Key Engineering Materials Vol. 928

Abstract: Diamond-like carbon (DLC) is widely studied for various applications such as optoelectronics, energy, aerospace, and medicine. It’s hard, chemically inert, and optically transparent. Due to its superior antireflection properties, DLC films are more suited for photovoltaic technology. Here in this work, we report a facile, high speed, and low-cost method of DLC film development from an aqueous solution via electrodeposition. The effect of applied voltage and solution concentration on the properties of DLC film was analyzed. The morphology, shape, and uniformity of the DLCs were analyzed with optical and electron microscopies. The presence of C-H, C-C, and C=C bonds in the DLC films was confirmed from FTIR and Raman spectroscopies. Whereas the optical behavior was analyzed with a UV-Vis-NIR spectrophotometer. The DLC films were deposited at 2.7 V, 4V, 6V, 8V, and 10V, and it was shown that for a fixed electrolyte concentration and electrode spacing, the applied voltage can be adjusted to obtain varying deposition rates. Likewise, the solution concentration was varied in the 2 vol.% to 10 vol.%, and it was demonstrated that by increasing the solution concentration the deposition rate increases. The increase in the deposition rate was evidenced by an increase in the deposition current as well as the roughness of the films. It was noticed that smaller-sized, well-defined, and more uniform DLC films were obtained at lower concentrations and low voltage levels. The band gap was varied between 2.91ev to 3.39ev. It was clearly shown that reflection reduced remarkably after depositing DLC film on the substrate surface. This work demonstrates that DLC film has a potential to utilized as an antireflection layer in photovoltaic application.

Abstract: Among the second-generation solar cells, thin-film solar cells based on CIGS, CZTS and CdTe are well known due to their higher efficiencies, low cost and simple fabrication techniques. In this proposed work, we are committed to developing CZTS and ZnS thin films for second-generation thin-film photovoltaics. CZTS and ZnS have energy band gap of 1.4-1.5 eV and 3.6 eV, while their light absorption coefficients are 10⁴ cm^-1 and 3.3×10⁴ cm^-1 respectively. In thin film solar cells, absorber layer consists of photovoltaic material that should have optimal bandgap close to 1.34 eV. Moreover, a wide band gap window layer acts as p-n junction with absorber layer. In this work, we are reporting the synthesis of CZTS and ZnS thin films via wet chemistry route using spin coating method. Deposition of thin films on SLG substrate were carried out at specific process parameters such as spinning speed, concentration of precursors and annealing temperature to get the optimized thin film suitable for photovoltaic applications. Surface morphology and elemental compositional analysis investigated through SEM and EDX spectroscopy respectively. Electrical and optical properties were examined via Hall effect measurements and UV-VIS NIR spectrophotometry respectively. In addition micro-chemical and functional group analysis were conducted by FRIT spectroscopy. Keywords: Thin film PV, wet-chemistry, CZTS, ZnS

Abstract: Transparent conducting oxides (TCOs) are wide band gap semiconductors having found their use in optoelectronics, flexible electronics, flat panel displays, electrochromic windows, transparent heater windows, and many more. Aluminum (Al) doped zinc oxide (AZO) is an important TCO material which is being widely investigated for such applications. Its optoelectronic properties can be tuned by adjusting the Al content. In this work we study the variation patterns of the electrical conductivity and the optical transparency of AZO thin films with altering the Al content between 0 and 8 at%. The AZO thin films were prepared by wet chemical synthesis from its stabilized sol of zinc acetate dihydrate and aluminum nitrate nonahydrate dissolved in an ethanol and methanol mix. The morphological, electrical, and optical characteristics of these films were explored employing optical microscopy, Hall effect measurements, and UV-Vis-NIR spectrophotometry, respectively. We found out that annealing induces cracks into the AZO thin films and can severely degrade its electrical conductivity. Therefore, it’s imperative to control the Al content as well as the film morphology and structure. Before studying the effects of the Al content, the cracks were mitigated by optimizing the deposition and annealing conditions. The films were spin coated from its sol at 3000 RPM for 30 seconds. The films were dried at 100 °C and were subsequently annealed at 450°C. Since annealing induced cracks, therefore three coats were applied and annealed each time to mitigate the number of transverse cracks across the thickness of the film. The crack minimization was also confirmed by the enhancement in electrical conductivity. For the uniform crack-free AZO films, the Al doping was found to significantly modify the electronic behavior of the films. We expect an initial increase in the conductivity up to around 2 at% Al doping beyond which a decrease in conductivity is expected due to Al₂O₃ formation.

Abstract: Titanium dioxide (TiO₂) has been exploited extensively as it shows remarkable performance in photocatalytic applications. TiO₂ thin films can be deposited onto window glass which is workable for self-cleaning applications. In this article, we have studied the role of substrate temperature for spray pyrolysis (SP) of TiO₂ thin films for studying self-cleaning applications. For thin film deposition, TiO₂ sol is prepared by the sol-gel synthesis technique. The samples are deposited at room temperature and 250 °C, respectively. The samples are characterized via Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and UV-Vis spectroscopy to determine the morphology, surface roughness, and optical properties of the thin films. SEM and AFM results show that samples deposited at 250 °C (pre-heated samples) have uniform size distribution, and defined grain boundaries, respectively. The results also show that the pre-heated sample is highly transparent in the visible region of the solar spectrum when analyzed by the steady-state UV-Vis spectrophotometer. The wettability of the prepared thin films is measured, and the results show that the pre-heated sample shows a hydrophilic character. The self-cleaning property of prepared thin films is evaluated by the photodegradation of Rhodamine B dye. It is observed that the pre-heated substrates show better photoactivity in presence of UV light irradiation. Hence, due to their hydrophilic nature and better photoactivity, these pre-heated thin films deposited by spray pyrolysis can be employed as efficient coatings for self-cleaning glass applications.

Abstract: The DFT calculations were performed to study the structural, electronic and optical properties of Si. The open source computer code, Quantum ESPRESSO used is for first-principles electronic structure modeling based on density functional theory. The optimized crystal lattice constant, band gap, density of states (DOS) and band structure were calculated. The calculated value of lattice constant was in good agreement with reported value. The band gap was calculated using three different pseudo potentials. The HSE hybrid functional provided best agreement of band gap calculations with literature values. The pseudo-dielectric functions was also calculated to estimate the optical properties including refractive indices, extinction coefficients, reflectivities and absorption coefficients in the spectral energy ranges from 0 eV to 10 eV.

Abstract: Dye-sensitizer is heart of dye-sensitized solar cell which is an electron donor when receiving irradiation on the surface. Dye can be classified as synthesized dye or natural dye. Natural dye can be obtained colourfully from various plants such as tomato, carrot, grapes, chili, and so on. In this work, three selected plants which are locally available were chosen for three different colours. Green colour was extracted from leaves of Tiliacora triandra (Ya-nang). Red colour was from flower of Hibiscus sabdariffa (Rosella), and purple colour was from fruits of Morus alba (Mulberry). They were dried and ground into powder and their colour was extracted using 95%-ethanol. The concentrations of solution were 5, 10, and 20 g of powder per 200 ml for solvent extraction of each colour. These three colours were tested for absorbance index indicating ability of light absorption. The absorbance index curves showed that Ya-nang in green colour gave the best absorbance index having several peaks at 458 and 675 nm wavelengths for the 20 g/200 ml solution. While Rosella in red colour showed the peak at 534 nm wavelength and Mulberry in purple colour gave peak at 666 nm wavelength. The characteristics from the absorbance index curves also revealed the type of colour pigment in the dye. It was found that Ya-nang in green dye, Rosella in red dye and Mulberry in purple dye contained “Chlorophyll”, “Anthocyanin” and “Betacyanin” pigments respectively. Moreover, the area under the curves obviously indicated that the area of the green colour from Ya-nang was higher than those of Rosella and Mulberry. This implied that Ya-nang had higher potential to be dye-sensitizer and would give better dye sensitized solar performance compared with Rosella and Mulberry.