Terahertz Spectroscopy

Beard, Matthew C.; Turner, Gordon M.; Schmuttenmaer, Charles A.

doi:10.1021/jp020579i

Cited by 557 publications

(414 citation statements)

References 95 publications

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“…Figure 1a shows the complex conductivity σ (ω) for PbS (E gap = 0.42 eV) following excitation at 266 nm (4.66 eV), as inferred from the Fourier transforms of the time-domain fields 27 . We follow previous authors 26,27 and fit the data in Fig. 1a with the Drude model of free carriers,…”

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confidence: 94%

“…Whereas time-resolved optical and infrared spectroscopies are ideally suited to probe carrier populations in colloidal quantum dots 2,5,7,8,10,11,14,15,[17][18][19][22][23][24] , light of terahertz frequencies interacts strongly with free carriers and allows for the direct characterization of carrier density and mobility [25][26][27] . Here, we quantify carrier multiplication in bulk PbSe and PbS on ultrafast timescales using terahertz time-domain spectroscopy 26 (THz-TDS). We directly determine the number of photogenerated electron-hole pairs per absorbed photon, η, for various photon energies from the ultraviolet to the infrared, picoseconds after photo excitation, allowing for a direct comparison with quantum-dot results.…”

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confidence: 99%

“…Second, the amount of generated electron-hole pairs is determined using THz-TDS measurements 26 . This technique has been used extensively to determine carrier dynamics in semiconductors, as it provides the frequency-dependent complex conductivity σ (ω) of charge carriers 25,27 .…”

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Assessment of carrier-multiplication efficiency in bulk PbSe and PbS

Pijpers¹,

Ulbricht²,

Tielrooij³

et al. 2009

Nature Phys

257

362

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One of the important factors limiting solar-cell efficiency is that incident photons generate one electron-hole pair, irrespective of the photon energy. Any excess photon energy is lost as heat. The possible generation of multiple charge carriers per photon (carrier multiplication) is therefore of great interest for future solar cells 1 . Carrier multiplication is known to occur in bulk semiconductors, but has been thought to be enhanced significantly in nanocrystalline materials such as quantum dots, owing to their discrete energy levels and enhanced Coulomb interactions 1-3 . Contrary to this expectation, we demonstrate here that, for a given photon energy, carrier multiplication occurs more efficiently in bulk PbS and PbSe than in quantum dots of the same materials. Measured carriermultiplication efficiencies in bulk materials are reproduced quantitatively using tight-binding calculations, which indicate that the reduced carrier-multiplication efficiency in quantum dots can be ascribed to the reduced density of states in these structures.Carrier multiplication is the process in which the absorption of a single, high-energy photon results in the generation of two or more electron-hole pairs. The excess energy of the initially excited electron is used to excite a second electron over the bandgap, rather than being converted into heat through sequential phonon emission. Carrier multiplication is important for the operation for high-speed electronic devices 4 , but is especially relevant for solar cells 1 , because relaxation of hot carriers through phonon emission is a common loss mechanism in bulk semiconductor solar cells. In this context, semiconductor quantum dots are promising building blocks for future solar cells 1 . In addition to the size-tunability of the quantum-dot optical properties, the carrier-multiplication efficiency in quantum dots was reported to be much higher than in bulk materials, where the process is generally referred to as impact ionization. It has been argued that carrier multiplication is more efficient in nanostructured semiconductors owing to quantum-confinement effects causing (1) a slowing of the phonon-mediated relaxation channel 1 and (2) enhanced Coulomb interactions 2 , resulting from forced overlap between wavefunctions and reduced dielectric screening at the quantum-dot surface 3 . In recent years, several femtosecond spectroscopy studies have revealed highly efficient carrier multiplication in PbSe and PbS (refs 2, 5-9) (refs 18, 19) quantum dots. In initial studies, carrier-multiplication efficiencies may have been overestimated owing to several experimental complications, including too high excitation fluences (generating multiple carriers by sequential absorption of multiple photons), lack of stirring of quantum-dot suspensions (causing photo-induced charging) and sample-to-sample variability 19 . Furthermore, recent tight-binding calculations 20 suggest carrier multiplication in quantum dots is not only not enhanced relative to bulk, but is actually lower. Answering the...

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confidence: 99%

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Assessment of carrier-multiplication efficiency in bulk PbSe and PbS

Pijpers¹,

Ulbricht²,

Tielrooij³

et al. 2009

Nature Phys

257

362

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“…UV-visible 3 spectrophotometry and ellipsometry can be used to determine the bandgap and absorption coefficient. The minority carrier lifetime and mobility can be measured through time resolved photoluminescence, 44 terahertz spectroscopy, 36,45,46 and photoconductivity decay measurements. 47 Challenges also exist in performing quantum mechanical calculations on some of these materials.…”

Section: Introductionmentioning

confidence: 99%

Perovskite-Inspired Photovoltaic Materials: Toward Best Practices in Materials Characterization and Calculations

et al. 2017

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ABSTRACT:Recently, there has been an explosive growth in research based on hybrid lead-halide perovskites for photovoltaics owing to rapid improvements in efficiency. The advent of these materials for solar applications has led to widespread interest in understanding the key enabling properties of these materials. This has resulted in renewed interest in related compounds and a search for materials that may replicate the defect-tolerant properties and long lifetimes of the hybrid lead-halide perovskites. Given the rapid pace of development of the field, the rises in efficiencies of these systems have outpaced the more basic understanding of these materials. Measuring or calculating the basic properties, such as crystal/electronic structure and composition, can be challenging because some of these materials have anisotropic structures, and/or are composed of both heavy metal cations and volatile, mobile, light elements. Some consequences are beam damage during characterization, composition change under vacuum, or compound effects, such as the alteration of the electronic structure through the influence of the substrate. These effects make it challenging to understand the basic properties integral to optoelectronic operation. Compounding these difficulties is the rapid pace with which the field progresses. This has created an ongoing need to continually evaluate best practices with respect to characterization and calculations, as well as to identify inconsistencies in reported values to determine if those inconsistencies are rooted in characterization methodology or materials synthesis. This article describes the difficulties in characterizing hybrid lead-halide perovskites and new materials, and how these challenges may be overcome. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 2 challenges discussed. The focus in this article is on crystallography, composition measurements, photoemission spectroscopy and calculations on perovskites and new, related absorbers. We suggest how some of the important artifacts could be avoided and how the reporting for each technique could be streamlined between groups to ensure reproducibility as the field progresses.

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“…Terahertz (THz) technology [1,2] is a fast-growing field with applications in biology and medicine [3], medical imaging [4], material spectroscopy and sensing [5], security, monitoring and spectroscopy in pharmaceutical industry, and high-data-rate communications. Situated between millimeter-wave and infrared range, the terahertz region with the frequency range from 100 GHz up to 10 THz is a scientifically rich but as yet not fully explored part of the electromagnetic spectrum.…”

Section: Introductionmentioning

confidence: 99%

Signal Recovery in Pulsed Terahertz Integrated Circuits

Heidari¹,

Neshat²,

Saeedkia³

et al. 2010

PIER

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Abstract-In this article, a time-domain calibration procedure is proposed for pulsed Terahertz Integrated Circuits (TIC) used in onchip applications, where the conventional calibration methods are not applicable. The proposed post-detection method removes the unwanted linear distortions, such as interfering echoes and frequency dispersion, by using only one single-port measurement. The method employs a wave-transfer model for analysis of the TIC, and the model parameters are obtained by a proposed blind estimation algorithm. A complete implementation of the method is demonstrated for a fabricated TIC, when used in an on-chip sensing application. The features of interest in the measured signal, such as absorption lines, can be masked or weakened by the distortion of the THz signal happening in a TIC. The proposed signal recovery approach improves the detection of those otherwise hidden features, and can significantly enhance the performance of existing TICs. To show the effectiveness of the proposed de-embedding method, numerical results are presented for simulated and measured signals. The method presented in this article is enabling for accurate TIC applications, and can be utilized to optimally design novel TIC structures for specific purposes.

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Terahertz Spectroscopy

Cited by 557 publications

References 95 publications

Assessment of carrier-multiplication efficiency in bulk PbSe and PbS

Assessment of carrier-multiplication efficiency in bulk PbSe and PbS

Perovskite-Inspired Photovoltaic Materials: Toward Best Practices in Materials Characterization and Calculations

Signal Recovery in Pulsed Terahertz Integrated Circuits

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