High Rate Growth Of Hydrogenated Amorphous And Microcrystalline Silicon For Thin Film Silicon Solar Cells Using Dynamic Very High Frequency Plasma Enhanced Chemical Vapor Deposition

Author : Emmanuel U. Onyegam
Publisher :
ISBN 13 :
Total Pages : 242 pages
Book Rating : 4.:/5 (919 download)

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Download or read book Remote Plasma Chemical Vapor Deposition for High Efficiency Heterojunction Solar Cells on Low Cost, Ultra-thin, Semiconductor-on-metal Substrates written by Emmanuel U. Onyegam and published by . This book was released on 2014 with total page 242 pages. Available in PDF, EPUB and Kindle. Book excerpt: In the crystalline Si solar cell industry, there is a push to reduce module cost through a combination of thinner substrates and increased cell efficiency. Achieving solar cells with sub-100 [greek small letter mu]m substrates cost-effectively is a formidable task because such thin substrates impose stringent handling requirements and thermal budget due to their flexibility, ease of breakage, and low yield. Moreover, as the substrate thickness decreases the surface passivation quality dictates the performance of the cells. Crystalline Si heterojunction (HJ) solar cells based on hydrogenated amorphous silicon (a-Si:H) have attracted significant interest in recent years due to their excellent surface passivation properties, potential for high efficiency, low thermal budget and low cost. HJ cells with ultra-passivated surfaces showing > 700 mV open-circuit voltages (Voc) and > 20% conversion efficiency have been demonstrated. In these cells, it has been identified that high-quality a-Si:H films deposited by a low-damage plasma process is key to achieving such high cell performance. However, the options for low-damage plasma deposition process are limited. The main objectives of this work are to develop a low-plasma damage a-Si:H thin film deposition process based on remote plasma chemical vapor deposition (RPCVD) and to demonstrate high efficiency HJ solar cells on bulk substrates as well as on ultra-thin silicon and germanium substrates obtained by a novel, low-cost semiconductor-on-metal (SOM) technology. This manuscript presents a detailed description of the RPCVD system and the process leading to the realization of high quality a-Si:H thin films and high efficiency HJ solar cells. First, p-type a-Si:H thin films are developed and optimized, then HJ solar cells are subsequently fabricated on bulk and ultra-thin Si and Ge SOM substrates without intrinsic a-Si:H passivation. Single HJ cells on ~ 500 μm bulk Si and ~25 μm ultra-thin substrates exhibited conversion efficiencies of [greek small letter eta] = 16% (Voc = 615 mV, Jsc = 34 mA/cm2, and FF = 77%) and [greek small letter eta] = 11.2% (Voc = 605 mV, Jsc = 29.6 mA/cm2, and FF = 62.8%), respectively. The performance of the ~25 [greek small letter mu] m cell was further improved to [greek small letter eta] = 13.4% (Voc = 645 mV, Jsc = 31.4 mA/cm2, and FF = 66.2%) by implementing the dual HJ architecture without front side i-layer passivation. For single HJ cells based on Ge substrates, the results were [greek small letter eta] = 1.78 % (Voc = 148 mV, Jsc = 35.1 mA/cm2, and FF = 1.78%) on ~500 [greek small letter mu]m bulk Ge, compared to [greek small letter eta] =5.3% (Voc = 203 mV, Jsc = 44.7 mA/cm2, and FF = 5.28%) on ~ 50 μm Ge SOM substrates. Respectively, the results obtained on ultra-thin SOM substrates are among the highest reported in literature for based on comparable architecture and substrate thickness. In order to achieve improved cell performance, dual HJ cells with i-layer passivation of both surfaces were fabricated. First, optimized RPCVD-based i-layer films were developed by varying the deposition temperature and H2 dilution ratio (R). It was found that excellent surface passivation on planar substrates with as-deposited minority carrier lifetimes > 1 ms is achievable by using deposition temperature of 200 oC and moderate dilution ratio 0.5 ≤ R ≤ 1, even without the more rigorous RCA pre-cleaning process typically used in literature for achieving comparable results. Subsequently, dual HJ solar cells with i-layer films were demonstrated on planar and textured bulk Si substrates showing improved conversion efficiencies of [greek small letter eta] = 17.3% (Voc = 664 mV, Jsc = 34.34 mA/cm2 and FF = 76%) and [greek small letter eta] = 19.4% (Voc = 643 mV, Jsc = 38.99 mA/cm2, and FF = 77.5%), respectively.