Solar Cells最新文献

We present a summary of our defect chemical microscopic model for the effect that annealing in air or oxygen has on CuInSe₂ and CdTe-based polycrystalline thin film solar cells and explain its generalization to other chalcogenide-based semiconductor devices. The summary includes a hypothesis for specific O₂ (molecule) and surface interaction. From the point of view of device performance, the model provides a specific chemical explanation for the conclusions obtained from device analyses that the performance of the present generation of polycrystalline cells of this type is mainly limited by recombination at grain surfaces and boundaries.

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.

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.

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.

The availability of commercial spectroscopic ellipsometers (SE) has been restricted to the UV-visible range from 250–900 nm. Although this is useful for many applications, it must be extended to the near IR region (up to 1700 nm) for the study of the optical behavior of most photovoltaic materials. This paper discusses the development of a broad band (300–1700 nm) SE which has been used to measure the optical characteristics of various materials. Among these are the polycrystalline thin film materials, CuInSe₂ and CdTe (for which single crystal samples have also been investigated), and materials for high efficiency cascade solar cells including InP, InGaAs and InGaAsP. Most of these data are not presently available over such a wide spectral range.

Experimentally, a rotating polarizer-fixed analyzer ellipsometer with an a.c. detection system has been developed for accurate measurement of ψ and Δ, the relevant ellipsometric parameters, in the near IR. This approach has certain advantages over the rotating analyzer-fixed polarizer systems including reduced sensitivity to room light. The analytical methods include the use of a specially developed computer modeling program which gives ψ and Δ for a given set of values related to the film thickness (which may be finite or zero) and to the optical properties of the substrate.

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.

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.

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.