Unravel the Impact of Anchoring Groups on the Photovoltaic Performances of Diketopyrrolopyrrole Sensitizers for Dye-Sensitized Solar Cells

Two D-pi-A dyes, one with a strong electron withdrawing cyanoacrylic acid DPP 20 and one with a weak acceptor carboxylic acid DPP 21 have been synthesized and characterized for their application in dye-sensitized solar cells. This allows us to understand the role of electron withdrawing strength of the acceptor anchoring groups on the optoelectronic properties of chromophoric pi spacer molecules, e.g., the diketopyrrolopyrrole (DPP) moiety. The low energy absorption maxima of DPP 20 is red-shifted by only 7 nm compared to DPP 21, implying a minimal role for the strong acceptor in the light harvesting properties of colored pi spacer molecules. Because of this small red shift, J(SC) of DPP 20 is marginally higher than that of DPP 21. However, because of the higher fill factor of DPP 21, the overall power conversion efficiency (PCE = 7.65%) is higher than that of DPP20 (7.34%). The data reveal that a weak acceptor is adequate to achieve good light harvesting as well as provide good photovoltaic efficiencies for colored pi spacer sensitizers.

Photovoltaic performance and long-term stability of dye-sensitized mesoscopic solar cells

Michael Graetzel

The efficiency of electric power generation by dye-sensitized mesoscopic photovoltaic cells has been progressing steadily over the last years reaching now 11% in full sunlight. An important question for practical applications concerns the stability of these devices under prolonged exposure to light or heat. Strikingly stable operation can be obtained by judicious selection of the sensitizer, electrolyte and sealant rendering feasible a service life of at least 20 years under normal outdoor conditions. The sensitizer playing a central role in the light energy conversion process, we analyze the kinetic requirements for it to sustain the required one hundred million turnovers. We also review recent results on the use of self-assembled monolayers of amphiphilic sensitizers and co-adsorbents to enhance the thermal robustness of the device.

2006

Interface engineering in solid-state dye-sensitized solar cells

Jessica Krüger

Dye-sensitised nanocrystalline solar cells are currently subject of intense research in the framework of renewable energies as a low-cost photovoltaic device. In particular dye-sensitised cells based on spiro-MeOTAD have gained attention as promising approach towards an organic solid-state solar cell. However, the efficiency in such dye-sensitised solid-state solar devices (SSD) was so far only ca. 10 % of the values reported at AM1.5 for the classical dye-sensitised solar cell with an electrolyte hole transporting medium (DSSC). The objective of the present work is to study the limitations that emerge from the exchange of the electrolyte by the solid-state system and that oppose photovoltaic photon-to-electron conversion as high as for the DSSC. Interfacial charge recombination is an important loss mechanism in dye-sensitised solar cells. This is particularly true for SSD, as the solid hole-transporting medium is less efficient in screening of internal fields which assist recombination. A variety of strategies were tested in the SSD to minimise interfacial charge recombination. The most promising approach was the blending of the hole-transporting medium with tert.-butylpyridine (tBP) and lithium ions. Optical and electrochemical techniques, such as nanosecond laser spectroscopy, impedance spectroscopy and photovoltaic characterisation measurements, were used to study the impact of the additives on the SSD. Both lithium ions as well as tBP were found to increase the open circuit potential of the SSD. At the same time tBP was found to considerably lower the current output. The interaction of the additives was studied and their concentration in the spiro-MeOTAD medium optimised. The doping of the spiro-MeOTAD film, which was intended to support the hole transport, was found to enhance interfacial recombination significantly. The morphological properties of the TiO2, in particular layer thickness, particle size and film porosity, play a more important role in the SSD than in the DSSC. Penetration of the hole conductor into the TiO2 pores and electron diffusion length are coupled to these properties. As a result the light harvesting cannot be controlled at will via the TiO2 film thickness and the active surface area for dye adsorption. An enhanced light harvesting for thin TiO2 layers offers advantages for the charge transport and the formation of the interpenetrating network. The dye uptake in presence of silver ions was found to increase the dye loading and to significantly improve the device performance of thin TiO2 layer devices. The mechanism of this simple dye modification technique was studied by a variety of spectroscopic techniques. From spectroscopic evidence it is inferred that the silver is binding to the sensitiser via the ambidentate thiocyanate, allowing the formation of ligand-bridged dye complexes. The beneficial influence of the silver ions on the photovoltaic performance was not limited to the application of the standard N3 dye nor to the spiro-MeOTAD. SSDs were furthermore studied by frequency resolved techniques. Intensity modulated photocurrent spectroscopy (IMPS) and intensity modulated photovoltage spectroscopy (IMVS) were performed over a wide range of illumination intensities. The IMPS and IMVS responses provide information about charge transport and electron-hole recombination processes respectively. For the range of light intensities investigated, the dynamic photocurrent response appears to be limited by the transport of electrons in the nanocrystalline TiO2 film rather than by the transport of holes in the spiro-MeOTAD. The diffusion length of electrons in the TiO2 was found to be 4.4 μm. This value was almost independent of the light intensity as a consequence of the fact that the electron diffusion coefficient and the rate constant for electron-hole recombination both increase in the same way with light intensity but with opposite sign. The results of this work provide for a substantial improvement of the overall photovoltaic performance compared to earlier results for this type of SSD. However, this study reveals also that high conversion efficiencies as are measured for DSSC are not likely to be reachable with the spiro-MeOTAD system due to the significantly slower charge transport in the spiro-MeOTAD compared to the electrolyte redox mediator.

EPFL2003

Carbazol-phenyl-phenothiazine-based sensitizers for dye-sensitized solar cells

Yi Ding, Felix Thomas Eickemeyer, Wei Huang, Yunfei Jiao, Lukas Pfeifer, Dan Ren, Zhongjin Shen, Shaik Mohammed Zakeeruddin

Heteroaromatic units are commonly used as pi-spacers for sensitizers in dye-sensitized solar cells (DSSCs). Here, we investigated the influence of the pi-spacer on the photophysical properties, electrochemical properties and photovoltaic performance of carbazol-phenyl functionalized phenothiazine donor based organic dyes, i.e., CPPC-No (without a pi-spacer), CPPC-Th (thiophene as the pi-spacer), CPPC-Fu (furane as the pi-spacer), CPPC-Ph (phenyl as the pi-spacer) and CPPC-Py (pyridine as the pi-spacer). The solar to electric power conversion efficiency (PCE) achieved with these dyes strongly depends on the type of pi-spacer, increasing in the order of CPPC-Ph < No < Py < Th < Fu. The width of the absorption band and the photocurrent density increased with the electron richness of the pi-spacer. Theoretical calculations indicated that the planar molecular configuration of CPPC-Fu promotes the intramolecular charge transfer process. As a result, the CPPC-Fu based device showed a PCE of 7.4% under one sunlight illumination. The dyes using thiophene and pyridine as pi-spacers showed slightly lower performance, however, still significantly higher than the PCE achieved with the control compound lacking a pi-spacer. By contrast, introducing a phenyl group as the pi-spacer led to a decrease in device efficiency. These results give deeper insight into the effect of different pi-spacers on DSSC performance, showing the importance of molecular engineering in developing high efficiency organic dyes.

ROYAL SOC CHEMISTRY2021

Interface engineering in solid-state dye-sensitized solar cells

Jessica Krüger

EPFL2003