Optical Properties Of A Si Essay
The optical properties of the a-Si:H are given by its absorption coefficient and optical band gap. a-Si:H absorbs 100 times more than c-Si. This is because it behaves like a direct gap semiconductor. 1 micrometer thick layer is sufficient to absorb 90% of usable solar energy. Another advantage is that its optical band gap can be varied by varying hydrogen content.
The electrical properties of a-Si: H are characterized in terms of dark conductivity and photoconductivity. The low values of electron and hole mobilities and the high mobility gap of a-Si:H lead to low dark conductivity.
In order to manipulate the type of electrical conductivity and its magnitude, doping is carried out. Doping is adding a controlled amount of impurity atoms. The doping materials used are boron for p-type and phosphorous for n-type.
The two functions of the doped layers in a-Si:H are as below:
• To set up an internal electric field across an active intrinsic a-Si:H layer.
• To establish low loss ohmic contacts between a-Si:H part and the external electrodes.
Tandem cell using a-Si
A multi-junction photovoltaic cell can be produced with the combination of a layer of silicon combined with layers of other allotropic forms of silicon. Tandem cell is formed with the combination of only two layers (two o-n junctions). The cells’ overall efficiency increases by stacking layers on top of each other.
A layer of microcrystalline silicon is combined with…