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Photoinduced Charge Transfer Processes at Organic-Semiconductor Interfaces

Research output: Book/ReportDoctoral thesisCollection of Articles

Details

Original languageEnglish
PublisherTampere University
Number of pages55
ISBN (Electronic)978-952-03-1102-5
ISBN (Print)978-952-03-1101-8
Publication statusPublished - 24 May 2019
Publication typeG5 Doctoral dissertation (article)

Publication series

NameTampere University Dissertations
Volume67
ISSN (Print)2489-9860
ISSN (Electronic)2490-0028

Abstract

The growing energy demand of the mankind has lead to the extensive use of fossil fuels. The burning of oil, coal and natural gas has increased the global temperature and atmospheric carbon dioxide percentage to a dangerously high level. Therefore, carbon-neutral energy sources such as solar cells are needed.

In order to design more effective solar cells, a deep understanding of the primary photochemical processes in the cells is needed. Ultrafast time-resolved spectroscopy, especially transient absorption methods, are a very useful tool for investigating the reaction kinetics in order to optimize the solar cell performance.

In this thesis, kinetics of the photoinduced processes at the interface of an organic monomolecular layer and a semiconductor are studied. Such structures may be used as the active material e.g. in dye-sensitized solar cells. Two different types of organic–semiconductor hybrids were prepared: fullerenes (C60) immobilized on colloidal semiconductor quantum dots (QDs), and zinc phthalocyanine (ZnPc) derivatives on nanostructured titanium dioxide (TiO2) and zinc oxide (ZnO) surfaces. The driving force of photocurrent generation in these systems is a photonic excitation leading to an electron transfer reaction across the organic–semiconductor interface. The observed electron transfer rates vary from a few picoseconds in ZnPc monolayers on TiO2 to ca. 100 ps in QD–fullerene systems.

Phthalocyanine derivatives are very attractive sensitizing dyes for solar cell applications because of their excellent stability and strong absorption in the red part of the spectrum. A drawback with these compounds is their tendency towards aggregation. It reduces the solar cell efficiencies due to intra-aggregate losses. There are two common methods for aggregation-reduction: the use of molecular co-adsorbates and substitution of the phthalocyanine core with bulky side groups. Both mechanisms were observed to lower the degree of aggregation in the ZnPc samples. The substitution method proved to be more efficient in terms of the lifetime of the charge-separated state.

To more realistically mimic a solar cell, a hole-transporting material (HTM) was used. Its effect on the primary photoinduced reactions in the phthalocyanine–semiconductor samples was studied. With the chosen HTM, spiro-MeOTAD, the charge separation was observed to occur first at the phthalocyanine–HTM interface, followed by electron injection into the semiconductor material.

Complete solar cell samples were prepared in order to link the ultrafast spectroscopy results to actual solar cell performance. A correlation between the degree of aggregation and the produced photocurrent was confirmed. The less aggregated samples produce a higher photocurrent per number of absorbed photons. This study indentifies bottlenecks in modern hybrid organic–semiconductor solar cell design and suggests solutions for improving the solar cell performance.

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