Singlet Fission Dynamics in the 5,12-Bis(phenylethynyl)tetracene Thin Film

Abstract

The singlet fission (SF) process is a promising route to overcome the Shockley and Quiesser efficiency limit of organic solar cells and hence has attracted significant research interest in the recent past. Efficient SF in low-cost organic materials with broad visible absorption is desirable to potentially boost organic solar cell efficiency. Herein, we report the ultrafast and efficient SF property of a new molecular material, namely, the 5,12-bis(phenylethynyl)tetracene (BPET) thin film. Steady-state and time-resolved optical studies on the thin film of BPET revealed prompt deactivation of majority of singlet excitons with a lifetime of about 1.0 ps. Detailed analysis of transient spectral features suggests ultrafast relaxation of the singlet exciton to correlated triplet pair formation within 200 fs followed by free triplet exciton generation on the picosecond time scale. However, a slower SF component is also identified originating from the amorphous phase in the thin-film sample. Overall, high SF yield (∼90%) with long-lived triplet (∼3 ms) is estimated from spectroscopic measurements. Efficient SF in BPET as compared to parent tetracene is attributed as the influence of phenylethynyl substituents to alter the energy levels and intermolecular coupling. Wide spectral coverage of visible absorption, solution processability, and high triplet yield (∼180%) by the SF process are expected to make BPET a suitable candidate in SF-based organic solar cells.

Photophysics of Dibenzo[a,l]pentacene upon Monomer to Thin Film Transformation: Enhanced Visible Light Absorption and Ultrafast Singlet Fission

“…A thin film of DBPn was prepared by the physical vapor deposition method under low pressure conditions (∼10 –6 mbar). , The deposition of the sample was done by employing SMART COAT 3 from Hind High Vac. (India).…”

“…Singlet fission (SF) is a sought-after property of organic semiconductors having potential for overcoming the Shockley–Quisser efficiency limit of organic photovoltaics. − It is a process where a photogenerated singlet exciton is converted to two triplet excitons resulting in more than 100% excitonic yield, − paving the way for fabrication of efficient organic solar cells. However, this property is restricted to only a few classes of organic molecules due to stringent energetic and molecular coupling criteria. − Efficient SF is exhibited by molecular systems having the lowest excited singlet (S 1 ) energy levels twice that of the lowest triplet (T 1 ) energy level, that is, it must fulfill E­(S 1 ) ≥ 2E­(T 1 ) energetic criteria. − In addition, SF involves an electron spin-exchange process, which requires an orbital overlap and introduces the strict requirement of strong intermolecular coupling. , Polyacenes, carotenoids, isobenzofurans, perylenebisimides, terylenes, and diketopyrrolopyrrole derivatives are mostly reported to exhibit this exceptional property. − …”

“…Chemistry of organic molecules is blessed with the option of incorporation of various substituents or molecular tailoring to obtain desired properties from a material. Substantial efforts have been invested to favorably alter the SF property by incorporating various chemical substituents in SF molecules. ,,,,,,,, Such studies not only showed the way to achieve a better SF yield but also provided us better molecular insights on the dynamical process. Among polyacenes, pentacene is well known to show an ultrafast SF process with nearly 200% triplet yield ,, and hence it has been systematically tested for solar cell performance and witnessed to show efficiency in the range of 1.8–4.7%. , …”

Comparative Study of Exciton Dynamics in 9,9′-Bianthracene Nanoaggregates and Thin Films: Observation of Singlet–Singlet Annihilation-Mediated Triplet Exciton Formation

“…It is observed that, for the high excitation energy of 4 μJ/pulse, the triplet ESA band achieves the maximum signal by 0.3 ps, while for 0.8 μJ/pulse, the triplet ESA band is found to grow even up to 20 ps delay. Such fast triplet exciton formation is reported for systems having the scope of singlet fission process. − However, considering the endoergic nature (2 E T1 – 2 E S1 ∼ 0.6 eV) of this present anthracene derivative, the possibility of involvement of singlet fission is very unlikely. S 2 - or hot S 1 -mediated singlet fission was also reported recently for some polyaromatic systems. , However, in the present case, reduction in the triplet formation rate was observed upon the reduction of the excitation intensity.…”

“…There are reports where the singlet fission is found to occur within the 100 fs timescale. 55,59 Hence, following similar spin exchange mechanism, triplet formation is also not unjustified during the singlet−singlet exciton annihilation process. Here, one triplet excitonic level forms, which is lower in energy as compared to the S 1 energy level, and the other triplet state (T m ) formed in the same equation will have a higher energy level as compared to S 1 .…”