A Simple Synthetic Route to Obtain Pure Trans-Ruthenium(II) Complexes for Dye-Sensitized Solar Cell Applications

We report a facile synthetic route to obtain functionalized quaterpyridine ligand and its trans-dithiocyanato ruthenium complex, based on a microwave-assisted procedure. The ruthenium complex has been purified using a silica chromatographic column by protecting carboxylic acid groups as iso-butyl ester, which are subsequently hydrolyzed. The highly pure complex exhibits panchromatic response throughout the visible region. DFT/time-dependent DFT calculations have been performed on the ruthenium complex in solution and adsorbed onto TiO2 to analyze relative electronic and optical properties. The ruthenium complex endowed with the functionalized quaterpyridine ligand was used as a sensitizer in dye-sensitized solar cell yielding a short-circuit photocurrent density of more than 19mAcm(-2) with a broad incident photon to current conversion efficiency spectra ranging from 400 to 900nm, exceeding 80% at 700nm.

New Paradigm in Molecular Engineering of Sensitizers for Solar Cell Applications

Takeru Bessho, Mohammad Khaja Nazeeruddin, Ursula Röthlisberger, Ivano Tavernelli

A novel thiocyanate-free cyclometalleted ruthenium sensitizer for solar cells is designed and developed. Upon anchoring to nanocrystalline TiO2 films, it exhibits a remarkable incident monochromatic photon-to-current conversion efficiency of 83%. The solar cell employing a liquid-based electrolyte exhibits a short circuit photocurrent density of 17 mA/cm(2), an open circuit voltage of 800 mV, and a fill factor of 0.74, corresponding to an overall conversion efficiency of 10.1% at standard AM 1.5 sunlight. To understand the structural, electronic, and optical properties of the cyclometalleted ruthenium sensitizer, we have investigated using density functional theory (DFT) and time-dependent DFT (TDDFT). Our results show the HOMO is located mostly on ruthenium and cyclometalated ligand, while the LUMO is on 4-carboxylic acid-4'-carboxylate-2,2'-bipyridine. Molecular orbitals analysis confirmed the experimental assignment of redox potentials, and TDDFT calculations allowed assignment of the visible absorption bands. The present findings provide new design criteria for the next generation of ruthenium sensitizers and help foster widespread interest in the engineering of new sensitizers that interact effectively with the I-/I-3(-) redox couple.

2009

Molecular engineering of hybrid sensitizers incorporating an organic antenna into ruthenium complex and their application in solar cells

Mohammad Khaja Nazeeruddin, Xiang Xu

Two novel hybrid sensitizers incorporating an organic antenna into a ruthenium complex have been synthesized and applied successfully in nanocrystalline TiO2 based solar cells, which yielded solar to electricity conversion efficiencies of 8.20-9.16% under AM 1.5 sunlight.

2008

Investigation on Functionalized Ruthenium-Based Sensitizers to Enhance Performance and Robustness of Dye-Sensitized Solar Cells

Nuttapol Pootrakulchote

It has been increasingly aware to the world today that reserves of fossil fuels are limited and their use has serious environmental side effects. Encouraged by this realization was the evolution of the use of cleaner alternative energy, among which Dye-sensitized solar cells (DSCs) are potentially attractive candidates for the lower cost of producing devices which convert an abundant amount of energy from the sun into electricity. Dye-sensitizer in DSCs plays a crucial role as the chlorophyll in plants; to harvest solar light and transfer the energy via electron transfer to a suitable material (TiO2 in this case) to produce electricity. The topic of interest for this thesis is to further enhance the photovoltaic performance and the robustness of DSCs by tuning the optical properties of the dye-sensitizer (Ruthenium complex, in this case) using several strategies including an extension of the π-conjugation system, an introduction of antenna molecules and a modification of the Ru-complex structure. This work focuses on the DSC device fabrication and photovoltaic characterization in order to investigate more insight into structure-property-device performance relationship. New benchmarks for high performance DSCs with ruthenium complex sensitizers with π-extension in their ancillary ligands were presented. The overall conversion efficiency of 9.6% and 8.5% have been achieved with Ru-based sensitizer containing ethylenedioxythiophene, using low-volatile electrolyte and solvent-free electrolyte, respectively. The Rusensitizer functionalized with hexylthio-bithiophene unit exhibited a conversion efficiency of 9.4% with low-volatile electrolyte. All these devices showed good stability under prolonged light soaking at 60 °C. Extending π-conjugation of the anchoring ligand with thiophene units in monoleptic Ru-sensitizer also yields an impressive conversion efficiency of 6.1% using 3-µm-thin mesoporous TiO2 film in corporate with low-volatile electrolyte. DSC devices based on ruthenium sensitizers functionalized with thienothiophene- and EDOT-conjugated bridge, together with carbazole moiety on the ancillary ligands were found efficient with conversion efficiencies of 9.4% and 9.6%, respectively, in presence of a volatile electrolyte. The carbazole-functionalized ruthenium-based DCSs also performed excellently in the stability test using a low-volatile electrolyte. Furthermore, the Ru-complexes synthesized by click-chemistry in association with triazole-derivative moieties were successfully used as DCS sensitizers. DSC devices sensitized with these dyes provided the overall conversion efficiency close to 10% with volatile electrolyte. Further studies with solvent-free electrolyte showed notable device stability under extending full sunlight intensity at 60 °C. The results presented here provide a fertile base for further investigation, which will focus on improving the spectral response of ruthenium dye-sensitizer to full sunlight by searching for new strategies to modify the sensitizer with more efficient functional groups. The target is to reach higher conversion efficiency of DSC devices while retaining their stability under standard reporting conditions.

EPFL2012

Investigation on Functionalized Ruthenium-Based Sensitizers to Enhance Performance and Robustness of Dye-Sensitized Solar Cells

Nuttapol Pootrakulchote

EPFL2012