4. (13 points total) a) To increase efficiency of a Photovoltaic (solar) cell, i
ID: 2266284 • Letter: 4
Question
4. (13 points total) a) To increase efficiency of a Photovoltaic (solar) cell, is it desirable to have longer or shorter carrier lifetimes? Explain why? (5 points) b) Efficiency of photovoltaic solar cells can be improved using multi-layer structures consisting of layers of different semiconductor materials. (8 points) How would you arrange the sequence of different materials in a multi-layer solar cell. Would you place the wider bandgap material on top of the narrower bandgarp materials or vice versa? Why? And how does it help absorb a higher portion of the light energy?Explanation / Answer
a) To increase the efficiency of a solar cell, we need to have higher carrier lifetime.
This is because Higher carrier lifetime reduces the recombination effect . When an electron gets excited, it liberates itself from the electron-hole pair and moves freely. After deexcitation, it again recombines with a hole to become the electron-hole pair. Thus, inorder to avoid quick recombination, a carrier should have higher life time.
In a multi layer, each junction acts as a resist. Thus, the higher the carrier lifetime,. the higher is the chance for the carrier to pass through the junction.
b) Materials are arranged in the DECREASING order of the Bandgap.
i.e. Higher bandgap material should be placed on the top of the lower bandgap material. The higher the Bandgap, it absorbs the higher wavelengths. Ideally, the absorption starts from UV region and extends till the IR region. The rays from UV region with higher wavelengths are absorbed by the topmost layer and the rays from the IR region with lower wavelengths are absorbed by the lowermost layer in the solar cell.
If placed in the increasing order, only the solar spectrum consisting of lower wavelengths are absorbed.
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