Enhancement Efficiency of Solar Cells Based on Quantum Dots: A Theoretical Study

Abstract

Improving the conversion efficiency of solar cells is a key way to make solar cells cost-competitive with conventional sources of energy because the cost of electricity produced from solar cells depends on their efficiency. According to Shockley-Queisser limit, all single junctions cells have a theoretical efficiency limit of 33.7%. Efficiency losses are associated with light that either is not energetic enough or too energetic for the generation of an electron-hole pair. In other words, the two most important loss mechanisms in single bandgap solar cells are the inability to convert photons with energies below the bandgap energy (Eg) into electricity and the thermalization of photon energies exceeding Eg. These two mechanisms alone represent the loss of about half of the incident solar energy during the conversion. Intermediate band (IB) located inside the forbidden band of the host semiconductor, making it possible to increase the absorption of photons with energy lower than the band gap energy Eg, can be a solution for the first problem. The effect of IB on the efficiency of solar cells was discussed. Our aim is to show how IB can improve the efficiency of solar cells based on Quantum Dots (QDs), i.e, the efficiency of a solar cell can be greatly increased through additional optical absorption. In this paper, two cases were discussed applying a numerical model. Our model, used to calculate solar cells efficiency and to plot the current-voltage (I-V) characteristics and the power-voltage (P-V) characteristics curves, is mainly based on the principles of the detailed balance between absorption and emission of solar light and well separated quasi-Fermi levels. The first case is an idealized case where the recombinations are assumed to be entirely radiative. In the second case, the model takes into account the non-radiative recombinations introduced by the increase in the density of defects caused by the increase in the size of the QDs. It has been shown that Consideration of size leading to the reduction of the defects is one of important key solution to increase the efficiency of intermediate band solar cells (IBSC).