The Impact Of Preparation, Modification And Structural Defects On Photoelectronic Properties Of Copper-based Multicomponent Thin Film Devices

In recent years, copper-based Cu(In,Ga)Se2(CIGS) thin film solar cells have attracted a lot of attention for the advantages in conversion efficiency, cost and radiation tolerance, etc. The enterprises usually use low cost sputtering method to prepare CIGS thin film in mass production. In this paper, the optoelectronic properties, recombination mechanism, distribution and uniformity of electronic defect states in CIGS solar cells prepared by sputtering have been investigated. The CIGS solar cells have been modificated by rapid thermal annealing(RTA) process and spectral modulation. The incident light spectrum was modulated by adding Er doped Zn O(Er:Zn O) thin films with the function of down conversion. The research methods and major results are summarized as follows:(1) The CIGS thin film, deposited by two-step sputtering, is the chalcopyrite structure for its(112) preferred orientation. The secondary ion mass spectrometry result suggests that the interaction and inter-diffusion between layers have taken place and the boundaries between layers are ambiguous. X-ray diffraction(XRD) result shows that the Cd Inx Sey compounds have been formed in Cd S/CIGS interface. The conversion efficiency of CIGS solar cell with small area of 0.46 cm2 is 12.2%. When the area of CIGS solar cell is greater than 1 cm2, the efficiency decreases with the area increasing.(2) The recombination mechanism and distribution of electronic defect states in CIGS solar cells are analyzed by photoluminescence(PL), electroluminescence(EL) and capacitance-voltage(C-V) characterization. The PL and EL are non-destructive methods to detect the signal of photons emitted by radiative transitions in materials. There are three main kinds of recombination mechanism in CIGS solar cells: the recombination through electronic defect states in CIGS thin films, the tunneling assisted recombination in spaced charged region and the Cd S/CIGS interface states recombination. The temperature dependent PL and EL results indicate that there are three types of defects in CIGS thin films: In Cu donor defects,(In Cu+VCu) defect-complexes and VSe donor defects. The In Cu and(In Cu+VCu) are deep leveldefects which are the non-radiative recombination centers at room temperature. The non-radiative recombination centers have a negative effect on photovoltaic device performance. The(In Cu+VCu) and VSe defect levels in CIGS band gap provide the channels for tunneling assisted recombination in spaced charged region. The calculation from C-V data shows that the Cd S/CIGS interface state density is 2.2×1013 cm-2.(3) The formula derivations of PL and EL intensities show that the different excitation ways(laser or electricity) have different vertical distribution regions of the excess carriers. Therefore, the vertical distribution of recombination and electronic defect states in CIGS solar cells can be analyzed by the combination of PL and EL. When the CIGS solar cells are excited by lasers, the excess carriers distribute in “penetration region” and “diffusion region”, and the distribution regions are wide, so the PL signal is mainly from radiative recombination inner in CIGS thin films. Under forward bias excitation, the electrons in Zn O film conduction band are injected into CIGS layer conduction band, and the distribution region of excess carriers is a narrow region near Cd S/CIGS interface, so EL is more sensitive to Cd S/CIGS interface recombination than PL. Moreover, AFORS-HET software simulation results show that the defects of CIGS thin film have larger effects on the short circuit current(JSC) than open circuit voltage(VOC) while the Cd S/CIGS interface recombination have larger influences on VOC of solar cells.(4) To analysis the three dimensional uniformity of electronic defect states, the mapping of optoelectronic performance, PL and EL characterizations are performed on CIGS solar cells. Over all, the large area CIGS solar cells, prepared by sputtering, have better device performance and uniformity in the center region than in the edge region. In the center region, the CIGS thin film has good uniformity and the defect states in Cd S/CIGS interface is slightly non-uniform, and the performance is that the difference in JSC and PL is very small while the difference in VOC and EL is larger. In the edge region of CIGS solar cells, the electronic defects in both CIGS thin films and Cd S/CIGS interface are non-uniform, so the detected luminescence spectrum and optoelectronic performances of different solar cells are quite different.(5) The continuous RTA processes are carried out to optimize the CIGS solar cells. After annealed 30 seconds at 400 °C under nitrogen atmosphere, the conversion efficiency of CIGS solar cells is increased by 43%. The improvement of CIGS device performance is ascribed to the reduction of defects in CIGS thin film and Cd S/CIGS interface. The reduction of these defect states reduces the recombination, leakage and parasitic resistance loss of CIGS device, which significantly increases the JSC, VOC and fill factor. The CIGS solar cells are damaged after 500 °C RTA treatment, causing drastic reduction in all optoelectronic parameters.(6) The Er:Zn O thin films with photon conversion function are prepared by the magnetron sputtering method at room temperature. To modulate the incident light spectrum, the Er:Zn O films are deposited on the surface of CIGS solar cells, which improve the utilization of the incident photon and JSC of CIGS-based devices. The optimum sputtering power and work pressure for Er:Zn O thin films are 140 W and 1.5 Pa, respectively. The thickness of Er:Zn O thin films is 184 nm for the best possible of CIGS solar cells. When we superimpose Er:Zn O thin film upon CIGS solar cells, the spectral response current is increased in the range of 500-700 nm, and the JSC is increased by 10%.