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.
Reduction of Tin Oxide by Hydrogen Radicals
The effect of a reducing hydrogen ambient on textured tin oxide thin films on glass substrates has been investigated. Hydrogen treatments were done at 230 and 430 °C by hot wire (HW) and rf plasma-...
rPET detectors design and data processing
Small animal PET systems based on rotating planar detectors posses some interesting advantages for high sensitivity, high resolution imaging. We have designed the rPET detectors based on MLS crystals assembled on a 30times30 matrix optically coupled to a flat-panel PS-PMT. Weighted position readout circuits pre-process the 64 signals from the 8times8 anodes matrix, which are digitized using a charge-integrating converter. The amplification electronics, including the trigger output for coincidence detection, and the high voltage supply are integrated in a three PCBs stack that forms the base attached to the back of the PMT. The whole assembly is enclosed in a light tight, lead (Pb) shielded aluminum box. The detectors are mounted on a rotating gantry with more than 180 degrees rotation span. The digitized events are screened and histogramed, and a modified center of gravity algorithm removes from the position calculation those signals with poor signal to noise ratio. Apparent mean crystal size on the 511 keV field-flood images is 0.6 mm, mean peak-to-valley ratio is better than 8, and intrinsic resolution is 1.5 mm at the central row, with the energy window wide open. Sensitivity (CPS) for a pair of these detectors set in coincidence at 160 mm distance is 1%