Mohammed K. Nazeeruddin

Cyclopentadithiophene Bridged Donor-Acceptor Dyes Achieve High Power Conversion Efficiencies in Dye-Sensitized Solar Cells Based on the tris-Cobalt Bipyridine Redox Couple

Keywords: cobalt ; dye-sensitized solar cells ; electrochemistry ; photovoltaics ; sensitizers ; Photovoltaic Cells ; Transport ; Recombination ; Electrolyte ; Performance ; Mediators Reference EPFL-ARTICLE-166673doi:10.1002/cssc.201100120View record in Web of Science Record created on 2011-06-08, modified on 2017-05-12

Preparation of phosphonated polypyridyl ligands to anchor transition-metal complexes on oxide surfaces: application for the conversion of light to electricity with nanocrystalline TiO2 films

To anchor transition-metal compounds onto metal oxide surfaces 2,2′:6′,2″-terpyridine-4′-phosphonic acid (4′-PO3H2-terpy) is synthesized; strong surface adhesion as well as efficient charge-transfer sensitization of nanocrystalline TiO2 films has been observed with a ruthenium complex involving this ligand.

Blue-coloured highly efficient dye-sensitized solar cells by implementing the diketopyrrolopyrrole chromophore.

The paradigm shift in dye sensitized solar cells (DSCs) – towards donor- π bridge-acceptor (D-π-A) dyes – increases the performances of DSCs and challenges established design principles. Framed by this shifting landscape, a series of four diketopyrrolopyrrole (DPP)-based sensitizers utilizing the donor-chromophore-anchor (D-C-A) motif were investigated computationally, spectroscopically, and fabricated by systematic evaluation of finished photovoltaic cells. In all cases, the [Co(bpy)3]3+/2+ redox-shuttle afforded superior performance compared to I3−/I−. Aesthetically, careful molecular engineering of the DPP chromophore yielded the first example of a high-performance blue DSC – a challenge unmet since the inception of this photovoltaic technology: DPP17 yields over 10% power conversion efficiency (PCE) with the [Co(bpy)3]3+/2+ electrolyte at full AM 1.5 G simulated sun light.

Regeneration and recombination kinetics in cobalt polypyridine based dye-sensitized solar cells, explained using Marcus theory

Regeneration and recombination kinetics was investigated for dye-sensitized solar cells (DSCs) using a series of different cobalt polypyridine redox couples, with redox potentials ranging between 0.34 and 1.20 V vs. NHE. Marcus theory was applied to explain the rate of electron transfer. The regeneration kinetics for a number of different dyes (L0, D35, Y123, Z907) by most of the cobalt redox shuttles investigated occurred in the Marcus normal region. The calculated reorganization energies for the regeneration reaction ranged between 0.59 and 0.70 eV for the different organic and organometallic dyes investigated. Under the experimental conditions employed, the regeneration efficiency decreased when cobalt complexes with a driving force for regeneration of 0.4 eV and less were employed. The regeneration efficiency was found to depend on the structure of the dye and the concentration of the redox couples. [Co(bpy-pz)2](2+), which has a driving force for regeneration of 0.25 eV for the triphenylamine based organic dye, D35, was found to regenerate 84% of the dye molecules, when a high concentration of the cobalt complex was used. Recombination kinetics between electrons in TiO2 and cobalt(iii) species in the electrolyte was also studied using steady state dark current measurements. For cobalt complexes with highly positive redox potentials (>0.55 V vs. NHE) dark current was found to decrease, consistent with electron transfer reactions occurring in the Marcus inverted region. However, for the cobalt complexes with the most positive redox potentials an increase in dark current was found, which can be attributed to recombination mediated by surface states.

Effect of coadsorbent on the photovoltaic performance of squaraine sensitized nanocrystalline solar cells

The effect of chenodeoxycholic acid as the coadsorbent with a squaraine sensitizer on TiO(2) nanocrystalline solar cells was investigated, and it was found that the coadsorbent prevents the squaraine sensitizer from aggregating on the TiO(2) nanoparticles but reduces dye loading leading to an interdependent photovoltaic performance. Analysis of the absorption spectra, and incident monochromatic photon-to-current conversion efficiency data showed that the load of squaraine sensitizer as well as the appearance of H-aggregates is strongly dependent on the molar concentration of chenodeoxycholic acid coadsorbent. The open circuit voltage of the solar cells with chenodeoxycholic acid increases due to the enhanced electron lifetime in the TiO(2) nanoparticles coupled with the band edge shift of TiO(2) to negative potentials.

Catechol as an efficient anchoring group for attachment of ruthenium–polypyridine photosensitisers to solar cells based on nanocrystalline TiO2 films

The Ru(II)–polypyridyl complexes [Ru(H2L)(terpy)][PF6]2 (1) and [Bu4N][Ru(H2L)(NCS)3] (2) (H2L=4′-(3,4-dihydroxyphenyl)-2,2′:6′,2″-terpyridine), in which H2L is coordinated as a terpyridyl fragment with a catechol site pendant from the C4′ position, adhere effectively to nanocrystalline TiO2 (anatase) surfaces via the pendant catechol group; incident photon-to-current conversion efficiency values of up to 50% were obtained in their photocurrent action spectra, suggesting that the catechol unit may be a convenient and effective anchoring group for attaching dyes to TiO2-based photovoltaic cells.

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 19 mA cm(-2) with a broad incident photon to current conversion efficiency spectra ranging from 400 to 900 nm, exceeding 80 % at 700 nm.

CHAPTER 6:Chemistry of Sensitizers for Dye-sensitized Solar Cells

In this chapter we have introduced operating principles of dye-sensitized solar cells, molecular engineering aspect of sensitizers and redox mediators. The design strategies of ruthenium sensitizers consisting of polypyridyl ligands with, and without thiocyanate ligands are demonstrated. Organic sensitizers based on donor–π-spacer–acceptor (D-π-A) architecture, in which electron-rich (donor) and electron-poor (acceptor) are connected through a conjugated (π) bridge and the anchoring group is attached with the acceptor part, donor–chromophore–acceptor family diketopyrrolopyrrole (DPP) and ullazine sensitizers and their photovoltaic properties are discussed. Molecular engineering aspect of porphyrin core with the bulky donor and strong acceptor groups to obtain panchromatic response is shown. In the last section we highlighted organic–inorganic hybrid perovskites for thin-film photovoltaics, which came to the limelight because of their high efficiency, low cost and the ease to make these materials solution processable yielding over 15% efficiency.