Research On Preparation And Theoretical Calculation Of Copper Indium Diselenide Thin Film Materials For Solar Cell Application

Since the beginning of 21st century, global-scale energy and environmental issues have become increasingly prominent. Solar energy has the advantages of inexhaustible and clean energy, thus is effective solution to energy shortages, environmental pollution and the greenhouse effect. Thin film solar cells combine the high conversion efficiency as compared with Si based solar cells and the advantage of low cost. Meanwhile, copper indium diselenide (CuInSe2, CIS) is considered as the most promising materials for thin film solar cell application, owing to its ability to adjust optical band gap, high optical absorption coefficient (~6×105/cm), strong anti-radiation characteristics and long-term stability. In this thesis, we focus on some fundamental issues of CIS thin film materials for solar cell application, including the experimental and theoretical calculation parts.In the experimental part, the preparation of CIS thin film by electrodeposition and selenization of sputtered Cu-In alloy precursors is studied, and the effects of key process parameters on the properties of CIS thin films are analyzed. In the electrodeposition process, the work mechanism and the control of performance of CIS thin films by process parameter are studied. Also, the influence of follow-up vacuum annealing on the crystallization, surface morphology and composition profile of electrodeposited CIS thin films is investigated. Furthermore, the effect of selenization temperature on the properties of CIS thin films prepared by selenization of sputtered Cu-In alloy precursors is studied, revealing the selenium incorporation, phase change and evolution of surface and cross-section morphology of CIS thin films during selenization process. The three stage growth model of CIS thin films is also proposed. Moreover, it is revealed that CIS thin films deposited onto MoNx thin film substrate exhibit strong preferred orientation along (112) plane, and the enhancement of preferred orientation is attributed to the reduced lattice mismatch between MoNx and CIS with increasing N2 partial pressure during reactive sputtering of MoNx thin films.In the theoretical calculation part, we employ first principles calculation based on density functional theory (DFT) to study some basic physics problems in CIS. First, the electronic structure modification of CIS induced by structural deformation and composition alchemy is studied, in which the structural deformation is related to distortion of lattice constants (a and c) and anion displacementμand composition alchemy is related to substitution of components with the same group elements. The energy level movements induced by structural deformation are integrated by alteration of chemical bonds, and the changes of lattice parameter and band gap induced by composition alchemy are demonstrated. Also, the defect physics in CIS are studied by calculating the formation energy of intrinsic and extrinsic defects to evaluate the most favorite defect mechanism. Two stable polytypes of ordered defect compound CuIn5Se8 derived from chalcopyrite and CuAu type CIS are first reported in this thesis. Moreover, the roles of sodium induced extrinsic point defect and defect pairs in CIS are explained. Another topic is to determine the band alignment between CIS and II-VI group compounds (i.e. CdS, CdSe, ZnO, ZnS and ZnSe). The valence band offsetΔEv and conduction band offsetΔEc are calculated by means of average electrostatic potental.This thesis's finds have positive significance to advance the large scale application of CIS thin film solar cells.