Silicon Heterojunction Solar Cells and p-type Crystalline Silicon Wafers: A Historical Perspective

The first reports of both boron-oxygen (BO)-related light-induced degradation (BO-LID) and amorphous/crystalline silicon heterojunction (SHJ) solar cell fabrication date back to the early 1970s. However, the complete development of the "modern" SHJ structure took place well before BO defect stabilization processes were developed. Due to the susceptibility of p-type Czochralski (Cz)-grown silicon to BO-LID, such wafers were deemed unsuitable for SHJ solar cells. In addition to stability issues, lower charge carrier lifetimes due to contamination and challenges with surface passivation posed barriers to the adoption of p-type wafers in SHJ applications. Herein, these three key challenges are discussed in detail. Kinetic modeling and experimental results reveal the severe impact of BO-LID in p-type SHJ solar cells and provide possible explanations as to why earlier attempts using p-type wafers might have failed. The role of gettering and advanced hydrogenation in stabilizing BO defects in SHJ solar cells is demonstrated experimentally. Finally, a summary of the effective surface recombination velocities reported in the literature for hydrogenated intrinsic amorphous silicon passivation of p- and n-type crystalline silicon wafers is presented. Based on these findings, the potential of p-type wafers to enable a next-generation of high-efficiency solar cells featuring carrier-selective contacts is discussed.