Pushing the efficiency limit of low-cost, industrially relevant Si solar cells to > 22.5% by advancing cell structures and technology innovations (Final Report)

Technical Report · Fri May 07 04:00:00 EDT 2021

DOI:https://doi.org/10.2172/1834738· OSTI ID:1834738

Rohatgi, Ajeet ^[1]; Ok, Young Woo ^[2]; Upadhyaya, Ajay D. ^[2]; Upadhyaya, Vijaykumar D. ^[2]; Rounsaville, Brian C. ^[2]; Ying-Yuan, Huang ^[2]; Madani, Keeya ^[2]; Choi, Wookjin ^[2]

---

1. Georgia Institute of Technology, Atlanta, GA (United States); Georgia Institute of Technology
2. Georgia Institute of Technology, Atlanta, GA (United States)

The overall objective of this program is to achieve ~23% bifacial n-type cell efficiencies by developing and implementing optimized homogeneous or selective boron (B) emitter on front and tunnel oxide passivated contact (TOPCon) on rear side, in combination with advanced fine-line screen-printing metallization with floating busbars. During this research project, first we developed a technology roadmap to drive the 21% n-PERT cell efficiency from 21% to 23% by transforming the cell design to n-TOPCon and establishing the requirements for each layer, including B emitter, rear n-TOPCon, n-base Si and screen-printed contacts. Next, consistent with our roadmap, we developed advanced homogeneous implanted B emitter (150-180 Ω/⌫) passivated with ALD Al₂O₃ layer capped with PECVD SiN_x/SiO_x double-layer antireflection coating This gave a very low recombination current density of 10-15 fA/cm² prior to metallization. In addition, we demonstrated metallized recombination current density of ~31 fA/cm² for this advanced homogeneous B emitter with industrial screen-printed, fire-through contacts with 40 μm wide grid lines, floating busbars and implementation of an advanced Ag-Al paste which resulted in local or reduced area metal-Si contact under the grid lines with virtually no emitter surface etching. This paste reduced the full area metallized J_oe,metal from >1100 fA/cm² to ~700 fA/cm². We also developed novel processes for the formation of p⁺/p⁺⁺ selective B emitter by a) single B diffusion with selective etch back and b) two-steps diffusion with implanted B in field region and APCVD B diffusion under the metal grid. We achieved very low un-metallized recombination current density (J₀) of ~18 fA/cm² for the selective p⁺⁺p⁺ emitters (30/150 Ω/⌫) and metallized J₀ of ~28 fA/cm² with ~3% screen-printed metal contact to p⁺⁺ regions. Next, we developed the technology for n-TOPCon by growing phosphorus-doped LPCVD and PECVD poly-Si on top of ~ 15Å chemically grown (NAO) tunnel oxide. After an optimized anneal at 875 °C for 30 min, passivated n-TOPCon have unmetallized J₀ of ~ 5 fA/cm² which went down further to ~ 1 fA/cm² after a 700 Å SiN_x capping layer and simulated contact firing cycle at 770 °C. After screen-printed fire-through metallization on this n-TOPCon with ~13% metal coverage, metallized J₀ value increased to only ~5 fA/cm² which is among the lowest reported value to the best of our knowledge for screen-printed metallization. Finally, we integrated all the above technology innovations and enhancements and demonstrated low-cost manufacturable screen-printed n-TOPCon bifacial Si solar cell with ~23% efficiencies. Based on the experimental and theoretical understanding developed in this project, we have developed a new technology roadmap that shows that implementation of busbarless contacts, 10-20 ms bulk lifetime Si, and selective B emitter or selective TOPCon on the front can drive ~23% efficient cells achieved in this research to ~25% at low-cost.

• Research Organization: Georgia Institute of Technology, Atlanta, GA (United States)
• Sponsoring Organization: USDOE Office of Energy Efficiency and Renewable Energy (EERE)
• DOE Contract Number: EE0007554
• OSTI ID: 1834738
• Report Number(s): Final-Technical-Report
• Country of Publication: United States
• Language: English

Related Subjects

14 SOLAR ENERGY
TOPCon
energy
photovoltaics
renewable
silicon cells
solar