The multicrystalline grain size of CH3NH3PbI3 thin films can be manipulated by using different precursor concentrations which dominate the competition between the nucleation and crystal growth during the one-step spin-coating process with the toluene washing treatment. A comprehensive study was carried out to explore the correlation between the structural properties and the exciton dynamics in CH3NH3PbI3 thin films fabricated using various precursor concentrations from 1.25 M to 1.65 M. The experimental results show that the precursor concentration influences the grain size and exciton dynamics of the resultant CH3NH3PbI3 thin films which were fabricated on top of poly(3,4-ethylehedioxythiophene)-polystyr ene sulfonate/lTO/glass substrates. The higher average power conversion efficiency (PCE) of 13.66% achieved can be explained as being due to the formation of a larger CH3NH3PbI3 grain size which thus results in higher photocurrent generation and lower voltage loss. It is predicted that the PCE can be further improved when the voltage loss across the CH3NH3PbI3 thin film is minimized. (C) 2016 Elsevier Ltd. All rights reserved.