Molecular Design to Control Device Behavior of Organic Electronics

Abstract

Self-assembled monolayers of phosphonates (SAMPs) were designed, studied, and integrated into three different types of organic electronics. Excellent device characteristics were measured for a pentacene-based organic thin-film transistor (OTFT) where the SiO2 gate dielectric is terminated with a self-assembled monolayer of 9,10-dinaphthyl-2-phosphonoanthracene in which calculated molecular spacings are about 0.7 nm. This creates channels that are on the order of the "thickness" of an aromatic &#960; system, which could allow for intercalation of pentacene units, favoring a &#960;-stacking motif for this first pentacene layer. Devices made using a series of SAMPs of such anthracene analogs showed improved mobilities, up to 4.7 cm<super>2</super>/Vs, which were strongly SAMP structure-dependent.

SAMPs of diphosphonoanthracenes and a novel self-assembled organophosphonate duplex ensemble were synthesized on nanometer-thick SiO2-coated, highly doped silicon electrodes. The duplex ensemble was synthesized by first treating the SAMP prepared from an aromatic diphosphonic acid with a titanium alkoxide to form a titanium complex-terminated one; this was followed by addition of a second equivalent of the aromatic diphosphonic acid. SAMP homogeneity, roughness, and thickness were evaluated by AFM; SAMP film thickness and the structural contributions of each unit in the duplex were measured by X-ray reflection (XRR). The duplex was compared with aliphatic and aromatic monolayer SAMPs to determine the effect of stacking on electrochemical properties; these were measured by impedance spectroscopy using aqueous electrolytes in the frequency range 20 Hz to 100 kHz, and data were analyzed using resistance&#8722;capacitance network based equivalent circuits. For the 11-hydroxyundecylphosphonate SAMP, CSAMP = 2.6 ± 0.2 &#956;F/cm<super>2</super>, consistent with its measured layer thickness (ca. 1.1 nm). For the anthracene-based SAMPs, CSAMP = 6&#8722;10 &#956;F/cm<super>2</super>, which is primarily attributed to a higher effective dielectric constant for the aromatic moieties (&#949; = 5&#8722;10) compared to the aliphatic one; impedance spectroscopy measured the additional capacitance of the second aromatic monolayer in the duplex (2ndSAMP) to be CTi/2ndSAMP = 6.8 ± 0.7 &#956;F/cm<super>2</super>, in series with the first. The preparation of titanium or hafnium oxide thin-films is described in which the silicon substrate is first treated with vapor of titanium or hafnium tetra(tert-butoxide) and then thermolyzed to give several monolayers of titanium or hafnium oxides that are attached to the silicon surface. The thickness of this oxide layer can be controlled by the vapor exposure time.

Two series of homologous SAMPs and self-assembled monolayers of carboxylates (SAMCs) that are based on oligothiophene dyes were synthesized on TiO2 thin film electrodes and were compared for their use in dye-sensitized solar cells (DSSCs). (Cyanovinyl)carboxylate- and phosphonate-terminated oligothiophene were also compared. In each case, the phosphonate dyes were compared with carboxylate analogs to determine the effect of the anchoring groups on photochemical properties in solution, which were measured by UV/Vis absorption spectroscopy. The growth of SAMPs or SAMCs on TiO2 electrodes was accomplished either by "tethering by aggregation and growth" (T-BAG) or by solution dipping. Surface roughness and homogeneity, elemental composition, and thickness of the SAMP or SAMC monolayers were evaluated by AFM, XPS, and ellipsometry. Molecular loadings and chemical absorptivity of SAMPs and SAMCs were determined by QCM analysis, and the stability of bonding between the dyes and the TiO2 was evaluated by measuring desorption processes; the carboxylates underwent significant dissociation in aqueous media but the phosphonates did not. DSSCs were prepared from each congener and were studied under irradiation from a AM 1.5G solar light source. The DSSC made using a SAMP of sexithiophene (6TP) had total power conversion efficiency (&#951;) up to 5%.