**7. Conclusion**

Various TiO2/CH3NH3PbI3-based photovoltaic devices were fabricated and characterized. Especially, effects of metal doping and halogen doping to the perovskite and TiO2 were investigated. Microstructure analysis indicated the changes of the perovskite structure, which resulted in the improvement of photovoltaic properties of the devices. Various elemental dopings to the perovskite structure could be studied further both by experiments and theoretical calculations as follows: Cs, Rb, and K doping to the CH3NH3 positions for stability of the structure; Ge, Sn, and Sb doping to the Pb positions for improvement of the semiconducting properties; Cl, Br, and F doping to the I positions for enhancement of carrier mobility.
