High structural quality of crystalline organic semiconductors is the basis of their superior electrical performance. Recent progress in quasi two-dimensional (2D) organic semiconductor films challenges bulk single crystals because both demonstrate competing charge-carrier mobilities. As the thinnest molecular semiconductors, monolayers offer numerous advantages such as unmatched flexibility and light transparency as well  they are an excellent platform for sensing. Oligothiophene-based materials are among the most promising ones for light-emitting applications because of the combination of efficient luminescence and decent charge-carrier mobility. Here, we demonstrate single-crystal monolayers of unprecedented structural order grown from four alkyl-substituted thiophene and thiophene–phenylene oligomers. The monolayer crystals with lateral dimensions up to 3 mm were grown from the solution on substrates with various surface energies and roughness by drop or spin-casting with subsequent slow solvent evaporation. Our data indicate that 2D crystallization resulting in single-crystal monolayers occurs at the receding gas–solution–substrate contact line. The structural properties of the monolayers were studied by grazing-incidence X-ray diffraction/reflectivity, atomic force and differential interference contrast microscopies, and imaging spectroscopic ellipsometry. These highly ordered monolayers demonstrated an excellent performance in organic field-effect transistors approaching the best values reported for the thiophene or thiophene–phenylene oligomers. Our findings pave the way for efficient monolayer organic electronics highlighting the high potential of simple solution-processing techniques for the growth of large-size single-crystal monolayers with excellent structural order and electrical performance competing against bulk single crystals.

In an effort to fabricate In–Ga–Zn oxide (IGZO) thin-film transistors (TFTs) that combine high performance and high stability, we optimize sputtering conditions to create devices based on different IGZO phases: amorphous, c-axis-aligned crystalline (CAAC), and a transition between them, which is introduced here as protocrystalline IGZO. For this, we study the performance of TFTs based on thin films of IGZO sputtered at different substrate temperatures T sub and oxygen flow ratios R O2 . While T sub is the principal phase-determining parameter, R O2 can be further optimized to enhance IGZO TFT characteristics. For both amorphous IGZO and CAAC IGZO, the best TFT performance and the best TFT bias stress stability are found under different sputtering conditions. In contrast, the protocrystalline IGZO shows a convergence of the highest TFT performance and the best bias stress stability, observed for an IGZO film sputtered at T sub = 200 °C and R O2 = 20%.

Polycrystalline indium−gallium−zinc oxide (IGZO) in the spinel phase was obtained by physical vapor deposition (PVD), using reactive sputtering from an IGZO target with In/Ga/ Zn = 1:1:1 composition. The initial growth of spinel IGZO is investigated by X-ray diffraction measurements after annealing the film. Deposition of spinel IGZO initially starts as a mixed amorphous/c-axis-aligned crystalline (CAAC) film, after which a metastable spinel IGZO is formed. Using a template of polycrystalline spinel Ga 2 ZnO 4 , the growth of the spinel phase is immediately achieved and enables the electrical characterization of pure spinel IGZO channels in scaled thin-film field-effect transistors. The average effective channel field-effect mobility of spinel IGZO of 50 ± 10 cm 2 /(V s) is slightly higher than amorphous IGZO in the same devices. This is in line with a slightly lower effective electron mass, as is calculated with density functional theory. The calculated total energies and band gaps have similar values to CAAC-IGZO. This metastable nature identifies spinel IGZO as an intermediate phase before the onset of CAAC-IGZO formation during PVD. Spinel IGZO is an interesting alternative to amorphous IGZO (a-IGZO) and CAAC-IGZO because of potentially higher robustness to oxygen vacancy formation.