Analysis of hydrogen distribution and migration in fired passivating contacts (FPC)

TL;DR

This study investigates hydrogen distribution and migration in fired passivating contacts for silicon solar cells, revealing key mechanisms of hydrogen effusion, re-introduction, and their impact on passivation quality and efficiency.

Contribution

It provides new insights into hydrogen behavior in FPC structures, highlighting the role of interface pile-up and oxide type in passivation performance.

Findings

Hydrogen almost completely effuses during firing but can be reintroduced by hydrogenation.

Higher hydrogen content correlates with increased open-circuit voltage (V_OC).

Oxide type significantly influences passivation quality, achieving V_OC up to 739 mV.

Abstract

In this work, the hydrogenation mechanism of fired passivating contacts (FPC) based on c-Si/SiO$_{x}$/nc-SiC$_{x}$(p) stacks was investigated, by correlating the passivation and local re-distribution of hydrogen. Secondary ion mass spectroscopy (SIMS) depth profiling was used to assess the hydrogen (/deuterium) content. The SIMS profiles show that hydrogen almost completely effuses out of the SiC$_{x}$(p) during firing, but can be re-introduced by hydrogenation via forming gas anneal (FGA) or by release from a hydrogen containing layer such as SiN$_{x}$:H. A pile-up of H at the c-Si/SiO$_{x}$ interface was observed and identified as a key element in the FPC's passivation mechanism. Moreover, the samples hydrogenated with SiN$_{x}$:H exhibited higher H content compared to those treated by FGA, resulting in higher iV$_{OC}$ values. Further investigations revealed that the doping of the SiC$_{x}$ layer does not affect the amount of interfacial defects passivated by the hydrogenation process presented in this work. Eventually, an effect of the oxide's nature on passivation quality is evidenced. iV$_{OC}$ values of up to 706 mV and 720 mV were reached with FPC test structures using chemical and UV-O$_{3}$ tunneling oxides, respectively, and up to 739 mV using a reference passivation sample featuring a ~25 nm thick thermal oxide.