Enhancement of Charge Transport of a Dye-Sensitized Solar Cell Utilizing TiO2 Quantum Dot Photoelectrode Film

A dye-sensitized solar cell (DSC) is the third generation of solar technology, utilizing TiO2 nanoparticles with sizes of 20–30 nm as the photoelectrode material. The integration of smaller nanoparticles has the advantage of providing a larger surface area, yet the presence of grain boundaries is inevitable, resulting in a higher probability of electron trapping. This study reports on the improvement of charge transport through the integration of quantum dot (QD) TiO2 with a size of less than 10 nm as the dye absorption photoelectrode layer. The QD TiO2 samples were synthesized through sol–gel and reflux methods in a controlled pH solution without surfactants. The synthesized samples were analyzed using microscopic, diffraction, absorption, as well as spectroscopic analyses. A current–voltage and impedance analysis was used to evaluate the performance of a DSC integrated with synthesized TiO2 as the photoelectrode material. The sample with smaller crystallite structures led to a large surface area and exhibited a higher dye absorption capability. Interestingly, a DSC integrated with QD TiO2 showed a higher steady-state electron density and a lower electron recombination rate. The shallow distribution of the trap state led to an improvement of the electron trapping/de-trapping process between the Fermi level and the conduction band of oxide photoelectrode material, hence improving the lifetime of generated electrons and the overall performance of the DSC.

Far-IR to deep-UV adaptive supercontinuum generation using semiconductor nano-antennas via carrier injection rate modulation

Supercontinuum generating sources, which incorporate a non-linear medium that can generate a wideband intensity spectrum under high-power excitation, are ideal for many applications of photonics such as spectroscopy and imaging. Supercontinuum generation using ultra-miniaturized devices is of great interest for on-chip imaging, on-chip measurement, and for future integrated photonic devices. In this study, semiconductor nano-antennas are proposed for ultra-broadband supercontinuum generation via analytical and numerical investigation of the electric field wave equation and the Lorentz dispersion model, incorporating semiconductor electron dynamics under optical excitation. It is shown that by a rapid modulation of the carrier injection rate for a semiconductor nano-antenna, one can generate an ultra-wideband supercontinuum that extends from the far-infrared (Far-IR) range to the deep-ultraviolet (Deep-UV) range for an infrared excitation of arbitrary intensity level. The modulation of the injection rate is achieved by high-intensity pulsed-pump irradiation of the nano-antenna, which has a fast nonradiative electron recombination mechanism that is on the order of sub-picoseconds. It is shown that when the pulse period of the pump irradiation is of the same order with the electron recombination time, rapid modulation of the free electron density occurs and electric energy accumulates in the nano-antenna, allowing for the generation of a broad supercontinuum. The numerical results are compared with the semiempirical second harmonic generation efficiency results for validation and a mean accuracy of 99.7% is observed. The aim of the study is to demonstrate that semiconductor nano-antennas can be employed to achieve superior supercontinuum generation performance at the nanoscale and the process can be programmed in an adaptive manner for continuous spectral shaping via tuning the pulse period of the pump irradiation.

Neighboring Atom Collisions in Solid-State High Harmonic Generation

High harmonic generation (HHG) from solids shows great application prospects in compact short-wavelength light sources and as a tool for imaging the dynamics in crystals with subnanometer spatial and attosecond temporal resolution. However, the underlying collision dynamics behind solid HHG is still intensively debated and no direct mapping relationship between the collision dynamics with band structure has been built. Here, we show that the electron and its associated hole can be elastically scattered by neighboring atoms when their wavelength approaches the atomic size. We reveal that the elastic scattering of electron/hole from neighboring atoms can dramatically influence the electron recombination with its left-behind hole, which turns out to be the fundamental reason for the anisotropic interband HHG observed recently in bulk crystals. Our findings link the electron/hole backward scattering with Van Hove singularities and forward scattering with critical lines in the band structure and thus build a clear mapping between the band structure and the harmonic spectrum. Our work provides a unifying picture for several seemingly unrelated experimental observations and theoretical predictions, including the anisotropic harmonic emission in MgO, the atomic-like recollision mechanism of solid HHG, and the delocalization of HHG in ZnO. This strongly improved understanding will pave the way for controlling the solid-state HHG and visualizing the structure-dependent electron dynamics in solids.

I-V Curves of an Apigenin Dye and Their Analysis by a New Parabolic Function

A new parabolic function for I-V curves’ analysis has been proposed. The new “analytical tool” provides a simple way to describe photophysical processes at an approximately monolayer surface of a dye-sensitized solar cell. It may now be possible to estimate factors such as hole–electron recombination, surface defects, and electron diffusion at the semiconductor layer. The theoretical approach that was previously reported by our group for predicting the photovoltaic performance of potential dye sensitizers has also been validated. The experimental photovoltaic and DFT/TD-DFT data of apigenin and those of the highly rated black dyes were used for the validation.

Synergetic Effect of Carbon Dot at Cellulose Nanofiber for Sustainable Metal-free Photocatalyst

Metal-free photocatalyst was synthesized by attaching carbon dot (CD) to cellulose nanofiber (CNF) via simple in-situ synthesis. The graphitic core and functional groups of CD on CNF was controlled along the precursor concentration, while the fibrous structure of CNF was intact. The optical property of the samples was profoundly analyzed focusing on the electron recombination pathway. It reveals that the excited electrons in the CD transferred to the CNF and delayed the radiative recombination. The electron-hole pairs were efficiently separated in the composite where abundant amide group and sulfide bonding existed. The CD on the CNF surface improved the morphological stability of CNF under UV irradiation as well. As a result, the composite showed superior photocatalytic performance to degrade 98% of MB molecules within 25 min. The aerogel synthesized from CDCNF had good re-usability without sacrificing its photocatalytic effect. The synthesized CDCNF composite showed superior possibility for applying cellulose nanofiber to sustainable metal-free photocatalyst.

Synergetic Effect of Carbon Dot at Cellulose Nanofiber for Sustainable Metal-free Photocatalyst

Metal-free photocatalyst was synthesized by attaching carbon dot (CD) to cellulose nanofiber (CNF) via simple in-situ synthesis. The graphitic core and functional groups of CD on CNF was controlled along the precursor concentration, while the fibrous structure of CNF was intact. The optical property of the samples was profoundly analyzed focusing on the electron recombination pathway. It reveals that the excited electrons in the CD transferred to the CNF and delayed the radiative recombination. The electron-hole pairs were efficiently separated in the composite where abundant amide group and sulfide bonding existed. The CD on the CNF surface improved the morphological stability of CNF under UV irradiation as well. As a result, the composite showed superior photocatalytic performance to degrade 98% of MB molecules within 25 min. The aerogel synthesized from CDCNF had good re-usability without sacrificing its photocatalytic effect. The synthesized CDCNF composite showed superior possibility for applying cellulose nanofiber to sustainable metal-free photocatalyst.

Polymer Nanocomposites for Photocatalytic Degradation and Photoinduced Utilizations of Azo-Dyes

Specially designed polymer nanocomposites can photo-catalytically degrade azo dyes in wastewater and textile effluents, among which TiO2-based nanocomposites are outstanding and extensively explored. Other nanocomposites based on natural polymers (i.e., chitosan and kaolin) and the oxides of Al, Au, B, Bi, Fe, Li, and Zr are commonly used. These nanocomposites have better photocatalytic efficiency than pure TiO2 through two considerations: (i) reducing the hole/electron recombination rate by stabilizing the excited electron in the conducting band, which can be achieved in TiO2-nanocomposites with graphene, graphene oxide, hexagonal boron nitride (h-BN), metal nanoparticles, or doping; (ii) decreasing the band energy of semiconductors by forming nanocomposites between TiO2 and other oxides or conducting polymers. Increasing the absorbance efficiency by forming special nanocomposites also increases photocatalytic performance. The photo-induced isomerization is exploited in biological systems, such as artificial muscles, and in technical fields such as memory storage and liquid crystal display. Heteroaryl azo dyes show remarkable shifts in photo-induced isomerization, which can be applied in biological and technical fields in place of azo dyes. The self-assembly methods can be employed to synthesize azo-dye polymer nanocomposites via three types of interactions: electrostatic interactions, London forces or dipole/dipole interactions between azo dyes, and photo alignments.

Effect of Combination of Natural Dyes and the Blocking Layer on the Performance of DSSC

Over the years, researchers have been working on replacing sensitized dye for dye sensitized solar cells (DSSC), because of its low production cost, biodegradability, and non-toxicity. However, the overall performance of natural dye-based DSSCs is low compared to the DSSCs sensitized with Ruthenium based dyes. The combination of natural dyes with an optimized choice of the extracting solvents and the proper volume ratio of mixture of the dyes, enhances inherent properties, such as absorption and adsorption of the dyes. It also allows the device to utilize photon energy more efficiently over the entire visible wavelength. As a result, DSSC sensitized with the dye mixture shows higher absorbance, and cumulative absorption properties over the whole visible region than the DSSC fabricated with individual dyes and showed higher photocurrent. Another effective way to improve cell efficiency is by using a blocking layer. The blocking layer increases the photocurrent, is mainly due to the improvement of the electron recombination at the transparent conducting oxide/electrolyte interfaces. Also, the blocking layer’s compact structure creates an effective pathway for electron transportation; thus, the device’s photocurrent increases. Additionally, a slight improvement in the open-circuit voltage and fill factor was observed, thus cell efficiency enhances significantly. By both the proper ratio of dye mixture and the blocking layer improves cell performance of DSSC and opens a new pathway for future studies.