Solar Harvesting the Bright Way: Optimising Photon Multiplication with Novel Chromophores
The abundance of solar energy incident on the Earth’s surface offers immense potential for meeting current and future global energy demands. Photovoltaic cells, used for the harvesting of solar energy, are constrained by the thermodynamic Shockley-Queisser limit to a maximum of 34% efficiency. Photon multiplication offers a way to circumvent this limit by splitting a single high energy photon into two or more photons of lower energy. Singlet fission, an excitation multiplication process that occurs in some organic molecules, can be used to harness the excess energy of photons, followed by energy transfer to a quantum dot for re-emission, acting as a photon multiplication system.
Singlet fission-based photon multipliers were recently developed, transferring triplets generated by singlet fission in a tetracene derivative to a lead sulfide quantum dot for emission, with a 17 ± 1% photoluminescence efficiency. Herein, we explore ways to improve this photon multiplication efficiency in a tetracene/PbS quantum dot-based system, through controlled design and synthesis of a range of novel tetracene chromophores to optimise quantum dot surface passivation. We functionalised PbS and PbS/CdS quantum dots with these novel tetracenes and measured the efficiencies of photon multiplication. We found that the addition of less tetracene to the quantum dot surface allowed efficient singlet fission without disrupting passivation. The overall photon multiplication efficiency using a PbS quantum dot emitter was found to be 36 ± 4%. The photon multiplication efficiency was further improved to 61 ± 6% with the addition of a CdS shell to aid in passivation of the emissive PbS core. As far as we are aware, these values are the highest reported singlet fission-based photon multiplication efficiencies to date.
The primary limitation of current singlet fission photon multipliers is the use of quantum dots with only moderate emission efficiencies. While PbS emission efficiencies range from 10 – 70%, CsPbX3 (where X = Cl, Br or I) can reach close to 100% emission efficiency. Herein, we designed and synthesised a range of anthracene derivatives with different binding functionalities in an attempt to optimise surface functionalisation of highly emissive CsPbI3 nanocrystals. We used Stern-Volmer analysis on the synthesised anthracene derivatives and CsPbI3 nanocrystals. We found strong binding of all the anthracene derivatives, although, these systems were limited by the low (<30%) energy transfer efficiencies achieved, irrespective of binding group. Preliminary investigation into the use of other quantum dot systems found an energy transfer efficiency of 72% from a carboxylic acid anthracene into quantum confined CsPbBr3 perovskite nanocrystals.
Another limitation to photon multiplication systems based on singlet fission in tetracene is the lack of absorption across the 360 – 450 nm region. Herein, we designed and synthesised two perylene- tetracene dyads to improve absorption across this region. Spectral analysis of these dyads found significant increases in absorption, without causing significant changes to the tetracene peak wavelength or shape. Energy transfer from perylene to tetracene was shown, with efficiencies estimated between 36 – 95%, demonstrating the ability for perylene sensitisation of tetracene. One of the synthesised sensitisers was then added to a tetracene/PbS quantum dot photon multiplier, both in solution and in solid state, and was found to negatively impact the emission efficiency of the PbS quantum dots in both cases.
We also carried out an investigation into the unexpected solvent tuneable emission that was observed in the above perylene-tetracene dyads. We designed and synthesised two novel perylene-acene molecules for investigation into the scope of this tuneability. We performed solvent-dependent absorption and emission spectroscopy and it was found that this effect was observed across a series of perylene-tetracene molecules. This effect was extended into a perylene-pentacene dyad, offering excellent tuneability across the entire visible region. Transient absorption spectroscopy was used to investigate the mechanism of emission wavelength tuneability. It was proposed that tuneability arises from a charge separated state only accessible in more polar solvents that quickly decays once occupied.
This thesis looked at optimising photon multiplication by controlling surface passivation of PbS quantum dots. This was successfully achieved, recording the highest emission efficiency of a photon multiplier to date. While photon multiplication was not able to be shown with an anthracene/perovskite nanocrystal system, energy transfer from anthracene to a nanocrystal was optimised, reaching transfer efficiencies as high as 72%. Through the synthesis of a perylene-tetracene sensitiser, we improved the absorption of tetracene through intramolecular energy transfer. Furthermore, these dyads were also used to create a novel class of tuneable dyes offering moderate emission efficiencies.
