Analysis of Tunneling Current in Heterojunction with Intrinsic Thin Layer (HIT) Solar Cells

Abstract

Heterojunction solar cells have proved to exceed the efficiency barrier that homojunction solar cells have tried to cross for decades. Among the most efficient heterojunction solar cell structures, the Heterojunction with Intrinsic Thin Layer or HIT is the most popular. The simplest form of HIT structure is: Amorphous P-type Semiconductor/Intrinsic Semiconductor/Crystalline Ntype semiconductor, or vice versa. In a heterojunction, due to the difference in bandgap of the materials, a potential barrier forms that creates obstacle for the electrons/holes to travel from one side to the other. Such trapping of electrons/holes reduces the efficiency of the solar cell. However, a fraction of these electrons/holes pass the potential barrier through Quantum Tunneling and create a tunneling current, which contributes to the overall current; hence improving the efficiency. In this document, the tunneling current of a P-type a-Si/I-Si/N-type c-Si HIT structure is studied. The length of the P-type amorphous layer, intrinsic layer and the N-type crystalline layer is varied to study how the thickness of each layer impacts the tunneling current. Also, the impact of doping densities of the P-side and N-side on the tunneling current is analyzed. Varying these parameters (thickness and doping) an optimized combination of a simple HIT structure is proposed for which the tunneling current is maximum. By analyzing the effects of thickness and doping densities, it is seen that the maximum tunneling current is observed for5 nm P-side thickness and 4 nm intrinsic thickness; with doping density 1019cm-3 at P-side and 1013 cm-3 at N-side.