High-Rate Plasma Process for Microcrystalline Silicon: Over 9% Efficiency Single Junction Solar Cells
- PDF / 560,477 Bytes
- 12 Pages / 595 x 842 pts (A4) Page_size
- 78 Downloads / 200 Views
A8.1.1
High-Rate Plasma Process for Microcrystalline Silicon: Over 9% Efficiency Single Junction Solar Cells Takuya Matsui, Akihisa Matsuda and Michio Kondo Research Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST) 1-1-1 Umezono, Tsukuba, 305-8568, Japan ABSTRACT This paper presents microcrystalline silicon (µc-Si:H) p-i-n (superstrate-type) solar cells fabricated by 100 MHz plasma-enhanced chemical vapor deposition (PECVD) at i-layer deposition rates of >2 nm/s. Under high-rate conditions, in particular, the deposition pressure is found to play a dominant role in determining short circuit current (Jsc) of solar cell. With an increase in deposition pressure from 3 to 7-9 Torr, Jsc increases by more than 50% due to a significant improvement in the long wavelength (>600 nm) responses, which essentially leads to high efficiency (~8%) solar cells in the 2-3 nm/s deposition rate range. Further progress in solar cell efficiency has been made by the improvement of TCO/p and p/i interfaces. As a result, efficiency reaches 9.13% (Jsc=23.7 mA/cm2, Voc=0.528 V, FF=0.73) with a 2.3 µm-thick i-layer grown at 2.3 nm/s. Transmission electron microscopy and secondary-ion mass spectroscopy studies reveal that samples prepared at lower pressure (~4 Torr) comprise many grain boundaries due to disordered grain growth, which induces an anomalous incorporation of atmospheric impurities (predominantly oxygen) after exposing sample to air. In contrast, the high-pressure process (>7 Torr) provides denser grain columns coalesced with [110]-oriented crystallites, which in turn inhibits impurities from penetrating deeper in the film. Based on above results, we propose that the less post-oxidation behavior associated with the denser microstructure of high-pressure-grown µc-Si:H is responsible for the excellent charge collection in p-i-n solar cells.
INTRODUCTION Hydrogenated microcrystalline silicon (µc-Si:H) films for photovoltaic application are of considerable current interest because of their wide-range spectral sensitivities and excellent stabilities against light exposure. In the last decade, remarkable progress in solar cell efficiency has been achieved employing low-temperature (~200°C) deposition techniques such as plasma-enhanced chemical vapor deposition (PECVD) [1-6] and hot wire chemical vapor deposition [7]. These low-temperature processes allow direct µc-Si:H deposition on inexpensive foreign substrates in large area, making it possible for low-cost solar cell fabrication. Unfortunately, µc-Si:H solar cell requires relatively thick absorber layer (at least 2 µm with light
A8.1.2
trapping) to gain sufficient low-energy-photon absorption because material exhibits indirect optical transition behavior. Thus, high-rate µc-Si:H deposition is of technological importance in view of further low cost manufacturing of silicon-based thin-film solar cells. However, high-efficiency (> 8%) µc-Si:H solar cells reported so far can only be achieved at limited deposition rates typically as low
Data Loading...
