Page - 48 - in Photovoltaic Materials and Electronic Devices

a Ru complex in which a tpy was covalently bound to rhodamine B through an ethanolaminespacer, thuspursuinganenergytransferby“reverse“FRET. 6. SummaryandOutlook The literature offers multiple choices in order to tune the photoelectrochemical propertiesof terpyridine-basedcomplexessuchas theBlackDye, rangingfromthe modificationof thedonorandacceptor ligands to theexchangeof themetalcenter withothercations. Theincreaseofthemolarextinctioncoefficienthasbeencommonly pursuedbyextendingthepi-conjugationonthe ligands. Differentanchoringmoieties were compared, among which COOH turned out as one of the most effective groups. Isothiocyanatewasoftensubstitutedbydifferentancillary ligands inorder to improve long-term stability and the synthetic yield of complexation; bidentate and tridentate ligands that exploit coordination through N or C atoms have been tested in order to achieve a better sensitization. Tetradentate ligands have been used inorder to furtherenlarge thespectralabsorptionproperties. Few outlines can be depicted in this scenario for the design of future complexes: (1)betterstabilitycanbeachievedavoidingtheuseofmonodentateSCNancillary ligands; (2) better performances are offered in the case of heteroleptic complexes (the homoleptic ones have an unfavourable symmetric charge distribution); (3) hydrophobic substitutions on the ligands are able to reduce the electron recombination; (4) a better coupling between the complex and semiconductor can be achieved when COOH moieties are used as attaching groups. Overall, a wise approach is requested in order to tune the energy levels far enough to reach panchromatic absorption, but not too much in order not to exceed the limit for a good regeneration rate by the electrolyte and a good electron injection driving force. Furthermore, the use of tpy complexes nowadays goes beyond the traditional role as sensitizers. Cobalt complexes have been reported as redox mediators, by exploiting the interaction of the EDOT-substituted complex with a PEDOT-covered counter-electrode (PEDOT: poly(3,4-ethylenedioxythiophene) [169]. By finely tuning the single DSC components and their interaction, a further increase ofDSCperformanceswillbepossible. Acknowledgments: The authors gratefully acknowledge financial support of the DSSCX project (PRIN2010-2011,20104XET32) fromMIURandUniversitàdiTorino(RicercaLocale ex-60%,Bando2014). Author Contributions: DS, NB and PQ conceived and drafted the review. DS and CM screened the search results and extracted data from papers. CB and GV coordinated and supervised the project. All authors analyzed and approved the final version of the manuscript. Conflictsof Interest: Theauthorsdeclarenoconflictof interest. 48