-compensating defects. Then, we demonstrate that applying proper illumination during the growth of CZTS will help achieve a higher hole density and simultaneously suppress the formation of the SnZn antisite significantly, which are beneficial for the performance of CZTS solar cells.

Effect of CZTS/CCZTS Stacked Structures Prepared through Split-Cycle on the Performance of Flexible Solar Cells

Flexible CZTS solar cells have attracted significant attention in recent years owing to their high earth element composition and high photovoltaic conversion efficiency. However, these cells are associated with limitations common in compound thin-film solar cells, such as heterojunction interface combination centers, material defects, and energy level mismatch. In the current study, a green and simple solution spin-coating method is proposed for the preparation of flexible solar cells with multicycle CZTS/CCZTS absorber stacked structures to effectively circumvent the limitations of flexible devices. The results showed that the optimized Zn/Cd ratio results in CCZTS films with large grain longitudinal growth. However, lots of small broken grains were formed at the bottom of the absorber owing to the large lattice spacing, leading to the formation of cell leakage channels. The surface morphology of the absorber film was improved by preparing a stacked structure of CZTS and CCZTS with different periods while suppressing the longitudinal grain delamination. The stacked structure also helps to match the energy band of CZTS/CCZTS/CdS heterojunction. The top layer of CCZTS forms a buffered “energy band ladder” in the energy band of the CZTS/CdS heterojunction, thus suppressing the rapid carrier recombination caused by the Cliff-type energy band bending. A flexible solar cell was thus prepared to obtain a high photoelectric conversion efficiency of 6.51%, and the cells showed excellent resistance to mechanical bending.

Fabrication of CZTS thin films and solar cells via single-step Co-evaporation method

Despite remarkable enhancements that have been achieved during the past years, the photovoltaic performance of CZTS is far below the theoretical expectation. It appears that the profound breakthrough has to be made internally in the aspect of absorber itself, indicating the significance of fabrication strategies. Therefore, this study adopts the single-step co-evaporation method, which has been proved essentially successful for CIGS, to simultaneously achieve on-site deposition and considerably handy control of the film growth process. On this basis, we demonstrate that ZnS segregation can be alleviated through subtle growth profile modification. According to the evolution of series resistance and the conversion efficiency, the carrier transport blocking behavior which arises from ZnS is distinctly reduced. Furthermore, we achieve substantial control of the Sn-contained volatile phases so that the chemical equilibrium is beneficially shifted and the decomposition of CZTS is suppressed, bringing in significantly improved photovoltaic performance compared to the references. A power conversion efficiency of 4.3% is achieved with a considerable open-circuit voltage of 608 mV. The results in this study validate the potential along with the convenience of the single-step co-evaporation method.

Effects of annealing following back contact metal layer formation on CZTS solar cell properties

Cu2ZnSnS4 (CZTS) is interesting as a light absorption layer for thin-film solar cells because it consists of only earth-abundant materials such as copper, zinc, tin, and sulfur. The poor adhesion between CZTS and molybdenum (Mo), which is the back metal contact material, causes CZTS to peeling off during the chemical bath deposition (CBD) for the formation of CdS as a buffer layer. This induces severe degradation of the solar cell performance. In this study, we investigated the effect of annealing following back contact metal layer formation on CZTS stability and solar cell performance. By annealing Mo/soda lime glass at 400 °C in atmosphere, Mo was oxidized, and the surface roughness of Mo increased. On the Mo surface, the CZTS peeling off was suppressed during the CBD process, resulting in an improvement in the shunt resistance.

Exploration of CZTS-based solar using the ZrS2 as a novel buffer layer by SCAPS simulation

In this paper, we report on numerical analysis of the CZTS based solar cells performed by the solar cell capacitance simulator (SCAPS-1D). The main objective of this study is to reveal the prospect of using a novel nontoxic n-type ZrS2 transition metal dichalcogenides (TMDCs) as a buffer layer, the first of its kind. The different photovoltaic parameters of the proposed Al-doped ZnO (AZO)/ZrS2/CZTS structure are studied in detail versus each layer's thickness and carrier concentration. Our results reveal an optimized PCE of about 17.61% at corresponding thicknesses of 0.05 μm, 0.1 μm, and 0.3 μm for the window, absorber, and buffer layers, respectively. Additionally, the impact of the energy bandgap (Eg) of the ZrS2 buffer layer on the cell performance is investigated, with an optimized Eg of 1.54 eV. The results are discussed in terms of the conduction band alignments at the CZTS/ZrS2 buffer absorber interface. Furthermore, the photovoltaic cell performance is evaluated versus the defect level of the ZrS2 buffer layer. It has been figured out that deep defect levels beyond 1 × 1015 cm−3 diminish the Jsc. The results revealed that the ZrS2 could be a practical alternative for fabricating nontoxic CZTS solar cells.

High-Quality Zno/Ag/Zno Multilayer by Sputtering and Czts Solar Cells with Excellent Flexibility

High-Quality Zno/Ag/Zno Multilayer by Sputtering and Czts Solar Cells with Excellent Flexibility

Multi-modal characterization of kesterite thin-film solar cells: experimental results and numerical interpretation

We report a multi-modal study of the electrical, chemical and structural properties of a kesterite thin-film solar cell by combining the spatially-resolved X-ray beam induced current and fluorescence imaging techniques for the evaluation of a fully functional device on a cross-section. The data allowed the correlation of the chemical composition, defects at interfaces and inhomogeneous deposition of the layers with the local charge-collection efficiency of the device. We support our observations with Monte Carlo simulations of high-energy X-ray interactions with the semiconductor device, and finite-volume modeling of the charge-collection efficiency.

Impact of the secondary phase ZnS on CZTS performance solar cells using simulation program

In the present work ,Scaps 1d software is used to study the effect of ZnS secondary phase on the performance of solar cells. The standard CZTS-based devices are composed of a multilayer thin film formed on soda-lime glass (SLG), Mo/MoS2/CZTS/Cds/i-ZnO/ZnO Al. a metallic molybdenum (Mo) back electrode, MoS2 built between the absorber and the back contact; a p-CZTS absorber layer; an n-type buffer layer, typically Cds to form the junction of the solar cell, a TCO doped ZnO layer and a transparent ZnO:Al front contact. Ni/Al metal contact grids complete the cell. An ultrathin layer of ZnS is placed in the standard solar cell structure between the Cds buffer layer and the CZTS absorber layer to represent the second phase (SP) above CZTS [1].Cell characteristics such as ƞ, FF, Jsc , and Voc were studied. The formation of ZnS on the surface of CZTS has negative effects on the solar cell parameters where the conversion efficiency ƞ decreases.The impact of this layer on the performance of CZTS solar cells is also investigated through the results obtained from the analysis of the complex impedance (Z*) and the modulus (M*). An equivalent circuit of the solar cell consisting of series and parallel resistors has been evolved to study the electrical behavior around each interface of this structure.This new procedure has been very beneficial for the identity of the most important parameters of the diffusion and recombination process.

Enhancing CZTS solar cell parameters using CZTSe BSF layer and non-toxic SnS2/In2S3 buffer layer

Direct bandgap, high absorption coefficient and good electronic properties are the main reasons why the quaternary semiconductor Copper Zinc Tin Sulphide (shortly named CZTS) reached a high interest in the minds of researchers. In this work, a numerical modeling of CZTS solar cells using SCAPS-1D was executed investigating two major points. One, the possibility of replacing the traditional and toxic CdS as buffer layer for CZTS cells, we suggested both Tin sulphide (SnS2) and Indium sulphide (In2S3) as they showed good conductivity, optical transparency and optimal bandgap which make them appropriate to be buffer layer materials. Two, the enhancement that could be brought in by adding a back surface field (BSF) in-between the absorber layer and the back contact of the cell, and here we suggested CZTSe for its good optoelectronic properties, nontoxicity, earth-abundant components. The substrate cell configuration Mo/CZTSe/CZTS/(SnS2 and In2S3)/ZnO/FTO is used to observe the effect of temperature, absorber, buffer, BSF layer thicknesses, absorber’s concentration of acceptors, series resistance and back contact on the cell performance and to optimize the results following a valid procedural approach. Efficiencies of 32,55% and 32,50% are reached for SnS2 and In2S3 respectively. These simulation results could be very helpful to improve our knowledge of this type of solar cells and thus control better the fabrication process.

Ultrathin wide band gap kesterites.

Kesterite Cu2ZnSnS4 (CZTS), used for thin film solar cells, has a band gap energy around 1.5-1.6 eV with possibilities for further increase through alloying. In some applications for wide band gap solar cells, reduced absorber thickness can be beneficial, to allow partial light transmission. Reduced thickness can also be beneficial to reduce bulk recombination, and so called ultrathin solar cells (<700 nm thick) have been studied for several materials systems. Here, we report performance for CZTS devices down to 250 nm thickness and show that performance loss from thickness reduction is relatively small, partly due to short minority carrier diffusion length. Insertion of thin passivation layers (Al2O3, SiO2 or HfO2) at the Mo/CZTS interface gives improved performance of ultrathin devices, from 4.7% to 5.6% efficiency for best performing cells having 250 nm thick CZTS with Mo as compared to Mo/Al2O3 back contact. The approach of NaF post deposition for making isolating passivation layers conductive is tested for the first time for CZTS and is shown to work. For fabrication of CZTS devices on transparent ITO back contact, the insertion of passivation layers can reduce diffusion of indium into CZTS, but device performance is lower than on Mo back contacts.

Conduction Band Offset Effect on the Cu2ZnSnS4 Solar Cells Performance

Among the causes of the degradation of the performance of kesterite-based solar cells is the wrong choice of the n-type buffer layer which has direct repercussions on the unfavorable band alignment, the conduction band offset (CBO) at the interface of the absorber/buffer junction which is one of the major causes of lower VOC. In this work, the effect of CBO at the interface of the junction (CZTS/Cd(1-x)ZnxS) as a function of the x composition of Zn with respect to (Zn+Cd) is studied using the SCAPS-1D simulator package. The obtained results show that the performance of the solar cells reaches a maximum values (Jsc = 13.9 mA/cm2, Voc = 0.757 V, FF = 65.6%, ɳ = 6.9%) for an optimal value of CBO = -0.2 eV and Zn proportion of the buffer x = 0.4 (Cd0.6Zn0.4S). The CZTS solar cells parameters are affected by the thickness and the concentration of acceptor carriers. The best performances are obtained for CZTS absorber layer, thichness (d = 2.5 µm) and (ND = 1016 cm-3). The obtained results of optimizing the electron work function of the back metal contact exhibited an optimum value at 5.7 eV with power conversion efficiency of 13.1%, Voc of 0.961 mV, FF of 67.3% and Jsc of 20.2 mA/cm2.

Influence of “Productive” Impurities (Cd, Na, O) on the Properties of the Cu2ZnSnS4 Absorber of Model Solar Cells

Abstract The study focuses on the optical properties of the CZTS multicomponent semiconductor absorber with 3 % “production” impurities of Cd, Na, O within the framework of the density functional theory using the generalized gradient approximation and the SCAPS program, as well as investigates their influence on the performance and efficiency of CZTS-solar cells. The results showed that the introduction of Cd, Na, O impurities would lead to a decrease in the intensity of the absorption bands at 2.06 eV and 2.55 eV. The density of states CZTS: (Cd, Na, O) was determined from first principles, and it was revealed that impurities of Cd and O atoms would lead to a decrease in the band gap (to 0.9 eV and 0.79 eV), and an increase in Na impurity absorption (1.2 eV). It was also found that a decrease in the band gap led to a decrease in the open circuit voltage, and it was also shown that “industrial” impurities led to a decrease in the efficiency of energy conversion of solar cells to 2.34 %.

Numerical modeling of AZTS as buffer layer in CZTS solar cells with back surface field for the improvement of cell performance

This paper presents the use of AZTS, a non-toxic and earth abundant material as buffer layer in CZTS solar cells due to its good lattice match. We have performed simulation on the experimentally fabricated AZTS/CZTS hetero-junction device by adding CZTS as back surface field (BSF) layer. In this work, first we have reconstructed the AZTS/CZTS experimentally designed solar cell then BSF layer is added to boost the solar cell performance in SCAPS simulator. Several parametersof AZTS buffer layer such as generation/recombination rate and doping concentration are varied during simulationto enhance the performance of investigated solar cells structures. It has been noticed that addition ofBSF layer increasesthe efficiency of AZTS/CZTS solar cell from 4.51% to 5.05% along with improvement of both Voc and FF.

Bilayer CZTS/Si absorber for obtaining highly efficient CZTS solar cell

The good photovoltaic (PV) properties of Cu2ZnSnS4 (CZTS) and silicon (Si) can be used combinedly as a potential route for enhancing the efficiency of CZTS solar cell. In this work, a bilayer CZTS/Si absorber structure is presented to replace the conventional single CZTS absorber layer based solar cell. The numerical tool SCAPS-1D is utilized first to validate the structure with the help of J-V characteristics, generation rate, quantum efficiency and band diagrams. A similar tool is used afterwards to optimize the proposed structure considering different parameters such as layer thickness, defect density, carrier concentration and operating temperature. The highest yielded efficiency of the bilayer structure is 18.30% (VOC = 0.883 V, JSC = 25.90 mA/cm2). The thickness of both CZTS and Si layers are optimized and its effect on the performance parameters are discussed. The p-type doping concentrations of both layers are studied for the suitable range and optimized. Also, the electric field at the CZTS/Si interface is estimated. Next, the defect state density and operating temperature are varied to see the solar cell performances. The studied results provide a clear indication of the performance improvement of the CZTS solar cell using the bilayer CZTS/Si absorber structure.