Quantum Dot Solar Cells. Semiconductor Nanocrystals as Light Harvesters
18 Oct 2008-Journal of Physical Chemistry C (American Chemical Society)-Vol. 112, Iss: 48, pp 18737-18753
TL;DR: In this paper, three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell, (ii) polymer−smiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell.
Abstract: The emergence of semiconductor nanocrystals as the building blocks of nanotechnology has opened up new ways to utilize them in next generation solar cells. This paper focuses on the recent developments in the utilization of semiconductor quantum dots for light energy conversion. Three major ways to utilize semiconductor dots in solar cell include (i) metal−semiconductor or Schottky junction photovoltaic cell (ii) polymer−semiconductor hybrid solar cell, and (iii) quantum dot sensitized solar cell. Modulation of band energies through size control offers new ways to control photoresponse and photoconversion efficiency of the solar cell. Various strategies to maximize photoinduced charge separation and electron transfer processes for improving the overall efficiency of light energy conversion are discussed. Capture and transport of charge carriers within the semiconductor nanocrystal network to achieve efficient charge separation at the electrode surface remains a major challenge. Directing the future resear...
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TL;DR: PL activation (PLA) and darkening over populations of single colloidal ZnS-capped CdSe QDs under continuous illumination, which is described well by a simple consecutive elementary reaction (CER) scheme in the 1 to 10 kW cm(-2) excitation intensity regime.
Abstract: Semiconductor nanocrystals or quantum dots (QDs) exhibit a number of unique optical properties including fluorescence intermittency (FI), photoluminescence (PL) enhancement, and darkening. Here we report PL activation (PLA) and darkening over populations of single colloidal ZnS-capped CdSe QDs under continuous illumination, which is described well by a simple consecutive elementary reaction (CER) scheme in the 1 to 10 kW cm2 excitation intensity regime. The scheme allows determination of rate constants for both fluorescence activation and decay processes as well as the measurement of initial bright, fluorescent and dark, nonfluorescent QD fractions. The latter parameters can function as a "quality control" on the total population of detectable QDs in an imaging experiment or a synthesis. We further show reversible PLA at low intensities <0.5 kW cm2 and a photoinduced conditioning of the QD that results in increased rates of PLA following repeated cycles of illumination and an induction period that precedes photoactivation upon initial exposure to light. By interrogating individual QD fluorescence trajectories, the population fractions found exclusive to each of three illumination cycles, common to any two cycles and fluorescent in all three cycles, show that only a small number of QDs (5%) remain fluorescent through multiple cycles of photoactivation and recovery.
31 citations
TL;DR: A simple synthetic approach based on solvothermal technique and hydrogenation was developed for preparation of TiO 2 nanocrystals with different morphologies such as elongated rhombic, dog-bone, oval and core-shell structured oval.
31 citations
TL;DR: This study indicates that the generation of strongly anchored CdS and CdTeS QDs on a TiO(2) nanowire surface is achievable without introduction of a linker molecule, whose presence is known to decrease the electron injection efficiency.
Abstract: An easy process was developed to synthesize TiO2 nanowires sensitized with CdS and CdTeS quantum dots (QDs) requiring no pretreatment of the TiO2 nanowires prior to nanoparticle generation. CdS and CdTeS nanoparticles were firstly grown by an in situ colloidal method directly onto the TiO2 surface, hence not requiring subsequent functionalization of the QDs. The resulting nanostructure assembly and composition was confirmed by transmission electron microscopy (TEM), x-ray photoelectron spectroscopy (XPS) and Raman spectroscopy. Successful decoration of the TiO2 nanowires by the QDs was observed by TEM, while XPS spectra provided clear evidence for the coexistence of CdS and CdTeS QDs and TiO2 nanowires. The electronic structure of the TiO2 nanowires was preserved as indicated by Raman spectroscopy. Preliminary photocurrent measurements showed that inclusion of Te in CdS QDs improved the photocurrent efficiency. Compared to bare TiO2 nanowires, CdS/TiO2 nanoassemblies showed an enhancement in photocurrent efficiency of 300% while CdTeS/TiO2 presented an improvement of 350%. This study indicates that the generation of strongly anchored CdS and CdTeS QDs on a TiO2 nanowire surface is achievable without introduction of a linker molecule, whose presence is known to decrease the electron injection efficiency.
30 citations
TL;DR: In this paper, a systematic study of the temperature and electric field dependence of conductivity in films of alkyl-ligand-terminated silicon nanocrystals (Si NCs) is presented.
Abstract: Silicon nanocrystals (Si NCs) have shown great promise for electroluminescent and photoluminescent applications. In order to optimize the properties of Si NC devices, however, electronic transport in Si NCs films needs to be thoroughly understood. Here we present a systematic study of the temperature and electric field dependence of conductivity in films of alkyl-ligand-terminated Si NCs, which to date have shown the highest potential for device applications. Our measurements suggest that the conductivity is limited by the ionization of rare NCs containing donor impurities. At low bias, this ionization is thermally activated, with an ionization energy equal to twice the NC charging energy. As the bias is increased, the ionization energy is reduced by the electric field, as determined by the Poole-Frenkel effect. At large bias and sufficiently low temperature, we observe cold ionization of electrons from donor-containing NCs, with a characteristic tunneling length of about 1 nm. The temperature- and electric-field-dependent conductance measurements presented here provide a systematic and comprehensive picture for electron transport in lightly doped nanocrystal films.
30 citations
Cites background from "Quantum Dot Solar Cells. Semiconduc..."
... cycling. In general, the conductance of an insulating disordered system, like our NC films, decreases with decreasing temperature and can typically be described by T exp 0 T G ªº§· J v«»¨¸ «»¬© , (1) where the temperature exponent γ depends on the transport mechanism and T 0 is a characteristic temperature.25 Situations with γ = 1, in general, correspond to nearestneighbor hopping (NNH), where th...
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TL;DR: The photocatalytic deposition of antimony(III) sulfide on the surface of TiO 2 films on the conductive FTO glass was investigated in this article, where the photoprocessor yields particles of amorphous and almost stoichiometric Sb 2 S 3 with a size of 150-300nm and spherical micrometer particles enriched with Sb with a Sb:S ratio that decreases gradually from 13:1 in the particle center to 5:1 at the surface.
Abstract: The photocatalytic deposition of antimony(III) sulfide on the surface of TiO 2 films on the conductive FTO glass was investigated. The photoprocess yields particles of amorphous and almost stoichiometric Sb 2 S 3 with a size of 150–300 nm and spherical micrometer particles enriched with Sb with a Sb:S ratio that decreases gradually from 13:1 in the particle center to 5:1 at the surface. Thermal treatment of the FTO/TiO 2 /Sb 2 S 3 films at 350 °C results in the transformation of amorphous antimony(III) sulfide into crystalline stibnite without extensive changes in the film morphology. The crystalline FTO/TiO 2 /Sb 2 S 3 films exhibit photoelectrochemical activity when illuminated in the spectral range of λ > 350 nm in aqueous solutions of ascorbic acid as a sacrificial electron donor. The maximal photocurrent density generated by the TiO 2 /Sb 2 S 3 heterostructure is higher than the photoresponse of the starting FTO/TiO 2 film by a factor of 3.6. Deposition of an intermediary layer of In x (OH) y S z on the interface between TiO 2 and Sb 2 S 3 results in a further 3-fold increase in the photoelectrochemical activity of the FTO/TiO 2 /Sb 2 S 3 heterostructure.
30 citations
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TL;DR: In this article, an upper theoretical limit for the efficiency of p−n junction solar energy converters, called the detailed balance limit of efficiency, has been calculated for an ideal case in which the only recombination mechanism of holeelectron pairs is radiative as required by the principle of detailed balance.
Abstract: In order to find an upper theoretical limit for the efficiency of p‐n junction solar energy converters, a limiting efficiency, called the detailed balance limit of efficiency, has been calculated for an ideal case in which the only recombination mechanism of hole‐electron pairs is radiative as required by the principle of detailed balance. The efficiency is also calculated for the case in which radiative recombination is only a fixed fraction fc of the total recombination, the rest being nonradiative. Efficiencies at the matched loads have been calculated with band gap and fc as parameters, the sun and cell being assumed to be blackbodies with temperatures of 6000°K and 300°K, respectively. The maximum efficiency is found to be 30% for an energy gap of 1.1 ev and fc = 1. Actual junctions do not obey the predicted current‐voltage relationship, and reasons for the difference and its relevance to efficiency are discussed.
11,071 citations
TL;DR: Many potential applications have been proposed for carbon nanotubes, including conductive and high-strength composites; energy storage and energy conversion devices; sensors; field emission displays and radiation sources; hydrogen storage media; and nanometer-sized semiconductor devices, probes, and interconnects.
Abstract: Many potential applications have been proposed for carbon nanotubes, including conductive and high-strength composites; energy storage and energy conversion devices; sensors; field emission displays and radiation sources; hydrogen storage media; and nanometer-sized semiconductor devices, probes, and interconnects. Some of these applications are now realized in products. Others are demonstrated in early to advanced devices, and one, hydrogen storage, is clouded by controversy. Nanotube cost, polydispersity in nanotube type, and limitations in processing and assembly methods are important barriers for some applications of single-walled nanotubes.
9,693 citations
TL;DR: In this paper, the carrier collection efficiency and energy conversion efficiency of polymer photovoltaic cells were improved by blending of the semiconducting polymer with C60 or its functionalized derivatives.
Abstract: The carrier collection efficiency (ηc) and energy conversion efficiency (ηe) of polymer photovoltaic cells were improved by blending of the semiconducting polymer with C60 or its functionalized derivatives. Composite films of poly(2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and fullerenes exhibit ηc of about 29 percent of electrons per photon and ηe of about 2.9 percent, efficiencies that are better by more than two orders of magnitude than those that have been achieved with devices made with pure MEH-PPV. The efficient charge separation results from photoinduced electron transfer from the MEH-PPV (as donor) to C60 (as acceptor); the high collection efficiency results from a bicontinuous network of internal donor-acceptor heterojunctions.
9,611 citations
TL;DR: In this paper, the authors describe recent progress in the theory of nanoparticle optical properties, particularly methods for solving Maxwell's equations for light scattering from particles of arbitrary shape in a complex environment.
Abstract: The optical properties of metal nanoparticles have long been of interest in physical chemistry, starting with Faraday's investigations of colloidal gold in the middle 1800s. More recently, new lithographic techniques as well as improvements to classical wet chemistry methods have made it possible to synthesize noble metal nanoparticles with a wide range of sizes, shapes, and dielectric environments. In this feature article, we describe recent progress in the theory of nanoparticle optical properties, particularly methods for solving Maxwell's equations for light scattering from particles of arbitrary shape in a complex environment. Included is a description of the qualitative features of dipole and quadrupole plasmon resonances for spherical particles; a discussion of analytical and numerical methods for calculating extinction and scattering cross-sections, local fields, and other optical properties for nonspherical particles; and a survey of applications to problems of recent interest involving triangula...
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TL;DR: Semiconductor nanocrystals prepared for use as fluorescent probes in biological staining and diagnostics have a narrow, tunable, symmetric emission spectrum and are photochemically stable.
Abstract: Semiconductor nanocrystals were prepared for use as fluorescent probes in biological staining and diagnostics. Compared with conventional fluorophores, the nanocrystals have a narrow, tunable, symmetric emission spectrum and are photochemically stable. The advantages of the broad, continuous excitation spectrum were demonstrated in a dual-emission, single-excitation labeling experiment on mouse fibroblasts. These nanocrystal probes are thus complementary and in some cases may be superior to existing fluorophores.
8,542 citations