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Title: Silicon solar cells made by a self-aligned, selective-emitter, plasma-etchback process

Abstract

A potentially low-cost process for forming and passivating a selective emitter. The process uses a plasma etch of the heavily doped emitter to improve its performance. The grids of the solar cell are used to mask the plasma etch so that only the emitter in the region between the grids is etched, while the region beneath the grids remains heavily doped for low contact resistance. This process is potentially low-cost because it requires no alignment. After the emitter etch, a silicon nitride layer is deposited by plasma-enhanced, chemical vapor deposition, and the solar cell is annealed in a forming gas. 5 figs.

Inventors:
; ;
Publication Date:
Research Org.:
Sandia Corporation
Sponsoring Org.:
USDOE, Washington, DC (United States)
OSTI Identifier:
321276
Patent Number(s):
US 5,871,591/A/
Application Number:
PAN: 8-742,378
Assignee:
Sandia Corp., Albuquerque, NM (United States) SNL; SCA: 140501; PA: EDB-99:028897; SN: 99002064387
DOE Contract Number:
AC04-94AL85000
Resource Type:
Patent
Resource Relation:
Other Information: PBD: 16 Feb 1999
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; SILICON SOLAR CELLS; FABRICATION; PASSIVATION; DOPED MATERIALS; CHEMICAL VAPOR DEPOSITION; ANNEALING

Citation Formats

Ruby, D.S., Schubert, W.K., and Gee, J.M. Silicon solar cells made by a self-aligned, selective-emitter, plasma-etchback process. United States: N. p., 1999. Web.
Ruby, D.S., Schubert, W.K., & Gee, J.M. Silicon solar cells made by a self-aligned, selective-emitter, plasma-etchback process. United States.
Ruby, D.S., Schubert, W.K., and Gee, J.M. 1999. "Silicon solar cells made by a self-aligned, selective-emitter, plasma-etchback process". United States. doi:.
@article{osti_321276,
title = {Silicon solar cells made by a self-aligned, selective-emitter, plasma-etchback process},
author = {Ruby, D.S. and Schubert, W.K. and Gee, J.M.},
abstractNote = {A potentially low-cost process for forming and passivating a selective emitter. The process uses a plasma etch of the heavily doped emitter to improve its performance. The grids of the solar cell are used to mask the plasma etch so that only the emitter in the region between the grids is etched, while the region beneath the grids remains heavily doped for low contact resistance. This process is potentially low-cost because it requires no alignment. After the emitter etch, a silicon nitride layer is deposited by plasma-enhanced, chemical vapor deposition, and the solar cell is annealed in a forming gas. 5 figs.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = 1999,
month = 2
}
  • A potentially low-cost process for forming and passivating a selective emitter. The process uses a plasma etch of the heavily doped emitter to improve its performance. The grids of the solar cell are used to mask the plasma etch so that only the emitter in the region between the grids is etched, while the region beneath the grids remains heavily doped for low contact resistance. This process is potentially low-cost because it requires no alignment. After the emitter etch, a silicon nitride layer is deposited by plasma-enhanced, chemical vapor deposition, and the solar cell is annealed in a forming gas.
  • Photovoltaic cells and methods for making them are disclosed wherein the metallized grids of the cells are used to mask portions of cell emitter regions to allow selective etching of phosphorus-doped emitter regions. The preferred etchant is SF{sub 6} or a combination of SF{sub 6} and O{sub 2}. This self-aligned selective etching allows for enhanced blue response (versus cells with uniform heavy doping of the emitter) while preserving heavier doping in the region beneath the gridlines needed for low contact resistance. Embodiments are disclosed for making cells with or without textured surfaces. Optional steps include plasma hydrogenation and PECVD nitridemore » deposition, each of which are suited to customized applications for requirements of given cells to be manufactured. The techniques disclosed could replace expensive and difficult alignment methodologies used to obtain selectively etched emitters, and they may be easily integrated with existing plasma processing methods and techniques of the invention may be accomplished in a single plasma-processing chamber.« less
  • Photovoltaic cells and methods for making them are disclosed wherein the metallized grids of the cells are used to mask portions of cell emitter regions to allow selective etching of phosphorus-doped emitter regions. The preferred etchant is SF.sub.6 or a combination of SF.sub.6 and O.sub.2. This self-aligned selective etching allows for enhanced blue response (versus cells with uniform heavy doping of the emitter) while preserving heavier doping in the region beneath the gridlines needed for low contact resistance. Embodiments are disclosed for making cells with or without textured surfaces. Optional steps include plasma hydrogenation and PECVD nitride deposition, each ofmore » which are suited to customized applications for requirements of given cells to be manufactured. The techniques disclosed could replace expensive and difficult alignment methodologies used to obtain selectively etched emitters, and they may be easily integrated with existing plasma processing methods and techniques of the invention may be accomplished in a single plasma-processing chamber.« less
  • Plasma-enhanced chemical vapor deposition (PECVD) is a cost-effective, performance-enhancing technique that can provide surface passivation and produce an effective antireflection coating layer at the same time. To gain the full benefit from improved emitter surface passivation on cell performance, it is necessary to tailor the emitter doping profile so that the emitter is lightly doped between the gridlines, but heavily doped under them. This selectively patterned emitter doping profile has historically been obtained by using expensive photolithographic or screen-printed alignment techniques and multiple high-temperature diffusion steps. We built on a self-aligned emitter etchback technique first described by Spectrolab. We includedmore » PECVD-nitride deposition because the low- recombination emitter produced by the etchback requires good surface passivation for improved cell performance. The nitride also provides a good antireflection coating. We studied whether plasma-etching techniques can use standard screen-printed gridlines at etch masks to form self-aligned, patterned-emitter profiles on multicrystalline (MC-Si) cells from Solarex Corp. This investigation determined that reactive ion etching (RIE) is compatible with using standard, commercial, screen printed gridlines as etch masks to form self- aligned, selectively-doped emitter profiles. This process results in reduced gridline contact resistance when followed by PECVD treatments, an undamaged emitter surface easily passivated by plasma-nitride, and a less heavily doped emitter between gridlines for reduced emitter recombination. This allows for heavier doping beneath the gridlines for even lower contact resistance, reduced contact recombination, and better bulk defect gettering. Our results found improvement of half a percentage point in cell efficiency when the self-aligned emitter etchback was combined with the PECVD-nitride surface passivation treatment.« less
  • We studied whether plasma-etching techniques can use screen printed gridlines as etch masks to form self-aligned, patterned-emitter profiles on multicrystalline silicon (mc-Si) cells from Solarex Our initial results found a statistically significant improvement of about half an absolute percentage point m cell efficiency when the self-aligned emitter etchback was combined with a PECVD-nitride surface passivation treatment. Some additional improvement in bulk diffusion length was observed when a hydrogen passivation treatment was used in the process. We attempted to gain additional benefits from using an extra-heavy phosphorus emitter diffusion before the gridlines were deposited. However, this required a lusher plasma-etch powermore » to etch back the deeper diffusion and keep the etch time reasonably short. The higher power etch may have damaged the surface and the gridlines so that improvement due to surface passive and reduced gridline contact resistance was inhibited.« less