Higher Electron Diffusion Coefficient Research Articles

Multifunctional nitrogen-doped graphene quantum dots incorporated into mesoporous TiO2 films for quantum dot-sensitized solar cells

Nitrogen-doped graphene quantum dots (N-GQDs) synthesized by a hydrothermal method were incorporated into TiO2 mesoporous films to boost the photovoltaic performance of quantum dot-sensitized solar cells (QDSSCs). Cadmium selenide quantum dots (CdSe QDs) were then deposited on the N-GQDs-incorporated TiO2 films using a successive ionic layer adsorption and reaction (SILAR) technique. The N-GQDs serve multiple functions for enhancing light-harvesting, facilitating electron transportation, and suppressing charge recombination. The non-zero gap N-GQDs both generate excitons as co-sensitizer and provide more adsorption sites for CdSe QDs. Furthermore, the N-GQDs provide efficient electron pathways between TiO2 nanoparticles and passivate the surface defects, giving rise to a high electron diffusion coefficient and long electron lifetime. The resulting QDSSC with the N-GQDs showed significantly improved power conversion efficiency (4.88%) compared to the QDSSC (3.92%) without N-GQDs.

Pt-decorated zinc oxide nanorod arrays with graphitic carbon nitride nanosheets for highly efficient dual-functional gas sensing

In this work, well-aligned ZnO nanorods were grown on the substrate of exfoliated g-C3N4 nanosheets via a microwave-assisted hydrothermal synthesis, and then Pt/ZnO/g-C3N4 nanostructures were obtained after the deposition of Pt nanoparticles. The growth of vertically ordered ZnO nanorods was occurred on g-C3N4 nanosheets through the bonding interaction between Zn and N atoms, which was confirmed by XPS, FT-IR data and molecular orbital theory. The Pt/ZnO/g-C3N4 nanostructures sensor exhibited the remarkable sensitivity, selectivity, and fast response/recovery time for air pollutants of ethanol and NO2. The application of Pt/ZnO/g-C3N4 nanostructures could be used as a dual-functional gas sensor through the controlled working temperature. Besides, the Pt/ZnO/g-C3N4 nanostructures sensor could be applied to the repeating detection of ethanol and NO2 in the natural environment. The synergistic effect and improved the separation of electron-hole pairs in Pt/ZnO/g-C3N4 nanostructures had been verified for the gas sensing mechanism. Additionally, Pt/ZnO/g-C3N4 nanostructures revealed the excellent charge carriers transport properties in electrochemical impedance spectroscopy (EIS), such as the longer electron lifetime (τn), higher electron diffusion coefficient (Dn) and bigger effective diffusion length (Ln), which also played an important role for Pt/ZnO/g-C3N4 nanostructures with striking gas sensing activities.

Optimization of a HOT LWIR HgCdTe Photodiode for Fast Response and High Detectivity in Zero-Bias Operation Mode

Fast response is an important property of infrared detectors for many applications. Currently, high-temperature long-wavelength infrared HgCdTe heterostructure photodiodes exhibit subnanosecond time constants while operating under reverse bias. However, nonequilibrium devices exhibit excessive low-frequency 1/f noise that extends up to MHz range, representing a severe obstacle to their widespread application. Present efforts are focused on zero-bias operation of photodiodes. Unfortunately, the time constant of unbiased photodiodes is still at the level of several nanoseconds. We present herein a theoretical investigation of device design for improved response time and detectivity of long-wavelength infrared HgCdTe photodiodes operating at 230 K in zero-bias mode. The calculation results show that highly doped \tp-type HgCdTe is the absorber material of choice for fast photodiodes due to its high electron diffusion coefficient. The detectivity of such a device can also be optimized by using absorber doping of NA = 1 × 1017 cm−3. Reduction of the thickness is yet another approach to improve the device response. Time constant below 1 ns is achieved for an unbiased photodiode with absorber thickness below 4 μm. A tradeoff between the contradictory requirements of achieving high detectivity and fast response time is expected in an optically immersed photodiode with very small active area.

Significant improvement of photocurrent in dye-sensitized solar cells by incorporation thiophene into electrolyte as an inexpensive and efficient additive

For the first time, thiophene as an efficient additive is applied in electrolyte of dye-sensitized solar cells (DSSCs). In the cell based on 2-cyano-3-(4-(diphenylamino)phenyl)acrylic acid (TPA) sensitizer, addition of 1 M thiophene to the electrolyte (instead of 4-tert-butylpyridine (TBP)) increases significantly photocurrent density (Jsc) from 4.62 to 7.80 mA cm−2. Consequently, overall conversion efficiency (η) enhances from 2.17% to 3.18% with a 46% improvement. For DSSC based on 2-cyano-3-(2′-(5′,10′,15′,20′-tetraphenylporphyrinato zinc(II))yl)acrylic acid (Zn-1) with modified electrolyte, Jsc increases from 4.18 to 8.23 mA cm−2 and η improves from 1.46% to 1.96% that shows an enhancement of 34% in η. Intensity modulated photocurrent spectroscopy (IMPS) indicates the faster electron transfer in the DSSC based on thiophene additive compared to the standard device. Also, the higher electron diffusion coefficient (Dn) of the DSSC with modified electrolyte shows the thiophene additive facilitates the electron transfer. Based on cyclic voltammetry (CV) and UV–vis data, thiophene forms a molecular complex with iodine molecule in the electrolyte solution that has a remarkable effect on the redox couple oxidation–reduction reactions which improves Jsc. A significant improvement in Jsc with a small decrease in Voc and thereby an increase in η are observed from addition thiophene.

Sr, Zn co-doped TiO2 xerogel film made of uniform spheres for high-performance dye-sensitized solar cells

This study comes up with the facile preparation of Sr,Zn co-doped TiO2 xerogel film for boosting the short circuit current density of dye-sensitized solar cells (DSCs). The film contains 2.5-μm-diameter spheres assembled from 60 nm nanoparticles. X-ray photoelectron spectroscopy (XPS) shows that Sr2+ and Zn2+ ions to be well incorporated into the TiO2 crystal lattice without forming specific strontium and zinc compositions. The crystallite size, phase composition, and band structure of the spheres depend on the dopants concentration. Isolated energy levels near valence band as a result of the foreign ions introduction improve the photocatalytic activity of the prepared TiO2 spheres, enhancing the short circuit current density of the cells. The DSC co-doped with 0.075 at.% Sr and 0.4 at.% Zn showed the highest power conversion efficiency of 7.87 % and short circuit current density of 18.75 mA cm−2 thanks to lower charge transfer resistance (2.16 Ω cm2), lower electron transit time (1.19 ms), and higher electron diffusion coefficient (18.1 × 104 cm2 S−1) compared to the other cells, demonstrated by electrochemical impedance spectroscopy (EIS). The concept of the simultaneous introduction of alkaline earth ions and transition ions into TiO2 xerogel films will open up a new insight into the fabrication of high performance DSCs.

Enhanced performance of dye-sensitized solar cells aided by Sr,Cr co-doped TiO2 xerogel films made of uniform spheres

One-pot preparation of Sr,Cr co-doped TiO2 xerogel film for boosting the short circuit current density of dye-sensitized solar cells (DSCs) is reported. The 2.5-μm-diameter spheres are assembled from 60nm nanoparticles by a modified sol–gel method. X-ray photoelectron spectroscopy (XPS) shows that Sr2+ and Cr3+ ions to be well incorporated into the titania crystal lattice without forming specific strontium and chromium compositions. The crystallite size, phase composition, and band structure of the particles depend on the dopants concentration. Isolated energy levels near valence band as a result of the transition ion (i.e., Cr) introduction, in conjunction with the local lattice distortions owing to the alkaline earth ion (i.e., Sr) insertion, improves the photocatalytic activity of the prepared TiO2 spheres, enhancing the short circuit current density of the cells. The DSC co-doped with 0.075 at.% Sr and 2.5 at.% Cr (i.e., S7C25 solar cell) showed the highest power conversion efficiency of 7.89% and short circuit current density of 18.58mA/cm2 thanks to lower charge transfer resistance (2.35Ωcm2), lower electron transit time (1.26ms), and higher electron diffusion coefficient (17.1×104cm2S−1) compared to the other cells, demonstrated by electrochemical impedance spectroscopy (EIS). The concept of simultaneously introduction of alkaline earth ions and transition ions into TiO2 lattice will open up a new insight into the fabrication of high performance DSCs.

Low crystallinity TiO2 film with inherent low oxygen vacancy for sensitized solar cells

Instead of perfect crystallization or surface defect-removing treatments, this study suggests an alternative route of oxygen vacancy reduction, i.e. amorphization, to improve the electron transport properties of TiO2 mesoporous films. In this paper, a low crystallinity TiO2 mesoporous film with inherent low content of oxygen vacancies is used as the photoanode of dye-sensitized solar cells (DSCs). Electrochemical impedance spectroscopy (EIS) analysis indicates the high electron diffusion coefficient and long electron life time of the low crystallinity TiO2 photoanode. An extraordinary high open photovoltage of 860mV is achieved owing to the low oxygen vacancy. And also the charge recombination of the internal cell significantly decreases, thus enhances the utilization efficiency of dye, which has been characterized with the high ratio of short circuit photocurrent to dye-loading.

High-efficiency cascade CdS/CdSe quantum dot-sensitized solar cells based on hierarchical tetrapod-like ZnO nanoparticles

Quantum dot-sensitized solar cells (QDSCs) constructed using cascade CdS/CdSe sensitizers and the novel tetrapod-like ZnO nanoparticles have been fabricated. The cascade co-sensitized QDSCs manifested good electron transfer dynamics and overall power conversion efficiency, compared to single CdS- or CdSe-sensitized cells. The preliminary CdS layer is not only energetically favorable to electron transfer but behaves as a passivation layer to diminish the formation of interfacial defects during CdSe synthesis. On the other hand, the anisotropic tetrapod-like ZnO nanoparticles, with a high electron diffusion coefficient, can afford a better carrier transport than traditional ZnO nanoparticles. The resultant solar cell yielded an excellent performance with a solar power conversion efficiency of 4.24% under simulated one sun (AM1.5G, 100 mW cm(-2)) illumination.

Photovoltaic Behavior of Dye-sensitized Long TiO2Nanotube Arrays

Long nanotube (NT) arrays, prepared by electrochemical anodization of Ti foils, have been utilized as dye-adsorbing electrodes in dye-sensitized solar cells (DSCs). By anodizing for 1-24 hr and subsequent annealing, highly crystallized and tightly-adhered NT arrays were tailored to 11-150 lengths, ~90 nm innerpore diameter and ~30 nm wall thickness. I-V curves revealed that the photovoltaic conversion efficiency () was proportional to the NT length up to 36 . Beyond this length, the ) was proportional to the NT length up to was still steadily increased, though at a much lower rate. For example, an of 5.05% at 36 was increased to 6.18% at 150 . Transient photoelectron spectroscopic analyses indicated that NT array-based DSCs revealed considerably higher electron diffusion coefficient () and life time () than those with nanoparticles (NP). Moreover, the electron diffusion lengths () of the photo-injected electrons were considerably larger than the corresponding NT lengths in all the cases, suggesting that electron transport in NT arrays is highly efficient, regardless of tube length.

Highly efficient dye-sensitized solar cell with a ZnO nanosheet-based photoanode

Zinc oxide (ZnO) nanosheets (ZnO-NS) were prepared for the photoanode of a dye-sensitized solar cell (DSSC), first by directly growing layered hydroxide zinc carbonate (LHZC) on an FTO substrate using a chemical bath deposition (CBD) method and then by transforming the LHZC into ZnO through pyrolysis at 300 °C. A light-to-electricity conversion efficiency (η) of 6.06% was achieved for the DSSC with ZnO-NS as its photoanode, under 100 mW cm−2 illumination, and this (η) was found to be much higher than that of the DSSC with ZnO nanoparticles (ZnO-NP) as the photoanode (2.92%). The far superior performance of the DSSC with ZnO-NS is essentially attributed to (i) higher effective electron diffusion coefficient of ZnO-NS (3.59 × 10−3 cm2s−1) than that of ZnO-NP (1.12 × 10−3 cm2s−1), and to (ii) higher dye loading on ZnO-NS (2.66 × 10−7 mol cm−2) than that on ZnO-NP (1.99 × 10−7 mol cm−2); this higher electron diffusion coefficient and dye-loading are attributed to the specific morphology of the ZnO-NS. A further improvement in the efficiency of the DSSC with ZnO-NS could be achieved through the electrophoretic deposition (EPD) of a very thin layer (3 μm) of titanium dioxide nanoparticles (TiO2-NPs of average size 14 nm) onto the ZnO-NS layer (12 μm). Notwithstanding a decrease in the effective electron diffusion coefficient (3.07 × 10−3 cm2s−1) in the TiO2-NP/ZnO-NS film, with reference to that in the ZnO-NS film (3.59 × 10−3 cm2s−1), a far higher cell efficiency was obtained in favor of the cell with TiO2-NP/ZnO-NS (7.07%), compared to that of the cell with bare ZnO-NS (6.06%); this enhancement in the η of the cell with TiO2-NP/ZnO-NS is ascribed to an increased dye-loading in favor of its cell (3.92 × 10−7 mol cm−2), with reference to that in the case of the cell with bare ZnO-NS (2.66 × 10−7 mol cm−2). As against the common ruthenium dyes, such as N3 and N-719, a metal-free dye, coded as D149, was used in this research. The efficiency achieved for the best DSSC in this work is the highest ever reported for a DSSC with ZnO as the main semiconductor material.

Improved Electron Diffusion Coefficient in Electrospun TiO2 Nanowires

TiO2 nanowires with a diameter of ∼150 nm, length of ∼3−4 μm, and aspect ratio of 10:1, were prepared by ultrasonically dispersing electrospun continuous nanofibers in monocarboxylic acids. The resulting pastes were used for making nanowire films on conducting glass substrates with thicknesses in the range of 500 nm to 100 μm, good adhesion, and high nanowire packing. These films were used to fabricate dye-sensitized solar cells using the D131 dye and the iodide/triiodide electrolyte. Transient photocurrent measurements showed a high electron diffusion coefficient in those nanowire films. The measured diffusion coefficient in those TiO2 nanowires was orders of magnitude higher than that observed in nanoparticles under similar experimental conditions. The charge-transport mechanism in the nanowire sample is discussed in support with the measured open-circuit voltage decay curves.

TiO2 Surface State Control for Dye Sensitized Solar Cells with High Efficiency and the Solidification -Fabrication of Charge Carrier Path

AbstractImprovement of photovoltaic performance for dye sensitized solar cells (DSC) is discussed in terms of electron-path and ion-path. In order to make electron path, we focused on passivation of TiO2 surface states which are observed by thermally stimulated current (TSC). The TiO2 surface was well-passivated with dye molecules under pressurized CO2 atmosphere. It was found that DSC cells prepared by a CO2 process (Cell-CO2) had higher efficiency than those prepared by a conventional dipping process (Cell-DIP) and the higher efficiency was associated with low TiO2-surface state, high electron diffusion coefficient and long electron life time in TiO2 for the Cell-CO2. In addition, dye-staining under pressurized CO2 atmosphere had advantages over a conventional dipping process on rapid dye-uptake and less dye aggregation. In order to fabricate ion-path in solidified electrolyte, we focused on surface modification of nano-materials. Surface of nano-materials such as TiO2-nanoparticles and porous alumina films were modified with imidazolium iodide moieties consisting of long alkyl chains which render surface-molecules self-organized. Redox-species are concentrated on the self-organized molecules and make ion-path. We propose quasi-solid electrolyte system consisting of two layers having different charge carrier concentration to keep high photoconversion efficiencies even after solidification.