Low-Temperature $p$-Type Microcrystalline Silicon as Carrier Selective Contact for Silicon Heterojunction Solar Cells

Abstract

Silicon heterojunction (SHJ) solar cells have reached record efficiency, particularly in all-back contacted architectures. Despite this, two-side contacted SHJ cells still suffer from parasitic absorption and series resistance losses in the amorphous silicon contacts. An alternative to the doped amorphous silicon layer is microcrystalline silicon, which exhibits improved transparency and charge transport, while maintaining the superior passivation quality of all-silicon contact stacks. However, depositing thin, highly crystalline films has remained a challenge until recently. In this work, we use deposition temperatures $p$ -type $\mu$ c-Si:H contact layers. With these layers, we demonstrate $J_{\text{sc}}$ gains of 1 mA/cm $^2$ , while reducing series resistance below 1 $\Omega$ cm $^2$ , leading to screen printed 4 cm $^2$ cells with certified $\eta =\text{23.45}\%$ . Using a suite of device and material characterization techniques, we show that reduced deposition temperature leads to an increase in crystalline volume fraction from 35% to 55% for $p$ -type films, which mitigates parasitic absorption in the front contact and facilitates hole extraction. These improvements are explained as resulting from higher transparency in the $p$ -type layer accompanied by higher band bending in the c-Si wafer. These findings provide a method to improve SHJ solar cells performance, while offering insight into the importance of band bending considerations when optimizing heterojunction designs.