Organic (opto)electronic materials are of interest due to their low cost and tunable properties; a broad range of their applications, from photovoltaics to three-dimensional displays, have been demonstrated. One of the promising directions in organic semiconductor research is to use a singlet fission process to enhance efficiency of (opto)electronic devices such as solar cells. Singlet fission (SF) is a carrier multiplication process that involves spin-allowed conversion of one singlet exciton (created by absorption of one photon) into two triplet excitons that produce two pairs of charge carriers. SF photovoltaic devices can potentially break the Shockley-Queisser energy conversion limit in silicon, and in the past decade there has been significant research on many SF systems which meet the basic SF energy requirement, where the energy of the singlet state is comparable to double of the energy of the triplet state.

 Acenes and anthradithiophenes are benchmark organic (opto)electronic and SF materials which have served as model systems for understanding physical mechanisms of SF, charge photogeneration, and charge transport. However, one of the bottlenecks that hindered wide-range commercialization of these materials is their relatively low stability with respect to environmental factors such as exposure to light and oxygen. Modifications of the molecular structure that improve the stability often lead to enegy level alignment and film morphology that are detrimental for optoelectronic properties. Therefore, for the development of SF-based optoelectronic devices, it is important to establish the relationship between the SF properties and photodegradation and to find a way to enhance stability of the molecules without sacrificing the optoelectronic performance. This project explored the new routes for enhancing stability of benchmark SF organic semiconductors, functionalized acene and anthradithiophene (ADT) derivatives, using molecular packing engineering and strong coupling of molecular excitations to microcavities.

The project resulted so far in 6 published research articles (2 invited), 3 full-size conference proceeding papers, 9 national/international (3 invited) and 6 regional meeting presentations, and 4 invited seminars.

Intellectual Merit

We developed a comprehensive picture of singlet fission – photochemistry – molecular packing connections in functionalized tetracene- and pentacene-based films and crystals.  In particular, we established the links between the molecular morphology conducive to SF and the propensity of these morphologies for photodegradation, both via formation of the endoperoxide in the presence of air due to reaction with oxygen and via photodimerization in the absence of air. We demonstrated that it is possible to create an efficient SF material while maintaining good photostability and that ‘slip-stacked’ molecular packing motifs, which have been a staple of molecular design principles for the best SF materials over the past decade, should be avoided to enhance the photostability. Novel bis-acene derivatives with a near-infrared absorption and extraordinary stability were reported and characterized. It was determined that magnetic fields slow down the degradation via endoperoxide formation in air in various acenes, which highlights the important role of triplet states (that are manipulated by magnetic fields) in this reaction.

We achieved strong coupling between excitons in acene-based thin films and cavity photon in various microcavities and examined it depending on the temperature and magnetic field, revealing considerably weaker effects of external parameters on the light-matter hybrid (polariton) states as compared to cavity-uncoupled molecular states; this robustness of optical properties of polaritons can be utilized in applications. We observed enhanced (suppressed) photodimerization in cavities at room (elevated) temperature as compared to control samples, which we attribute in part to the selectivity of cavity coupling to a particular molecular population.

Broader impact  

Results of the project promote understanding of fundamental physics and chemistry of SF and light-matter interactions in organic materials which is important for applications ranging between photovoltaics and polariton lasers. The PI has actively involved undergraduates, as well as female and minority students, in research, with one of the students (Madalyn Gragg) winning the 2024 Barry Goldwater Scholarship. The PI’s group has developed hands-on activities for high-school and middle-school students and hosted various events via OSU Juntos program for Latinx students and Tribal Youth program for Native American students and their families in her lab. We leveraged the project to continue developing a state-of-the-art ultrafast laser center funded by NSF-MRI award (DMR-1920368) to OSU team which includes the PI. The team involves five faculty members from OSU Physics (2), Chemistry (1), and EECS (2) and four graduate student superusers (two are from the PI’s group). The project enabled important calibrations of the experimental setups and superuser training.  The facility, key capabilities of which (ultrafast pump-probe and fluorescence microscopy in a cryomagnet) have leveraged the project, has been advertised to a broad base of users, as it is now available to researchers nation-wide as a shared facility.

The grant has supported and/or enabled research training for 7 physics graduate students (2 female, 1 minority, 1 veteran) and 6 physics undergraduate students (1 female, 2 non-binary). 

Last Modified: 12/17/2024
Modified by: Oksana Ostroverkhova