Performance Analysis of Ultra-Thin Silicon Based Tunnel Junctions for Tandem Solar Cell Applications

Abstract

Nowadays silicon solar cells have an efficiency of up to 20 % and in order to increase the efficiency of them, fabrication of multi-junction thin film solar cells with different band gaps is one of the most promising approaches. The silicon based tandem solar cells are third generation new style solar cells with ultra-high efficiency. The sub-cells in a tandem solar cell have different energy band gaps. In order to match the currents between sub-cells, tunnel junctions are used to connect the sub-cells. This work will concentrate on simulating the tunnel junction for application as part of multi-junction solar cell. In this way dopant concentration is changed and the tunnel junction current-voltage characteristics and their Energy band diagram in different dopant levels under equilibrium condition for moderate and usual doping have been calculated. An n++-Si/p++-Si tunnel junction is selected to simulate the overall characteristics of cell by numerical finite element method. We have simulated a symmetric silicon tunnel junction with thickness of 25 nm for n-type and 25 nm for p-type silicon by changing doping value from 1 × 10e20 cm−3 to 2 × 10e20 cm−3. The simulation results show that the doping concentration of 2 × 10e20 cm−3 is suitable for both sides.