Richard D. Schaller

New Aspects of Carrier Multiplication in Semiconductor Nanocrystals

One consequence of strong spatial confinement of electronic wave functions in semiconductor nanocrystals (NCs) is a significant enhancement in carrier-carrier Coulomb interactions. This effect leads to a number of novel physical phenomena including ultrafast decay of multiple electron-hole pairs (multiexcitons) by Auger recombination and high-efficiency generation of mutiexcitons by single photons via carrier multiplication (CM). Significant recent interest in multiexciton phenomena in NCs has been stimulated by studies of NC lasing, as well as potential applications of CM in solar-energy conversion. The focus of this Account is on CM. In this process, the kinetic energy of a hot electron (or a hot hole) does not dissipate as heat but is, instead, transferred via the Coulomb interaction to the valence-band electron, exciting it across the energy gap. Because of restrictions imposed by energy and translational-momentum conservation, as well as rapid energy loss due to phonon emission, CM is inefficient in bulk semiconductors, particularly at energies relevant to solar energy conversion. On the other hand, the CM efficiency can potentially be enhanced in zero-dimensional NCs because of factors such as a wide separation between discrete electronic states, which inhibits phonon emission (phonon bottleneck), enhanced Coulomb interactions, and relaxation in translational-momentum conservation. Here, we investigate CM in PbSe NCs by applying time-resolved photoluminescence and transient absorption. Both techniques show clear signatures of CM with efficiencies that are in good agreement with each other. NCs of the same energy gap show moderate batch-to-batch variations (within approximately 30%) in apparent multiexciton yields and larger variations (more than a factor of 3) due to differences in sample conditions (stirred vs static solutions). These results indicate that NC surface properties may affect the CM process. They also point toward potential interference from extraneous effects such as NC photoionization that can distort the results of CM studies. CM yields measured under conditions when extraneous effects are suppressed via intense sample stirring and the use of extremely low pump levels (0.02-0.03 photons absorbed per NC per pulse) reveal that both the electron-hole creation energy and the CM threshold are reduced compared with those in bulk solids. These results indicate a confinement-induced enhancement in the CM process in NC materials. Further optimization of CM performance should be possible by utilizing more complex (for example, shaped-controlled or heterostructured) NCs that allow for facile manipulation of carrier-carrier interactions, as well as single and multiexciton energies and dynamics.

Optical properties of PbSe nanocrystal quantum dots under pressure

The optical properties of PbSe nanocrystal quantum dots (NQDs) were studied as a function of applied hydrostatic pressure over the range from ambient to 5.4GPa. PbSe NQDs exhibit an energy gap that is dominated by quantum confinement. Despite such strong confinement, the authors find that the energy gaps of 3, 5, and 7nm diameter PbSe NQDs change monotonically with pressure with a dependence that is almost entirely determined by the bulk deformation potential. The sizable dependence of the NQD energy gap with pressure invites applications in the areas of high speed pressure sensing and tunable IR lasers.

Nanocomposite solar cells based on conjugated polymer/PbSe quantum dot

We report a novel type of nanocomposite of conjugated polymer (regio-regular polythiophene) with infrared-sensitive, PbSe quantum dots (QD), which have size-tunable lowest-energy absorption bands between 0.3 and 1 eV. Thin film devices show very good diode characteristics and sizable photovoltaic response with an open circuit voltage, Voc, of ~ 0.3-0.4 V and short circuit current density, Jsc, of ~ 0.2mA/cm2, which is significantly higher than recently reported in PbS QD-based devices. This is the evidence of a quite efficient photoinduced charge transfer between the polymer and QD, with infrared sensitivity. Photocurrent under reverse bias is significantly enhanced to Jph ~ 1 mA/cm2 indicating that the polythiophene/PbSe QD system can be used as effective infrared photodetectors. Detailed spectroscopic studies of photoresponse over a wide spectral range are presented. Quenching of photoluminescence by PbSe QDs has also been studied to gain more understanding of energy and charge transfer in this system.

Efficient production of multiexcitons from single photons in semiconductor nanocrystals for low-cost, high efficiency photovoltaics

Solar cells represent a clean source of renewable energy. However, in order to make them competitive with traditional energy sources, the efficiency-to-cost ratio must be increased appreciably. Increases in power conversion efficiency have typically relied on iterative improvements in material quality (for both Si and non-Si systems) and/or device engineering aspects including, e.g., the use of tandem architectures. There exist, however, approaches that can potentially lead to leaps in photovoltaic performance through the use of new principles for conversion of solar energy into electricity. Proposed methods include the development of impurity band and intermediate band devices, hot electron extraction, and carrier multiplication.