Improved Surface Passivation by Wet Texturing, Ozone‐Based Cleaning, and Plasma‐Enhanced Chemical Vapor Deposition Processes for High‐Efficiency Silicon Heterojunction Solar Cells

Abstract

Silicon heterojunction (SHJ) solar cells rely on excellent surface passivation of the crystalline wafer. This article reports on the development of wet chemical processes varying the texturing and optimizations of the final clean processes for Czochralski–silicon wafers used in SHJ solar cells. Three different additives are used to modify both the pyramid size and the texture homogeneity on the wafers. As an alternative to standard Radio Corporation of America (RCA) sequence, ozonized deionized (DI) water‐based procedures are used to clean the silicon surfaces, reducing process time and the amount of chemicals to obtain the same cleaning quality. In addition, two different amorphous silicon (a‐Si:H) passivation stacks are discussed in this article demonstrating an improvement in the open‐circuit voltage, Voc, by 15 mV. Combining these wet chemistry improvements with optimized process conditions for the passivating a‐Si:H layers deposited by plasma‐enhanced chemical vapor deposition (PECVD), allows to obtain high‐efficiency devices both in small‐ and full‐area solar cells. On 6 in. full‐area solar cells (215.3 cm2 aperture area), open‐circuit voltages, fill factors, and efficiency values of 738 mV, 81.4%, and 23.2% are obtained, respectively; for 4 cm2 cell size, the best obtained values in equivalent devices are 741 mV, 80.6%, and 23.2%.

Toward High Solar Cell Efficiency with Low Material Usage: 15% Efficiency with 14 μm Polycrystalline Silicon on Glass

“…This approach shows a new path to achieving high V OC values on LPC‐Si and will be the subject of future work. For more information, the reader is referred to a recent publication by Morales‐Vilches et al., in which V OC values of 740 mV have been demonstrated on small (4 cm 2 ) and full‐area (244 cm 2 ) cells on monocrystalline Czochralski silicon (CZ‐Si) wafers with silicon heterojunction architecture …”

“…Further improvements in V OC can still be expected with improved a‐Si:H contact layers and reduced recombination at crystallographic defects . In particular, the approach of depositing three very thin layers of a‐Si(i), each with a short hydrogen plasma treatment, will be optimized for LPC‐Si as it has demonstrated V OC values of 740 mV on CZ‐Si wafers …”

Optimization of Silicon Heterojunction Interface Passivation on p‐ and n‐Type Wafers Using Optical Emission Spectroscopy

“…have to be tuned to enhance SiH 3 radical formation in the plasma. In addition to these plasma processing conditions, in situ hydrogen plasma treatment (HPT), [ 17 ] postannealing, [ 18 ] and a multilayer approach [ 19–21 ] have been exploited to enhance the chemical passivation.…”

Selective rounding for pyramid peaks and valleys improves the performance of SHJ solar cells

“…12,13 In recent years, ozone-based bath has been introduced as an alternative cleaning process, which is more environmentally friendly than traditional RCA cleaning. 14,15 The ozone-based method of chemical polishing is carried out in an O 3 + HF solution, avoiding pollution emission of nitrogen. However, the influences of these two CP etching methods on the pyramid profile have not yet been compared in detail.…”