High Efficiency Amorphous Silicon Based Solar Cells: A Review
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HIGH EFFICIENCY AMORPHOUS SILICON BASED SOLAR CELLS:
A REVIEW
RAJEEWA R. ARYA Solarex Corporation, Thin Film Division, 826 Newtown-Yardley Road, Newtown, PA 18940 ABSTRACT Over the past twelve years large strides have been made in improving the conversion efficiency of amorphous silicon based solar cells from 2.4% to 13%. The history and status of the material and device developments that have led to this five-fold improvement in the conversion efficiency are reviewed. Prospects for future improvements are discussed. INTRODUCTION The greatest impediment to large-scale use of photovoltaics in terrestrial applications is the cost of solar cells. Thin film solar cells, particulary hydrogenated amorphous silicon based solar cells, have emerged as an effective low cost alternative to crystalline or polycrystalline silicon solar cells. Amorphous silicon solar cells have already established widespread use in consumer electronics and are increasingly finding use in other terrestrial applications. The attractiveness of hydrogenated amorphous silicon as a low cost solar cell material stems from its unusual optoelectronic properties which are well suited for solar cell applications. It has an optical bandgap of about 1.7eV and its absorption coefficient is such that a large fraction of the incident solar radiation is absorbed in less than one micron thick film to create useful electron-hole pairs. Moreover, it can be readily doped p- or n-type and forms alloys with carbon and nitrogen to give wider bandgap material and with germanium and tin to give narrower bandgap material. The material is produced by glow-discharge decomposition of silane or disilane mixed with dopant gases such as diborane or phosphine. The discharge can be produced either with a d.c. or an r.f. glow-discharge at pressures in the range of .1 to lTorr. The deposition temperature is in the range of 100-3000 C. The deposition process is simple and can be scaledup to uniformly deposit over areas of several square feet onto glass or metal substrates. In amorphous semiconductors, the disorder in the lattice tends to broaden the band edges and introduce states inside the forbidden gap. For example, in pure amorphous silicon (deposited by thermal evaporation o01by 3 sputtering) the density of states within the gap is on the order of 10 cm eV Such a high density of gap states greatly reduces the carrier lifetime. It also pins the Fermi level that prevents doping thereby rendering the matepial useless for device applications. Hydrogenated amorphous silicon produced by glow-discharge of silane has two remarkable features: (i) the incorporation of hydrogen (and more recently fluorine) as a monovalent bond terminator allows one to reduce ýbe 1 rge density of electronically active dangling-bond defects from about 10 cm- eV-I in pure amorphous silicon to about 10 16 cm- 3 eV-1 optimized hydrogenated amorphous silicon as shown in Figure 1, and (ii) due to the low density of defect states within the gap it is possible to dope the material p- or n- type by adding a few p
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