N. A. Khan

Prospects of Zinc Sulphide as an alternative buffer layer for CZTS solar cells from numerical analysis

Zinc Sulphide (ZnS) is a promising candidate to be an alternative buffer layer to the commonly used cadmium sulphide (CdS) in CZTS solar cells. In this study, buffer layer parameters like layer thickness and buffer layer bandgap have been investigated by Analysis of Microelectronic and Photonic Structures (AMPS-1D) to find out the higher conversion efficiency. A promising result has been achieved with an efficiency of 14.49% (with Voc = 0.81 V, Jsc = 28.85 mA/cm2 and Fill factor = 67.5) by using ZnS as a buffer layer. It is also found that the high efficiency of CZTS absorber layer thickness is between 2 μm and 4 μm. From the simulation results, it is revealed that higher efficiency can be achieved for the buffer layer bandgap around 3.10 eV - 3.25 eV. This result can be explained by the practical work as the bandgap of ZnS is largely dependent on the preparation conditions and stoichiometry. In conclusion, numerous influences of buffer layer are investigated in CZTS solar cell that can lead to the fabrication of high efficiency devices.

Design optimization of CdTe thin film solar cells from numerical analysis

In this paper, a modified structure for CdTe thin film solar cell was proposed by numerical analysis with an addition of a novel ZnO buffer to improve the conversion efficiency. The CdS window layer was reduced to 50 nm together with the insertion of zinc oxide (ZnO) as the buffer layer to prevent forward leakage current. The thickness of CdTe absorber layer was varied from 1000 nm to 5000 nm and as well as the operating temperature was also varied from 25°C to 165°C. The numerical simulation was done by Analysis of Microelectronic and Photonic Structures (AMPS-1D) simulator. The highest conversion efficiency obtained was 20.27% (Voc = 1.08 V, Jsc = 24.93 mA/cm2, FF = 0.83) with 5000 nm CdTe absorber layer and 50 nm CdS window layer. Moreover it was observed that with the increase in operating temperature, the normalized efficiency decreased linearly at a gradient of 0.2%/°C, which reveals that the CdTe thin film solar cell has higher stability.

Prospects of novel CdZnTe thin film solar cells from numerical analysis

The efficiency of CdTe based solar cell can be increased using ternary CdZnTe material as absorber layer. Cd1-xZnxTe has tunable bandgap depending on the composition. In this work the bandgap of CdZnTe layer (1.57 eV) which is in the optimum range, can be achieved with Zn composition of x=0.1. First the carrier concentration of absorber layer in the baseline case is increased then the thicknesses of absorber layer and window layer in the conventional baseline case are reduced and optimized. Finally an optimized cell structure is proposed. After optimization, the total thickness of the baseline case cell is reduced by factor four and results high efficiency. The cell structure in both baseline case and modified cell is: (SnO2/CdS/CdZnTe/Back Contact), however some material parameters are different. The performance parameters are found better in the optimized cell structure. We also investigated the effect of ZnO buffer layer and the operating temperature on the performance parameters.

Influence of thermal annealing on CdTe thin film deposited by thermal evaporation technique

Effects of thermal annealing on the structural and optical properties of thermally evaporated CdTe thin films are presented here. Thin films of CdTe were deposited on soda lime glass substrates at room temperature by varying the deposition current ranging from 25 Amperes to 30 Amperes by thermal evaporation. The grown samples were annealed at the annealing temperature of 400°C for 15 minutes in a vacuum furnace of nitrogen ambient with pressure of 250–300 mTorr. The structural, optical and electrical properties of the grown samples were investigated by XRD, AFM and UV-VIS spectrometry. The as-deposited films prepared at deposition current 25A shows polycrystalline nature whereas the films prepared at 28A and 30A exhibit cubic crystallinity with (111) preferential orientation around 2θ=23.8°. The surface roughness of the films is also highly affected by the thermal annealing as observed from the AFM images. The band gap has been found around 1.5 eV for the as-deposited film whereas the band gap decreased approximately to 1.4 eV after thermal annealing treatment.

Structural and electrical characteristics of room temperature sputtered ZnO

Thin films of ZnO were deposited on cleaned soda lime glass substrates by using RF magnetron sputtering technique for deposition time of 20mins, 40mins, 60mins, 80mins and 100mins at room temperature. The structural and electrical properties of ZnO thin films were obtained from X-ray diffraction (XRD), Atomic force microscopy (AFM) and Hall Effect measurement. XRD results confirm the presence of (002) crystal orientation of ZnO and improvement of grain size was observed for films with higher deposition time. The average and the R.M.S roughness have been measured from the AFM images and the Hall Effect measurement confirms the carrier concentration, resistivity and mobility. Hall Effect data revealed that the mobility of the carriers of ZnO thin films increased with higher deposition time.

Investigation of the annealing time effects on Cu deposited CdTe thin films for photovoltaic application

Firstly, Cadmium Telluride (CdTe) thin films have been deposited on cleaned soda lime glass substrates at 300°C by using the RF magnetron sputtering technique. After that, Cu thin film was deposited for 5 minutes at 200°C on top of CdCl₂ treated CdTe thin films by sputtering. Subsequently, CdTe and Cu stacks were annealed at 400°C for 15 minutes, 20 minutes and 25 minutes in a vacuum furnace. The influence of different annealing times on the structural, topographical and electrical properties of Cu sputtered CdTe thin films were then examined by XRD, AFM and Hall Effect measurement, respectively. XRD patterns reveal that, one CdTe peak corresponding to the (111)cub reflection planes at 2θ=23.8^o and another low intensity Cu₂Te peak representing (200)^hex hexagonal reflection planes at around 2θ=24.8^o were found for all the annealing times. Surface roughness and topography were viewed from the AFM images. Noteworthy changes were observed in the films surface roughness due to the different annealing times. The surface roughness values imply rising trend for lower annealing times. Bulk carrier density was in the order of 10¹⁸cm^-3. The highest carrier concentration of 7.1×10¹⁸cm^-3 was achieved for the films annealed for 15 min.