Behavior of Lead Sulfide Photocells in the Ultraviolet*

Smith, Abbott; Dutton, David

doi:10.1364/josa.48.001007

Cited by 46 publications

(35 citation statements)

References 9 publications

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“…After normalizing to the 800 nm slope, for which we have verified that η = 1, one finds η for the different photon energies. Figure 2b shows η versus photon energy for PbS, as inferred from our terahertz measurements, together with the reported values for a PbS photovoltaic cell 21 . The number of extra carriers created by carrier multiplication is over 1.5 times larger than concluded in ref.…”

mentioning

confidence: 85%

“…PbSe and PbS are arguably the most important materials in the carrier-multiplication discussion, as their small bulk bandgap values result in an optimal energy gap for quantum dots of these materials to use the excess energy of visible photons. However, reports of bulk carrier-multiplication efficiencies in PbSe and PbS are, respectively, absent or dated 21 . For bulk PbS, carrier-multiplication efficiencies have been inferred in the 1950s from device photocurrent measurements, which require charges to move over large distances.…”

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confidence: 99%

“…The number of extra carriers created by carrier multiplication is over 1.5 times larger than concluded in ref. 21. As explained in the Supplementary Information, of carriers in defects.…”

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confidence: 99%

“…After calculating the electronic structure of bulk PbSe and PbS in tight binding, we compute the matrix elements of the screened Coulomb interaction between two-particle states derived from tight-binding orbitals, as described in ref. 21. The impact ionization rates are obtained using the Fermi golden rule and are used as inputs in the numerical simulation of hot-carrier relaxation and carrier multiplication.…”

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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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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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mentioning

confidence: 85%

mentioning

confidence: 99%

“…The number of extra carriers created by carrier multiplication is over 1.5 times larger than concluded in ref. 21. As explained in the Supplementary Information, of carriers in defects.…”

mentioning

confidence: 99%

mentioning

confidence: 99%

See 2 more Smart Citations

Assessment of carrier-multiplication efficiency in bulk PbSe and PbS

Pijpers¹,

Ulbricht²,

Tielrooij³

et al. 2009

Nature Phys

257

362

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“…Impact ionization must also compete with carrier cooling and as a result the threshold for I.I. is higher than expected from purely momentum and energy conservation arguments, for example, in bulk PbS it was found to be ∼ 4.5E g in one experiment [42]. For silicon, which has been the subject of extensive theoretical modeling as well as numerous electronic and optical measurements, the threshold for the onset of I.I.…”

Section: Carrier-carrier and Carrier-lattice Interactions In Bulk Semmentioning

confidence: 99%

Multiple exciton generation in semiconductor nanocrystals: Toward efficient solar energy conversion

Beard¹,

Ellingson²

2008

Laser & Photonics Reviews

142

131

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Within the range of photon energies illuminating the Earth's surface, absorption of a photon by a conventional photovoltaic semiconductor device results in the production of a single electron-hole pair; energy of a photon in excess of the semiconductor's bandgap is efficiently converted to heat through electron and hole interactions with the crystal lattice. Recently, colloidal semiconductor nanocrystals and nanocrystal films have been shown to exhibit efficient multiple electron-hole pair generation from a single photon with energy greater than twice the effective band gap. This multiple carrier pair process, referred to as multiple exciton generation (MEG), represents one route to reducing the thermal loss in semiconductor solar cells and may lead to the development of low cost, high efficiency solar energy devices. We review the current experimental and theoretical understanding of MEG, and provide views to the nearterm future for both fundamental research and the development of working devices which exploit MEG. MEG E g E hνAbsorption of a single photon with energy in excess of two times the band gap, Eg, produces multiple excitons at the band edge. TEM picture of typical PbSe nanocrystals.

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