Design Rule of Electron- and Hole-Selective Contacts for Polycrystalline Silicon-Based Passivating Contact Solar Cells

A crystalline silicon (c-Si) solar cell with a polycrystalline silicon/SiOx (poly-Si/SiOx) structure, incorporating both electron and hole contacts, is an attractive choice for achieving ideal carrier selectivity and serving as a fundamental component in high-efficiency perovskite/Si tandem and interdigitated back-contact solar cells. However, our understanding of the carrier transport mechanism of hole contacts remains limited owing to insufficient studies dedicated to its investigation. There is also a lack of comparative studies on the poly-Si/SiOx electron and hole contacts for ideal carrier-selective solar cells. Therefore, this study aims to address these knowledge gaps by exploring the relationship among microstructural evolution, dopant in-diffusion, and the resulting carrier transport mechanism in both the electron and hole contacts of poly-Si/SiOx solar cells. Electron (n+ poly-Si/SiOx/substrate)- and hole (p+ poly-Si/SiOx/substrate)-selective passivating contacts are subjected to thermal annealing. Changes in the passivation properties and carrier transport mechanisms of these contacts are investigated during thermal annealing at various temperatures. Notably, the results demonstrate that the passivation properties and carrier transport mechanisms are strongly influenced by the microstructural evolution of the poly-Si/SiOx layer stack and dopant in-diffusion. Furthermore, electron and hole contacts exhibit common behaviors regarding microstructural evolution and dopant in-diffusion. However, the hole contacts exhibit relatively inferior electrical properties overall, mainly because both the SiOx interface and the p+ poly-Si are found to be highly defective. Moreover, boron in the hole contacts diffuses deeper than phosphorus in the electron contacts, resulting in deteriorated carrier collection. The experimental results are also supported by device simulation. Based on these findings, design rules are suggested for both electron and hole contacts, such as using thicker SiOx and/or annealing the solar cell at a temperature not exceeding the critical annealing temperature of the hole contacts.