High-Efficiency Microcrystalline Silicon and Microcrystalline Silicon-Germanium Alloy Solar Cells
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High-Efficiency Microcrystalline Silicon and Microcrystalline Silicon-Germanium Alloy Solar Cells
Takuya Matsui and Michio Kondo Research Center for Photovoltaic Technologies, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki, 305-8568, Japan
ABSTRACT This paper presents our material studies on hydrogenated microcrystalline silicon (ȝc-Si:H) and microcrystalline silicon-germanium alloy (ȝc-Si1-xGex:H) thin films for the development of high efficiency p-i-n junction solar cells. In ȝc-Si:H solar cells, we have evaluated the structural properties of the intrinsic ȝc-Si:H layers grown by plasma-enhanced chemical vapor deposition at high deposition rates (>2 nm/s). Several design criteria for the device grade ȝc-Si:H are proposed in terms of crystallographic orientation, grain size and grain boundary passivation. Meanwhile, in ȝc-Si1-xGex:H solar cells, we have succeeded in boosting the infrared response of solar cell upon Ge incorporation up to x~0.2. Nevertheless, a degradation of solar cell parameters is observed for large Ge contents (x>0.2) and thick i-layers (> 1 ȝm), which is attributed to the influence of the Ge dangling bonds that act as acceptorlike states in undoped ȝcSi1-xGex:H. To improve the device performance, we introduce an oxygen doping technique to compensate the native defect acceptors in ȝc-Si1-xGex:H p-i-n solar cells. INTRODUCTION Hydrogenated microcrystalline silicon (ȝc-Si:H) grown by plasma-enhanced chemical vapor deposition (PECVD) is extensively employed as a stable bottom cell absorber in hydrogenated amorphous silicon (a-Si:H)/ȝc-Si:H double junction tandem solar cells [1-5]. As μc-Si:H is a material exhibiting indirect optical transition, a relatively thick absorber layer (>2 μm) is necessary for efficient absorption in the infrared wavelengths, which in turn requires the high-rate deposition for industrial production. To achieve both high deposition rate and high efficiency, we have developed a deposition process based on a combination of high-pressure depletion (HPD) [6] and very-high-frequency (VHF) [1,7] glow discharge techniques. This process allows growing μc-Si:H at high rates (> 2 nm/s) while preserving excellent film qualities in terms of compact microstructure and less post-deposition oxidation [8]. As a result, efficiencies of 8-9% have been demonstrated for the μc-Si:H single junction p-i-n solar cells at deposition rates between 2 and 3 nm/s.
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Despite the successful material combination of a-Si:H and μc-Si:H, the stabilized efficiency of the a-Si:H/μc-Si:H tandem solar cells is still limited as low as ~12% [9]. The one of the major limitations of efficiency is the weak light absorption in the infrared wavelengths in μc-Si:H bottom cell. To extend the spectral sensitivities of solar cells into longer wavelengths, hydrogenated microcrystalline silicon-germanium alloys (μc-Si1-xGex:H) have been proposed [10-13] as a low-band-gap absorber in multijunction structures such as a-Si:H/μc-Si1-xGex:H [14] and a-Si:H/μc-Si:H/μc-Si1-xGex:H [1
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