Abstract
Nanostructured dye-sensitized solar cells (DSSCs) are promising photovoltaic devices because of their low cost and transparency. Ruthenium polypyridine complexes have long been considered as lead sensitizers for DSSCs, allowing them to reach up to 11% conversion efficiency. However, ruthenium suffers from serious drawbacks potentially limiting its widespread applicability, mainly related to its potential toxicity and scarcity. This has motivated continuous research efforts to develop valuable alternatives from cheap earth-abundant metals, and among them, iron is particularly attractive. Making iron complexes applicable in DSSCs is highly challenging due to an ultrafast deactivation of the metal–ligand charge-transfer (MLCT) states into metal-centered (MC) states, leading to inefficient injection into TiO2. In this review, we present our latest developments in the field using Fe(II)-based photosensitizers bearing N-heterocyclic carbene (NHC) ligands, and their use in DSSCs. Special attention is paid to synthesis, photophysical, electrochemical, and computational characterization.
Highlights
Replacement of fossil fuels is one of the major concerns mankind is currently facing
We focus on the synthesis of both homo- and heteroleptic complexes, discuss their photophysical, electrochemical, and computational characterization, and we describe the performance of these complexes as photosensitizers in dye-sensitized solar cells (DSSCs)
These results clearly indicate that even if the interfacial energy level alignment is favorable for an efficient electron injection step, the primary charge transfer associated to the lowest-energy metal–ligand charge-transfer (MLCT) state goes in the opposite direction with respect to the injection into the TiO2 conduction band (CB)
Summary
Replacement of fossil fuels is one of the major concerns mankind is currently facing. Inorganics 2018, 6, x FOR PEER REVIEW partial transparency [1,2,3] Their principle is to collect photons using a dye, called a sensitizer, linked to a photoanode The oxidized formsource of the for mediator is regenerated by reduction thedye’s cathode, forming a current electrical circuits. States that allowmetal—ligand the electron injection into the semiconductor conduction band (CB). MLCT states are states are populated with a long relatively long lifetime to the of competitive relaxation populated with a relatively lifetime due to the due absence of absence competitive relaxation pathways When they linked to asince photoanode, state energyelectron is highinjection enough, When they are linked to aare photoanode, the MLCTsince state the energy is high enough, electron.
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