Quantification of dyes generating photocurrent and/or photoluminescence in dye-sensitized solar cells using laser scanning microscopy

Abstract

Photoconversion processes such as electron injection (photooxidation) and dye regeneration (reduction) in dye-sensitized solar cells (DSSCs) occur at considerably inhomogeneous semiconductor/dye/electrolyte interfaces, implying a very high heterogeneity of interfacial photoconversion kinetics. Herein, we present a temporally and spatially resolved investigation of DSSCs comprising a cover glass photoanode with a 100-nm thick TiO2 layer loaded with the metal-free organic dye sensitizer MK-2, which is performed by employing laser scanning microscopy (LSM) for the simultaneous measurement of the photocurrent (PC) and photoluminescence (PL) of DSSCs under short-circuit conditions. Analysis of PL decay curves and the excitation rate dependences of PC and PL obtained for local (or submicrometric) areas of the MK-2-DSSC allows disclosing and quantifying three types of dyes coexisting in the DSSCs: (i) a dye that only generates PC (“PC-dye,” 75% of total dye molecules in the DSSC), (ii) a dye that generates both PC and PL (“PCPL-dye,” 20%), and (iii) a dye that only generates PL (“PL-dye,” 5%). Considering recent theoretical reports on cyanoacrylic dyes, we propose that the PC-dye and the PCPL-dye are covalently bound on a TiO2 surface with different adsorption modes (presumably bidentate and tridentate bridging configurations), whereas the PL-dye is noncovalently trapped within a mesoporous TiO2 film.