Performance Enhancement by the Introduction of Additional Narrow Band Gap Bottom Layer of aSi0.64Ge0.36: H on Proposed p + aSi:H/i-aSi: H/n+ aSi0.73Ge0.27: H Thin Film Solar Cells

This work is the continuation of our previous work entitled "Investigations on optical, material and electrical properties of aSi:H and aSiGe:H in making proposed n+aSi:H/iaSi:H/p+aSiGe:H graded band gap solar cells." In this work, we present an additional bottom layer made of increased germanium content: aSi0.64Ge0.36:H to the previously recommended p+aSi:H/i-aSi:H/n+aSi0.73Ge0.27:H photovoltaic cell to strengthen the absorption spectrum and thereby boosting the attainment of the solar cell. Moreover, the overall active layer thickness is reduced from 430 nm of previous work to 395 nm of proposed work. This work includes the fabrication of samples of epitaxially grown aSiGe:H thin films of varying band gap made with Plasma Enhanced Chemical Vapour Deposition (PECVD) technique succeeded by their characterisation. The establishment of band gap tailoring by varying the germane (GeH4) gas flow rate is thoroughly investigated through optical characterisation. The growth chemistry of PECVD made aSi0.64Ge0.36:H layer has been analysed and the presence of respective radicals has been verified using Fourier Transform Infra Red (FTIR) spectroscopy. In accordance with the measured band gaps, p+ aSi:H/i-aSi:H/n+aSi0.73Ge0.27:H/naSi0.64Ge0.36:H solar cell has been proposed. A comprehensive inquiry on optimisation of the recommended structure has been made by varying the optical band gap and thickness of the bottom most aSi0.64Ge0.36:H layer of the structure. All the cell parameters including open circuit voltage (Voc), short circuit current density (Jsc), maximum power point voltage (Vm), maximum power point current density (Jm), Fill factor (FF) and conversion efficiency (η) has been calculated using SCAPS1D solar simulator. Furthermore, C–V characteristics and Mott-Schottky plot of the proposed structure has been evaluated. The introduction of narrow band gap amorphous silicon germanium (aSi0.64Ge0.36:H) at the bottom has remarkably enhanced Jsc and η to 15.54 mA/cm2 and 15.15% respectively, which is better compared to reported amorphous silicon photovoltaic cells having single junction.