Preparation of Narrow-gap a-Si:H Solar Cells by VHF-PECVD Technique
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1245-A07-07
Preparation of narrow-gap a-Si:H solar cells by VHF-PECVD technique Do Yun Kim1, Ihsanul Afdi Yunaz1, Shunsuke Kasashima1, Shinsuke Miyajima1, and Makoto Konagai1, 2 1 Department of Physical Electronics, Tokyo Institute of Technology, Japan 2 Photovoltaics Research Center (PVREC), Tokyo Institute of Technology, Japan ABSTRACT Optical, electrical and structural properties of silicon films depending on hydrogen flow rate (RH), substrate temperature (TS), and deposition pressure (PD) were investigated. By decreasing RH and increasing TS and PD, the optical band gap (Eopt) of silicon thin films drastically declined from 1.8 to 1.63 eV without a big deterioration in electrical properties. We employed all the investigated Si thin films for p-i-n structured solar cells as absorbers with ilayer thickness of 300 nm. From the measurement of solar cell performances, it was clearly observed that spectral response in long wavelength was enhanced as Eopt of absorber layers decreased. Using the solar cell whose Eopt of i-layer was 1.65 eV, the highest QE at long wavelength with the short circuit current density (Jsc) of 16.34 mA/cm2 was achieved, and open circuit voltage (Voc), fill factor (FF), and conversion efficiency (η) were 0.66 V, 0.57, and 6.13%, respectively. INTRODUCTION As the world has come under pressure to stop burning fossil fuel, a solar cell, which is an environment-friendly and unlimited energy source, has received a great deal of attention. For now, wafer-based crystalline silicon solar cells have occupied by over 90% of PV markets [1], however, Si-based thin film solar cells have emerged as promising candidates to replace bulk crystalline silicon solar cells. Si-based thin film solar cells have notable advantages of low production cost, large-scale deposition, low-temperature production, and so on. Especially, the most noticeable advantage is that it is possible to use wide range of solar radiation spectra by stacking a few number of unit cells on the top of the others, so called a multi-junction solar cell [2]. In this manner, it is an important issue to control the optical band gap of an absorber layer of the each unit cell. The optical band gap of silicon films can be easily tuned by incorporating various elements such as carbon [3] and oxygen [4] for a wide-gap solar cell, and germanium [5] for a narrow gap solar cell. Also, the optical band gap of silicon thin films depends largely on hydrogen contents (CH) of the films [6]. In this study, we have fabricated narrow gap silicon films and solar cells without an introduction of any germanium sources. We have systematically investigated a dependence of optical, electrical and structural properties of Si thin films and solar cells on hydrogen flow rate (RH), substrate temperature (TS) and deposition pressure (PD). EXPERIMENTAL DETAILS Silicon films were prepared with very high frequency plasma enhanced chemical vapor deposition (VHF-PECVD, 60 MHz) from a gas mixture of silane (SiH4) and hydrogen (H2) on the Corning 7059 glass and Si (100) substrate
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