Progress on hot carrier cells

Conibeer, Gavin; Ekins-Daukes, N. J.; Guillemoles, Jean‐François; König, Dirk; Cho, Eun‐Chel; Jiang, Chu-Wei; Shrestha, Santosh; Green, Martin A.

doi:10.1016/j.solmat.2008.09.034

Cited by 118 publications

(95 citation statements)

References 20 publications

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“…For example, within our ab initio framework there is no need to employ deformation potentials to describe e-ph scattering rates. This work does not aim to suggest strategies to mitigate thermalization losses in solar cells [21,22], but rather aims to study hot carriers using accurate ab initio calculations.We compute hot carrier relaxation times and mean free paths in Si, and map these quantities in the BZ. Our calculations indicate that hot carrier thermalization in Si is dominated by phonon emission processes with a time scale of ∼100 fs for carriers near the band edges, while faster relaxation times of ∼10 fs are found at higher energies away from the band edges.…”

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confidence: 99%

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Ab InitioStudy of Hot Carriers in the First Picosecond after Sunlight Absorption in Silicon

et al. 2014

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Hot carrier thermalization is a major source of efficiency loss in solar cells. Because of the subpicosecond time scale and complex physics involved, a microscopic characterization of hot carriers is challenging even for the simplest materials. We develop and apply an ab initio approach based on density functional theory and many-body perturbation theory to investigate hot carriers in semiconductors. Our calculations include electron-electron and electron-phonon interactions, and require no experimental input other than the structure of the material. We apply our approach to study the relaxation time and mean free path of hot carriers in Si, and map the band and k dependence of these quantities. We demonstrate that a hot carrier distribution characteristic of Si under solar illumination thermalizes within 350 fs, in excellent agreement with pump-probe experiments. Our work sheds light on the subpicosecond time scale after sunlight absorption in Si, and constitutes a first step towards ab initio quantification of hot carrier dynamics in materials. DOI: 10.1103/PhysRevLett.112.257402 PACS numbers: 78.56.-a, 71.20.Mq, 78.47.db, 88.40.H-Single-junction solar cells based on crystalline Si are rapidly approaching the Shockley-Queisser efficiency limit [1,2]. While the Carnot efficiency of ∼95% sets the ultimate limit for solar energy conversion at room temperature, practical efficiency limits in ordinary photovoltaic (PV) solar cells are significantly lower; e.g., the ShockleyQueisser limit for Si is close to 30% [2]. The main factors limiting efficiency are carrier thermalization and absorption losses [3,4]. For the case of Si under AM1.5 solar illumination [5], nearly 25% of incident solar energy is lost to heat as the nonequilibrium ("hot") carriers generated by sunlight absorption thermalize to the edges of the band gap. Not only is hot carrier thermalization the main source of loss in most PV materials, it is also difficult to prevent, control, and understand with microscopic detail due to the subpicosecond time scale typical of hot carrier relaxation [6]. This scenario is common to other technologies employing hot carriers, including electronics, optoelectronics, and renewable energy devices beyond PV [7][8][9][10][11].The leading mechanisms involved in hot carrier thermalization consist of inelastic electron-phonon (e-ph) and electron-electron (e-e) scattering processes [12]. Relaxation times for e-ph and e-e interactions in semiconductors have been studied extensively by model Hamiltonians with selected phonon modes, simplified electronic band structures, deformation potentials, and/or empirical pseudopotentials [13][14][15][16][17][18]. Hot carrier dynamics in semiconductors has been investigated experimentally using pump-probe optical measurements [19,20].This work has two main goals. First, we present an ab initio approach based on density functional theory (DFT) and many-body perturbation theory to investigate hot carriers in materials. Second, we apply this framework to study hot carrier thermalization a...

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Ab InitioStudy of Hot Carriers in the First Picosecond after Sunlight Absorption in Silicon

et al. 2014

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“…hot carriers | semiconductors | GaAs | ultrafast | electron-phonon scattering H ot carriers (HCs) generated by the absorption of light or injection at a contact are commonly found in many advanced technologies (1)(2)(3)(4)(5)(6)(7)(8)(9). In electronics, the operation of high-speed devices is controlled by HC dynamics, and HC injection is a key degradation mechanism in transistors (10,11).…”

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“…In electronics, the operation of high-speed devices is controlled by HC dynamics, and HC injection is a key degradation mechanism in transistors (10,11). In solar cells and plasmonics, recent work has focused on extracting the kinetic energy of HCs before cooling (7,9), a process defined here as the energy loss of HCs, ultimately leading to thermal equilibrium with phonons. HC dynamics is also crucial to interpret time-resolved spectroscopy experiments used to study excited states in condensed matter (12).…”

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Ab initio study of hot electrons in GaAs

Bernardi

Vigil‐Fowler

Ong

et al. 2015

Proc. Natl. Acad. Sci. U.S.A.

122

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Hot carrier dynamics critically impacts the performance of electronic, optoelectronic, photovoltaic, and plasmonic devices. Hot carriers lose energy over nanometer lengths and picosecond timescales and thus are challenging to study experimentally, whereas calculations of hot carrier dynamics are cumbersome and dominated by empirical approaches. In this work, we present ab initio calculations of hot electrons in gallium arsenide (GaAs) using density functional theory and many-body perturbation theory. Our computed electron-phonon relaxation times at the onset of the Γ, L, and X valleys are in excellent agreement with ultrafast optical experiments and show that the ultrafast (tens of femtoseconds) hot electron decay times observed experimentally arise from electron-phonon scattering. This result is an important advance to resolve a controversy on hot electron cooling in GaAs. We further find that, contrary to common notions, all optical and acoustic modes contribute substantially to electron-phonon scattering, with a dominant contribution from transverse acoustic modes. This work provides definitive microscopic insight into hot electrons in GaAs and enables accurate ab initio computation of hot carriers in advanced materials.hot carriers | semiconductors | GaAs | ultrafast | electron-phonon scattering H ot carriers (HCs) generated by the absorption of light or injection at a contact are commonly found in many advanced technologies (1-9). In electronics, the operation of high-speed devices is controlled by HC dynamics, and HC injection is a key degradation mechanism in transistors (10, 11). In solar cells and plasmonics, recent work has focused on extracting the kinetic energy of HCs before cooling (7, 9), a process defined here as the energy loss of HCs, ultimately leading to thermal equilibrium with phonons. HC dynamics is also crucial to interpret time-resolved spectroscopy experiments used to study excited states in condensed matter (12). This situation has sparked a renewed interest in HCs in a broad range of materials of technological relevance.Experimental characterization of HCs is challenging because of the subpicosecond timescale associated with the electron-phonon (e-ph) and electron-electron (e-e) scattering processes regulating HC dynamics. For example, HCs can be studied using ultrafast spectroscopy, but microscopic interpretation of time-resolved spectra requires accurate theoretical models. However, modeling of HCs thus far has been dominated by empirical approaches, which do not provide atomistic details and use ad hoc parameters to fit experiments (13,14). Notwithstanding the pioneering role of these early studies, the availability of accurate ab initio computational methods based on density functional theory (DFT) (15) and many-body perturbation theory (16) enables studies of HCs with superior accuracy, broad applicability, and no need for fitting parameters.Hot electrons in gallium arsenide (GaAs) are of particular interest because of the high electron mobility and multivalley character of the c...

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“…7). However, thermalization, driven by electron-phonon interactions, is usually a very fast process on the order of picoseconds, and slowing it to a rate that allows efficient charge collection at elevated electron energies remains a challenge in material physics 9,10 .…”

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Photon-enhanced thermionic emission from heterostructures with low interface recombination

et al. 2013

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Photon-enhanced thermionic emission is a method of solar-energy conversion that promises to combine photon and thermal processes into a single mechanism, overcoming fundamental limits on the efficiency of photovoltaic cells. Photon-enhanced thermionic emission relies on vacuum emission of photoexcited electrons that are in thermal equilibrium with a semiconductor lattice, avoiding challenging non-equilibrium requirements and exotic material properties. However, although previous work demonstrated the photon-enhanced thermionic emission effect, efficiency has until now remained very low. Here we describe electron-emission measurements on a GaAs/AlGaAs heterostructure that introduces an internal interface, decoupling the basic physics of photon-enhanced thermionic emission from the vacuum emission process. Quantum efficiencies are dramatically higher than in previous experiments because of low interface recombination and are projected to increase another order of magnitude with more stable, low work-function coatings. The results highlight the effectiveness of the photon-enhanced thermionic emission process and demonstrate that efficient photon-enhanced thermionic emission is achievable, a key step towards realistic photon-enhanced thermionic emission based energy conversion.

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Progress on hot carrier cells

Cited by 118 publications

References 20 publications

Ab InitioStudy of Hot Carriers in the First Picosecond after Sunlight Absorption in Silicon

Ab InitioStudy of Hot Carriers in the First Picosecond after Sunlight Absorption in Silicon

Ab initio study of hot electrons in GaAs

Photon-enhanced thermionic emission from heterostructures with low interface recombination

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