Switching of Dye Loading Mechanism in Electrochemical Self-Assembly of Cu­SCN-D­AST Hybrid Thin Films
Authors: Yoshida Tsukasa , Tsuda Yuki , Nakamura Tenshou , Uda Kyota , Sun He , Sun Lina , Okada Shuji , Masuhara Akito , Stadler Philipp , Sariciftci Niyazi Serdar , White Matthew Schuette 
Published: February 2020

We have found electrochemical self-assembly (ESA) of inorganic / organic hybrid thin films in which the inorganic is CuSCN, known to be a wide bandgap p-type semiconductor, whereas the organic is 4-N,N-dimethylamino-4’-N’-methylstilbazolium chromophore (abbreviated as DAS+) as its salt with tosylate (DAST) is known to exhibit second-order nonlinear optical property for terahertz emitters [1]. DAS+ is also known to yield a layered inorganic-organic hybrid crystal in a (DAS)(Cu5I6) composition [2]. Strong dipole-dipole interaction of the DAS+ chromophores aligned between the CuI layers results in a spontaneous photocarrier generation and ambipolar transport in a single absorber solar cell. Thus, CuSCN/DAS hybrid thin films can be attractive alternatives for such opto-electrical applications, when ordered arrangement of DAS+ chromophore is achieved during ESA.
In our previous study, switching of dye loading mechanism has been suggested, depending on DAS+ concentration in the bath [1]. In low DAS+ concentration range, the loading is limited by diffusion so that DAS+ is entrapped within CuSCN crystal grains, while surface reaction of hybridization begins to limit the dye loading in high DAS+ concentration range, resulting in formation of unique nanostructures as well as phase separation of inorganic and organic domains. In this study, electrochemical analysis by employing rotating disk electrode (RDE) has been performed to verify the mechanism of ESA and also to explore the limit of DAS+ loading.
Electrodeposition of CuSCN undergoes as limited by transport of 1 : 1 complex between Cu2+ and SCN- ions, namely {[Cu(SCN)]+} species, near 100% Faradic efficiency with marginal influence by the presence of DAS+. Thus, the rate of CuSCN growth is always proportional to the concentration of the active species, and also to ω1/2 (ω = angular speed of rotation of RDE). On the other hand, the rate of DAS+ precipitation, v(DAS)(SCN), should also be proportional to ω1/2 under the regime of diffusion limited loading with a given [DAS+], and is independent of CuSCN formation rate. When the rate surface complex formation begins to limit the rate of DAS+ precipitation, the amount of DAS+ loading should become proportional to the rate of CuSCN deposition, if a second order rate law as shown below holds,
v(DAS)(SCN) (mol s-1 cm-2) = k[DAS+][site] (1)
where, k (mol-1 cm3 s-1) is the second order reaction rate constant for formation of surface complex and [site] is the surface concentration of newly formed CuSCN sites, which should be expressed as,
[site] (mol cm-2) = A × 0.62 × {[Cu(SCN)]+} × D{[Cu(SCN)]+}2/3 × ν(methanol, 298 K)-1/6 × ω1/2 (2)
which is simply derived from Levich equation for diffusion limited electrodeposition of CuSCN, multiplied by a proportionality constant, A (s), to express the activity of the surface site.
The amount of DAS+ loaded into the film electrodeposited for a given [DAS+] under variation of {[Cu(SCN)]+} in the bath was examined, and indeed such a trend as predicted from the above-mentioned model was found (Fig. 1). In the high {[Cu(SCN)]+} range, DAS+ loading is independent of {[Cu(SCN)]+} and appears proportional to [DAS+], because of the diffusion limited loading mechanism. When {[Cu(SCN)]+} goes below certain concentration, DAS+ loading changes proportionally to {[Cu(SCN)]+}, for a given [DAS+], but is also proportional to [DAS+] for a given {[Cu(SCN)]+}, as expected for the surface reaction limitation expressed by Eq. (1). The switching from surface reaction to diffusion limitation occurs at the lower {[Cu(SCN)]+} when [DAS+] goes lower, as recognized by the shift of the border between {[Cu(SCN)]+} dependent and independent parts.
[1] Yuki Tsuda et al., Monatshefte für Chemie, 148, 845-854 (2017)
[2] Elena Cariati et al., Adv. Mater., 13, 1665-1668 (2001).

Figure 1