KEYWORDS: Photovoltaics, Scanning electron microscopy, FT-IR spectroscopy, X-ray diffraction, Spectroscopy, Solar energy, Solar cells, Photoluminescence
Non-toxic, low cost, and environment-friendly solar absorber materials is a great subject of interest for research community for photovoltaic applications. The preparation of these solar cell materials by adopting a greener synthesis route still remains a challenge. Herein we present a surfactant free, cheaper, non-toxic and highly scalable one pot hydrothermal synthesis of Cu2ZnSnS4 (CZTS) ink for its efficacy in photovoltaic application. The properties of CZTS ink were analyzed by utilizing Photoluminescence (PL), Fourier Transform Infrared (FTIR) spectroscopy, Scanning Electron Microscopy (SEM), and X-ray diffraction (XRD) characterization techniques. The highly pure kesterite structure of CZTS was confirmed by the diffraction peaks observed using XRD. The calculated lattice parameters and from the XRD data are 0.54 nm and 1.09 nm respectively having a full width half maxima (FWHM) of 0.81 corresponding to the highly intense diffraction peak (112). The recorded PL spectra having intense PL main peak at 823 nm confirms the highly luminescent behavior of our synthesized CZTS ink. The formation of a very good surface morphology has been confirmed by SEM image. The observed FTIR peaks at 1456 cm -1 and 712 cm -1 govern the existence of functional groups C-H and C-C bending vibration respectively. This interesting study shows the ability to synthesize surfactant free, low cost and high performance CZTS ink for its utility in photovoltaic domain.
Kesterite solar cells require a novel high-research implementation to replace the costlier Copper Indium Gallium Selenide (CIGS) solar cells. This study, attempts to demonstrate the performance improvement of kesterite solar cells using multiple quantum wells (MQWs). A numerical simulation approach using Atlas software from Silvaco is used. Firstly, a baseline model of the best performing Cu2ZnSnS4(CZTS) solar cell Mo/CZTS/CdS/i-ZnO/ITO with 11% power conversion efficiency (PCE) is implemented. Further, to exploit the use of MQWs, Cu2ZnSn(SxSe1-x)4 (CZTSSe) with 40% sulfur content is added as well material in a series of wells while keeping the CZTS as the barrier material. This structural modification facilitates the absorption of lower energy photons by the lower bandgap well material. Further, MQW induced quantized energy levels and higher electric fields help to increase the carrier collection, thereby increasing the solar cell's short circuit current density (Jsc) and overall power conversion efficiency (PCE). A detailed study on the effect of well and barrier thickness on the solar cell performance is done, and a well thickness of 5 nm and a barrier thickness of 10 nm was chosen for further optimization. The number of wells is also optimized to 70, which results in the highest performance of the solar cell. This structural modification and optimization remarkably improved Jsc by 48.76% (rel.) and PCE by 34.72% (rel.) compared to solar cells without nanostructures. Moreover, with an optimized structure, an external quantum efficiency (EQE) of over 95% is achieved with the optimized structure.
In this study, the author proposed a new technique for strain minimization, called linear alloy technique (LAT), for the symmetric dot-in-a-well (DWELL) heterostructure. Here, three different DWELL InAs QDs heterostructures with 6 nm thick InxGa1-xAs as well material have been simulated using 8 band k.p. model-based Nextnano software. Here, the first sample is analog alloyed DWELL heterostructure having In0.15Ga0.85As well (Sample A), the second sample is digital alloyed DWELL heterostructure where the well layer is divided into three sub-layers of 2nm thickness with indium composition varied from 45% to 15% in the step of 15%(Sample D), and the third sample is linear alloyed DWELL heterostructure where indium composition is varied from 45% to 15% (Sample L) in linear fashion have been studied. The Lower the magnitude of hydrostatic strain better will be carrier confinement. The more the biaxial strain, the more the heavy-hole and light-hole band splitting, which reduces the transition energy gap. The computed biaxial strain is increased by 1.52% and 2.21%, and the magnitude of hydrostatic strain is reduced by 3.66% and 1.13% in sample Lcompared with samples A and D, respectively. Strain inside the well layer of sample L reduces more smoothly than samples A and D, respectively. The computed PL emission wavelength for all three samples are 1329, 1418, and 1419 nm for the samples A, D, and L, respectively. Hence, this proposed technique can be the best choice for fabricating future optoelectronicbased devices.
In this study, we minimize the strain by using the new technique called linear alloy technique (LAT) for the Stranski-Krastanov (SK) quantum dot heterostructure. Here, three different SK InAs QDs heterostructures with 6 nm thick capping layer (CL) having InxGa1-xAs as capping material have been simulated using the 8-band k.p. model-based Nextnano software. Here, the first sample is analog alloyed SK QDs heterostructure having In0.15Ga0.85As capping (Sample A1), the second sample is digital alloyed SK QDs heterostructure where CL is divided into three sub-layers each of 2nm thickness with indium composition varied from 45-30-15% (Sample D1), and the third sample is linear alloyed SK QDs heterostructure where indium composition is varied from 45% to 15% (Sample L1) in a linear fashion, have been studied. The biaxial and hydrostatic strain is computed for all three heterostructures and compared. The biaxial strain is improved by 2.03% and 2.0%, and hydrostatic strain is reduced by 3.49% and 0.071% inside the QD region of sample L1 compared with samples A1 and D1, respectively. Additionally, digital sample D1 offers a step-wise strain reduction inside CL compared to analog sample A1. However, sample L1 offers an even more relaxed strain inside CL than samples A1 and D1, respectively. The PL emission wavelength is observed at 1317, 1372, and 1379 nm for samples A1, D1, and L1, respectively. Hence the linear alloy technique is useful for making future optoelectronic devices where strain reduction is the main factor.
Cadmium Sulphide(CdS) has been the most preferred n-type buffer layer and Indium Tin Oxide(ITO) is the popular window layer in kesterite solar cells. Cadmium being toxic and Indium being a rare earth element, continuous efforts are being made to replace these materials from kesterite solar cells structure. In this work, ZnS, ZnSe, and Zn0.8Sn0.2O are considered as possible alternatives for CdS. Similarly, Aluminium doped Zinc Oxide(AZO) is considered as an alternative for ITO. Firstly, a cell model with CdS and ITO (Mo/CZTSSe/CdS/ZnO/ITO) is developed using SCAPS-1D software. To optimise the performance parameters namely open-circuit voltage(Voc), short-circuit current density(Jsc), fill factor(FF), and the power conversion efficiency (PCE) for irradiation under normal working conditions, thickness and the composition ratio of the absorber layer(CZTSSe) are evaluated through numerical simulations. PCE of 14.51% is achieved for a 40% of Sulphur content and 2 um thickness of Cu2ZnSn(SxSe1-x)4 when CdS is used as the buffer layer. For the same structure, replacing ITO with AZO results in a PCE of 14.62%. Use of Cadmium-free buffer layers ZnS, ZnSe, and Zn0.82Sn0.18O with ITO as window layer result in PCE of 13.98%, 14.28%, and 14.53%, respectively. For the Cadmium-free buffer layers, an improvement in PCE is achieved when ITO is replaced by AZO, with the highest being 14.62% for Zn0.82Sn0.18O. This can be attributed to the smaller conduction band offset, which reduces the recombination of photogenerated carriers and improves the carrier transport in the solar cell. The above results indicate that the Zn0.8Sn0.2O and AZO can be potential candidates for the buffer layer and window layer, respectively, for high-performance and cheap kesterite solar cells.
This work reports the performance improvement of the CZTSSe solar cell by using a back surface field (BSF) layer between the back contact and absorber layer. Firstly, a cell model with Cadmium (Cd) free buffer structure (Mo/CZTSSe/Zn(O, S)/ZnO/ITO) is developed using SCAPS-1D software. To improve the performance, thickness and composition ratio of the absorber (CZTSSe) and buffer (Zn(O, S)) layer are optimized through simulations. The efficiency of 14.39% is achieved for a Sulphur content of 40% and 70% in CZT(SxSe1-x)4 and Zn(O1-x Sx) respectively. Further performance improvement is attempted by using a back surface field (BSF) layer between the back contact and the CZTSSe absorber layer. The P+-MoSe2, P+ - Si0.75Ge0.25, and SnSe layers are used as BSF layers to investigate their effects on performance improvement. Inclusion of the BSF layer gives further scope for optimization of the absorber layer thickness. It is observed that the use of SnSe as a BSF layer produces maximum power conversion efficiency of 17%. These findings will be helpful for the research community working in the area of high-performance and low-cost CZTSSe based solar cells.
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