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• Device modeling of lead free CH 3 NH 3 SnI 3 solar cell is comprehensively performed. • Effect of different kesterite based HTLs and zinc based ETLs on the solar cell are studied. • Dependency on the band alignment, absorption, electric field, recombination, defects, thickness, doping, temperature and reflecting are investigated. • Among 24 proposed optimized structures, ZnO/MASnI 3 /CNTS achieved the highest PCE of 29.24% with J sc of 29.44 mA cm−2, V oc of 1.12V and FF of 88.82%. • Reasons for low power conversion efficiencies of the structures were also analyzed. Methylammonium tin triiodide (MASnI 3) perovskite solar cells (PSC) have gained a lot of interest due to their edge over conventional Pb-based PSC in terms of less-toxic nature, wider optical absorption range and smaller bandgap. In this work, 24 novel n-i-p heterostructures of MASnI 3 PSC have been analyzed in detail with various zinc-based electron transport layers (ETL) and kesterite-based hole transport layers (HTL). The proposed device architecture (FTO/ETL/CH 3 NH 3 SnI 3 /HTL/Back contact) performance was first enhanced by optimizing the thickness and then by the doping concentration of each layer via SCAPS-1D simulator under AM 1.5G illumination. The energy band alignment of the different charge transport layers (CTL) with MASnI 3 was analyzed in detail to understand its working principle. Moreover, the effects of optical absorption, band offsets, electric field, defect density, interface defects, temperature, rear reflective coating, and electrodes are monitored to characterize the performance of each cell structure. Among all the proposed structures, ZnO/MASnI 3 /CNTS based perovskite solar cell performed outstandingly well with J sc of 29.44 mA cm−2, V oc of 1.12 V, FF of 88.82 %, and PCE of 29.24 %. These simulations provided important insights into the mechanism of carrier transport and the factors affecting the performance of solar cells. The results in this paper will help the research community in fabricating highly efficient eco-friendly solar cells. [ABSTRACT FROM AUTHOR]

The low open-circuit voltage is one of the main reasons to deteriorate the efficiency of copper zinc tin sulfide (CZTS) based solar cells. The double-layer structures (pp +) for CZTS cells are beneficial for reaching high open-circuit voltage due to the presence of the back surface field. Our simulation-based study on ZnS/CZTS-bilayer solar cell reveals the effect of band gap of dual CZTS layers, the thickness of different layers, front and back contact metal work-function, which helps to optimize the solar cell for these parameters. The different experimental conditions and techniques offer a wide range of shunt and series resistance for the fabricated cell. Our device reported the highest efficiency of 20.34% and 20.19% for shunt-series resistance pair of (1424.05 Ω -cm 2 , 1.97 Ω -cm 2) and (840 Ω -cm 2 , 1.5 Ω -cm 2) respectively. The double CZTS layer based device shows an enhancement in cell efficiency compared to a single layer. • To achieve the optimized CZTS-double layered based solar cell is our aim. • The effect of layer properties, metal work-function on cell performance is discussed. • We optimized the device for layer thickness, band gap, and metal work function. • The device is simulated for different shunt-series resistance pair. • The optimized cell is simulated with (η = 20. 19 %) and without (η = 16. 49 %) BSF layer. [ABSTRACT FROM AUTHOR]

Reliable measurement routines are crucial for power rating and yield prediction of photovoltaic emerging thin-film technologies. Copper-Zinc-Tin-Sulfur-Selenium (CZTSSe) thin-film photovoltaic devices are an emerging technology made of abundant elements. Still, sufficient stabilization methods prior to electric power measurement are missing in the international standardization, while existing standards for other thin-film technologies do not work properly for CZTSSe. This study investigated methods for achieving power stabilization of the CZTSSe solar devices. Three complementary stabilization routines for the kesterite-based solar devices were investigated as an alternative to the existing international device testing standards: rapid annealing, dark electric biasing and different operating points under illumination. The typical number of stabilization cycles for power stabilization was between 3 and 6 cycles of rapid annealing, dark electric bias and illumination with a power loss of -19.5%, -11.4%, and -1.9%, for the respective methods. The dark electric bias method was found to provide the most reliable average result for power stabilization. All stabilization methods proved to have the potential to work sufficiently in stabilizing the CZTSSe devices for standardized power measurement. • Stabilization of CZTSSe solar devices: rapid annealing, dark-biasing, illumination. • While promising, rapid annealing at T ≤ 100 °C leads to strong degradation. • Dark bias at T > 25 °C and current bias < 0.66 Isc gives fast stabilization. • The best illumination method is stabilization based on IEC 61215-2 but with 25 °C. • All methods allow to stabilize devices with the 2% criterion of the standard. [ABSTRACT FROM AUTHOR]

In this research, performance enhancement of three terminal heterojunction bipolar transistor structure based solar cell (HBTSC) is investigated via implementing perovskite layer as emitter absorber region and nanorods within emitter. For this study, a p-n-p hybrid HBTSC structure has been proposed with MAPbI 3 /CdS/ACZTSe materials for photon absorber layer, and MgF 2 for anti-reflection layer (ARL). The purpose of this research is mainly to optimize emitter (MAPbI 3) and CdS nanorods thickness of HBTSC structure therefore boosting device efficiency through optoelectronic and electrical simulations. During obtaining the results, the effects of the surface, Shockley–Read–Hall (SRH) and radiative recombination mechanisms has been considered. Two new proposed structures based on planar MAPbI 3 /CdS/ACZTSe & MAPbI 3 /CdS nanorods/CdS/ACZTSe perovskite–kesterite based hybrid HBTSCs has been introduced. The maximum efficiency (η) of 23.45% and short-circuit current density (J s c) of 39.93 mAcm−2 are obtained for planar MAPbI 3 /CdS/ACZTSe based perovskite–kesterite HBTSCs with open-circuit voltage (V o c) of 760.05 mV and fill factor (F F) of 77.28%, respectively. As for MAPbI 3 /CdS nanorods/CdS/ACZTSe based hybrid HBTSCs with η of 27.91%, J s c of 42.03 mAcm−2, V o c of 786.8 mV, and F F of 84.39%, a total improvement of 4.46% is achieved when compared to planar hybrid perovskite–kesterite HBTSCs structure. • Top emitter absorber layer is engineered to design 3-T hybrid HBTSCs. • Optimized absorber layers thickness and doping concentration to obtain maximum PCE. • Implemented optimized n-CdS nanorods to obtain the highest PCE of 27.91%. • Overall PCE improvement of 4.46% is achieved by implementing CdS nanorods in MAPbI 3. [ABSTRACT FROM AUTHOR]

• An optimized design of Kesterite solar cell with double CZTS layers was investigated. • Numerous loss mechanisms in bulk CZTS and interface CdS/CZTS were considered. • Several window Cd-free materials were investigated in order to circumvent the CdS/CZTS interface drawback. • The optimized design, compared with the conventional one efficiency 8.8%, exhibit a conversion efficiency of 10.68%. • The suggested design technique enabled the characterization and the identification of the optimal design. In this paper, we suggest a potential high efficiency Kesterite solar cell with double CZTS layers including efficient band offset alignment. Several loss processes like radiative recombination, existence of defect and traps densities in bulk CZTS and interface CdS/CZTS are considered in the modeling framework. The prediction capability of the adopted modeling framework is assessed with respect to experimental results where a good matching can be distinguished. An optimal solar cell design is investigated by studying the effect of interfacial traps, beside doping and thickness of different device layers. In order to circumvent the CdS/CZTS interface drawback, several window Cd-free materials such as IGZO, TiO 2 , ZnO, WS 2 , ZnS are scrutinized. Our findings show that ZnO with high optical and electrical characteristics can significantly boost the solar cell performance. Moreover, in order to tune the band alignment at ZnO/CZTS interface, we explore an appropriate electron affinity of ZnO by varying Mg content where we find that pristine ZnO with (χ = 4.13 eV) establish an optimal band offset with a spike like conformation. The achieved results prove the efficiency of the proposed design in mitigating the unwanted effect of the recombination. By comparing these outcomes with the conventional device efficiency 8.8%, we achieved an enhanced efficiency of 10.68%. The proposed structure may provide advantages such as enhanced absorption behavior and decreased recombination effects. [ABSTRACT FROM AUTHOR]

• Thermal annealing of Cu 2 ZnSnS 4 solar cell in air improved the solar cell characteristics. • The band gap of Cu 2 ZnSnS 4 decreased with thermal annealing in air. • Cd diffusion was observed from CdS to to Cu 2 ZnSnS 4. • Increase in external quantum efficiency was observed with hot-plate annealing. In this work, a detailed study of the effect of thermal treatment (hot plate) on a Mo/CZTS/CdS/i-ZnO/ITO solar cell was carried out, which promotes the diffusion of the Cd contained in the window layer of CdS towards the absorber layer of CZTS. The thermal treatment of the Mo/CZTS/CdS/i-ZnO/ITO solar cell was carried out at temperatures ranging from 100 °C to 400 °C and times varying from 3 to 15 min in air atmosphere. The CZTS was grown from deposits of metallic precursors (Cu/Sn/Cu/Zn) by sputtering and annealing at two ramps: 200° C for 45 min with an argon flow of 1 mbar and 550 °C for 5 min at 1 bar in an atmosphere of S + Sn. For the complete solar cells with a hotplate treatment of 280 °C for 6 min an increase of 2.7% to 5.2% of conversion efficiency was observed compared to the solar cell without hotplate treatment. The band gap CZTS decreased from 1.56 to 1.43 eV and Jsc increased from 12.0 mA/cm2 to 15.4 mA/cm2. [ABSTRACT FROM AUTHOR]

In this paper, first principle calculations of Cu 2 NiGeS 4 (CNGS) are carried out to explore the structural, electronic and optical properties of kesterite compound. Both mBJ+U and HSE potentials are used to calculate the band gap energy. The first approach gives a value of 1.78 eV and the second one a value of 1.76 eV. Our numerical simulation shows that the CNGS exhibits a remarkable high absorption coefficient of the order of 10 4 cm − 1 , leading to a promising absorber material for photovoltaic devices. Additional optical properties such as refraction index and dielectric function are also calculated in this work. Furthermore, CNGS-based solar cell simulation was performed through SCAPS software. The calculated values of short-circuit current density J s c , open-circuit voltage V o c , Fill factor FF and power conversion efficiency show that CNGS can be a potential candidate for solar cell application. • Determination of the structural properties of CNGS kesterite structure. • Determination of the electronic and optoelectronic properties of CNGS kesterite structure. • Determination of the current–voltage characteristic of CNGS solar cell baseline. • Improving the solar cell performance with MoS 2 interlayer. [ABSTRACT FROM AUTHOR]

The indispensability and innovations in thin-film-based solar cells are gaining attention among researchers due to their tremendous growth as a sustainable energy source. The development of solar cells instigates in the 19th century after confirming usable electrical energy from sunlight. Currently, the solar cell holds a replaceable position in many applications. Solar cells stand as an easier way to use the enormous source of renewable energy. This review addresses the exceptional physicochemical properties of thin-film materials deposited via spray pyrolysis technique, followed by its effective use in solar cell applications. The study rolled over the various generations of solar cells and discoursed the relevant approaches in the field in detail. As the review proceeds, the copper and non-copper thin film-based discussion took the way and explained its advancements, pros, and cons in photovoltaics. Finally, the review elucidates the recent proceedings, the impending scopes, and the recycling of thin-film based Photovoltaics. Succinctly, the investigation visualizes the state of photovoltaics and persuading further research in this field. [ABSTRACT FROM AUTHOR]

• The absorber ACZTSe/CZTSe and CZTS/ACZTS tandem based solar cell is proposed. • Optimization of layer parameters carried to attain maximum efficiency. • Impact of temperature, series, shunt and Capacitance frequency analysis on Ag incorporated Kesterite cell performance is investigated. In this work, the analytical results pertaining to modeling of silver substituted Kesterite absorber (Cu 1-x Ag x) 2 ZnSnSe 4 (ACZTSe) and (Cu 1-x Ag x) 2 ZnSnS 4 (ACZTS) materials are demonstrated. The basic approach adopted here is the tailoring of bandgap and electron affinity of the modified Kesterite structure so as to enhance the performance. The promising Ag incorporated materials exhibit improvement in open circuit voltage which is one of the bottlenecks in the Kesterite structure. The pure Cu 2 ZnSnS 4 (CZTS) and Cu 2 ZnSnSe 4 (CZTSe) are considered and modeled as the baseline structures for simulation in SCAPS. To improve the performance parameters, the analysis was carried out for tandem structures with silver substituted in place of copper in the Kesterite material. As Ag has got favorable atomic properties which makes it an ideal candidate to substitute against copper. The equations of bandgap and electron affinity for Ag doped CZTS and CZTSe with a tuning factor of [ A g ] A g + [ C u ] is modeled for optimizing the tandem structures. The front linear grading with reverse linear bandgap profile is used for the improvement of the overall performance of the device with variation in absorber layer thickness, acceptor doping density. The modeled Ag substituted tandem devices ACZTSe/CZTSe and CZTS/ACZTS results in efficiency of 19.37% and 23.50% respectively. Finally, the impact of temperature variation on Capacitance-Frequency calculation along with the influence of series and shunt on the device performance is investigated and discussed. [ABSTRACT FROM AUTHOR]

[Display omitted] • CZTS films sulfurized under various partial-pressures of sulfur-vapor (P s). • Sulfurization was performed in evacuated ampoules containing sulfur-powder. • Sub atmospheric sulfurization-pressure leads to a CZTS film with better morphology. • MoS 2 thickness dominated by diffusion of sulfur through the film irrespective of P s. • Bandgap decreased and disorder increased with P s. Thin-films of Cu 2 ZnSnS 4 (CZTS), a promising photoabsorber for photovoltaics, were obtained by sulfurization of the precursor films at 550 °C in evacuated and sealed quartz ampoules under various partial pressures (22.3–232.5 kPa) of sulfur-vapor, and its effects on the morphological and optical properties were studied. The precursor films were obtained on the molybdenum (Mo) coated soda-lime glass substrate via solution-chemistry. The observed changes in the morphology have been explained based on the amount of CuS formed and its conversion to sulfur-poor Cu 2 S. A decrease in the grain size from 448 to 223 nm was observed with the increase in partial pressure of sulfur-vapor. The thickness of the molybdenum sulfide (MoS 2) layer that forms at the CZTS – Mo interface during sulfurization, increased from 62 to 479 nm. The structural disorder, estimated using Raman spectroscopy, increased with the increase in the partial pressure of sulfur-vapor. [ABSTRACT FROM AUTHOR]

References. Bandgap and Absorption coefficient Image: Jiang, M., Yan, X., 2013. Cu2ZnSnS4 thin film solar cells: present status and future prospects. Solar Cells—Research and Application Perspectives. Crystal structure Image: Chen, S., Walsh, A., Gong, X.G., Wei, S.H., 2013. Classification of lattice defects in the kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 earth‐abundant solar cell absorbers. Advanced materials 25(11), 1522–1539. Schematics of the band diagram for CIGS and CZTS Image: Romero, M. J., Du, H., Teeter, G., Yan, Y., Al-Jassim, M. M., 2011. Comparative study of the luminescence and intrinsic point defects in the kesterite Cu2ZnSnS4 and chalcopyrite Cu (In, Ga) Se2 thin films used in photovoltaic applications. Physical Review B, 84(16), 165324. Capacitance –Voltage Image: Yang, K. J., Sim, J. H., Son, D. H., Jeon, D. H., Hwang, D. K., Nam, D., Cheong, H., Kim S. Y., Kim J. H., Kim D. H., and Kang, J. K., 2017. Comparison of chalcopyrite and kesterite thin-film solar cells. Journal of Industrial and Engineering Chemistry, 45, 78–84. [Display omitted] • Various types of defects and interfaces are discussed in detail. • The effect of loss mechanisms on the kesterite-based solar cells are discussed. • Establish a correlation between simulation and experimental results. • Various approaches to reduce the loss mechanism are discussed in detail. Kesterite Cu 2 ZnSnS 4 (CZTS), Cu 2 ZnSnSe 4 (CZTSe), and its alloys Cu 2 ZnSn(S x ,Se 1−x) 4 (CZTSSe, 0 < x < 1), are cost-effective thin-film photovoltaic materials to meet future energy demands. The presence of more elements in these quaternary materials than binary or ternary materials provides flexibility in the fundamental physical and chemical properties. The huge V oc losses have limited the performance of devices. On the other hand, progressive studies for obtaining high efficiency kesterite-based Cu 2 ZnSnS 4 , Cu 2 ZnSnSe 4 , and Cu 2 ZnSn(S x ,Se 1−x) 4 devices have been ongoing over the past few years, but a concrete understanding is lacking. In this review, the lattice defect and loss mechanisms in kesterite materials and their impact on solar cell performance are analyzed systematically. The origin and the cause of atomic defects, deep disorders and interface properties, and their influence on the solar cell performance are discussed. In addition, a comprehensive correlation between the experimental and simulation methods has been attempted to determine the loss mechanisms in kesterite materials. This review also focuses on some approaches to overcome the loss mechanisms. These findings are expected to provide a path for better device performance. [ABSTRACT FROM AUTHOR]

• Facile growth of phase pure CZTS films by RF magnetron sputtering is reported. • Effect of sulfurization parameters on the phase and microstructural evolution is studied. • I-V characteristics of Ag/p-CZTS/Mo Schottky diode are analysed through thermionic emission model. While single target sputter deposition appears very attractive to prepare kesterite Cu 2 ZnSnS 4 (CZTS) thin films for photovoltaic applications, the growth of secondary phases arising as a consequence of stoichiometric deviation stemming from variation in sputter yield of elements must be prevented. Here, we demonstrate growth of phase pure kesterite CZTS thin films by RF magnetron sputtering of a single target by intuitively manipulating the target composition and post-sulfurization process carried out in a quasi-open environment using sulfur flakes. The influencing mechanism of post-sulfurization process was elucidated from systematic variation in the dwell time, temperature and the sulfur amount. A high temperature or a shorter dwell time yielded a small-grained microstructure associated with the presence of secondary phases. Phase pure CZTS films with better microstructural features were obtained for sulfurization at 500 °C for 60 min with 1.0 g of sulfur flakes. This film exhibited an optical bandgap of ∼1.58 eV indicating its photovoltaic potential. A device in the Mo/CZTS/Ag configuration showed typical features of a Schottky junction. The obtained current–voltage characteristic was analyzed to estimate saturation current, ideality factor and series resistance in correlation with the properties of the CZTS film. [ABSTRACT FROM AUTHOR]

• Performance evaluation of Kesterite solar cell without defects and trap charges. • Analysis of kesterite solar cell in presence of Defect densities and trap charges. • Performance analysis of solar cell under Tail and Gaussian Distributions of defects. • Analysis of efficiency and quantum efficiency of solar cell. Kesterite materials Cu 2 ZnSn(S x Se 1−x) 4 , Cu 2 ZnSnS 4 and Cu 2 ZnSnSe 4 have received considerable attention as absorber layers for photovoltaic cell applications. But they exhibit efficiencies lower than 12%. In this work, different possible loss mechanisms that affect the solar cell performance are analysed. Numerical investigation is made on the influence of different loss mechanisms such as radiative recombination, presence of traps and defect densities on the performance of the device. A comparative between results on solar cells based on Cu 2 ZnSnS 4 and Cu 2 ZnSnSe 4 absorber layers is presented. The recombination mechanisms are found to play a vital role in degrading the performance of the solar cell. Additionally, trap centers and density of defect states are distributed to the absorber layer. It is observed that efficiency of the solar cell is severely affected and degraded by 10 % in the presence of defect states. [ABSTRACT FROM AUTHOR]

[Display omitted] • Indoor bifacial photovoltaics (i-BPVs) emerge as a promising solution for efficient and economical light energy harvesting. • Perovskite based i-BPVs show exceptional potential for indoor applications because of high-power conversion efficiency and design flexibility. • Developing perovskite-based BPV integrated IoT systems for indoor use is crucial in the current scenario, despite challenges such as stability, toxicity, and commercialization. Bifacial photovoltaics (BPVs) have received tremendous attention as a potential contender for efficient and cost-effective light energy harvesting. Recently, advancements in BPV technologies have broadened their scope, allowing them to harness artificial indoor light energy efficiently from both top and bottom sides. This innovative approach has demonstrated its effectiveness in energy harvesting through a single-cell design. Among various available PV technologies, thin-film perovskite photovoltaics (PPVs) exhibit exceptional promise for indoor applications in low-light environments. They offer high-power conversion efficiency and ease of design, making them a superior choice when compared to other emerging indoor PV technologies such as kesterite and dye-sensitized PVs. To improve the performance of the indoor BPVs, it is necessary to further review several characteristics relating to their materials, architecture, processing, and indoor characterizations. Additionally, a comprehensive understanding of charge-transfer mechanisms, the variability in indoor lighting sources, the development of standardized indoor simulators, assessment of materials toxicity, and considerations of scalability are essential elements that must be addressed to advance this technology. In summary, this review examines recent developments, emerging trends, challenges, and provide suggestions for the further advancement of i-BPVs, also highlighting their potential as reliable and sustainable indoor energy-harvesting solution. [ABSTRACT FROM AUTHOR]

• A Temperature of at least 530 °C is required for a high crystalline kesterite absorber. • CdS buffer layer is completely alloyed at 550 °C. • Crystallinity has a critical effect on the defect density and photovoltaic performance. Kesterite based on Cu 2 ZnSnS 4 composition is considered as a promising absorber material for the next generation of photovoltaics due to raw materials abundance and low toxicity. Developing a superstrate architecture using kesterite as an absorber could be the key to a better performance, which allows new ways of engineering the formation of a kesterite thin film. In this work, we study the effects of the sulfurization temperature on the crystallization of kesterite film when is fabricated in a superstrate architecture, and how this affects the performance of a solar cell. We noted that the temperature affects the final composition of the absorber due to cadmium diffusion at 500 °C from the CdS layer, while the temperature above 530 °C is essential for a complete Zn incorporation into the crystal structure. The crystallinity is highly affected, and a temperature of 550 °C is found to be favorable for the crystal growth, and the fabricated device gave improved performance. Impedance spectroscopy measurements suggest the performance enhancement is due to the reduction of defects and an increase of the depletion width in the p-n junction. [ABSTRACT FROM AUTHOR]

[Display omitted] • A single junction CdS(n)/(Ag x Cu 1–x) 2 ZnSnSe 4 (p)/Cu 2 ZnSnSe 4 (p +) solar cell is presented. • Photoconversion efficiency of 18.63% is achieved from the optimized cell including different recombination mechanisms. • A relative improvement of ~47.6% in the performance of selenium rich low bandgap CZTSe solar cell is reported. • (Ag x Cu 1–x) 2 ZnSnSe 4 (ACZTSe) is used as the main absorber for better cationic ordering. • ITO is used as the back electrode to improve band bending and Schottky contact. • CZTSe is used as the back-surface field layer to prevent surface recombination. Direct bandgap Cu 2 ZnSnSe 4 (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)/ (Ag x Cu 1–x) 2 ZnSnSe 4 (p) /Cu 2 ZnSnSe 4 (p +) solar cell where we use (Ag x Cu 1–x) 2 ZnSnSe 4 (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. [ABSTRACT FROM AUTHOR]

[Display omitted] • A novel kesterite solar cell was designed and analyzed by SCAPS 1D. • Replacement of Mo by ITO reduces the Schottky barrier height. • Back surface field enhances minority carrier life time and diffusion length. • Alternative buffer produces an offset of 0 eV at buffer/absorber interface. • A systematic optimization of cell design boosted the efficiency up to 20%. Among the several obstacles which impede efficiency enhancement of CZTS solar cells, the sub-optimized Mo back contact and the unfavorable conduction band offset at CZTS/CdS interface are the critical ones, contributing largely to high surface recombination and low open circuit voltage. In this article, a numerical simulation model was used for device structure analysis and performance optimization. A substrate configuration of CZTS solar cell was considered. Benchmarking study of the reference cell gives a conversion efficiency of 8.3% which matches well with the reported experimental value, validating the accuracy of our simulation model. Thereafter, we have implemented several modifications in the experimental device structure including an alternative back contact, a back surface passivation layer, a Cd-free buffer layer with tunable conduction band minimum, and a wide band gap ZnO-based alloy material which enhances the spectral response to shorter wavelength contributing to higher efficiency. A systematic optimization of device structure results in the increase of J SC and V OC due to the increment in diffusion length and optimal free carrier collection which ultimately increases the power efficiency from 8.4 to 20.6%. [ABSTRACT FROM AUTHOR]

The double-ion-doping strategy is significantly better than single ion doping strategy and undoped sample for CZTSSe solar cells. [Display omitted] • Double-ion-doping strategy is studied in CZTSSe solar cells. • Fine-grain layer of CZTSSe film eliminated by Li+&Ag+ doping. • Grain size of CZTSSe film increased by Li+&Cd2+ doping. • Double-ion-doping strategy is better than single-ion-doping. Ion doping strategy is a promising method to enhance the efficiency of Cu 2 ZnSn(S,Se) 4 (CZTSSe) solar cells, however, most of the reported works focus on studying the single ion doping in CZTSSe absorber layers. Here, Li+&Ag+ and Li+&Cd2+ double-ion-doping strategy are applied to improve the microstructure and device performances. We found that the double-ion-doping strategy can obviously increase the grain size, reduce the thickness of fine-grain layers and enhance the device performances. Furthermore, the PCEs of Li+&Ag+ and Li+&Cd2+ doped CZTSSe solar cells are up to 8.87% and 8.39%, respectively. It shows an encouraging improvement of over 43.5% and 35.8% enhancement compared with the traditional method (6.18%), and significantly higher than single ion doping strategy (7.60%, 7.99% and 7.11% for Li+, Ag+, and Cd2+ doped CZTSSe solar cells). This work provides a new solution to improve the microstructure and device performances of CZTSSe solar cells. [ABSTRACT FROM AUTHOR]

• The CZTS/CZTSe bilayer using constant band gap grading is proposed and shows enhanced performance. • For the first time an interfacial layer of MoS 2 is used between the two-absorber layer giving an enhanced performance. • Optimization of layer parameters carried to attain maximum efficiency. • Impact of temperature on cell performance is investigated. Thin film solar cells based on Cu 2 ZnSn(S,Se) 4 are very promising, because they contain earth abundant elements and show high absorptive behavior. However, the performance of these solar cells are limited and needs to be improved in order to reach efficiencies as high as that reported for Cu(In,Ga)Se 2 and CdTe based solar cells. To obtain enhanced performance, the present study investigates the output effect of fixed bilayer CZTS/CZTSe structure with band gap 1.5 and 0.9 eV respectively and the effect of MoS 2 interfacial layer between CZTS and CZTSe layer to avoid unintentional elemental diffusion. The idea of bilayer CZTS/CZTSe and CZTS/MoS 2 /CZTSe structure was exploited to achieve higher open circuit voltage and efficiency by band engineering mechanism compared to single layer. In this numerical study the focus is given to fixed band gap model as the structure is practically feasible to realize with the associated materials. The thin interfacial layer introduced between the two layers ultimately diminishes the recombination process due to the back induced electric field. Finally, the fixed bandgap CZTS/MoS 2 /CZTSe exhibits better performance with open circuit voltage of 0.88 V and efficiency of 21.1%. [ABSTRACT FROM AUTHOR]

• Binary CdS and SnS 2 were prepared via CBD and Hydrothermal procedures, respectively. • XRD diffraction peaks of CdS and SnS 2 thin films are confirmed polycrystalline nature. • The surface homogeneity of SnS 2 exhibit big grains and coalescence than CdS thin films. • The resultant band gaps to be 2.45 eV, 2.20 eV for SnS 2 and CdS thin films, respectively. • A remarkable photocurrent (140 µA/ cm2) observed for SnS 2 as compare to CdS (80 µA/ cm2) thin films. Binary compound tin disulfide (SnS 2) and cadmium sulfide (CdS) are the potential candidates used as a buffer layer for copper indium gallium selenide (CIGS) and copper zinc tin sulfide (CZTS) thin-film device. Herein, both compounds have been successfully prepared through simple hydrothermal (HD) and chemical bath deposition (CBD) techniques, respectively. The prepared samples were characterized by different available techniques like X-ray diffraction (XRD), atomic force microscopy (AFM), surface electron microscopy (SEM), energy dispersive spectroscopy (EDS), transmittance electrons microscopy (TEM), UV–Visible spectroscopy and photoelectrochemical (PEC) analysis. The XRD analysis confirms the polycrystalline nature of the prepared thin films. AFM analysis showed that the SnS 2 display better roughness (60 nm), grain size (75 nm) than CdS roughness (23 nm), grain size (41 nm) thin films. SEM and EDS studies revealed near stoichiometry behavior of elemental composition of the films. The optical absorption spectrum showed the direct bandgap of CdS 2.45 eV and 2.20 eV for SnS 2 thin films. The PEC analysis revealed that the SnS 2 thin films exhibit two times higher photoresponse (140 µA) as compare to CdS (80 µA) thin films. The SnS 2 high photocurrent could be attributed to the small band gap and increase in grain size which can trap more incident light. Based on the results the SnS 2 used as a buffer layer can be a good choice for an efficient photovoltaic device. [ABSTRACT FROM AUTHOR]

• Cu 2 ZnSnS 4 thin films were deposited by direct drop-casting of the precursor solution. • An in-depth characterization of both CZTS thin films and PV devices is discussed. • The addition of KCl was experimentally demonstrated to improve device performance. • SCAPS simulation was performed to deeply-understand the device behaviour. The PV research is nowadays focused on finding low cost and easily processable thin-film materials, trying to move towards lighter and more versatile technologies. Among them, Cu 2 ZnSnS 4 (CZTS) drew the attention of the scientific community thanks to its structural similarity to the already reported and well-studied Cu(In,Ga)Se 2 but with the great advantage to use cheap and earth-abundant elements. In this work, we report CZTS-based solar cells produced with a simple and straightforward sol–gel process, demonstrating how further expensive and industrially non-scalable instruments are not needed in order to provide a working device. Direct drop-casting of a precursor solution onto the substrate, followed by gelation and final annealing in Ar atmosphere, generates the desired phase of the material without any further source of sulphur. The precursor solution contains metal acetate salts which play a primary role in networking the metals in the gel matrix, together with thiourea and dimethylsulfoxide. With this methodology, only the strictly necessary amount of reagents is used saving until 2–3 orders of magnitude of starting materials compared to other wet deposition techniques and working devices were obtained showing η = 1.1%. The addition of KCl as a dopant into the precursor solution was proven to be beneficial for the grain growth and material quality, so the performance of the corresponding devices rises it up to η = 1.75%. The synthesised thin films have been fully characterised by UV–Vis absorption, μ-Raman, X-ray diffraction and Energy Dispersive Spectroscopy measurements. Their morphology has been investigated by Scanning Electron Microscopy, proving the high quality of the material. The so-produced PV devices with a standard SLG/Mo/CZTS/CdS/ZnO/AZO architecture show interesting and promising results, especially in terms of short circuit current density. [ABSTRACT FROM AUTHOR]

• The predeposited CdS buffer layer has a significant effect on the quality of the selenized CZTS films. • During the selenization Cd diffuses into the CZTSSe absorbing material and improves its crystallinity and quality. • CZCTSSe colar cells of superstrate architecture demonstrate 3.3% power conversion efficiency. In this article, we present a systematic investigation of the influence of CdS layer thickness on the structural and compositional properties of CZTSSe films and photovoltaic performance of superstrate solar cell devices. The selenization of the CZTS films results in Cd diffusion into the absorbing layer. This leads to a significant change in the lattice parameters and crystallinity, and also causes the inhomogeneous distribution of S and Se ions along the thickness of the CZTSSe layer and formation of CZCTSSe kesterite phase. Alternatively, an increase in Cd content for a certain amount also leads to the enhancement of the device performance parameters of CZCTSSe films which are attributed to the improved crystallinity, morphology and, possibly, graded band-gap. The achieved power conversion efficiency of 3.3% is among the highest for the spray pyrolysis deposited solid-state CZTSSe solar cells of superstrate architecture. [ABSTRACT FROM AUTHOR]

• Numerous novel nip structures based MAPb I 3 perovskite solar cells are proposed in SCAPS environment. • Eight ETL and six HTL layers were applied to device structure TiO 2 / MAPbI 3 / S p i r o - O M E T A D. • Total forty-nine device structures were analyzed in SCAPS. • PCE varied from 2% to 26.69% for each of forty-nine device structures. • ETL / MAPbI 3 / C N T S device structure having PCE greater than 24.84% to the maximum value of 26.69%. In this work, detailed analysis and guidelines were provided using solar cell capacitance software (SCAPS) to characterize possible novel n i p structures for M A P b I 3 based perovskite solar cells having device power conversion efficiency (PCE) greater than 22%. To accomplish this task, we first optimize the performance of T i O 2 / M A P b I 3 / S p i r o - M e O T A D device structure by finding the optimal parameters of thickness and doping concentration for each layer. After device optimization, we first apply eight different electron transport layers (ETL) to the given device structure by replacing each with T i O 2 in the SCAPS environment and analyzing the effect of each ETL on device performance. After evaluating device structure with different ETL layers, we then apply six different kesterite and quaternary compounds as a possible candidate for the HTL layer. From an analysis, we found forty-nine new possible n i p device structures for each ETL and HTL layer having PCE variation from a value of 2% to 26.69%. Among them the best possible n i p structures, we achieved are E T L / M A P b I 3 / C N T S having PCE greater than 24.84% to the maximum value of 26.69%. [ABSTRACT FROM AUTHOR]

• Promotion of Ge incorporation into CZTSe lattice via Na, detected by XRD and Raman. • Thicker NaF layer results in higher Ge content, E g and shorter depletion width. • Nano-Mo/nano-V 2 O 5 /FTO as good back contact for semi-transparent kesterite devices. • Semi-transparent CZTGSe-based solar cells with 5.6% efficiency. Cu 2 ZnSn 1-x Ge x Se 4 (CZTGSe) thin films have been grown onto Mo/SLG and Mo/V 2 O 5 /FTO/SLG substrates using thermal co-evaporation followed by a subsequent thermal annealing. A NaF precursor layer was evaporated prior to the deposition of the kesterite absorber layer. In the samples grown on Mo/SLG, it has been found that Na promotes Ge incorporation into the Cu 2 ZnSnSe 4 lattice. The high concentration of incorporated Ge leads to the segregation of Sn-Se secondary phases as well as to an accumulation of Sn next to the Mo layer. The use of 12 and 16 nm NaF thick precursor layers prior to the CZTGSe deposition leads to absorber band gaps of 1.30 and 1.34 eV, and to device performances of 4.7 and 4.0%, respectively. A higher Na content, furthermore, caused the formation of bigger grains, a higher charge carrier concentration and a shorter depletion width. A 12 nm NaF precursor layer was used for the devices grown on FTO-based substrates, producing an optimal back contact that allows achieving efficiencies of 5.6% and transmittance of 30% in the near infrared range. This enhanced performance can be associated with the absence of secondary phases and Ge distribution through the absorber layer. The formation of a MoSe 2 layer at the back interface in all the investigated devices seems to play a crucial role to improve the solar cell efficiency. [ABSTRACT FROM AUTHOR]

• Comparison and validation of the simulation with the experimental result of Mo/CZTS/CdS/i-ZnO/ZnO:Al/Al thin-film solar cell. • Introducing of CZTSe and CZTSSe intermediate layers (Mo/CZTSSe/CZTS/CdS/i-ZnO/ZnO:Al/Al) for the purpose of increasing efficiency and open-circuit voltage. Using the thin layers mentioned above and optimizing the structure as P+PN, the following efficiencies are respectively obtained: 15.98%, and 17.81%. • In the proposed structure with CZTS 0.2 Se 0.8 intermediate layer, open-circuit voltage is increased to 986 mV by proper band alignment at back contact/CZTSSe interface and through reduction in series resistance recombination. • Optimization of each of the two proposed structures to obtain the highest performance for the devices. • Simulations demonstrate that at the bias voltage of 0.8 V for CZTSSe/CZTS structure, i.e. after employing the CZTS 0.2 Se 0.8 intermediate layer, the recombination current total density is acquired as 6.47 mA/cm2. There have been significant efforts to enhance the performance of CZTS thin-film solar cells. However, the increase in series resistance, formation of the unfavorable MoS 2 between the absorber layer and back-contact, and recombination losses are some of the most effective factors for the low values of the efficiency and open-circuit voltage parameters. In this paper, five experimental CZTS solar cells with high efficiency of 8.4–11% have been mainly studied and discussed. Afterwards, two experimental works have been reproduced to validate our results. Additionally, with introducing the thin CZTS x Se (1-x) layer between the Mo back-contact and CZTS absorber layer, the efficiency and open-circuit voltage of solar cells have been notably improved. There is a significant decrease in recombination losses (less than 12 mA/cm2) after using CZTS x Se (1-x) intermediate layers between the absorber layer and back-contact. Using the CZTSe, and CZTS 0.2 Se 0.8 thin layers and optimizing the structure as p+pn structure (p+-CZTS x Se (1-x) /p-CZTS/n-CdS), the following efficiencies (and open-circuit voltage) are achieved as: 15.98% (921 mV) and 17.81% (986 mV), respectively. Simulations demonstrate that at 0.8-V bias for p+-CZTS 0.2 Se 0.8 /p-CZTS/n-CdS structure, the recombination current total density is reduced to 6.47 mA/cm2. To obtain the highest performance in the proposed structures, the absorber layer carrier concentration should be 1 × 1016 cm−3 and the intermediate layer carrier concentration needs to be within the range of 1 × 1017 cm−3 to 1 × 1018 cm−3. Furthermore, the optimum thickness of CZTSe and CZTSSe intermediate layers in the proposed structures is 50 nm. [ABSTRACT FROM AUTHOR]

• Kesterite Cu 2 ZnSnS 4 nanoparticles were synthesized by two-phase synthesis method. • Kesterite nanoparticles were used for fabrication of inorganic HTM layer of C-PSC. • 53% increase in PCE of C-PSC was obtained by employing kesterite HTM layer. In the present work, Cu 2 ZnSnS 4 (known as kesterite) nanoparticles were synthesized by two-phase method. X-ray diffraction measurement confirmed formation of kesterite crystalline phase. Size of nanoparticles in colloidal suspension was determined by dynamic light scattering method and it was 9.2 ± 2.5 nm. The obtained nanoparticles were used as inorganic hole-transporting material (HTM) in paintable carbon-based PSC (C-PSC). Power conversion efficiency (PCE) was increased from 6.1% ± 1.1% for HTM-free paintable C-PSC to 9.3% ± 0.9% for paintable C-PSC with kesterite HTM. This significant increase in PCE was due to the higher hole-extraction rate at carbon counter-electrode, in the presence of kesterite HTM, which was confirmed by different measurement methods such as incident photon to current conversion efficiency, photoluminescence spectroscopy and electrochemical impedance spectroscopy. Although hot-injection synthesized kesterite nanoparticles have been previously used as inorganic HTM in Ag and Au-based PSCs, using kesterite nanoparticles, synthesized by a cheaper method, i.e. two-phase method, in C-PSCs was reported in the present work, for the first time. [ABSTRACT FROM AUTHOR]

• CZTS thin films with Cu-rich composition was synthesized using sequential evaporation technique. • Sequentially deposited thin films were subjected to sulfurization with temperature ranging from 520 °C to 640 °C. • Resulting thin films were assigned to various Cu-rich off-stoichiometries based on their composition. • Cu-rich CZTS thin films have been evaluated for its implication in photovoltaic application. We report synthesis of Copper Zinc Tin Sulfide (CZTS) thin films using sequential evaporation technique. The as-deposited films were subjected to various sulfurization temperatures ranging from 520 °C to 640 °C. This paper presents a detailed analysis of temperature dependent phase and defect formation in Cu-rich off-stoichiometric CZTS thin films. CZTS thin films moved from F-type to C-type with decreasing Cu/Zn disorder and defect concentration with increasing sulfurization temperature. The resulting films showed less off-stoichiometry in contrast to initial choice of intended off-stoichiometry exhibiting the structural flexibility within the compositional limits. Despite being off-stoichiometric, all the samples exhibited the tetragonal CZTS phase with preferred orientations. Raman analysis showed the disappearance and reappearance of secondary phases with increasing sulfurization temperature. EDS analysis suggested that composition of the films moved towards better stoichiometry till 580 °C before deteriorating again. Composition and lattice parameter values suggested Cu-rich off-stoichiometric nature of sulfurized CZTS thin films. AFM study showed dependence of roughness with sulfurization temperature. Band gap was obtained in the range of 1.28 eV to 1.48 eV from UV–Vis spectroscopy measurement. PL analysis revealed the asymmetric shape and blue shift of the peaks. Band-to-tail recombination becomes pronounced with increase in sulfurization temperature. XPS analysis showed elements are in expected oxidation states. [ABSTRACT FROM AUTHOR]

• Single phase (Ag x Cu 1−x) 2 ZnSn(S,Se) 4 monograins were synthesized in molten flux. • Adding Ag to CZTSSe increases carrier concentration and decreases carrier lifetime. • Low Ag contents (x ≤ 0.02) in CZTSSe lead to higher solar cell device efficiencies. • The Ag incorporation changed the dominant radiative recombination channel in CZTSSe. • The Ag incorporation decreased the collection width as evaluated by the EBIC study. The open circuit voltage (V OC) deficit of Cu 2 ZnSn(S,Se) 4 (CZTSSe) kesterite solar cells is higher than that of the closely related Cu(InGa)Se 2 solar cells. One of the most promising strategies to overcome the large V OC deficit of kesterite solar cells is by reducing the recombination losses through appropriate cation substitution. In fact, replacing totally or partially Zn or Cu by an element with larger covalent radius one can significantly reduce the concentration of I–II antisite defects in the bulk. In this study, an investigation of the impact of partial substitution of Cu by Ag in CZTSSe solid solution monograins is presented. A detailed photoluminescence study is conducted on Ag-incorporated CZTSSe monograins and a radiative recombination model is proposed. The composition and structural quality of the monograins in dependence of the added Ag amount are characterized using Energy Dispersive X-ray Spectroscopy and X-Ray Diffraction method, respectively. The Ag-incorporated CZTSSe monograin solar cells are characterized by temperature dependent current-voltage and electron beam induced current methods. It was found, that low Ag contents (x ≤ 0.02) in CZTSSe lead to higher solar cell device efficiencies. [ABSTRACT FROM AUTHOR]

• Single step ED technique was used to fabricate CZTS kesterite thin films. • Growth of CZTS on different substrates such as Mo, FTO, and ITO were investigated. • Virtuous crystallinity on Mo substrates and phase purity was found around 450 °C. • The band gaps of the annealed samples are found around 1.47 eV. • Device performance was investigated for different substrates in SCAPS-1D. Kesterite C u 2 Z n S n S 4 (C Z T S) with an optimal band gap of 1.5 e V is an auspicious material to be used as absorber layer high efficiency thin film photovoltaic cells. Effect of substrates on the morphology and structural properties of CZTS kesterite thin films were analyzed by depositing CZTS on Molybdenum, Indium doped tin oxide, and Fluorine doped tin oxide via electrochemical deposition method. The electrolyte contains C u S o 4 , Z n S o 4 , S n S o 4 and N a 2 S 2 O 3 as precursors, with N a 3 C 6 H 5 O 7 and C 4 H 6 O 6 as complexing agents. Electrochemical depositions were carried out at room temperature with a voltage of −1.05 V vs. Ag/AgCl reference electrode. Films were annealed at a temperature around 450 °C and then characterized by X-ray diffraction. The characterization shows the development of CZTS kesterite structure, with a good crystallinity on Mo substrates and phase purity, which were also confirmed by Raman spectroscopy and scanning electron microscopy. Then optical measurements showed that the deposited thin films present a bandgap of around 1.47 eV. Correspondingly, the effect of metal contact work function for these substrates were also investigated with the aid of device modeling software SCAPS. The analysis shows that for given solar cell structure, back contact/CZTS/CdS/ZnO, Mo substrates presented better performance. [ABSTRACT FROM AUTHOR]

• The effective back-contact barrier of CZTSe: nanoscale Ge is improved and characterized. • The mechanism of the back-contact improvement is demonstrated with device models. • The model of MoSe 2 interface defect at back-contact agrees with the empirical data. • The efficiency is improved up to 8.3% by including nanolayer Ge at back contact. Using temperature-dependent measurements and device modeling, we systematically study the effective back-contact barrier of CZTSe devices to improve the property of the back-contact interface. By comparing with CZTSe devices with various nanoscale Ge configurations, CZTSe nanoscale Ge bi-layers devices show the improved power conversion efficiency by 1.1%. DC magnetron sputtering is used to fabricate CZTSe: nanolayer Ge devices. Critical device parameters are characterized to understand the impact of nanoscale Ge films on the back-contact device characteristics. Based on empirical results, modeling is performed for the influence of MoSe 2 defects on the effective back-contact barrier. Analysis of experimental results of Ge bi-layers devices with the improved back-contact barrier makes a good agreement with modeling and Sentaurus TCAD simulation at the 95% confidence-level. The conversion efficiency of CZTSe: nanoscale Ge bi-layers devices is improved up to 8.3%. [ABSTRACT FROM AUTHOR]

• A first working monolithic CZTS/Si tandem cell is presented and characterized. • A MoS 2 /FTO/ZnO intermediate contact between top and bottom cells is developed. • Limiting factors of this device are addressed and investigated to overcome them. Earth abundant Cu 2 ZnSnS 4 (CZTS) semiconductor can find a promising application as wide-bandgap top cell absorber in CZTS/Silicon tandem devices. The coupling between the top and the bottom cells in a monolithic device requires the development of a proper intermediate connection able to ensure: (i) high transparency in the infrared region (ii) good electric contacts and (iii) good chemical stability under thermal treatments used for the CZTS growth, in order to prevent elements interdiffusion and silicon degradation. To this purpose, some multilayered structures based on MoS 2 and different Transparent Conductive Oxides (TCOs) were tested as intermediate connection in CZTS/Silicon tandem devices. The first working monolithic tandem cell, with open circuit voltage of about 950 mV and an efficiency of 3.5%, was obtained using a MoS 2 /FTO/ZnO trilayer structure as intermediate contact between the top and the bottom cells. Some limiting factors of this device were addressed and investigated in order to increase the tandem cell efficiency. [ABSTRACT FROM AUTHOR]

• CZTSSe absorber film deposited by spray pyrolysis using DMF solution. • The higher amount of Se leads to better device performance. • The superstrate CZTSSe solar cell device shows 3.1% power conversion efficiency. In this work, the influence of Se proportion on structural and photovoltaic characteristics of the spray pyrolysis deposited CZTSSe film, and superstrate solar cell device are reported. We observe that the sulfur to selenium ratio significantly impacts the crystallinity and concomitant electrical properties of CZTSSe films. The higher amount of Se leads to better device performance. We found that the superstrate solar cell devices with S/(S + Se) ratio of about 0.2 achieved a power conversion efficiency (PCE) of 3.1%, which is among the highest for the spray pyrolysis deposited solid state CZTSSe solar cells of superstrate architecture. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • CBTSSe/CdS/ACZTSe HBT architecture-based solar cell is proposed. • The maximum efficiency is found to be 21.63% including recombination mechanisms. • The optimized thicknesses of CBTSSe, CdS and ACZTSe layers are estimated. • The requirement of minority carrier diffusion length is reduced. • The proposed configuration utilizes the whole solar spectrum efficiently. • Efficiency comparable to dual-junction tandem cell is reported. Abstract We propose a heterojunction bipolar transistor (HBT) architecture for solar cell using earth-abundant CBTSSe, CdS and ACZTSe materials. The proposed CBTSSe/CdS/ACZTSe structure has a pnp transistor architecture where high band gaps CBTSSe/CdS form the emitter/base junction and ACZTSe acts as low bandgap collector. Using optoelectronic simulations, we investigate the performance of the proposed solar cell and optimize different layer thicknesses to maximize its efficiency. The optimized efficiency of the designed solar cell is found to be 21.63% with 735 mV open circuit voltage, 38.39 mAcm−2 short circuit current and 76.7% fill factor including Shockley–Read–Hall (SRH), surface and radiative recombination mechanisms. The optimized thicknesses of CBTSSe, CdS and ACZTSe layers are calculated to be 300 nm, 50 nm and 800 nm, respectively. The significance of this HBT solar cell lies in its simplicity of the structure that doesn't require tunneling junction and reduces the number of layers to achieve efficiency comparable to dual junction kesterite solar cells. The design and analysis presented in this article would help in achieving highly efficient inorganic solar cells at low cost and less fabrication complexities. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Facile growth of phase pure CZTS films from non-toxic solutions is reported. • Reaction pathway leading to suppression of secondary phases and the formation of single phase CZTS has been established. • Role of the chelating agent has been examined. • Prepared films show >275% improvement over dark current at 6 V bias upon white light illumination. Abstract Non-toxic solution based approaches need to suppress the formation of secondary phases in the growth of Cu 2 ZnSnS 4 (CZTS) thin films, which otherwise detrimentally affect the device performance especially by limiting the open circuit voltage. While growing CZTS films from ethanol based solutions containing common metal salts, we have conclusively established the reaction pathway leading to the formation of the single phase kesterite CZTS from the precursor solution via intermediate solid state binary and ternary compounds during deposition and the subsequent sulfurization treatment. Differently with the previous reports, we found that the presence of monoethanolamine (MEA) - a routinely used complexing agent - in the initial stage of solution mixing, inhibits complete reduction (oxidation) of Cu2+ (Sn2+), which subsequently affected the reaction pathway resulting in undesirable secondary phases. The activation energy for the decomposition of the precursor complex was found to be smaller, surmounted in the early formation of the single phase CZTS in the absence of MEA. Having a bandgap of 1.44 eV, these phase pure CZTS films exhibited very high photoresponse behavior (about 275% change in current upon white light illumination at a bias of 6 V). [ABSTRACT FROM AUTHOR]

Highlights • Effect of MoS 2 interfacial layer between CZTS and Mo back contact is studied. • Simulation wxAMPS-1D is used by varying electronic parameters of p and n-MoS 2. • CZTS device underperforms without any MoS 2 due to high back contact barrier. • Transition in efficiency is noticed when carrier concentration exceeds 1016 cm−3. • MoS 2 with lower affinity and band gap is ideal to induce desirable band alignment. Abstract In this study, the effects of transition metal dichalcogenide, MoS 2 interfacial layer formation between the Cu 2 ZnSnS 4 (CZTS) absorber layer and Mo back contact in a conventional CZTS thin film solar cell (TFSC) structure have been studied by numerical simulation using wxAMPS-1D software. The goal of this study is to elucidate the effects of both n and p-type MoS 2 on the overall CZTS solar cell's performance from the viewpoint of metal-semiconductor junction and heterojunction band alignment. Interestingly, CZTS device, regardless of p or n-type MoS 2 largely outperforms device without any MoS 2 due to lower back contact barrier value. Significant transition in efficiency is noticed when acceptor (increases efficiency) or donor (decreases efficiency) concentration has a transition from 1016 cm−3 to higher concentration of 1018 cm−3 or more. Also, effect of variable electron affinity and band gap of MoS 2 has been discussed from band alignment perspective. Generally, MoS 2 layer with lower electron affinity and band gap is preferred to induce desirable band alignment and subsequently result in higher efficiency. All-in all, the formation of p-type MoS 2 in CZTS solar cells can be tuned to improve the cell performance mainly by doping with higher acceptor doping concentration and limiting layer thickness. However, the detrimental effect of n-MoS 2 can be prevented by maintaining thinner layer in the vicinity of ∼30 nm with low to moderate donor doping (<1016 cm−3). [ABSTRACT FROM AUTHOR]

Abstract Critical raw material-free Cu 2 ZnSn(S,Se) 4 photovoltaic has a potential to offer long-term, sustainable, annual deployment in the gigawatt to terra watt scale. This review article presents the current progress in the kesterite Cu 2 ZnSn(S.Se) 4 thin film photovoltaic fabrication utilizing pre-synthesised Cu 2 ZnSn(S,Se) 4 powder. Apart from the frequently reported kesterite thin film fabrication routes utilizing molecular precursor, nanoparticle and vacuum-based deposition, the kesterite thin film materials and photovoltaic fabrication has also been reported to utilise raw powder material. This alternatives route proposes to provide a highly anticipated large scale thin film photovoltaic deployment by taking advantage of the abundancy of commercial raw powder materials as sources for Cu-Zn-Sn-S-Se and of the high throughput character of solution-based film deposition technique. The powder-to-film approach possesses a typical value chain of (i) kesterite powder synthesis, (ii) powder size reduction (milling), (iii) slurry formulation, (iv) slurry deposition, and (v) sintering. Current status of each value chain is reviewed and discussed. It is shown that the presented kesterite route through raw powder utilisation is promising for further upscaling toward industrial and commercial kesterite photovoltaic fabrication. [ABSTRACT FROM AUTHOR]

Highlights • Recent work on CZTS solar cells from compound sputtered precursors is reviewed. • Studies of back contacts for CZTS should include influence on Na transport and re-optimization of the annealing process. • Variation in composition and chalcogen pressure during annealing can point to what defects are limiting performance. • For sulfide CZTS, CdS buffer layer must be replaced with a material with higher conduction band minima. Abstract Cu 2 ZnSn(S,Se) 4 (CZTS) thin film solar cells have reached efficiencies of up to 12.6% and current research is focused on understanding reasons for device limitations. At Uppsala University, a CZTS synthesis route based on compound sputtering and annealing in elemental vapors is used. Variation of chemical composition and annealing conditions is used as a tool to try to understand defect-related material- and device properties. Front and back contacts are also studied with focus on energy band matching at the hetero-interface using atomic layer deposition buffer layers and chemical stability of the back contact. In this review, we discuss recent work from our group, with reference to related work in the literature and with regards to areas for future work. [ABSTRACT FROM AUTHOR]

Highlights • Na is not detected at the CZTSe surface by XPS. • Na concentration in the grain interior (30 ppm by APT) is lower than in the film (140 ppm by ICP-MS). • Na improves efficiency, open-circuit voltage and fill factor of CZTSe solar cells. • Na induces a shallow acceptor, which significantly increases the apparent doping concentration. Abstract Sodium is typically used during the synthesis of kesterite thin films to enhance the performance of solar cells. As sodium tends to affect grain growth and morphology, it is difficult to analyse solely the electronic effects of sodium as dopant. To decouple the structural and electronic effects from each other, two processes were designed in this work to successfully incorporate sodium into a vacuum-processed Cu 2 ZnSnSe 4 absorber without changing the morphology. A thin layer of NaF is deposited before precursor deposition (Pre-NaF) or after absorber synthesis to undergo a post deposition treatment (NaF-PDT). While composition and distribution of matrix elements remain unchanged, the sodium concentration is increased upon sodium treatment up to 140 ppm as measured by inductively coupled plasma mass spectrometry. X-ray photoelectron spectroscopy showed that the surface composition was not altered. Within its detection limit, sodium was not present at the absorber surface. For a Pre-NaF sample measured with atom probe tomography a sodium concentration of 30 ppm was measured in a grain, suggesting that sodium might segregate at grain boundaries. The additional sodium content in the film leads to an increased acceptor concentration, which results in improved open-circuit voltage and fill factor. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • CZTSe are deposited combining co-electrodeposition of CZT films and RTP selenization. • CZT compositions required for PV cells are achievable with a short deposition time. • The use of RTP treatments reduces selenization time by a factor of 5, favoring cost and energy saving. • Best solar cells achieve efficiencies over 5% and reaches quantum yields of 70–80%. • Low open circuit value is attributed to the low quality of the Mo/CZTSe interface. Abstract In this work, a fast two-step process combining co-electrodeposition of a CuZnSn precursor plus a reactive selenization step with a rapid thermal annealing to synthesize Cu 2 ZnSnSe 4 thin films is presented. By tuning the electrochemical procedure is feasible to obtain with a short electrodeposition time (15 min) the precursor composition required for high efficiency solar cell devices. XRD characterization reveals that the precursor is mainly composed of binary Cu-Zn and Cu-Sn alloys that after thermal treatment completely react to form a kesterite thin film. Raman spectra and Raman mappings demonstrate that best quality kesterite films are obtained after selective chemical etching necessary to remove secondary phases, like ZnSe and SnSe, which are detrimental for solar cell performance. Best solar cells prototypes achieve efficiencies of 5% with a current density of 30.9 mA/cm2, an open circuit potential of 327 mV and a fill factor of 51.5%. The low value of open circuit voltage is attributed to the presence of voids and partial delamination observed in the Mo/Cu 2 ZnSnSe 4 interface. [ABSTRACT FROM AUTHOR]

Highlights • Surface phases of the CZTSSe were identified by XPS, Raman and ternary phase diagram. • The mechanism of how the secondary phases affect the PCE of CZTSSe solar cell was elucidated. • The PCE of CZTSSe solar cell has an increase of 26% by etching. Abstract Cu 2 ZnSn(S,Se) 4 (CZTSSe) films with smooth surface were prepared by solution approach and etched (etched-CZTSSe) through using KMnO 4 in a H 2 SO 4 based medium followed by Na 2 S. Two types of solar cells with conventional structure were fabricated with CZTSSe and etched-CZTSSe as absorber, respectively. It is demonstrated by XRD, XPS, EDS and Raman measurement that the bulk CZTSSe is of kesterite structure while its surface contains a small amount of Cu 2 ZnSn 3 Se 8 and ZnSe secondary phases besides kesterite CZTSSe. On the surface, the Cu 2 ZnSn 3 Se 8 is completely removed after etching while the ZnSe is partially. It is found that the elimination or reduction of the secondary phases increase the shunt resistance greatly, leading to a larger open-circuit voltage (V oc) which results in the improvement of power conversion efficiency (PCE) for the etched-CZTSSe-based solar cell. The PCE has a maximum increase of 26% by etching. An intensive study has been made for the influencing mechanism of etching on the PCE of solar cells. [ABSTRACT FROM AUTHOR]

Graphical abstract Highlights • Copper, Zinc and Tin sulfides were synthesized by solution method employing Na 2 S as sulfur source. • Structural, optical and morphological analysis indicated major portion of CZTS. • Secondary phases such as SnS 2 , SnS and Cu 3 SnS 4 at 350° and 450 °C. • Secondary phases vanishing significantly at 550° and 650 °C. • High absorption coefficient for CZTS in the range of 104–105 cm−1. • Electrochemical analysis supported that the CZTS samples could be used as absorber material in thinfilm solar cells. Abstract Copper, Zinc and Tin sulfides were synthesized by solution method employing Na 2 S as sulfur source. Individual sulfides were sulfurized in non-vacuum condition at 350°, 450°, 550° and 650 °C to obtain CZTS. Structural, optical and morphological analysis indicated major portion of CZTS along with secondary phases such as SnS 2 , SnS and Cu 3 SnS 4 at 350° and 450 °C whereas the secondary phases vanishing significantly at 550° and 650 °C. The optical studies indicated high absorption coefficient for CZTS in the range of 104–105 cm−1. Electrochemical analysis demonstrated that the CZTS samples prepared by the current methodology could be very well used as absorber material in thinfilm solar cells. [ABSTRACT FROM AUTHOR]

A new design for perovskite solar cell using kesterite mesostructure is introduced and simulated to enhance the performance of perovskite solar cells. The NiO and ZnO used as inorganic p-type and n-type carriers transporting layers, respectively. Plasmonic network is employed to improve the absorbance of kesterite mesoscopic perovskite solar cell. Plasmonic network which induces near field into the absorber layer and the kesterite mesostructure which enhances light absorption and hole transportation, lead to higher light confinement inside the perovskite absorber layer. They are able to broaden the absorbance spectrum and enhance the external quantum efficiency of the bare planar perovskite solar cell without any light-trapping structure by 29%. [ABSTRACT FROM AUTHOR]

The pure kesterite CZTSe absorbers are vital for CZTSe solar cells. However, the secondary phases such as Zn-Se, Cu-Se, Sn-Se and Cu-Sn-Se, which are almost harmful to CZTSe solar cells, often appear in the CZTSe absorbers. To investigate the phase transition pathways of as-sputtered Cu-Zn-Sn-Se precursors during selenization can provide a basis for the reduction of secondary phases and the preparation of high-performance CZTSe solar cells. In the work, the sputtered Cu-Zn-Sn-Se precursors were annealed in a selenium-containing atmosphere at temperatures from 200 to 580 °C for 30 min. The phase transition pathways were analyzed and presented. Low or high selenization temperatures were unfavorable for CZTSe growth. When the temperature were below 530 °C, the crystallization would be insufficient. Meanwhile, when the selenization temperature were above 560 °C, the CZTSe films would decompose during selenization and degrade rapidly. 530 °C seemed to be a relatively appropriate selenization temperature although the grain size was still not large enough at this temperature. By continuously inletting H 2 Se gas during the whole selenization process, CZTSe decomposition could be suppressed even at high temperature of 550 °C. And thus the grain size was improved and CZTSe solar cell with the efficiency of 6.78% was obtained. [ABSTRACT FROM AUTHOR]

Currently, the conversion efficiency of Cu 2 ZnSn(S, Se) 4 (CZTSSe) thin film solar cell is mainly limited by its low open-circuit voltage. In this work, the performances of CZTSSe thin film solar cell were calculated by the simulation program AFORS-HET v2.5 to reveal some useful ways to enhance the open-circuit voltage. First, several factors affecting the open-circuit voltage were analyzed. The open-circuit voltage of CZTSSe solar cell was significantly improved by reducing the defect states, interface states, parasitic resistance effect, and formation of MoS 2 layer, and adjusting the hole concentration or band gap of CZTSSe absorber. Then, the band gap gradient was introduced into the CZTSSe absorber to extend the built-in electric field and enhance absorptions, leading to benign impacts on the open-circuit voltage of CZTSSe solar cell. Finally, a back surface field was considered for the CZTSSe solar cell. The effects of CZTSSe and amorphous silicon back surface fields depended on the defect states. The advantage of back surface field was prominent when the defect density was low. It was necessary to reduce the defects before considering a back surface field. In the calculations, the best CZTSSe solar cell had an open-circuit voltage deficit of only 0.134 V. The numerical analyses in this work provide some methods to improve the open-circuit voltage and conversion efficiency CZTSSe thin film solar cell. The proposed methods offer guidance for experimental works. [ABSTRACT FROM AUTHOR]

• Ag 2 BeSnX 4 (S, Se,Te)-based kesterite solar cell modeling: a D F T investigation and S c a p s - 1 d analysis. • The values of the absorption coefficient are remarkably high of the order (104 cm−1) for the 3 structure. • The effects of various variables, such as layer thickness and acceptor concentrations, were studied. • The efficiencies of the three cells are 18.271%, 25.152%, 14.628% for Ag 2 BeSnS 4 , Ag 2 BeSnSe 4 , Ag 2 BeSnTe 4 , respectively. • Intriguing photovoltaic measurements are provided by the study of two-terminal monolithic solar cells Ag 2 BeSnS 4 / Ag 2 BeSnTe 4 , including a η of 19.41% and superb FF. In the present work, computational analysis based on density functional theory is carried out for the optical properties of new kesterite materials Ag 2 BeSnX 4 (S, Se, Te). We used a generalized gradient approximation (GGA) function. On the three structures studied here, the absorption coefficient values are remarkably high of order (104 cm−1). For photovoltaic applications, Ag 2 BeSnX 4 (S, Se, Te) has emerged as a viable choice. The Scaps is used to simulate the Mo / I T O / Ag 2 B e S n X 4 (S , S e , T e) / C d S solar cell. On the Ag 2 BeSnX 4 (S, Se, Te) layer, the effects of various variables, such as layer thickness and acceptor concentrations, were investigated. Simulation results reveal that the optimum thickness and concentration of the absorption layer in Ag 2 B e S n X 4 (S , S e , T e) -based solar cells are 1.25, 1.00, 0.75 µm 1017, 1018, 1018 cm−3, for Ag 2 BeSnS 4 , Ag 2 BeSnSe 4 , Ag 2 BeSnTe 4 respectively. The efficiencies of the three cells equal to 18.271 %, 25.152 %, 14.628 % for Ag 2 BeSnS 4 , Ag 2 BeSnSe 4 , Ag 2 BeSnTe 4 respectively. Intriguing photovoltaic measurements are provided by the investigation of two-terminal monolithic solar cells, including a η of 19.41 % and a superb FF. We hope that our theoretical findings will inspire researchers working on photovoltaic systems to experimentally materialize these materials. [ABSTRACT FROM AUTHOR]

The electronic and optical properties of Cu 2 NiGeS 4 (CNGS) are examined using the first-principle DFT calculations. A unique mBJ + U potential method is used for the band gap energy calculation of CNGS. With a remarkably high absorption coefficient (10 4 cm − 1), CNGS has become a promising candidate for photovoltaic applications. SCAPS-1D tool is used to simulate a thin-film solar cell with a Mo/MoS 2 /Cu 2 NiGeS 4 (CNGS)/CdS/ZnO/ZnO:Al structure. The impact of various factors, such as layer thickness, donor and acceptor concentrations, and defect density on the CNGS layer was explored. This study also explores the combination of suitable buffer layers (such as CdS, ZnS, and their alloy Cd 1 − x Zn x S), along with different doping concentrations and thicknesses, to be used as suitable buffer layers in the CNGS solar cell. The simulation outcomes suggest that the optimal thickness for the absorption layer in CNGS solar cells is between 2000 and 2400 nm, while the ideal thickness for MoS 2 is 100 nm. The buffer layer should be between 20 and 50 nm. Keeping the defect density of CNGS below 10 14 cm − 3 is crucial for high efficiency. The optimized results yield an efficiency conversion rate of 20.05%, a 66.77% fill factor, a short-circuit current of 29.67 mA/cm 2 and an open-circuit voltage of 0.983 V. • Determining electronic and optical properties of CNGS. • Optimizing thickness, doping, and defects in CNGS material. • Investigating the effect of different buffer layers. • The optimized CNGS-based solar cell exhibits an efficiency of 20.05% [ABSTRACT FROM AUTHOR]