The film morphology and device performance of planar heterojunction solar cells based on the molecular donor material α-sexithiophene (6T) are investigated. Planar heterojunctions of 6T with two different acceptor molecules, the C 60 fullerene and diindenoperylene (DIP), have been prepared. The growth temperature of the 6T bottom layer has been varied between room temperature and 100 °C for each acceptor. By means of X-ray diffraction and X-ray absorption, we show that the crystallinity and the molecular orientation of 6T is influenced by the preparation conditions and that the 6T film templates the growth of the subsequent acceptor layer. These structural changes are accompanied by changes in the characteristic parameters of the corresponding photovoltaic cells. This is most prominently observed as a shift of the open circuit voltage (V oc ): In the case of 6T/C 60 heterojunctions, V oc decreases from 0.4 to 0.3 V, approximately, if the growth temperature of 6T is increased from room temperature to 100 °C. By contrast, V oc increases from about 1.2 V to almost 1.4 V in the case of 6T/DIP solar cells under the same conditions. We attribute these changes upon substrate heating to increased recombination in the C 60 case while an orientation dependent intermolecular coupling seems to change the origin of the photovoltaic gap in the DIP case.

We present depth-resolved grazing incidence x-ray diffraction, grazing incidence small angle scattering and x-ray reflectivity studies on the structure of mixed C(60) and diindinoperylene (DIP) films as a function of the mixing ratio. We observe enhanced out-of-plane order and smoothing of the mixed films compared to pure films upon coevaporation of DIP:C(60) thin films (in different mixing ratio) which otherwise phase separate. The mixing ratio of molecules can be tuned to alter the in-plane crystallite size as well as the interisland distances of the mixing molecules. Real-time in situ grazing incidence x-ray diffraction measurements show the kinetics and thickness dependence of phase separation, which appears to proceed only after a certain thickness. The crystallite grain size of the individual phase separated components is significantly larger at the top of the film than at the bottom with implications for the understanding of devices.

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...

Thickness and substrate dependence of film growth, morphology, unit-cell structure, and electronic structures was thoroughly investigated for picene, the zigzag connected 5-ring molecule, by employing complementary techniques of in situ real-time X-ray reflectivity/diffraction, in situ electron spectroscopies, and atomic force microscopy. A different kind of thickness dependent structural transition was observed on SiO2 and graphite, resulting in a distinct electronic structure. On SiO2 picene films with 3D crystalline domains are formed with nearly upright molecular orientation from the initial growth stage. With increasing the film thickness the in-plane dimensions of the unit cell in the initially grown domains become smaller (in other words, more compressed), and, at the same time, crystalline domains with a more relaxed structure are nucleating on top of the compressed domains. In spite of such structural changes, the electronic structure, namely energy position of the highest occupied molecular orbital and threshold ionization potential (IPT), is not significantly altered. On graphite, on the other hand, we found a transition from a 2D (layer) to a 3D (island) growth mode with a variation of the molecular orientation from flat-lying to tilted one. The IPT changes significantly between the 2D and 3D growth regime in contrast to the SiO2 system. The origin of the different IPTs of these picene thin films is discussed. The present results are compared with other planar π-conjugated compounds, in particular pentacene which is a structural isomer of picene and shows electronic properties strongly different from picene thin films.

Diindenoperylene (DIP) and tetraphenyldibenzoperiflanthene (DBP) are two commonly used donor materials in organic solar cell devices. Despite their structural similarities, DIP films are crystalline, exhibiting good charge and exciton transport, whereas DBP films are amorphous and have lower carrier mobility and a short exciton diffusion length. However, DBP reveals a distinctly higher absorption due to the lying orientation of its transition dipole moments. In this paper, we investigate the influence of solvent vapor annealing (SVA) on the solar cell performance of both materials. In general, SVA induces a partial re-solubilization of the material leading to enhanced crystallinity of the treated layer. For DBP, extended annealing times result in a strong aggregation of the molecules, creating inhomogeneous layers unfavorable for solar cells. However, in DIP cells, SVA leads to an increase in fill factor (FF) and also a slight increase in short-circuit current density (J SC ) due to interface roughening. The best results are obtained by combining solvent vapor annealed DIP layers with strongly absorbing DBP and C 70 on top. Through this device architecture, we obtain the same increase in FF in addition to a higher gain in J SC , elevating the power conversion efficiency by a factor of 1.2 to more than 4 %. FIG. 1. Round crystallites formed in a 50 nm thick DBP-layer by annealing for 10 minutes in chloroform vapor.could be observed. Therefore, in-plane (grazing incident X-ray diffraction) measurements were recorded, but also in this configuration no peaks indicating DBP crystallinity are observable. The results of both, out-of-plane and in-plane measurements are shown in the supplementary material. Other groups have already reported on "more crystalline" DBP, achieved through different techniques. Growing on a crystalline template 38 or on a heated substrate 40 as well as applying organic vapor phase deposition (OPVD) using a hot inert carrier gas 21 were reported to result in DBP layers of higher order. However, in these cases the crystallinity could neither be visualized by means of XRD 38 , nor via reflection high energy electron diffraction 40 (RHEED) nor by selected area electron diffraction 21 (SAED).Next, we investigated the surface properties of 15 nm films of DBP annealed for various durations via AFM. There was no difference for layers growing either on ITO/HIL1.3 or on glass so the reorganization of the molecules is independent of the substrate. These layers were treated by SVA for 4, 8 and 12 minutes, respectively, and then compared with each other as well as with an untreated sample. As reported previously 23,40 , the pristine DBP layer has an extremely smooth surface with a root-mean-square (RMS) roughness of merely RMS = 0.63 nm. However, SVA causes a strong aggregation of the DBP molecules. After 4 minutes of SVA treatment, the RMS roughness increases more than thirtyfold to 21.65 nm. 7 Evaluating the performance of solar cells fabricated with annealed DBP films confirms these problems. For elevate...