Organic chromophore aggregates for solid-state photon upconversion

Abstract

Photon upconversion, a process that creates the excited states at higher energy (shorter wavelength) with excitation at lower energy (longer wavelength), has numerous potentials in photovoltaic, photocatalysis and bioimaging applications. Triplet-triplet annihilation based upconversion (TTA-UC) has attracted significant attentions due to its advantages of harnessing non-coherent and low-power excitation, coupled with intense absorption of the excitation light and high upconversion quantum yield. Two chemical components are required for TTA-UC: the first component, the sensitizer, absorbs a low energy photon and transfers it to the second component, the emitter, through the triplet-triplet energy transfer (TTET). Two or more triplet excited states generated in emitters come together and annihilate to emit one photon of higher energy than that of the initially absorbed photon. To date, the most efficient TTA-UC has been achieved in solution, which allows the rapid diffusion of chromophores, however, the solution-based systems are impractical for real-world applications. Therefore, solid-state upconversion systems are highly desirable for real technical applications. Current endeavours to improve the solid-state upconversion efficiency have been mainly focused on increasing the triplet exciton migration. The efforts to understand the parameters to develop new emitter molecules with efficient TTA and high fluorescence quantum yield in solid-state systems are limited.

This work aims to advance our understanding of the effect of molecular geometry on TTA-UC, improve the overall upconversion efficiency in solid-state systems and approach the ultimate goal of developing an efficient solid-state upconversion solar cell device.

Firstly, the effect of molecular geometry on the TTA upconversion performance was investigated by employing four new 9,10-diphenylanthracene (DPA) derivatives as the emitters and platinum octaethylporphyrin (PtOEP) as the triplet sensitizer. These new emitter molecules containing multiple DPA subunits linked together via a central benzene core. The upconversion quantum yield values were determined both in solution and solid-state systems. In solution, the meta-substitute dimer exhibited the highest upconversion quantum yield among the new DPA derivatives, but all were inferior to the benchmark DPA-PtOEP couple. The different upconversion quantum yield values between the new DPA derivatives were mainly attributed to the statistical probability for obtaining a high energy singlet excited state from TTA, f, both for inter- and intra- molecular processes. The f value was strongly related to the interactions between the excited state chromophores and also dependent on the magnetic field. Of the dimers, the m-dimer exhibited the largest fint (statistical probability for obtaining a singlet excited state from intramolecular TTA), indicating the highest proportion of upconversion was from intramolecular TTA. The linkage through the meta position resulted in the weakest exchange-coupling between the triplet excited states, leading to a largest magnetic field effect (MEF%). The fall (statistical probability for obtaining a singlet excited state from both inter- and intra- molecular TTA) values of the dimers and trimer were smaller than that of DPA, which were consistent with the upconversion quantum yield results. These results demonstrated that multichromophoric emitters are not necessarily better than single chromophores. In the solid-state, the much lower upconversion quantum yield of new DPA derivatives compared to the DPA reference was attributed to the much smaller fluorescence quantum yield and slower TTET from the sensitizer to the emitter.

Further investigation of the effect of arrangement of chromophores in multi-chromophore molecules on TTA-UC was carried out by utilizing tetraphenylethene 9,10-diphenylanthracene (TPE-DPA) derivatives as the emitters. With the TPE chromophore in these multi-chromophore molecules, the new TPE-DPA derivatives exhibited aggregation induced emission (AIE) behaviour as their fluorescence quantum yield increased as increasing the proportion of water in THF solution. Various fluorescence quantum yield values obtained in PMMA, neat films and nanoparticle dispersions indicating that different aggregation states were formed. Upconversion emission was detected from the TPE-DPA/PtOEP nanoparticle dispersions with the gem-DPA/PtOEP showing the highest upconversion emission intensity. A higher upconverted emission intensity was observed in aerated (compared to deaerated) solutions of the derivatives following UV light irradiation, which was attributed to oxidation of the TPE moiety. The effect of oxygen on the photophysics of the TPE-DPA derivatives was further investigated under the UV light irradiation. The TPE moiety in the gem-DPA was found to be more easily oxidized than that in the mono- and cis-DPA. A possible oxidation route was proposed: weak intersystem crossing in TPE-DPA derivatives created a triplet population which can sensitize formation of highly reactive singlet oxygen. This photosensitized singlet oxygen preferred to react with TPE moiety first forming endoperoxide. In the absence of the TPE moiety, the molecules showed enhanced fluorescence in solution.

Finally, the solid state upconversion quantum yield of the DPA/PtOEP systems was improved by decreasing the sensitizer’s aggregation and phase separation. Two new DPA derivatives (bDPA-1 and bDPA-2), with bulky isopropyl groups on the phenyl rings, were used as the emitters. Under the sensitization of the PtOEP, TTA-UC performance was comprehensively investigated in various systems: toluene solution, polyurethane thin film and crystals. Only a small difference of upconversion efficiency was observed in toluene solution and polyurethane thin film of b-DPAs compared to the DPA reference. Interestingly, a greater than 10 times improvement in the upconversion quantum yield was obtained in bDPA-2/PtOEP crystals with low excitation intensity threshold compared to that of the DPA/PtOEP crystals. The dispersibility of the sensitizer in emitter crystals was improved in bDPA-2 due to its slower rate of crystal formation. Apart from the excellent TTA-UC performance of the b-DPAs, they also show outstanding stability under UV light irradiation, which is ideal for the real-world long-term application of TTA-UC. Based on the results presented herein, we are moving closer towards the development of highly efficient solid-state upconversion devices for harvesting solar energy.