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SP Al-BSF solar cells efficiency of samples from different locations from a) low-and high-resistivity B-doped Cz ingots b) Ga-doped Cz ingot a) b)
Source publication
A systematic study of the variation in resistivity and lifetime on cell performance, before and after light-induced degradation (LID), was performed along the B- and Ga-doped Czochralski (Cz) ingots. Screen-printed solar cells with Al-back surface field were fabricated and analyzed from different locations on the ingots. Despite the large variation...
Context in source publication
Citations
... received the B.S. degree from the School of Materials Science and Engineering, Korea University, Seoul, South Korea, in 1999 and the M.S. and Ph.D. degrees from the Gwangju Institute of Science and Technology, Gwangju, South Korea, in 2001 and 2005, respectively. After receiving Ph.D. degree, he was with the Solar Cell Laboratory, Korea University, Seoul, as a Postdoctoral Fellow for two years and then with the Semiconductors and Solar Cells Laboratory, Australian National University, Canberra, Australia, as a Visiting Researcher for one year. ...
Solar cells fabricated on boron (B)-doped Czochralski (Cz) Si wafers in the photovoltaic industry are known to suffer from light-induced degradation (LID) in efficiency. This paper reports on promising LID-free large-area indium (In)-doped Cz Si solar cells. Two different commercial-grade B-doped Cz materials were included for comparison. To study the impact of LID on the cell structure, ion-implanted large-area (239 and 242.22 cm2) screen-printed full aluminum (Al) back-surface field (BSF) baseline cells, as well as higher performance passivated emitter rear cells (PERC) with oxide passivation and local Al BSF, were fabricated. In-doped PERC cells achieved 20.3% efficiency, while the B-doped cells gave efficiencies of 20.7% and 20.5% from low- (2 Ω · cm) and high-resistivity (6.2 Ω · cm) substrates, respectively. It was found that initial efficiency of In-doped PERC cells was ∼0.2% lower due to lower bulk lifetime and higher back-surface recombination velocity. However, In-doped PERC cells showed no LID and surpassed the B-doped PERC cell efficiency by 0.3-0.5% after 0.8-sun 48-h illumination at 37 °C.
... To avoid the degradation, the use of other p-type dopants, such as gallium, was proposed and investigated. 2,3) The doping of silicon with indium, is reported in literature at least since 1979 when a gradient-transport-solution growth process was used to investigate the solubility limit of indium in silicon and to characterize the indium doped crystal. 4,5) Recently further investigations were performed on the crystal growth process of indium-doped silicon related to the complications associated with the use of indium as dopant. ...
Indium is becoming one of the most important dopant species for silicon crystals used in photovoltaics. In this work we have investigated the behavior of indium in silicon crystals grown by the Czochralski pulling process. The experiments were performed by growing 200 mm crystals, which is a standard diameter for large volume production, thus the data reported here are of technological interest for the large scale production of indium doped p-type silicon. The indium segregation coefficient and the evaporation rate from the silicon melt have been calculated to be 5 × 10−4 ± 3% and 1.6 × 10−4 cms−1, respectively. In contrast to previous works the indium was introduced in liquid phase and the efficiency was compared with that deduced by other authors, using different methods. In addition, the percentage of electrically active indium at different dopant concentrations is calculated and compared with the carrier concentration at room temperature, measured by four-point bulk method.
... LID can be reduced or eliminated by choosing n-type material, reducing oxygen and boron concentrations, and replacing B dopant by Ga or In. Considerable research has been done on n-type solar cells, reduction of oxygen [6] and introduction of Ga dopant [7][8]10]. However, very little is known about the performance of In-doped Si cells [9,10] and their LID behavior. ...
We have developed a high-throughput Si CVD epitaxy system for photovoltaic application. The system was designed with high throughput, low energy cost, and high utilization of precursors, thus enabling extremely low cost Si epitaxial film for photovoltaic application. The system will be capable of processing a large batch of >320 wafers in less than an hour. The novel system architecture effectively utilizes resources such as electricity and process gases, and the processing cost can be as low as $0.22 per wafer for 50 μm deposition. Initial results confirmed good power utilization and excellent film properties. As an example of application, solar cells have been fabricated with epitaxial films on wafers from ingots pulled from metallurgical Si in Czochralski method.
Purpose:
Intensity modulated radiation therapy (IMRT) allows the delivery of escalated radiation dose to tumor while sparing adjacent critical organs. In doing so, IMRT plans tend to incorporate steep dose gradients at interfaces between the target and the organs at risk. Current quality assurance (QA) verification tools such as 2D diode arrays, are limited by their spatial resolution and conventional films are nonreal time. In this article, the authors describe a novel silicon strip detector (CMRP DMG) of high spatial resolution (200 microm) suitable for measuring the high dose gradients in an IMRT delivery.
Methods:
A full characterization of the detector was performed, including dose per pulse effect, percent depth dose comparison with Farmer ion chamber measurements, stem effect, dose linearity, uniformity, energy response, angular response, and penumbra measurements. They also present the application of the CMRP DMG in the dosimetric verification of a clinical IMRT plan.
Results:
The detector response changed by 23% for a 390-fold change in the dose per pulse. A correction function is derived to correct for this effect. The strip detector depth dose curve agrees with the Farmer ion chamber within 0.8%. The stem effect was negligible (0.2%). The dose linearity was excellent for the dose range of 3-300 cGy. A uniformity correction method is described to correct for variations in the individual detector pixel responses. The detector showed an over-response relative to tissue dose at lower photon energies with the maximum dose response at 75 kVp nominal photon energy. Penumbra studies using a Varian Clinac 21EX at 1.5 and 10.0 cm depths were measured to be 2.77 and 3.94 mm for the secondary collimators, 3.52 and 5.60 mm for the multileaf collimator rounded leaf ends, respectively. Point doses measured with the strip detector were compared to doses measured with EBT film and doses predicted by the Philips Pinnacle treatment planning system. The differences were 1.1% +/- 1.8% and 1.0% +/- 1.6%, respectively. They demonstrated the high temporal resolution capability of the detector readout system, which will allow one to investigate the temporal dose pattern of IMRT and volumetric modulated are therapy (VMAT) deliveries.
Conclusions:
The CMRP silicon strip detector dose magnifying glass interfaced to a TERA ASIC DAQ system has high spatial and temporal resolution. It is a novel and valuable tool for QA in IMRT dose delivery and for VMAT dose delivery.
The objective of this thesis is to understand and improve optical and electrical confinement to achieve cost-effective high-efficiency thin p-type Si solar cells. Optical confinement is achieved by front surface texturing in conjunction with an internal reflective layer on the back surface. Electrical confinement is obtained through the use of a high-lifetime material coupled with high-quality passivation on both surfaces. This research is divided into five tasks. In the first task, Ga-doped Cz Si was investigated to achieve a high and stable lifetime. It was found that for 1 ohm-cm nominal-resistivity screen-printed Al-back surface filed (BSF) cells, the Ga-doped ingot gave ~1.5% higher absolute efficiency after light-soaking relative to the B-doped counterpart. The benefit of using Ga is therefore quite explicit. In the second task, the screen-printed Al-BSF was investigated to explore its potential and limitations for achieving high-efficiency cells. It was found that there exists a critical alloying temperature for a given Al-thickness, above which the Al-BSF becomes non-uniform and cell performance starts to degrade. This puts a limit on the quality of the Al-BSF that can be achieved. An alternative way of back passivation involving dielectric/metal layers was therefore explored. In Task three, two key requirements for achieving high-efficiency dielectric back-passivated cells were established through device modeling. These are (1) a formation of a high-quality BSF underneath the local back contact through vias in the dielectric and (2) a high-quality dielectric passivation with either a moderate positive charge density or a high negative charge density. Task four involved the development of a metallization technique through vias in the dielectric to achieve a high-quality contact and an efficient internal reflector in conjunction with a high-quality local BSF. Further, a novel dielectric system composed of a spin-on SiO<SUB>2</SUB> layer capped with SiN<SUB>x</SUB> was developed that exhibited excellent passivation and a moderate positive charge density. The final task involved fabrication and analysis of dielectric back-passivated cells. The new dielectric and process sequence developed in this thesis resulted in screen-printed solar cells with efficiency as high as 19% with the potential for 20% efficient cells on 100-µm thick Si substrates. Ph.D. Committee Chair: Rohatgi, Ajeet; Committee Member: Begovic, Miroslav; Committee Member: Doolittle, William; Committee Member: Gaylord, Thomas; Committee Member: Liu, Meilin
Photovoltaics (PV) offers a unique opportunity to solve energy and environmental problems simultaneously since the solar energy is essentially free, unlimited, and not localized any part of the world. Currently, more than 90% of PV modules are produced from crystalline Si. However, wafer preparation of cast multicrystalline Si materials account for more than 40% of the PV module manufacturing cost, which can be significantly reduced by introducing the ribbon-type Si materials. Edge-defined film-fed grown (EFG) and String Ribbon Si materials are among the promising candidates for the cost-effective PV because they are grown directly from the Si melt, which eliminates the need for ingot slicing and chemical etch for surface preparation. However, the growth of these ribbon Si materials leads to relatively high concentration of metallic impurities and structural defects, resulting in very low as-grown carrier lifetime of less than 5 µs. Therefore, the challenge is to produce high-efficiency cells on EFG and String Ribbon Si by enhancing the carrier lifetime during the cell processing and to understand the effect of electrically active defects on cell performance through in-depth device characterization and modeling. The research tasks of this thesis focus on the understanding, development, and implementation of defect passivation to enhance the bulk carrier lifetime in ribbon Si materials for achieving high-efficiency cells. It is shown in this thesis that the release of hydrogen from SiNx layer is initially rapid and then slows down with time. However, the dissociation of hydrogen from defects continues at the same pace. Therefore, a short firing provides an effective defect passivation. An optimized hydrogenation process produces a record high-efficiency ribbon Si cells (4.0 cm2) with photolithography (18.3%) and screen-printed (16.8%) contacts. However, active defects are still present even after the optimized hydrogenation process. An analytical model is developed to assess the impact of inhomogeneously distributed active defects on cell performance, and the model is applied to establish the roadmap for achieving high-efficiency ribbon Si cells in the presence of defects. Finally, PC1D simulations reveal that the successful implementation of the surface texturing can raise the cell efficiency to 18%. Ph.D. Committee Chair: Dr. Ajeet Rohatgi; Committee Member: Dr. Bernard Kippelen; Committee Member: Dr. Gabriel Rincon-Mora; Committee Member: Dr. Miroslav Begovic; Committee Member: Dr. W. Brent Carter
