# Quantum Dot Solar Cells: Small Beginnings Have Large Impacts
## 2 Timescales of Excited State Processes
### Absorption and Multiple Exciton Generation
Known as impact ionization (II) in bulk semiconductors, CM was observed to be of significance only at photon energies far into the UV range (\(>\)3 eV) which contributes to less than 1% of the solar spectrum. This is because the strong coupling of electron and phonon states in bulk semiconductors provides a quicker cooling pathway for hot carriers through phonon emission (schematically represented by the red wavy lines in pathway 1b of Figure 3) and the need for the crystal lattice momentum conservation raises the threshold energy required for CM. The quantum confinement of charge carriers in QDs however has been expected to help overcome these limitations. Due to the small crystal size and quantum confinement along all three dimensions in QDs, the need for crystal momentum conservation is minimized while these discrete energy levels separated by multiple optical phonon energies makes the relaxation of hot carriers via phonon emission a rather slow process. Furthermore, MEG is a carrier scattering process mediated via the carriers' Coulomb interaction. As this Coulomb interaction is enhanced due to confinement of the electron and hole wavefunctions in QDs, the rate of CM is also increased [77]. The possibility of using the multiple excitons to produce higher photocurrents has motivated researchers to use such QD semiconductors in solar cells [79; 80; 81]. However, in addition to the generation of multiple excitons, the effective separation of the exciton into charge carriers and their extraction into electron- and hole-accepting contacts are equally important in realizing the true potential of MEG-based solar cells [82]. Using time-resolved microwave conductivity (TRMC) measurements, [PERSON] et al. [83] showed that varying the excitation energy and measuring the photoconductance leads to a signal that correlates directly to the fraction of absorbed photons (i.e., the mobility was constant as a function of excitation energy but the number of free carriers depends on the fraction of photons absorbed). At excitation energies above \(2E_{g}\), the yield of carriersexceeded 100% indicating that free charges were produced through CM (pathway 2b of Figure 3). These excess charges can also be used to drive chemical reactions (not just solar cells) with yields greater than 100% [84]. One concept to increase the efficiency of CM is through band alignment engineering [85]. By combining different materials in a quasi-type II heterojunction (e.g., PbSe/CdSe core/shell QDs), the carrier cooling rate can be reduced so the MEG rate becomes proportionally more dominant and the MEG yield thus increases [86]. However, one drawback of this approach is that the hole is effectively stuck in the core of the QD and not free to move to contacts precluding applications requiring long-range carrier transport. Alternatively to core/shell nanocrystals, Janus particles (anisotropic particles where each side of the particle is a different material) could retain the same benefits while allowing access to each carrier. [PERSON] et al. have shown a built-in potential difference in these heterostructured nanocrystals with scanning Kelvin probe measurements that can be explained by having a space-charge region where 10-15 charges have transferred across the nanoscale junction [87; 88]. Furthermore, [PERSON] et al. [89] report a controllable cation exchange synthesis to produce Janus particles of various PbE and CdE (E = S, Se, Te) compositions that have been applied to this type of study [90]. More simplistically, [PERSON] et al. [91] fabricated films of mixed CdSe/PbSe QDs and observed hot-electron transfer between the QDs indicating the energy states of neighboring QDs are coupled. Further understanding and control via wavefunction engineering, opens up many new possibilities to employ advanced photophysics to applied optoelectronic devices. For instance, the exciton can spatially delocalize over the surrounding QDs if the energy landscape is favorable.