Lawrence Renna

Lawrence Renna

DV Group

The Discovery of Ionic Transport in Hybrid Organic-Inorganic Perovskites and its

Implications on Solar Cell and Non-Volatile Memory Applications

1:30PM

Hybrid Organic-Inorganic Perovskites (HOIPs) exploded onto the materials science scene around 2012, when they were finally exploited to realize high efficiency solar cells. HOIPs are a material composed of a 3-D network of corner-sharing metal (Pb or Sn)-halide (Cl, Br, and I) octahedra, with a cation (Cs, methylammonium (MA), or formamidinium (FA)) in the octahedral voids. Since then there has been an inordinate amount of research on HOIPs for functional materials. During the rush to make new and better electronic devices, one striking fact about HOIPs had been over looked — that is there poor stability under illumination.

Our investigations began with an observation, we found that upon illumination the power conversion efficiency (PCE) of HOIP devices decreases. However, this degradation was found to be quasi-reversible; upon turning the light off for 15 min the PCE increased, but to a value less than the original PCE. To understand the quasi-reversible degradation under light, we performed electrochemical impedance spectroscopy (EIS), which can decouple different processes occurring in a material, by their phase response to an applied AC field. We find, under illumination that the EIS Nyquist plots contain two features: 1) A high-frequency semicircle attributed to electronic processes, and 2) A low-frequency constant phase element (linear component), which we attributed to ion diffusion, and modelled with a Warburg element for 1-D ion diffusion. We next sought to probe the origin of this ion diffusion by changing the ratio of MA to FA. By increasing the size of the organic cation, the ionic diffusion coefficient also increased, thus allowing us to determine the chemical identity of the mobile ion as the organic cation, MA and or FA. We also conducted temperature-dependent EIS studies to calculate activation energies (E_a) for ionic transport. For MAPbI₃ we observed a discontinuous E_a with increasing temperature. Using X-ray diffraction structural studies, we correlated this observation with a volume increase and a discontinuous tetragonal to cubic phase change. Upon prolonged illumination be observed a decrease in HOIP and an increase in PbI₂, this is due to lattice instability when ions leave their octahedral voids. We used this phenomenon to conceive a novel type of memory-resistor, so-called memristor. The device utilizes interface properties to store information by adsorbing ions, and recall information by desorbing ions. This is a fantastic “lemonade out of lemons” moment, where we found that ion diffusion is detrimental to solar cell performance, but can be utilized to fabricate new devices with non-linear electrical properties for memory applications.

To conclude, this work was the first to identify one of the causes of poor stability in HOIPS as degradation of the perovskite structure due to light- and heat-accelerated ion diffusion. Using EIS we have characterized this ion diffusion, and identified the organic counterion as a mobile one. We next utilized the detrimental (in solar cells) ion diffusion to our benefit to fabricate a novel memristor device.