Electron Spin Resonance of Field-Induced Polarons in Regioregular Poly(3-alkylthiophene) Using Metal–Insulator–Semiconductor Diode Structures

Direct observation of the charge carrier concentration in organic field-effect transistors by electron spin resonance

“…Such information, if obtained by a microscopic method, can provide a direct evidence for the microscopic origin of the field-induced charge carriers in operating FETs, which should be associated with the organic molecules at the device interface. Electron spin resonance ͑ESR͒ spectroscopy is a particularly suitable method for this purpose since the direct observations of the field-induced charge carriers in organic devices have been reported for the MIS diodes of RR-P3HT and RR-P3OT [13][14][15] or for the FET devices of pentacene in the absence of drain-source voltages. 16,17 In this letter, we report on the ESR measurements of the concentration of the field-induced charge carriers in the FET devices of RR-P3HT during the device operation.…”

“…The devices were fabricated on n + -Si substrates with SiO 2 gate insulators 15 instead of quartz glass substrates with aluminum gates and Al 2 O 3 gate insulators used in the earlier ESR studies of field-effect devices. 13,14,16 An n + -Si wafer ͑10-20 ⍀ cm, ͗100͘ axis, and 0.5 mm thick͒ was cut out to 3 ϫ 30 mm 2 rectangular substrates. A relatively high value of the surface resistivity of the wafer was chosen so that the deterioration of the quality factor ͑Q-factor͒ of the ESR cavity is minimized.…”

“…Other details of the device fabrication are described elsewhere. [13][14][15] FET characteristics were measured by using Keithley 2400 and 6487 source measure units to apply V gs and V ds , respectively. ESR measurements were performed by using a Bruker E-500 spectrometer equipped with a TE 011 cylindrical cavity.…”

Electron spin resonance observation of charge carrier concentration in organic field-effect transistors during device operation

“…Here, the applied gate voltage is sufficiently small not to induce the spinless states, which has been observed in the high gate voltage region in previous FI-ESR studies due to the formation of spinless bipolarons (or polaron pairs). [26][27][28]37 Thus, N spin agrees with the charge concentration.…”

“…26 So far, this method has been applied to various OFETs of poly(3-alkylthiophene), [26][27][28][29][30] pentacene, [31][32][33] thienothiophene-based high-mobility materials, 34,35 and rubrene single crystals, 36 etc., giving microscopic information such as spin-charge relation, carrier wave function, motional effect of charge carriers, and local molecular orientation at the device interface. In particular, direct determination of spin concentration within the channel during device operation played a crucial role in clarifying the operation mechanism of OFETs, as was demonstrated for P3HT transistors using top-contact geometry, 37 and subsequently, for rubrene single-crystal transistors with bottom-contact geometry; 38 the spin concentration exhibited a clear decrease by applying V ds together with V gs in the linear region, whereas no V ds dependence was observed for the saturation region.…”