Optical polycrystalline thin films

Chemical vapor deposition [37,38], and thermal or anodic oxidation of Ti substrates [39,40,41] have been used to prepare polycrystalline thin films of Ti02. For example, thin films of Ti02 prepared by anodic oxidation of Ti, followed by electrodeposition of In20s from 0.5 M 102(504)3 show enhanced optical absorption up to 500 nm [42] with the In203 modified electrode showing enhanced photocurrent and photovoltage partially due to the low electrical resistance (10 Q) and reduced overvoltage of the photoanode. 

The optical propagation losses in the vacuum evaporated benzylic amide [2] catenane thin films, measured in planar waveguide configuration [47, 48] were found to be PL = 2.8 0.1 dB/cm at A = 1.32 (xm and PL = 4.0 0.1 dB/cm at A = 1.55 (xm, respectively. These values were determined by a two prism method [49]. As for polycrystalline thin films these value are significantly smaller han usually observed. It shows the ability of these molecules to form good optical quality thin films by using these technologically friendly technique. It shows also that the crystallites are very small, tens to a few hundreds of nanometers size. 

The substituted five-ring OPVs have been processed into polycrystalline thin films by vacuum deposition onto a substrate from the vapor phase. Optical absorption and photoluminescence of the films are significantly different from dilute solution spectra, which indicates that intermolecular interactions play an important role in the solid-state spectra. The molecular orientation and crystal domain size can be increased by thermal aimealing of the films. This control of the microstructure is essential for the use of such films in photonic devices. 

Fair and Forsyth [96] investigated the optical absorption in polycrystalline thin films prepared by the novel technique of exposing thin films of vacuum-deposited lead to gaseous HN3 in the presence of water vapor. The optical absorption spectrum taken at 15°K shows considerable structure (Figure 15). 

Dumas and Schlenker (1974) have studied Smi iNd,tSe (x<0.15) polycrystalline thin films obtained by thermal evaporation. At 290 K the samples are of S.C. type for x <0.11, and of M. type for x >0.11. Their optical absorption shows the presence of three peaks. Some resistance measurements are given versus temperature and composition. 

Spray pyrolysis technique has been used to deposit polycrystalline thin films comprising of CuInSa nanocrystals onto glass substrates. p-XRD studies demonstrate that the films have a chalcopyrite structure with preferred orientation along (112) lattice plane. Average diameter of the nanocrystals, as determined by SEM and TEM images, was found to be about 40-60 nm while band gap calculated through optical absorption studies was found to be 1.55 eV. A solar device fabricated by using these films demonstrated a power conversion efficiency of 7.60%. 

Nason et al. [20] reported that polycrystalline thin films (1-10 /am) of the nonlinear material 2-methyl-4-nitroaniline were deposited on Ag, Cu, and Si by conventional and ionized cluster beam deposition. The use of ions was found to promote a highly oriented film, with only a single x-ray diffraction orientation (112) being observed at reduced temperatures (-50 C). The effect of the ions on the physical microstructure of the films was to densify the film and reduce the size of the microcrystals. The films had powder-like optical properties and displayed a strong second harmonic from an N YAG laser. 

The most attractive feature of a-6T and a-8T crystals is undoubtedly the access that they provide to optical dichroism (absorption and luminescence) and charge transport anisotropy (carrier mobility) so that comparison can be made with what is currently observed in polycrystalline thin films and disordered polythiophenes. Beside a-6T and a-8T, a few studies have also been done on doped Q -4T(Q -Me)2 single crystals. 

Fig. 11.5 Typical film texture of polycrystalline thin films of a liquid crystalline terthiophene derivative, 8-TTP-8, observed by polarized optical microscope, atomic force microscope, and confocal laser microscope...

Fig. 11.6 Texture and surface profile of polycrystalline thin film of 8-Tp-BTBT (30 nm) observed by optical microscope and confocal laser microscope (upper left), and microscope textures of the films of 8-Tp-BTBT (upper right) and lO-BTBT-10 (bottom right) after heating at 150 °C for 5 min...

These model compounds can also be used in device fabrication, since thin films of appropriate thickness can be obtained by sublimation and subsequent deposition onto a substrate in vacuum. Electrical as well as optical properties of such devices have turned out to be strongly dependent on both the molecular packing within the crystallites and the polycrystalline morphology. Understanding and control of this aspect is one of the current scientific challenges. 

The potentiostatic electrodeposition of iron selenide thin films has been reported recently in aqueous baths of ferric chloride (FeCb) and Se02 onto stainless steel and fluorine-doped TO-glass substrates [193], The films were characterized as polycrystalline and rich in iron, containing in particular a monoclinic FesSea phase. Optical absorption studies showed the presence of direct transition with band gap energy of 1.23 eV. 

It was reported recently [216] that optical-quality PbTe thin films can be directly electrodeposited onto n-type Si(lOO) substrates, without an intermediate buffer layer, from an acidic (pH 1) lead acetate, tellurite, stirred solution at 20 °C. SEM, EDX, and XRD analyses showed that in optimal deposition conditions the films were uniform, compact, and stoichiometric, made of fine, 50-100 nm in size, crystallites of a polycrystalline cubic structure, with a composition of 51.2 at.% Pb and 48.8 at.% Te. According to optical measurements, the band gap of the films was 0.31 eV and of a direct transition. Cyclic voltammetry indicated that the electrodeposition occurred via an induced co-deposition mechanism. 

The optical properties of electrodeposited, polycrystalline CdTe have been found to be similar to those of single-crystal CdTe [257]. In 1982, Fulop et al. [258] reported the development of metal junction solar cells of high efficiency using thin film (4 p,m) n-type CdTe as absorber, electrodeposited from a typical acidic aqueous solution on metallic substrate (Cu, steel, Ni) and annealed in air at 300 °C. The cells were constructed using a Schottky barrier rectifying junction at the front surface (vacuum-deposited Au, Ni) and a (electrodeposited) Cd ohmic contact at the back. Passivation of the top surface (treatment with KOH and hydrazine) was seen to improve the photovoltaic properties of the rectifying junction. The best fabricated cell comprised an efficiency of 8.6% (AMI), open-circuit voltage of 0.723 V, short-circuit current of 18.7 mA cm, and a fill factor of 0.64. 

Fig. 3.19. Experimental (dotted lines) and best-model (solid lines) IRSE spectra of two polycrystalline Al-doped ZnO thin films grown by magnetron sputtering on metallized polyimide foil [43], The best-model free-charge-carrier concentration, optical mobility, and thickness parameters are indicated...

In MgO thin films, O vacancies can be generated by electron bombardment or sputtering with Ar. In polycrystalline MgO samples, O vacancies can be created by thermal treatment of hydroxylated surfaces. The dehydroxylation occurs at the expense of a lattice O anion with consequent formation of a surface vacancy or F center. Anion vacancies on the MgO surface, the centers corresponding to the removal of O, are difficult to observe, as these centers do not have specific spectral properties. However, they can act as electron traps and their existence can be deduced by doping the material with excess electrons to form the corresponding Fg and Fg centers which can be detected either by EPR (Fg" ") [130-133] or by optical (Fg+ and Fg) [134-137] spectroscopies. In the F+ or F centers one or two electrons are associated with the defect they are localized in the vacancy [39-43,50] by the MP. 

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