Singlet exciton fission in a modified acene with improved stability and high photoluminescence yield

Abstract

We report a fully efficient singlet exciton fission material with high ambient chemical stability. 10,21-Bis(triisopropylsilylethynyl)tetrabenzo[a,c,l,n]pentacene (TTBP) combines an acene core with triphenylene wings that protect the formal pentacene from chemical degradation. The electronic energy levels position singlet exciton fission to be endothermic, similar to tetracene despite the triphenylenes. TTBP exhibits rapid early time singlet fission with quantitative yield of triplet pairs within 100 ps followed by thermally activated separation to free triplet excitons over 65 ns. TTBP exhibits high photoluminescence quantum efficiency, close to 100% when dilute and 20% for solid films, arising from triplet-triplet annihilation. In using such a system for exciton multiplication in a solar cell, maximum thermodynamic performance requires radiative decay of the triplet population, observed here as emission from the singlet formed by recombination of triplet pairs. Combining chemical stabilisation with efficient endothermic fission provides a promising avenue towards singlet fission materials for use in photovoltaics.

Parallel triplet formation pathways in a singlet fission material

“…Strikingly, we even observe similar triplet features—polarization pattern and spectral width—in H-like film 2 , which shows no evidence for SF and no detectable optical signatures of triplet formation. Because of this surprising behavior, we additionally considered the role of orientation of both samples relative to the external magnetic field, which has been shown to impact the trEPR spectra of triplets in other SF materials 73 . In neither film did we observe the appearance of SF spectral signatures at other angles.…”

Down‐conversion materials for organic solar cells: Progress, challenges, and perspectives

“…[45] Congreve et al demonstrated that the SF process could cross the external quantum efficiency over the unity of OSCs. [46] There are a few materials have been reported for SF application in the literature called polydiacetylene, [47] carotenoids, [48] pentacene, [49] tetracene, [50] anthracene, [51] and 10,21-bis(tri-isopropylsilylethynyl) tetrabenzo[a,c,l,n] pentacene, [52] and so forth, and it is very challenging to find the right material. The spectral converter materials were introduced in the solar cells to assist the higher to lower photons energy conversion.…”

Permanent Dipole Moments Enhance Electronic Coupling and Singlet Fission in Pentacene

“…This has led to increased interest in the engineering of the properties of single molecules and molecular aggregates via chemical modification. In the context of a rational design of SF chromophores, several key parameters have been identified, such as electronic coupling strength, molecular arrangement, and the energetic difference of singlet and triplet states. − Current approaches toward an optimization of SF chromophores employ, for example, the addition of bulky side groups to modify the molecular packing in aggregates ,− or the use of dimers of SF chromophores for a controlled variation of the relative molecular arrangement and, thus, of the electronic coupling strength. − Alternatively, the energy difference between singlet and triplet states has been modified by the substitution of carbon atoms of the SF chromophore backbone with heteroatoms, such as nitrogen or sulfur, − or by fluorination of the SF chromophore. − The latter is in the general context of the optimization of organic semiconductors for optoelectronic devices, a well-established approach to increase stability against oxidation or to control energy levels, molecular packing, and charge transport. − …”