Unique Quantization Effects in Quantum Dots and Quantum Dot Solar Cells for PV and Solar Fuels
Arthur Nozik a b, Matt Beard b, Joey Luther b, Octavi Semonin b, Justin Johnson b, Mark Hanna b
a University of Colorado, Boulder, 215 UCB University of Colorado, Boulder, 00309, United States
b NREL, 16253 Denver West Parkway, Golden, 80401, United States
Proceedings of International Conference Solution processed Semiconductor Solar Cells (SSSC14)
Oxford, United Kingdom, 2014 September 11th - 12th
Organizers: Henry Snaith and Samuel Stranks
Invited Speaker, Arthur Nozik, presentation 001
Publication date: 1st July 2014

In quantum dots (QDs), quantum rods (QRs) and unique molecular chromophores that undergo singlet fission (SF) the relaxation pathways of photoexcited states can be modified to produce efficient multiple exciton generation (MEG) from single photons . We have observed efficient MEG in PbSe, PbS, PbTe, and Si QDs and efficient SF in molecules that satisfy specific requirements for their excited state energy levels. We have studied MEG in close-packed QD arrays where the QDs are electronically coupled in the films and thus exhibit good transport while still maintaining quantization and MEG. We have developed simple, all-inorganic QD solar cells that produce large short-circuit photocurrents and respectable power conversion efficiencies via both nanocrystalline Schottky junctions and nanocrystalline p-n junctions. These solar cells also showed for the first time external quantum yields (QYs) for photocurrent that exceed 100% in the photon energy regions of the solar spectrum where MEG is possible (i.e., energy conservation is satisfied); the photocurrent internal QYs from MEG as a function of photon energy match those determined via time-resolved spectroscopy and the results settle controversy concerning MEG. Recent analyses of the dramatic effects of solar concentration combined with MEG on the conversion efficiency of solar cells will also be discussed. The properties required for nanocrystals and SF molecules to achieve the high solar conversion efficiencies predicted by theory will be presented. Regarding production of solar fuels, all viable systems must have the following features: (1) two photosystems arranged either in a Z-scheme analogous to biological photosynthesis, or two tandem p-n junctions connected in series where sufficient photopotential (1.23 V + overvoltage for H2O splitting) is generated to drive the redox reactions; (2) strong absorption of solar photons; (3) efficient separation of the photogenerated e-h pairs, (4) efficient transport to and collection of the separated carriers at electrocatalytic surfaces; (5) low overvoltages; (6) appropriate alignment of the redox potentials in the photoelectrodes with those of the fuel-producing reactions; and (7) resistance to dark-and photo-corrosion achieving long-term photostability. Cells with buried junctions in a tandem p-n configuration or a Z-scheme can achieve these requirements.

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