PhD Defense: “Charges and Excitons in Field-Induced Organic and Pervoskite Light Emitting Devices.” March 30, 2018, at 10 AM

Junwei Xu
Public Presentation in Olin 101
Friday, March 30, 2018, at 10:00 AM
David L. Carroll, PhD, Advisor

The defense will follow.

ABSTRACT

Charge balance in organic light emitting structures is essential to simultaneously achieving high brightness and high efficiency. In DC-driven OLEDs, this is relatively straight forward. However, in the newly emerging, capacitive, field-activated AC-driven organic devices, charge balance can be a challenge. We introduced the concept of gating the compensation charge in AC-driven organic devices and demonstrated that this can result in exceptional increases in device performance. To do this we replaced the insulator layer in a typical field-activated organic light emitting device with a nanostructured, wide band gap semiconductor layer. This layer acts as a gate between the emitter layer and the voltage contact. Time resolved device characterization shows that, at high-frequencies (over 40,000Hz), the semiconductor layer allows for charge accumulation in the forward bias, light generating part of the AC cycle and charge compensation in the negative, quiescent part of the AC cycle.

We also showed that solution-grown films of CsPbBr₃ pervoskite nanocrystals imbedded in wide band-gap Cs₄PbBr₆ can be incorporated as the recombination layer in light-emitting diode structures. More importantly, optical response was studied to shed light on why the very poor light emitter CsPbBr₃ becomes a high-efficiency fast emitter when imbedded as nanocrystals in the wider gap host Cs₄PbBr₆.  Kinetics at high carrier density of pure (extended) CsPbBr₃ and the nano-inclusion composite were measured and analyzed, indicating second order kinetics in extended and mainly first order kinetics in the confined CsPbBr₃, respectively. In these terms, the nano-confinement might be viewed as enforcing mainly geminate recombination of electrons and holes in a material (CsPbBr₃) that did not support stable excitons at room temperature.  The resulting first-order recombination competes with trapping much more effectively than does bimolecular recombination at the moderate carrier densities typical of LEDs and usual photo-excitation.  Analysis of absorption strength of this all-perovskite, all-inorganic imbedded nanocrystal composite relative to pure CsPbBr₃ indicated enhanced oscillator strength consistent with earlier published attribution of the subnanosecond exciton radiative lifetime in nano-precipitates of CsPbBr₃ in melt-grown CsBr host crystals and CsPbBr₃ evaporated films.