Physics, Chemistry and Optimization of Perovskite Light Emitting Diodes

Abstract

The desire for high color quality, low cost and stable lighting and display technology has driven intense research and development of better light emitting materials and devices. Organic-inorganic hybrid perovskites have demonstrated the potential to develop into a new generation of light emitting diodes (LEDs) that have unique advantages such as high color purity, wide emission wavelength tunability, solution processability, and easy fabrication at low cost. In this work, we focus on understanding the physics, chemistry and processing of perovskite materials, and how best to exploit that understanding to optimize device performance of perovskite LEDs.

We begin by investigating the defect physics and crystallization mechanisms of perovskites and demonstrating a facile method for preparing perovskite nanocrystalline films in situ, which involves the use of large organoammonium halides as additives to confine crystal growth, achieving ultra-smooth perovskite films with small grain sizes. Based on extensive theoretical calculations and supported directly by experimental evidence, we identify important design considerations for the choice of the additives, and find that molecular structure and dipole moment are important for both efficient defect passivation and improving material flexibility, overcoming the common tradeoffs between optoelectronic and mechanical properties. Using all these various techniques, it is shown how record efficiencies have been achieved for highly robust and flexible perovskite LEDs. In addition, we show that, due to the high refractive index of the perovskite layer, waveguiding loss is significant in perovskite LEDs. Using a thinner perovskite layer can effectively suppress waveguiding and improve light outcoupling.

The second major theme of this work involves understanding the stability and degradation mechanisms of perovskite devices. The roles of material properties, device structures and operational conditions are approached through extensive in-situ and ex-situ characterization techniques. Our findings revealed that Joule heating plays an important role in device degradation, which is closely related to ionic motion within perovskites. Adopting a thinner perovskite layer could significantly improve operational stability by reducing Joule heating. Finally, we determine that perovskites are considerably redox active, and that redox chemistry dictates material and device degradation, which brings important insight regarding electrode and buffer layer choices in perovskite devices.