Ionic Liquid Electrodeposition And Performance Of CIGS Thin Films

Cu_1.00In_xGa_1-xSe_2.00（CIGS） thin film solar cel s have a wide application prospect for semiconductor thin film batteries. CIGS absorbing layer is the key of the thin film solar cell material. Electrodeposition has attracted significant interests to prepare CIGS thin films due to its easy operation, simple equipment and controlled continuous production. Ionic liquids are thermodynamic stable and with wide electrochemical window when ionic liquids are used as electrolytes it is easier to get a stable sedimentary CIGS thin film. Therefore, the research of new ionic liquid electrodeposition process of CIGS thin film has very important theoretical and practical significance. In this thesis, the electrodeposition technological parameters and semiconductor performance of CIGS thin films from ionic liquid were studied in detail and multiple metal deposition mechanism of ionic liquid electrodeposition was investigated.According to the characteristics of Cu, In, Ga, Se and the nature of the ionic liquid, 1-butyl-3-methylimidazolium trifluoromethanesulfonate（[BMIm][Tf O]） ionic liquid was chosen as electrolyte solution for preparing CIGS thin films via electrodeposition method. The surface morphologies, composition and structural properties of the obtained CIGS thin films were determined by SEM, TEM, ICP, XRD and Raman. The stoichiometric composition of the electrodeposition CIGS thin film was Cu_1.00In_0.83Ga_0.23Se_1.64 and the surface microscopy was uniform and denserstructure. The crystal ine phase of CIGS film was proved as chalcopyrite structure and mainly preferential growth along the（112） plane and the lattice distance of（112） plane was 0.328 nm. The photoelectric properties of CIGS thin films were tested and evaluated via UV-visible absorption spectra and Hall effect measurement. The results showed that the obtained CIGS thin film was a p-type semiconductor, the band gap was approximately 1.55 e V and the carrier concentration and Hall coefficient of the CIGS thin films were 2.74×1020 cm^-3 and 2.28×10^-2 cm³·C^-1, respectively.Single ionic liquid used as electrolytes showed relative defects of large viscosity and low conductivity, therefore cosolvent system was studied. Propyl alcohol was final y choosen as cosolvent for [BMIm][Tf O]- alcohol mixed electrolyte system. [BMIm][Tf O]- 30 Vol% propyl alcohol mixed electrolyte solution was used to prepare CIGS thin films via a constant potential electrodeposition method. The composition, surface morphologies, structural and photoelectric properties of the obtained CIGS thin films were determined by ICP, SEM, TEM, XRD, Raman, UV-Vis and Hall Effect. The obtained CIGS thin film was a p-type semiconductor, the surface morphologies of the obtained Cu_1.00In_0.75Ga_0.10Se_1.72 thin films were evenly and compact. CIGS thin films were chalcopyrite structure and exhibited higher crystal ization degree and more orderly crystal growth along the（112） plane with the lattice distance of 0.340 nm. The band gap of the CIGS thin film is 1.48 e V, the carrier concentration and average Hall coefficient of the CIGS thin films are 6.37×10¹⁹ cm^-3 and 9.81×10^-2 cm³·C^-1, respectively. Compared with the carrier concentration of ionic liquid system of CIGS thin film electrodeposition, the carrier concentration of ionic liquid [BMIm] [Tf O]-30 Vol % propyl alcohol mixed system is smaller.The surface morphologies of CIGS thin films obtained by constant potential electrodeposition method showed poor uniformity and crystal grain size due to the existence of a large number of clusters structure. Therefore, pulse electrodeposition was studied. Pulse electrodeposition process parameters with a peak current density of 2m A·cm^-2, duty ratio of 50% and frequency of 2k Hz were chosen for preparing of CIGS thin films. The CIGS thin films were prepared by using pluse electrodeposition method in [BMIm][Tf O]- 30 Vol% propyl alcohol mixed electrolyte solution. The obtained CIGS thin film showed as a p-type semiconductor and the uniformity of the obtained Cu_1.00In_0.51Ga_0.10Se_2.04 thin films was improved and grain size was smal. CIGS thin films were mechanical mixed chalcopyrite structure compounds and no impurity phase was found and the lattice distance of（112） plane was 0.328 nm. The band gap of the CIGS thin films was 1.35 e V and the carrier concentration and average Hall coefficient of the CIGS thin films are 1.64×1022 cm^-3 and 3.81×10^-4 cm³·C^-1, respectively.The electrochemical behaviors and electrocrystallization performance of Cu²⁺, In³⁺, Ga³⁺ and Se⁴⁺ ions on glassy carbon electrode were investigated owing to the complex co-depositon mechanism of quaternary system in ionic liquid. The Cu-InGa-Se co-deposition mechanism was studied by examining the cyclic voltammograms combining with I^t curves under different step potentials. Cyclic voltammograms of unitary systems, binary systems, ternary systems and quaternary systems were studied in ionic liquid [BMIm][Tf O] electrolyte solution. The results showed that the reduction reaction of Cu²⁺ occurred firstly, followed by the inductive codeposition of Se onto Cu and the deposition of In and Ga occurred on the Se at last. I^t curves of ionic liquid [BMIm][Tf O] and [BMIm][Tf O]-30 Vol% propyl alcohol mixed electrolyte system demonstrated that electrocrystallization process of several kinds of Cu²⁺, In³⁺, Ga³⁺, Se⁴⁺ ions on glassy carbon electrode in ionic liquid [BMIm][Tf O] electrolyte system was controlled by diffusion, and the growth process was consistant with the three-dimensional instantaneous nucleation mechanism. The electrocrystallization process of several kinds of Cu²⁺, In³⁺, Ga³⁺, Se⁴⁺ ions on glassy carbon electrode in ionic liquid [BMIm][Tf O]-30 Vol% propyl alcohol mixed electrolyte system was controlled by diffusion, and the growth process was firstly consistant with the three-dimensional continuous nucleation mechanism, and then charged into instantaneous nucleation mechanism as the deposition potential shifted to negative.