B. von Roedern

A new modular multichamber plasma enhanced chemical vapor deposition system

Abstract The present work reports on a new modular UHV multichamber PECVD system with characteristics which prevent both the incorporation of residual impurities and cross contamination between different layers. A wide range of intrinsic and doped hydrogenated amorphous silicon (a-Si:H) materials have been produced and single junction pin solar cells with an efficiency greater than 10% have been readily obtained with little optimization. The system contains three UHV modular process zones (MPZs); the MPZs and a load lock chamber are located around a central isolation and transfer zone which contains the transport mechanism consisting of an arm with radial and linear movement. This configuration allows for introduction of the substrate into the MPZs in any sequence so that any type of multilayer device can be produced. The interelectrode distance in the MPZs can be adjusted between 1 and 5 cm. This has been found to be an important parameter in the optimisation of the deposition rate and of the uniformity. The multichamber concept also allows individually optimized deposition temperatures and interelectrode distances for the various layers. The system installed in Utrecht will be employed for further optimization of single junction solar cells and for research and development of stable a-Si:H tandem cells.

Current status of polycrystalline thin-film PV technologies

Thin-film solar cell technologies based on CdTe and CIS have made significant technical progress. Most of the improvements have occurred at the level of solar cells, modules, systems, and reliability testing in both CdTe and CIS. Total-area efficiencies of 14.7% for a thin-film CdTe solar cell deposited on a low-cost, sodalime glass fabricated by Golden Photon, Inc. was verified by NREL. Siemens Solar Industries has fabricated a world-record, aperture-area efficiency of 11.1% and a power output of 40.6 W for a thin-film CIGSS module. Solar Cells, Inc. has fabricated a large-area, thin-film CdTe module with an aperture-area efficiency of 9.1% and power output of 61.3 W; Golden Photon, Inc., has also fabricated a power module with an aperture-area efficiency of 9.2% and power output of 31.0 W. Key research issues for CdTe and CIS are discussed in this paper. Several polycrystalline thin-film arrays based on CdTe and one based on CIGSS have been deployed worldwide. Stability data indicated by both thin-film CIS and thin-film CdTe modules and systems are encouraging. Many companies worldwide are actively pursuing early commercialization efforts based on both speciality products and power modules.

Performance test of amorphous silicon modules in different climates - year four: Progress in understanding exposure history stabilization effects

In a round robin outdoor exposure experiment carried out in three different climates, we have previously demonstrated that amorphous silicon (a-Si) PV modules reach higher stabilized performance levels in warmer climates. The four-year experiment involved three identical sets of thin-film a-Si modules from various manufacturers deployed outdoors simultaneously in three sites with distinct climates. Each PV module set spent a one-year period at each site before a final period at the original site where it was first deployed. The experiment aimed to determine the light-induced degradation and stabilization characteristics of a-Si regarding specific history of exposure, and to compare degradation rates in different climates. We propose that after the initial sharp degradation associated with the Stabler-Wronski effect (SWE) has passed, the subsequent stabilized performance levels attained will depend largely on light exposure and a characteristic temperature associated within a coherent time-scale. PV modules which were first deployed at the lowest-temperature site for one year, reaching a stabilized state, and were then further deployed at higher temperature sites for two more years, experienced considerable recovery in output parameters (Pmax and FF). However, when further deployed back at the original, lowest-temperature site, performance degraded back to the first year, original level.

Progress and issues in polycrystalline thin-film PV technologies

Substantial progress has occurred in polycrystalline thin-film photovoltaic technologies. However, the transition to first-time manufacturing is still under way, and technical problems continue, This paper focuses on the promise and the problems of the copper indium diselenide and cadmium telluride technologies, with an emphasis on continued R&D needs for the near-term transition to manufacturing and for next-generation improvements. In addition, it highlights the joint R&D efforts being performed in the US Department of Energy/National Renewable Energy Laboratory Thin-Film Photovoltaic Partnership Program.

Second controlled light-soaking experiment for amorphous silicon modules

Dual-junction and triple-junction amorphous silicon modules from three manufacturers were subjected to light soaking at 1-sun intensity at 50/spl deg/C, loaded to the maximum power point, for 1000-2000 hours, with annealing to 70/spl deg/C in the dark after 1000 hours. Performance characterization was done periodically, both under a pulsed solar simulator and outdoors. Aperture-area efficiencies as high as 9.1% were obtained after 1000 hours of light-soaking. The power output after 1000 hours of light soaking and subsequent partial annealing ranged from 77% to 92% of the initial power output. The recovery in power due to annealing was 4%-6.5%. For a-Si/a-Si-type modules, stabilized performance was reached before 1000 hours. The validity of the results is discussed in detail.<<ETX>>

Model for Staebler-Wronski degradation deduced from long-term, controlled light-soaking experiments

Long-term light-soaking experiments of amorphous silicon photovoltaic modules have now established that stabilization of the degradation occurs at levels that depend significantly on the operating conditions, as well as on the operating history of the modules. The authors suggest that stabilization occurs because of the introduction of degradation mechanisms with different time constants and annealing activation energies, depending on the exposure conditions. Stabilization will occur once a sufficient accumulation of different degradation mechanisms occurs. They find that operating module temperature during light-soaking is the most important parameter for determining stabilized performance. Next in importance is the exposure history of the device. The precise value of the light intensity seems least important in determining the stabilized efficiency, as long as its level is a significant fraction of 1-sun.

Study of light induced instability in intrinsic hydrogenated amorphous silicon films by the photomixing technique

By using the photomixing technique we have found that the drift mobility (μd) of intrinsic hydrogenated amorphous silicon (a‐Si:H) films produced by both glow discharge and hot wire techniques increases with increasing electric field, while the lifetime (τ) decreases with increasing electric field, and the μdτ product is essentially independent of the electric field. We have also found an empirical relationship that a greater field dependence of the drift mobility of an a‐Si:H film in the annealed state indicates a poorer stability of the photoconductivity upon light soaking.

Polycrystalline Thin-Film Photovoltaics: From the Laboratory to Solar Fields

We review the status of commercial polycrystalline thin-film solar cells and photovoltaic (PV) modules, including current and projected commercialization activities

Polycrystalline thin-film photovoltaic technologies: from the laboratory to commercialization

Substantial technical progress has occurred in the area of polycrystalline thin-film photovoltaic technologies based on cadmium telluride (CdTe) and copper indium gallium diselenide (CuInGaS/sub 2/, [CIGS]) in the past few years. We report here the many technical advances made in the area of materials research, device development, manufacturing technology, module and system deployment and testing, and early commercialization activities worldwide related to CdTe and CIGS products for specialty and power applications. Also, the various technical issues related to CdTe and CIS are elucidated. NREL scientists have achieved record total-area efficiency of 15.8% for a novel thin-film CdTe solar cell, and world-record total-area efficiency of 18.8% has been achieved fora thin-film CIGS, solar cell. Details of cell processing are given. World-record power output of 915 W and aperture-area efficiency of 10.6% have been achieved by BP Solar, Fairfield, California for a thin-film CdTe power module. Also, a world-record aperture-area conversion efficiency of 12.1% for a thin-film CIGSS power module fabricated by Siemens Solar Industries, Camarillo, California. Both efficiencies and power outputs have been independently confirmed by NRFL.

Electronic properties of hydrogenated amorphous silicon–germanium alloys and long-range potential fluctuations

Abstract The charge transport properties and microstructure of hydrogenated amorphous silicon–germanium alloys (a-SiGe:H) prepared by the hot-wire chemical vapor deposition (HWCVD) process as a function of alloy composition have been investigated in detail by employing the photoconductive frequency mixing and small angle X-ray scattering techniques. Evidence for the presence of long-range potential fluctuations in a-SiGe:H alloys is revealed from the measurements of electric field dependence of the drift mobility. The effect of the long-range potential fluctuations is enhanced by the addition of Ge to the alloy system that results in the deterioration of the opto-electronic properties of a-SiGe:H alloys. Through the drift mobility field dependence, the depth and range of the potential fluctuations as a function of alloy composition are determined, and subsequently the charged defect density. It was found that at a composition of ∼10% Ge in Si, the photoresponse begins to decrease monotonically with increasing Ge content due to the decreases in the drift mobility and lifetime as a result of an increase in the concentration of charged defects, which lead to the long-range potential fluctuations whose depth increases, while the range decreases. The sharp changes in these parameters are demonstrated to be attributed to a concurrent abruptly increased structural heterogeneity due to the introduction of microvoids.