Studies on Synthesis and Properties of Novel Conjugated Molecules as Interface Modifiers of P3HT/TiO2 Layered Heterojunction
Date Issued
2010
Date
2010
Author(s)
Teo, Chin-Sien
Abstract
The main purpose of this research is to synthesize a series of conjugated molecules serving as an TiO2 modifier in poly(3-hexylthiophene) (P3HT)/titania (TiO2) layer heterojunction devices. By improving the charge separation and transfer between P3HT organic donor and TiO2 inorganic acceptor, the LUMO levels of interface modifiers (IMs) should be located between the LUMO levels of P3HT and conduction band of TiO2. Therefore, combining the strategies of structural design and principle of electronic structure, we developed a series of novel conjugated molecules that composed of three parts: (i) benzathiadiazole and thiophene moieties as the conjugated backbone for energy levels tuning, (ii) a hexyl chain to increase solubility, lipophilicity and inhibit π-stacking aggregation, and (iii) cyanoacetic acid as an electron-accepting and TiO2 anchoring substituents.
We first utilized Kumada coupling to engineer the hexyl chain onto different positions on thiophene thus obtaining various derivative structures, and subsequently combined benzathiadiazole and thiophene moieties through Stille coupling to form conjugated molecules. Next, we performed a Vilsmeier-Haack formylation to yield an additional aldehyde group on the conjugated backbone. Finally, we introduced cyanoacetic acid by Knoevenagel condensation to form SL-series conjugated modifiers. Structural characterization of SL-series was achieved by 1H NMR, 13C NMR, elemental analysis (EA) and electron impact mass spectroscopy (EI-MS). Optical and electronic properties of these molecules were determined by UV-vis spectroscopy and cyclic voltammetry (CV). Furthermore, the contact angle of SL-series modified TiO2 on FTO film were measured by contact angle analyzer. The quenching effect on FTO/TiO2:IMs/P3HT was determined by photoluminescence measurement.
Our analytical studies reveal that the UV-vis spectra exhibit a bathochromic effect with an increment in the conjugated moieties. While introducing the hexyl chain effectively increases the solubility of the conjugated molecules, the electron-density donation of hexyl chain also reflects a bathochromic effect on the UV-vis absorption. We also observed an inhibition toward the formation of π-π stacking aggregation in TiO2:IMs films attributed to conjugated molecules bearing a hexyl chain. Furthermore, contact angle analysis shows that, hexyl chian-containing modifiers form a higher average water contact angle than those without hexyl chain, suggesting a better lipophilicity on the TiO2 surfaces in the presence of conjugated molecules consisting hexyl chain which in turn increases its compatibility with organic materials.
Collectively, our study concluded that modified TiO2 films containing the SL-series compounds have a higher quenching ratio in comparison to their unmodified counterpart. These results can further extend to the notion that conjugated molecules can serve as an interface modifier between organic donor and inorganic acceptor to warrant a better performance in the heterojunction photovoltaic devices.
Subjects
conjugated molecules
interface modifier
TiO2
P3HT
layer heterojunction
SDGs
Type
thesis
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