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0989-A15-03

Triple Junction n-i-p Solar Cells with Hot-Wire Deposited Protocrystalline and Microcrystalline Silicon Ruud E.I. Schropp, Hongbo Li, Ronald H.J. Franken, Jatindra K. Rath, Karine van der Werf, Jan Willem Sch¸ttauf, and Robert L. Stolk Faculty of Science, Utrecht University, Department of Physics and Astronomy, SID - Physics of Devices, P.O. Box 80.000, Utrecht, 3508 TA, Netherlands ABSTRACT We have implemented a number of methods to improve the performance of protoSi/proto-SiGe/µc-Si:H triple junction n-i-p solar cells in which the top and bottom cell i-layers are deposited by Hot-Wire CVD. Firstly, a significant current enhancement is obtained by using textured Ag/ZnO back contacts developed in house instead of plain stainless steel. We studied the correlation between the integrated current density in the long wavelength range (650-1000 nm) with the back reflector surface roughness and clarified that the rms roughness from 2D AFM images correlates well with the long wavelength response of the cell when weighted with a Power Spectral Density function. For single junction 2-µm thick µc-Si:H n-i-p cells we improved the short circuit current density from the value of 15.2 mA/cm2 for plain stainless steel to 23.4 mA/cm2 for stainless steel coated with a textured Ag/ZnO back reflector. Secondly, we optimized the µc-Si:H n-type doped layer on this rough back reflector, the n/i interface, and in addition we used a profiling scheme for the H2/SiH4 ratio during i-layer deposition. The H2 dilution during growth was stepwise increased in order to prevent a transition to amorphous growth. The efficiency that was reached for a single junction µc-Si:H n-i-p cell was 8.5%, which is the highest reported value for hot-wire deposited cells of this kind, whereas the deposition rate of 2.1 ≈/s is about twice as high as in record cells of this type so far. Moreover, these cells are shown to be totally stable under light-soaking tests. Combining the above techniques, a rather thin triple junction cell (total silicon thickness 2.5 µm) has been obtained with an efficiency of 10.9%. Preliminary light-soaking tests show that these triple cells degrade by less than 4%. INTRODUCTION Hot-wire chemical vapor deposition (HWCVD) has become a viable method for the preparation of high-quality silicon and silicon alloy materials for application in thin film transistors and solar cells. At Utrecht University, we have developed HWCVD intrinsic protocrystalline silicon (proto-Si:H), which is characterized by an enhanced medium range structural order and a higher stability against light-soaking [1] compared to amorphous silicon, and microcrystalline silicon (µc-Si:H), which is characterized by a low density of states [2] at a crystalline volume fraction of ~40% as determined by Raman spectroscopy. These materials were successfully applied in thin film solar cells on plain stainless steel [3,4]. To enhance the efficiency, multibandgap structures are required [5]. We first developed proto-Si/µc-Si/µc-Si triple junction cells, but it appeared