On the development of single and multijunction solar cells with hot-wire CVD deposited active layers

Abstract

We present an overview of the scientific challenges and achievements during the development of thin film silicon based single and multijunction solar cells with hot-wire chemical vapor deposition (HWCVD) of the active silicon layers. The highlights discussed include the development of Ag/ZnO coatings with a proper roughness and morphology for optimal light trapping in single and multijunction thin film silicon solar cells, studies of the structural defects created by a rough substrate surface and their influence on the performance of nc-Si:H n-i-p single junction solar cells, and studies of the phase change during the growth of nc-Si:H by HWCVD and the use of a 'reverse' H 2 profiling technique to achieve nc-Si:H single junction n-i-p cells with high performance. Thus far, the best AM1.5 efficiency reached for n-i-p cells on stainless-steel with HWCVD i-layers is 8.6% for single junction nc-Si:H solar cells and 10.9% for triple junction solar cells. The opportunities for further improvement of cell efficiency are also discussed. We conclude that the uniqueness of HWCVD and of the i-layers deposited with this technique require some adjustments in the strategy for optimization of single or multijunction solar cells, such as using a reverse H 2 profiling technique for the deposition of nc-Si:H i-layers. However, the output performance of solar cells with HWCVD deposited i-layers is close to those with i-layers deposited by other techniques. The difference between the best nc-Si:H ni-p cells obtained so far in our lab and the reported best n-i-p cells with PECVD i-layers can be mainly attributed to the differences in the rough substrates and to the use of rather thin i-layers.

Reversibility of silicidation of Ta filaments in HWCVD of thin film silicon

“…It was found by several groups that a lower filament temperature is beneficial for the material quality [8,9]. However, under the conditions used in this study, high quality a-Si:H [4,5] and µc-Si:H [6] material are obtained at high deposition rates (1 nm/s and 0.2 nm/s respectively).…”

“…15 cm length, in electrical series connection, they were first annealed in vacuum (background pressure b10 − 7 mbar) for 3 h at a constant current of 12.5 A (2050°C) to remove impurities. Depositions were performed at a constant current of 10.5 A (~1750°C) with a partial silane pressure of either 2 Pa during typically 10 min (a-Si:H [4,5]) or 0.25 Pa for almost 3 h (µc-Si:H [6]). Prior to every subsequent exposure to silane, the filaments were annealed for 30 min at a constant current of 12.5 A (1950-2050°C).…”

Nano-patterned glass superstrates with different aspect ratios for enhanced light harvesting in a-Si:H thin film solar cells

“…The results indicate that the optical absorption of the a-Si:H layer increases with increasing aspect ratios due to their broadband absorption. Nevertheless, the electrical properties are restricted by steep surface roughness [15]. Enhanced photovoltaic characteristics are demonstrated for the nanostructured glass superstrate with a moderate aspect ratio of 0.16.…”

Black thin film silicon

“…12 Considerable attention is currently given to the correlation of the light-trapping effects with the morphology of the TCOSi interface. [12][13][14][15][16] In particular, it has been realized that the size of the features in the texture is important for the scattering mechanism at the interface and thus for the resulting light-trapping efficiency. 15 The typical features of the presently used textures have lateral sizes between several lm and $700 nm.…”

“…It was also shown that silicon thin films grown over very rough surfaces become defective. 16 Another important limitation of this absorption enhancement approach is that the performance of the back reflectors, used to additionally improve the absorption, deteriorates with increasing roughness. 16 Therefore, the optimization of solar cells grown on textured substrates with respect to a geometrical absorption enhancement is a matter of compromise.…”