1,721,287 research outputs found
Understanding exciton diffusion in organic solar cells
No full textOrganic solar cells are promising candidates for future energy production. However,
many challenges remain to optimise efficiency. One particular challenge lies in our understanding of the factors affecting exciton diffusion and the length scale of this process. This is particularly relevant to morphology optimisation for organic solar
cells. A powerful tool to gain insight into this is femtosecond photophysical measurements.
This thesis details work undertaken at the University of St Andrews between
September 2009 and December 2010 towards a Masters of Philosophy. It includes
an introduction to the field and a description of experimental methods used.
Experimental chapters discuss measuring the optical constants of P3HT, a study of
low concentration volume quenching, surface quenching measurements using
titanium dioxide, surface quenching measurements using other quenchers and an
investigation of the thickness dependence of the photoluminescence decay time in
thin polymer films
Photophysics of linear and star-shaped oligofluorenes and their application in lasers
No full textThis thesis presents a study of the photophysical properties of a number of fluorene molecules used for organic semiconductor lasers. These results are then combined with lasing results to assess what the important properties in an organic semiconductor laser material are.
Photophysical measurements were performed on a family of oligofluorenes; results show a redshift in the peak absorption and emission wavelengths with increasing length. There is also an increase in the molar extinction coefficient and photoluminescence quantum yields of the molecules. Transition dipole moments also increase with length, but fluorescence scales slower than absorption due to self-trapping occurring at longer molecular lengths.
This study was then expanded to two families of star-shaped molecules with fluorene arms and differing cores. These molecules have three arms connected to either a central benzene unit or a larger truxene core. These molecules show an increase in PLQY and roughly three times higher molar extinction coefficients than comparable linear oligofluorenes. The star-shaped molecules PLQY and transition dipole moments are both greater than their linear oligofluorene counterparts.
Energy transfer was then studied in the truxene-cored molecules, which showed that the symmetry of the molecule was broken due to interactions with the solvent. Energy transfer was observed on two timescales; a fast 500 fs process which is attributed to a localisation onto a single arm to emit, and a 3-10 ps second decay component, and was assigned to resonant energy transfer between the arms. Both decays were found to be wavelength dependent.
Lasing results were then obtained for the benzene cored molecules. It was found that star-shaped molecules present improved lasing characteristics with lower ASE and lasing thresholds. These results were compared with those obtained for truxene-cored molecules whose rigid core provides them with better lasing and ASE characteristics
Material and device design for organic optoelectronics
No full textThis thesis describes investigations into the photophysical properties of luminescent
materials and their application in optoelectronic devices such as light emitting diodes
and photodetectors. The materials used were all solution processable because of the
interest in low cost processing of organics.
I have investigated the photophysics of 1,4,5,8,9,12-hexamethyltriphenylene, a
triphenylene derivative which has its luminescence enhanced by the addition of
methyl groups. These groups change the planar shape of the triphenylene molecule
into a twisted one, changing the symmetry of the molecule and increasing its dipole
moment in absorption and emission by ~4 fold. This increased its rate of radiative deexcitation
by ~20 times. In addition, the twisted shape of the molecule prevents
intermolecular interactions and concentration effects from affecting the luminescence.
This results in an efficient solid-state photoluminescence quantum yield of 31%.
This thesis also includes an investigation into phosphorescent polymer dendrimers,
designed to have suitable viscosities in solution for inkjet printed OLED applications.
A photophysical study of the intra-chain aggregation effects on the luminescence was
undertaken in both homopolymers and copolymers with high energy gap spacer units.
Using double dendrons to increase the steric protection of the luminescent cores, the
best homopolymers achieved 12.1% external quantum efficiency (39.3 cd/A) at 100
cd/m² brightness and the best co-polymer achieved 14.7% EQE (48.3 cd/A) at 100
cd/m². This compares favourably with 11.8% EQE for the best phosphorescent
polymer and 16% for the best solution processed dendrimer OLED previously
reported.
Finally I have applied a solution processed enhancement layer to silicon photodiodes
to enhance their ultraviolet response. Using a blend of materials to give favourable
absorption and emission properties, 61% external quantum efficiency was achieved at
200 nm, which is better than the 20-30% typical for vacuum deposited lumogen
enhancement layers used commercially
Material and device design for organic photovoltaics
Get PDFThis thesis presents novel materials for photovoltaic conversion. The materials described are solution-processable organic semiconductors and have been used in the fabrication of organic photovoltaic cells (OPVs).
The widely used PEDOT:PSS layer was investigated in P3HT and PTB7 photovoltaics. By doping, the efficiencies recorded were amongst the highest reported in the field using a conventional architecture.
Two low band-gap BODIPY-based polymers were introduced and shown to have properties favourable for optoelectronics. Photovoltaics consisting solely of the polymers as the active component surpassed the performance expected without the use of an acceptor, indicating ambipolar behaviour, which was verified by charge carrier mobility measurements. When blended with an acceptor, the devices demonstrated a short-circuit current density similar to that of P3HT, a well-studied and successful OPV material. They also revealed a broad spectral response and were shown to operate as photodiodes.
Two small molecules containing diketopyrrolopyrrole (DPP) and BODIPY were introduced and characterised. The addition of thiophenes red shifted the absorption but did not result in a sufficient bathochromic shift. Instead, a propensity to aggregate limited the performance. PLQY measurements showed the aggregation to quench luminescence. The study demonstrated the importance of controlling aggregation for efficient devices.
Two solution-processable small molecules with a germanium-bridged spiro centre were investigated, and the molecular, electrochemical and optical properties discussed. The small molecule with shorter conjugation length exhibited an interesting packing motif shown to be favourable for charge transport. The mobility measurements were an order of magnitude higher than those reported for sexithiophene, a small molecule analogue, and the same order of magnitude as P3HT. The two-dimensional charge transporting nature of the material was verified with two independent techniques: time of flight (TOF) and organic field-effect transistor (OFET) measurements. The mobility of the material was found to vary with annealing, a result of morphological changes. These were studied with optical, electron and scanning probe microscopies. By controlling the morphology with the implementation of a well-defined annealing method, it was possible to improve the performance of OFETs and planar-heterojunction OPVs. Solution-processed bulk-heterojunction OPVs were fabricated, characterised and optimised with Ge spiro molecules. A PCE similar to that of P3HT, 2.66 %, was achieved for the one, whilst a PCE of 1.60 % was obtained for the other. The results are encouraging, and there is scope for improvement by increasing the overlap between the absorption and solar spectrum, for example
Solid state optical conjugated polymer amplifier; with ultrafast gain switching
No full textConjugated polymers are organic materials which are attractive as optoelectronic devices
because they have a combination of broad band emission, high gain, versatility
in processing, are ductile and can be electrically pumped. This thesis describes work
conducted on such conjugated polymers as amplifier devices.
The conjugated polymers used in this thesis were MEH-PPV, F8BT, GP1302 and
ADS233YE. The amplifier devices used were grating coupled and end coupling waveguides.
Amplification of light was demonstrated and characterised on single and multiple
pulses using the grating coupled structure. Single pulse measurements obtained
gains of 21 and 17 dB in a 1 mm long waveguide using the conjugated polymers MEHPPV
and F8BT. Annihilation rate was also analysed in the single pulse method with
MEH-PPV, giving a value of
γ ≈ (3 ± 0.1) x 10⁻⁹ cm³/s.
Amplification of a single pulse led to demonstrate amplification and the capability
of the amplifier to function with multiple pulses, which resulted in F8BT being used
as the gain medium. An average gain of 18 dB was obtained with F8BT in a 1 mm
waveguide channel. Amplification was also investigated with end coupled waveguides.
This led into investigating a suitable material or suitable combination of material for
amplification with the waveguides.
Switching of an amplified pulse was attempted on F8BT and GP1302 in the amplifier
device at 5 kHz. Switching of F8BT was problematic which lead to attempt
switching in GP1302 which was a co-polymer of PFO and F8BT. A 70 % switching
effect was obtained with GP1302. Gain recovery dynamics of F8BT, GP1302 and
ADS233YE was also investigated. These measurements established a switching rate
of 500 GHz for GP1302 and ADS233YE, and F8BT showed partial gain recovery
indicating the presence of long lived species.
Switching was also attempted on a polymer laser. This resulted in a 100 % switched
pulse with a combination of weak pump and strong switch pulse of 40 nJ and 2 μJ
respectively. And a strong pump and weak switch pulse of 200 and 50 nJ respectively.
Temporal delay of the switch pulse relative to the pump pulse resulted in re-timing
of the laser output. Amplification and switching of light pulses were also attempted at a higher repetition
rate of 50 kHz with F8BT, GP1302, ADS233YE and MEH-PPV. This resulted
in strong amplification of light in MEH-PPV and F8BT with gains of 21 and 13
dB respectively in a waveguide length of 422 μm. Weak amplification of light in
ADS233YE and GP1302 was also obtained with a maximum gain of 8 and 3 dB
respectively. Switching was attempted on MEH-PPV and ADS233YE
Measurements of exciton diffusion in conjugated polymers
No full textThe exciton diffusion length, which is the distance an exciton can diffuse in its lifetime, is an important parameter that has a critical impact on the operation of many organic optoelectronic devices, including organic solar cells, light emitting diodes and lasers. Knowledge of the exciton diffusion length can be a powerful aid for the design and optimisation of these devices.
This thesis details the development of techniques based on time-resolved fluorescence for measuring the exciton diffusion in organic semiconductors. Two main methods were used to investigate exciton diffusion in the conjugated polymers P3HT, MEH-PPV and F8BT: the surface quenching technique and exciton-exciton annihilation. In particular, the surface quenching technique was adapted to avoid some of the potential pitfalls that have plagued earlier measurements. Using a titania quencher, measurements were performed using the surface quenching technique and fitted with an exciton diffusion model, allowing the calculation of the exciton diffusion length. Results from measurements of the exciton-exciton annihilation rate, which is a diffusion controlled process, where in good agreement with those from surface quenching, confirming the robustness of this twofold approach.
A novel method for the control of the β-phase conformation in PFO films was used to produce films containing varying concentrations of β-phase. Exciton-exciton annihilation was used to investigate exciton diffusion in these films, revealing a gradual rise with increasing β-phase fraction due to improved interconnectivity. This work demonstrates how simple processing techniques can be used to control both film morphology and the exciton diffusion.
The thickness dependence of the photoluminescence lifetime in conjugated polymers is a phenomenon that has so far received little attention and, thus, remained unexplained. This study demonstrates that it is not due to exciton quenching by external factors, but can be explained by a change in the morphology with decreasing film thickness
Magnetic resonance spectroscopy of organic photovoltaic cells
Get PDFOrganic photovoltaics (OPV) have the potential advantages of low-cost, flexibility and
high throughput production. However, at present their efficiency is lower than other thin
film technologies and they are susceptible to degradation which limits cell lifetimes.
Magnetic resonance spectroscopy is a powerful technique to study the key processes
involved in the operation of OPV cells. In this thesis a range of electron paramagnetic
resonance (EPR) methods are used to investigate the processes which influence cell
efficiencies.
The understanding of degradation pathways and how they influence cell performance is
important if OPV cells are to reach commercialisation. The efficiency of PTB7:PC₇₁BM
cells is severely reduced when exposed to ambient atmosphere during processing.
Current-voltage analysis was combined with EDMR spectroscopy to investigate the
source of this performance loss. This investigation revealed that exposure of
PTB7:PC₇₁BM films to the solvent additive DIO and ambient atmosphere leads to
electron trap formation on the PC₇₁BM which acts as a recombination centre.
Using time resolved EPR spectroscopy the variation of charge separation across blends
of the DTS family of small molecule electron donors with PC₆₁BM is investigated.
Charge separation is found to be slowest in the [1,2,5]thiadiazolo[3,4-c]pyridine blend.
This slower separation is accompanied by a higher population of triplet excitons formed
by back electron transfer. This finding demonstrates that back electron transfer is a loss
mechanism in these molecular systems when charge separation is slow.
The EPR signatures of negative polarons on two high efficiency non-fullerene acceptors,
ITIC and IDTBR, are identified using multifrequency light induced EPR spectroscopy.
The polaron signatures of ITIC and IDTBR were found to overlap with polarons on P3HT
at all three microwave frequencies. Using multifrequency simulations the negative
polaron signatures and g-tensors of ITIC and IDTBR were determined for the first time
Ultrafast organic lasers and solid-state amplifiers
No full textThis thesis presents an investigation of the lasing dynamics and optical
amplification devices using conjugated polymers. Spectroscopic studies of conjugated
polymers and dendrimers were also performed. Conjugated polymers and dendrimers are
materials with great potential as display materials and tuneable lasers due to their broad spectra and high optical gains.
The effect of conjugation is studied in MEH-PPV and an anisotropy measurement of two different cored dendrimers has been shown to verify a theoretical prediction on
their depolarisation. Singlet emission from a highly efficient phosphorescent dendrimer is also observed and is the first known report of fluorescence from this class of dendrimers.
Conjugated polymers exhibit optical gain over broad spectral ranges, which has
led to much interest in their potential as novel laser gain media. Investigations into lasing from conjugated polymers has been confined mainly to studying the lasing properties and not the temporal dynamics of the laser pulses. In this work an investigation into the lasing
dynamics of a 2D-DFB conjugated polymer laser is demonstrated with the first subpicosecond laser pulses observed for a polymer laser. A novel encapsulated laser
fabricated via a soft lithography route was also studied and exhibited laser pulse of 6 ps duration.
The high gain observed over broad spectral ranges also means that these materials
are suitable for use as optical amplifiers. Broadband gain in a conjugated polymer
solution was demonstrated with a gain of 30 dB accessible across a 60 nm wavelength
range. In the solid state the limited thickness of films (~ 100 nm) and the uneven nature of the film edges had limited the ability to study the amplification of a probe signal. The first practical solid state conjugated polymer amplifier has been demonstrated. The device uses grating structures to couple a probe signal into and out of the gain region. The gain dynamics of different length amplifiers were studied and an 18 dB gain was observed in a
300 µm device length using a conjugated polymer blend of RedF and F8BT. Further
work on a conjugated polymer MEH-PPV led to a 21dB gain in a 1 mm device
Using time-resolved fluorescence to investigate exciton harvesting in organic photovoltaic blends
Get PDFThis thesis is an investigation of the photophysical processes that occur in organic photovoltaic blends in the time between light being absorbed and free charges being generated. The purpose of all solar cells is to generate a photocurrent. The free charges, as they flow out of the device, make up the photocurrent, so understanding the processes by which they are created is vitally important to organic photovoltaic research. The main experimental method used was time-resolved fluorescence spectroscopy. This technique was used to probe the exciton population with respect to time for a variety of blends of organic semiconductors, including the high performance photovoltaic materials PCDTBT, PTB7, C71-PCBM and P3HT. The main goal of the work was to characterise the exciton diffusion lengths of these materials by developing a technique called volume quenching. Volume quenching involves blending a small quantity of quenching material into a thin film of semiconducting material. These introduced quenching sites render excitons unemissive on contact. Thus, from the drop in fluorescence compared with the ‘unquenched’ material, it was possible to work out what proportion of the initial excitons have encountered a quenching site in the blends. The results can then be fitted to quantify how diffusive the excitons are -i.e. how far they move. By looking at the rate constant of the quenching process and how it varies with respect to time, quencher concentration and quencher type, it was possible to generate a wealth of additional information, not just about exciton diffusion, but about all the inter-related processes that contribute to exciton harvesting. These processes included the measurement of long-range energy transfer from the donor to the acceptor, electron transfer at the interface with the acceptor and the understanding of nanomorphology of donor-acceptor heterojunctions
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