Considerable research into organic small molecules (OSM) have taken place due to its cheap large scale fabrication cost compared to inorganic semiconductors, tunable optical and electronic properties, and its potential to be designed flexible, transparent, and light weight. OSM are a class of organic semiconductors that form extended crystals. These crystals are held together by weak Van der Waals forces which allow large amplitude inter- and intra- molecular motions. The motions cause large electronic coupling fluctuations, which results in unfavorable electronic properties. With the infinite design space afforded to OSM, it is difficult to make a distinction between molecular design and detrimental molecular motions. Thus, it is important to have full knowledge of the phonon spectrum to design molecules with favorable electronic properties. Inelastic neutron scattering (INS) is a spectroscopy technique well-suited to study the dynamics of OSM. Here, we use INS in conjunction with density functional theory to highlight the phonon modes that are detrimental to the electronic coupling. Design principles are considered to reduce the infinite design space that dampens those detrimental phonons. We then expand the capabilities of INS by designing and building a sample changer stick, and two photochemistry sample sticks. The sample changer stick can simultaneously cool four samples, and measure each individually. The first photochemistry sample stick uses an LED above the sample container within the closed-cycle refrigerator on the VISION neutron spectrometer at Oak Ridge National Laboratory. The second incorporates a laser and tumbler sample mixing system to increase the density of photoexcited sample. We show simple test cases using an acene and 405 nm photoresin for the two designs. Lastly, an experimental and computational study of an multiexciton generation mechanism, singlet fission (SF), in tetracene and pentacene are studied with varying excitation energies. The findings suggest the electronic states shifts due to the excimer deformations leading to energetically favorable SF.

Organic semiconductors have the potential to create flexible, transparent electronic devices. Unfortunately, these materials suffer from low mobilities because atomic vibrations, known as phonons, localize charge carriers. In order to improve mobility in these materials, it is essential to understand which types of phonons are present and how they affect charge transport. Inelastic neutron scattering (INS) can measure these motions directly, but the results cannot be interpreted without computing the phonon modes from an experimental structure. These calculations typically rely on density functional theory (DFT) which is highly accurate but computationally costly. This limits calculations to highly-ordered small molecules. To try and push past these limits, we used six different computational methods to compute the phonon modes ranging from DFT to molecular dynamics (MD) and machine learning (ML). We find that while nothing can compete with DFT, density functional tight binding (DFTB) and DFTB with a machine learning component produce decent results at a fraction of the cost. Then, we apply INS along with our optimized DFT-based method to a metal-organic framework (MOF) system. These MOFs are known to have defects that play an important role in their catalytic activity, but characterizing these defects remains an ongoing challenge. By measuring the phonons with INS and simulating them with DFT, we are able to identify peaks that correspond to defects in the MOF. By using a variety of structures, we are able to identify the structure of the MOF along with the types of defects present. Finally, we develop a novel workflow, ElPh, which allows us to couple our phonon calculations to the electronic structure in small-molecule organic semiconductors. This allows us to understand how all of the phonons work together to limit charge mobility. In addition, we developed a novel analysis technique that shows how each atom limits charge transport. This novel approach allows us to discover design rules for the first time--making meaningful progress toward designing new, high-performance materials.