Electron-rotor interaction in Organic-Inorganic Lead Iodide Perovskites - Dr. Gang Liu

JULY 14, 2016

New work co-led by Dr. Gangliu from HPSTAR found carrier-rotor coupling effect in perovskite organic-inorganic hybrid lead iodide (CH3NH3PbI3) compounds from isotope effect. The discovery of the electron-rotor interaction would help to establish the theoretical foundation governing various energy transport, conversion, and storage sciences. The story is published in The Journal of Physical Chemistry Letters.

Understanding of electron-lattice interaction is crucial for establishing the mechanistic foundation governing various energy transport, conversion, and storage sciences such as superconductivity, thermoelectricity, photovoltaics, and supercapacitors. The emerging organic-inorganic hybrid lead iodide perovskite materials (e.g., CH3NH3PbI3 or noted as MAPbI3) promise a low-cost and high-efficiency photovoltaic technology. While successfully demonstrating their high power conversion efficiency, hybrid perovskites also offer a new platform to explore some unprecedented fundamental interactions between carriers and lattice.

The structure of hybrid lead iodide perovskite materials can be pictured as the individual cationic organic rotors electrostatically pinned in the cages of a solid-state anionic inorganic framework. Therefore, it is intriguing in a fundamental sense to investigate how photo-induced carriers in the inorganic framework interplay with the neighboring cationic rotors.

Using multiple methods, the team discovered a new type of electron-lattice interaction, namely, electron-rotor interaction, occurring in solid-state hybrid perovskite lead triiodide.

“This interaction is fundamentally different from the well-known electron-phonon interaction, in which the lattice vibration interactions with conduction electrons”, said Gang Liu.

In organic-inorganic hybrid perovskites, cationic methylammonium is electrostatically pinned in the cages formed by the anionic PbI3- framework. However, these organic rotors can undergo rapid rotation in the cage. Thus, the coupling between the cationic side of the rotors and the light-induced free electrons in the PbI3- framework, as the formation of polarons, is effectively dependent on the rotational momentum of the rotors, as evidenced by our time-resolved photoluminescence study, in which the carrier lifetimes of isotopic hybrid perovskite lead triiodides exhibit a trend of CH3NH3PbI3 > CH3ND3PbI3 ≈ CD3NH3PbI3 > CD3ND3PbI3, in good agreement with the trend for their rotational frequencies.

Density functional theory calculation suggests that polar on trapping energy, a key factor that influences carrier lifetime, exhibits a strong dependence on the orientation of the methylammonium.

Caption: Schematic depiction of the CH3NH3⁺ cationic rotors in the cage formed by the anionic PbI3^- framework.

“This finding can provide an innovative alternative for better materials by design—for example, better perovskite materials that can overcome the current constraints of lead toxicity and instability ”, presented in the paper.