Jörg Horzel

Development of RTP for industrial solar cell processing

Rapid Thermal Processing (RTP), originally developed for processing microelectronic devices has been investigated in the recent decade for its potential in the production of Si solar cells. This paper will discuss the use of RTP for industrial Si solar cells with screen-printed contacts. Printed metal contacts require adapted emitters when good fill factors should be achieved. Multi-crystalline Si substrates require to adapt the temperature ramps of RTP to avoid minority carrier lifetime degradation from activated defect centres. Finally, industrial processing requires high throughput that cannot be achieved with conventional RTP equipment. This paper will present an advanced selective emitter process and a recently developed continuous RTP system that meet for the first time the requirements to make RTP compatible with industrial solar cell processing. The limits of industrial RTP solar cell processing will be discussed.

Oxidation enhanced diffusion for screen printed silicon solar cells

If the worlds 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 (Joe) as well as final solar cell devices. We report that increasing oxidation significantly decreases phosphorus surface concentration (Ns) while increasing junction depth. The final cell results show that increasing oxidation increases both open circuit voltage (Voc) as well as fill factor (FF), however the current density (Jsc) is reduced partly due to front reflection loss. The cells studied in this paper are fabricated on 149cm2, 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/cm2 and a fill factor of 79.3%.

Integration of high sheet resistance homogeneous emitters in a process flow for PERC-type solar cells with Cu contacts

This paper presents the results of the integration of a high sheet resistance (120 Ω/sq) homogeneous emitter with an industrial feasible process flow on large area p-type CZ-Si PERC solar cells with plated Ni-Si/Cu front contacts. Focus is put on the junction isolation performed by either applying a mask prior to POCl3 diffusion or by selectively removing the rear side emitter in an inline 1-sided wet-chemical etching tool after diffusion. A metallization sequence [1,2] applying Ni and Cu plating, is performed to contact the moderately doped emitters. The best experimental split in this comparison resulted in excellent average conversion efficiency of 20.2%.