Selective detection of circularly polarised light using chiral organic semiconductors
File(s)
Author(s)
Ward, Matthew David
Type
Thesis or dissertation
Abstract
Chiral organic semiconductors with sensitivity to circularly polarised light (CPL) handedness will enable miniaturised, low-cost, and wavelength-tuneable CPL photodetectors. These devices have the potential to revolutionise several technologies, including biosensing, quantum optics, and spintronics. In this thesis, chiral materials are applied to organic photodetector architectures to identify design rules that relate device architecture to conventional and CPL-selective figures of merit (FoM).
This thesis demonstrates the first published chiral fullerene phototransistor, reporting a photocurrent dissymmetry (g_ph) of ≤1.27±0.06 for fullerenes with absorption dissymmetry factors (g_abs) of ∼10^{−3}. This behaviour supports previous reports of g_ph enhancement relative to g_abs and is attributed to a CPL-selective shift of phototransistor threshold voltages.
This thesis also demonstrates a novel CPL-selective photodiode architecture using a blend of achiral poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2) and chiral 1-aza[6]helicene (aza[6]H). Using this architecture, we show that conventional and CPL-selective photodetector FoM are antagonistic. We also show that CPL-selective FoM depend on the chiral donor layer and the
achiral acceptor layer. Device optimisation yields photodiodes with the fastest response time reported to date (t_{rise/fall} ≈ 7 μs) with high CPL selectivity (g_ph > 0.1). We then model the behaviour of our F8T2:aza[6]H blends using transfer matrix optics (TMO). We find that TMO alone is insufficient to relate the device architecture to measured g_ph. We also explore the possibility of CPL-selective charge transport using time-of-flight mobility measurements. Hole transport is not observed to be CPL-selective and is highly dispersive, with a mobility of ≳0.3 cm^2V^{−1}s^{−1}.
Finally, we develop two novel circularly polarised imaging systems, capable of spatially mapping circular dichroism in solid-state samples. The optimised system has a resolution of 100 μm and a sensitivity of 250 mdeg. This allows the processing of chiral samples to be observed in situ to accelerate processing optimisation.
This thesis demonstrates the first published chiral fullerene phototransistor, reporting a photocurrent dissymmetry (g_ph) of ≤1.27±0.06 for fullerenes with absorption dissymmetry factors (g_abs) of ∼10^{−3}. This behaviour supports previous reports of g_ph enhancement relative to g_abs and is attributed to a CPL-selective shift of phototransistor threshold voltages.
This thesis also demonstrates a novel CPL-selective photodiode architecture using a blend of achiral poly(9,9-dioctylfluorene-alt-bithiophene) (F8T2) and chiral 1-aza[6]helicene (aza[6]H). Using this architecture, we show that conventional and CPL-selective photodetector FoM are antagonistic. We also show that CPL-selective FoM depend on the chiral donor layer and the
achiral acceptor layer. Device optimisation yields photodiodes with the fastest response time reported to date (t_{rise/fall} ≈ 7 μs) with high CPL selectivity (g_ph > 0.1). We then model the behaviour of our F8T2:aza[6]H blends using transfer matrix optics (TMO). We find that TMO alone is insufficient to relate the device architecture to measured g_ph. We also explore the possibility of CPL-selective charge transport using time-of-flight mobility measurements. Hole transport is not observed to be CPL-selective and is highly dispersive, with a mobility of ≳0.3 cm^2V^{−1}s^{−1}.
Finally, we develop two novel circularly polarised imaging systems, capable of spatially mapping circular dichroism in solid-state samples. The optimised system has a resolution of 100 μm and a sensitivity of 250 mdeg. This allows the processing of chiral samples to be observed in situ to accelerate processing optimisation.
Version
Open Access
Date Issued
2023-06-05
Date Awarded
2024-09-01
Copyright Statement
Creative Commons Attribution NonCommercial Licence
License URL
Advisor
Campbell, Alasdair
Nelson, Jenny
Fuchter, Matthew
Sponsor
Engineering and Physical Sciences Research Council
Grant Number
EP/L016702/1
Publisher Department
Physics
Publisher Institution
Imperial College London
Qualification Level
Doctoral
Qualification Name
Doctor of Philosophy (PhD)