Investigation Of Small Molecule Organic Solar Cells

Organic solar cell is a new type cell and has become a hotspot all over the world now. Compared with inorganic cells,organic solar cells have lots of advantages,such as light weight,low cost,mechanical flexibility,large area fabrication,etc. These will definitely decrease the cost of photovoltaic industry. Organic cells will be an effective complementarity of the photovoltaic area, and will promote the economic development.In this paper,the emphasis is focused on the study of small molecule organic solar cells based on CuPc/C60. The preparations of small molecule cells,such as the design of ITO electrodes,the fabrication of the electrodes,the pretreatment of the ITO surface,the techniques of vacuum evaporation of the organic materials and metallic cathode were introduced. Single layer Schottky cells and bilayer heterojunction cells were fabricated,using CuPc as a donor and C60 as an acceptor. CuPc forms a Schottky junction with Al. The excitons generate in the CuPc layer,and dissociate to electrons and holes near the interface of CuPc/Al. CuPc forms a heterojunction with C60. The excitons generate both in CuPc and C60 layer,and dissociate at the interface of CuPc/C60. Either ITO/CuPc/Al cells or ITO/CuPc/C60/Al cells show a few abilities of power conversion. However,the short-circuit current density,fill factor and power conversion efficiency of the bilayer heterojunction cells are much better than that of the single layer Schottky cells,due to the broader spectrum response,lower density of the interface-state,lower recombination probability of the carriers and excitons.In the theory chapter of this paper,the optical propagation matrixes at the interface of two different films and inside a single thin film were calculated. Then the intensity distribution inside CuPc/C60 cells was analyzed. The results of the intensity distribution inside the cell of ITO(220nm)/CuPc(20nm)/C60(40nm)/Al(100nm) were given out,in which the light wavelengths used were 630nm and 450nm,respectively. The relationship between the quantum yield of the effective excitons and the thicknesses of CuPc and C60 was obtained. The optimum structure shows as ITO(220nm)/CuPc(15nm)/C60(40nm)/ Al(100nm). This result has been validated by amount of experiments. The experimental result is well coincident with the theoretical result at the thickness of C60. However,there is an error of 5nm at the thickness of CuPc. This error should be accepted since there are many assumptions and predigestions in the analysis process. This can be said that the methods and predigestions in the analysis are reasonable.An ultra-thin LiF layer was introduced between the C60 film and the Al cathode. The performances of the cells are improved a lot. The ultra-thin LiF layer doesn't change the light intensity distribution inside the cells evidently. But the electrical characteristics of the cells have been changed. The functions of LiF film are explained by MIS structure in this paper. It is an exciton block layer,as well as an electron transmission layer. It also prevents the C60 film from the destruction of Al atoms with high energy. The optimum thickness (1.5nm) of LiF buffer layer was obtained via amount of experiments. If the buffer layer is too thin,it couldn't block the excitons effectively. But instead it would block the electrons if it is too thick.The stability of the cells based on CuPc/C60 was studied,including the CuPc/C60 cells with and without LiF buffer layer. The suggestion of encapsulation time was given in this paper. For a CuPc/C60 cell without a LiF buffer layer,the performances will improve a little after placing in the clean air for a while. That is because an ultra-thin Al2O3 is formed between C60 and Al. Its function is a cathode buffer layer like LiF. However with the increase of the thickness of Al2O3,the performances of the devices decline. So the encapsulation time we suggested is about an hour later after fabrication. When a LiF buffer layer is added,the Al2O3 between LiF and Al would increase the thickness of the insulation layer directly. Thus the cells with a LiF layer will attenuate directly after fabrication without encapsulation. In addition,the compactness of LiF is worse than that of Al2O3. So the attenuation speed of the cells with a LiF layer is larger than those without LiF. Thus we suggest that the cells with a LiF buffer layer should be encapsulated immediately after fabricationSince the limitation of the equipments,the performances of the cells in the experiments are not very well. So the emphasis of this paper is to explain some physical processes of the devices,and to give some suggestions.