PECVD BSG Diffusion Sources for Simplified High-Efficiency N-PERT BJ and BJBC Solar Cells

We investigate boron silicate glasses (BSG) deposited by plasma-enhanced chemical vapor deposition (PECVD) as a boron diffusion source on n-type wafers for the simplified fabrication of crystalline Si solar cells by the codiffusion processes. By varying the SiH
₄
/B
₂
H
₆
gas flow ratio and the layer thickness of the PECVD BSG layers, we obtain sheet resistivities in a wide range from 30 to 500 Ω/□ after thermal B drive-in. Emitter saturation current densities as low as J
_0e
= 4 fA/cm
²
(for R
_sheet
= 236 Ω/□) are demonstrated using PECVD BSG layers as diffusion sources. A boron concentration in the PECVD BSG of up to 6.4 × 10
²¹
cm
^-3
is measured by plasma profiling time-of-flight mass spectrometry (PP-TOFMS). A process simulation model of the B diffusion from the PECVD BSG into the Si substrate reproduces the experimental B concentration profile in the Si, measured both by PP-TOFMS and electrochemical capacitance- voltage (ECV) measurements. We fabricate industrial-type passivated emitter and rear totally diffused back-junction (PERT BJ) solar cells, as well as back-junction back-contact cells on n-type wafers. Applying codiffusion from PECVD BSG layers and thus a lean process flow including only one high-temperature step, we demonstrate n-PERT BJ cells with conversion efficiencies of up to 19.85%.