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High efficiency silicon solar cell developed at IMEC

by Steve Bush, 1 November 2007

Belgium research organisation IMEC has reported 17.4 per cent efficiency in thin multi-crystalline silicon solar cells. The cells were made using the lab’s i-PERC (industrial-passivated emitter and rear cells) process which to save cost shuns high-purity semiconductor-type processing in favour of more industrial techniques. The record-breaker is a 100cm sq. 180µm thick device with a short circuit current of 35.22mA/cm sq. and an open circuit voltage of 629.8mV.

I-PERC competes with traditional aluminium-alloyed back surface field (Al-BSF) cells where aluminium is deposited directly onto the back of the silicon. With i-PERC, a comparatively low-quality rear SiO2/SiN dielectric is deposited using plasma-enhanced chemical vapour deposition (PECVD). Compared with thermal oxide growth, PECVD is faster and cooler.

The dielectric is laser perforated, then screen-printed with aluminium which is fired to form local BSF contacts through the perforations.  Isolating a large part of the silicon from the aluminium reduces rear surface recombination, leading to an increase in short circuit current and open circuit voltage.

The i-PERC rear contact also improves light confinement as it is more reflective than Al-BSF at infra-red wavelengths below 1,000nm. At 1,100nm, rear reflectance, according to IMEC, is 64 per cent with Al BSF, compared with 91 per cent for i-PERC. This represents a net gain of 0.9mA/cm sq. – or four per cent in overall efficiency.

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