- Title
- Graded heterojunction for improved performance and stability of organic solar cells
- Creator
- Poh, Chung-How
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2014
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The photoactive layer of an organic solar cell is typically 100 to 200 nm thick. At such a low dimensional thickness the light interference effect becomes significant. There are regions within the photovoltaic structure where the localized light intensity is low. In some instances, a relatively transparent material such as one made from titanium dioxide has been engineered into the solar cell structure so that the photoactive layer is strategically positioned at a location where the light intensity is high. The experimental work in this thesis begins by observing the existence of the interference effect in a P3HT-PCBM BHJ solar cell. An optical simulation package is employed to calculate the light intensity developed in the photoactive layer when its thickness is varied. Actual solar cells with the same varying thicknesses are fabricated. The physical solar cells are noted for their short circuit currents and the open circuit voltages. The open circuit voltages are found to vary from 0.617 to 0.642 V, depicting a linear correlation to the calculated field intensity in the photoactive layer. Additionally, the experimental short circuit current changes according to the expected profile obtained from the optical simulation. The observations demonstrate that the performance of the physical solar cell can be influenced by the optical interference effect arising from the low dimensional nature of its photoactive layer. Consequently, the next phase of investigation involves incorporating an optically transparent region into the solar cell so that the thickness and the position of the photoactive layer can be independently optimized and therefore allowing more light to dissipate within that layer. The PCBM component which is widely used in the construction of organic solar cells and with a light absorption at a low 345 nm wavelength is seen as an attractive candidate for this purpose. The standard BHJ morphology from which the photoactive layer of the organic solar cell is normally constructed is found to be inadequate in this case as the morphology involves no extended region composing purely of the transparent PCBM. On the other hand, the graded heterojunction architecture is found to be an elegant structure in the pursuit of the concept of using the naturally occurring PCBM in the solar cell as an optical spacer. The graded structure offers a region of interdiffused zone consisting of the P3HT and the PCBM, and adjacent to that is a layer of rather pristine PCBM. According to optical simulation a 20 nm PCBM region in combination with a photoactive layer (the interdiffused zone) of 60 nm will put the photoactive layer into alignment with an intense band of light which has developed within the solar cell structure. However, the intermixed zone appears to be only 40 nm thick for the as-fabricated graded structure, while the pristine PCBM region is estimated to be 30 nm in thickness. Fortunately, it is noted that thermal interdiffusion can be used to transform this semi-optimized graded structure into a structure whose thickness parameters are as required by the optical model to achieve optimum light harvesting. The as-fabricated graded solar cell structure is placed on a digitally controlled hot plate held at 140 °C. The rate of interdiffusion at the set temperature is initially unknown. However, after a series of experiments, the depletion rate of the PCBM layer is estimated to be 0.5 nm per minute, representing a rather fine structural control at the nano-scale. At this rate of depletion, a heating time of 20 minutes is required to reduce the intrinsic PCBM layer to 20 nm thick. After the graded structure has been tuned, the obtained conversion efficiency is 3.4 %. This contrast well with 3.2 % observed earlier with the bulk heterojunction morphology. When the graded structure is used, i.e. when there is a layer of the transparent PCBM as optical spacer incorporated into the solar cell structure, the open circuit voltage is observed to increase to 0.666 V. The highest open circuit voltage obtained is 0.676 V when a slightly thicker interdiffused region is chosen, however the thicker version of the photoactive region only leads to poorer fill factor (due to higher series resistance) and thus the overall conversion efficiency has remain unchanged at 3.4 %. Thus, the proof of concept effort in this thesis demonstrates that the PCBM is capable of acting as an optical spacer, leading to increased open circuit voltages observed with the graded solar cells. Apart from its natural capacity to accommodate an optical spacer, the graded architecture is also noted to have longer operating lifetime in ambient air, compared to the solar cells having the bulk heterojunction morphology. The improved stability is thought to be the added PCBM layer, which made the solar cell more robust to the diffusions of the metal species from the neighboring metal electrode. With some modification, it is anticipated that gold nanoparticles can also be incorporated into the graded solar cell and therefore offering an extended absorption spectrum for the P3HT-PCBM solar cell. Test with a bilayer P3HT-C₆₀ solar cell indicates that the structure has improved EQE at 395 and 675 nm wavelengths. These enhancements can be attributed to the optical absorption of gold and light scattering in the presence of the nanoparticles.
- Subject
- solar cells; PCBM; photoactive layer; light absorption
- Identifier
- http://hdl.handle.net/1959.13/1055318
- Identifier
- uon:15867
- Rights
- Copyright 2014 Chung-How Poh
- Language
- eng
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
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| View Details Download | ATTACHMENT02 | Thesis | 3 MB | Adobe Acrobat PDF | View Details Download |