Orderly Growth Of Six Thiophene Thin Films And Their Application To Solar Cells, Flexible Substrate

Recently,great importance has been attached to the application of organic semiconductors in microelectronic and optoelectronic devices due to its potential low cost,low weight,flexibility,and easy processing. Sexithiophene(6T)is one of the organic semiconductors with the best holes transport mobility,thin film of which is promising candidate as semiconducting layer for organic thin film transistors,as holes transport layer for orgainc light emitting diodes,or as donor for organic thin film solar cells.Optimization of related film deposition processes and devices is critical.Through optimizing evaporation conditions such as substrate temperature and growth rate detected by in situ quartz crystal microbalance measurement,well-ordered sexithiophene films were deposited on both flexible insulating polyvinylpyrrolidone(PVP)layers and flexible transparent conductive indium tin oxide(ITO)layers.X-ray diffraction (XRD)investigations showed that too low or too high growth rate led to declined film crystallinity.On PVP substrate,well-ordered sexithiophene film was deposited at growth rate 10nm/min and substrate temperature 90℃.The main axis of sexithiophene molecules was found parallel to PVP substrate,and thin film crystallinity declined with the decrease of substrate temperature.Scanning electron microscope(SEM)investigations indicated that sexithiophene film deposited at room temperature was constituted of stalk grains with 10nm magnitude width and 100nm magnitude length, while sexithiophene film deposited at higher temperature was constituted of flake grains with size of 1μm magnitude.The latter was obviously crystalized better than the former,which was consistent with the XRD results.On ITO substrate,well-ordered sexithiophene film was deposited at growth rate 10nm/min and substrate temperature 50℃.Substrate temperature notably influenced molecule orientation of sexithiophene on ITO layers.At room temperature there existed an inclined angle between the main axis of sexithiophene molecules and ITO substrate surface.With the increase of substrate temperature,the main axis tended to parallel to substrate surface,and the UV/Vis absorption peak at 514nm occured redshift.Flexible thin film solar cells based on 6T/C₆₀heterojunction were vacuum deposited on ITO substrates based on the previously optimized preparation process of sexithiophene films.The thickness of 6T layers with the planar heterojunction devices ITO/6T/C₆₀/Al was varied from 20nm to 40nm without changing the 60nm thick C₆₀layer,and the optimum photovoltaic properties were obtained at 30nm with short circuit current (Isc)of 0.380 mA·cm^-2,open circuit voltage(Voc)of 0.330V,fill factor (FF)of 0.41,and photovoltaic conversion efficiency(η)of 0.51%. Introduction of a 6nm thick CuPc layer between C₆₀and Al layers resulted in higher FF andη.Single source evaporation was applied to deposit the bulk heterojunction thin film 6T:C₆₀,and the planar-mixed heterojunction device with structure of IYO/6T(20nm)/6T:C₆₀(20nm)/C₆₀(50nm)/Al performed Isc of 0.405 mA·cm^-2,Voc of 0.364V,FF of 0.39,andηof 0.58%.