A pinboard by
Emily Westbrook

Graduate student, University of Cincinnati


Improving triplet-tripet annihilation upconversion in solid state applications

Solar panels are fairly inefficient, usually reaching no higher than 30% of their capability. Triplet-triplet annihilation upconversion is a process that turns lower energy light into higher energy light. This process could be used to improve solar panels by reusing some of the light that is not used by the solar panel material and making it usable. However, this process is usually done in solution, which is not feasible for application in solar panels. So my research is focused on improving the results from materials as solids. Our strategy is to bind the molecules responsible for the process to a polymer which can be easily made into a film without the usual separation of the molecules. The triplet-triplet annihilation upconversion process requires the molecules to be close together, but separation of those molecules usually occurs in the solid state. Attaching the molecules to a polymer locks them into position so they cannot separate when put in the solid state, allowing the process to be as efficient as possible. We are trying to vary the amount of the molecules attached to each polymer to determine the best performing combination, which can be used as an additional layer in solar panels to make them more efficient and potentially cheaper.


Improving triplet-triplet-annihilation based upconversion systems by tuning their topological structure.

Abstract: Materials capable to perform upconversion of light transform the photon spectrum and can be used to increase the efficiency of solar cells by upconverting sub-bandgap photons, increasing the density of photons able to generate an electron-hole pair in the cell. Incoherent solar radiation suffices to activate upconverters based on sensitized triplet-triplet annihilation, which makes them particularly suited for this task. This process requires two molecular species, sensitizers absorbing low energy photons, and emitters generating higher frequency photons. Successful implementations exist in solutions and solids. However, solid upconverters exhibit lower efficiency than those in solution, which poses a serious problem for real applications. In the present work, we suggest a new strategy to increase the efficiency of sensitized upconverters that exploits the solid nature of the material. We show that an upconversion model system with molecules distributed as clusters outperforms a system with a random distribution of molecules, as used in current upconverters. Our simulations reveal a high potential for improvement of upconverter systems by exploring different structural configurations of the molecules. The implementation of advanced structures can push the performance of solid upconverters further towards the theoretical limit and a step closer to technological application of low power upconversion.

Pub.: 17 Nov '14, Pinned: 29 Jun '17

Switching of the triplet-triplet-annihilation upconversion with photoresponsive triplet energy acceptor: photocontrollable singlet/triplet energy transfer and electron transfer.

Abstract: A photoswitchable fluorescent triad based on two 9,10-diphenylanthracene (DPA) and one dithienylethene (DTE) moiety is prepared for photoswitching of triplet-triplet annihilation upconversion. The DPA and DTE moieties in the triad were connected via Click reaction. The DPA unit in the triad was used as the triplet energy acceptor and upconverted fluorescence emitter. The fluorescence of the triad is switched ON with the DTE moiety in open form [DTE-(o)] (upconversion quantum yield ΦUC = 1.2%). Upon UV irradiation, photocyclization of the DTE-(o) moiety produces the closed form [DTE-(c)], as a result the fluorescence of DPA moiety was switched off (ΦUC is negligible). Three different mechanisms are responsible for the upconverted fluorescence photoswitching effect (i.e., the photoactivated fluorescence resonance energy transfer, the intramolecular electron transfer, as well as the photoactivated intermolecular triplet energy transfer between the photosensitizer and DTE-(c) moiety). Previously, the photoswitching of TTA upconversion was accomplished with only one mechanism (i.e., the triplet state quenching of the photosensitizer by DTE-(c) via either the intermolecular or intramolecular energy transfer). The photophysical processes involved in the photochromism and photoswitching of TTA upconversion were studied with steady-state UV-vis absorption and fluorescence emission spectroscopies, nanosecond transient absorption spectroscopy, electrochemical characterization, and DFT/TDDFT calculations.

Pub.: 17 Dec '14, Pinned: 29 Jun '17

Near-Infrared-to-Visible Photon Upconversion Enabled by Conjugated Porphyrinic Sensitizers under Low-Power Noncoherent Illumination.

Abstract: We report four supermolecular chromophores based on (porphinato)zinc(II) (PZn) and (polypyridyl)metal units bridged via ethyne connectivity (Pyr1RuPZn2, Pyr1RuPZnRuPyr1, Pyr1RuPZn2RuPyr1, and OsPZn2Os) that fulfill critical sensitizer requirements for NIR-to-vis triplet-triplet annihilation upconversion (TTA-UC) photochemistry. These NIR sensitizers feature: (i) broad, high oscillator strength NIR absorptivity (700 nm < λ(max(NIR)) < 770 nm; 6 × 10(4) M(-1) cm(-1) < extinction coefficient (λ(max(NIR))) < 1.6 × 10(5) M(-1) cm(-1); 820 cm(-1) < fwhm < 1700 cm(-1)); (ii) substantial intersystem crossing quantum yields; (iii) long, microsecond time scale T1 state lifetimes; and (iv) triplet states that are energetically poised for exergonic energy transfer to the molecular annihilator (rubrene). Using low-power noncoherent illumination at power densities (1-10 mW cm(-2)) similar to that of terrestrial solar photon illumination conditions, we demonstrate that Pyr1RuPZn2, Pyr1RuPZn2RuPyr1, and Pyr1RuPZnRuPyr1 sensitizers can be used in combination with the rubrene acceptor/annihilator to achieve TTA-UC: these studies represent the first examples whereby a low-power noncoherent NIR light source drives NIR-to-visible upconverted fluorescence centered in a spectral window within the bandgap of amorphous silicon.

Pub.: 12 May '15, Pinned: 29 Jun '17

Triplet–Triplet Annihilation Photon Upconversion in Polymer Thin Film: Sensitizer Design

Abstract: Efficient visible-to-UV photon upconversion via triplet–triplet annihilation (TTA) is accomplished in polyurethane (PU) films by developing new, powerful photosensitizers fully functional in the solid-state matrix. These rationally designed triplet sensitizers feature a bichromophoric scaffold comprising a tris-cyclometalated iridium(III) complex covalently tethered to a suitable organic small molecule. The very rapid intramolecular triplet energy transfer from the former to the latter is pivotal for achieving the potent sensitizing ability, because this process out-competes the radiative and nonradiative decays inherent to the metal complex and produces long-lived triplet excitons localized with the acceptor moiety readily available for intermolecular transfer and TTA. Nonetheless, compared to the solution state, the molecular diffusion is greatly limited in solid matrices, which even creates difficulty for the Dexter-type intramolecular energy transfer. This is proven by the experimental results showing that the sensitizing performance of the bichromophoric molecules strongly depends on the spatial distance separating the donor (D) and acceptor (A) units and that incorporating a longer linker between the D and A evidently curbs the TTA upconversion efficiency in PU films. Using a rationally optimized sensitizer structure in combination with 2,7-di-tert-butylpyrene as the annihilator/emitter, the doped polyurethane (PU) films demonstrate effective visible-to-UV upconverted emission signal under noncoherent-light irradiation, attaining an upconversion quantum yield of 2.6%. Such quantum efficiency is the highest value so far reported for the visible-to-UV TTA systems in solid matrices.

Pub.: 15 Apr '16, Pinned: 29 Jun '17

Pd–Porphyrin Oligomers Sensitized for Green‐to‐Blue Photon Upconversion: The More the Better?

Abstract: A series of directly meso‐meso‐linked Pd–porphyrin oligomers (PdDTP‐M, PdDTP‐D, and PdDTP‐T) have been prepared. The absorption region and the light‐harvesting ability of the Pd–porphyrin oligomers are broadened and enhanced by increasing the number of Pd–porphyrin units. Triplet–triplet annihilation upconversion (TTA‐UC) systems were constructed by utilizing the Pd–porphyrin oligomers as the sensitizer and 9,10‐diphenylanthracene (DPA) as the acceptor in deaerated toluene and green‐to‐blue photon upconversion was observed upon excitation with a 532 nm laser. The triplet–triplet annihilation upconversion quantum efficiencies were found to be 6.2 %, 10.5 %, and 1.6 % for the [PdDTP‐M]/DPA, [PdDTP‐D]/DPA, and [PdDTP‐T]/DPA systems, respectively, under an excitation power density of 500 mW cm−2. The photophysical processes of the TTA‐UC systems have been investigated in detail. The higher triplet–triplet annihilation upconversion quantum efficiency observed in the [PdDTP‐D]/DPA system can be rationalized by the enhanced light‐harvesting ability of PdDTP‐D at 532 nm. Under the same experimental conditions, the [PdDTP‐D]/DPA system produces more 3DPA* than the other two TTA‐UC systems, benefiting the triplet–triplet annihilation process. This work provides a useful way to develop efficient TTA‐UC systems with broad spectral response by using Pd–porphyrin oligomers as sensitizers.

Pub.: 03 May '16, Pinned: 29 Jun '17

Enhanced Triplet–Triplet Annihilation Upconversion in Dual-Sensitizer Systems: Translating Broadband Light Absorption to Practical Solid-State Materials

Abstract: Photochemical upconversion (UC) of low-energy photons that would otherwise be wasted could drastically improve the efficiency of solar technologies by allowing them to harness a greater fraction of the solar spectrum. Although UC through the triplet–triplet annihilation (TTA) mechanism operates efficiently under low-power irradiation such as sunlight, its ability to improve solar device efficiencies is limited by the narrow light absorption bands of its sensitizer chromophores. This bottleneck on UC performance can be overcome by employing multiple sensitizers in tandem, but such an approach has thus far been studied exclusively in solution-based TTA-UC systems requiring intensive deoxygenation and sealing procedures. This study presents the first dual-sensitizer TTA-UC system in a solid-state host suitable for practical applications. We fabricate thin polyurethane films containing two benchmark TTA-UC sensitizers in a range of different concentrations and characterize their red-to-blue and green-to-blue UC performance as a function of excitation intensity. The broadband absorption of the dual-sensitizer films significantly enhances their performance under simultaneous low-intensity excitation of the two sensitizers, giving rise to anti-Stokes fluorescence surpassing the combined anti-Stokes fluorescence of the films’ single-sensitizer analogues. We circumvent trade-offs between light absorption and TTA-UC performance at high sensitizer concentrations by harnessing the films’ unique versatility to produce an alternative “multijunction” TTA-UC system comprising overlaid single-sensitizer films, thereby achieving strong broadband light absorption and superior TTA-UC performance.

Pub.: 20 Dec '16, Pinned: 29 Jun '17