Low cost back contact heterojunction solar cells on thin c-Si wafers. integrating laser and thin film processing for improved manufacturability
- Univ. of Delaware, Newark, DE (United States)
An interdigitated back contact (IBC) Si wafer solar cell with deposited a-Si heterojunction (HJ) emitter and contacts is considered the ultimate single junction Si solar cell design. This was confirmed in 2014 by both Panasonic and Sharp Solar producing IBC-HJ cells breaking the previous record Si solar cell efficiency of 25%. But manufacturability at low cost is a concern for the complex IBC-HJ device structure. In this research program, our goals were to addressed the broad industry need for a high-efficiency c-Si cell that overcomes the dominant module cost barriers by 1) developing thin Si wafers synthesized by innovative, kerfless techniques; 2) integrating laser-based processing into most aspects of solar cell fabrication, ensuring high speed and low thermal budgets ; 3) developing an all back contact cell structure compatible with thin wafers using a simplified, low-temperature fabrication process; and 4) designing the contact patterning to enable simplified module assembly. There were a number of significant achievements from this 3 year program. Regarding the front surface, we developed and applied new method to characterize critical interface recombination parameters including interface defect density Dit and hole and electron capture cross-section for use as input for 2D simulation of the IBC cell to guide design and loss analysis. We optimized the antireflection and passivation properties of the front surface texture and a-Si/a-SiN/a-SiC stack depositions to obtain a very low (< 6 mA/cm2) front surface optical losses (reflection and absorption) while maintaining excellent surface passivation (SRV<5 cm/s). We worked with kerfless wafer manufacturers to apply defect-engineering techniques to improve bulk minority-carrier lifetime of thin kerfless wafers by both reducing initial impurities during growth and developing post-growth gettering techniques. This led insights about the kinetics of nickel, chromium, and dislocations in PV-grade silicon and to achieving millisecond lifetimes in kerfless silicon materials. Laser fired contacts to n-Si were developed for the first time using a Al/Sb/Ti metal stack giving contact resistances < 5 mOhm-cm2 when fired through several different dielectric layers. A new 2 step laser+chemical etch isolation technique was developed using a sacrificial top coating which avoids laser damage to Si passivation. Regarding the heterojunction emitter, analysis of front FHJ (1D) and IBC (2D) cells with range of p-layer conditions found that a 2-stage high/low doped p-layer was optimum: the low doped region has lower defects giving higher Voc and the high doped region gave a better contact to the metal. A significant effort was spent studying the patterning process and its contribution to degradation of passivation and reproducibility. Several promising new cleaning, contact and deposition patterning and processing approaches were implemented leading to fabrication of several runs with cells having 19-20% efficiency which were stable over several months. This program resulted in the training and support of 12 graduate students, publication of 21 journal papers and 14 conference papers.
- Research Organization:
- Univ. of Delaware, Newark, DE (United States)
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE), Renewable Power Office. Solar Energy Technologies Office
- Contributing Organization:
- Massachusetts Institute of Technology, Cambridge, MA (United States); IPG, Manchester, NH (United States)
- DOE Contract Number:
- EE0005314
- OSTI ID:
- 1224531
- Report Number(s):
- UD-0005314
- Country of Publication:
- United States
- Language:
- English
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