Using the nano-scale to capture lost energy
In solar cell operation, the extra energy of photons with hv > EG is lost to heat. The ideal situation would be to use those photons with hv > 2EG to generate multiple carriers. Furthermore, all the energy of photons with hv < Eg is lost. The ideal situation would be to use several of these photons together to generate a carrier. If these objectives of making better use of photon energies could be attained meaningfully, then the ultimate efficiency analysis results discussed in Section 3.3.1 would be very favorably improved. Nano-scale materials engineering offers the possibility of attaining these goals.
As discussed in Chapter 2, semiconductor nanoparticle （quantum dot） structures in the sub-10-nm range open the door, through quantum confinement effects to band gap tuning and to the possibility of absorption that leads to multiple exciton generation （MEG）. With MEG, one supra-band gap photon （hv > 2EG） can produce more than one exciton. This is extremely attractive since it makes use of the otherwise wasted excess energy possessed by the hot species resulting from supra-band gap photon absorption. Quantum dots of PbSe, PbS, PbTe, and CdSe have been shown to exhibit MEG,45 and, therefore, offer the possibility of free carrier multiplication （CM）. As discussed in Chapter 2, the modeling of CM in quantum dots is based on a MEG process analogous to impact ionization; however, other models have also been put forward.45,46 The minimum threshold for MEG found in this modeling is the expected 2EG, which means low band gap QD materials can be used to fully utilize the whole photon-rich region of the solar spectrum （see Fig. 1.1）. once multiple excitons have been created from one supra-band gap photon in a quantum dot, the issues are how the excitons can be dissociated and how the free electrons and holes can be collected. In other words, how do we get to the free carrier multiplication stage? The dissociation required is analogous to that undergone by the exciton produced by absorption at a dye molecule in the DSSC cell of Figure 3.14g. In that cell, the excited dye molecule's exciton relaxes by emitting an electron to the levels at the semiconductor conduction band edge and a hole to the anion level in the electrolyte. In quantum dots, the analogous collection of free carriers out of MEG appears to be hampered by QD surface states and short exciton lifetimes.47 The use of QD absorbers for CM has been tried in several cell structures, including tandem cells, but with limited success so far, apparently due to these collection issues.47
In an effort to capture the lost energy of sub-band gap photons, there have been proposals （dating back to 1960） to introduce states in the band gap to allow two-photon generation processes.48,49 As discussed in Chapter 2, two-photon processes based on delocalized intermediate bands （IB） in the gap are expected to be the most effective. Such bands, if properly designed, are believed to be able to support a two-step photon absorption-free carrier generation process without enhancing the competing loss mechanism of nonradiative recombination. Such IB levels are proposed to be attainable with quantum dot structures immersed in a wider band gap absorber.50 An analysis of IB two-photon generation based on quantum dot structures for up to 1000 suns has shown its potential also for concentrator applications.