University Crystalline Silicon Photovoltaics Research and Development

The overall goal of the program is to advance the current state of crystalline silicon solar cell technology to make photovoltaics more competitive with conventional energy sources. This program emphasizes fundamental and applied research that results in low-cost, high-efficiency cells on commercial silicon substrates with strong involvement of the PV industry, and support a very strong photovoltaics education program in the US based on classroom education and hands-on training in the laboratory

String Ribbon Silicon Solar Cells with 17.8% Efficiency

Presented at the 3rd World Conference on Photovoltaic Energy Conversion; Osaka, Japan; May 11-18, 2003.We have fabricated 4 cm(2) cells on String Ribbon Si wafers with efficiencies of 17.8% using a combination of laboratory and industrial processes. These are the most efficient String Ribbon devices made to date, demonstrating the high quality of the processed silicon and the future potential for industrial String Ribbon cells. Cofiring PECVD (Plasma Enhanced Chemical Vapor Deposition) silicon nitride (SiN(x)) and Al was used to boost the minority carrier lifetime of bulk Si. Photolithography front contacts were used to achieve low shading losses and low contact resistance with a good blue response. The firing temperature and time were studied with respect to the trade-off between hydrogen retention and aluminum back surface field (Al-BSF) formation. Bulk defect hydrogenation and deep Al-BSF formation took place in a very short time (~1 sec) at temperatures higher than 740 degrees C

Effective Interfaces in Silicon Heterojunction Solar Cells

Presented at the 31st IEEE Photovoltaic Specialists Conference, Orlando, Florida; January 3-7, 2005. ©2005 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.Thin hydrogenated amorphous silicon (a-Si:H) layers deposited by hot-wire chemical vapor deposition (HWCVD) are investigated for use in silicon heterojunction (SHJ) solar cells on p-type crystalline silicon wafers. A requirement for excellent emitter quality is minimization of interface recombination. Best results necessitate immediate a-Si:H deposition and an abrupt and flat interface to the c-Si substrate. We obtain a record planar HJ efficiency of 16.9% with a high Voc of 652 mV on p-type float-zone (FZ) silicon substrates with HWCVD a-Si:H(n) emitters and screen-printed Al-BSF contacts. H pretreatment by HWCVD is beneficial when limited to a very short period prior to emitter deposition

Lifetime Enhancement and Low-Cost Technology Development for High-Efficiency Manufacturable Silicon Solar Cells

Presented at the 11th Workshop on Crystalline Silicon Solar Cell Materials and Processes; Estes Park, Colorado; August 19-22, 2001.A low-cost, manufacturable defect gettering and passivation treatment, involving simultaneous anneal of a PECVD SiN(x) film and a screen-printed Al layer, is found to improve the lifetime in Si ribbon materials from 1-10 µs to over 20 µs. Our results indicate that the optimum anneal temperature for SiN(x)-induced hydrogenation is 700°C for EFG and increases to 825°C when Al is present on the back of the sample. This not only improves the degree of hydrogenation, but also forms an effective back surface field. Controlled rapid cooling was implemented after the hydrogenation anneal and contact firing to improve the retention of hydrogen at defect sites using RTP. RTP contact firing improved the performance of ribbon solar cells by 1.3-1.5% absolute when compared to slow, belt furnace contact firing. Enhanced hydrogenation and rapid heating and cooling resulted in screen-printed Si ribbon cell efficiencies approaching 15%. A combination of screen-printed Al and a two minute RTP anneal in an oxygen ambient produced simultaneously a high quality rapid thermal oxide (RTO) and an aluminum back surface field (Al-BSF) with a back surface recombination (BSRV) of 200 cm/s 2-3 Ohm-cm single and multicrystalline silicon solar cells. In addition, RTO/SiN(x) stack passivation was found to be superior to SiN(x) surface passivation. RTO/SiN(x) passivation reduces the BSRV to ~10 cm/s on 1-2 Ohm-cm p-type single crystal Si and also lowers the Joe of 40 and 90 Ohm/sq emitters by a factor of three and ten, respectively. Integration of RTP emitters, screen-printed RTP Al-BSF and RTO produced 19% and 17% efficient monocrystalline cells with photolithography and screen-printed contacts, respectively

Understanding and Implementation of Hydrogen Passivation of Defects in String Ribbon Silicon for High-Efficiency, Manufacturable, Silicon Solar Cells

Photovoltaics offers a unique solution to energy and environmental problems simultaneously. However, widespread application of photovoltaics will not be realized until costs are reduced by about a factor of four without sacrificing performance. Silicon crystallization and wafering account for about 55% of the photovoltaic module manufacturing cost, but can be reduced significantly if a ribbon silicon material, such as String Ribbon Si, is used as an alternative to cast Si. However, the growth of String Ribbon leads to a high density of electrically active bulk defects that limit the minority carrier lifetime and solar cell performance. The research tasks of this thesis focus on the understanding, development, and implementation of defect passivation techniques to increase the bulk carrier lifetime in String Ribbon Si in order to enhance solar cell efficiency. Hydrogen passivation of defects in Si can be performed during solar cell processing by utilizing the hydrogen available during plasma-enhanced chemical vapor deposition (PECVD) of SiNx:H films. It is shown in this thesis that hydrogen passivation of defects during the simultaneous anneal of a screen-printed Al layer on the back and a PECVD SiNx:H film increases the bulk lifetime in String Ribbon by more than 30 ?A three step physical model is proposed to explain the hydrogen defect passivation. Appropriate implementation of the Al-enhanced defect passivation treatment leads to String Ribbon solar cell efficiencies as high as 14.7%. Further enhancement of bulk lifetime up to 92 ?s achieved through in-situ NH3 plasma pretreatment and low-frequency (LF) plasma excitation during SiNx:H deposition followed by a rapid thermal anneal (RTA). Development of an optimized two-step RTA firing cycle for hydrogen passivation, the formation of an Al-doped back surface field, and screen-printed contact firing results in solar cell efficiencies as high as 15.6%. In the final task of this thesis, a rapid thermal treatment performed in a conveyer belt furnace is developed to achieve a peak efficiency of 15.9% with a bulk lifetime of 140 ?Simulations of further solar cell efficiency enhancement up to 17-18% are presented to provide guidance for future research.Ph.D.Committee Chair: Rohatgi, Ajeet; Committee Member: Carter, William Brent; Committee Member: Liu, Meilin; Committee Member: Wang, Z.L.; Committee Member: Wong, C.P

Implied-V(oc) and Suns-V(oc) Measurements in Multicrystalline Solar Cells

Presented at the 29th IEEE Photovoltaic Specialists Conference; New Orleans, Louisiana; May 17-24, 2002. ©2002 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.Identifying loss mechanisms and predicting device performance are key goals of device and process characterization. Photoconductance measurements allow the extraction of the Implied V(oc) and Suns V(oc), which together can be used for process monitoring, for loss analysis and to identify the potential device performance in the absence of unwanted defects. In this paper, we measure the Implied V(oc) and Suns V(oc) from solar cells with a range of different substrates and at different stages in processing. These measurements are used to analyze the correlation with the actual V(oc) to determine the impact of both non-idealities such as depletion region recombination, and expected effects such as lifetime changes, both during processing and in the final devices

PECVD SiN(x) Induced Hydrogen Passivation in String Ribbon Silicon

Presented at the 28th IEEE Photovoltaic Specialists Conference; Anchorage, Alaska; September, 2000. ©2000 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in other works must be obtained from the IEEE.To improve the bulk minority carrier lifetime in String Ribbon silicon, SiN(x) induced defect passivation during a post deposition anneal is investigated. Our results indicate that SiN(x) induced hydrogen passivation is very effective when the SiN(x) film is annealed in conjunction with a screen-printed AI layer on the back. In addition, it is found that controlled rapid cooling can be used to enhance the defect passivation process. A model is proposed which relates the high temperature passivation to the release of hydrogen from the SiN(x) film, the injection of vacancies from backside AI alloying, and the retention of hydrogen at defect sites. High efficiency screen-printed String Ribbon solar cells (>14.5%) are fabricated utilizing the simultaneous SiN(x)/AI anneal in a belt furnace for hydrogenation and AI-BSF formation, followed by RTP firing of screen-printed contacts to improve the retention of hydrogen at defects

Cost and Technology Roadmaps for Cost-Effective Silicon Photovoltaics

Presented at the 12th International Workshop on the Physics of Semiconductor Devices; New Delhi, India; December 16-20, 2003.The cost of photovoltaics (PV) is expected to decrease by a factor of two to four within the next two decades, making PV an integral part of the solution to the problems of fossil fuel depletion and growing energy demand. This paper describes cost and technology roadmaps for achieving 17–18%-efficient crystalline Si solar cells at a competitive manufacturing cost of less than $1/W

High Efficiency Mono-Crystalline Solar Cells with Simple Manufacturable Technology

Presented at the 21st European Photovoltaic Solar Energy Conference and Exhibition; Dresden, Germany; September 4-8, 2006.This paper describes the analysis and optimization of phosphorus-doped n(+) emitters for Si solar cells with screen-printed contacts to improve the uniformity of contact formation. Analysis of the simulated emitters showed that J(oe) increases with the increase in phosphorus surface concentration. Cells fabricated on emitter having a higher surface concentration and shallower junction depth, were on an average 0.3% (absolute) higher in efficiency and 0.5 mA/cm (2) higher in J(sc) values. Internal quantum efficiency analysis showed that the J(sc) enhancement was due to better short wavelength response in these cells. In addition the fill factors were also slightly higher in the cells with higher surface concentration and shallower junction depth. SEM analysis showed larger (~1.5μm) and more uniformly distributed Ag crystallites on the surface of cells with emitter that had higher surface concentration. This may lead to a more tolerant contact firing process and result in a higher yield of high-efficiency cells. Furthermore, use of emitters with higher phosphorus surface concentration and shallower junction depth reduces the cell processing time appreciably leading to high throughput and cost savings in cell manufacturing. We were able to tailor the emitter profile and the firing conditions of a commercially available front silver paste to obtain good average FF’s of 77.7% in conjunction with short circuit current (J(sc)) of 34.8 mA/cm (2) and an open circuit (V(oc)) of 619 mV and efficiency of ~17% on 149 cm (2) Czochralski silicon wafers

Beneficial Impact of Low Frequency PECVD SiN(x):H-Induced Hydrogenation in High-Efficiency String Ribbon Silicon Solar Cells

Presented at the 19th European Photovoltaic Solar Energy Conference and Exhibition, Paris, France; June 7-11, 2004.PECVD SiN(x):H-induced hydrogenation of bulk defects in String Ribbon Si during RTP anneal is investigated in this study to enhance the carrier lifetime and understand the role of the plasma excitation frequency and an in-situ NH3 plasma pretreatment before SiN(x):H deposition. The results show that a low frequency SiN(x):H film with a NH3 plasma pretreatment annealed in RTP at 740°C for 60 seconds enhances the lifetime in String Ribbon Si from 5-6 μs to 90-100 μs. Secondary ion mass spectroscopy underneath SiN(x):H films deposited with deuterated ammonia (ND3) and silane shows greater deuterium incorporation in Si under the low frequency SiN(x):H film. Thus, hydrogen incorporated in Si during SiN(x):H deposition may act as an additional source that enhances hydrogen defect passivation during subsequent RTP treatments. In addition, the effect of the anneal time during RTA for hydrogenation is studied in an effort to reduce the hydrogenation time and improve the retention of hydrogen at defects in Si. The RTA time for hydrogenation is reduced to one second without loss of lifetime enhancement and leads to the fabrication of high-efficiency String Ribbon solar cells (17.9%) with photolithography-defined contacts. A rapid belt furnace contact co-firing scheme is developed based on the short RTA and produces screen-printed 4-cm2 String Ribbon solar cells with efficiencies as high as 15.9%