Effect of Buffer Structure on the Performance of a-Si:H/a-Si:H Tandem Solar Cells
- PDF / 250,577 Bytes
- 6 Pages / 432 x 648 pts Page_size
- 76 Downloads / 231 Views
Effect of Buffer Structure on the Performance of a-Si:H/a-Si:H Tandem Solar Cells C.H. Hsu, C.Y. Lee, P.H. Cheng, C.K. Chuang, C.C. Tsai Department of Photonics, National Chiao Tung University, Hsinchu, Taiwan ABSTRACT The study focuses on the influence of the hydrogenated amorphous silicon carbide (aSiC:H) buffer layer in hydrogenated amorphous silicon (a-Si:H) single-junction and tandem thinfilm solar cells. By increasing the undoped a-SiC:H buffer layer thickness from 6nm to 12nm, the JSC in single-junction cell was significantly improved, and the efficiency was increased by 4.5%. The buffer layer also effectively improves the efficiency of the a-Si:H/a-Si:H tandem cells by 7% as a result of the increase in open-circuit voltage (VOC) and short-circuit current (JSC). Although the bottom cell absorbs less short-wavelength photons, the wider-bandgap doped and buffer layers were still necessary for improving the cell efficiency. Presumably, this is because these wider-bandgap layers allow more photons to reach the bottom cell. Also, they can reduce interface recombination. INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) has received much attention due to its superior properties for the thin-film solar cells. The high absorption coefficient of a-Si:H allows it to absorb light with less material. The bandgap around 1.75eV also makes it suitable for the effective absorption of the solar spectrum. However, the Staebler-Wronski effect (SWE) [1] influences the long-term stability of the solar cells, resulting in a decrease in the cell efficiency [2]. The amount of the degradation depends on the material quality and the film thickness. By the use of a-Si:H/a-Si:H tandem structure, the absorber is divided into two devices [3]. The thinner absorber not only reduces the degree of SWE, but also creates a stronger build-in electric field to assist the carrier transport. Wieder et al. [4] had reported an initial cell efficiency of 9.2% with a relative decrease of 8% after 900 hours light soaking due to a more effective carrier extraction in thinner undoped layers. Furthermore, other materials with lower bandgaps, such as hydrogenated amorphous silicon germanium (a-SiGe:H) or hydrogenated microcrystalline silicon (ȝc-Si:H) can be used in the bottom cells. However, a-SiGe:H uses costly germane and ȝc-Si:H requires longer deposition time. As a comparison, the a-Si:H/a-Si:H solar cell has a lower production cost. A high efficiency a-Si:H single-junction thin-film solar cell normally contains a widebandgap window layer to allow more high energy photons to be absorbed in the undoped layer. However, the bandgap difference between doped and undoped a-Si:H at the p/i interface hinders the hole transportation [5]. A wide-gap undoped layer was therefore used to accommodate the band offset. Such a buffer layer also significantly reduces the p/i interface recombination, prohibits the electron from moving into p-layer [6] and may prevent the boron diffusion into absorber. As a result, the buffer layer can significantly improve the VOC, ac
Data Loading...
