Defects in semiconductors can play a vital role in the performance of electronic devices, with native defects often dominating the electronic properties of the semiconductor. Understanding the relationship between structural defects and electronic function will be central to the design of new high-performance materials. In particular, it is necessary to quantitatively understand the energy and lifetime of electronic states associated with the defect. Here, we apply firstprinciples density functional theory (DFT) and many-body perturbation theory within the GW approximation to understand the nature and energy of the defect states associated with a charged nitrogen vacancy on the electronic properties of gallium nitride (GaN), as a model of a well-studied and important wide gap semiconductor grown with defects. We systematically investigate the sources of error associated with the GW approximation and the role of the underlying atomic structure on the predicted defect state energies. Additionally, analysis of the computed electronic density of states (DOS) reveals that there is one occupied defect state 0.2 eV below the valence band maximum and three unoccupied defect states at energy of 0.2-0.4 eV above the conduction band minimum, suggesting that this defect in the +1 charge state will not behave as a carrier trap. Furthermore, we compare the character and energy of the defect state obtained from GW and DFT using the HSE approximate density functional, and find excellent agreement. This systematic study provides a more complete understanding of how to obtain quantitative defect energy states in bulk semiconductors.

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We present a first-principles many-body perturbation theory study of the role of inter-molecular coupling on the optoelectronic properties of a one-dimensional p-stacked nanowire composed of perylene-3,4,9,10-tetracarboxylic diimide (PTCDI) molecules on a DNA-like backbone. We determine that strong inter-molecular electronic coupling results in large bandwidths and low carrier effective masses, suggesting a high electron mobility material. Additionally, by including the role of finite temperature phonons on optical absorption via a newly presented approach, we predict that the optical absorption spectrum at room temperature is significantly altered from room temperature due to allowed indirect transitions, while the exciton delocalization and binding energy, a measure of inter-molecular electronic interactions, remains constant. Overall, our studies indicate that strong inter-molecular coupling can dominate the optoelectronic properties of π-conjugated 1D systems even at room temperature.

Germanium Selenide (GeSe) is a van der Waals-bonded layered material with promising optoelectronic properties, which has been experimentally synthesized for 2D semiconductor applications. In the monolayer, due to reduced dimensionality and, thus, screening environment, perturbations such as the presence of defects have a significant impact on its properties. We apply density functional theory and many-body perturbation theory to understand the electronic and optical properties of GeSe containing a single selenium vacancy in the −2 charge state. We predict that the vacancy results in mid-gap "trap states" that strongly localize the electron and hole density and lead to sharp, low-energy optical absorption peaks below the predicted pristine optical gap. Analysis of the exciton wavefunction reveals that the 2D Wannier-Mott exciton of the pristine monolayer is highly localized around the defect, reducing its Bohr radius by a factor of four and producing a dipole moment along the out-of-plane axis due to the defect-induced symmetry breaking. Overall, these results suggest that the vacancy is a strong perturbation to the system, demonstrating the importance of considering defects in the context of material design.