Deposition of Optimal a-Si:H and a-SiGe:H by HWCVD Using the Same Filament Temperature and Substrate Temperature
- PDF / 124,832 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 0 Downloads / 152 Views
A9.48.1
Deposition of Optimal a-Si:H and a-SiGe:H by HWCVD Using the Same Filament Temperature and Substrate Temperature A.H. Mahan, Y. Xu, and L.M. Gedvilas NREL, 1617 Cole Blvd., Golden, CO 80401 ABSTRACT The incorporation of high Ge content a-SiGe:H into a low bandgap solar cell device commonly involves the use of bandgap (Ge) profiling. In previous work using the hot wire (HWCVD) technique, device quality low bandgap (ETauc = 1.25eV) a-SiGe:H films were deposited at low Tsub (~200-250°C) using a thin (0.38 mm diameter) W filament operating at Tfil ~ 1750-1800°C. The film Ge contents were ~55-70 at.%. However, higher bandgap films containing little or no Ge and deposited under the same low temperature (Tsub,Tfil) conditions were of decidedly inferior quality to those deposited using higher temperatures (Tfil~2000°C, Tsub~360°C). Although a single junction n-i-p solar cell with an efficiency ~ 5.85% was fabricated using these materials, it was clear that our best ‘end point’ materials were not used in this device. In addition, W filament alloying at this low Tfil severely limits film reproducibility and filament lifetime. This work explores deposition of device quality low bandgap a-SiGe:H and (high bandgap) aSi:H, both at the same low Tsub, using a tantalum (Ta) filament operating at low Tfil. Film material properties are presented. INTRODUCTION Hydrogenated amorphous silicon germanium (a-SiGe:H) has been used as a mid bandgap material in single junction solar cells, and also as the bottom cell in a tandem solar cell structure, and excellent efficiency results have been reported (1). Of considerable recent interest has been a-SiGe:H deposited by the hot wire CVD (HWCVD) technique. Initial results showed that ‘midgap’ a-SiGe:H, with a Tauc’s bandgap (ETauc) of ~ 1.60-1.65 eV, could be deposited at high deposition rates (10Å/s), and could be successfully incorporated into a double tandem solar cell. Initial and light soaked efficiencies for these cells were 11.5% and 9.7% respectively (2). More recently, deposition of narrow bandgap films has also showed considerable promise (3,4), in that a standard measurement used to reflect material quality, the (σL/σD) conductivity ratio, was considerably enhanced for HWCVD films compared to films of similar bandgap deposited by PECVD. In particular, for a Tauc’s bandgap of ~ 1.25 eV, σL/σD values for PECVD films are at most 10-15, while those for the HWCVD films are typically > 125150 and have exceeded 350 in one case. To successfully incorporate narrow bandgap a-SiGe:H material into a solar cell, bandgap profiling must be used. The viability of this approach using HWCVD films was previously demonstrated, as a single junction cell η of 5.85% was achieved in a single junction device (5). At the same time, however, these results suggested that further improvements could be achieved. In particular, the best ‘end points’ of the graded bandgap cell were not used in the same device, since the best a-Si:H and a-SiGe:H films have been deposited using quite different filament (Tfil)
Data Loading...
