Elucidating the structural details of molecular assemblies formed on surfaces is essential to understand a variety of fundamental surface and interface phenomena as well as develop new types of functional surfaces and interfaces. Examples include molecular electronic devices and adsorption of biological molecules and materials. In the past the vast majority of such studies have been performed for thiol-based self-assembled monolayers (SAMs) which can be formed on gold surfaces by immersion. In contrast, the use of thiol-based SAMs on other metal and semiconducting surfaces has been limited technologically. Meanwhile, carboxylic and phosphonic acids have been frequently employed as an anchoring group for the immobilization of functional molecules on semiconducting surfaces. However, their studies have focused mainly on function rather than structure. As such, adsorption/desorption and exchange behaviour of functional molecules on semiconducting surfaces and their structure-function relationship have not been fully addressed.

As a promising alternative to silicon-based solar cells, considerable efforts have been devoted to the development of dye-sensitized solar cells (DSSCs) where a SAM of dye is formed on a highly porous semiconducting electrode. In particular, porphyrin-sensitized solar cells (PSSCs) have drawn much attention in DSSCs because of potential advantage of porphyrins over ruthenium dyes in terms of cost, molecular design, and light-harvesting properties. On the basis of push-pull concept as well as asymmetrical pi-elongation, cell performances of PSSCs have been improved rapidly. In particular, push-pull porphyrins have been developed intensively in recent years with the aim of better matching between the absorption and solar spectra, reaching a power conversion efficiency more than 10%. To achieve further improvement of the cell performances, it is also important to elucidate close relationship between the dye structure, film structure, photophysics and photovoltaic properties, providing a guideline for the rational molecular design of high-performance dyes.

During the course of our studies, we accidently encountered “memory effects” of the photovoltaic properties in the porphyrin SAMs on the TiO₂ that could be regulated by choice of the sensitization solvents (MeOH vs. t-BuOH/MeCN) with and without the porphyrins. Although there are some examples of memory effects of SAMs induced by external stimuli, “memory effects” of photovoltaic properties by SAMs have not been reported in organic photovoltaics devices. Herein we report the first example of the memory effects in porphyrin SAMs on TiO₂ electrodes that can be controlled by sensitization solvents, which have been evaluated by the photovoltaic properties of PSSCs, complemented by measurements of electron transfer kinetics and SAM composition and thickness. CN-labeled ZnP carboxylic acid (CNMP) and the reference porphyrin (MP) were used in this study.

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