Copyright 2004 by Marcel Dekker, Inc. All Rights Reserved.

2. Quantum-Dot-Sensitized Nanocrystalline TiO 2 Solar CellsThis configuration is a variation of a recent promising new type of PV cell thatis based on dye-sensitization of nanocrystalline TiO 2 layers [164–166]. In thislatter PV cell, dye molecules are chemisorbed onto the surface of 10–30 nmsizeTiO 2 particles that have been sintered into a highly porous nanocrystalline10–20-Am TiO 2 film. Upon photoexcitation of the dye molecules,electrons are very efficiently injected from the excited state of the dye intothe conduction band of the TiO 2 , affecting charge separation and producing aphotovoltaic effect.For the QD-sensitized cell, QDs are substituted for the dye molecules;they can be adsorbed from a colloidal QD solution [167] or produced in situ[168–171]. Successful PV effects in such cells have been reported for severalsemiconductor QDs, including InP, CdSe, CdS, and PbS [167–171]. Possibleadvantages of QDs over dye molecules are the tunability of optical propertieswith size and better heterojunction formation with solid hole conductors.Also, a unique potential capability of the QD-sensitized solar cell is theproduction of quantum yields greater than one by impact ionization (inverseAuger effect) [172].3. Quantum Dots Dispersed in Organic Semiconductor PolymerMatricesRecently, photovoltaic effects have been reported in structures consisting ofQDs forming junctions with organic semiconductor polymers. In one configuration,a disordered array of CdSe QDs is formed in a hole-conductingpolymer—MEH–PPV {poly[2-methoxy, 5-(2V-ethyl)-hexyloxy-p-phenylenevinylene]}[173]. Upon photoexcitation of the QDs, the photogenerated holesare injected into the MEH–PPV polymer phase and are collected via anelectrical contact to the polymer phase. The electrons remain in the CdSe QDsand are collected through diffusion and percolation in the nanocrystallinephase to an electrical contact to the QD network. Initial results show relativelylow conversion efficiencies. [173,174], but improvements have beenreported with rodlike CdSe QD shapes [175] embedded in poly(3-hexylthiophene)(the rodlike shape enhances electron transport through the nanocrystallineQD phase). In another configuration [176], a polycrystalline TiO 2layer is used as the electron-conducting phase, and MEH-PPV is used toconduct the holes; the electron and holes are injected into their respectivetransport mediums upon photoexcitation of the QDs.A variation of these configurations is to disperse the QDs into a blend ofelectron- and hole-conducting polymers [177]. This scheme is the inverse oflight-emitting-diode structures based on QDs [178–182]. In the PV cell, eachtype of carrier-transporting polymer would have a selective electrical contactto remove the respective charge carriers. A critical factor for success is toCopyright 2004 by Marcel Dekker, Inc. All Rights Reserved.