Hierarchical i-p and i-n porous heterojunction in planar perovskite solar cellsElectronic supplementary information (ESI) available: Effect of substrate on the GISAXS experiments; interface (surface) area determined by Porod's law. GISAXS profiles of two-step processing of PbI2 layer (step I) and perovskite layer. 1D XRD pattern of PbI2 and two-step processed perovskite. See DOI: 10.1039/c5ta02184g

A hierarchical pore network in planar CH 3 NH 3 PbI 3 perovskite is demonstrated herein. Quantitative characterizations by grazing incidence small angle X-ray scattering (GISAXS) with modeling and complementary microscopic observations provide insight at various length scales. It is a pore structure comprised of nano-scaled primary pores aggregating into meso-scaled fractal networks within the perovskite layer. Its structural evolution and mechanistic interpretation are explored with respect to different preparation methods/steps. The time-of-flight secondary ion mass spectrometer (TOF-SIMS) results suggest the infiltration of hole transporting materials (HTM) or electron transporting materials (ETM) deposited on top at different length scales. The inter-penetrating perovskite/HTM or perovskite/ETM form i-p or i-n one-sided porous heterojunctions, respectively, over the typically regarded planar-stacked heterojunction. They show distinctive photovoltaic characteristics and behaviors in which the large i-n interfaces at the nanoscale lead to highly efficient, hysteresis-free and reliable solar cell devices. The morphology-performance correlation is helpful for associated design of device architecture and processing toward higher efficiency and stability. A hierarchical pore network is discovered in CH 3 NH 3 PbI 3 perovskite solar cell, which forms an i-p or i-n porous heterojunction with infiltrated hole transporting materials or electron transporting materials, respectively.