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Organic-inorganic metal halide perovskite devices have generated significantinterest for LED, photodetector, and solar cell applications due to theirattractive optoelectronic properties and substrate-choice flexibility1-4. Thesedevices exhibit slow time-scale response, which have been explained by pointdefect migration5-6. In this work, we report the discovery of a roomtemperature intrinsic amplification process in methylammonium lead iodideperovskite (MAPbI3) that can be triggered by few photons, down to a singlephoton. The electrical properties of the material, by way of photoresponse, aremodified by an input energy as small as 0.19 attojoules, the energy of a singlephoton. These observations cannot be explained by photo-excited electronicband-to-band transitions or prevailing model of photo-excited point defectmigration since none of the above can explain the observed macroscopic propertychange by absorption of single or few photons. The results suggest theexistence of an avalanche-like collective motion of iodides and theiraccumulation near the anode, which we will call ionic impact ionization (I3mechanism). The proposed I3 process is the ionic analog of the electronicimpact ionization, and has been considered impossible before becauseconventionally it takes far more energy to move ions out of their equilibriumposition than electrons. We have performed first principle calculations to showthat in MAPbI3 the activation energy for the I3 mechanism is appreciably lowerthan the literature value of the activation energy for the electronic impactionization. The discovery of I3 process in perovskite material opens uppossibilities for new classes of devices for photonic and electronicapplications.

Discovery of Ionic Impact Ionization (I3) in Perovskites Triggered by a Single Photon