Computational Design and Modeling of Molecular Organic Semiconductors for Solar Cell and Lighting Applications

Abstract

In this thesis, we study the optoelectronic properties, including energy levels, charge transport, and optical emission, of organic semiconductors by computational methods.By functionalizing octasilsesquioxanes (SQ8) with pentacene, we construct two organic-inorganic hybrid molecules, i.e. dipentacene-SQ8 and monopentacene-SQ8. Unlike the herringbone pattern in crystalline pentacene, the pentacene segments in the predicted crystal structures of the hybrid molecules assume parallel configurations, leading to enhanced orbital overlap between pentacene segments. A multi-scale hopping model based on Fermi’s golden rule is developed to simulate the charge transport in these crystals. The simulated hole mobility in crystalline dipentacene-SQ8 can be as high as 11775 cm2/Vs at room temperature, compared to 17 cm2/Vs for crystalline pentacene. We use density functional theory (DFT) to identify design principles for energy level tuning in donor/acceptor conjugated polymers (CPs). We observe that increasing the electron withdrawing strength of the acceptor unit for a given donor drops the lowest unoccupied molecular orbital (LUMO) level, but keeps the highest occupied molecular orbital (HOMO) level almost unchanged. Conversely, increasing the electron donating strength of the donor unit for a given acceptor raises the HOMO level while keeping the LUMO level unchanged. We identify strong correlations between the frontier orbital energy levels, the amount of charge transfer between the donating and accepting units and the degree orbital localization in CPs.We investigate the influence of the conjugation length of organic molecules on phosphorescence. In experiments phosphorescence efficiency decreases as the conjugation length increases. Our time-dependent density functional theory (TDDFT) calculations reveal that the intersystem crossing (ISC) rate between first singlet (S1) and first triplet (T1) is reduced when increasing the conjugation length. Molecular orbital analysis shows that singlets are more localized than triplets over the conjugation backbone. This results in a larger spatial separation between singlets and triplets when increasing the conjugation length, leading to diminished ISC efficiency and eventually reduced phosphorescence.These discoveries help us identify the underlying design principles of organic semiconductors, thus enhancing the efficiency of new material development.