The Photophysical Properties of Magnesium Modified Alq3 for Solar Cell Applications
Date Issued
2009
Date
2009
Author(s)
Lin, Chien-Hung
Abstract
Organic solar cell research has developed during the past 30 years, but especially in the last decade it has attracted scientific and economic interest triggered by a rapid increase in power conversion efficiencies in low-cost organic materials. To overcome this limitation of the photo-to-carrier generation, enhancements of electron conductivity, exciton separation and exciton diffusion have been suggested. A suitable electron acceptor in solar cells, which can replace the high-cost and air-instable fullerene C60, must has suitable energy to match p-type layer and cathode and owns high electron conductivity. n this thesis, the low molecular weight tris-(8-hydroxyquinoline)aluminum (Alq3) has been modified with magnesium (Mg) incorporation that altered the nature of electronic characteristics in it. This Mg modified Alq3 can provide an electron accepter and a good electron transporting layer in photovoltaic devices, which conduction enhancement of the current with variations of more than 3 orders of magnitude can be observed. The energy level of Mg-Alq3 shifts 0.64 eV and forms charge-transfer condition to promote the exciton separation at p-n junction. Detail optical and vibrational studies depict the quenching of the fluorescence, originating from singlet state transition of short lifetime (2-14 ns) excitons upon Mg incorporation. This is accompanied by enhanced phosphorescence, due to a triplet state transition, having longer lifetimes (0.5 ms) and hence an estimated excitonic diffusion length of ~90 nm to boost the performance of organic solar cell devices. Optimized Mg:Alq3 layer, when introduced in the device, improves the power conversion efficiencies by several orders to 0.15% compared to the pure Alq3 device. The improvement in the photovoltaic performance has been attributed to the carrier transport, high HOMO and the superior exciton diffusion length.
Subjects
organic solar cells
organic photovoltaic
SDGs
Type
thesis
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