How the Nature of Triphenylamine-Polyene Dyes in Dye-Sensitized Solar Cells Affects the Open-Circuit Voltage and Electron Lifetimes

Three donor-linker-acceptor triphenylamine-based cyanoacrylic acid org. dyes used for dye-sensitized solar cells (DSCs) were examd. with respect to their effect on the open-circuit voltage (Voc). The previous study showed a decrease in Voc for DSCs based on dyes with increased mol. size (increased linker conjugation). The authors study the origin of Voc with respect to (a) conduction band (ECB) positions of TiO2 and (b) degree of recombination between electrons in TiO2 and electrolyte acceptor species at the interface. These parameters were studied as a function of dye structure, dye load, and I2 concn. Two types of behavior were identified: the smaller polyene dyes show a surface-protecting effect preventing recombination upon increased dye loading, whereas the larger dyes enhance the recombination. How the different dye structures affect the recombination is discussed in terms of dye surface blocking and intermol. interactions between dyes and electrolyte acceptor species.

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

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