Effect of an Electric Field on the Growth and Optoelectronic Properties of Quasi-One-Dimensional Organic Single Crystals of 1-(Phenylazo)-2-naphthol

Organic single-crystal thin films (2−10 μm thick) of 1-(phenylazo)-2-naphthol (Sudan I) were grown by capillary filling from its molten state (∼133 °C) into cells constructed of two parallel pieces of indium tin oxide (ITO) coated glass followed by slow cooling to room temperature. The solidified crystals were needle-shaped, a few tens of micrometers in diameter, and were aligned parallel to both ITO surfaces. Needle-shaped crystals of Sudan I were also grown from solution. X-ray diffraction analysis revealed a monoclinic crystal with space group, P21/c (No. 14), having a = 5.8225(15), b = 17.377(5), c = 24.598(5) Å, β = 92.37(2)°, V = 2486.7(11) Å3, Z = 8, ρ = 1.33 g cm-3. The planar molecules are stacked parallel to one another to form molecular columns along the needle axis. The pattern of crystal growth can be changed by the application of an electric field between the two ITO electrodes during solidification to produce crystal needles tilted at an angle relative to the ITO surface. The electrical conductivity parallel to the crystal needle axis is over 100 times higher than that perpendicular to it due to the better π−π overlap among the molecules along the needle. The electric-field-induced reorientation of the quasi-one-dimensional molecular crystals increased the conductivity of the ITO/Sudan I/ITO cells by 8−9 times. Moreover, the short-circuit photocurrent generated from these cells was enhanced by 14 times, indicating that the interfacial separation power of photon-produced electron/hole pairs was also improved substantially. This is the first example of optimization of the optoelectronic properties of molecular crystal-based devices through the manipulation of crystal orientation by an electric field.