Growth of Competing Crystal Phases of α-Sexithiophene Studied by Real-Time in Situ X-ray Scattering

Abstract

We report on a real-time in situ study of the growth of α-sexithiophene on silicon oxide substrates. Synchrotron-based X-ray diffraction experiments were performed during and directly after the growth in order to monitor the growth process. We observed a coexistence of two different crystal phases for different substrate temperatures. For films prepared at 233 and 308 K a disordered phase (β-phase) seems to be dominant compared to films prepared at 373 K where the so-called lowtemperature bulk crystal phase (LT-phase) is dominant. From real-time measurements during growth we observed a temperature and film thickness dependent effect on the fraction of both phases in one sample. At 373 K the film growth begins primarily in the β-phase, and above a certain thickness the film growth proceeds mainly in the LT-phase. However, at 308 K the film growth is dominated by the β-phase for the entire thickness. We show that for kinetically limited growth conditions (high deposition rate and/or low substrate temperature) substrate induced growth effects are dominant. ■ INTRODUCTIONOrganic electronics are an attractive alternative to common inorganic devices, inter alia due to potentially low preparation costs, low-temperature processing, and the possibility of using flexible substrates. 1−7 Organic semiconductors (OSCs) are also used as active layers in organic light-emitting diodes, organic photovoltaics (OPV), and organic field effect transistors (OFETs). For small molecule OSCs, the crystallinity of the thin films and the relative orientation of the individual molecules constituting the thin film are crucially important for the efficiencies of such devices. Oligothiophenes are an important class of OSC materials. In particular, α-sexithiophene (6T) (Figure 1a) is considered very promising. 6T has shown a high open circuit voltage in combination with diindenoperylene in OPV cells 8 and high hole mobility of up to 4 × 10 −2 cm 2 V −1 s −1 in OFETs. 9 As in many other small molecule OSCs, the crystal structure and the crystal defect density are crucial for the performance. 10−12 In this study we focus on the growth and structure of 6T on native silicon oxide (nSi).In general, depending on the substrate, the preparation conditions and the sample state (i.e., thin film or single crystal), there are several phases of 6T with different crystal structures reported. 13−16 In thin films on substrates with low interaction energies, mostly the so-called low-temperature phase (LTphase) of single crystals, reported by Horowitz et al., 16 and the thin film β-phase 14,15 are found. Figure 1b depicts the unit cell of the LT crystal phase. For 6T on SiO 2 several studies of growth, structure, and charge transport report that 6T molecules are oriented mostly perpendicularly to the substrate. 14,15,17−21 Different anisotropic growth scenarios on fused silica and stretched polyethylene substrates were measured via absorption spectroscopy by Oelkrug et al. 22 On TiO 2 the formation of domains of the LT-phase 16 was observed. 23,24 Highly ordered...

Templating Effects of α-Sexithiophene in Donor–Acceptor Organic Thin Films

“…This confirms the estimation that the film consists of only β phase crystallites, which is in agreement with real-time growth results, showing that close to the substrate the film growth is dominantly in the β phase. 54 Thicker films (nominally 20 nm) were prepared at two different substrate temperatures, 373 and 308 K. The film structure is in agreement with previous reports. 54 The out-ofplane data of the film prepared at 373 K (black curve in Figure 2) show only damped, weak oscillations in the low-q z range, indicating a roughness of 5.3 nm.…”

“…54 Thicker films (nominally 20 nm) were prepared at two different substrate temperatures, 373 and 308 K. The film structure is in agreement with previous reports. 54 The out-ofplane data of the film prepared at 373 K (black curve in Figure 2) show only damped, weak oscillations in the low-q z range, indicating a roughness of 5.3 nm. The XRR data are dominated mostly by Bragg reflections of the LT phase.…”

“…The Journal of Physical Chemistry C Article the literature. 54 In summary, three templating layers with different structural properties are identified: (a) films prepared at 373 K, consisting mainly of the 6T LT phase and having a large D coh∥ , (b) films prepared at 308 K, showing mainly β phase domains and D coh∥ is roughly half the size of D coh∥ of the 373 K film, and (c) films prepared at 233 K with even smaller D coh∥ and additional fractions of lying-down oriented 6T. The contact plane of the lying-down 6T cannot be unambiguously determined; it could either be the (0 1 0) or the (−4 1 1) plane, which are also reported for lying-down 6T on Cu (1 1 0).…”

Delayed phase separation in growth of organic semiconductor blends with limited intermixing

“…In both systems in the blends dominated by the shorter compound (PIC respectively 6T) phase separation occurs. Interestingly in the pure PIC [37] and 6T [27] thin films different polymorphs nucleate, thus this could be another reason for the phase separation.…”

“…Polymorphism and coexistence of polymorphs are rather typical and fundamental issues in molecular crystals, which are notoriously hard to predict and understand theoretically. Therefore, a solid experimental study is even more important [21][22][23][24][25][26][27][28]. For mixed systems, the spectrum of possible scenarios is even broader and its rationalization more challenging.…”