- Title
- Development of novel binary and ternary blend organic photovoltaics
- Creator
- Thameel, Mahir Noori
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2019
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- Over the last few years, promising development has been reported in organic photovoltaics. Typically, two organic semiconductors are blended as the active layer to absorb light and generate charges. To further improve the power conversion efficiency (PCE) a third material with complementary absorption spectrum can also be incorporated to enhance photon harvesting. Such a ternary system gives rise to more complex charge transfer and exciton dissociation mechanisms, which are explored in this thesis. In this thesis poly(3-hexyl thiophene): Phenyl-C60-butyric acid methyl ester: 2,4-bis [4-(N, N-diisobutylamino)- 2,6-dihydroxyphenyl] squaraine (P3HT:PCBM:DIBSq) system is used as a ternary model system. First, the exciton dissociation mechanism in the P3HT:PCBM:DIBSq material blend is analysed. The energy level diagram of this ternary system indicates that sufficient driving force is available at the interface between P3HT and DIBSq to dissociate excitons. However, an in depth study of charge generation in P3HT:PCBM:DIBSq films and photovoltaic devices shows that the P3HT:DIBSq interface is not capable of generating free charge carriers. An examination of energy transfer from P3HT to DIBSq in solution shows that hetero-energy transfer occurs efficiently. Indeed, energy transfer is found to occur in the solid state as well, which allows for a two-step exciton dissociation mechanism. A series of ternary (P3HT:PCBM:DIBSq) and binary (P3HT:PCBM) bulk hetero-junction (BHJ) organic photovoltaic (OPV) devices were fabricated to optimize their device performance. Binary P3HT:PCBM devices were found to have an optimized PCE of 3.8%. By comparison ternary devices with a DIBSq concentration of less than 1.5% by weight exhibited an improved PCE of 4.2% . As the DIBSq content is increased beyond 1.5 %, no further performance improvement is seen and the PCE steadily decreases. Transient electrical measurements reveal that the presence of DIBSq does not significantly decrease the charge carrier mobility, bimolecular recombination rate or charge generation efficiency of the blend. The charge collection efficiency, on the other hand, was found to steadily decrease with increasing DIBSq content. A morphological study using TEM and AFM shows that while the addition of DIBSq to the P3HT:PCBM blends does not alter the microstructure of the P3HT phase, DIBSq aggregates are formed. These aggregates increase in size as the DIBSq content increases and subsequently hinder charge extraction. Hence, the improvement in light absorption and charge generation due to DIBSq is completely negated by the reduction in charge extraction at high DIBSq concentrations. At low DIBSq concentrations (<1.5 wt. %), the advantage of the increased absorption due to DIBSq outweighs the negative effect on charge extraction and gives rise to an overall improvement in PCE. In general, the optimum performance in ternary systems is expected at the highest dye content that does not cause significant aggregation. Finally, ternary nanoparticles consisting of P3HT, PCBM and DIBSq were synthesized and their optical and structural properties characterised using transmission electron microscope (TEM), scanning electron microscope (SEM), atomic force microscope (AFM) and scanning transmission X-ray microscope (STXM) measurements. A series of photovoltaic devices were fabricated using aqueous P3HT:PCBM:DIBSq nanoparticles with DIBSq concentration varying from 0.5 wt.% to 22 wt.%. The marked difference in the morphology of the ternary nanoparticles compared to the BHJ structures means that a higher DIBSq concentration can be achieved before the PCE suffers. Devices with 14.3 wt.% DIBSq exhibited the highest performance with PCE of 1.1%, while the equivalent binary devices had a PCE of 0.41%. Enhanced optical properties, beneficial energy transfer and morphological properties of the ternary nanoparticles showed that the ternary concept is a promising strategy for improving the performance of aqueous nanoparticulate photovoltaic devices.
- Subject
- organic photvoltaics; ternary organic solar cells; sequaraine; P3HT:PCBM:DIBSq
- Identifier
- http://hdl.handle.net/1959.13/1408811
- Identifier
- uon:35889
- Rights
- Copyright 2019 Mahir Noori Thameel
- Language
- eng
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|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 16 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 372 KB | Adobe Acrobat PDF | View Details Download |