Direct imaging of dopant distributions across the Si-metallization interfaces in solar cells: Correlative nano-analytics by electron microscopy and NanoSIMS

Abstract

The overall efficiencies of screen printed monocrystalline Si solar cells are limited by electrical losses across the Si-metallization interface. The process of metallization affects the emitter and space charge region of the solar cell, particularly with respect to the dopant distributions. Until now, direct imaging of dopant distributions across the interface has not been reported mainly because the concentrations of dopants are far below the detection limit of conventional analytical tools. In the present study, we harness the high-resolution (100nm) high-sensitivity chemical imaging with Nano Secondary Ion Mass Spectrometry (NanoSIMS) and correlate with microstructural and electrical properties to elucidate the factors limiting the overall cell efficiencies. We analysed two sets of p-type solar cells fabricated from identical starting materials, the only difference being the firing temperature. It was found that the overall efficiency of cells fired at 900°C was ∼17% while the efficiency of cells fired at 960°C was only 13.6%.In phosphorus (P) ion maps, the P emitter structure was found to be well-preserved by NanoSIMS in cells fired at 900°C, it was completely disintegrated in the overfired cells and thereby increasing the contact resistance. The passivation layer (SiNX) was found to be disintegrated in the overfired cell and furthermore, below the metallization, a diffusion cloud was observed wherein boron (B) rich domains extend over several µm. In the overfired cell the disintegration of the SiNX layer identified in the SEM correlated with the disintegrated emitter structure (P) analysed by NanoSIMS. This implies that the disintegration of the passivation layer leads to a diffusion of the dopants resulting in the loss of overall cell efficiency. Thus, our new comprehensive approach provides unprecedented insights into the factors limiting the overall efficiencies in solar cells.