Dual-junction a-Si/a-Si (same band-gap tandem) solar cells have lower material cost (no GeH4) than tandem cells using a-SiGe, and have slightly higher efficien­cies (0.5-1 % absolute) than a-Si single junction cells. Same band-gap a-Si/a-Si tandems have been in production for decades. Multiband-gap dual junction (a-Si/a – SiGe: Doughty and Gallagher 1990, or a-Si/nc-Si Yamamoto et […]

To enhance the fill factor of cells with a-SiGe i-layers, band gap grading is used to enhance the collection of holes (Guha et al. 1989; Zimmer et al. 1998). An asym­metric V-shaped band-gap profile is created by adjusting the Ge content across the i – layer. Wider-band-gap material is deposited closest to the p-layer. Such […]

The band-gap of a-SiGe alloys can be continuously adjusted between 1.7 and 1.1 eV by varying the percentage of Ge. However, the optoelectronic quality (i. e. defects and carrier transport) of a-SiGe degrades rapidly when the a-SiGe bandgap is re­duced below about 1.4 eV. Figure 41.8 shows the J-V characteristics of a series of a-SiGe […]

41.7.1 Advantages of Multi-junction Solar Cells A-Si:H solar cells can be fabricated in a stacked structure to form multi-junction solar cells. Figure 41.7(1) shows a tandem cell with two junctions (i. e. two pin photodiodes) in series. These multi-junction cells can have higher solar conversion efficiency than single-junction cells and are presently used in most […]

Back reflectors and substrate texturing are used to improve the power output of most thin film solar cells. Figure 41.6a shows the “quantum efficiencies” measured for a 250-nm-thick a-Si:H solar cell with varying texturing and back reflectors (Hegedus et al. 1996), along with absorptances calculated from the optical absorption coeffi­cient spectrum for a-Si:H. The dashed […]

The power generation is determined by the depth of the absorption for the more than 70 % of the blue components (this depth is approximately first 200 nm), and by the distance a photo-generated carrier can travel before it recombines. This distance is given by hx = ixdFt with the drift mobility, F the acting […]

41.6.1 Electronic Structure of a pin cell Due to the presence of a “built-in” electric field F(x) within the a-Si:H-based pin solar cell device, the bands are tilted, for EC according to eF(x) = dEC(x)dx; however, these built-in fields cannot be measured directly. The built-in fields across the i – layer stems from the space […]

As was discussed in Sect. 41.4.7, a-Si-based alloys can be deposited using a gas mixture of SiH4 with other gases such as GeH4, CH4, O2, or NO2, and NH3 for obtaining a-SiGex, a-SiCx, a-SiOx and a-SiNx, respectively. Among these alloys, only a-SiC, as a wide band-gap p-layer, and a-SiGe, as a low-band-gap absorber layer, have […]