ate. This is generally accompanied by a mobility drop of more than one order of magnitude. [20] In contrast, spiro-substituted CTMs combine the high morphologic stability commonly only observed in polymeric systems with the high charge mobility of low-molecular-weight CTMs. They open a new synthetic route to stable, electronically active materials with improved properties for the design of high-performance optoelectronic devices.
Experimental
Spiro-m-TTB and spiro-TAD were used as received from Covion Semiconductors GmbH/Frankfurt (>99.9 % pure; according to high-performance liquid chromatography and 1 H NMR). The syntheses are described elsewhere [23,25] .
Conducting glass sheets, coated with fluorine doped SnO 2 (1 mm; Asahi) were used as substrates and structured with a mixture of 4 N HCl and granulated zinc. The structured substrates were cleaned by subsequent ultrasonification in acetone, ethanol, Hellmanex, and distilled water. A thin, compact layer (50±100 nm) of TiO 2 was applied via spray pyrolysis of a 0.02 M ethanolic solution of titanium(IV)bis(acetylacetonate)diisopropoxide at 450 C. A thin, 3±4 mm thick film of the hole-conducting material was evaporated onto the sample from a tantalum crucible. Typical deposition rates were 1 nm/s at a typical background pressure of 10 ±6 torr. Finally a 40 nm thick gold layer was vapor deposited on top of the organic layer. The sample was not exposed to ambient atmosphere between the two evaporation steps.
Mobilities were measured with a conventional TOF setup. The sample was placed inside a cryostat (Oxford Instruments), equipped with quartz windows and kept under nitrogen atmosphere. A frequency-doubled ruby laser (6 ns pulse width, 354 nm excitation wavelength, 0.1 Hz) was used as the excitation light source. Optically neutral density filters were used to adjust the excitation energy. The exposures were such that the maximum charge injected into the sample was always less than 5 % of the charge stored on the capacitor, represented by our device. The displacement current was measured in the external circuit by a time-resolved measurement of the potential drop over a variable resistance, using a Tektronix digital oscilloscope. From the absorption coefficient of the CTMs and TiO 2 we calculated that more than 90 % of the excitation light is adsorbed in the TiO 2 layer and the first 10 % of the organic film.