Thin film silicon alloys with enhanced stability made by PECVD and HWCVD for multibandgap solar cells
This paper presents the development of high deposition rate protocrystalline Si:H and /spl mu/c-Si:H as obtained by hot wire chemical vapor deposition (HWCVD, or catalytic CVD) and very high frequency plasma enhanced CVD (VHF PECVD) and protocrystalline SiGe:H as obtained by conventional PECVD. Using the latter two low bandgap materials as absorber layer, it is feasible to obtain stable devices showing virtually no light-induced changes in fill factor. We have obtained stable proto-SiGe:H cells using high H/sub 2/ dilution. A disadvantage, however, is the very low deposition rate for proto-SiGe:H (/spl sim/0.6 /spl Aring//s). By HWCVD, much higher deposition rates have been achieved for silicon-based thin films (both proto-Si:H and pc-Si:H). During the last few years, improved control of parameters is also obtained in HWCVD, which presently makes this method interesting for reducing the production cost of thin film solar cells. Recently, we have demonstrated that protocrystalline Si:H can be obtained from undiluted silane at 250 /spl deg/C in HWCVD at a high deposition rate of 1 nm/s. The light induced degradation in single junction n-i-p cells is less than 10%. Here, we also present for the first time p-i-n cells on commercial SnO/sub 2/:F with proto-Si intrinsic layers deposited at rates up to 3 nm/s, with again remarkable stability. Further, HWCVD microcrystalline materials with a crystalline fraction V/sub f/ of /spl sim/50% possess excellent electronic quality: the dangling bond density extracted from computer simulations, is only 2 /spl times/ 10/sub 15/ cm/sup -3/ and therefore we could obtain fill factors of 0.77 and 0.75 in dual-junction stacked /spl mu/c-Si:H//spl mu/c-Si:H cells, and proto-Si:H//spl mu/c-Si:H 'micromorph' n-i-p type solar cells, respectively. Both VHF PECVD /spl mu/c-Si:H made in the high pressure depletion regime and HWCVD /spl mu/c-Si:H have excellent potential for multibandgap solar cells.
p-i interface engineering and i-layer control of hot-wire a-Si:H based p-i-n solar cells using in-situ ellipsometry
Abstract In this paper we report on the effect of monitoring the i-layer region near the p-i interface with the help of in-situ kinetic and spectroscopic ellipsometry on the performance of hot-wire deposited hydrogenated amorphous silicon p-i-n solar cells. It is very clearly observed that the microstructure at the p-i interface region in terms of the SiSi bond packing density and surface roughness significantly affects the cell performance. The filament temperature, T Fil , was the main parameter varied to control the above mentioned two properties near the p-i interface as well as in the bulk i-layer. In order to achieve significant enhancement in the cell performance we extended the idea of the “soft start”, earlier employed for the glow discharge deposited solar cells, to the hot-wire deposited i-layer. We were able to control the i-layer properties at the p-i interface and in the bulk independently and correlate these to the cell performance. It is shown that a major increase in cell performance can be achieved by improving the microstructure of the growing film directly at the p-i interface. Most interestingly, no significant deterioration in cell efficiency has been observed if only the p-i interface was properly controlled but the i-layer was of lower quality. These results are also shown to be consistent with model calculations of a numerical simulation. Our results therefore provide a clue to prepare hot-wire a-Si:H based solar cells with high efficiency and in the whole at high growth rates, which is needed for a more economic a-Si:H solar cell production.
Models of a Total Criminal Justice System
The need to examine the total criminal justice system—police, prosecution, courts, and correction agencies—in an integrated way constitutes a central problem in improving law enforcement. Too, any such analysis must reflect the feedback into society of offenders released at various stages in the system. This paper formulates a model for the criminal justice system in one state; it depicts the flow of arrested persons through the system as a function of type of crime, and provides a basis for apportioning costs to system components and to types of crime. The model's feedback feature includes the probability of rearrest as a decreasing function of age and a crime-switch matrix reflecting the successive-crime distribution. The results from the model include a cost distribution by crime type, criminal-career costs, an examination of the courses of criminal careers, and estimates of the sensitivities of costs and offender flows within the system to changes in its controllable variables.