Amorphous-nanocrystalline silicon thin films: structural and optical properties

Istražene su strukturne i optičke osobine tankih filmova amorfno-nanokristalnog silicija debljine 100-400 nm pripremljenih kemijskom depozicijom pojačanom radiofrekventnim izbojem u plazmi iz smjese silana i vodika. Promjenom parametara depozicije u odnosu na pripremu slojeva amorfnog silicija što uključuje povišenu snagu izboja (100-180 W/m2) i jako razrijeđenu smjesu silana sa vodikom (94-95 % udio vodika) dobiveni su slojevi a-nc-Si:H sa volumnim udjelom nanokristalne faze od potpuno amorfnog do 38 % udjela (Raman), sa raspodjelom veličine nanokristala čija se prosječna vrijednost kretala u rasponu od 2 do 9 nm i slijedila log-normalnu raspodjelu (HRTEM). Rast slojeva a-nc-Si:H određuju procesi depozicije i jetkanja tako da u početnoj fazi rasta nastaje potpuno amorfan sloj a s vremenom se kroz procese jetkanja stvaraju uređene jezgre iz kojih dalje rastu nanokristali. Tako su pripremljeni slojevi potpuno amorfni uz podlogu, a u smjeru prema površini udio i veličina nanokristala se povećava (GISAXS, GIWAXS). Sa povećanjem udjela nanokristala u amorfnoj matrici povećava se broj defekata na graničnim plohama nanokristala sa amorfnom matricom. Pokazano je da optička svojstva (koeficijent apsorpcije, širina optičkog procjepa) ovisi o volumnom udjelu i veličini nanokristala što je posljedica kvantnih efekata povezanih sa malim dimenzijama nanokristala. Kontroliranjem veličine i udjela nanokristala silicija uronjenih u amorfnu matricu, postignuta je varijacija u širini energijskog procjepa od vrijednosti karakteristične za amorfni silicij (1.7 eV) pa do 2.1 eV za uzorak sa najvećim udjelom nanokristala. Ugradnjom takvih slojeva u sunčeve ćelije postignut je plavi pomak maksimuma kvantne efikasnosti do Δλ = 20 nm u odnosu na amorfni silicij što je uz poboljšanu otpornost na degradaciju efikasnosti tijekom izlaganja svjetlu bitno za primjenu anc- Si:H u sunčevim ćelijama treće generacije.Structural and optical properties of amorphous-nanocrystalline silicon thin films, 100-400 nm thick, prepared by plasma-enhanced chemical-vapor deposition using silane and hydrogen mixture were investigated. By using increased discharge power (100-180 W/m2) and highly diluted mixture of silane and hydrogen (94-95% hydrogen), nanocrystalline silicon thin films with crystallinity from completely amorphous up to 38% percent and wide log-normal size distribution of nanocrystals with average size 2-9 nm were obtained. Thin film growth appears to be governed both by deposition and plasma etching forming completely amorphous layer in the beginning stages of deposition. During the etching process ordered centers for nanocrystals formation are formed. In that way the samples are completely amorphous close to substrate, and the crystallinity and size of nanocrystals are larger close to the surface. Defects density (mainly dangling bonds that are mostly concentrated at grain boundaries) increased with the volume contribution of nanocrystals. Optical properties (absorption coefficient and optical band gap) are highly correlated to the size and crystallinity of nanocrystals. That is result of quantum size effect. By varying size and volume fraction of silicon nanocrystals incorporated in amorphous matrix it were obtained samples with increased optical bandgap from 1.7 eV (value characteristic for a-Si:H) up to 2.1 eV for the most crystalline sample. Those samples are incorporated in p-i-n structure of thin film solar cells. It results in the blue shift of up to 20 nm in the quantum efficiency maximum, compared to the value characteristic for completely amorphous silicon

Novel, Simple and Low-Cost Preparation of Ba-Modified TiO2 Nanotubes for Diclofenac Degradation under UV/Vis Radiation

A novel low-cost synthesis of barium-modified TiO2 nanotube (TNT) arrays was used to obtain an immobilized photocatalyst for degradation of diclofenac. TNT arrays were prepared by electrochemical anodization of titanium thin films deposited on fluorine-doped tin oxide (FTO) coated glass by magnetron sputtering, ensuring transparency and immobilization of the nanotubes. The Ba-modifications were obtained by annealing solutions of Ba(OH)2 spin coated on top of TNT. Three different concentrations of Ba(OH)2 were used (12.5 mM, 25 mM and 50 mM). The crystalline structure, morphology and presence of Ba were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. Ba-modified TiO2 nanotubes (BTNT) were tested for photocatalytic degradation of diclofenac under UV/Vis radiation and it was proven that all of the Ba-modified samples showed an increase in photocatalytic activity with respect to the unmodified TNTs. The most efficient photocatalyst was the sample prepared with 25 mM Ba(OH)2 which showed 90% diclofenac degradation after 60 min. This result was in agreement with cyclic voltammetry measurements that showed the largest increase in photo-oxidation current densities for the same sample due to the increased generation of •OH radicals obtained by a more efficient photogenerated charge separation

Simulating the Performance of a Formamidinium Based Mixed Cation Lead Halide Perovskite Solar Cell

Abstract: With the aim of decreasing the number of experiments to obtain a perovskite solar cell (PSC) with maximum theoretical efficiency, in this paper, PSC performance was studied using the program solar cell capacitance simulator (SCAPS-1D). The PSC with the architecture ITO/TiO2/perovskite/spiro- MeOTAD/Au was investigated, while the selected perovskite was mixed cation Rb0.05Cs0.1FA0.85PbI3. The analysis was based on an experimentally prepared solar cell with a power conversion efficiency of ~7%. The PSC performance, verified by short-circuit current density (J sc ), open-circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE), was studied by optimization of the simulation parameters responsible for improvement of the cell operation. The optimized parameters were absorber layer thickness, doping, defect concentration and the influence of the resistivity (the net effect of ohmic loss, Rs and the leakage current loss represented by the resistivity, Rshunt). The results of SCAPS-1D simulations estimated the theoretical power conversion efficiency of 15% for our material. We have showed that the main contribution to improvement of solar cell efficiency comes with lowering ohmic resistivity of the cell as well as doping and defect concentration, because their concentration is proportional to recombination rate

Influence of RF excitation during pulsed laser deposition in oxygen atmosphere on the structural properties and luminescence of nanocrystalline ZnO:Al thin films

Thin ZnO:Al layers were deposited by pulsed laser deposition in vacuum and in oxygen atmosphere at gas pressures between 10 and 70 Pa and by applying radio-frequency (RF) plasma. Grazing incidence small angle x-ray scattering and grazing incidence x-ray diffraction (GIXRD) data showed that an increase in the oxygen pressure leads to an increase in the roughness, a decrease in the sample density, and changes in the size distribution of nanovoids. The nanocrystal sizes estimated from GIXRD were around 20 nm, while the sizes of the nanovoids increased from 1 to 2 nm with the oxygen pressure. The RF plasma mainly influenced the nanostructural properties and point defects dynamics. The photoluminescence consisted of three contributions, ultraviolet (UV), blue emission due to Zn vacancies, and red emission, which are related to an excess of oxygen. The RF excitation lowered the defect level related to blue emission and narrowed the UV luminescence peak, which indicates an improvement of the structural ordering. The observed influence of the deposition conditions on the film properties is discussed as a consequence of two main effects: the variation of the energy transfer from the laser plume to the growing film and changes in the growth chemistry