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Geometrical and energetical structural changes in organic dyes for dye-sensitized solar cells probed with photoelectron spectroscopy and DFT
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
Uppsala University, Disciplinary Domain of Science and Technology, Chemistry, Department of Chemistry - Ångström, Physical Chemistry.
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2016 (English)In: Physical Chemistry, Chemical Physics - PCCP, ISSN 1463-9076, E-ISSN 1463-9084, Vol. 18, no 1, 252-260 p.Article in journal (Other academic) Published
Abstract [en]

The effects of alkoxy chain length in triarylamine based donor acceptor organic dyes are investigated with respect to the electronic and molecular surface structures on the performance of solar cells and the electron lifetime. The dyes were investigated when adsorbed on TiO2 in a configuration that can be used for dye sensitized solar cells (DSCs). Specifically, the two dyes D35 and D45 were compared using photoelectron spectroscopy (PES) and density functional theory (DFT) calculations. The differences in solar cell characteristics when longer alkoxy chains are introduced in the dye donor unit are attributed to geometrical changes in dye packing while only minor differences were observed in the electronic structure. A higher dye load was observed for D45 on TiO2. However, D35 based solar cells result in higher photocurrent although the dye load is lower. This is explained by different geometrical structures of the dyes on the surface.

Place, publisher, year, edition, pages
2016. Vol. 18, no 1, 252-260 p.
National Category
Physical Chemistry
Identifiers
URN: urn:nbn:se:uu:diva-230853DOI: 10.1039/c5cp04589dISI: 000368755500027OAI: oai:DiVA.org:uu-230853DiVA: diva2:742135
Funder
Swedish Research CouncilCarl Tryggers foundation Swedish Energy AgencyStandUp
Available from: 2014-08-31 Created: 2014-08-31 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Interfaces in Dye-Sensitized Solar Cells Studied with Photoelectron Spectroscopy at Elevated Pressures
Open this publication in new window or tab >>Interfaces in Dye-Sensitized Solar Cells Studied with Photoelectron Spectroscopy at Elevated Pressures
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

With an increasing demand for renewable energy sources, research efforts on different solar cell technologies are increasing rapidly. The dye-sensitized solar cell (DSC) is one such technology, taking advantage of light absorption in dye molecules. The liquid based DSC contains several interesting and important interfaces, crucial for the understanding and development of the solar cell performance. Examples of such interfaces include dye-semiconductor, electrode-electrolyte and solute-solvent interfaces. Ultimately, complete interfaces with all these components included are of particular interest. One major challenge is to understand the key functions of these systems at an atomic level and one way to achieve this is to use an element specific and surface sensitive tool, such as photoelectron spectroscopy (PES). This thesis describes the use and development of PES for studying interfaces in the DSC.

The materials part of the thesis focuses on interfaces in DSCs studied with PES and the methodology development parts focus on methods to use PES for investigations of solvated heterogeneous interfaces of interest for photoelectrochemical systems such as the DSC. More specifically, beginning with standard vacuum techniques, dye molecules bound to a semiconductor surface have been studied in terms of energy level alignment, surface coverage and binding configuration. To increase the understanding of solvation phenomena present in the liquid DSC, liquid jet experiments have been performed in close combination with theoretical quantum calculations. As a step towards an in-situ method to measure a complete, functioning (in operando) solar cell, methodology development and measurements performed with higher sample pressures are described using new high pressure X-ray photoelectron spectroscopy techniques (HPXPS).

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2014. 75 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1173
Keyword
Dye-sensitized solar cells, interfaces, solvation, photoelectron spectroscopy, HPXPS, HP-HAXPES, liquid jet
National Category
Physical Chemistry
Research subject
Chemistry with specialization in Physical Chemistry
Identifiers
urn:nbn:se:uu:diva-230855 (URN)978-91-554-9022-5 (ISBN)
Public defence
2014-10-17, Häggsalen, Ångström laboratory, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2014-09-25 Created: 2014-08-31 Last updated: 2015-01-23
2. Towards Mixed Molecular Layers for Dye-Sensitized Solar Cells: A Photoelectron Spectroscopy Study
Open this publication in new window or tab >>Towards Mixed Molecular Layers for Dye-Sensitized Solar Cells: A Photoelectron Spectroscopy Study
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The increasing demand for renewable energy has led to substantial research on different solar cell technologies. The dye-sensitized solar cell (DSC) is a technology utilizing dye molecules for light absorption. Dye molecules are adsorbed to a mesoporous semiconductor surface and after light absorption in the dye, charge separation occurs at this interface. Traditionally, DSCs have used layers of single dye species, but in recent efforts to enhance power conversion efficiency, more complex molecular layers have been designed to increase the light absorption. For example, the most efficient DSCs use a combination of two dye molecules, and such dye co-adsorption is studied in this thesis.

A key to highly efficient DSCs is to understand the dye/semiconductor interface from a molecular perspective. One way of gaining this understanding is by using an element specific, surface sensitive technique, such as photoelectron spectroscopy (PES).

In this thesis, PES is used to understand new complex dye/semiconductor interfaces. Dyes adsorbed to semiconductor surfaces are analyzed using PES in terms of geometric and electronic surface structure.  The investigations ultimately target the effects of co-adsorbing dyes with other dyes or co-adsorbents.

PES shows that Ru dyes can adsorb in mixed configurations to TiO2. Co-adsorption with an organic dye affects the configuration of the Ru dyes. As a consequence, shifts in energy level alignment and increased dye coverage are observed. The dyes are affected at a molecular level in ways beneficial for solar cell performance. This is called collaborative sensitization and is also observed in todays most efficient DSC.

Dye molecules are generally sensitive to high temperatures and the substantial decrease in power conversion efficiency after heat-treatment can be understood using PES. Furthermore, comparing two mesoscopic TiO2 morphologies used in DSCs show differences in trap state density in the band gap, explaining the photovoltage difference in DSCs comprising these morphologies. Using mixed molecular layers on NiO results in significant improvements of p-type DSC power conversion efficiency. PES shows that changed adsorption configuration contribute to this effect.

This thesis shows that PES studies can be used to obtain insight into functional properties of complex DSC interfaces at a molecular level.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2016. 81 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1409
Keyword
dye-sensitized solar cell, DSC, mesoscopic solar cell, photoelectron spectroscopy, PES, XPS, interface, TiO2, NiO, co-adsorption, co-adsorbent, collaborative sensitization, mixed molecular layers
National Category
Condensed Matter Physics
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics
Identifiers
urn:nbn:se:uu:diva-301164 (URN)978-91-554-9664-7 (ISBN)
Public defence
2016-10-06, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 10:15 (English)
Opponent
Supervisors
Available from: 2016-09-14 Created: 2016-08-18 Last updated: 2016-09-22

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Eriksson K., SusannaEllis, HannaOscarsson, JohanLindblad, RebeckaEriksson, Anna I. K.Johansson, ErikBoschloo, GerritHagfeldt, AndersRensmo, Håkan

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