Hole-Selective Contact Optimization for Silicon Heterojunction Solar Cells: The Role of Doping and Thickness of a Boron-Doped a-Si:H Layer

Abstract

In silicon heterojunction (SHJ) solar cell fabrication, the hole-selective layer is crucial in minimizing the contact resistivity (ρ_c) along with charge carrier recombination. This study shows the effect of varying the diborane doping and thickness of p-a-Si:H in reducing the series resistance (R_s) and enhancing the fill factor (FF) of SHJ cells. Experimental results indicate the criticality of an optimal diborane doping of ∼3%, supported by a dark conductivity of ∼2 × 10^–5 S/cm. A lower hole-selective (i-a-Si:H/p-a-Si:H/ITO) ρ_c of ∼432 mΩ·cm² was observed after p-a-Si:H layer optimization due to proper c-Si band bending, facilitating an effective carrier transport. Furthermore, a comprehensive analysis of R_s components identifies the hole-selective contact as the primary contributor, responsible for ∼49% of the device’s total R_s of ∼860 mΩ·cm². The carrier transport was quantified by evaluating the light R_s with the dark R_s correction at different p-layer thicknesses. This has led to a front emitter SHJ cell’s open-circuit voltage (V_oc) of ∼725 mV, an FF of ∼79.20%, and a power conversion efficiency of ∼22.16%. Simulations using TCAD explained the effects of p-layer doping (work function) and thickness on carrier screening length, energy band bending at the device interfaces, and V_oc and efficiency of the SHJ cell, providing a theoretical basis for experimental observations.