Title: Tetragonal CH3NH3PbI3 Is Ferroelectric,
Abstract: Halide perovskite (HaP) semiconductors are revolutionizing photovoltaic (PV)
solar energy conversion by showing remarkable performance of solar cells made
with esp. tetragonal methylammonium lead tri-iodide (MAPbI3). In particular,
the low voltage loss of these cells implies a remarkably low recombination rate
of photogenerated carriers. It was suggested that low recombination can be due
to spatial separation of electrons and holes, a possibility if MAPbI3 is a
semiconducting ferroelectric, which, however, requires clear experimental
evidence. As a first step we show that, in operando, MAPbI3 (unlike MAPbBr3) is
pyroelectric, which implies it can be ferroelectric. The next step, proving it
is (not) ferroelectric, is challenging, because of the material s relatively
high electrical conductance (a consequence of an optical band gap suitable for
PV conversion!) and low stability under high applied bias voltage. This
excludes normal measurements of a ferroelectric hysteresis loop to prove
ferroelctricity s hallmark for switchable polarization. By adopting an approach
suitable for electrically leaky materials as MAPbI3, we show here ferroelectric
hysteresis from well-characterized single crystals at low temperature (still
within the tetragonal phase, which is the room temperature stable phase). Using
chemical etching, we also image polar domains, the structural fingerprint for
ferroelectricity, periodically stacked along the polar axis of the crystal,
which, as predicted by theory, scale with the overall crystal size. We also
succeeded in detecting clear second-harmonic generation, direct evidence for
the material s non-centrosymmetry. We note that the material s ferroelectric
nature, can, but not obviously need to be important in a PV cell, operating
around room temperature.