Amorphous Silicon Alloy Solar Cells Near the Threshold of Amorphous-to-Microcrystalline Transition
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Amorphous Silicon Alloy Solar Cells Near the Threshold of Amorphous-to-Microcrystalline Transition Jeffrey Yang, Kenneth Lord, Subhendu Guha, and S.R. Ovshinsky United Solar Systems Corp., 1100 West Maple Road, Troy, MI 48084 ABSTRACT A systematic study has been made of amorphous silicon (a-Si) alloy solar cells using various hydrogen dilutions during the growth of the intrinsic (i) layer. We find that the opencircuit voltage (Voc) of the cells increases as the dilution increases; it then reaches a maximum before it decreases dramatically. This sudden drop in Voc is attributed to the transition from amorphous silicon to microcrystalline inclusions in the i layer. We study i-layer thicknesses ranging from 1000 Å to 5000 Å and find that the transition occurs in all thicknesses investigated. Based on this study, a-Si alloy p i n solar cells suitable for use in the top cell of a high efficiency triple-junction structure are made. By selecting an appropriate dilution, cells with Voc greater than 1 V can be achieved readily. Solar cells made near the threshold not only exhibit higher initial characteristics but also better stability against light soaking. We have compared top cells made near the threshold with our previous best data, and found that both the initial and stable efficiencies are superior for the near-threshold cells. For an a-Si/a-Si double-junction device, a Voc value exceeding 2 V has been obtained using thin component cells. Thicker component cells give rise to an initial active-area efficiency of 11.9% for this tandem structure. INTRODUCTION One of the most effective techniques used to obtain high quality amorphous silicon (a-Si) alloys is the use of hydrogen dilution during film growth [1-6]. The resultant material exhibits a more ordered microstructure and gives rise to high efficiency solar cells. A stable 13% cell efficiency has been reported using a triple-junction structure [7]. Hydrogen dilution is also known to promote the growth of microcrystallites. We have recently reported that the best material is obtained at a dilution just below the threshold of amorphous-to-microcrystalline transition [8]. This material is characterized by an improved intermediate range order and may contain a small-volume fraction of microcrystallites. The critical dilution has also been shown to depend on the film thickness [9-11]. For a given dilution, the material quality is found to improve as the thickness of the film grows [9]. It is therefore important to determine the threshold of amorphous-tomicrocrystalline transition for a given intrinsic layer thickness under a given set of deposition conditions. The conventional way to determine the amorphous-to-microcrystalline transition is to use spectroscopic tools such as transmission electron microscopy [8], Raman spectroscopy [8,12], Xray diffraction [9,13], ellipsometry [10], or infrared spectroscopy [14]. We have shown [9] that the open-circuit voltage (Voc) of a solar cell also decreases dramatically as the transition takes place; measurement of Voc can, therefore,
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