Solar References
1) “Effect of high temperature steam annealing for SiO2 passivation” by Abe, Y. et.al.,
Solar Energy Materials & Solar Cells 65 (2001) 607-612
The reduction of surface recombination velocity of minority carriers in single and poly-crystalline Si solar cells is very important to obtain high-efficiency solar cells. To reduce the solar cell processing costs, it is desired to use thinner device structures. However, for thinner cells the energy conversion efficiency is reduced by increased surface recombination velocity. SiO2 passivation and forming gas annealing (FGA) are commonly used to post-anneal. FGA is performed at 3% H2 at 400-450C for 30 minutes.
Commonly, hydrogen is used to anneal at the SiO2/Si interface. Hydrogen is needed for the process, but can be from molecular, atomic, or water vapor source. Hydrogen diffuses to the SiO2/Si interface.
In the paper, high temperature steam was used as the hydrogen source for post anneal. Process parameter were 400 to 450C for 5 minutes at 1 bar. Flow was 13.4 slm. which was sufficient to get significant improvement over FGA and comparable to Hydrogen Radical Anneal, HRA.
Improvement was attributed to a decrease of the interface state density. Reduced interface density slowed surface recombination velocity.
2) “Silicon Oxide/ Silicon Nitride Stack System for 20% Efficient Silicon Solar Cells” by Schultz et.al. 31st IEEE PVSC Orlando, Fl, 2005
Thick Oxides of greater than 100 nm have led to high efficiency solar cells from mono and multicrystalline silicon at greater than 20%. The good performance of the oxide is due to good surface passivation by reduction of the density at the interface.
For solar cells to be cost effective efficiencies have to increase and wafer thickness must decrease. To increase efficiency, performance of the rear surface optical ( internal reflectance) and electrical quality ( Passivation) must be improved.
Oxide Passivation prevents recombination at the rear by reducing the density of the interface states and is not affected by local contacts. SiN and thin oxides were not as efficient.
3) “Annealing in water vapor as a new method for improvement of silicon thin film properties” by S. Honda, et. al., J. Non-Crystalline Solids 352 (2006) 955-958
Hydrogenated microcrystalline silicon suffers from low deposition rates (1 nm/s). Polycrytalline films ( poly-Si) can be deposited at much higher rates but requires high growth temperatures (1050C) and thus contain no hydrogen for passivating defects at grain boundaries, resulting in low efficiency. Defect passivation by plasma hydrogenation is widely used, but it is too slow and expensive for PV production, and prolonged hydrogenation also creates new defects on the surface.
The researchers believe they are the first to try H20 vapor treatment to passivate silicon layers for solar cells. H2O treatment occurred in quartz tube at 300C from 5 minutes to 7 hours.
Defects have a profound influence on the barriers at the grain boundaries and thus also on the electron mobility. For this reason we have characterized the electronic properties of the films mainly by measuring the Hall effect after removal of the oxide layer. Hall mobility was improved from 4.45 cm-2/Vs to 15 cm-2/Vs with H2O vapor in one hour. A difference in passivation between water vapor and hydrogen was suspected due to the decrease in crystal disorder with H2O compared to H2, where it is expected H2 plasma damages the poly-Si. In addition, H2 content in bulk substrate was found to be 6%, while it was less than 1% after 7 hours with H2O vapor. H2O passivates without creating defects or crystal disorder found with HRA.
4) “Boron Doping Effects on the Electro-optical Properties of Zinc Oxide Thin Films Deposited by Low-Pressure Chemcial Vapor Deposition Process” J. Steinhauser , et. al., Mater. Res. Soc. Symp. Proc. Vol.928 2006 MRS
Boron doped zinc oxide films are used as transparent conductive oxide (TCO) to contact thin film silicon solar cells. In order to contact the solar cell and let the light enter into the absorbing part of the cell they have to exhibit good electrical (high conductivity) and optical (high transmittance) properties. In addition to these characteristics, they also have to scatter the light at the TCO-cell interface in order to increase the effective absorption of the light within the active layer of the cell.
Zinc oxide is n-type with 80% better transmittance. Large grains appear at the growing surface as large pyramids which yield an as-grown surface texture that efficiently diffuses the light that enters into the solar cell.
The ZnO layer was deposited by LP-CVD on 0.7 mm thick Schott AF45 glass substrates. Diethylzinc (DEZ) and water vapor (which was not diluted in a carrier gas) were used as pressures and directly evaporated in the system. DEZ and H2O flows were set to 13.5 sccm and 16.5 sccm respectively.
Highly doped boron ZnO films were limited by scattering within the grain, such as ion impurity scattering. Referred to as Free Carrier Absorption, FCA, increasing Boron doping reduces transmittance in the near infrared region. Undoped and low doped ZnO films were mainly limited by grain boundaries scattering, which was reduced as grain size increased.
In order to reduce as much as possible FAC effect which degrades optical transmittance in the near infrared range while still maintaining low resistivity values, it is mandatory to have ZnO films with high electron mobility. This is possible 36 cm2V/s for thick low doped ZnO films having a large grain size (lambda = 500 nm).
5) “High-efficiency p-i-n a-Si:H solar cells with low boron cross-contamination prepared in a large-area single-chamber PECVD reactor” U. Kroll et al. Thin Solid Films 451-452 (2204) 525-530.
Plasma deposition of aSi:H p-i-n solar cells in a single chamber reactor leads to considerable simplifications and reduced costs compared to multi-chamber processes. However, deposition of the i-layer over the p-layer can lead to boron contamination from the chamber walls. Boron can contaminate the initial i-layer and the critical p-i interface and thereby weaken the strength of the electrical field in the i-layer close to the p-i interface. This lead to less efficient carrier separation just in this zone and leads to a reduced collection efficiency in the solar cell and thereby a deterioration of the cell performance.
A Si:H buffer layer was inserted between the p-i layer. The chamber was flushed with water vapor between the p- and i-layer (later with a buffer layer) at 200C at 1 mbar with no water vapor measured flow rate for 10 minutes. No plasma required and the substrate with the p-layer can remain loaded during treatment. Oxidation by direct oxygen injection did not work. Water vapor increased efficiency by 1.5% over vacuum pumping to 10.13% and reduced process time by 20 minutes to 30 minutes. Water vapor purge generated good carrier collection in the i-layer close to the p-layer and suggests a low boron contamination.
It is assumed the water vapor locks the boron oxide into the silicon making B electrically inactive. In addition to performance and time gain, we found additionally the oxidation treatment improves the process robustness with respect to boron cross contamination.
6) Fraunhofer ISE annual Report 2005
Efficiency values are limited by bulk recombination of charge carriers within the cell. In the materials, both localized impurities and extended defects such as dislocations or grain boundaries act as recombination centers. The behavior of these recombination centers changes during the manufacturing process.
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