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

In silicon heterojunction (SHJ) solar cell fabrication, the hole-selective layer is crucial in minimizing the contact resistivity (ρc) 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 (Rs) 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) ρc 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 Rs components identifies the hole-selective contact as the primary contributor, responsible for ∼49% of the device’s total Rs of ∼860 mΩ·cm2. The carrier transport was quantified by evaluating the light Rs with the dark Rs correction at different p-layer thicknesses. This has led to a front emitter SHJ cell’s open-circuit voltage (Voc) 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 Voc and efficiency of the SHJ cell, providing a theoretical basis for experimental observations.