- Title
- Methods and materials for the investigation and extension of the performance of polymer photovoltaic devices
- Creator
- Duck, Benjamin Clyde
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2011
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The focus of this thesis is on developing new experimental techniques for the study of charge generation in conducting polymers and exploring the low energy response of photovoltaic devices constructed from these semiconducting materials. The behaviour of photonic crystal fibre when pumped with femtosecond pulses sourced from a Ti:Sapphire system is characterised. Higher order mode structures within the output are observed for pumping wavelengths that lie below the group velocity dispersion zero of the fibre. Enhancement of the higher order mode structure is shown to occur when the pumping position is offset from the centre of the fibre core. A novel investigation technique is employed where the output of the fibre is simultaneously spatially and spectrally measured to obtain accurate and detailed information about the wavelength dependence of the structures. The higher order features are determined to originate from solitons formed in a higher order mode within the fibre. An additional rotational component is observed, with the new measurement technique, which is attributed to the geometry of the fibre itself. The commonly available conducting polymer blends of MEH-PPV:PCBM and P3HT:PCBM are investigated for their performance at energies below the absorption range of the component materials in isolation. So called charge transfer (CT) state absorption is shown in only the blended systems and is observed to result in photocurrent generation at energies as low as 1.16 eV when the blend is used as the active layer in a photovoltaic device. A response at these energies makes them a viable candidate for near infrared photodetector applications. Photocurrent action spectroscopy measurements show that the CT response of MEH-PPV:PCBM blends matches the performance of the overall response however the P3HT:PCBM devices show a different behaviour for excitation in the two regions. The CT response is correlated with the film morphology and, in-particular, is related to the total number of absorption sites available. For P3HT:PCBM devices, annealing affects the film morphology, increasing the amount of phase segregation in the film, and thus diminishes the CT absorption while improving overall response. Modelling of the energy of the CT state is performed to analyse the affect of composition and annealing. MEH-PPV:PCBM devices show a reduction in the energy of the CT state for increasing concentrations of PCBM, a result which agrees with the annealing results obtained in P3HT:PCBM, however this is not reflected in the P3HT:PCBM composition data. The discrepancy indicates that the P3HT:PCBM system has two competing processes determining the energy of the CT state. Characterisation of the performance of the new low bandgap polymer material pC6TP is performed when incorporated into photovoltaic devices. Binary blends of pC6TP:PCBM are shown to respond to annealing only when a low boiling point solvent is used and by varying the blend composition it is shown that a ratio of 1:2 produces an optimal device. Clear photocurrent response is observed for both pC6TP and PCBM excitation with the pC6TP component allowing photocurrent response to be measured at energies as low as 1 eV. Ternary blends of P3HT:pC6TP:PCBM are constructed and observed to behave like two cells (P3HT:PCBM and pC6TP:PCBM) in tandem. Modelling of the open circuit voltage shows that the polymer phase of the ternary device consists of both P3HT and pC6TP intimately intermixed. An energy difference between the onset of absorption and photocurrent is identified and attributed to the energy required to achieve full charge separation of the initially generated exciton. A new method for the measurement of transient photoinduced absorption spectra is developed based around the continuum generated within a photonic crystal fibre when pumped with femtosecond pulses. Various measurement methodologies are investigated with a system using two lock-in amplifiers measuring the signal from a narrow spectral slice of the continuum, which is found to be necessary in order to compensate for the instability of the continuum itself. The new method is shown to reproduce results from other authors and allows for low power high repetition rate investigations of conducting polymer systems. These measurements remove the obscuring signals from excitation-excitation interactions that occur at the higher excitation densities used in conventional time resolved measurement systems of this type.
- Subject
- polymer photovoltaic; ultrafast spectroscopy; microstructured optical fibre
- Identifier
- http://hdl.handle.net/1959.13/923581
- Identifier
- uon:9758
- Rights
- Copyright 2011 Benjamin Clyde Duck
- Language
- eng
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| View Details Download | ATTACHMENT02 | Thesis | 8 MB | Adobe Acrobat PDF | View Details Download |