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Abstract
Triplet-triplet annihilation photon upconversion (TTA-UC) in solid state assemblies are desirable since they can be easily incorporated into devices such as solar cells, thus utilizing more of the solar spectrum. Realizing this is, however, a significant challenge that must circumvent the need for molecular diffusion, poor exciton migration, and detrimental back energy transfer among other hurdles. Here, we show that the above-mentioned issues can be overcome using the versatile and easily synthesized oxotriphenylhexanoate (OTHO) gelator that allows covalent incorporation of chromophores (or other functional units) at well-defined positions. To study the self-assembly properties as well as its use as a TTA-UC platform, we combine the benchmark couple platinum octaethylporphyrin as a sensitizer and 9,10-diphenylanthracene (DPA) as an annihilator, where DPA is covalently linked to the OTHO gelator at different positions. We show that TTA-UC can be achieved in the chromophore-decorated gels and that the position of attachment affects the photophysical properties as well as triplet energy transfer and triplet-triplet annihilation. This study not only provides proof-of-principle for the covalent approach but also highlights the need for a detailed mechanistic insight into the photophysical processes underpinning solid state TTA-UC.
Highlights
In 1960, Shockley and Queisser1 published the theoretical efficiency limit for single junction solar cells as ≈32%, a limit that comes about mainly due to the mismatch between the solar spectrum and the bandgap of the solar cell materials
We show that TTA-UC can be achieved in the chromophore-decorated gels and that the position of attachment affects the photophysical properties as well as triplet energy transfer and triplet–triplet annihilation
The self-assembly relies to a large extent on the aromatic rings 1 and 2 and assures the close proximity between chromophores required for TTA (Fig. 2)
Summary
In 1960, Shockley and Queisser published the theoretical efficiency limit for single junction solar cells as ≈32%, a limit that comes about mainly due to the mismatch between the solar spectrum and the bandgap of the solar cell materials. A few years later, Parker and Hatchard published their seminal papers on a phenomenon that today is considered a promising way to overcome the Shockley–Queisser limit, photon upconversion through triplet– triplet annihilation (TTA-UC).. In the TTA-UC process, two low energy photons are combined to form one high energy photon, enabling utilization of photons with lower energy than the solar cell bandgap.. Sensitized TTA-UC, schematically illustrated, relies on a series of energy transfer reactions.. A triplet excited sensitizer (3S∗) is formed through photon absorption and intersystem crossing (ISC). The 3S∗ interacts with a ground state annihilator (A0) through a Dexter type triplet–triplet energy transfer (TET) reaction yielding a triplet excited annihilator (3A∗). Thereafter, two 3A∗’s interact in the triplet–triplet annihilation (TTA) process to form one singlet excited annihilator (1A∗), which can release its excess energy as photons, and one ground state annihilator (A0)
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