Assembly of CdS quantum dots onto mesoscopicTiO2 films for quantum dot-sensitized solar cell applications

Abstract

Colloidal cadmium sulfide (CdS) quantum dots (QDs) were prepared and surfacemodified by mercaptosuccinic acid (MSA) to render a surface with carboxylicacid groups (MSA–CdS). The MSA–CdS QDs were then assembled onto bareTiO2 mesoporous filmsusing the carboxylic groups/TiO2 interaction. The TiO2 film was also surface modified by 3-mercaptopropyl trimethoxysilane (MPTMS) or3-aminopropyl-methyl diethoxysilane (APMDS) to prepare, respectively, a thiol (–SH) or amino(–NH2) terminated surface for binding with the CdS QDs. The experimental results showedthat the MPTMS-modified film has the highest adsorption rate and adsorptionamount to the CdS QDs, attributable to the strong thiol/CdS interaction. Incontrast, the adsorption rate and incorporated amount of the QDs on the bareTiO2 film are much lower than for the silane-modified films. The incidentphoton-to-current conversion efficiency (IPCE) obtained for the CdS-sensitizedTiO2 electrode was about 20% (at 400 nm) for the bareTiO2, 13% for theMPTMS–TiO2, and6% for APMDS–TiO2. The current–voltage measurement under dark conditions reveals a higher dark current on theMPTMS- and APMDS-modified electrodes, indicating a lower coverage ratio of CdS on theseTiO2 films. This result is attributed to the fast adsorption rate of CdS QDs on the bottleneck of amesopore which inhibits the transport of the QDs deep into the inner region of a pore. For the bareTiO2 film, the lower incorporated amount of CdS but higher energy conversionefficiency indicates the formation of a better-covered CdS QDs monolayer.The moderate adsorption rate of MSA–CdS QDs using the carboxylicacid/TiO2 interaction is responsible for the efficient assembly of QDs onto the mesoporousTiO2 films.