Chapter 15: Quantum Structured Solar Cells

This chapter discusses quantum structured solar cells. The relaxation dynamics of photo-excited electrons in semiconductor quantum dots (QDs) can be greatly modified compared to the bulk form of the semiconductor. The cooling dynamics of highly energetic (hot) electrons created by absorption of supra-bandgap photons can be slowed. This slowed cooling is caused by a so-called "phonon bottleneck" when the energy spacing between quantized levels in the QD is greater than the LO-phonon energy thus, inhibiting hot-electron relaxation (cooling) by electron-phonon interactions. To produce the slowed hot-electron cooling via the phonon bottleneck, it is necessary to block an Auger process that could bypass the phonon bottleneck and allow fast electron cooling. The Auger cooling process involves the transfer of the excess electron energy to a hole, which then cools rapidly because of its higher effective mass and closely-spaced energy levels. Slowed cooling in QDs could lead to their use in solar cell configurations wherein impact ionization (the formation of two or more electron-hole pairs per absorbed photon) or hot-electron separation, transport, and transfer can become significant, thus producing enhanced photocurrents or photovoltages and corresponding enhanced conversion efficiencies with thermodynamics limits of 66% (one sun). Three configurations for QD solar cells are described in the chapter that can produce either enhanced photocurrent or photovoltage.