The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

Quantum dot (QD) solar cells have the potential to increase the maximum attainable thermodynamic conversion efficiency of solar photon conversion up to about 66% by utilizing hot photogenerated carriers to produce higher photovoltages or higher photocurrents. The former effect is based on miniband transport and collection of hot carriers in QD array photoelectrodes before they relax to the band edges through phonon emission. The latter effect is based on utilizing hot carriers in QD solar cells to generate and collect additional electron–hole pairs through enhanced impact ionization processes. Three QD solar cell configurations are described: (1) photoelectrodes comprising QD arrays, (2) QD-sensitized nanocrystalline TiO 2 , and (3) QDs dispersed in a blend of electron- and hole-conducting polymers. These high-efficiency configurations require slow hot carrier cooling times, and we discuss initial results on slowed hot electron cooling in InP QDs.

/pdf/quantum-dot-solar-cells-4f2pqetwm9.pdf

Quantum dot solar cells

This article reviews recent theoretical and experimental advances in the fundamental understanding and active control of quantum fluids of light in nonlinear optical systems. In the presence of effective photon-photon interactions induced by the optical nonlinearity of the medium, a many-photon system can behave collectively as a quantum fluid with a number of novel features stemming from its intrinsically nonequilibrium nature. A rich variety of recently observed photon hydrodynamical effects is presented, from the superfluid flow around a defect at low speeds, to the appearance of a Mach-Cherenkov cone in a supersonic flow, to the hydrodynamic formation of topological excitations such as quantized vortices and dark solitons at the surface of large impenetrable obstacles. While the review is mostly focused on a specific class of semiconductor systems that have been extensively studied in recent years (planar semiconductor microcavities in the strong light-matter coupling regime having cavity polaritons as elementary excitations), the very concept of quantum fluids of light applies to a broad spectrum of systems, ranging from bulk nonlinear crystals, to atomic clouds embedded in optical fibers and cavities, to photonic crystal cavities, to superconducting quantum circuits based on Josephson junctions. The conclusive part of the article is devoted to a review of the future perspectives in the direction of strongly correlated photon gases and of artificial gauge fields for photons. In particular, several mechanisms to obtain efficient photon blockade are presented, together with their application to the generation of novel quantum phases.

Quantum fluids of light

Here, we will first briefly summarize the general principles of QD synthesis using our previous work on InP as an example. Then we will focus on QDs of the IV-VI Pb chalcogenides (PbSe, PbS, and PbTe) and Si QDs because these were among the first QDs that were reported to produce multiple excitons upon absorbing single photons of appropriate energy (a process we call multiple exciton generation (MEG)). We note that in addition to Si and the Pb-VI QDs, two other semiconductor systems (III-V InP QDs(56) and II-VI core-shell CdTe/CdSe QDs(57)) were very recently reported to also produce MEG. Then we will discuss photogenerated carrier dynamics in QDs, including the issues and controversies related to the cooling of hot carriers and the magnitude and significance of MEG in QDs. Finally, we will discuss applications of QDs and QD arrays in novel quantum dot PV cells, where multiple exciton generation from single photons could yield significantly higher PV conversion efficiencies.

/pdf/semiconductor-quantum-dots-and-quantum-dot-arrays-and-4c2cspcqy1.pdf

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

Beyond 11% Efficiency: Characteristics of State‐of‐the‐Art Cu2ZnSn(S,Se)4 Solar Cells

Carrier capture and relaxation processes in self-assembled 15-nm ${\mathrm{In}}_{0.5}$${\mathrm{Ga}}_{0.5}$As/GaAs quantum dots are investigated by means of time-resolved photoluminescence spectroscopy. In a systematic study of photoluminescence rise times and barrier decay times (variation of temperature, excitation energy, and excitation density) we aim to identify the physical mechanisms responsible for fast carrier capture and relaxation in quantum dots. Both processes are separated by using appropriate excitation energies. Carrier capture and relaxation are shown to proceed with rates as high as \ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}${10}^{10}$ ${\mathrm{s}}^{\mathrm{\ensuremath{-}}1}$ at low temperature even if less than one electron-hole pair per dot and excitation pulse is created. We interpret our results in terms of multiphonon processes at low excitation densities and in terms of Auger processes at high excitation densities. \textcopyright{} 1996 The American Physical Society.

Rapid carrier relaxation in self-assembled inxga1-xas/gaas quantum dots

We present experimental results concerning optical transitions and carrier dynamics (capture and relaxation) in self assembled InAs/GaAs quantum dot structures grown by metalorganic vapor phase epitaxy.Photoluminescence(PL) measurements at high excitation level reveal optical transitions above the ground state emission. These transitions are found to originate from occupied hole states by solving the quantum doteigenvalue problem. Time‐resolved studies after non‐resonant pulse excitation exhibit a relaxation ladder of the excited carriers from the GaAs barrier down to the ground state of the quantum dots. From both the continuous‐wave measurements and the PL‐decay curves we conclude that the carrier relaxation at non‐resonant excitation is mediated by Coulomb interaction (Auger effect). PL‐decay curves after resonant pulse excitation reveal a longer rise time compared to non‐resonant excitation which is a clear indication of a relaxation bottleneck inside the quantum dots. We interpret the rise time (≊ 400 ps) in this case to originate from relaxation via scattering by acoustic phonons. The PL‐decay time of the ground state emission ≊700 ps is interpreted as the excitonic lifetime of the quantum dot.

Optical transitions and carrier relaxation in self assembled InAs/GaAs quantum dots

Room-temperature recombination dynamics has been investigated in a large set of different Cu(In,Ga)Se2 absorber films and compared to the electrical device characteristics of the respective solar cell modules. For a given cell preparation process, a characteristic relation between the low-injection minority-carrier lifetime of the absorber layers and the conversion efficiency of the solar cells is observed: Long lifetimes correlate with high open circuit voltages and conversion efficiencies, while no significant influence of the lifetime on the short circuit current is found.

Minority-carrier lifetime and efficiency of Cu(In,Ga)Se2 solar cells

Three-dimensionally confined Bragg microcavities exhibiting several fully quantized photon modes have been investigated by time-resolved photoluminescence spectroscopy. A strong decrease of the exciton lifetime with decreasing lateral dimension is observed and shown to be mainly due to coupling of the long-lived excitons with short-lived photon modes. We observe similar decay times of all photon modes for a fixed structure size which amount to approximately half of the corresponding exciton lifetimes. This behavior is explained by incoherent Auger-type scattering processes populating the photon modes on the low-energy side of the exciton line.

B. Ohnesorge

Papers

Rapid carrier relaxation in self-assembled inxga1-xas/gaas quantum dots

Optical transitions and carrier relaxation in self assembled InAs/GaAs quantum dots

Minority-carrier lifetime and efficiency of Cu(In,Ga)Se2 solar cells

Enhancement of spontaneous emission rates by three-dimensional photon confinement in Bragg microcavities