Defect and Band Gap Engineering of Amorphous Silicon Solar Cells
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DEFECT AND BAND GAP ENGINEERING OF AMORPHOUS SILICON SOLAR CELLS R.E.I. SCHROPP*, J. DAEY OUWENS*, M.B. VON DER LINDEN*, C.H.M. VAN DER WERF*, W.F. VAN DER WEG*, AND P.F.A. ALKEMADE** * Department of Atomic and Interface Physics, Debye Institute, Utrecht University, P.O. Box 80,000, 3508 TA Utrecht, The Netherlands ** DIMES, Section Submicron Technology, Delft University of Technology, P.O. Box 5046, 2600 GA Delft, The Netherlands
ABSTRACT This paper demonstrates that the incorporation of an unoptimized, wide band gap a-SiC:H layer near the p-type emitter layer in addition to a graded bandgap "buffer" layer, leads to improved fill factors and open circuit voltages, in spite of the increased number of recombination sites at the p/i heterojunction. The as deposited efficiency as a function of a-SiC:H thickness shows an optimum of 10.5 % at a thickness of 10 - 20 A. We have further improved this type of cell by incorporating a reverse carbon graded p-type layer and have thus achieved efficiencies in excess of 11.0 %. The cells are all amorphous and do not comprise antireflective coatings or enhanced back reflectors. A new defect engineering scheme to accomplish enhanced stabilized efficiencies of amorphous silicon solar cells is also proposed here. INTRODUCTION General Background In amorphous silicon pt-i-n+ solar cells, the construction of the front 10 - 20 nm has a dominant influence on the overall performance. In 1981, a major improvement in the efficiency was obtained by incorporating a carbonated wide band gap p-layer, thus improving its window properties [1]. In 1986, the use of intentionally graded "buffer" p+/i interfaces was reported to enhance the performance in the blue region of the spectrum [2]. Recently, the window properties of the p+-layer have been further improved by using profiling schemes within the p+-layer [3], and by employing oxygenated p+-layers and buffer layers [4]. Researchers have speculated on the possible origins of the efficiency enhancement due to the p+/i buffer layer. The most frequently found explanations range from the classical concept of relaxation of band gap discontinuity by reduced bond distortion to the modified boron diffusion profiles as a result of the buffer layer [2,5]. In general, the improved quantum efficiencies in the blue region, the higher open circuit voltage (V0 ,) and fill factor values have usually been attributed to a reduced interface density of carrier recombination centers near the junction [2,6]. As pointed out recently [7], the reduced recombination losses can also be interpreted as due to the prevention of access to interfacial defects rather than to actual defect minimization. New Solar Cell Optimization Approach We have undertaken a new solar cell optimization scheme from the viewpoint of spatially separating electrons and holes at the p+/i junction in order to prevent recombination, rather than merely minimizing the density of recombination sites. The new optimization scheme employed here involves the addition of a very thin unoptimized, wide band ga
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