Dye-sensitized solar cells with hole-stabilizing surfaces: “inorganic” versus “organic” strategies
Two 2,2′:6′,2′′-terpyridine ligands (9 and 10) incorporating second-generation diphenylamino-dendrons have been synthesized and characterized; one ligand contains chromophoric benzothiadiazole domains. Using the ‘surface-as-ligand, surface-as-complex’ strategy, zinc(II)-containing sensitizers [Zn(Lanchor)(Lancillary)]2+ with carboxylic or phosphonic acid anchors (1 and 2, respectively) have been assembled and tested in n-type DSCs. The solid-state absorption spectra of dye-functionalized electrodes show a broad spectral response for all the dyes with enhanced intensity for those containing the benzothiadiazole units. However, the [Zn(Lanchor)(Lancillary)]2+ dyes perform poorly, exhibiting very low values of the short-circuit current density (JSC) and open-circuit voltage (VOC). The external quantum efficiency (EQE) spectra confirm that electron injection occurs, but EQEmax is ≤3%. Non-optimal positioning of the thiadiazole domain in the dye probably contributes to the poor performances. Screening of DSCs containing FTO/TiO2 photoanodes without adsorbed dye shows that they generate small short-circuit current densities and open-circuit voltages which contribute significantly to parameters reported for badly performing dyes. An organic dye 11, structurally similar to 10 and containing a 2-cyanoacrylic acid anchor, is also reported. This exhibits a broad and intense spectral response between 300 and 600 nm, and shows efficient electron injection over a broad wavelength range. DSCs containing 11 are stable over a 17 day period and show global efficiencies of 3.93–4.57% (ca. 70% with respect to N719 set at 100%). Ground state DFT calculations reveal that the HOMO in each of [Zn(1)(9)]2+, [Zn(2)(9)]2+, [Zn(1)(10)]2+, [Zn(2)(10)]2+ and 11 is localized on the peripheral diphenylamino units, allowing for hole-transfer to the reduced electrolyte. In 11, a major contribution from the 2-cyanoacrylic acid anchoring group appears in the LUMO manifold; however, while the LUMO in each zinc(II) dye is localized on anchoring ligand 1 or 2, it is concentrated close to the metal centre which may contribute to poor electron injection.