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

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.

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

Over the last decade there have been spectacular developments in ultrafast laser technology, due to the introduction of solid state active materials and of new mode-locking and amplification techniques. These advances, together with the discovery of new nonlinear optical crystals, have fostered the introduction of ultrafast optical parametric amplifiers as a practical source of femtosecond pulses tunable across the visible and infrared spectral ranges. This article summarizes the recent progress in the development of ultrafast optical parametric amplifiers, giving the basic design principles for different frequency ranges and in addition presenting some advanced designs for the generation of ultrabroadband, few-optical-cycle pulses. Finally, we also briefly discuss the possibility of applying parametric amplification schemes to large-scale, petawatt-level systems.

Ultrafast optical parametric amplifiers

▪ 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.

We study the carrier dynamics in planar methyl ammonium lead iodide perovskite films using broadband transient absorption spectroscopy. We show that the sharp optical absorption onset is due to an exciton transition that is inhomogeneously broadened with a binding energy of 9 meV. We fully characterize the transient absorption spectrum by free-carrier-induced bleaching of the exciton transition, quasi-Fermi energy, carrier temperature and bandgap renormalization constant. The photo-induced carrier temperature is extracted from the transient absorption spectra and monitored as a function of delay time for different excitation wavelengths and photon fluences. We find an efficient hot-phonon bottleneck that slows down cooling of hot carriers by three to four orders of magnitude in time above a critical injection carrier density of ∼5 × 1017 cm−3. Compared with molecular beam epitaxially grown GaAs, the critical density is an order of magnitude lower and the relaxation time is approximately three orders of magnitude longer. Hot carriers in perovskites experience slow cooling.

Observation of a hot-phonon bottleneck in lead-iodide perovskites

A broadly tunable femtosecond optical parametric oscillator (OPO) based on KTiOPO4 is externally pumped by a self-mode-locked Ti:sapphire laser. The laser is capable of continuous tuning from 1.2 micrometers to 1.37 micrometers in the signal branch and 1.8 to 2.15 micrometers in the idler branch, when using one set of OPO optics. Other optics expand the tuning range of the OPO from 1.0 micrometers to 2.75 micrometers, for example, by using three sets of mirrors and two different crystals. Without prisms in the OPO cavity, 215 mW of chirped pulses is generated in the signal branch, while 235 mW is generated in the idler branch. The total conversion efficiency, as measured by pump depletion, is 50%. With prisms in the cavity, nearly transform-limited pulses of 135 femtoseconds are generated, which can be shortened to 75 fs by increasing the output coupling.

Ti:sapphire-pumped high repetition rate femtosecond optical parametric oscillator

An investigation of the hot-carrier relaxation in GaAs/(Al,Ga)As quantum wells and bulk GaAs in the high-carrier-density limit is presented. Using a time-resolved luminescence up-conversion technique with \ensuremath{\le}80-fs temporal resolution, carrier temperatures are measured in the 100-fs-to-2-ns range. Our results show that the hot-carrier cooling rates in the quantum wells are significantly slower than in the bulk for carrier densities greater than 2\ifmmode\times\else\texttimes\fi{}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$. A comparison is made with previous publications to resolve the confusion concerning the difference in cooling rates in quasi-two- and three-dimensional systems.

Comparison of hot-carrier relaxation in quantum wells and bulk GaAs at high carrier densities

The details concerning the resonator configuration, crystal parameters, and operating characteristics of high-repetition-rate and high-average-power broadly tunable femtosecond optical parametric oscillators are reviewed and discussed in some detail. We also report new results on an intracavity-doubled optical parametric oscillator with tunability from 580 to 657 nm in the visible and the first, to our knowledge, high-repetition-rate femtosecond optical parametric oscillator with the new nonlinear-optical crystal In:KTiOAsO4, which can potentially tune to 5.3 μm.

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Recent advances of the Ti:sapphire-pumped high-repetition-rate femtosecond optical parametric oscillator

We present what is to our knowledge the first demonstration of a femtosecond optical parametric oscillator using the new nonlinear crystal KTiOAsO4 Powers of as much as 75 mW in the signal branch and as much as 100 mW in the idler branch were coupled out of the cavity. Pulse widths as short as 85 fs in the signal branch and 150 fs in the idler branch were measured. The potential of tuning out to ~5 μm in the idler branch is discussed.

Optical parametric oscillation with KTiOAsO4.

A high-repetition-rate Ti:sapphire-pumped optical parametric oscillator based on the new nonlinear optical crystal CsTiOAsO4 is described. The operation of this optical parametric oscillator is characterized for a 90°-cut crystal by use of a type II interaction. Tuning from 1.46 to 1.74 μm is demonstrated, and there is the potential for tuning from 0.9 to 5 μm with angle tuning. Powers of 100 mW in the signal and the idler branches are obtained. Pulse widths as short as 64 fs are generated with and without prisms in the cavity.

P. E. Powers

Papers

Ti:sapphire-pumped high repetition rate femtosecond optical parametric oscillator

Comparison of hot-carrier relaxation in quantum wells and bulk GaAs at high carrier densities

Recent advances of the Ti:sapphire-pumped high-repetition-rate femtosecond optical parametric oscillator

Optical parametric oscillation with KTiOAsO4.

High-repetition-rate femtosecond optical parametric oscillator based on CsTiOAsO(4).