Increasing Efficiency with Multiple Exciton Generation

Abstract

In a conventional solar cell the energy of an absorbed photon in excess of the band gap is lost as heat as the photo-generated charges cool rapidly to the band edge; for a silicon-based cell under standard insolation this loss is equivalent to 47 % of incident power. However, in colloidal quantum dots an alternative process can become significant-multiple exciton generation (MEG). Here, immediately after the absorption of a photon photo-generated carriers can instead use this excess energy to produce additional electron-hole pairs, which can contribute to the photocurrent and thereby increase the efficiency of a solar cell. In this chapter, this phenomenon will be introduced and its potential for increasing the efficiency of quantum dot solar cells discussed. MEG was first investigated spectroscopically and the challenging techniques used will be described. Progress since the first demonstration of MEG in 2004 will then be reviewed, including the controversy over quantum yields and its resolution. The recent demonstration of devices benefiting from MEG will be detailed. Finally, several possible future directions for research will be discussed.