DOI: 10.1002/cssc.200800214

CuI versus RuII : Dye-Sensitized Solar Cells and BeyondNeil Robertson*[a]

Fossil fuels are a finite resource and areoverwhelmingly believed to lead to cli-mate-altering accumulation of CO2 in theatmosphere.[1] In this context, low-costand efficient conversion of solar energyhas emerged as a crucial goal, as solarenergy is the only renewable sourcewith the proven capacity to meet theworld’s increasing energy needs.[2] It haslong been known that RuII polypyridylcomplexes provide a number of excel-lent characteristics for the conversion ofsolar energy, including absorption in thevisible region by metal-to-ligand chargetransfer (MLCT), reversible RuII/RuIII redoxcouple, long-lived excited state lifetimeswith visible emission, tunable propertiesthrough ligand design and stable com-plexes under appropriate conditions.[3]

For several decades, much effort hasbeen dedicated to the exploitation ofthese properties for the conversion oflight into usable energy. For example,many studies have explored hydrogenproduction from water using a multi-component system comprising a Rupolypyridyl photosensitizer, a sacrificialelectron donor, an electron relay and acatalyst for H2 evolution. As well as com-prising separate chemical species, thesecomponents can be combined into com-plex assemblies of different functionalunits[4] and recently bimetallic RuII-PtII

complexes were shown to lead to photo-chemical hydrogen production fromwater using a sacrificial electron donor.[5]

Overall, however, conversion of visiblelight into chemical fuel using molecularphotocatalysts without the need for sac-rificial reagents remains an elusive chal-lenge. In contrast, however, high-efficien-cy conversion of solar radiation to elec-trical power has been achieved using

dye-sensitized solar cells (DSSCs) basedon high-surface-area nanocrystalline TiO2

films with attached sensitizer dyes thatallow efficient light harvesting in the visi-ble spectrum where solar radiationpeaks.[6] The excited-state sensitizer in-jects an electron into the TiO2 conduc-tion band. The sensitizer is re-reducedby a solution or solid-state electrolyteleading to regeneration of the dye to itsinitial state. The resulting separation ofelectrons into the TiO2 conduction bandand holes to the electrolyte gives rise tothe cell potential. Although classes ofsensitizer dye have included organic,phthalocyanines, porphyrin compoundsand metal complexes of Os, Pt, Fe andRe,[7, 8] devices with a solar-to-electricpower-conversion efficiency of over 10 %have been achieved only within theruthenium polypyridyl class of dyes, suchas the commonly used [Ru ACHTUNGTRENNUNG{4,4’- ACHTUNGTRENNUNG(CO2H)-bipy}2ACHTUNGTRENNUNG(NCS)2] (N3; bipy = bipyridine).[9]

There has been much discussion inrecent years concerning the use of CuI

polypyridyls as alternative functionalspecies to the ubiquitous RuII systems.[10–

14] These offer a similar set of key electro-chemical, synthetic and photophysicalproperties (Figure 1) for application insolar energy conversion as well as otherpossible technologies including light-emitting diodes,[15] light-emitting electro-

chemical cells,[16] supramolecular archi-tectures and supramolecular machines.[17]

Crucially, CuI polypyridyl complexesdiffer from other redox-active first-rowtransition-metal analogues in having along-lived MLCT excited state that isoften emissive. This behaviour is due tothe filled d10 subshell of CuI preventingrapid non-radiative decay of the MLCTexcited state via metal-centered excitedstates, which for first-row transitionmetals are at a similar energy. The lowcost and high abundance of Cu as com-pared with Ru leads to an obvious ad-vantage in practical technological appli-cations provided that similar functionalutility can be achieved.

An important feature of CuI complexesconcerns the expected flattening of theapproximately tetrahedral geometrywhen oxidized or in the MLCT excitedstate (where the metal is effectively oxi-dized and the ligand is reduced). Thisfeature necessitates some steric con-strain in functionally useful CuI systemsthrough, for example, 2,9-substitution onbipyridyl ligands (Figure 1) to preventgeometric changes that can slow elec-tron-transfer processes and facilitaterapid, non-radiative decay of the MLCTexcited state. Note that the use of suchgeometric constraint to enable reversibleCuI/CuII electrochemistry has parallelswith the entatic state of blue copperprotein active sites.[18]

Initial attempts to use geometricallyconstrained CuI-based dyes as the sensi-tizer (Figure 2 a, b) in nanocrystallineDSSCs led to low-efficiency devices.[19, 20]

With hindsight, however, these can berationalized. Sensitizer dyes require to bestrongly anchored to the semiconduct-ing oxide and also require appropriateelectronic communication between thedye and the nanocrystalline material.These requirements are both normallyachieved through carboxylic acid groups,and studies of RuII polypyridyl sensitizershave generally shown the 4,4’-positions

Figure 1. Summary of the key properties of RuII

(n = 3, e.g. R = H) and CuI (n = 2, R = bulky group)polypyridyl complexes including bipyridines (asshown), phenanthrolines and mixed-ligand com-plexes.

[a] Dr. N. RobertsonSchool of Chemistry, University of EdinburghKing’s Buildings, West Mains RoadEdinburgh EH9 3JJ (UK)Fax: (+ 44) 131-6504743E-mail : neil.robertson@ed.ac.uk

ChemSusChem 2008, 1, 977 – 979 � 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 977

on the bipyridyl ligand to be most favor-able location for these groups.[8] This canoffer an explanation for the poor perfor-mance in the initial studies of CuI com-plexes as the acid functional groups inboth examples (Figure 2 a, b) are in otherpositions. Recently, however, Besshoet al. reported the first examples of CuI-bipyridyl complexes as dyes for nano-crystalline DSSCs with the anchoringgroups located in the 4,4’-positions ofthe bipyridyl unit.[21] This study has nowled to solar-to-electric power-conversionefficiencies of 1.9 % (Figure 2 c) and 2.3 %(Figure 2 d), with the latter enhancementattributed to better light-harvesting bythe more conjugated dye.

Although the efficiencies achievedusing these complexes remain aroundfour times lower than those using thebest RuII dyes, the greater abundance ofCu offers considerable opportunity forreductions in cost. Furthermore, the CuI

approach has yet to undergo the bar-rage of investigation and optimizationthat has been applied to RuII sensitizersand it can be envisaged that great im-provements are yet to come now thatthe possibility of significant efficiencieshas been demonstrated. There are sever-al straightforward design strategies thatcan now be used to follow up these re-sults. For example, all the sensitizersshown in Figure 2 are symmetrical with

two identical ligands however asymme-try in the sensitizer can lead to direction-ality in the light-harvesting transitiongiving more favorable charge-transfer ki-netics in the DSSC device. AsymmetricCuI complexes have yet to be exploredin DSSCs, although interesting exampleswith high emission quantum yields havebeen demonstrated in, for example,light-emitting electrochemical cells.[16]

Also in the case of RuII complexes, thehighest-efficiency sensitizers are general-ly neutral or anionic whereas the CuI ex-amples reported to date are cationic.The charge on the sensitizer may play arole in determining crucial electron-transfer kinetics in the device, and neu-tral CuI complexes could be readily pre-pared using an anionic ligand in place ofone of the bipyridyl ligands. Further-more, the recent study by Bessho et al.emphasized the importance of extend-ing light harvesting by CuI sensitizers tolonger wavelengths to better utilize thesolar spectrum. They also showed en-hanced light harvesting through extend-ed conjugation within the bipyridylligand. CuI polypyridyl complexes in gen-eral have lower molar absorption coeffi-cients than RuII analogues, and strategiesto increase light harvesting offer muchscope for the enhancement of efficien-cies.

With attention to such criteria in thecoming years, we can anticipate furtherjumps in efficiency of DSSCs with sensi-tizers based on CuI. This exciting possi-bility can only add to the prospect of CuI

challenging the long-dominant positionof RuII not only in DSSCs but across sev-eral fields of photophysical, photochemi-cal and supramolecular applications.

Keywords: copper · dye-sensitized solarcells · N ligands · photovoltaics ·sensitizers

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Figure 2. CuI complexes studied as dyes in DSSCs.

978 www.chemsuschem.org � 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2008, 1, 977 – 979

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Received: October 23, 2008

Published online on November 28, 2008

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Cu Dyes for DSSCs