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Solution-processable organic semiconductors are central to developing viable printed electronics, and performance comparable to that of amorphous silicon has been reported for films grown from soluble semiconductors. However, the seemingly desirable formation of large crystalline domains introduces grain boundaries, resulting in substantial device-to-device performance variations. Indeed, for films where the grain-boundary structure is random, a few unfavourable grain boundaries may dominate device performance. Here we isolate the effects of molecular-level structure at grain boundaries by engineering the microstructure of the high-performance n-type perylenediimide semiconductor PDI8-CN2 and analyse their consequences for charge transport. A combination of advanced X-ray scattering, first-principles computation and transistor characterization applied to PDI8-CN2 films reveals that grain-boundary orientation modulates carrier mobility by approximately two orders of magnitude. For PDI8-CN2 we show that the molecular packing motif (that is, herringbone versus slip-stacked) plays a decisive part in grain-boundary-induced transport anisotropy. The results of this study provide important guidelines for designing device-optimized molecular semiconductors.

Polymer semiconductors such as the alkyl-substituted polythiophenes have long been recognized as solution-processable materials for device applications, but the carrier mobility of polymers is typically lower than insoluble organic small molecules such as pentacene. The lower mobility is generally attributed to less structural order; specifically the smaller and less-aligned crystals typical of polymer semiconductors should exhibit reduced intermolecular p-orbital overlap at grain boundaries.[1] The more disordered nature of polymer semiconductors has made it challenging to determine the details of their thin-film crystal structure.[2] Structure measurements are further complicated by the small volume of the thin, 20-50 nm, polymer semiconductor films used in organic field effect transistors. [3,4] X-ray diffraction (XRD) usually provides only primary index peaks, most often the and <0l0> series, that reveal crystal structure, orientation, and size, but exact atomic positions can rarely be determined. Moreover, it is clear that the thin-film crystal structure can differ from single crystal or powder structure.[5]Alkyl-substituted polythiophenes feature a backbone of sequentially bonded thiophene rings with linear alkane chains attached to their sides. In thin films, they self-assemble into lamellae; comparing typical unit-cell dimensions to molecular dimensions leads to a generally accepted layer-packing motif where planar backbones p stack in aromatic lamellae vertically segregated from lamellae of side chains. [6][7][8] Two critical but poorly characterized aspects of the crystal structure are the conjugated-plane tilt and the alkane side-chain configuration. In the lamellar motif, the conjugated planes must be roughly vertical (orthogonal to the lamella) so that they can p stack face-to-face within a quasi two-dimensional sheet. However, a variety of conjugated-plane tilts and a rich possibility of side-chain configurations-distinguished by varying tilt and degree of interdigitation-are consistent with this layer-packing motif and available diffraction data. Greater structural detail will support the development of new synthetic and processing strategies, because carrier transport critically depends on the intermolecular overlap of carrier band orbitals, which is controlled by the conjugated-plane spacing and tilt. Recently, poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophenes) (pBTTT) have been reported to exhibit hole mobility comparable to that of many small molecule semiconductors [9] and rivaling what is achievable in amorphous silicon. The structure of this material is indicated schematically in the insets of Figures 1 and 3a. The high performance of the pBTTTs was attributed to greater structural order than typical polymer semiconductors, and XRD indicated large and well-oriented crystals.[9] Here we exploit this crystallinity to study structural detail within ca. 25 nm thick films of a pBTTT with tetradecyl side chains (pBTTT-C 14 ). From first-principles energy minimization using density f...

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We discuss the energetics and structure of a laterally contracted Ga bilayer model for the Ga-rich pseudo-1ϫ1 phase of the GaN͑0001͒ surface. First-principles total energy calculations reveal that a laterally contracted overlayer of Ga atoms bonded to a 1ϫ1 Ga adlayer is energetically favorable in the Ga-rich limit. The calculations also show that the energy of this surface structure is very insensitive to the lateral position of the contracted layer with respect to the underlying Ga layer. The flatness of the energy surface suggests the presence of rapidly moving domain boundaries separating regions of the surface having different registries. Such motion may lead to the 1ϫ1 corrugation pattern seen in scanning tunneling microscopy images.

Reconstructions of the GaN͑0001 ¢ surface are studied for the first time. Using scanning tunneling microscopy and reflection high-energy electron diffraction, four primary structures are observed: 1 3 1, 3 3 3, 6 3 6, and c͑6 3 12͒. On the basis of first-principles calculations, the 1 3 1 structure is shown to consist of a Ga monolayer bonded to a N-terminated GaN bilayer. From a combination of experiment and theory, it is argued that the 3 3 3 structure is an adatom-on-adlayer structure with one additional Ga atom per 3 3 3 unit cell. [S0031-9007(97)04507-9]

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