Development of Nanocrystalline Silicon Based Multi-Junction Solar Cell Technology for Large Volume Manufacturing
Y. LiY. ZhangFeng-Chun LinJack XiaoZhou Xiao-minXiaoning RuChangtao PengMengnan QuYu CaoAnhong HuXixiang XuJ. Zhang
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Nanocrystalline material
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A two-junction device consisting of a 1.7-eV GaNPAs junction on a 1.1-eV silicon junction has the theoretical potential to achieve nearly optimal efficiency for a two-junction tandem cell. We have demonstrated some of the key components toward realizing such a cell, including GaNPAs top cells grown on silicon substrates, GaP-based tunnel junctions grown on silicon substrates, and diffused silicon junctions formed during the epitaxial growth of GaNP on silicon. These components have required the development of techniques for the growth of high crystalline quality GaNPAs on silicon by metal-organic vapor-phase epitaxy.
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A two-junction device consisting of a 1.7-eV GaNPAs junction on a 1.1-eV silicon junction has the theoretical potential to achieve nearly optimal efficiency for a two-junction tandem cell. We have demonstrated a monolithic III-V-on-silicon tandem solar cell in which most of the III-V layers are nearly lattice-matched to the silicon substrate. The cell includes a GaNPAs top cell, a GaP-based tunnel junction (TJ), and a diffused silicon junction formed during the epitaxial growth of GaNP on the silicon substrate. To accomplish this, we have developed techniques for the growth of high crystalline quality lattice-matched GaNPAs on silicon by metal-organic vapor-phase epitaxy.
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We recently demonstrated how the short-circuit current density of an a-Si:H/c-Si heterojunction solar cell can be significantly improved to above 40 mA/cm 2 by replacing the standard a-Si:H(p) emitter by a silicon oxide emitter containing p-doped silicon nanocrystallites. While we could obtain a conversion efficiency of 20.3%, the cell suffered from a lower fill factor of 72.9%, compared with 77.0% for our standard process. In this paper, we address this issue both theoretically and experimentally. We found that a thin (~3 nm) highly doped nanocrystalline silicon layer on top of the emitter can greatly improve the fill factor. Using 1-D device simulation, we explain the prevalent loss mechanism, which originates mostly from poor tunnel recombination at the transparent conducting oxide/emitter interface rather than in the bulk of the emitter. We suspect that have their origin in the lower effective dopant concentration of the nanocrystalline silicon oxide emitter. From the model, implications for further developments can be derived.
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A novel nanocrystalline Si (nc-Si)/ multicrystalline Si (mc-Si) heterojunction (HJ) solar cell has been fabricated using a simple structure. This paper presents the process development and fabrication of the HJ solar cell based on relatively thick (n + ) nc-Si emitter on a (p) mc-Si substrate without using any TCO layer or intrinsic buffer layer. Highly conductive nc-Si films with conductivities in the range of 80-150 Omega -1 cm -1 were developed by direct deposition of nc-Si on mc-Si substrates. In addition to a high-quality junction, nc-Si emitters can provide low resistive path for the lateral photocurrent in photovoltaic devices. Solar cells with size of 1cm 2 , with high quality junctions and fill factors of exceeding 70% are obtained
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Though thin film silicon has evolved into an important technology for photovoltaic industry, further increasing its conversion efficiency remains to be a key task. In this work, we report the progress we have made in developing compatible nanocrystalline Si (nc-Si) technology with our existing amorphous silicon germanium (a-SiGe) based multi-junction solar cell manufacturing lines. We have conducted experiments mainly on two types of nc-Si based solar cell structures, a-Si/a-SiGe/nc-Si triple-junction and a-Si/nc-Si double-junction device. Currently we are attaining initial total area efficiency of 10.7% and 12.4% for the triple- and double-junction structures, respectively, on substrate size of 0.79 m 2 (1.245m × 0.635m). Experimental results including study of crystalline volume fraction along nc-Si growth, individual component cell optimization and current match, development of superior tunnel-junction and contact layers are presented.
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A novel thin-film multijunction solar cell based on nanocrystalline silicon (nc-Si:H) is presented in this paper. Existing thin-film double junction solar cells are based on amorphous silicon carbide (aSiC:H) and amorphous silicon layers. Such solar cells have limited efficiency due to lower absorption and poor charge transport properties of the a-SiC:H layer. These solar cells have maximum achieved efficiency of about 8.8%. In this work, a-SiC:H has been replaced with nc-Si:H layer and the double junction solar cell has been redesigned. The proposed structure has been simulated with Silvaco TCAD (ATLAS). The simulated results indicated a step increase in the performance of the solar cell with open circuit voltage Voc=2.096 V and efficiency η = 10.2%. It was proven that the nc-Si:H is a suitable material for the development of an efficient thin film multijunction solar cell.
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Recent research activities at the University of Toledo (UT) in the fabrication of high-efficiency triple, tandem and single-junction solar cells, all employing high-quality a-SiGe cells, are reviewed in this paper. Incorporating various improvements in device fabrication, the UT group fabricated 1) triple-junction a-Si/a-SiGe/a-SiGe solar cells with 12.5% initial efficiency and 10.7% stable efficiency, tandem-junction a-Si/a-SiGe solar cells with 12.9% initial efficiency, and single-junction a-SiGe solar cells with 12.5-13% initial efficiency and 10.5% stable efficiency. This review also highlights recent UT work on the nanocrystalline silicon p-layer and light-assisted electrochemical shunt passivation process.
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