Solar Cells最新文献

The single-lifetime model is commonly used to describe recombination in photovoltaic materials. Here we describe two non-linear processes which affect the applicability of that model. Photon recycling is observed in direct band gap materials such as GaAs. This self-absorption and secondary emission of photons makes the effective radiative lifetime a function of device geometry. The saturation of recombination centers by minority carriers produces light intensity dependent lifetimes when the former are present. These effects need to be considered in device design and modeling.

This paper describes properties of microcrystalline silicon (μc-Si), and microcrystalline silicon-carbon (μc-SiC) thin films formed by the process of remote plasma-enhanced chemical-vapor deposition (PECVD). We discuss: (i) the way that the remote PECVD deposition process is applied to the deposition of μc-Si and μc-SiC thin films; (ii) the characterization and properties of the intrinsic and doped μc-Si thin film materials; (iii) the characterization and properties of the intrinsic and doped μc-SiC thin film materials; and (iv) the application of remote PECVD μc-Si and μc-SiC thin films in device structures.

This paper reviews the status of CuInSe₂ (CIS) module development. The potential of CIS for high power, thin film photovoltaic modules is demonstrated by the achievement of 14.1% active area cell efficiencies and unlaminated module power outputs of 10.5 W (11.2% aperture efficiency) on 940 cm² modules and 37.8 W (9.7% aperture efficiency) on 3900 m² modules. The definition of 0.4 m² CIS module pilot production is progressing.

Amorphous silicon technology offers an avenue for low-cost thin film photovoltaic applications. The performance of amorphous silicon-based solar cells is limited by light-induced degradation. Inadequate description of the electronic phenomena in these materials and devices hampers resolution of this problem. We are posing questions which should stimulate researchers to develop better descriptions for device performance and better microscopic models for defect sites. The issue of Staebler-Wronski degradation should not be addressed separately from initial performance, but research should focus on material and device properties in the stabilized state. The main focus of a-Si:H research sponsored by the U.S. Department of Energy will be improvement of stabilized performance, which we anticipate to accomplish through focused development of optimized multijunction device structures, combined with an improved understanding of materials.

Formation of polycrystalline thin film CuInSe₂ was achieved by the rapid thermal processing of vacuum-deposited copper, indium, and selenium. Films were fabricated and characterized in three composition regions: copper-poor (approximately 20 at.% Cu). stoichiometric (25 at.% Cu) and copper-rich (approximately 28 at.% Cu). Characterization results including X-ray diffraction analysis, electron probe for microanalysis, scanning electron microscopy, and optical reflection and transmission measurements are presented. The results show that nearly single-phase material has been formed from co-deposited precursors with a post-deposition annealing time of less than 2 min. The films have smooth morphologies amenable for photovoltaic device fabrication, optical absorption coefficients in the high 10⁴ cm⁻¹ range, and an optical band gap of 1.0 eV.

The performance of a number of electronic devices such as solar cells, power devices, and transistors is strongly influenced by minority charge-carrier lifetimes in the semi-conductor material. The conditions for float-zone growth of silicon crystals for these devices can be adjusted to achieve charge-carrier lifetimes as long as 20 ms. Low impurity levels are of course necessary, and the material must be free of dislocations and grain boundaries. Microdefects such as swirl defects (A- or B-type) and frozen-in defects are also carrier recombination centers, and thus must be controlled during crystal growth. The type and density of defects can be altered by changing the growth conditions. Long minority charge-carrier lifetimes are achieved at moderately high growth rates and low thermal gradients during crystal growth.

The Niagara Mohawk Power Corporation has begun operation of a photovoltaic (PV) system in upstate New York to study the summer peak load reduction capability of grid-connected PV systems serving commercial buildings. The roof-retrofitted system consists of a 151 m² polycrystalline silicon module area rated at 15.4 kW d.c., three one-axis trackers, and a high efficiency power conditioning unit. Preliminary results from the first two months of operation indicate PV system output is at a high fraction of capacity when the building experiences its electrical demand peaks. Ongoing studies are evaluating a cross-section of commercial customer load profiles in terms of the probability of peak demand reduction.

Testing of subgroups, trackers, and laminates was performed at the Carrizo Solar Photovoltaic Power Plant. Testing was performed to characterize the effects of ethylene vinyl acetate (EVA) degradation at the plant. EVA degradation is believed to have been accelerated by the high operating temperatures accompanying the use of mirrors to concentrate sunlight. Testing of 128 laminates revealed that degradation is highly non-uniform. Measured laminate peak power values ranged from 8.6 W to 47.8 W with an average value of 32.6 W. This average was 35.9% below the installed laminate rating at similar conditions. Mismatch between laminates contributed an additional power loss of 11.1%. Mismatch was found to increase with laminate temperature. One tracker was tested with and without mirror enhancement. It produced 3046.5 W with mirrors and 3275.9 W when the mirrors were covered. Preliminary results indicate that removal of the mirrors would increase the plant's peak power output on hot days.

A 4 kW photovoltaic (PV) power system composed of 144 Sovonics R100 amorphous-silicon alloy modules was constructed in southeastern Michigan in early 1987. During more than three years of continuous operation, the system and its components have been reliable and durable. Analysis of array performance has shown initial degradation, followed by stabilization. Cyclic efficiency variations have been found to be related to solar spectrum, ambient temperature and past history of temperature.