Phenoxazine dyes for dye-sensitized solar cells: relationship between molecular structure and electron lifetime

A series of metal-free organic dyes with a core phenoxazine chromophore have been synthesized and tested as sensitizers in dye-sensitized solar cells. Overall conversion efficiencies of 6.03-7.40% were reached under standard AM 1.5G illumination at a light intensity of 100 mW cm(-2) . A clear trend in electron lifetime could be seen; a dye with a furan-conjugated linker showed a shorter lifetime relative to dyes with the acceptor group directly attached to the phenoxazine. The addition of an extra donor unit, which bore insulating alkoxyl chains, in the 7-position of the phenoxazine could increase the lifetime even further and, together with additives in the electrolyte to raise the conduction band, an open circuit voltage of 800 mV could be achieved. From photoelectron spectroscopy and X-ray absorption spectroscopy of the dyes adsorbed on TiO(2) particles, it can be concluded that the excitation is mainly of cyano character (i.e., on average, the dye molecules are standing on, and pointing out, from the surface of TiO(2) particles).[on SciFinder (R)]

Phenoxazine Dyes for Dye-Sensitized Solar Cells and Relationship Between Molecular Structure and Electron Lifetime

Ulf Anders Hagfeldt, Xiao Jiang

Metal-free org. dyes with a core phenoxazine chromophore have been synthesized and tested as sensitizers in dye-sensitized solar cells. Overall conversion efficiencies of 6.03-7.40% were reached under std. AM 1.5 G illumination at a light intensity of 100 mW cm-2. A clear trend in electron lifetime could be seen; a dye with a furan-conjugated linker showed a shorter lifetime relative to dyes with the acceptor group directly attached to the phenoxazine. The addn. of an extra donor unit, which bore insulating alkoxyl chains, in the 7-position of the phenoxazine could increase the lifetime even further and, together with additives in the electrolyte to raise the conduction band, an open circuit voltage of 800 mV could be achieved. From photoelectron spectroscopy and x-ray absorption spectroscopy of the dyes adsorbed on TiO2 particles, the excitation is mainly of cyano character (i.e., on av., the dye mols. are standing on, and pointing out, from the surface of TiO2 particles).

2011

Geometrical and energetical structural changes in organic dyes for dye-sensitized solar cells probed using photoelectron spectroscopy and DFT

Anna Eriksson, Ulf Anders Hagfeldt

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.

Royal Soc Chemistry2016

Solid state nanocrystalline titanium oxide photovoltaic cells

The dye solar cell, is a novel photoelectrochemical solar cell presenting unique advantages, as the use of low cost materials and the potential simplicity of manufacturing. However, a liquid electrolyte is actually required for the transport of the photogenerated positive charges. A major breakthrough is the replacing of the liquid electrolyte by a solid-state hole conducting material – the aim of this work. First, the metal/TiO2 junction was investigated, with either silver or gold as rectifying contact. Sensitization was demonstrated, as action spectra were measured using different dyes. The evaporated metal could not enter the pores of the nanocrystalline TiO2 to contact all the dye molecules, hence only low photovoltaic performances were obtained. Furthermore, these metal/TiO2 junctions showed a high sensitivity to humidity and to forward bias polarization. The use of an electrically conducting polymer, poly(bithiophene), was studied in a metal/polymer junction and in TiO2/polymer devices with flat and nanocrystalline TiO2 electrodes. The photoelectric characterizations indicated that electrochemically reduced poly(bithiophene) was able to inject electrons into TiO2, i.e. sensitizing TiO2, and it acted as a hole conducting layer. Short-circuit current densities of 2-3 mA/cm2 were measured at an irradiation of 1.2 W/cm2. The open-circuit voltages were in the 250-350 mV range, depending on the light intensity and temperature. The use of a transparent hole conducting material, CuI, which is a wide bandgap p-type semiconductor, was demonstrated in a nanocrystalline junction made of highly porous sensitized TiO2 and solution-cast CuI. A short-circuit current of 1 mA/cm2 was achieved under one sun simulated light; the open-circuit voltage was 600 mV and the light-to-power conversion efficiency was 0.3 %. The peak quantum efficiencies of 8 % in the UV, and 3 % in the visible, showed that the nanocrystalline junction was working inefficiently, probably due to the poor electrical contact between the TiO2 nanocrystallites and the CuI crystals. Furthermore, the sensitizer was not tuned to inject holes into the valence band of CuI. As conclusion, a controlled-capillary pore based sensitized device is suggested. In this concept, all the internal surface of the TiO2 electrode coated with sensitizer is contacted with the transparent hole conducting layer, hence improving the photovoltaic properties of the device.

EPFL1996

Solid state nanocrystalline titanium oxide photovoltaic cells

EPFL1996