Design and Fabrication of Graded Bandgap Solar Cells in Amorphous Si and Alloys
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DESIGN AND FABRICATION OF GRADED BANDGAP SOLAR CELLS IN AMORPHOUS SI AND ALLOYS VIKRAM L. DALAL AND GREG BALDWIN Iowa State University, Dept. of Electrical and Computer Engr. Ames, Iowa 50011
ABSTRACT In this paper, we discuss the appropriate design conditions and fabrication technology for graded bandgap solar cells in a-Si:H and a-(Si,Ge) :H. In particular, we show that by carefully designing the buffer layers and the placement of the graded gap in the i layer of a p-i-n cell, one can fabricate a-Si:H solar cells which are more stable than standard design solar cells. Similarly, appropriate considerations of device physics and electric field profiles in devices made from a-(Si,Ge) :H alloys can result in high performance a-(Si,Ge) :H cells . We also show that using significant H dilution during growth of a-(Si,Ge) :H layers and cells results in both films and devices of high quality and reproducibility.
I.INTRODUCTION The stability and performance of a-Si:H and a-(Si,Ge) :H solar cells are of significant technological interest. It has been recognized for some time that in both these material systems, the device design has to allow for the poorer electronic properties of these materials, and the further degradation of these properties after light soaking.(I' 2 ) A device which is designed optimally for the initial performance may not be the best device after light soaking. This fact arises from the fact initially, one can make a good device incorporating a very low defect density material grown at low temperatures(200-250C). However, upon degradation, the defect density in this "good" material may increase by a factor of 50 or so, thereby significantly reducing the electric field in the middle portions of the device, and also reducing the mu-tau products of both holes and electrons.(2) To overcome this rapid degradation of the initially "good" material, it has been suggested that one should instead make devices in materials which are much more stable, even though these more stable materials may initially have a higher defect density than the nominally good materials currently used. In particular, the use of materials grown at higher temperatures(350-400C), which are more stable, with a lower saturated state density than the lower-temperature-grown materials(3,4).In this paper, we show how a combination of appropriate device design, using graded gaps, and the use of more stable materials, can lead to a high performance cell which is indeed more stable than a nominally good cell grown at lower temperatures.
II.
DEVICE DESIGN CONSIDERATIONS
In order to design the device properly, we need to study the device physics of a-Si:H cells. It is well known that in a p-i-n a-Si:H
Mat. Res. Soc. Symp. Proc. Vol. 297. 01993 Materials Research Society
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solar cell, the field in the i layer is non-uniform, with a high field near the interfaces, and a low field in the middle of the device. 5 2 ) The low field in the middle gets much smaller after the cell has degraded, simply because the defect density is higher. This e
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