SnO <sub> 2 </ Sub> Sol Growth Theory And Thin-film Photovoltaic Performance

Transparent conductive oxides(TCO) are attributed in nature to degenerate n-type semiconductor with wide band gap, which compromises exquisitely the conductivity of metals and the transparency of glassy insulates, and inherits transparent insulates but resembles metals closely, so being ascribed to glassy metal. Among a large TCO family, The three oxides,In2O3, SnO2, ZnO, have acquired pervasive approval in commercial use, serving as transparent electrodes of electro-optic displays, chemical sensors, and viasistors respectively. Under increasing emergency of power resources all over the world, it is ultimately urgent for building efficient window use to develop TCO coated glasses with excellent performance of heat reflectance, therefore this thesis is devoted itself to exploring a technique route employed for such efficient building glasses of energy conservation: preparing antimony-doped tin oxide(ATO) films by Sol-Gel processing(S-G), a wet chemical route, its contents focus on modeling S-G process and characterizing electrical and optical properties of the films, along with such a route of experimentation, testing characterization, modeling and establishing relevant theory.The research and application status of TCO materials across past several decades have been reviewed in this thesis through fields of transparent electrode, heat-reflection glass, preparation and characterization of nano-structures. By putting attention concentrated on the stability of sol solutions and preparation and characterization of heavy Sb doped SnO2 films, five aspects of researches have been done in depth in the thesis:sol solution chemistry, rheology, light scattering, drying and solidification of coated gel films and characterization of electrical and optical properties of final films. For sol solution stability, reaction chemistry of hydrolysis and condensation of precursor monomers has been studed based on control of reaction activity of precursor Sn(OBun)4 by acetylacetone (acacH), and the influences of water and catalyst addition ratios on the stability of sol solution. Further efforts concentrating on observing dynamic variations of shear viscosityηand intrinsic viscosity [η] with SnO2 sol aging time, hereby the dynamic variations of average size Rz and molecule mass M of sol aggregates with aging time were observed indirectively, from which much of microscopic information of aggregate fractal feature and growth dynamics were acquired.The third,making measures of Small Angle X-ray Scattering (SAXS) and Dynamic Light Scattering(DS, or Photo Correlation Spectrum:PCS) for sol solutions at different aging time before and after sol-gelling transition, taking photon as a probe to probe into sol aggregates in the size range of 1～2000nm with respect to the size distributions, configurations, organization architecture, self-similarity, and diffusion in solution, and then combining together with rheological measures and growth theory of molecule reaction/aggregation, growth processes were reasonably modeled for SnO2 sol aggregates, which funded the microscopic theory of SnO2 sol solution stability. The fourth wet gel films developed from the tin-alkyoxide-derived sol-gel dip-coating route will transform to porous films under drying and solidification procedure, by use of DTA and TG heat analysis technologies, XRD and BET structure analysis techniques, the restructure process of wet gel films during drying and solidification procedure referred to such as desorption of the absorbed, reconfiguration and crystallization of gel skeleton was investigated and modeled. Finally electrical and optical performance was characterized by means of measuring film square resistance, Hall resistance, carrier mobility, transparency/reflectance spectrum and film thickness for obtained SnO2 films, the influences of Sb dopant amount and heat treatment temperature on conductivity, carrier concentration and mobility of Sb-doped SnO2 was hereby pronounced and modeled. Hereto a number of original results of research related to technique method, growth dynamic process of sol aggregates, electronic band structures, electrical and optical properties and characterization technologies can take into conclusions.Process Approach It is shown from experimental surveys and the characterization of electrical and optical attributes that a transparent conductive porous SnO2:Sb film can be derived by S-G Dip-Coating route, which exhibits excellent electrical and optical performance of resistivity of p～10-2～-3 Q.cm, transparency in visible spectrum region of 85～90% and reflectance of 10～15%, n-carrier mobility of 10～80 cm2/v.s and carrier concentration of～1020cm～3,which are availably for efficient window glasses. From the view of craft, it has been perceived to control stability of sol solution for dip-coating, and sol aggregates growth process has been modeled reasonably, whose measuring method can be employed to test on line for state and performance of sol coating solutions. The incisive investigations on S-G processing lay a solid foundation for large area production of SnO2-coated transparent conductive oxides glasses.Porous Structures of Sb Doped SnO2 films Films derived from S—G Processing is proved nano-porous structures with relative density p/ps approximately of 0.7～0.8. Its carrier mobility is controlled by nano-skeleton scattering mechanism and impurity scattering mechanism together, film resistivity is larger 101～2 than that of dense films. For promotion of film conductivity it is key research aspect to improve its densification and to coarsen film skeletons. Wet gelling films are paved by loose aggregates of fractal structures, under drying and solidification at temperature between 20～700℃, SnO2 gels undergo a series of re-buildups but its nanoporous non-fractal structures keep invariant. The fractal structures of gelling films derived from sol-gel processing have been violated upon drying and solidifying processing, and solidified structures of films present non-fractal.As heat treatment route proceeds, the solvents and water absorbed on skeleton of gel desorb and volatilize away subsequently between 20～200℃,-acac chelates dissociate at 320～340℃,so as to make way for the relaxation of gel skeletons at 380～390℃, and the gel skeletons crystallize at 420～440℃into rutile phase. Above 550℃, skeletons become viscid and present creep, further restructuring. During the heat treatment process, the diameters of skeletons thicken from 2-3nm to 5-14nm, the pore sizes increase from lnm to 13nm, but the aspect ratio of skeletons keeps about 0.29～0.32 unchanged, the relative density p/ps of gels does not change substantially. Heat treatment processing alters largely the skeleton structures, cause to huge varieties of electricity of films. The film carrier concentration enhances by 26 times, and its resistivity reduces 4 number of grades when solidification temperature rises from 400 to 500℃. Heat treatment may actuate diffusion of Na+, K+ions into TCO layer to degrade the its performance.The sol stability mechanisms By incorporation of acacH to control hydrolysis reaction activity of monomer [Sn(OBun)]4,it is proposed that the monomer is attacked by nucleophilic substitution of accH to form chelated intermediate Sn(OBun)2(acac)2, which is responsible for the huge reduction of activity of initial monomer and make quite stable the sol system. After formation of the chelated intermediate, the functionality for hydrolysis of initial monomer reduces from 4 to 2, leading to the formation of quasi-linear chain polymers undergoing hydrolysis and condensation reactions. The chelated intermediate Sn(OBun)2(acac)2 is attacked by extra water, radicals of acid or ammonia incorporated as catalysts at-acac chelating ring by nucleophilic substitution, causing the branching of stem chains and further the curling and rolling of them. On the other hand,the substitution of-acac chelating rings increases the hydrolysis functionality of chelated intermediates surpassing over 2, which enhances activity of the intermediates violently and the growth pace of sol clusters, influencing mostly and even violating the sol stability and the configurations of sol clusters and aggregates. The more over 2 added water ratio is, the more violent the stem chain branches, sol system precipitates in five minute at added water ratio of 3. in such case, the growth model of sol clusters changes completely. As the same mechanism as the water addition, the radical ions of acids and ammonia catalyze and accelerate hydrolysis and condensation reactions in sol process. For the introduction of strong electron donors (Lewis strong base) such as F-ions, the sol aggregates precipitate in five minute.The Growth Process of Aggregates during Sol→Gel It has been shown from the surveys of small angle x-ray scattering(SAXS) and dynamic light scattering(DS) that monomers condense into primitive sol clusters by hydrolysis and coalescence at initial stage, when the clusters build up to a certain critical size of Rc≈8nm, clusters contact each other and cease to grow. These primitive clusters exist in sol solution though sol-gel process before gelling and after gelling.Sol clusters neighboring each other joint into a hierarchy of sol aggregates abiding by self-similar construction as the initial clusters contact each other, whose sizes list in order: Rc,R2,R3,R4,R5R6,R7..., the size of later generation is larger a constant times than that of last generation, depending on the specific way smaller aggregates build up larger aggregates. Hierarchy of sol aggregates constructs itself from primitive clusters across macro-size aggregates up to gelling, the configuration manner of any generation is similar to that of other generation, only different from their sizes. In the case of SnO2 sol system, DS surveys demonstrate the existence of a hierarchy of five generations of sol aggregates with Rn/Rn-1≈4-6.Near gelling, the hierarchy of sol aggregates coalesce raptly into infinite large aggregates of macroscopic size by jointing neighboringly and self-similarly bond-linking style, and sol system undergoes sol→gel transition into gelling bulk. Upon gelling bond-linkings take place principally among large aggregates, and the way aggregates coalesce change with respect to sol growth process, following percolation-linkage with fractal dimension D= 2.5, aggregates coalesce into space wetworks, average size Rz of sol aggregates tends to diverge as follows Rz∝(|1-t/tg|)-v.sol→gel transition belongs to a critical transition of percolation.As sol-gel route proceeds, the survivals of monomers in solution attack the surface of sol primitive aggregates, and condense onto it, its surface tends smooth and surficial dimension approach to 2.0. At the same time, the primitive clusters diffuse ceaselessly in solution, and diffuse onto the surfaces and percolate into interiors of large aggregates, derive them to reconstruct, thus the surfaces of large aggregates become smooth gradually. However, the reconstruction of small and large aggregates can also attribute to slow aggregation style, and contribute to the exponential growth of sol aggregates with aging time.Fractal configurations of sol aggregates The experimental data of dynamical relations of intrinsic viscosity of sol solutions with aging time [η]-t is in good agree with Flory-Bechtold's quasi-linear chain model ln[η]=(lnA+αlnΩ)+a ln[x/(1-x)] (x =t/tg),which verifys the proposal that the sol aggregates of SnO2 sol system belong to quasi-linear polymer chains. The fractal dimensions of such aggregates of linear chain are approximately 1.8, intervening between self-avoid random walk chain (SAW, D=5/3) and Guassian random walk chain(D=2.0). The fractal configurations and dimensions of SnO2 sol aggregates may be controlled by an adequate control of incorporation of water and catalysts. Before gelling, both primitive clusters formed in early stage by monomer reactions and hierarchy of aggregates constituted in later stage, keep the same fractal configuration though whole sol-gelling process.Slow Aggregation Growth Model (Exponential Growth) The sol aggregates in SnO2 sol solution build up in style of the well-known slow aggregation before gelling. The experimental surveys show that shear viscosityηof sol solution satisfies the law of exponential variations with aging time:η∝exp[λ't], that is, Lnη-t keeps linear relation, the average sizes of sol aggregates meet exponential relation with aging time t:Rz～eλt. In quite later stage, sol growth transits into gelling style of bond-linking percolation. The existence of hierarchy of aggregates confirms that the growth of sol aggregates is controlled by Oswald Ripening mechanism (small cluster—large cluster aggregation) and diffusion limited cluster aggregation(DLCA) growth model,whose fractal dimension of aggregates is approximately 1.8.Characterization Method Rheological dynamic measure may be used to characterize in situ the growth process of sol aggregates with availability and rapidity for liquid sol systems. The shear viscosityη, and intrinsic viscosity [η] are related to average size Rz and mass M of sol aggregates, rheological dynamic measures describe size and mass dynamical variations of sol aggregates with sol—gel process:Rz-t, M-t. Rheological dynamic characterization method, combining with dynamic light scattering (DS) developed recent years and other light scattering techniques, and under the theoretical frame of growth of molecule reaction/aggregation, may provides a lots of profound information of dynamics and structures related to geometrical configuration, growth mechanisms, fractal structure, growth mechanism of sol aggregates.Two novel research results have been developed in the thesis. Firstly, a film-coating processing method for large scale production of heavy doped SnO2 coated glasses with low loss, Sol—Gel processing derived from alkyoxide, is developed up, in which sol coating solutions keep stable over 1000h with process working model being setup, prepared TCO films have excellent performance suit for efficient building windows. The second original result is introduction of a characterization method for liquid sol solution, Rheological Measuring method, which may be employed for aggregates of sol solutions to measure dynamic variations of average size Rz and mass M with time:Rz-t, M-t, and can acquire dynamical information in deep insight. Its setup is simple and measure is easily operated, has practical availability for on-line testing. Combining with light scattering measure means, Rheological Measuring method is a good measure mean of characterization of growth and structures of sol aggregates with self-similarity for liquid sol solutions.