Photon upconversion via triplet–triplet annihilation (TTA-UC) is a promising strategy for increasing maximum theoretical solar cell efficiencies by a factor of 1.3. However, one factor limiting integrated TTA-UC solar cell performance is the transmission window between sensitizer and acceptor molecule absorption. Here we demonstrate that the incorporation of a singlet sensitizer (SS) into a self-assembled trilayer is an effective means of harnessing that previously transmitted light. A record TTA-UC photocurrent density of 0.315 mA cm–2 under 1 sun irradiation was achieved and is attributed to directional SS-to-sensitizer singlet energy transfer and sensitizer-to-acceptor triplet energy transfer, followed by TTA, excited-state electron transfer into TiO2, and regeneration by a redox mediator in solution. These results demonstrate that singlet sensitization-enhanced self-assembled trilayers are a promising strategy for enhancing broad-band light absorption and improving the performance of TTA-UC solar cells.

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Self-assembly of sensitizer and acceptor molecules has recently emerged as a promising strategy to facilitate and harness photon upconversion via triplet-triplet annihilation (TTA-UC). In addition to the energetic requirements, the structure and relative orientation of these molecules can have a strong influence on TTA-UC rates and efficiency. Here we report the synthesis of five different acceptor molecules composed of an anthracene core functionalized with 9,10- or 2,6-phenyl, methyl, or directly bound phosphonic acid groups and their incorporation into self-assembled bilayers on a ZrO2 surface. All five films facilitate green-to-blue photon upconversion with Φuc as high as 0.0023. The efficiency of TTA, and not triplet energy transfer, fluorescence, or losses via FRET, was primarily responsible for dictating the Φuc emission. Even for molecules having similar photophysical properties, variation in the position of the phosphonic acid resulted in dramatically different ΦTTA, Ith values, γTTA, and D. Interestingly, we observed a strong linear correlation between ΦTTA and the Ith value but the cause of this relationship, if any, is unclear.

Organic room temperature phosphorescent (ORTP) compounds have recently emerged as a promising class of emissive materials with a multitude of potential applications. However, the number of building blocks that give rise to efficient ORTP materials is still limited, and the rules for engineering phosphorescent properties in organic solids are not well understood. Here, we report ORTP in a series of N-substituted acridone derivatives with electron-donating, electron-withdrawing, and sterically bulky substituents. X-ray crystallography shows that the solid-state packing varies progressively between coparallel and antiparallel πstacking and separated π-dimers, depending on the size of the substituent. The detailed photophysical studies supported by DFT calculations reveal complex dynamics of singlet and triplet excited states, depending on the electronic effects of substituents and solid-state packing. The programmable molecular packing provides a lever to control the triplet−triplet annihilation that is manifested as delayed fluorescence in acridone derivatives with continuous (both parallel and antiparallel) π-stacking.

We explore a series of furan‐based non‐fullerene acceptors and report their optoelectronic properties, solid‐state packing, photodegradation mechanism and application in photovoltaic devices. Incorporating furan building blocks leads to the expected enhanced backbone planarity, reduced bandgap and red‐shifted absorption of these acceptors. Still, their position in the molecule is critical for stability and device performance. We found that the photodegradation of these acceptors originates from two distinct pathways: electrocyclic photoisomerization and Diels‐Alder cycloaddition of singlet oxygen. These mechanisms are of general significance to all non‐fullerene acceptors, and the photostability depends strongly and predictably on the molecular structure. Placement of furans next to the acceptor termini offers better photostability, well‐balanced hole/electron transport, and significantly improved device performance. Methylfuran as the linker led to the best photostability and power conversion efficiency (>14%), outperforming all furan‐based acceptors reported to date and all indacenodithiophene‐based acceptors. Our findings show the possibility of photostable furan‐based alternatives to the currently omnipresent thiophene‐based photovoltaic materials.

Thiele’s hydrocarbon was the first synthesized diradicaloid in the search for stable open-shell structures, but it remains sensitive to oxygen and light. We here report the synthesis of Thiele’s fluorocarbon (TFC) and its derivatives exhibiting exceptional thermal, oxidative, and photostability. TFCs have remarkable luminescent properties with yellow to NIR fluorescence and up to 100% quantum yields. X-ray crystallography and ESR spectroscopy confirm their closed-shell quinoidal ground state. As expected from their symmetric nonpolar structure, the TFCs’ absorption spectra show no solvent effect, but their emission reveals an extraordinarily large Stokes shift which increases with solvent polarity (from 0.9 eV in cyclohexane to 1.5 eV in acetonitrile). We show that this behavior is a result of sudden polarization, leading to a zwitterionic excited state.

We explore a series of furan‐based non‐fullerene acceptors and report their optoelectronic properties, solid‐state packing, photodegradation mechanism and application in photovoltaic devices. Incorporating furan building blocks leads to the expected enhanced backbone planarity, reduced bandgap and red‐shifted absorption of these acceptors. Still, their position in the molecule is critical for stability and device performance. We found that the photodegradation of these acceptors originates from two distinct pathways: electrocyclic photoisomerization and Diels‐Alder cycloaddition of singlet oxygen. These mechanisms are of general significance to all non‐fullerene acceptors, and the photostability depends strongly and predictably on the molecular structure. Placement of furans next to the acceptor termini offers better photostability, well‐balanced hole/electron transport, and significantly improved device performance. Methylfuran as the linker led to the best photostability and power conversion efficiency (>14%), outperforming all furan‐based acceptors reported to date and all indacenodithiophene‐based acceptors. Our findings show the possibility of photostable furan‐based alternatives to the currently omnipresent thiophene‐based photovoltaic materials.

Thiele’s hydrocarbon was the first synthesized diradicaloid in the search for stable open-shell structures but itself remains sensitive to oxygen and light. We here report the synthesis of Thiele’s fluorocarbon (TFC) and its derivatives exhibiting exceptional thermal, oxidative, and photostability. TFCs have remarkable luminescent properties with yellow to NIR fluorescence and up to 100% quantum yields. X-ray crystallography and ESR spectroscopy confirm their closed-shell quinoidal ground state. As expected from their uniform HOMO/LUMO distributions, TFCs’ absorption spectra show no solvent effect, but their emission reveals an extraordinarily large Stokes shift which increases with solvent polarity (from 0.9 eV in cyclohexane to 1.5 eV in acetonitrile). We show that this behavior is a result of sudden polarization leading to a zwitterionic excited state.