  Photovoltaic conversion efficiency (PCE) of halide perovskite solar cells has
risen spectacularly, yet the very crystalline structure of CH3NH3PbI3 remains
ambiguous after extensive researches, and its polar nature remains hotly
debated. Here we present compelling evidences that CH3NH3PbI3 crystals
self-grown on FTO/TiO2 substrate consist of ferroic domains with alternating
polar and nonpolar orders, in contrast to previous experimental and theoretical
expectations, and polar domains possess reduced photocurrent. It is found that
polar and nonpolar orders of CH3NH3PbI3 can be distinguished from their
distinct lateral piezoresponse, energy dissipation, first and second harmonic
electromechanical couplings, and temperature variation, even though their
difference in crystalline lattice is very subtle, and they possess two-way
memory effect through cubic-tetragonal phase transition. These findings resolve
key questions regarding polar nature of CH3NH3PbI3 and its implication on
photovoltaics, reconcile contradictory data widely reported, and point a
direction toward engineering ferroic domains for enhanced PCE.
