Why Lead Methylammonium tri-IODIDE perovskite-based solar cells requires   a mesoporous electron transporting scaffold (but not necessarily a hole   conductor)
  CH3NH3PbI3-based solar cells were characterized with electron beam-induced current (EBIC), and compared to CH3NH3PbI3-xClx ones. A spatial map of charge separation efficiency in working cells shows p-i-n structures for both thin film cells. Effective diffusion lengths, LD, (from EBIC profile) show that holes are extracted significantly more efficiently than electrons in CH3NH3PbI3, explaining why CH3NH3PbI3-based cells require mesoporous electron conductors, while CH3NH3PbI3-xClx ones, where LD values are comparable for both charge types, do not. 
Eran Edri, Saar Kirmayer, Alex Henning, Sabyasachi Mukhopadhyay,   Konstantin Gartsman, Yossi Rosenwaks, Gary Hodes, David Cahen
physics.chem-ph cond-mat.mtrl-sci