Morphology and performance of nanoparticle organic photovoltaics

Holmes, Natalie P.

Title: Morphology and performance of nanoparticle organic photovoltaics
Creator: Holmes, Natalie P.
Relation: University of Newcastle Research Higher Degree Thesis
Resource Type: thesis
Date: 2015
Description: Research Doctorate - Doctor of Philosophy (PhD)
Description: Nanoparticle organic thin films are an emerging area of organoelectronic research which offer the prospect of printing large area solar cells from water-based inks, as well as the ability to control the semi-conducting polymer morphology on the nanoscale. However, to date there has been little research into the composition and morphology of the photoactive layers in these devices. To realise the potential for nanoscale control of these films we must first develop an understanding of the factors which affect morphology in these devices. This thesis works on unravelling the material-morphology-performance relationships in NP OPV systems. This thesis first explores the intra- and inter-particle morphology of P3HT:PC₆₁BM nanoparticles prepared from a range of P3HT molecular weights (5 to 72 kg mol^-1 Mw), and reports a common core-shell structure. When thermal annealing treatments were applied to these nanoparticles and nanoparticle films, we found that the morphological changes which occurred were highly dependent upon the polymer molecular weight, from subtle changes at high P3HT molecular weight to more pronounced changes at low P3HT molecular weight. Diffusion of PC₆₁BM in the polymer matrix was found to govern the morphological evolution of annealed NP films, with both molecular PC₆₁BM diffusion as well as diffusion of PC₆₁BM NP core units observed, the latter only occurring after sufficient thermal energy had been supplied to the system. The lessons learnt from the molecular weight–morphology investigation were then applied to the fabrication of P3HT:PC₆₁BM nanoparticulate OPV devices, for the various molecular weight batches of P3HT. Power conversion efficiencies (PCEs) were observed to peak for the mid-range of molecular weights tested, this behaviour differed from the trend generally observed for bulk heterojunction (BHJ) devices, where high molecular weight polymers deliver the highest PCEs. A balance of competing phenomena account for the optimal NP OPV device performance with mid-range molecular weight P3HT. The functionality of a core-shell nanoparticle photoactive layer morphology was then investigated in detail, made possible by varying the ratio of donor-to-acceptor material in the nanoparticles and probing the origin of photocurrent. The new morphological variety that nanoparticles present further increases the complexity of the exciton dissociation, exciton separation and charge transport processes. Due to the core-shell domain structure in the OPV photoactive layer, charge separation at core-shell material interfaces and transport through percolation pathways in the joined shell network become increasingly more complicated processes. It was shown that when PC₆₁BM loading exceeded a 1:1 donor-acceptor ratio, the PCE was maintained, which is opposite to that reported in similar BHJ OPV studies. This difference was due to both an increase in functional PC₆₁BM volume and an improvement to charge percolation pathways through the joined shell network for charge extraction. Finally, the application of the low band gap polymer TQ1 to nanoparticle OPVs was investigated. This polymer proved to be more applicable to nanoparticle OPV fabrication than P3HT due to its more desirable material properties; namely a high glass transition temperature (T_g) and non-crystalline nature. TQ1:PC₇₁BM NP OPV device performance was optimised by the application of mild thermal annealing treatments in the range of the T_g (sub-T_g and post-T_g), both in the photoactive layer drying stages and post-cathode deposition annealing stages of device fabrication. In addition, the morphological changes caused by thermal treatments were investigated. We observed a type of morphological evolution in nanoparticle films for OPV photoactive layers that has not previously been reported: that of PC₇₁BM nano-pathway formation between dispersed PC₇₁BM-rich nanoparticle cores. These have the benefit of making the bulk film more conducive to charge percolation and extraction.
Subject: nanoparticle; morphology; polymer; organic photovoltaic; thesis by publication
Identifier: http://hdl.handle.net/1959.13/1310021
Identifier: uon:21974
Language: eng
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