*refreshments at 12:40pm
"Slow and Fast Dynamic Processes in Hybrid Perovskite Solar Cell Materials"
Hybrid perovskites have rapidly become an important class of semiconductors for solar cells and other optoelectronic devices, such as photodetectors, LEDs, and lasers. Perovskite solar cell efficiencies have skyrocketed in the past few years and are now approaching those of mature technologies such as silicon and cadmium telluride. Because perovskites can be solution processed from inexpensive materials, they hold great promise for low-cost solar cells as well as for lightweight and flexible devices.
Unlike conventional inorganic semiconductors, hybrid perovskites are a fundamentally dynamic material system. In perovskites, chemical and physical processes that occur across a vast range of timescales have influence on both film processing and electronic properties. In this talk, I will present two dynamic processes that occur on very different timescales and describe their impacts on the formation and performance of the perovskite methylammonium lead iodide (CH3NH3PbI3), which is a model compound for this class of materials.
In the first half of this talk, I will describe a slow process that leads to the formation of high-quality perovskites. A popular method for depositing perovskite films employs chlorine in the starting materials, even though almost no chlorine remains in the final film. I will show that this deposition proceeds via a crystalline intermediate phase that is an altogether novel material, and that the transformation from intermediate to perovskite requires the evaporation of chlorine through a self-regulating mechanism. While most perovskites form in seconds or minutes, this evaporation process occurs on the timescale of hours and the ability to retard the formation of the perovskite results in high quality films that exhibit impressive optoelectronic performance.
The second half of this talk will focus on a fast process that occurs in the perovskite crystal lattice. In solid materials, atoms collectively vibrate in well-defined ways, and these vibrations are called phonons. One reason that phonons are important is that they interact with electrons and thus can have substantial impacts on the operation of solar cells or other devices. I will demonstrate that acoustic phonons, the type that are responsible for transmitting heat, have extraordinarily short lifetimes in the perovskite methylammonium lead iodide. These short lifetimes have direct implications on the cooling and transport of electrons and reflect a key difference between hybrid perovskites and conventional inorganic semiconductors.