Organic bulk heterojunction solar cells with improved efficiency

Ding, Yanfei (2009) Organic bulk heterojunction solar cells with improved efficiency. Masters thesis, Memorial University of Newfoundland.

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Abstract

Organic solar cells have attracted considerable attention in recent years for their unique advantages, such as solution processing, flexibilities, and low cost for mass production. The highest power conversion efficiency reported so far is 6.5%. However, it is widely believed that the efficiency barrier of 10% has to be exceeded before polymer solar cells can be commercialized. Our study focuses on developing universal techniques for the improvement of the efficiency of organic solar cells and flexible polymer solar cells on two types of bulk heterojunction devices, i.e. poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), and (poly[2-methoxy,5-(2-ethylhexoxy)-l,4-phenylene vinylene]) (MEH-PPV) and PCBM. The universal techniques we propose and demonstrate to achieve efficiency improvement include treatment of anode electrode by chemical and optical techniques, optimization of buffer layer, and device processing with additives. All reported organic solar cells of high efficiencies have been achieved under inert environment, among which some devices are either encapsulated devices or tandem devices. The polymer solar cells studied in this thesis are unencapsulated single cells tested at ambient environment. The efficiency of the P3HT:PCBM device with ITO substrate irradiated by UV light has been enhanced to 3.65%, as compared with the efficiency of 2.01%) from the device fabricated on untreated ITO substrate. To improve the efficiency of organic solar cell, we reveal an approach to effectively adjust the properties of the buffer layer, Poly(ethylene-dioxythiopene):poly(styrenesulfonate) (PEDOT:PSS), achieving an improvement in the efficiency from 2.08% to 2.92%. The properties of MEH-PPV:PCBM devices processed with either of the two additives, dimethyl sulfoxide (DMSO) or 1,8-octanedithiol (ODT) demonstrate significant increases in the efficiency, i.e., from 2.5%) to 2.8% after doped 2 vol% of ODT, and from 2.5% to 3.15% by doping 5 vol% of DMSO, surpassing the highest efficiency of MEH-PPV:PCBM solar cells reported so far. -- We demonstrate the operation of flexible solar cells, which are fabricated on polyethylene erephthalate (PET) substrates. Size effect on the performance of flexible solar cells indicates that the efficiency of the solar cell increase in the device size. The relationship between the efficiency and the bending angle of the flexible solar cells as well as the mechanisms of the degradation in efficiency are evaluated for the finite time.

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