Study of Passivation in the Gap Region Between Contacts of Interdigitated-Back-Contact Silicon Heterojunction Solar Cells: Simulation and Voltage-Modulated Laser-Beam-Induced-Current
Recent record efficiency Si cells have had an interdigitated back contact (IBC) design where both positive and negative contacts are fabricated on the back side of the Si wafer. Many industry groups have investigated IBC cells but used photolithography to pattern the rear contacts although it is considered impractical for low cost, high volume manufacturing. Alternative patterning methods using lasers and mechanical masking have been utilized in fabrication of patterned regions in IBC Si solar cells to replace photolithography. Another exciting advance in Si solar cell technology has been replacing the high temperature diffusion of doped regions inside the crystalline Si wafer with low temperature deposition of very thin layers of doped amorphous hydrogenated Si (a-Si:H) on the Si wafer. This is called a Si heterojunction (HJ) device. The a-Si:H provides excellent passivation of surface defects and has produced the highest open circuit voltages (VOC) of any Si solar cell device structure. The Institute of Energy Conversion (IEC) demonstrated the first IBC-HJ solar cell combining these two strategies in 2007 and has fabricated IBC-HJ cells with 20% efficiency using three photolithography steps. Despite the demonstrated efficiency potential of this device by industry groups (>26%) there is no commercial production because of the challenges of patterning and processing the structure in an industrial environment. Our work sought to address that challenge. The objective of this 3 year project was to develop the processing for the IBC-HJ Si solar cell using lasers for patterning and contact formation instead of photolithography. Laser patterning enables rapid, contactless manufacturing of patterned regions. We intensively studied laser fired contacts, laser patterning of the a-Si multi-layer stacks and metal layers and the use of plasma shadow masks. After an exhaustive focus in the first year on the laser fired emitter (LFE) and contact (LFC), we were unable to achieve VOC greater than 660 mV, compared to the 720 mV required to meet our milestones. We discovered that an additional issue with our original IBC structure was the presence of an inversion layer at the back surface connecting the p and n regions which reduced the VOC and fill factor (FF). We developed innovative methods for characterizing the inversion layer. These two limitations in the original design lead to development of a new IBC-HJ process sequence and device structure which we called Plasma Masked Laser Processed (PMLP). It retained the original high efficiency features including manufacturability. The LFC was replaced with a standard n-type a-Si HJ contact. The inversion layer was eliminated by replacing the previous p-type stack in the gap with an n-type or SiN stack. The PMLP used laser ablation of a multi-layer a-Si stack followed by chemical etching to open the n-contact. It required deposition of a patterned stack through a mask in the plasma deposition chamber. This turned out to be a source of significant problems due to inevitable unwanted deposition ‘leakage’ under the mask. This formed a blocking contact on the emitter which significantly reduced Voc and FF. Several iterations in PMLP device structure and chemical etching steps lead to increasing the efficiency from 3 to 15% and VOC from 450 to 660 mV. The best IBC-HJ device we fabricated had only 15% efficiency while our standard front HJ devices had 20% efficiency. The relatively low efficiencies we obtained were due to the blocking barrier and challenges of plasma masking. While the PMLP device process and structure was evolving, the implied Voc (iVoc) remained consistently high. For example, the implied VOC (iVOC) was > 710 mV. This confirms that the laser ablation and etching steps were not degrading the passivation. This confirms that direct laser ablation and chemical etching of various dielectric layers on the back surface can be achieved with negligible (<10 mV) loss in i VOC. Originally, we thought this would be one of the biggest challenges. We showed that laser patterning via direct laser ablation has strong potential for IBC cell processing. Excellent passivation is needed regardless of the patterning or rear structure. Throughout the project we conducted parallel studies to push the quality and reproducibility of the passivation. This included a detailed study of Si wafer surface cleaning and texturing and development of a bilayer deposition with hydrogen plasma anneal treatment. These works lead to standard HJ cells with state-of-the-art iVOC > 750 mV, VOC =725 mV and efficiency=20%. These enhancements in Si passivation engineering are easily incorporated into a manufacturing environment. We developed electrodeposition (ED) of Cu for IBC contacts on thin laser patterned Ni layers. Careful optimization was needed to simultaneously minimize the degradation to VOC and yet ensure complete removal so there was no shorting across the ~150 um gap. ED processing conditions for uniform plating of 2-3 um thick Cu films with excellent adhesion on laser-scribed Ni contacts yielded a low contact resistance of ~5 mΩ.cm2. ED was used to fabricate HJ cells with rear IBC patterned Ni contacts with identical performance as standard evaporated Al metal. This confirmed that ED Cu on patterned Ni could provide similar low contact resistance and uniformity as standard Al metallization This project resulted in 4 PhD dissertations (3 at University of Delaware and 1 at University of Virginia), 5 publications in journals, and 8 presentations at conferences with publications in their proceedings. Here, most publications had joint authorship between UD and U Va attesting to the close collaboration between the groups.
- Research Organization:
- Univ. of Delaware, Newark, DE (United States)
- Sponsoring Organization:
- USDOE Office of Energy Efficiency and Renewable Energy (EERE)
- Grant/Contract Number:
- EE0007534
- OSTI ID:
- 1650139
- Alternate ID(s):
- OSTI ID: 1644682
- Journal Information:
- IEEE Journal of Photovoltaics, Vol. 8, Issue 2; ISSN 2156-3381
- Publisher:
- IEEECopyright Statement
- Country of Publication:
- United States
- Language:
- English
Web of Science
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