Thermalization Of Charge Carriers Research Articles

Sources of Thermal Power Generation and Their Influence on the Operating Temperature of Organic Solar Cells.

Thermal stability, closely associated with the operating temperature, is one of the desired properties for practical applications of organic solar cells (OSCs). In this paper, an OSC of the structure of ITO/PEDOT:PSS/P3HT:PCBM/ZnO/Ag was fabricated, and its current-voltage (J-V) characteristics and operating temperature were measured. The operating temperature of the same OSC was simulated using an analytical model, taking into consideration the heat transfer, charge carrier drift-diffusion and different thermal generation processes. The simulated results agreed well with the experimental ones. It was found that the thermalization of charge carriers above the band gap had the highest influence on the operating temperature of the OSCs. The energy off-set at the donor/acceptor interface in the bulk heterojunction (BHJ) was shown to have a negligible impact on the thermal stability of the OSCs. However, the energy off-sets at the electrode/charge-transporting layer and BHJ/charge-transporting layer interfaces had greater impacts on the operating temperature of OSCs at the short circuit current and maximum power point conditions. Our results revealed that a variation over the energy off-set range from 0.1 to 0.9 eV would induce an almost 10-time increase in the corresponding thermal power generation, e.g., from 0.001 to 0.01 W, in the cells operated at the short circuit current condition, contributing to about 16.7% of the total solar power absorbed in the OSC.

Energy management in hybrid organic-silicon nanostructured solar cells by downshifting using CdZnS/ZnS and CdZnSe/ZnS quantum dots

Management of the solar spectrum by downshifting has been a key theme in broadband solar cell research over the last few years. The absorption of high-energy photons that exceed the bandgap edge of Si causes thermalization of charge carriers, which is a major loss mechanism by which collected energy is underutilized in silicon solar cells. The spectral response of Si also falls in the UV region. Progress in overcoming these effects achieved by modifying the input spectrum will dramatically enhance power conversion efficiency (PCE). Herein we report the application of highly luminescent CdZnS/ZnS and CdZnSe/ZnS quantum dots (QDs) as downshifting materials for nanostructured silicon. The applied QDs show major potential for shifting the high energy in the ultraviolet region to low energy, which is then transferred to the underlying silicon nanostructures via radiative and nonradiative energy transfer. This energy transfer dramatically improves the cell efficiency from 10.4% to 13.5%, showing this approach to be a reliable solution for improving solar cell properties. A Novel hybrid organic/inorganic silicon nanotips solar cell employing CdZnS/ZnS QDs as a downshifting layer which exhibits a drastic enhancement in the cell characteristics due to the radiative and nonradiative energy transfer (RET & NRET) from the QDs to underlying silicon nanostructure. • Highly emissive CdZnS/ZnS and CdZnSe/ZnS core/shell QDs were prepared by hot injection in two growth steps. • Silicon nanowires were fabricated by metal-catalyzed chemical etching and modified to tapered nanotip structure. • The current QDs show high potential for downshifting in hybrid silicon solar cells and improve the PCE to 13.49%.

Charge-carrier thermalization in bulk and monolayer CdTe from first principles

While cadmium telluride (CdTe) thin films are being used in solar cell prototyping for decades, the recent advent of two-dimensional (2D) materials challenges the fundamental limit for thickness of conventional CdTe layers. Here, we report our theoretical predictions on photocarrier dynamics in an ultimately thin (about 1 nm) CdTe slab. It corresponds to a layer that is just a single unit cell thick, when the bulk parent crystal in the zinc blende phase is cleaved along the [110] facet. Using an \textit{ab-initio} method based on density functional theory (DFT) and the Boltzmann equation in the relaxation time approximation (RTA), we determine the thermalization time for charge carriers excited to a certain energy for instance through laser irradiation. Our calculations include contributions arising from all phonon branches in the first Brillouin zone (BZ), thus capturing all relevant inter- and intraband carrier transitions due to electron-phonon scattering. We find that the photocarrier thermalization time is strongly reduced, by one order of magnitude for holes and by three orders of magnitude for electrons, once the CdTe crystal is thinned down from the bulk to a monolayer. Most surprisingly, the electron thermalization time becomes independent of the electron excess energy up to about 0.5~eV, when counted from the conduction band minimum (CBM). We relate this peculiar behavior to the degenerate and nearly parabolic lowest conduction band that yields a constant density of states (DOS) in the 2D limit. Our findings may be useful for designing novel CdTe-based optoelectronic devices, which employ nonequilibrium photoexcited carriers to improve the performance.

Effect of SrF2:Pr3+-Yb3+ Nanophosphor on Downconversion of High Energy Photons for Efficient Dye-Sensitized Solar Cells

An emerging important class of photovoltaics for the next generation, the dye sensitized solar cells (DSSCs) has attracted great attention due to its low-cost, simple fabrication process and good performance. Though, one of the major loss mechanisms leads to lowering the power energy conversion efficiency is thermalization of charge carriers generated by the absorption of high energy photons. One promising novel strategy to decrease such thermalization loss is the use of downconversion (DC) luminescent materials as spectral converters. The rare-earth doped downshifting materials can converts higher energy photons into lower energy photons based on energy transfer from lanthanide ions to Yb3+ ions. In addition, the downconversion nanoparticles (DCNPs) are also act as scattering centers which can facilitates the light capture in photo-absorbing layer. With this aim, the rare earth doped downconversion SrF2: Pr3+-Yb3+ nanoparticles were synthesized by facile template free hydrothermal method. X-ray diffraction analysis confirms the formation cubic SrF2 phase. The prepared SrF2: Pr3+-Yb3+ down-shifting nanophosphor shows strong absorption in UV region and broad luminescence band emission in visible region evidenced from absorption and photoluminescence studies respectively. The enhancement in power conversion efficiency (PCE) was achieved by employing the synthesized downconverting nanomaterial as bottom layer in DSSC photoanode (Fig.1). The photovoltaic performance of the device was studied using standard solar simulator at 1 Sun intensity (AM 1.5 G) and DSSC with SrF2: Pr3+-Yb3+ shows improved efficiency of 9.066% than bare TiO2 based device (8.395%). The down-shifting nanophosphor material enhanced the efficiency of DSSC by 7.4% with 7.5% increment in photocurrent density compared to bare TiO2 based device. Figure 1

Heat mitigation in perovskite solar cells: The role of grain boundaries

We have simulated a perovskite solar cell by developing a comprehensive mathematical model which incorporates drift-diffusion and heat transfer equations. This model incorporates all the heat transfer mechanisms, including the thermal power generation due to the tail state recombination and thermalization of charge carriers. Our model is validated by calculating the current-voltage characteristics of a PSC of the structure of Glass/PEDOT: PSS/CH3NH3PbI3/PC60BM/Al and finding it in very good agreement with the experimental results. The influence of grain boundary diameter on the operating temperature of the PSC is investigated in detail. It is found that by increasing the diameter of the grain boundaries, the operating temperature of the PSC decreases at higher voltages and the open circuit voltage increases slightly. The thermal power generated by the thermalization is found to play the dominant role in the operating temperature followed by the thermal power generated at the grain boundaries due to the highest tail state recombination in the region. This study is expected to provide a deeper understanding about different factors that influence the operating temperature of perovskite solar cells.

Dispersive and steady-state recombination in organic disordered semiconductors

Charge carrier recombination in organic disordered semiconductors is strongly influenced by the thermalization of charge carriers in the density of states (DOS). Measurements of recombination dynamics, conducted under transient or steady-state conditions, can easily be misinterpreted when a detailed understanding of the interplay of thermalization and recombination is missing. To enable adequate measurement analysis, we solve the multiple-trapping problem for recombining charge carriers and analyze it in the transient and steady excitation paradigm for different DOS distributions. We show that recombination rates measured after pulsed excitation are inherently time dependent since recombination gradually slows down as carriers relax in the DOS. When measuring the recombination order after pulsed excitation, this leads to an apparent high-order recombination at short times. As times goes on, the recombination order approaches an asymptotic value. For the Gaussian and the exponential DOS distributions, this asymptotic value equals the recombination order of the equilibrated system under steady excitation. For a more general DOS distribution, the recombination order can also depend on the carrier density, under both transient and steady-state conditions. We conclude that transient experiments can provide rich information about recombination in and out of equilibrium and the underlying DOS occupation provided that consistent modeling of the system is performed.

Ultrafast Exciton Dynamics in Shape-Controlled Methylammonium Lead Bromide Perovskite Nanostructures: Effect of Quantum Confinement on Charge Carrier Recombination

We report ultrafast spectroscopy investigations of photoinduced exciton dynamics in three novel CH3NH3PbBr3 perovskite nanostructures: nanocrystals (0D), nanowires (1D), and nanoplatelets (2D). Aided by analysis of UV–visible absorption and photoluminescence spectra, features in the transient absorption (TA) spectra are assigned to different charge carrier processes, time constants of which are determined in fitting the transient kinetics. Immediately after photoexcitation, the charge carrier thermalization process occurs within the instrument response function time (fwhm ∼ 350 fs) and results in a quasi-equilibrium distribution of charge carriers. It is followed by charge carrier cooling on a sub-picosecond time scale (τc ∼ 300 fs). The ensuing charge carrier recombination process obeys the rate law of a second-order reaction, which suggests that it occurs mainly via the bimolecular nongeminate recombination process. Dependence of the charge recombination rate constant (kr) on the initial charge carrier ...

Visible and Near-Infrared Photothermal Catalyzed Hydrogenation of Gaseous CO2 over Nanostructured Pd@Nb2O5.

The reverse water gas shift (RWGS) reaction driven by Nb2O5 nanorod‐supported Pd nanocrystals without external heating using visible and near infrared (NIR) light is demonstrated. By measuring the dependence of the RWGS reaction rates on the intensity and spectral power distribution of filtered light incident onto the nanostructured Pd@Nb2O5 catalyst, it is determined that the RWGS reaction is activated photothermally. That is the RWGS reaction is initiated by heat generated from thermalization of charge carriers in the Pd nanocrystals that are excited by interband and intraband absorption of visible and NIR light. Taking advantage of this photothermal effect, a visible and NIR responsive Pd@Nb2O5 hybrid catalyst that efficiently hydrogenates CO2 to CO at an impressive rate as high as 1.8 mmol gcat−1 h−1 is developed. The mechanism of this photothermal reaction involves H2 dissociation on Pd nanocrystals and subsequent spillover of H to the Nb2O5 nanorods whereupon adsorbed CO2 is hydrogenated to CO. This work represents a significant enhancement in our understanding of the underlying mechanism of photothermally driven CO2 reduction and will help guide the way toward the development of highly efficient catalysts that exploit the full solar spectrum to convert gas‐phase CO2 to valuable chemicals and fuels.

Charge carrier thermalization in organic diodes

Charge carrier mobilities of organic semiconductors are often characterized using steady-state measurements of space charge limited diodes. These measurements assume that charge carriers are in a steady-state equilibrium. In reality, however, energetically hot carriers are introduces by photo-excitation and injection into highly energetic sites from the electrodes. These carriers perturb the equilibrium density of occupied states, and therefore change the overall charge transport properties. In this paper, we look into the effect of energetically hot carriers on the charge transport in organic semiconductors using steady state kinetic Monte Carlo simulations. For injected hot carriers in a typical organic semiconductor, rapid energetic relaxation occurs in the order of tens of nanoseconds, which is much faster than the typical transit time of a charge carrier throught the device. Furthermore, we investigate the impact of photo-generated carriers on the steady-state mobility. For a typical organic voltaic material, an increase in mobility of a factor of 1.1 is found. Therefore, we conclude that the impact of energetically hot carriers on normal device operation is limited.

Decoherence reduces thermal energy loss in graphene quantum dots

Intraband splitting energies in graphene quantum dots are on the order of the fundamental phonon energies, creating a single-phonon relaxation pathway. Yet, charge carrier thermalization is slow. Our time-domain, ab initio computations demonstrate that the slow rate of electronic relaxation is due to quantum decoherence. The relaxation through the discrete states occurs on a 30 ps timescale. Electron-hole recombination takes place within 660 ps, two orders of magnitude slower than a coherent mechanism. Disorder introduced by structural variations around the edge expedites coherence loss, increasing the lifetimes of excited charge carriers in graphene quantum dots.

Tm3+-sensitized up- and down-conversions in nano-structured oxyfluoride glass ceramics

Tm3+-sensitized up-conversion and down-conversion are studied in transparent glass ceramics embedded with β-YF3 nanocrystals. Upon excitation at 1220nm, where crystalline silicon (c-Si) solar cells no longer absorb, the sub-bandgap photons could be converted to the higher-energy ones via up-conversion. In addition, under excitation at 468nm (a wavelength close to the peak in the air-mass 1.5 global solar spectrum), one blue photon might be split in up to two near-infrared ones via down-conversion. In both cases, the frequency-converted photons match the spectral response of c-Si solar cell well. Hopefully, the investigated luminescent materials may act as the spectral conversion layers to reduce the sub-bandgap transmission and charge carrier thermalization losses of c-Si solar cells, and in turn, enhance the energy efficiency.

On the pathway of photoexcited electrons: probing photon-to-electron and photon-to-phonon conversions in silicon by ATR-IR

Photoexcitation and charge carrier thermalization inside semiconductor photocatalysts are two important steps in solar fuel production. Here, photoexcitation and charge carrier thermalization in a silicon wafer are for the first time probed by a novel, yet simple and user-friendly Attenuated Total Reflectance Infrared spectroscopy (ATR-IR) system.

Ultraviolet-blue to near-infrared downconversion of Nd^3+-Yb^3+ couple

To reduce energy losses by thermalization of charge carriers in a silicon solar cell, quantum-cutting luminescent materials are desired for the efficient downconversion of UV-visible radiation into near-IR radiation. In this Letter, quantum cutting involving emission of two near-IR photons for each UV-blue photon absorbed is demonstrated in Nd(3+)/Yb(3+):beta-YF(3) nanocrystals embedded in transparent bulk-glass ceramics. Upon excitation of an Nd(3+) ion with a UV-blue photon, Yb(3+) ions emit two near-IR photons through an efficient two-step energy transfer from Nd(3+) to Yb(3+) with Nd(3+):(4)F(3/2) acting as the intermediate state.

High-field electrothermal transport in metallic carbon nanotubes

We describe the electro-thermal transport in metallic carbon nanotubes (m-CNTs) by a semi-classical approach that takes into account the high-field dynamical interdependence between charge carrier and phonon populations. Our model is based on the self-consistent solution of the Boltzmann transport equation and the heat equation mediated by a phonon rate equation that accounts for the onset of non-equilibrium (optical) phonons in the high-field regime. Given the metallic nature of the nanostructures, a key ingredient of the model is the assumption of local thermalization of charge carriers. Our theory remarkably reproduces the room temperature electrical characteristics of m-CNTs on substrate and free standing (suspended), shedding light on charge-heat transport in these one dimensional nanostructures. In particular, the negative differential resistance observed in suspended m-CNTs under electric stress is attributed to inhomogeneous field profile induced by self-heating rather than the presence of hot phonons.

Modeling of Hole Transport in Poly[2-Methoxy-5-(2′-Ethylhexyloxy)-p-Phenylene Vinylene] Doped with Polar Additives

The theoretical model of the hole mobility in poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] doped with a photochromic polar additive is proposed. Our model involves description of the on-chain charge carrier states, charge carrier thermalization, and inter-chain hopping. While the on-chain states are calculated by means of quantum mechanical methods, the semi-classical Marcus approach is used for the description of inter-chain hopping. The dependence of the mobility on several parameters of the model is studied. The results predict the possibility to construct an efficient optoelectronic switch based on a polymer doped with a photochromic polar additive.

Eu-doped LGF Luminescent Down Converter Possible for TiO2Dye Sensitized Solar Cells

For improving solar efficiencies, down conversion of high-energy photons to visible lights is discussed. The losses due to thermalization of charge carriers generated by the absorption of high-energy photons, can largely be reduced in a solar cell if more than one electron-hole pair can be generated per incident photon. The solar cell was constructed of dye-sensitized anatase-based TiO<TEX>$_2$</TEX>, approximately 30nm particle size, 6<TEX>$\mu\textrm{m}$</TEX>thickness, and 6<TEX>${\times}$</TEX>6<TEX>$\textrm{mm}^2$</TEX> active area, Pt counter electrode and I<TEX>$_3$</TEX><TEX>$\^$</TEX>-/I<TEX>$_2$</TEX><TEX>$\^$</TEX>-/ electrolyte. After correction for losses due to light reflection and absorption by the conducting glass, the conversion of photons to electric current is practically quantitative in the plateau region of the curves. The incident photon to current conversion efficiency(IPCE) of N3 used as a dye in this work is about 80% at around 590nm and 610nm which is the emission spectrum of Eu doped LGF. The Eu doped LGF powder was prepared by conventional ceramic process, and used as a down converter for DSC after spin coated on the slide glass and fired.

Thermally Induced Transient Absorption of Light by Poly(3,4‐ethylenedioxythiophene):Poly(styrene sulfonic acid) (PEDOT:PSS) Films: A Way to Probe Charge‐Carrier Thermalization Processes

AbstractInfrared‐induced transient absorptions in the millisecond and sub‐picosecond time domains have been used to study the dynamics of charge carriers of a conducting polymer, poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonic acid) (PEDOT:PSS). On the millisecond timescale, the transient absorption is ascribed to a thermal effect induced by absorbed infrared light. The decay of the transient absorption is limited by the transport of heat from the polymer film to the substrate and corresponds to the decay kinetics of infrared‐induced changes in the resistivity of the material. Near 1.5 eV, the infrared‐induced absorption can be modeled in terms of an interband transition. The assignment of the optical transients in terms of carrier heating opens the possibility to study charge carrier thermalization processes using short laser pulses. Pump‐probe spectroscopy on a sub‐picosecond timescale shows that the initial thermalization of the excited charge carriers occurs with a time constant of less than 500 fs, i.e., faster than for noble metals.

Auger recombination and charge-carrier thermalization in Hg - n -cluster photoelectron studies.

Photoelectron spectra (PES) of Hg(-)(n) show strong dependence of spectral features on photon energy, i.e., peak tailing and band gap filling. This dependence suggests the existence of complex photoinduced processes in parallel with the direct photodetachment process. The "corrupted" PES, taken with intermediate photon energy, carry the signature of interband absorption followed by charge-carrier thermalization and Auger electron ejection in Hg(-)(n). These processes, so significant in the photophysics of bulk semiconductors and nanoparticles, have not yet been identified in clusters.

Computer-assisted study of carrier thermalization by hopping in disordered semiconductors

Techniques for the analysis of hopping transport in disordered semiconductors are outlined and applied to the thermalization of charge carriers within a particular distribution of localized states (N(E)). The transient current is superficially similar to that for the case of trap-limited band transport. However, techniques developed assuming the dominance of this process in the calculation of N(E) yield totally erroneous results. Also, the derived values of the attempt-to-escape frequency, νph, and capture cross section, σ, of the trapping centers are unrealistically small. This may well explain the anomalous results obtained in various experimental studies. The use of data obtained at different temperatures to calculate νph and σ is therefore proposed as a valuable new procedure for differentiating between hopping and trap-limited band transport.

Improving solar cell efficiencies by down-conversion of high-energy photons

One of the major loss mechanisms leading to low energy conversion efficiencies of solar cells is the thermalization of charge carriers generated by the absorption of high-energy photons. These losses can largely be reduced in a solar cell if more than one electron–hole pair can be generated per incident photon. A method to realize multiple electron–hole pair generation per incident photon is proposed in this article. Incident photons with energies larger than twice the band gap of the solar cell are absorbed by a luminescence converter, which transforms them into two or more lower energy photons. The theoretical efficiency limit of this system for nonconcentrated sunlight is determined as a function of the solar cell’s band gap using detailed balance calculations. It is shown that a maximum conversion efficiency of 39.63% can be achieved for a 6000 K blackbody spectrum and for a luminescence converter with one intermediate level. This is a substantial improvement over the limiting efficiency of 30.9%, which a solar cell exposed directly to nonconcentrated radiation may have under the same assumption of radiative recombination only.