Synthesis And Photoelectric Characteristics Of CdSe/TiO₂ Heterojunction Thin Films

Human beings demand for energy more and more with rapid economic development. But fossil resources are limited on earth, and will be faced to dry up. Utilization and development of new energy resources have been a major subject of present world. People are paying more and more attentions to solar cell prepared by TiO2. However, the large band gap of TiO2(anatase 3.2 eV,rutile 3.0 e V) can only response to ultraviolet light, electron hole pairs of TiO2 are particularly easy to recombine, which restrict extensive application of TiO2 in solar cell filed. Therefore, in order to improve TiO2 photoelectric properties,this paper studied the influence of CdSe as semiconductor composite to improve TiO2 photoelectric characteristics.(1) TiO2 nanorod array thin films(TNRs) were deposited on FTO glass by using hydrothermal method, with butyl titanate as titanium source and concentrated hydrochloric acid as hydrolysis inhibitor. The influence of titanium source concentration, acidity, reaction time and temperature was investigated.The optimum conditions are: 1 mL of butyl titanate, 30 mL of deionized water and 30 mL concentrated hydrochloric acid, 150 ~ 180℃, 18 h. Under theseconditions, rutile TNRs with a mean size of 120 nm, perpendicular to substrate and well dispersed, were prepared.(2) CdSe thin films were electrodeposited on ITO glass by using cyclic voltammetry. The crystal structure, morphology, optical and photoelectric properties of CdSe thin films prepared with different temperatures were characterized by X-ray diffraction(XRD), field emission scanning eletron microscopy(FESEM), atomic force microscopy(AFM), UV-Vis spectrophotometry and electrochemical. The results show that the samples, with smooth surface and uniform thickness, consist of nanoparticles with zinc blende phase. The size of CdSe nanoparticles grows as the annealing temperature increases. Thus, the absorption peak shows red shift and the band gap gradually decreases, as a result of quantum size effect. Photocurrent test of the samples shows obvious photoelectric response under visible light illumination, and the photocurrent density of the samples increases as the annealing temperature increases. The samples annealed at 350℃ has a smaller energy gap than as-prepared samples, decreasing from 1.88 to 1.75 eV, and correspondingly photocurrent density increases from 2.76 to 7.03 mA·cm-2.(3) With CdCl2·5H2O and Se O2 as raw material, CdSe/TiO2 heterojunction thin films were electrodeposited on TNRs by using cyclic voltammetry. The influence of concentration of Se O2, electrodeposition time, annealing temperature and time on CdSe/TiO2 heterojunction thin films’ morphology,structure and photoelectric properties was studied. The results show thatCdSe/TiO2 heterojunction thin films prepared with electrodepositing 4 cycles with 4 mmol SeO2 have 5.03% photoelectric conversion efficiency and 3.61mA·cm-2 photocurrent density. When CdSe/TiO2 heterojunction thin films were annealed at 450℃ for 5 h, nanorod-shell structure was formed. Further improving photoelectric conversion efficiency to 8.75% and photocurrent density are measured 5.16 mA·cm-2.