Microplasma Breakdown and Hot-spots in Silicon Solar Cells

Photovoltaic module hot-spot endurance tests performed at CEC JRC Ispra showed that the temperature distributions in cells subjected to reverse bias heating were non-uniform and reproducible. An investigation of the physical processes causing non-uniform power dissipation in reverse biased silicon solar cells, using IR imaging, scanning electron microscopy, and electrical measurements, is presented here. Hot-spots form in solar cells at defects causing microplasrna breakdown: avalanche breakdown, localized at electric field enhancing structural defects. Visible fight emission from microplasmas near cell front surfaces correlated with hot-spots revealed by IR imaging. Scanning electron microscopy studies of samples containing points of light emission found electron beam induced current (EBIC) enhancement at defects under reverse bias. Measurements of the electrical characteristics of these samples showed that the temperature coefficient of the breakdown voltage was negative, confirming the presence of avalanche multiplication. The defects found in the solar cell samples were pits (approximately 5 #m diameter) in the surface, occasionally contaminated with metals (nickel, aluminium, titanium) from the front contacts. Electron microscopic studies of defects in solar cells were performed in the Materials Science Department of Imperial College, London University. IR imaging studies, and electrical measurements were performed in the ESTI Sector of the Physics Division, CEC JRC Ispra.