Field-Induced Electron Spin Resonance of Site-Selective Carrier Accumulation in Field-Effect Transistors Composed of Organic Semiconductor Solid Solutions

The formation and characterization of solid solutions with guest (or impurity) molecules are difficult to realize in crystalline organic semiconductors. Here, we demonstrate that an operando field-induced electron spin resonance (FIESR) experiment allows the site-selective analysis of accumulated charge carriers in polycrystalline organic field-effect transistors (FETs), which comprise molecular solid solutions as channel semiconductor layers. We utilize vacuum-deposited dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT) thin films with various amounts of dibenzotetrathiafulvalene (DBTTF) as guest molecules. Our measurements reveal that the field-induced carriers are first trapped at guest DBTTF sites at low gate voltages and then begin to accumulate at host DNTT sites at higher gate voltages. Furthermore, we perform dispersion-corrected density-functional theory calculations to investigate the stability of the solid-solution crystals. It is shown that the DBTTF molecules can be stably incorporated by aligning molecular long axes within the crystal lattices of DNTT, the results of which agree well with the angle-dependent FIESR measurements. We demonstrate that the mobility, threshold voltage, and subthreshold swing are controlled by the concentration of guest molecules in solid-solution organic FETs.

Figure 1

(a) Device structure of DNTT-DBTTF field-effect transistors (FETs). Static magnetic field is applied at angle θ with respect to substrate plane. (b) Molecular structures and HOMO-LUMO levels of DNTT and DBTTF molecules. Coordinates indicate principal axes of g tensor. Energies are calculated using gaussian 16, based on DFT, with B3LYP functional and 6-31G(d) basis set.

Figure 2

(a) Field-induced ESR spectra of DNTT(99.8%)-DBTTF(0.2%) FET. (b) Density of spins accumulated at DNTT and DBTTF molecules separately. Right axis indicates corresponding spin concentration relative to number of DNTT molecules in first layer.

Figure 3

(a) Angle dependence of g factor for DNTT and DBTTF components in DNTT-DBTTF FET. (b) Angle-dependent ESR spectra of DNTT component in DNTT-DBTTF FET at V_G = −200 V at 100 K. (c) Angle-dependent ESR spectra of DBTTF component in DNTT-DBTTF FET at V_G = −60 V at room temperature. (d) Angle-dependent ESR spectra of pristine DBTTF FET at V_G = −200 V at 5 K. Insets show schematic illustrations of molecular orientations on substrates of DNTT-DBTTF and pristine DBTTF FETs. Dashed lines in (b)–(d) provide visual guide. (e) Temperature dependence of linewidth for DNTT and DBTTF components in DNTT-DBTTF FET.

Figure 4

(a) Optimized model of solid-solution crystal, which is obtained by replacing central DNTT molecule with guest DBTTF molecule in herringbone packing of DNTT. (b) Changes in sum of intermolecular-interaction energies of DBTTF molecule with six surrounding DNTT molecules (E_int) associated with change in herringbone angle. (c) Changes in E_int associated with change in position of central DBTTF molecule. (d) E_int calculated using crystal structures of DNTT and DBTTF and that calculated using optimized geometry of solid-solution crystal model.

Figure 5

(a) Field-induced ESR spectra of DNTT-DBTTF FETs with different DBTTF concentrations. Gate voltage is fixed at −200 V, and temperature is room temperature. Concentration of DBTTF is determined via ESR measurement. (b) Transfer characteristics of devices at room temperature. Channel length L = 0.5 mm, channel width W = 20 mm, and drain voltage V_D = −10 V. (c) Field-effect mobility, (d) threshold and turn-on voltages, and (e) subthreshold swing of devices. Dashed lines provide visual guide. Solid lines show calculated slope obtained based on simple assumption of charge accumulation in first monolayer.