Organic-inorganic hybrid CH3NH3PbI3 perovskite solar cells have been investigated intensely in the past few years with efficiency exceeding 20%.[1] The substitution from I to Br in CH3NH3PbI3 was proven to tune the band gap from 1.50 eV to 2.20 eV conveniently.This feature makes CH3NH3PbI3-xBrx very attractive in terms of fabricating colorful and semi-transparent devices.It was also found that CH3NH3PbI3-xBrx displays excellent photovoltaic property with improved long-term stability.[2] However,it is not easy to fabricate uniform and well crystalized CH3NH3PbI3-xBrx planar films.Reaction between PbX2(X=Br or I)and CH3NH3X(X=Br or I)usually leads to films with rough surface and low coverage,resulting in poor performance of the solar cells.[3] Here we report a one-step solution method to fabricate more uniform and crystalized CH3NH3PbI3-xBrx(x=0,0.45,0.51,0.60,1,2,3)film on FTO/TiO2 by using NH4Cl additive.The films with improved coverage and crystallization were confirmed by SEM and XRD.UV-vis spectrum showed that the absorbance of the films using NH4Cl was greatly enhanced comparing to those without NH4Cl.Efficiencies of the resulting planar solar cells can be largely improved.Especially when x = 0.60,the efficiency increased from 6.40%to 12.10%by 89%.As far as we know,this is the highest efficiency of planar heterojunction solar cell based on CH3NH3PbI3-xBrx.Furthermore,we also conducted in-situ XRD to explore the role played by NH4Cl in the film formation.To sum up,this work provides a convenient approach for the fabrication of high efficiency CH3NH3PbI3-xBrx based planar heterojunction solar cells.