Doctoral Dissertations

Orcid ID

0000-0001-6278-8233

Date of Award

5-2021

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Bin Hu

Committee Members

Bin Hu, David Mandrus, Mahshid Ahmadi, Tessa Calhoun

Abstract

Organic semiconductors and organic-inorganic hybrid perovskites have demonstrated versatile functionalities for optoelectronic and spintronic applications. Research in this dissertation focuses on the spin effects of excited states in emerging organic semiconductor and hybrid perovskite systems to understand the fundamental working principle. The investigation on the spin effects in excited states can provide insightful guidance for the development of the next-generation organic and hybrid perovskite optoelectronics and spintronics.

In organic semiconductors, the excited states, namely, excitons can be characterized by the total spin angular momentum S=Se+Sh of the constituent electron and hole, which determines the spin states of singlets and triplets. In chapter 2, magneto-optical effects were investigated in organic optoelectronic devices including microcavity organic light-emitting diodes (OLEDs) and single active layer organic photovoltaic devices. It has been demonstrated that magneto-optical effects can function as the in-operando technique to reveal the spin-related optoelectronic processes in devices by directly changing the spin states. Inspired by the successful demonstration of magneto-optical effects in organics, the comprehensive magneto-optical studies were performed in newly emerging thermally activated delayed fluorescence (TADF) molecules with intramolecular charge-transfer states to provide an in-depth understanding on the spin-dependent reverse intersystem crossing (rISC) process from triplets to singlets. The spin-orbital coupling (SOC) was experimentally revealed to be the dominating spin-mixing mechanism in TADF process, responsible for the efficient rISC to harvest non-radiative triplet states. In chapter 4, spin, energy, and polarization parameters were systemically investigated in exciplex systems with intermolecular charge-transfer states. It was found that spin, energy, and polarization parameters can develop the cooperative relationship in exciplex systems for high-efficiency OLEDs. Moreover, in the exciplex hosted iridium complexes doped systems, the extended polarization memory was revealed to be the key factor in determining the device efficiency through enhanced light out-coupling.

With total angular momentum J in organic-inorganic hybrid perovskites, spin (J) states can be manipulated through optical methods. Optically induced magnetization through photoexcited spin states at perovskite/ferromagnet interface was demonstrated in chapter 5. It was found that optically generated spin (J) states in perovskite by using circularly polarized photoexcitation can directly interact with the ferromagnet layer to induce magnetization in perovskite. In chapter 6, spin relaxation dynamics were investigated by using ultrafast laser spectroscopy. The long spin lifetime in nanoseconds was realized in quasi-2D hybrid perovskite at room temperature, which represents a new opportunity to control the optoelectronic processes by using spin states in hybrid perovskites.

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