Investigation of the relation between growth mechanism and material properties of amorphous silicon solar cells

Investigation of the relation between growth mechanism and material properties of amorphous silicon solar cells

Adejumo, K.O.

Wank, M. (mentor)
Swaaij, R.A.C.M.M. (mentor)

Electrical Engineering, Mathematics and Computer Science

Microelectronics

2009-07-10

Thin film silicon solar cells are produced by using plasma deposition techniques. With this technique a thin film layer of silicon, most commonly amorphous silicon (a-Si: H), can be deposited on a substrate. The material properties depend largely on the plasma properties and the growth mechanism. The investigation of the correlation between material properties and the ion bombardment, which are important for electronic device operation, is the sole aim of this thesis. Expanding Thermal Plasma Chemical Vapour Deposition (ETP-CVD) is used for the deposition of a-Si: H layers. With this deposition technique, growth rates in excess of 1nm/s can be achieved. Through this, the production cost of solar cells can be reduced because of the increase in the production output. The growth mechanism of a-Si:H layers deposited with this technique has been extensively investigated and it has been shown that SiH3 radicals contribute to about 90% of the growth. The growth mechanism of a-Si: H layers can be manipulated by applying a bias to the substrate. In this way, the contribution of ions to the growth and the ion energy can be varied. To induce ion bombardment during film growth, a non-sinusoidal pulse-shaped signal is applied to the substrate and the material properties, which include optical properties, structural properties, and electrical properties, are measured. Denser a-Si:H films are obtained with increasing ion energy, indicating material densification. Higher growth rate is observed because of the production of more radicals around the substrate. An increase in photoconductivity is obtained up to 100V, which correlates with the decrease in the Tauc band gap and the densification of the material. Increase in the ion bombardment leads to decrease in the microstructure, which indicates reduction of voids in the material. However, defect density and Urbach energy increases with the ion bombardment due to the creation of weak bonds which end up as defects in the a-Si:H films. The optimized layers of the measured samples are incorporated into solar cells. Solar cells that are produced with pulse-shaped bias voltage up to 50V yielded a conversion efficiency of 6.2%. The efficiency is lower at higher voltages, which is mainly caused by smaller Voc and FF because of high defect densities present in the intrinsic layers at higher voltages.

amorphous growth mechanism
material properties
Pulse shaped biasing

http://resolver.tudelft.nl/uuid:c3e295b7-c9fa-4ebd-b61b-727bb4e14aee

Student theses

master thesis

(c) 2009 O. Adejumo, K.