Influence Of Doping And Solvents On The Morphologies And Properties Of ZnO Films Deposited By AACVD

Transparent-conductive ZnO thin films have shown great application prospect inphotoelectricity field and have drawn a great deal of research attention, due to its low-cost,nontoxicity, excellent photoelectric properties and stability. Researches mostly focus onimproving its transmittance, conduction, surface morphology and cost reduction. Recently,synthesizing ZnO thin film with napped light trapping structure along (110) growthorientation has became an emphasis and difficulty for improving light absorptionefficiency of film solar cells.Traditional chemical vapor deposition technique was attached with an ultrasonicatomizer apparatus. ZnO thin films were successfully synthesized by the aerosol-assistedchemical vapor deposition technique. This dissertation mainly studied the influence ofsolvent and doping on the optical and electrical performance of transparent-conductiveZnO thin films. The influence of solvent species, properties and the introducing of H2Ointo the solvents on the deposition mechanism, doping types (Al3+cation n-type dopingand F-anion n-type doping; single element doping and codoping) and concentration on thefilm performance, and precursor solution pH on the behavior of H that doped into thefilms were systematically studied, for the purpose of exploring direct synthesizationmethod of ZnO thin films with high transmittance in visible range, low resistance andnapped structure from nontoxic and low cost solvent, without the use of activator,stabilizer, template and post treatment.The paper firstly discussed the effects of organic solvents methanol, ethanol,2-methoxyethanol, isopropanol and methanol/H2O mixture solvents. The results revealedthat, as the carrier of precursor, solvents act on the film performance by varing theviscosity and surface tension. The precursor solution with higher viscosity has a loweratomization rate, resulting in slow deposition. Film growth pattern, grain size distribution,agglomerating-separating degree and preferred growth plane vary for the samplesdeposited at different rates. The H2O introduced into the organic solvents influences on theoriginal droplet size and mass by changing the surface tension of precursor solutions. Thenthe behavior of precursors within the aerosol droplets is affected during the film formation, as well as the grain morphologies and film thicknesses. Due to the poor depositionefficiency, film can not be synthesized from the precursor solution with excess surfacetension. The average visible transmittance can be improved up to over90%by changingsolvent species and volume fraction of H2O in the organic solvents.The ZnO thin films were synthesized with different degree of Al doping, F dopingand Al-F codoping. The results indicated that the films deposited from the precursorsolutions with4at.%and5at.%Al3+concentration exhibit visible transmittance of over90%and relative lower resistance, comparing with undoped samples. Proper amount of Fdoping induces the napped light trapping structure at some area of the ZnO films, loweringthe sheet resistance significantly at the same time. However, visible transmittance isnotablely decreased. The sheet resistance of sample deposited from the precursor solutionwith4at.%F-concentration can be decreased to272/□, while the average transmittancein visible range is only81.7%. The cooperative doping effect can be achieved by Al-Fcodoping. The doping efficiency for both the impurity elements was enhanced. For thesamples deposited from the precursor solutions with Al:F:Zn=2.5:2.5:100-2:3:100(at.%),the donor defects density is increased, leading to the marked decrease of sheet resistanceto＜100/□, with average transmittance in visible range of＞90%. The relativehomogeneous napped structure along (110) direction is obtained as Al:F:Zn=2:3:100(at.%). Besides, green emissions at548nm are observed as Al:F:Zn=3:2:100-1:4:100(at.%) in the precursor solutions.The influence of pH in precursor solution on the Al doped ZnO film performance wasalso studied. The concentration-dependent behavior of H in the films is mainly affected bythe pH. The impurity H of different concentration results in different kinds and amount ofdefects which improve and reduce the films conduction to a certain extent. Combining thepH, volume fraction of H2O in the solvents and Al-F codoping concentration, orthogonaltest was carried out. Finally, the optimized deposition parameters were determined forsynthesizing ZnO thin films with excellent conduction, visible transmittance and nappedlight trapping structure.