Silicon solar cells textured by low damage RIE with natural lithography

Abstract

1 2 ABSTRACT RIE with natural lithography has two advantages over RIE with automasking: the process causes less surface damage and the process window is broader. We have systematically explored the parameter range of our process and identified a natural lithography RIE process which causes a minimum amount of surface damage. By using this RIE process and a wet chemical etch of a few nanometers, we reached a gain in shortcircuit current of 2.9% and no loss in open circuit voltage. This resulted in an absolute efficiency gain of 0.6% for the RIE textured wafers. INTRODUCTION Texturing of multicrystalline silicon solar cells byReactive Ion Etching (RIE) has been identified as an attractive method since it avoids the problems encountered in altemative texturing approaches such as wet acidic texturing or mechanical V-grooving. RIE acts independently of the. crystal orientation and defects, produces little chemical waste, and is applicable to wafers of any thickness 111 because it induces no mechanical stress. The most commonly used form of RIE is automasking. Automasking has the disadvantage that plasma parameters can only be adjusted in a small rangep]. Beyond this range, no automasking occurs. RIE used with an applied mask does not have that disadvantage. Natural lithography, using colloids as a mask, has the advantage over other masking techniques that the mask is easy to apply and to remove. Although automasking can yield cells of good quality [3.4], the subsequent processing sequence is in those cases rather complicated, involving long anneal steps and passivating oxides.In this paper automasking and natural lithography are compared and then the opitmization study o i natural lithography is discussed. EXPERIMENTAL METHODS: At RWTH Aachen cells were textured with RIE using Baysix 10x10 cm' wafers. This process includes a -microscopic, self-organised mask applied by spin coating [5,6] and an etch in a PECVD batch reactor. which is normally used for silicon nitride depositions. one group at a time. The RIE process was optimized for low reflection and low surface damage. At ECN the cell pmduction process was optimized on clean properties. The cells were processed using a standard firing-through siliconnitride process (see fig 1). All results were obtained with neighbouring multicrystalline wafers and a reference group was always included. The median is listed as the result of each group, which consists always of 20 wafers. IV and IQE were measured and results were checked on statistical significance using multifactor analysis of variance m. To monitor the texture with changes of the (reactor)process. reflectance measurements and SEM photography were used.

Novel low cost chemical texturing for very large area industrial multi-crystalline silicon solar cells

“…Different surface structures of mc-Si wafers have been formed after NaOH-NaOCl etching at different experimental conditions at a fixed etching time of 20 min as shown in SEM micrographs of figures 2 and 3. Actually, NaOH is a well-known anisotropic etchant widely used for mono-crystalline silicon solar cell texturing and MEMS application [12]. NaOCl is a strong oxidizing agent [13].…”