Abstract
Well-ordered 3D mesoporous indium tin oxide (ITO) films obtained by a templated sol-gel route are discussed as conductive porous current collectors. This paper explores the use of such films modified by electrochemical deposition of zinc oxide (ZnO) on the pore walls to improve the electron transport in dye-sensitized solar cells (DSSCs). Mesoporous ITO film were dip-coated with pore sizes of 20–25 nm and 40–45 nm employing novel poly(isobutylene)-b-poly(ethylene oxide) block copolymers as structure-directors. After electrochemical deposition of ZnO and sensitization with the indoline dye D149 the films were tested as photoanodes in DSSCs. Short ZnO deposition times led to strong back reaction of photogenerated electrons from non-covered ITO to the electrolyte. ITO films with larger pores enabled longer ZnO deposition times before pore blocking occurred, resulting in higher efficiencies, which could be further increased by using thicker ITO films consisting of five layers, but were still lower compared to nanoporous ZnO films electrodeposited on flat ITO. The major factors that currently limit the application are the still low thickness of the mesoporous ITO films, too small pore sizes and non-ideal geometries that do not allow obtaining full coverage of the ITO surface with ZnO before pore blocking occurs.
Highlights
Transparent conductive oxide (TCO) films have received significant attention in the past few years, as they are applicable as transparent electrodes in liquid crystal displays [1] and organic light-emitting diodes [2]
The solution was kept at a temperature of 70 °C and was not stirred, while the electrodeposition time was varied between 1.25 s and 10 s or 5 s and 20 s for mesoporous indium tin oxide (ITO) films prepared with poly(isobutylene)-b-poly(ethylene oxide) (PIB-PEO) 3000 or PIB-PEO 20000, respectively
The electrochemical deposition of zinc oxide (ZnO) in mesoporous ITO films for use in dye-sensitized solar cells (DSSCs) is strongly influenced by the pore size and pore geometry; allowing in case of small or thin pores only the formation of a rather incomplete ZnO layer on the inner ITO pore walls before blocking the pore entrances
Summary
Transparent conductive oxide (TCO) films have received significant attention in the past few years, as they are applicable as transparent electrodes in liquid crystal displays [1] and organic light-emitting diodes [2]. In an initial work successfully resulting in ordered mesoporous ITO films the commercially non-available KLE (H(CH2CH2CH2(CH)CH2CH3)89(OCH2-CH2)79OH) block copolymer was employed to prepare 3D arrangements of spherical mesopores of about 13 nm diameter. Proposed a concept of a collector-shell electrode, where a nanoparticular TCO film of (ITO or ATO) is coated with a thin layer of the semiconducting material This approach still suffers from grain boundaries between the TCO nanoparticles as well as from irregular pore shapes, narrow pore openings and a wide pore size distribution, ranging from micro- to macropores complicating the deposition of a homogeneous thin semiconducting layer on the pore walls as well as the wetting with the redox electrolyte.
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