Multiple Exciton Generation Effect Research Articles

Multiple exciton generation effect in photocatalytic overall water splitting

Multiple exciton generation effect in photocatalytic overall water splitting

Multiple exciton generation effect in photocatalytic overall water splitting

Multiple exciton generation effect in photocatalytic overall water splitting

Semiconducting Quantum Dots for Energy Conversion and Storage

AbstractSemiconducting quantum dots (QDs) have received huge attention for energy conversion and storage due to their unique characteristics, such as quantum size effect, multiple exciton generation effect, large surface‐to‐volume ratio, high density of active sites, and so on. However, the holistic and systematic understanding of the energy conversion and storage mechanism centering on QDs in specific application is still lacking. Herein, a comprehensive introduction of these extraordinary 0D materials, e.g., metal oxide, metal dichalcogenide, metal halides, multinary oxides, and nonmetal QDs, is presented. It starts with the synthetic strategies and unique properties of QDs. Highlights are focused on the rational design and development of advanced QDs‐based materials for the various applications in energy‐related fields, including photocatalytic H2 production, photocatalytic CO2 reduction, photocatalytic N2 reduction, electrocatalytic H2 evolution, electrocatalytic CO2 reduction, electrocatalytic N2 fixation, electrocatalytic O2 evolution, electrocatalytic O2 reduction, solar cells, metal‐ion batteries, lithium–sulfur batteries, metal–air batteries, and supercapacitors. At last, challenges and perspectives of semiconducting QDs for energy conversion and storage are detailedly proposed.

Recent Advances in Quantum Dots for Photocatalytic CO2 Reduction: A Mini-Review.

Solar energy–driven carbon dioxide (CO2) reduction to valuable solar fuels/chemicals (e.g., methane, ethanol, and carbon monoxide) using particulate photocatalysts is regarded as one of the promising and effective approaches to deal with energy scarcity and global warming. The growth of nanotechnology plays an eminent role in improving CO2 reduction (CO2R) efficiencies by means of offering opportunities to tailor the morphology of photocatalysts at a nanoscale regime to achieve enhanced surface reactivity, solar light absorption, and charge separation, which are decisive factors for high CO2R efficiency. Notably, quantum dots (QDs), tiny pieces of semiconductors with sizes below 20 nm, offering a myriad of advantages including maximum surface atoms, very short charge migration lengths, size-dependent energy band positions, multiple exciton generation effect, and unique optical properties, have recently become a rising star in the CO2R application. In this review, we briefly summarized the progress so far achieved in QD-assisted CO2 photoreduction, highlighting the advantages of QDs prepared with diverse chemical compositions such as metal oxides, metal chalcogenides, carbon, metal halide perovskites, and MXenes.

Ge quantum dot lattices in alumina prepared by nitrogen assisted deposition: Structure and photoelectric conversion efficiency

Thin films comprising three dimensional germanium (Ge) quantum dot lattices formed by nitrogen (N) assisted magnetron sputtering deposition in alumina (Al2O3) matrix have been studied for light harvesting purposes. In order to expand the application of this material it is necessary to reduce germanium oxidation and to achieve stabilization of the germanium/alumina interface. Effects of tuning the N concentration, substrate temperature and Ge sputtering power during the films preparation are monitored. It is shown that the N presence not only reduces Ge oxidation during annealing but also affects the internal structure, size and arrangement of Ge quantum dots. Additionally, the deposition temperature and Ge sputtering power are used to tune the Ge quantum dot size, separation and the regularity of their positions. It is shown that the optical and electrical properties of the films, especially their photo-induced current, and quantum efficiency are strongly tunable by the deposition conditions. Moreover, a significant photo-response and effect of multiple exciton generation effect is observed. The materials presented could be used as a sensitive layer for photodetectors or photovoltaic light harvesting devices. The presented tools can be used for future fine tuning of the material to achieve the optimal quantum efficiency.

Efficient PbSe Colloidal Quantum Dot Solar Cells Using SnO2 as a Buffer Layer.

PbSe colloidal quantum dots (CQDs) are widely used in solar cells because of their tunable band gap, solution processability, and efficient multiple exciton generation effect. The most efficient PbSe CQD solar cells use high-temperature-processed ZnO as the electron transport layer (ETL), limiting their applications in flexible photovoltaics. Currently, low-temperature solution-processed SnO2 has been demonstrated as an efficient ETL for high-efficient PbS CQD and perovskite solar cells because of less parasitic light absorption and higher electron mobility. Herein, we introduce low-temperature solution-processed SnO2 as ETL for PbSe CQD solar cells, and fabricate the PbSe CQD absorber layer with a one-step spin-coating method. The champion device with the structure of FTO (SnO2:F)/SnO2/PbSe-PbI2/PbS-EDT (1,2-ethanedithiol)/Au achieves a high open-circuit voltage of 577.1 mV, a short-circuit current density of 24.87 mA cm-2, a fill factor of 67%, and an impressive power conversion efficiency of 9.67%. Our results pave the way for the development of low-temperature flexible PbSe CQD solar cells.

Band Tunable CdSe Quantum Dot-Doped Metals for Quantum Dot-Sensitized Solar Cell Application

Quantum dots are drawing great attention as a material for the next-generation solar cells because of the high absorption coefficient, tunable band gap, and multiple exciton generation effect. In search of the viable way to enhance the power conversion efficiency of quantum dot-sensitized solar cells, we have succeeded in preparing the quantum dot solar cells with high efficiency based on CdSe:X (Mn2+ or Cu2+) nanocrystal by successive ionic layer absorption and reaction. The morphological observation and crystalline structure of photoanode were characterized by field-emission scanning electron microscopy, X-ray diffraction, and the EDX spectra. In addition, the electrochemical performance of photoelectrode was studied by the electrochemical impedance spectra. As a result, we have succeeded in designing QDSSCs with a high efficiency of 4.3%. Moreover, the optical properties, the direct optical energy gap, and both the conduction band and the valence band levels of the compositional CdSe:X were estimated by the theory of Tauc and discussed details. This theory is useful for us to understand the alignment energy structure of the compositions in electrodes, in particular, the conduction band and valence band levels of CdSe:X nanoparticles.

PbS QDs as Electron Blocking Layer Toward Efficient and Stable Perovskite Solar Cells

Colloidal PbS quantum dots (QD), which possess size-tunable bandgap, multiple exciton generation effect, and low-cost synthesis process, are regarded as potential candidates in thin film photovoltaic field. Meanwhile, perovskite solar cells (PSCs) are prospecting photovoltaic devices with high power conversion efficiency (PCE). However, the high-performance PSCs largely rely on expensive and unstable organic hole transporting material (HTM), such as spiro-OMeTAD. Here, we design a new PSC architecture with a perovskite/PbS quantum dots (QDs) heterojunction film, which promotes hole extraction and decreases carrier recombination. By optimizing the concentration of colloidal PbS QDs, a PCE of 11.32% can be achieved, which is far higher than the control HTM-free PSCs (6.96% in PCE). In addition, the device structure exhibits higher stability than the conventional PSCs based on the control one. These findings along with low-temperature fabrication ( <100 °C) offer an alternative route in HTM-free heterojunction structure for low-cost and high-performance flexible PSCs.

Quantum dot sensitized solar cell: Recent advances and future perspectives in photoanode

Quantum dot (QD) has emerged as a promising agent in the field of solar energy conversion due to its distinct size-dependent optoelectronic characteristics. As next generation solar cell having facile and low cost fabrication techniques, quantum dot sensitized solar cell (QDSSC) has great potential to meet global demand for clean energy. In this type of solar cell architecture, high performance is expected due to multiple exciton generation effect and tuning of energy band gap in QD. As reported photo conversion efficiency of QDSSC is still less than dye sensitized solar cells, more research on optimization of material selection and material engineering is required. In this review paper, structure & working of QDSSC along with interfacial charge transportation mechanism is presented. Various fabrication techniques related to QDSSCs have been briefly described. This article focuses on recent advances in photoanode of QDSSC. It includes various aspects of photoanode such as structural, morphological effect, carbon related materials, doping effect in metal oxides, alternative photo-anodic materials, surface treatment, blocking layer, new types of sensitizer etc. Issues related to photoanode corrosion and stability are reviewed. Limitations and future prospects have been discussed to fabricate more efficient and stable quantum dot sensitized solar cell.

Preparation and multiple exciton generation of hydrogenated nanocrystalline silicon film prepared at different powers

The nc-Si:H films prepared by PECVD under different powers was characterized by Raman spectra, XRD pattern, and transmission measurements. The XRD pattern displays that the nc-Si:H nanocrystallites films is successfully prepared on quartz glass substrates. The average grain sizes of the prepared samples at different powers were estimated from full width at half maximum of (111) peak. The optical band gap of nc-Si:H films indicate that the optical band gap can be modulated by power. In the end, a simple statistical model was used to explain the effect of multiple exciton generation in the prepared nc-Si:H semiconductor material. The results indicate that multiple exciton generation efficiency in nc-Si:H film is related with both the size of quantum dots and incident photon energy.

Role of electron confinement in the formation of Tamm surface levels in nanoparticles

We study the effect of electron confinement in a nanoparticle on the parameters of Tamm's levels. It is shown using the combination of the Tamm model and the Kronig-Penney model that an increase in the gap width in the electron spectrum of the crystal leads to an increase in the energy of the Tamm state and a decrease in the degree of localization of the wavefunction of the Tamm state. Some applications of the results on the properties of the Tamm level (e.g., the effect on the surface tension of a nanocluster, the manifestation of modifications considered here in the multiple exciton generation effect in quantum dots, the possible role of the shape of a nanoparticle during its growth, and the role of varying Tamm states in catalysis by nanopar� ticles) are indicated.

Effect of multiple exciton generation on ultraviolet–visible absorption of Ag–Cu clusters: Ab initio study

We demonstrate the effect of multiple exciton generation on the Ultraviolet–visible absorption of 7-atom Ag–Cu clusters using symmetry adapted cluster theory with configuration interaction. Multiple exciton generation (MEG) behavior is for the first time observed in metal clusters. The results indicate that multiple excitons appear in the visible and near ultraviolet light ranges. Single excitations are mainly contributions for the optical spectra, while the multiple excitons merely contribute for some peaks at the higher energies. However, occurrence of MEG enhances the optical absorption in Ag–Cu clusters. The optical spectra of pure Ag7 and Cu7 clusters obtained using SAC–CI is in excellent agreement with experiment spectra. As observed in AgnCu7−n (n=1–6) clusters, doping can tune MEG–related optical spectra by redshifting and suppressing the absorption peak.

Multiple exciton generation in PbSe and PbS nanocrystals incorporated into amorphous silicon p–n junction

AbstractThe effect of multiple exciton generation in semiconductor nanocrystals suggesting the possibility of efficient conversion of each absorbed high‐energy photon into at least two electron–hole pairs (excitons) is demonstrated in the photocurrent from lead selenide (PbSe) and lead sulfide (PbS) colloidal nanocrystals incorporated into amorphous silicon p–n junction. It is shown that employment of this effect for increasing efficiency of solar cells requires such energy bandgap alignment in the system that ensures confinement of the photogenerated charge carriers inside the nanocrystal layer.

Recent development in colloidal quantum dots photovoltaics

The increasing demand for sustainable and green energy supply spurred the surging research on high-efficiency, low-cost photovoltaics. Colloidal quantum dot solar cell (CQDSC) is a new type of photovoltaic device using lead chalcogenide quantum dot film as absorber materials. It not only has a potential to break the 33% Shockley-Queisser efficiency limit for single junction solar cell, but also possesses low-temperature, high-throughput solution processing. Since its first report in 2005, CQDSCs experienced rapid progress achieving a certified 7% efficiency in 2012, an averaged 1% efficiency gain per year. In this paper, we reviewed the research progress reported in the last two years. We started with background introduction and motivation for CQDSC research. We then briefly introduced the evolution history of CQDSC development as well as multiple exciton generation effect. We further focused on the latest efforts in improving the light absorption and carrier collection efficiency, including the bulk-heterojunction structure, quantum funnel concept, band alignment optimization and quantum dot passivation. Afterwards, we discussed the tandem solar cell and device stability, and concluded this article with a perspective. Hopefully, this review paper covers the major achievement in this field in year 2011–2012 and provides readers with a concise and clear understanding of recent CQDSC development.

Statistics, synergetics, and mechanism of multiple photogeneration of excitons in quantum dots: Fundamental and applied aspects

The effect of multiple exciton generation is analyzed based on statistical physics, quantum mechanics, and synergetics. Statistical problems of the effect of multiple exciton generation (MEG) are broadened and take into account not only exciton generation, but also background excitation. The study of the role of surface states of quantum dots is based on the synergetics of self-catalyzed electronic reactions. An analysis of the MEG mechanism is based on the idea of electronic shaking using the sudden perturbation method in quantum mechanics. All of the above-mentioned results are applied to the problem of calculating the limiting efficiency to transform solar energy into electric energy.

Advanced theory of multiple exciton generation effect in quantum dots

The theoretical aspects of the effect of multiple exciton generation (MEG) in quantum dots (QDs) have been analysed in this work. The statistical theory of MEG in QDs based on Fermi’s approach is presented, taking into account the momentum conservation law. According to Fermi this approach should give the ultimate quantum efficiencies of multiple particle generation. The microscopic mechanism of this effect is based on the theory of electronic “shaking”. According to this approach, the wave function of “shaking” electrons can be selected as Plato’s functions with effective charges depending on the number of generated excitons. From the theory it is known increasing the number of excitons leads to enhancement of the Auger recombination of electrons which results in reduced quantum yields of excitons. The deviation of the averaged multiplicity of the MEG effect from the Poisson law of fluctuations has been investigated on the basis of synergetics approaches. In addition the role of interface electronic states of QDs and ligands has been considered by means of quantum mechanical approaches. The size optimisation of QDs has been performed to maximise the multiplicity of the MEG effect.

Development of Inorganic Solar Cells by Nano-technology

Abstract Inorganic solar cells, as durable photovoltaic devices for harvesting electric energy from sun light, have received tremendous attention due to the fear of exhausting the earth’s energy resources and damaging the living environment due to greenhouse gases. Some recent developments in nanotechnology have opened up new avenues for more relevant inorganic solar cells produced by new photovoltaic conversion concepts and effective solar energy harvesting nanostructures. In this review, the multiple exciton generation effect solar cells, hot carrier solar cells, one dimensional material constructed asymmetrical schottky barrier arrays, noble nanoparticle induced plasmonic enhancement, and light trapping nanostructured semiconductor solar cells are highlighted.

Synergetics in multiple exciton generation effect in quantum dots

We present detailed analysis of the non-Poissonian population of excitons produced by multiple exciton generation (MEG) effect in quantum dots on the base of statistic theory of MEG and synergetic approach for chemical reactions. From the analysis we can conclude that a non-Poissonian distribution of exciton population is evidence of nonlinear and nonequilibrium character of the process of multiple generation of excitons in quantum dots at a single photon absorption.