Access Type

Dissertation/Thesis

Date of Award

January 2019

Degree Type

Dissertation

Degree Name

Ph.D.

Department

Chemistry

First Advisor

Vladimir Y. Chernyak

Abstract

Due to their various advantages, including lightweight, flexible, and cheap manufacturing, organic photovoltaic materials have gained enormous research interest. Over nearly two decades, the power conversion efficiency of organic solar devices has increased dramatically. However, it is still low compared to traditional inorganic semiconductors. In order to improve efficiency, a better understanding of the basic thermodynamic properties of the light-to-electricity power conversion process is needed. One nontrivial aspect of organic solar cells is the low dielectric constant, which leads to tightly-bound excitons upon vertical excitations. The separation of electron-hole pairs requires a larger driving force to overcome the Coulombic binding energy in organic semiconductors compared to their inorganic counterpart. A particular state called charge transfer state appears during the dissociation process of the bound excitons. The exact role of this particular type of state, whether a precursor to efficient charge separation or a detrimental process which hinders the generation of free charges, is still under debate. Extensive research has been performed to elucidate the mechanism of free charge carrier creation. Some studies show that the dielectric environment plays a significant role during the charge dissociation process by affecting the energetics of excited states. For example, MDMO-PPV: PCBM device becomes more efficient when the dielectric constant reaches a certain value (εr = 9). The aim of this work is to map out the alignment of excited states of a typical polymer: fullerene device, taking PCPDTBT: PCBM as a specific example system, and find out the characteristics of the charge transfer state under the influence of polar solvent.

Long-range corrected time-dependent density functional theory combined with the polarizable continuum model has been used to study the solvent effect on excited state properties of PCPDTBT: PCBM molecular system. Solvation model has been applied using the linear-response and state-specific approaches to account for the dielectric environment. Electronic transitions are characterized by their intrinsic properties based on a detailed analysis of the one-electron transition density matrices. The tools include a numerical value termed charge transfer character, contour plots of the transition density matrix, and natural transition orbital of each excited state. The results show that the influence of the solvent depends on the nature of the excitations. For excitonic states, which have a characteristic of local excitations, the solvent has little to no effects on the excitation energies according to both solvent schemes. In contrast, a different trend is observed for states with a significant amount of charge transfer. State-specific predicts a sufficient decline in the excitation energy as the dielectric constant increases such that the charge transfer state can be stabilized to the lowest excited state, whereas linear-response shows almost no change. The comparison of two solvent approaches is discussed.

It concludes that a protocol that combines TDDFT with long-range-corrected hybrid functional, CAM-B3LYP, Grimme’s empirical dispersion correction D3, and state-specific solvation model can effectively and efficiently predict the energetics of charge transfer state in organic photovoltaic materials. Future directions are also provided, including extension of the current calculation scheme to more polymer: fullerene molecular systems, application of other methods such as charge constraint density functional theory and range-separated hybrid functionals combined with polarizable continuum model, simulation of the charge dissociation process in a dynamic picture, and investigation of the solvent effect under the variation of optical dielectric constant instead of focusing only on the static permittivity.

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