Oxidation enhanced diffusion for screen printed silicon solar cells

Abstract

If the world's answer to alternative energy production is to be silicon photovoltaics, the fabricated devices need to be produced with the lowest dollar per watt peak. This may be translated to a high efficiency at competitive costs. An easily implementable approach to increase solar cell efficiency without incurring too much cost is to include thermal oxidation [1,2,3,4]. Thermally grown silicon oxide is well known in the microelectronics industry as one of the best dielectrics to passivate silicon surfaces. This paper will focus on two simultaneous properties related to thermal oxidation. The first is the phosphorus emitter and how it can be significantly altered even at low temperatures (800C). The second is passivation due to the thermal oxidation of both front and rear surfaces. A 60 and an 80 Ω/□ emitter (compatible with silver screen printed contact formation) will be investigated with various oxidation conditions using secondary ions mass spectroscopy (SIMS), scanning spreading resistance (SRP), emitter saturation currents (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oe</sub> ) as well as final solar cell devices. We report that increasing oxidation significantly decreases phosphorus surface concentration (N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> ) while increasing junction depth. The final cell results show that increasing oxidation increases both open circuit voltage (V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">oc</sub> ) as well as fill factor (FF), however the current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">sc</sub> ) is reduced partly due to front reflection loss. The cells studied in this paper are fabricated on 149cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> , 155μm thin 1.5 ohm.cm p-type Cz-Si wafers that have screen printed Ag front contacts and a thin thermal oxide on both sides with a rear deposited oxide/nitride stack The highest confirmed efficiency of the cells studied is 19.9% with a Voc of 654mV, Jsc of 38.4 mA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> and a fill factor of 79.3%.