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Semiconductor quantum dots and quantum dot arrays and applications of multiple exciton generation to third-generation photovoltaic solar cells.

Nozik-AJ; Beard-MC; Luther-JM; Law-M; Ellingson-RJ; Johnson-JC
Chem Rev 2010 Nov; 110(11):6873-6890
Many classes of semiconductor quantum dots (also called nanocrystals) have been synthesized, including groups II-VI, III-V, IV-VI, IV, and their alloys, various intergroup and intragroup core-shell configurations, and various nanocrystal shapes. Recent research has improved upon the nanocrystal syntheses regarding more friendly "green" chemistry, better size distributions and stability, lower synthesis temperatures, and more uniform nanocrystals in general. The application of QDs in solar photon conversion devices (solar cells for photovoltaics and solar fuels) to enhance their conversion efficiencies is a promising and increasingly active field of research. Such cells are termed "future generation" or in the case of PV: "third-generation" solar cells. One approach to enhance efficiency in QD-based PV cells compared to conventional bulk semiconductor-based PV is to create efficient multiple exciton generation from a large fraction of the photons in the solar spectrum. Enhanced MEG quantum yields have now been confirmed by several groups in isolated colloidal QDs of PbSe, PbS, PbTe, Si, InP, CdTe, core-shell CdSe/CdTe, and in QD arrays of PbS, PbTe, and PbSe. It is noted that MEG has also been reported in singlewall carbon nanotubes. The most common method for determining the MEG QY has been ps to ns time-resolved spectroscopy (transient absorption, bleaching, photoluminescence, and THz); steadystate photocurrent spectroscopy measurements are also under study. Discrepancies in the literature for reported MEG QY values for PbSe and CdSe are explained by variations in the surface chemistry of the QDs and in some cases the effects of charging of QDs when photogenerated electrons or holes are trapped at the surface producing a charged QD core. After accounting for these variations and effects, MEG has been confirmed in the various QD materials discussed here and for these materials have threshold photon energies for MEG ranging from 2.1Eg to 3Eg, and total QYs at 3.0Eg, for example, ranging from 120% to 200%. With these MEG characteristics, the improvement in PV power conversion efficiency is relatively minor; to achieve significant increased power conversion efficiency, the MEG threshold needs to be close to 2Eg with a sharp increase in QY that reaches approximately 200% at approximately 2.1Eg to 2.5 Eg and approximately 300% at 3Eg to 3.5Eg. In summary, three generic types of QD solar cells that could utilize MEG to enhance conversion efficiency can be defined: (1) photoelectrodes composed of QD arrays that form either Schottky junctions with a metal layer, a hetero p-n junction with a second NC semiconductor layer, or the i-region of a p-i-n device, (2) QD-sensitized nanocrystalline TiO2 films, and (3) QDs dispersed into a multiphase mixture of electron- and hole-conducting matrices, such as C60 and hole conducting polymers (like polythiophene or MEH-PPV), respectively. Additional research and understanding is required to realize the potential of MEG to significantly enhance solar cell performance.
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Chemical Reviews