Characterizing the Excited State Dynamics of Organic Materials for Efficient Energy Conversion: from Current to Photons and Vice-Versa

The use of organic semiconductor (OSC) materials for optoelectronic applications such as display technologies, consumer electronics, energy-storage, and photovoltaic conversion has drawn academic and industrial interest in recent years. The OSC rise in popularity and demand is due to their multiple advantageous properties over their inorganic counterparts. Some of these OSC properties are their ease of device processability, flexibility, and cost-efficiency. In addition, their synthetic flexibility allows for subtle modifications in their structure so their optical and electrochemical properties could be systematically tuned. The molecular structure dictates their function, therefore, fundamental understanding on how modulating their chemical structures will tune their opto-electrical properties is imperative to optimize their optoelectronic performance. In this dissertation, ultrafast spectroscopy and nonlinear optical methods were used to probe the excited state dynamics of OSC materials with 1) thermally activated delayed fluorescence (TADF) characteristics, and 2) light harvesting and low-bandgap materials for bulk heterojunction (BHJ) architectures. The former is a mechanism used for enhancing the efficiency of the third-generation organic light emitting diodes (OLEDs), while the latter composed the active layer of the third generation organic photovoltaic (OPV) devices. TADF is the idea that dark triplet excitons can be converted into emissive singlet exciton via a reverse intersystem crossing (rISC) mechanism, which the rISC is prompted by the combination of room thermal energy with the small energy gap between the singlet-triplet manifolds (ΔEST). This mechanism has been implemented into OLEDs. However, one of the major challenges hampering their commercialization is the lack of seep understanding on the overall TADF mechanism in such systems. Throughout the course of this thesis, the photophysical properties of multiple TADF emitters are elucidated. In addition, a new optical approach for the direct characterization of the rate of rISC (krISC) is introduced. This new methodology could be used to predict their respective device performances. The krISC in TADF emitters is believed to be critical for reducing the triplet-triplet annihilation (TTA) mechanism that cause efficiency roll-off at high operational voltage. In the case of the active materials for BHJ applications, the optical properties of light harvesting polymers and low-bandgap small molecules for non-fullerene acceptors (NFA) devices are elucidated. Specifically, the influence of heterocycles such as furan and thiophene as linkers on the photophysical properties of light harvesting polymers were probed with and emphasis in their charge transfer properties. These light harvesting polymers were based on benzodifuran (BDF) as the donor and diketopyrrolopyrrole (DPP) as the acceptor. The low-bandgap materials were designed to be acceptor-donor-acceptor ladder-type molecules based on perylene-diimide (PDI) as the acceptor and heteroacenes as the donor. These materials have also drawn interest for developing transparent solar cells. Their photophysical properties were investigated and an intense intramolecular charge-transfer character upon pi-conjugation lengthening was observed among them. In addition, fundamental investigation of the feasibility of intramolecular singlet exciton fission (iSF) and subsequent two-electron transfer from tetracyanimethylene quinoidal bithiophene (QOT2) to anthracene were investigated as well.PhDChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/153365/1/ricbacu_1.pd