Anchoring Energy Acceptors to Nanostructured ZrO₂ Enhances Photon Upconversion by Sensitized Triplet–Triplet Annihilation Under Simulated Solar Flux

Abstract

Photon upconversion by sensitized triplet−triplet annihilation (UC-STTA) is a promising strategy for boosting the theoretical maximum efficiency of single threshold solar cells, in particular, dye-sensitized solar cells (DSSCs). Here, we report a substantial increase in the efficiency of UC-STTA on a nanostructured surface, using noncoherent excitation light with intensities as low as 0.5 mW cm −2 , easily achieved under sun illumination. The studied surface was a mesoporous ZrO 2 film working as a proxy system for the study of photophysics relevant to DSSCs. A wellknown UC-STTA "emitter" dye, 9,10-diphenylanthracene (DPA), was chemically modified to yield methyl 4-(10-p-tolylanthracen-9yl)benzoate (MTAB), which was chemisorbed onto ZrO 2 . The "sensitizer" dye, platinum(II) octaethylporphyrin (PtOEP), was free in butyronitrile (BuN) solution surrounding the ZrO 2 nanostructure. A rigorous oxygen removal minimized photodegradation of the dyes and enhanced triplet−triplet annihilation efficiency. The system already approaches the so-called "strong annihilation limit" at light intensities below 8 mW cm −2 . Highly efficient triplet−triplet annihilation is a requisite for the use of UC-STTA in DSSCs. Time-resolved data show that the limiting process in the UC-STTA mechanism of the present system is the dynamic triplet energy transfer step from PtOEP in solution to MTAB on the surface of ZrO 2 . This result can guide the way toward a better understanding and further efficiency improvement of UC-STTA on nanocrystalline metal oxides.

An intermediate band dye-sensitised solar cell using triplet–triplet annihilation

“…S8.2). As discussed by Lissau et al 24 , this slow onset is due to the necessary build-up of triplet-excited BDCA prior to TTA occurring, which generates the injection-active BDCA singlet state. BDCA + absorption is centred around 700 nm, but is quite broad and featureless, making direct spectral analysis somewhat challenging.…”

Photochemical upconversion: present status and prospects for its application to solar energy conversion

“…Lissau et al reported upconverted fluorescence from the PtOEP/DPA system adsorbed onto ZrO 2 nanoparticles, but found their system to be limited by aggregation; sub-optimal dye loading and orientation; and oxygen quenching. An improved system featured functionalized DPA with carboxylate anchor groups on ZrO 2 nanoparticles floating in a butyronitrile solution containing the PtOEP sensitizer, and showed improved performance 144 .…”

Upconversion for Photovoltaics – a Review of Materials, Devices and Concepts for Performance Enhancement

“…Hybrid Surroundings : Lissau et al investigated TTA upconversion in an environment that mimics a dye‐sensitized solar cell. PtOEP in butyronitrile (BuN) solution was used as a sensitizer, while the well‐known emitter DPA was chemically modified to methyl 4‐(10‐p‐tolylanthracen‐9‐yl)benzoate (MTAB) and chemisorbed onto mesoporous ZrO 2 , with subsequent removal of oxygen . Under excitation at (534 ± 5) nm and a very low irradiance of 8 mW cm −2 , they observed an internal UCQY of 0.04% by comparison to Ru(bpy) 3 Cl 2 in air‐equilibrated water as a reference standard.…”