Solar-blind ultraviolet photodetector based on Ti-doped Ga2O3/Si p–n heterojunction

A solar-blind ultraviolet photodetector based on Ti-doped Ga₂O₃/Si p–n heterojunction is demonstrated for the first time. It is found that the heterojunction quality forming between Ga₂O₃ and Si becomes better after Ti incorporation in Ga₂O₃. The current–voltage and temporal response measurements show that the detector based on Ti-doped Ga₂O₃/Si p–n heterojunction has a responsivity of 0.382 A/W and a fast rise time of 73 ms as well, which are much better than those undoped Ga₂O₃/Si p–n heterojunction analogues.

     

Evaluation of the shifting of the line of sight and higher order aberrations of eyes with keratoconus after corneal cross-linking

Aim

To evaluate changes in the coordinates of the line of sight (LoS) and higher order aberrations (HOAs) of eyes with keratoconus, following corneal cross-linking (CXL).

St. Lucie cherry,yellow jasmine,and madder berries as novel natural sensitizers for dye‐sensitized solar cells

Natural dyes extracted from fruits, vegetables, flowers, and leaves are considered as promising alternative sensitizers to replace synthetic dyes for dye‐sensitized solar cells (DSSCs). Generally, solar activity of natural dyes stem from anthocyanin pigment. Carbonyl, carboxyl, and hydroxyl groups present in the anthocyanin molecule improve the adsorption ability of dye on TiO₂ and therefore facilitate charge transfer. Here, for the first time, novel natural dyes extracted from St. Lucie cherry, yellow jasmine, and madder berries are reported to act as sensitizer in DSSCs. These novel natural dye extracts are prepared by dissolving related fruits in ethanol. The ingredient of the dyes is identified by FT‐IR spectroscopy. Accordingly, FT‐IR spectrum reveals that novel natural dye extracts exhibit all the characteristic peaks of anthocyanin pigment. Specifically, St. Lucie cherry consists of more distinct carbonyl group than other sources. Also, photoanodes composed of three TiO₂ layers are prepared by using a spin‐coating method. Then, they are immersed into natural dyes and analyzed by conducting UV‐Vis spectroscopy. Compared with bare TiO₂, natural dye–loaded photoanodes demonstrate far higher absorption ability in the visible region. After fabrication of devices with different novel natural dye sensitizers, current‐voltage characteristics and electrochemical impedance spectroscopy measurements are performed. The best power conversion efficiency (PCE) of 0.19% is obtained by sensitization of St. Lucie cherry with an open‐circuit voltage (V_oc) of 0.56 V, short‐circuit current density (J_sc) of 181 μA cm^?2, and fill factor (FF) of 0.55. Furthermore, St. Lucie cherry–sensitized devices show the lowest charge transfer and highest recombination resistances. This result can be attributed to the obvious carbonyl group exhibited by St. Lucie cherry.     

Facile fabrication of low-cost low-temperature carbon-based counter electrode with an outstanding fill factor of 73% for dye-sensitized solar cells

Developing chemically inert, electrically conductive, and catalytically active counter electrodes (CEs) to replace conventional Pt-based ones is highly desirable for dye-sensitized solar cells. Herein, we reported a facile, cost-effective, and low-temperature synthesis pathway to develop carbon-based CEs. The performance of homemade carbon paste (H-CP)–based CE (H-CE) was compared with that of commercial carbon paste (C-CP)–based CE (C-CE) and Pt-based CE (Pt-CE). The scanning electron microscope (SEM) results showed that H-CE demonstrated a penetrable surface structure which facilitates the diffusion of electrolyte through the carbon electrode. This phenomenon enhanced the triiodide reduction with respect to C-CE having a compact structure that limits the electrolyte diffusion. The charge transfer properties and catalytic activities of the investigated devices were explored using electrochemical impedance spectroscopy and Tafel polarization measurements; the obtained results indicated that the device based on H-CE revealed relatively lower charge transfer resistance and higher exchange current density compared with C-CE-based device. The current-voltage measurements showed that the device based on H-CE has a power conversion efficiency of 2.70%, which was about 1.6 times higher than that of the device based on C-CE (1.68%). Furthermore, a fill factor of 73% was achieved for the device based on H-CE, which outperformed the Pt-based device (69%) and was among one of the highest values obtained in the literature. Also, a tape adhesion test performed on H-CP-coated glass substrate displayed its excellent robustness.     

Optimization of n nc-Si:H and a-SiNx:H layers for their application in nc-Si:H TFT

The n-type doped silicon thin films were deposited by plasma enhanced chemical vapor deposition (PECVD) technique at high and low H₂ dilutions. High H₂ dilution resulted in n⁺ nanocrystalline silicon films (n⁺ nc-Si:H) with the lower resistivity (ρ ∼0.7 Ω cm) compared to that of doped amorphous silicon films (∼900 Ω cm) grown at low H₂ dilution. The change of the lateral ρ of n⁺ nc-Si:H films was measured by reducing the film thickness via gradual reactive ion etching. The ρ values rise below a critical film thickness, indicating the presence of the disordered and less conductive incubation layer. The 45 nm thick n⁺ nc-Si:H films were deposited in the nc-Si:H thin film transistor (TFT) at different RF powers, and the optimum RF power for the lowest resistivity (∼92 Ω cm) and incubation layer was determined. On the other hand, several deposition parameters of PECVD grown amorphous silicon nitride (a-SiN_x:H) thin films were changed to optimize low leakage current through the TFT gate dielectric. Increase in NH₃/SiH₄ gas flow ratio was found to improve the insulating property and to change the optical/structural characteristics of a-SiN_x:H film. Having lowest leakage currents, two a-SiN_x:H films with NH₃/SiH₄ ratios of ∼19 and ∼28 were used as a gate dielectric in nc-Si:H TFTs. The TFT deposited with the NH₃/SiH₄∼19 ratio showed higher device performance than the TFT containing a-SiN_x:H with the NH₃/SiH₄∼28 ratio. This was correlated with the N−H/Si−H bond concentration ratio optimized for the TFT application.     

Effect of heat treatment on the stress and structure evolution of plasma deposited boron nitride thin films

Boron nitride (BN) thin films are deposited at 573 K by plasma enhanced chemical vapor deposition (PECVD) with ammonia (NH₃) and hydrogen diluted diborane (15% B₂H₆ in H₂) source gases. UV-visible and Fourier transform infrared (FTIR) spectroscopies together with surface profilometry are used for the film characterization. These films are hydrogenated (BN:H) whose hydrogen content is pursued following the 1.5 h annealing process at 748 K, 923 K and 1073 K under nitrogen atmosphere. Hydrogen escape with the rising annealing temperature is observed together with increases of the compressive stress, band gap and Urbach energies. Films are composed of the hexagonal BN (h-BN) clusters that grow dominantly parallel to the substrate surface with some non-parallel planes at the edges of the clusters, which are embedded in an amorphous tissue (the so-called turbostratic structure, t-BN). Annealing seems to promote non-parallel planes, thus creating more stressful and distorted network. Most of hydrogen atoms are removed from the film annealed at 1073 K and wurtzite BN (w-BN) phase is formed with volume fraction of 57%. As a consequence or in parallel of hydrogen reduction, high compressive stress causes the cracking of the films.     

Characterization and mechanical properties of TiO2 nanotubes formed on titanium by anodic oxidation

The well-ordered titanium dioxide (TiO₂) nanotube array surfaces were formed at different voltages such as 20 V, 40 V, 60 V, 80 V and 100 V for 1 h on cp-Ti by anodic oxidation (AO) technique. And then, to improve crystallinity of the surface, heat treatment was applied at 450 °C for 1 h to all surfaces without any morphological changing. The surface and cross sectional morphology, elemental structure, phase composition, functional groups, roughness and thickness, wettability and mechanical results were investigated by SEM, EDX, XRD, FT-IR, AFM, contact angle measurement device and nanoindentation tester, respectively. Mainly, anatase- and rutile-TiO₂ phases were obtained at post-heat treatment whereas only, Ti phase was detected on AO surfaces at pre-heat treatment. All nanotube structures and the elements of Ti and O were uniformly distributed through the whole surface. The roughness and thickness of tube structures usually increased with increasing voltage values and measured. The roughness and thickness values were measured as 10.67–111.97 nm and 0.21–1.92 μm, respectively. TiO₂ nanotube surfaces exhibited hydrophobic behaviors with respect to plain Ti surface. Furthermore, mechanical properties such as hardness and elastic modulus of the coating produced at minimum voltage were great compared to ones at higher voltage and plain Ti surface under a Berkovich indenter due to phase structure, homogeneity and density of nanotube structures.     

Electroluminescence properties of a PIN structure made by nearly stoichiometric a-SiCx:H active layer

a-SiC_x:H PIN diode has been fabricated within a single pump-down process under the same deposition conditions used for doped and undoped PECVD grown thin films, whose optical and electrical properties are determined and compared with a-Si:H. Current-voltage characteristics of PIN diode are evaluated and concluded to be limited by tunnelling of holes at p-i interface into valence band tail states. Electroluminescence measurements revealed radiative monomolecular recombinations. Deconvolution of the luminescence spectra is utilized to analyse recombination mechanism to be dominated by the transitions between band tails and deep states, which are created by the large density of both silicon and carbon dangling bonds, probable in the stoichiometric a-SiC_x:H film. Finally, a small luminescence peak around 1.9 eV would be an evidence of reduced probability of tail to tail transitions, than that of the transitions between tail and deep states.