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The hot carrier solar cell (HCSC) concept has been proposed to overcome the Shockley Queisser limit of a single p–n junction solar cell by harvesting carriers before they have lost their surplus energy. A promising family of materials for these purposes is metal halide perovskites (MHP). MHPs have experimentally shown very long cooling times, the key requirement of a HCSC. By using ensemble Monte Carlo simulations, light is shed on why cooling times are found to be extended. This article concentrates on the role of thermalization in the cooling process. The role of carrier–phonon and carrier–carrier interactions in thermalization and cooling is specified, while showing how these processes depend on material parameters, such as the dielectric constant and effective mass. It is quantified how thermalization acts as a cooling mechanism via the cold background effect. The importance of a low degree of background doping is to achieve the observed extended cooling times. Herein, it is mapped out how perovskites should be tuned, their material parameters, carrier concentration, and purity, in order to realize a HCSC. It contributes to the debate on the cooling times in MHPs and the suitability of tin perovskites for HCSCs. [ABSTRACT FROM AUTHOR]

Hot carrier solar cells are a concept of photovoltaic devices, which offers the opportunity to harvest solar energy beyond the Shockley-Queisser limit. Unlike conventional photovoltaic devices, hot carrier solar cells convert excess kinetic energy into useful electrical power rather than losing it through thermalisation mechanisms. To extract the carriers while they are still “hot”, efficient energy-selective contacts must be developed. In previous studies, the presence of the hot carrier population in a p-i-n solar cell based on a single InGaAsP quantum well on InP substrate at room temperature has been demonstrated by means of complementary optical and electrical measurements, leading to an operating condition for this device beyond the limit for classical device operation. This result allows to design a new generation of devices to increase the hot carrier conversion contribution. In this work, we study InGaAs/AlInAs type II heterojunction as a selective contact for a future hot carrier solar cell device epitaxially grown on (001) oriented InP substrate. Two p-i-n solar cells have been grown by molecular beam epitaxy on InP. The absorber is a 50 nm-thick InGaAs layer surrounded by AlInAs barriers, all lattice-matched to InP. Two architectures are compared, the first with two symmetrical AlInAs barriers and the second with a single InGaAs quantum well in the center of the n-side barrier to allow electron tunneling across the barrier. Electrical characteristics under laser illumination with two different wavelengths have been measured to investigate the effect of the selective contact compared to the barrier. This preliminary study of InGaAs/AlInAs-based selective contacts show that such III–V combination is adapted for a future hot carrier solar cell in the InP technology.

The hot carrier solar cell aims to significantly boost the power conversion efficiency through fully utilizing the carrier thermalization energy loss. To realize such ultraefficient solar cells, it requires that the excess energy of excited "hot" carriers is captured for power generation by reducing the rate of, or even preventing, carrier cooling. It has been known that phonon bottleneck effects (PBE) play the most decisive role in reducing the carrier thermalization rate. However, the mechanisms underlying PBE are complex and vary in different material systems and are influenced by many factors such as illumination intensity and carrier density (extrinsic), electronic band structure, and phonon dispersion (intrinsic) and quantum confinement (intrinsic). III–V semiconductors are the most popular photovoltaic materials for ultraefficient thin film solar cells due to their high crystal quality and adjustable electronic band structure. Such features reduce some of the complexity of the study of PBE and hot carrier dynamics. Therefore, it is appropriate to identify the PBE mechanisms in these III–V semiconductors. This manuscript systematically reviews the mechanisms underlying PBE in III–V semiconductors in both bulk and nanostructures. There is a tendency for an enhanced PBE in low‐dimensional III–V semiconductors due to quantum and other confinement effects. Multiple quantum wells seem the most promising material system for hot carrier solar cells. [ABSTRACT FROM AUTHOR]

High-energy incoming photons can be absorbed and concluded to generate Hot Carriers. In normal solar cells, these carriers are scattered by electron–electron and electron-lattice mechanisms and rapidly lose extra energy and approach to conduction band energy edge. This event plus other loss mechanisms cause that the efficiency of the solar cells to be limited to 33% theoretically. If one makes the possibility for carriers that can be extracted rapidly, before scattering and releasing extra energy to the lattice, the efficiency of solar cells is enhanced considerably. This type of solar cell is named hot carrier solar cells (HCSCs). To this end and improvement the conversion efficiency, multilevel energy selective contacts (ESCs) as a new concept and new mechanism in solar cells are used. In the other words, several appropriate energy levels as carrier extraction contacts in the conduction band are introduced. Here, we use multilevel ESCs, and based on our simulation it is shown that the maximum efficiency of 75% is achievable for low bandgap materials. For a typical material such as Si, the maximum efficiency is increased to 60% using ten ESCs. [ABSTRACT FROM AUTHOR]

Hafnium Oxynitride belongs to the group IVB compounds with high permittivity and large acoustic impedance. In this work, hafnium oxynitride films have been synthesized using plasma-enhanced atomic layer deposition on Si and Quartz substrates. XRD results show the presence of mixed cubic and monoclinic phases with an optimum crystallization occurring at 850 °C. The thin films show strong absorption in the UV–visible spectrum suggesting semiconductor behaviour. The optical properties of the spectrophotometer and spectroscopic ellipsometry agree with the XRD observations. We also report the first observation of experimentally derived photoluminescence (PL) from hafnium oxynitride thin films synthesized using plasma-enhanced atomic layer deposition. The PL spectrum is consistent with the XRD results with two absorption peaks around 576 nm and 705 nm, corresponding to cubic and monoclinic phases, respectively. Also, the PL results match very well with the theoretical value of the band gap of cubic and monoclinic phases of Hf 2 ON 2. The Raman spectrum shows a phonon band gap around 242–263 cm−1, consistent with the theoretically reported value for cubic Hf 2 ON 2. [Display omitted] • The as-deposited films of hafnium oxynitride were partially annealed because of N 2 plasma exposure for 15 s. • XRD and Photoluminescence spectra suggested the presence of mixed phases: cubic and monoclinic. • The electronic and phononic band gap matched very closely with the theoretical band gaps of cubic and monoclinic Hf 2 ON 2. • XPS data suggested the chemical states of Hf, N, O to be of Hf-N-O bonds in the core and Hf-O-N bonds in the surface of the thin films. • The absorption coefficient calculated using the spectrophotometry data overlapped with that calculated from extinction coefficients. [ABSTRACT FROM AUTHOR]

Hot carrier solar cells promise theoretical power conversion efficiencies far beyond the single junction limit. However, practical implementations of hot carrier solar cells have lagged far behind those theoretical predictions. Reciprocity relations for electroluminescence from conventional single junction solar cells have been extremely helpful in driving their efficiency ever closer to the theoretical limits. In this work, we discuss how the signatures of a functioning hot carrier device should manifest experimentally when driven in reverse, that is, in electroluminescent mode. Hot carrier properties lead to deviations of the dark I–V from the Shockley diode equation that is typical for conventional single junction solar cells. These deviations are directly linked to an increase in temperature of the carriers and therefore the temperature measured from electroluminescence spectra. We also elucidate how the behaviour of hot carrier solar cells in the dark depends on whether Auger processes play a significant role, revealing a stark contrast between the regime of negligible Auger recombination (carrier conservation model) and dominant Auger recombination (impact ionisation model) for hot carrier solar cells. [ABSTRACT FROM AUTHOR]

Increasing photovoltaic conversion efficiency by overcoming the fundamental loss mechanisms due to hot carrier thermalisation remains one of the challenges in solar energy research. A large proportion of dynamic energy is lost due to thermalisation where hot carriers lose their energy to lattice vibrations within picoseconds. Extending hot carrier lifetimes will potentially improve the efficiency of photovoltaic devices. In this study, the bulk hafnium nitride film is proposed as a potential hot carrier absorber from its phononic properties. Hafnium nitride thin films were deposited by sputtering on quartz substrates. The films are polycrystalline as determined from X-ray diffraction and from X-ray photoelectron spectroscopy the chemical composition of the films are estimated to be HfN 0.9 . The absorption coefficient of the film exhibits strong amplitude below 500 nm and free electron absorption at longer wavelengths. Transient absorption spectroscopy was used to investigate carrier dynamics in the visible and near-infrared spectral regions. For the first time we present evidence of up to nanosecond long lifetimes of photo-excited carriers observed in hafnium nitride. We propose that the long lived excited states are due to restricted phonon decay pathways in hafnium nitride. The long lived excited states in the bulk hafnium nitride film provide a simple route to devices designed to utilise hot carriers efficiently. [ABSTRACT FROM AUTHOR]

The selective extraction of hot electrons to suitable energy contacts is a key aspect towards the development of hot carrier solar cells. Here we employ Time Resolved THz Spectroscopy (TRTS) to evaluate the extent to which hot electron transfer (HET) takes place from the 1P e states of colloidal 3 nm PbSe quantum dots molecularly linked by 3-mercaptopropionic acid to mesoporous SnO 2 and TiO 2 sensitized films. For PbSe–3MPA–SnO 2 samples, we show that the efficiency of hot electron transfer is negligibly small at room temperature, i.e. within the ∼10% detection limit of our measurements. The impact of spurious signals on TRTS data arising from carrier dynamics regarding QDs aggregates – which can be misinterpreted as HET – is discussed in detail. In contrast, in line with previous reports, hot electron transfer is observed to take place from the P e states of colloidal PbSe QDs in the PbSe–3MPA–TiO 2 system, with an efficiency ⩾80%. These results are rationalized in terms of a stronger donor–acceptor coupling between QD and oxide for the TiO 2 electrode when compared with a SnO 2 electrode, a factor that ultimately defines the kinetic competition between electron transfer rate towards the oxide and intraband cooling within the QDs. [ABSTRACT FROM AUTHOR]

Hot-carrier solar cells are envisioned to utilize energy filtering to extract power from photogenerated electron–hole pairs before they thermalize with the lattice, and thus potentially offer higher power conversion efficiency compared to conventional, single absorber solar cells. The efficiency of hot-carrier solar cells can be expected to strongly depend on the details of the energy filtering process, a relationship which to date has not been satisfactorily explored. Here, we establish the conditions under which electron–hole separation in hot-carrier solar cells can occur reversibly, that is, at maximum energy conversion efficiency. We thus focus our analysis on the internal operation of the hot-carrier solar cell itself, and in this work do not consider the photon-mediated coupling to the Sun. After deriving an expression for the voltage of a hot-carrier solar cell valid under conditions of both reversible and irreversible electrical operation, we identify separate contributions to the voltage from the thermoelectric effect and the photovoltaic effect. We find that, under specific conditions, the energy conversion efficiency of a hot-carrier solar cell can exceed the Carnot limit set by the intra-device temperature gradient alone, due to the additional contribution of the quasi-Fermi level splitting in the absorber. We also establish that the open-circuit voltage of a hot-carrier solar cell is not limited by the band gap of the absorber, due to the additional thermoelectric contribution to the voltage. Additionally, we find that a hot-carrier solar cell can be operated in reverse as a thermally driven solid-state light emitter. Our results help explore the fundamental limitations of hot-carrier solar cells, and provide a first step towards providing experimentalists with a guide to the optimal configuration of devices.

Abstract: We have calculated impact ionization (II) and Auger recombination (AR) lifetimes in hot carrier solar cells (HC-SCs) in operation, and found that these lifetimes are much longer than the average retention times of photo-generated carriers in the cells at an appropriate range of applied voltage, under practical conditions of 500–1000 times-concentrated solar irradiation and carrier thermalization times of several hundred picoseconds. This means that the particle conservation (PC) model, in which II and AR are completely excluded, can be applied to predict the conversion efficiency. In contrast, the PC model does not stand under the ideal condition of the maximally concentrated irradiation and no thermalization of carriers, as previously pointed out. [Copyright &y& Elsevier]

Abstract: Hot carrier solar cells are a third generation solar cell device where electrons and holes, heated by solar radiation, are removed from the absorber via low-dimensional energy selective contacts before they can thermalise to the band edge. Here, a new model is presented for calculating the performance of these devices, which takes into account the energy spectrum of the contacts. It is shown that efficiency is maximised with a certain ideal number of contacts and that the energy spectra of these should be narrow. [Copyright &y& Elsevier]

The hot carrier solar cells (HCSCs) is one of the most promising advanced concept solar cells. It aims to prevent or reduce the dominant energy loss from hot carrier thermalization, so that its theoretical efficiency limit is no longer limited by the Shockley-Queisser efficiency limit of 31% and could in principle reach 66% under one sun conditions. Such cells consist of two essential components: a hot carrier absorber (HCA) and energy selective contacts (ESCs), which play the role of reduction of thermalization rate and extraction of hot carriers, respectively. However, no operational device of such cells with the expected high efficiency have yet been realized at present. In order to attain a significant breakthrough on HCSCs, it is vital to clarify the interplay between the mechanisms underlying the thermalization in both bulk and low dimensional semiconductor candidates for HCA. This paper reviews mechanisms affecting the thermalization processes including quantum confinement effect, phonon bottleneck effect (or hot phonon effect) and carrier screening in terms of electronic and phononic band structures via different characterization techniques. Several promising III-V semiconductors and perovskite material systems in bulk and quantum well structures are systematically reviewed. It has been found that phonon bottlenecks can play a key role in interrupting thermalization processes by restricting phonon interactions and their ability to dissipate hot carrier energy into the lattice. Several other mechanisms affecting thermalization could eventually be attributed to different types of phonon bottleneck effect. [ABSTRACT FROM AUTHOR]

The search for a superior material with greater energy conversion efficiency as compared to the existing material systems for photovoltaic applications has always been the quest of research community. A systematic study of the optoelectronic properties of zinc blende (ZB) phase boron antimonide (BSb) using the full-potential linearized augmented plane wave (FPLAPW) basis sets in the density functional theory (DFT) with the Modified Becke-Johnson (mBJ) exchange correlation (XC) potential is carried out to investigate its potential as a photovoltaic material. Structural optimization is done considering the Perdew-Burke-Ernzerh of generalized gradient approximation (PBEsol) XC functional. Taking into account the optimized lattice constant of 5.2306 Å, which is consistent with previous experimental as well as theoretical findings, the fundamental electronic and optical parameters such as electronic density of state (DOS), band diagram, complex-dielectric function, refractive index, extinction coefficient, and optical absorption coefficient are computed. The effective mass of charge carriers for conduction and valence band have been calculated along the crystallographic symmetric directions. The calculated wider direct-gap (Γ−Γ) of 2.8048eV, and a comparatively narrower indirect-gap (X−Γ) of 0.99694eV are in excellent agreement with the available literature. These distinct and fascinating optoelectronic properties of BSb can be very useful for realizing solar cells with better energy conversion efficiency. [ABSTRACT FROM AUTHOR]

A fundamental understanding of hot electron transport is critical for developing efficient hot-carrier-based solar cells. There have been significant efforts to enhance hot electron flux, and it has been found that a key factor affecting the hot electron flux is the lifetime of the hot electrons. Here, we report a combined study of hot electron flux and the lifetime of hot carriers using a perovskite-modified plasmonic nanodiode. We found that perovskite deposition on a plasmonic nanodiode can considerably improve hot electron generation induced by photon absorption. The perovskite plasmonic nanodiode consists of MAPbI 3 layers covering a plasmonic-Au/TiO 2 Schottky junction that is composed of randomly connected Au nanoislands deposited on a TiO 2 layer. The measured incident photon-to-electron conversion efficiency and the short-circuit photocurrent show a significantly improved solar-to-electrical conversion performance of this nanodiode. Such an improvement is ascribed to the improved hot electron flux in MAPbI 3 caused by effective light absorption from near-field enhancement of plasmonic Au and the efficient capture of hot electrons from Au nanoislands via the formation of a three-dimensional Schottky interface. The relation between the lifetime and flux of hot electrons was confirmed by femtosecond transient absorption spectroscopy that showed considerably longer hot electron lifetimes in MAPbI 3 combined with the plasmonic Au structure. These findings can provide a fundamental understanding of hot electron generation and transport in perovskite, which can provide helpful guidance to designing efficient hot carrier photovoltaics.