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.

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Quantum dot solar cells

The science describing semiconductor−liquid interfaces is highly interdisciplinary, broad in scope, interesting, and of importance to various emerging technologies. We present a review of the basic physicochemical principles of semiconductor−liquid interfaces, including their historical development, and describe the major technological applications that are based on these scientific principles.

Physical Chemistry of Semiconductor−Liquid Interfaces

▪ Abstract Photoexcitation of a semiconductor with photons above the semiconductor band gap creates electrons and holes that are out of equilibrium. The rates at which the photogenerated charge carriers return to equilibrium via thermalization through carrier scattering, cooling by phonon emission, and radiative and nonradiative recombination are important issues. The relaxation processes can be greatly affected by quantization effects that arise when the carriers are confined to regions of space that are small compared with their deBroglie wavelength or the Bohr radius of bulk excitons. The effects of size quantization in semiconductor quantum wells (carrier confinement in one dimension) and quantum dots (carrier confinement in three dimensions) on the respective carrier relaxation processes are reviewed, with emphasis on electron cooling dynamics. The implications of these effects for applications involving radiant energy conversion are also discussed.

Spectroscopy and hot electron relaxation dynamics in semiconductor quantum wells and quantum dots.

Modulation spectroscopy is a powerful method for the study and characterization of a large number of semiconductor configurations, including bulk/thin film, microstructures (heterojunctions, quantum wells, superlattices, quantum dots), surfaces/interfaces and actual device structures in addition to semiconductor growth/processing. Furthermore, the influence of external perturbations such as temperature, electric fields, hydrostatic pressure, uniaxial stress, etc. can be investigated. This optical technique utilizes a very general principle of experimental physics, in which a periodically applied perturbation (either to the sample or probe) leads to sharp, derivative-like spectral features in the optical response of the system. Because of the richness of the derivative-like spectra, the information in the lineshape fits, room temperature performance and relative simplicity of operation this method is becoming increasingly more important as a tool to study these materials and structures. This article will review developments in the field during the last decade.

Modulation spectroscopy of semiconductors: bulk/thin film, microstructures, surfaces/interfaces and devices

In the presence of an electric field, the dielectric constant of a semiconductor exhibits Franz–Keldysh oscillations (FKO), which can be detected by modulated reflectance. Although it could be a powerful and simple method to study the electric fields/charge distributions in various semiconductor structures, in the past it has proven to be more complex. This is due to nonuniform fields and impurity induced broadening, which reduce the number of detectible Franz–Keldysh oscillations, and introduce uncertainties into the measurement. In 1989, a new structure, surface–undoped–doped (s‐i‐n+/s‐i‐p+) was developed, which allows the observation of a large number of FKOs and, hence, permitting accurate determination of electric fields. We present a review of the work on measuring electric fields in semiconductors with a particular emphasis on microstructures using the specialized layer sequence. We first discuss the general theory of modulation techniques dwelling on the approximations and their relevance. The cas...

Franz–Keldysh oscillations in modulation spectroscopy

The Franz–Keldysh oscillations induced by the electric field in the depleted zone below the GaAs surface are studied by photoreflectance spectroscopy. The electric field is precisely controlled by a molecular beam epitaxy grown buried highly doped layer and the pinned position of the Fermi level at the surface. It is shown that the electric field value as derived from theory is in disagreement with the value derived from electrostatic calculations. Consequently a determination of the Fermi level pinning is only possible from a measurement of both n‐ and p‐doped samples.

Franz–Keldysh oscillations originating from a well‐controlled electric field in the GaAs depletion region

Strained single quantum wells composed of GaAs/InGaAs/GaAs were grown by molecular beam epitaxy and characterized at room temperature by photoreflectance and at 6 and 77 K by photoluminescence spectroscopy. For the InGaAs/GaAs heterojunction, utilizing a band offset ratio of 85:15 (conduction band:valence band) for the intrinsic (nonstrained) interface and a contribution of the hydrostatic compression to the valence band movement corresponding to the pressure sensitivity of the spin orbit band, excellent agreement is found between calculated excitonic transition energies and those found by experiment at all temperatures studied. Our analysis indicates that material parameters and the combined strain components used to calculate band structure are not temperature dependent to our degree of sensitivity. An empirical equation, which differs slightly from that for bulk InGaAs crystals, describing the nonstrained band‐gap energy as a function of In fraction at 77 K is presented. The difference between band offset ratios for the intrinsic and strained heterojunction are found to be significant and the relative merits of each are discussed.

Strain effects and band offsets in GaAs/InGaAs strained layered quantum structures

Novel methods of GaAs surface passivation are investigated. Passivation is acheived by simple chemical treatments using aqueous solutions of Na2S, KOH, RuCl3, and K2Se. GaAs pn homojunction solar cells are used to evaluate the effectiveness of these passivation techniques. A significant reduction in minority‐carrier surface recombination velocity is demonstrated. In the best case, the surface recombination velocity decreased from 5×106 cm/s (untreated surface) to 103 cm/s. In addition, we observe improvements in solar cell photogenerated current, short wavelength spectral response, open‐circuit voltage, and junction ‘‘dark’’ current.

Study of novel chemical surface passivation techniques on GaAs pn junction solar cells

Photoluminescence (PL) and spatially resolved cathodoluminescence (CL) techniques are used to characterize the stress in narrow GaAs stripes grown on Si platforms. Since no overgrowth occurs over the recess edges, the GaAs stripes are grown unconstrained along one dimension. A duplication of the optical transitions is found in the PL spectrum from a region containing embedded GaAs and stripes. The peaks in the high‐energy shoulders of the PL spectrum are identified by CL measurements with high spatial resolution as the luminescence contribution of the GaAs stripes. They are submitted to lower internal stress values. A study on the geometrical dependence of the strain regime shows that a nonuniform biaxial strain field with a dominant longitudinal component is present in the stripes. The uniform biaxial strain, found in GaAs on Si (001), is present at stripe intersections.

Optical characterization of stress in narrow gaas stripes on patterned si substrates

Photoreflectance spectra have been used to characterize miniband formation in GaAs/ Alx Ga1−x As superlattices with wide wells (275–255 A) and withb arriers as thin as 17 A. Thirty‐two optical transitions are resolved in the photoreflectance spectra of the 17 A barrier sample. These experimental transitions match all those theoretically predicted from the selection rule Δn=0, including Γ‐ and Π‐type transitions arising from miniband dispersion; these results imply sample perfection. A sample with a 40 A barrier exhibits forbidden transitions with Δn≠0; these additional transitions, together with the narrow width of the minibands for 40 A barriers, create difficulty in resolving the miniband structure.

D. J. Arent

Papers

Franz–Keldysh oscillations originating from a well‐controlled electric field in the GaAs depletion region

Strain effects and band offsets in GaAs/InGaAs strained layered quantum structures

Study of novel chemical surface passivation techniques on GaAs pn junction solar cells

Optical characterization of stress in narrow gaas stripes on patterned si substrates

Miniband dispersion in GaAs/AlxGa1−xAs superlattices with wide wells and very thin barriers