Thermodynamic study on the properties of ferroelectric thin films

Jinbo Zhang

Supervisor: Yuxiang Zheng

Ferroelectric films are greatly important for various technological applications. One of the significant phenomena observed in ferroelectric thin films is the size effect. It has been shown that phase-transition temperature changes noticeably with decreasing film thickness. In addition, effective polarization and coercive field are also affected by the film thickness. Many experimental and theoretical studies have been devoted to this effect and result is that a surface layer may exist in the film, which is different from the interior due to the limitations of processing techniques and the interaction with substrate.

To clarify the influence of surface imperfections on the properties of ferroelectric thin films, a generalized Ginzburg–Landau–Devonshire (GLD) theory is used to study the properties of second-order phase transition in ferroelectric thin films sandwiched between two metal electrodes with a natural boundary condition. By taking into account the effect of the imperfect surface layer , the temperature and film-thickness dependence of the spontaneous polarization and the dielectric susceptibility are calculated.

It has been demonstrated that the depolarization field makes the polarization distribution more uniform. In addition, an asymmetric hysteresis loop can be obtained due to the existence of asymmetrical imperfect surface layers.

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Preparation of CuIn0.8Ga0.2Se2 Thin Films by Pulsed Laser Deposition and its Application in Fabricating Hybrid Solar Cells

Yu Zhao

Supervisor: Jiada Wu

Polycrystalline CuIn0.8Ga0.2Se2 (CIGS) thin films were deposited on ITO  substrates by pulsed laser deposition at different substrate temperatures. X-ray diffraction (XRD) patterns and Raman spectra showed that the CIGS films deposited at room temperature were amorphous and their crystallinity was improved with the substrate temperature increased from 200 to 500 ℃. The results of Raman spectroscopy and X-ray photoelectron spectroscopy indicated that the ODC model is also applicable to the CIGS thin films deposited by PLD. The optical absorption spectra showed that the absorption coefficients of the as-deposited CIGS thin films were all in the order of 105 cm-1 in a wavelength range of 300-900 nm. When the substrate temperature is 500 ℃,the conduction type of CIGS films translated from n type to p type.The CIGS thin films with improved electrical properties, i.e. carrier concentration of ~1018 cm-3 and carrier mobility of 100 cm2V-1S-1 were used to  fabricate hybrid solar cells in combination with organic materials PEDOT:PASS and PCBM .Hybrid solar cells device with structure of ITO/CIGS/PEDOT:PSS/PCBM/Al was fabricated by layer-by-layer process and the short-circuit current density (JSC), open-circuit voltage (Voc), fill factor (FF) and power conversion efficiency (PCE) are respectively 0.389 mA/cm2, 0.327V, 0.249and 0.033%.

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[3] M.C.Scharber, D. Muhlbacher, M. Koppe, P .Denk, A.J.Heeger and C.J.Bra

   bec,.Adv.Mater.2006,18,789.