Abstract
The effect of p-layer and i-layer characteristics such as thickness and doping concentration on the electrical behaviors of the a-Si:H/a-SiGe:H thin film heterostructure solar cells such as electric field, photogeneration rate, and recombination rate through the cell is investigated. Introducing Ge atoms to the Si lattice in Si-based solar cells is an effective approach in improving their characteristics. In particular, current density of the cell can be enhanced without deteriorating its open-circuit voltage. Optimization shows that for an appropriate Ge concentration, the efficiency of a-Si:H/a-SiGe solar cell is improved by about 6% compared with the traditional a-Si:H solar cell. This work presents a novel numerical evaluation and optimization of amorphous silicon double-junction (a-Si:H/a-SiGe:H) thin film solar cells and focuses on optimization of a-SiGe:H midgap single-junction solar cell based on the optimization of the doping concentration of the p-layer, thicknesses of the p-layer and i-layer, and Ge content in the film. Maximum efficiency of 23.5%, with short-circuit current density of 267 A/m2and open-circuit voltage of 1.13 V for double-junction solar cell has been achieved.
Highlights
Hydrogenated amorphous silicon-germanium alloys are widely used in multijunction solar cells, where their main advantage is the capability of shifting the optical band gap to lower energies by increasing the germanium concentration in the film [1,2,3]
The band offset of EV and EC at the aSi/SiGe heterointerface is dependent on the Ge concentration in the film
In addition to less surface recombination velocity, the smaller band gap (Eg) in high Ge concentration SiGe film has a larger light absorption coefficient, which can lead to more electron-hole pair generation and higher JSC
Summary
Hydrogenated amorphous silicon-germanium alloys (aSiGe:H) are widely used in multijunction solar cells, where their main advantage is the capability of shifting the optical band gap to lower energies by increasing the germanium concentration in the film [1,2,3]. In order to avoid drastic reduction of the short-circuit current (JSC) in a cell with small thickness, it is necessary to increase absorbance of the material and optimize the cell structure [4]. In addition to less surface recombination velocity, the smaller band gap (Eg) in high Ge concentration SiGe film has a larger light absorption coefficient, which can lead to more electron-hole pair generation and higher JSC. Both of them can result in higher cell efficiency of the a-SiGe-based solar cell [4].
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