Approaching isotropic transfer integrals in crystalline organic semiconductors

Dynamic disorders, which possess a finite charge delocalization, play a critical role in the charge transport properties of high-mobility molecular organic semiconductors. The use of two-dimensional (2D) charge transport in crystalline organic semiconductors can effectively facilitate reducing the sensitivity of charge carriers to thermal energetic disorders existing in even single crystals to enhance the carrier mobility. An isotropic transfer integral among adjacent molecules enables a dimensional transition from quasi-one-dimensional to 2D for charge transport among molecules. Herein, a tuned molecular packing, especially molecular rotation, was achieved in highly crystalline organic thin films via a brush-coating method. This tuned molecular packing was favorable for approaching isotropic transfer integrals. Consequently, high-performance organic transistors with a carrier mobility up to $21.5\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ and low angle dependence were obtained. This work presents a unique modulation of molecular packing at the molecular scale to enable less sensitivity of the charge transport to dynamic disorders, providing an alternative route for enhancing the electrical performance of organic electronic devices.

Figure 1

Crystal property characterizations and molecular packing of the obtained ${\mathrm{C}}_8-\mathrm{BTBT}$ films. (a) The illustration of the coating process. The inset is the comsul simulation of the microbar-coating method that shows the flow behavior of the solution around the hairs. The color is coded to indicate the scale of velocity. (b) Optical microscope (OM) image of a microbar-coated thin film. (c,d) Cross-polarized OM images of the strained thin films. (e) AFM image of the ${\mathrm{C}}_8-\mathrm{BTBT}$ films. (f) HRTEM image of the ${\mathrm{C}}_8-\mathrm{BTBT}$ films and (g) its corresponding SAED patterns and (h) filtered IFFT image of the selected area in (f).

Figure 2

Crystal property characterizations and molecular packing of the obtained ${\mathrm{C}}_8-\mathrm{BTBT}$ films. (a) XRD and (b) XRR patterns characterized along the parallel and perpendicular direction of deposition in microbar-coated films. (c) HRAFM image of the microbar-coated sample. The inset presents the oriented molecular structure according to (d) the FFT image. (e) The GIWAXS patterns of the obtained ${\mathrm{C}}_8-\mathrm{BTBT}$ films. (f) Comparison of a single ${\mathrm{C}}_8-\mathrm{BTBT}$ molecule in obtained films (green) and single crystals (red).

Figure 3

GIWAXS images of ${\mathrm{C}}_8-\mathrm{BTBT}$ films for varying writing speeds. The (hkl) indices of ($11l$), ($02l$), and ($12l$) are shown on the top. The dashed lines represent the positions of scattering patterns of ${\mathrm{C}}_8-\mathrm{BTBT}$ single crystals. The shift of red arrows on the ($01l$) plane corresponding to the dashed line suggest the unit-cell distortion of ${\mathrm{C}}_8-\mathrm{BTBT}$ molecular packing. The number of scattering patterns in the ($01l$) plane and the corresponding intensity also indicate the packing property along with the increasing speeds. Small shifts of ($02l$) and ($12l$) are observed in the sample deposited at 3 mm/s.

Figure 4

Charge transport properties from DFT calculations. (a) Top view projection along the $c$ axis showing dimers $\mathit{AA}$ and $\mathit{AB}$ in the crystal structure of ${\mathrm{C}}_8-\mathrm{BTBT}$. (b) Schematic illustration depicting the largest four charge transport routes between ${\mathrm{C}}_8-\mathrm{BTBT}$ molecules. (c) Charge transport route analysis of both dimers $\mathit{AA}$ and $\mathit{AB}$ in the nonequilibrium films and single crystals. (d) Calculated $J$ in the $a$ axis and $ab$ plane.

Figure 5

Structural defects and disorders in the nonequilibrium ${\mathrm{C}}_8-\mathrm{BTBT}$ films. (a) HRAFM image of microbar-coated ${\mathrm{C}}_8-\mathrm{BTBT}$ films. Some mismatches and defects can be observed (white circles). (b) Filtered IFFT image of the selected area in Fig. 1, showing the intermolecular periodicity within the columns. Some fuzzy distorted areas are easily observed, indicating the existence of defects and lattice mismatch in the ${\mathrm{C}}_8-\mathrm{BTBT}$ films. (c) Schematic of charge transfer showing the effective charge transport formed by virtue of the in-plane 2D pathways when the hole carriers encounter structural defects or thermal disorders in the obtained ${\mathrm{C}}_8-\mathrm{BTBT}$ films.

Figure 6

Electrical characterizations of OFETs based on distorted ${\mathrm{C}}_8-\mathrm{BTBT}$ films. (a) Transfer characteristics measured in the saturation regime ($V_{\mathrm{D}}=-20\mathrm{V}$). The channel width and length are 80 and 40 µm, respectively. (b) Gate-voltage dependence of the carrier mobility in the ${\mathrm{C}}_8-\mathrm{BTBT}$ transistors. The inset shows a schematic illustration of a bottom-gate top-contact transistor. (c) $I_{\mathrm{D}}$ and $I_D^{1/2}$ measured in the saturation regime ($V_{\mathrm{D}}=-20\mathrm{V}$) under various temperatures. (d) Temperature dependence of ${\mu }_{\mathrm{FET}}$ calculated from the transfer curves.

Figure 7

(a) Carrier mobility of OFETs based on distorted ${\mathrm{C}}_8-\mathrm{BTBT}$ films as a function of angle between channel and the deposition direction. (b) Distortion-induced molecular rotation. ${\mathrm{C}}_8-\mathrm{BTBT}$ molecular packing in dimers $\mathit{AA}$ and $\mathit{AB}$ in single crystals and unique polymorphic films, showing the relative molecular rotation in dimers.