- Title
- Morphological studies of organic photovoltaic blends
- Creator
- Berriman, Garth
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2015
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The main focus of this thesis has been to develop techniques for probing the active layer morphology of organic photovoltaic devices, as well as to develop a model to explain the phase separation observed in blended film devices containing PCBM. The first chapter describes the use of a near-field scanning optical microscope (NSOM) to generate and map local photocurrents in phase segregated P3HT:PCBM organic photovoltaic devices. Photocurrents are generated at three wavelengths, which are then used to determine the composition of the film based on the absorption coefficient for each wavelength in the pure materials. Then, based on the composition profile (which is corroborated with the use of scanning transmission X-ray microscopy (STXM)), it is shown that the absorption coefficients for the pure materials are insufficient to explain the photocurrent generated by 633 nm light. The difference is attributed to the creation of interface states upon blending of the P3HT and PCBM. The second experimental chapter describes the development of a new characterisation technique based on the near-field scanning photocurrent mapping used in the first chapter. The goal was to develop a technique capable of generating photocurrent maps at multiple wavelengths simultaneously, rather than sequentially. By simultaneously measuring the photocurrent generated by each wavelength, the technique is able to overcome several shortcomings associated with sequentially scanning the device. Two approaches are trialled, the first being a method which involves sequentially exposing the device to each wavelength before moving the probe to the next point in the scan. An electronic shutter system is used to control which wavelength is transmitted through the tip. This simultaneous-sequential multi-wavelength NSPM technique suffered from several technical flaws which prevented routine use of the instrument. The second approach taken to achieve simultaneous photocurrent maps at multiple wavelengths is to use a frequency division multiplexing technique. Each wavelength is modulated, using an optical beam chopper, at a unique frequency which in turn modulates the photocurrent at the optical modulation frequency. The current generated by each wavelength is then obtained by demodulating the photocurrent signal based on the modulation frequencies. The third experimental chapter describes the development of a technique capable of generating composition and thickness maps of thin organic films. The technique works by focussing several wavelengths of light onto a film and measuring the intensity of the transmitted light at each wavelength. The intensity of the transmitted light is used to determine the optical absorption of each wavelength. Each wavelength is modulated at its own unique frequency, allowing the intensity of each wavelength to be recovered using the Welch technique for estimating the periodogram. This frequency division multiplexing technique allows for the simultaneous measurement of multiple wavelengths, and shows a reduction in the variance of the measured light intensity over the technique used in Chapter 4. The instrument is demonstrated on an annealed P3HT:PCBM film, as these results can readily be compared to composition maps generated using scanning transmission X-ray microscopy (STXM). The composition maps compare favourably with the published STXM results and the blend ratio used while the thickness maps are self-consistent and in agreement with the measured film thickness. The final experimental chapter is focussed on gaining an understanding of the PCBM phase which forms upon the thermal annealing of P3HT:PCBM bulk heterojunction devices. A variety of theoretical, experimental and modelling results are combined to create a model which describes the formation of PCBM aggregates. AFM, XRD and SEM are combined to show that the aggregates are actually porous and composed of elliptical nano-crystallites of PCBM and are not a dense single crystal as is sometimes assumed. The diffusion constant of PCBM is measured and used to show that the dominant form of PCBM diffusion within BHJ devices is not molecular diffusion but in fact involves the diffusion of these elliptical nano-crystallites. The nano-crystallite diffusion which takes place in BHJ devices is used to reconcile disparate values for the diffusion constant in the literature. Finally, a basic diffusion model is used to explain the shape of PCBM aggregates.
- Subject
- organic photovoltaic devices; morphology; PCBM; phase seperation
- Identifier
- http://hdl.handle.net/1959.13/1059812
- Identifier
- uon:16699
- Rights
- Copyright 2015 Garth Berriman
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
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| View Details Download | ATTACHMENT02 | Thesis | 6 MB | Adobe Acrobat PDF | View Details Download |