Solar Energy Harvesting

Abstract

In this chapter, we describe in a very abbreviated form the most important ways to harvest photonic energy using semiconductor physics. To understand this chapter, it is essential to first study the chapter which explains the workings of a p-n junction, namely, Chap. 9. Here, we explain how the basic mechanism works and the crucial role played by photonic absorption, the bandgap, and the high-field region inside the p-n junction. The charge collection model is presented in general and in a number of simple limits, giving formulae which are easy to handle. High-efficiency solar cells are shown to be associated with cells which are designed to optimally harvest a large portion of the sun’s spectrum. The subject is still a hot research topic, and given the huge problem associated with pollution and global warming, the rewards for making cheaper and more efficiency solar cells are very high and in some countries already a matter of life or death. The interested student is encouraged to continue and study more specialized texts on the subject.

Problems

• 1. Using the Hecht formula Eq. 11.1, calculate the carrier collection efficiency η given that the width of the device is 5 μm, the drift velocity is 10⁵ cm/s, and the recombination time is 1 ns.

• 2. In your own words, explain how the multijunction cell illustrated in Fig. 11.5 a works. How do the two absorbing junctions cooperate to optimize the collection of light over a wider spectrum?

• 3. Define the power collection efficiency Λ in terms of its components and explain why a square-shaped IV curve is better than a triangular one.

• 4. If the carrier collection efficiency is 1, what is the single most important factor which limits the solar cell performance in a single junction system?