Presentations
Talks
Pyrene-Stabilized Acenes as Intermolecular Singlet Fission Candidates: Importance of Exciton Wave-Function Convergence
ACS Spring Meeting, April 5th 2021, virtual
Singlet fission (SF) is a photophysical process considered as a possible scheme to bypass the Shockley–Queisser limit by generating two triplet-state excitons from one high-energy photon. Polyacene crystals, such as tetracene and pentacene, have shown outstanding SF performance both theoretically and experimentally. However, their instability prevents them from being utilized in SF-based photovoltaic devices. In search of practical SF chromophores, we use many-body perturbation theory (MBPT) within the GW approximation and Bethe–Salpeter equation to study the excitonic properties of a family of pyrene-stabilized acenes. We propose a criterion to define the convergence of exciton wave-functions with respect to the fine k-point grid used in the BerkeleyGW code. An open-source Python code is presented to perform exciton wave-function convergence checks and streamline the double Bader analysis of exciton character. We find that the singlet excitons in pyrene-stabilized acenes have a higher degree of charge transfer character than in the corresponding acenes. The pyrene-fused tetracene and pentacene derivatives exhibit comparable excitation energies to their corresponding acenes, making them potential SF candidates. The pyrene-stabilized anthracene derivative is considered as a possible candidate for triplet–triplet annihilation because it yields a lower SF driving force than anthracene.
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Assessing Zethrene Derivatives as Singlet Fission Candidates Based on Multiple Descriptors
APS March Meeting, March 17th 2021, virtual
Singlet fission (SF) is a process where one singlet exciton splits into two triplet excitons. Utilizing SF may potentially increase the efficiency of solar cells beyond the Shockley–Queisser limit. To discover new SF materials, predictive descriptors for SF performance are needed. We consider multiple descriptors to assess several zethrene derivatives as candidate materials for intermolecular SF in the solid state. The descriptors include single molecule multiradical characters, many-body perturbation theory calculations of the thermodynamic driving force for SF and the singlet exciton charge transfer character in crystals, and a kinetic model based on molecular dimers extracted from the crystal structures. The zethrenes are compared to acenes known to exhibit SF with respect to these descriptors. The results indicate that all zethrene and heptazethrene derivatives studied here may exhibit SF. In particular, 7,14-bis(2,4,6-trimethylphenyl)dibenzo[de,mn]naphthacene (Z-T) emerges as a promising candidate. Its SF driving force is higher than tetracene, whose fission process is slightly endoergic, but lower than pentacene. Its singlet exciton charge transfer character is close to pentacene, and its crystal packing leads to a higher SF rate than other zethrene derivatives. Therefore, it may undergo fast SF with high energy efficiency. The approach of considering multiple descriptors may be useful for evaluating additional candidate materials for SF.
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Posters
Phenylated Acene Derivatives as Candidates for Intermolecular Singlet Fission
Electronic Strcture Workshop 2019, May 22nd 2019, University of Illinois at Urbana-Champaign
Singlet fission (SF), a spin-conserving process where one singlet exciton is converted into two triplet excitons, may improve the efficiency of organic photovoltaics. Only a few materials have been experimentally observed to undergo intermolecular SF, most of which are acenes and their derivatives. Using many-body perturbation theory in the GW approximation and the Bethe–Salpeter equation, we systematically investigate the electronic and excitonic properties of tetracene, pentacene, and their phenylated derivatives in the gas phase and solid state. Their potential for SF is evaluated with respect to the thermodynamic driving force and the singlet exciton charge-transfer character. In both the gas phase and solid state, pentacene and its derivatives are more promising than tetracene analogues. Within a family of molecules containing the same acene backbone, increasing the number of phenyl side groups is detrimental for the SF driving force in the gas phase. However, in the solid state, the SF driving force and the exciton character are modulated by intermolecular interactions present within different packing arrangements. Molecules with a higher number of phenyl side groups often form crystals with less cofacial interactions between the acene backbones. These crystals are found to exhibit a higher SF driving force and a higher degree of singlet exciton charge-transfer character. In particular, 5,7,12,14-tetraphenylpentacene, 1,4,6,8,11,13-hexaphenylpentacene, and 1,2,3,4,6,8,9,10,11,13-decaphenylpentacene emerge as promising candidates for intermolecular SF in the solid state.
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On the Possibility of Singlet Fission in Crystalline Quaterrylene
Electronic Strcture Workshop 2017, June 28th 2017, Princeton University
Singlet fission (SF), the spontaneous down-conversion of a singlet exciton into two triplet excitons residing on neighboring molecules, is a promising route to improve organic photovoltaic (OPV) device efficiencies by harvesting two charge carriers from one photon. However, only a few materials have been discovered that exhibit intermolecular SF in the solid state, most of which are acene derivatives. Recently, there has been a growing interest in rylenes as potential SF materials. We use many-body perturbation theory in the GW approximation and the Bethe-Salpeter equation to investigate the possibility of intermolecular SF in crystalline perylene and quaterrylene. A new method is presented for determining the percent charge transfer (%CT) character of an exciton wave-function from double-Bader analysis. This enables relating exciton probability distributions to crystal packing. Based on comparison to known and predicted SF materials with respect to the energy conservation criterion (ES-2ET) and %CT, crystalline quaterrylene is a promising candidate for intermolecular SF. Furthermore, quaterrylene is attractive for OPV applications, thanks to its high stability and narrow optical gap. Perylene is not expected to exhibit SF; however, it is a promising candidate for harvesting sub-gap photons by triplet-triplet annihilation.
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