Dixit, Ambesh

Two-dimensional layered transition metal dichalcogenide exhibit important characteristics such as suitable bandgap, high absorption coefficient, and favourable electron transport properties for their uses in nano-electronic such as ultrathin solar cells. We adopted a hybrid simulation approach, where density functional calculations are performed for optoelectronic properties of semiconductor materials and macroscopic device simulation is carried out to evaluate photovoltaic response. We investigated electronic and optical properties of bulk WS2, WSe2 and noticed very high absorption coefficient, making them suitable absorber materials for solar cell. Further, the electronic and optical properties of an ultrathin WSSe Janus layer are investigated using density functional theory and noticed low reflectance and high bandgap, supporting its usefulness as a buffer layer for W(S/Se)2 absorbers. The computed density functional results for W(S/Se)2 and Janus WSSe are used to simulate the photovoltaic response of WSSe/W(S/Se)2 solar cell using macroscopic device simulation. The photovoltaic performance of a single junction solar cell is optimized for W(S/Se)2 absorbers and WSSe Janus buffer materials. The effect of absorber layer thickness, carrier concentration, and contact work function is evaluated to understand the solar cell performance. We noticed that interface recombination speed between absorber and buffer layer and minority carrier lifetime are affecting the solar cell performance. The maximum efficiency of about ~17.73% and 18.87% is noticed for optimized WSSe/WS2 and WSSe/WSe2 solar cell. The present study will provide a new approach to design, develop, and optimize a solar cell and evaluate the impact of different materials parameters on solar cell performance.

We investigated a single heterojunction solar cell with stannite phase Cu2HgSnS4 as a new potential absorber material using the hybrid density functional theory for materials parameter and macroscopic device simulation studies for the photovoltaic response. The lattice constants are optimized using PBEsol and GGA exchange–correlation approach with mBJ parameterization and considering spin–orbit coupling effect on heavy element Hg, while strong correlation of Cu and Hg 3d electrons are taken into account by Hubbard parameter U = 0.52 Ry. The computed bandgap is ~1.33 eV. The effective mass of electrons and holes in the respective band edges (electron in conduction band and the hole in the valence band) is 0.25 m0 and 0.91 m0, respectively. Further, the computed materials optoelectronic parameters are used to optimize the device performance by introducing a variation in minority carrier lifetime, defect concentration in Cu2HgSnS4 absorber, Cu2HgSnS4/CdS interface, and the absorber thickness to achieve a realistic photovoltaic response. The optimal conversion efficiency is 11.6% after taking more realistic parameters in the considered single-junction solar cell. However, the maximum photovoltaic response >17% can be achieved by controlling absorber and interface defects together with optimal carrier concentration.

Quantum Dot Sensitized Solar Cells are considered as the potential third generation solar cells due to their suitable optoelectronic properties for photovoltaic response. The possibility of size and composition tunability makes quantum dots as relevant absorber materials to match the wider solar spectrum more efficiently. In conjunction, the possibility of multiple electron-hole pair generations at the cost of single photon i.e. multiple carrier generation is showing potential to overcome the theoretical single junction power conversion efficiency limitations. Quantum dot sensitized solar cells are showing power conversion efficiencies up to 12%, very close to its counterpart dye sensitized solar cells. However, QDSSCs efficiencies are still lagging behind the conventional solid state single junction solar cells. In this review, we will discuss the initial evolution of quantum dot sensitized solar cells with their microscopic working principles. The review will also address development of key building blocks and factors such as various interfaces in QDSSCs, carrier transport and recombination across different interfaces, affecting the power conversion efficiency. Further, fundamental concepts of carrier multiplication and possible theoretical models for multiple exciton generation are discussed towards their impact on the power conversion efficiencies of quantum dot sensitized solar cells.

The Quaternary Chalcogenide material Cu2ZnSn(S,Se)4 showed all the optimum material properties suitable for photovoltaic application. Yet, the current development has slowed down due to band-tailing issues and challenges to overcome. It causes the potential fluctuation for conduction band minima and valence band maxima. We introduced large ionic radii Sr in place of Zn to address the band tailing issue and demonstrated that Cu2SrSnS4 (CSTS) material is a promising alternative of Cu2ZnSn(S,Se)4 for solar cell application using a hybrid computational approach. The structural and optoelectronic properties of Cu2SrSnS4 are computed using the density functional approach. The direct bandgap of Cu2SrSnS4 of ~ 1.78 eV and significant absorption coefficient in the desired spectral range makes it a suitable absorber material for heterojunction solar cell. The computed materials properties are used to investigate the single junction photovoltaic device performance by introducing the realistic defect densities, recombination rate, electron affinity, and back electrode work function with two different buffer layers (CdS and ZnS). The devices, i.e., AZO/ZnO/CdS/CSTS/Mo and AZO/ZnO/ZnS/CSTS/Mo showed > 17.71% and > 20.12% photoconversion efficiency under optimized conditions. These devices exhibit nearly identical Jsc, whereas the device with ZnS buffer layer showed relatively larger Voc. Further, graphene as the ETL layer is evaluated and showed possible alternative to the conventional AZnO/ZO layers. This study shows the potential of Cu2SrSnS4 (CSTS) for an alternative absorber-based single heterojunction photovoltaic device with a large efficiency.

Mössbauer analysis is carried out on CZTS samples, subjected to a low temperature processing at 300 °C (S1) and high temperature processing at 550 °C under sulfur environment (S2). Loss of Sn is observed in sample S2 due to high temperature thermal treatment. The isomer shifts obtained in the Mössbauer spectra confirms the existence of Sn at its 4 + valence state in both the samples. Relatively high quadriple splitting value is observed in S1 with respect to S2, suggesting dislocations and crystal distortion present in S1, which are reduced drastically by high temperature annealed S2 sample. The fabricated solar cell with S1 and S2 absorbers showed significant improvement in efficiency from ∼0.145% to ∼1%. The presence of excess Sn in S1 allows enhanced recombination and the diode ideality factor shows larger value of 4.23 compared to 2.17 in case of S2. The experiments also validate the fact that S1 with Sn rich configuration shows lower acceptor carrier concentration as compared to S2 because of enhanced compensating defects in S1.