Amorphous Silicon Alloy Materials and Solar Cells Near the Threshold of Microcrystallinity
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ABSTRACT One of the most effective techniques used to obtain high quality amorphous silicon alloys is the use of hydrogen dilution during film growth. The resultant material exhibits a more ordered microstructure and gives rise to high efficiency solar cells. As the hydrogen dilution increases, however, a threshold is reached, beyond which microcrystallites begin to form rapidly. In this paper, we review some of the interesting features associated with the thin film materials obtained from various hydrogen dilutions. They include the observation of linear-like objects in the TEM micrograph, a shift of the principal Si TO band in the Raman spectrum, a sharp, low temperature peak in the H2 evolution spectrum, a shift of the wagging mode in the IR spectrum, and a narrowing of the Si (111) peak in the X-ray diffraction pattern. These spectroscopic tools have allowed us to optimize deposition conditions to near the threshold of microcrystallinity and obtain desired high quality materials. Incorporation of the improved materials into device configuration has significantly enhanced the solar cell performance. Using a spectral-splitting, triple-junction configuration, the spectral response of a typical high efficiency device spans from below 350 nm to beyond 950 nm with a peak quantum efficiency exceeding 90%; the triple stack generates a photocurrent of 27 mA/cm 2 . This paper describes the effect of the improved materials on various solar cell structures, including a 13% active-area, stable triple-junction device. INTRODUCTION Over the last two decades, photovoltaic (PV) technology using amorphous silicon (a-Si) alloy materials has advanced significantly to a stage where large-scale manufacturing is taking place worldwide [1]. Most of the products to date are aimed at terrestrial applications such as battery charging and building-integrated photovoltaics. On the other hand, due to the tremendous growth in the field of telecommunication, intensive efforts are underway to qualify the lightweight inexpensive thin-film solar cells for space applications [2]. Today, state-of-theart a-Si alloy triple-junction modules on thin, flexible substrates are being seriously evaluated in orbit such as those from United Solar Systems Corp. (United Solar) on board the Mir space station. Despite the substantial progress made in recent years, the biggest challenge for both terrestrial and extraterrestrial applications today is achieving even higher conversion efficiencies. In the United States, a National Thin-film Partnership Team including members from the PV industry, universities, and national laboratories was organized in 1992 by the National Renewable Energy Laboratory (NREL) to coordinate and address various aspects of the thinfilm materials and devices [3]. A broad spectrum of activities, ranging from fundamental material studies to novel multijunction designs, has been addressed. In the last several years, the most important finding from these activities for improving stabilized cell efficiencies using plasma-enhanced chemical vapo
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