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

Abstract

Charge carrier concentration in operating organic field-effect transistors (OFETs) reflects the electric potential within the channel, acting as a key quantity to clarify the operation mechanism of the device. Here, we demonstrate a direct determination of charge carrier concentration in the operating devices of pentacene and poly(3-hexylthiophene) (P3HT) by field-induced electron spin resonance (FI-ESR) spectroscopy. This method sensitively detects polarons induced by applying gate voltage, giving a clear FI-ESR signal around g = 2.003 in both devices. Upon applying drain-source voltage, carrier concentration decreases monotonically in the FET linear region, reaching about 70% of the initial value at the pinch-off point, and stayed constant in the saturation region. The observed results are reproduced well from the theoretical potential profile based on the gradual channel model. In particular, the carrier concentration at the pinch-off point is calculated to be β/(β + 1) of the initial value, where β is the power exponent in the gate voltage (V gs ) dependence of the mobility (μ), expressed as μ ∝ V β−2 gs , providing detailed information of charge transport. The present devices show β = 2.6 for the pentacene and β = 2.3 for the P3HT cases, consistent with those determined by transfer characteristics. The gate voltage dependence of the mobility, originating from the charge trapping at the device interface, is confirmed microscopically by the motional narrowing of the FI-ESR spectra.

Low-temperature carrier dynamics in high-mobility organic transistors of alkylated dinaphtho-thienothiophene as investigated by electron spin resonance

“…More details of the FI-ESR measurements were presented elsewhere. 22,27,29 The devices exhibited a standard operation at room temperature; the average mobility in the saturation regime was 2.2 cm 2 /V s with the maximum value of 3.5 cm 2 /Vs. Figure 2(a) shows the temperature dependence of transfer characteristics obtained with a drain voltage (V d ) of À1 V. With decreasing temperature, the output current becomes smaller and the onset voltage (V on ) shifts to larger negative values, which may be ascribed to increasing the effect of traps.…”

“…[20][21][22][23][24][25][26][27][28][29][30] In particular, ESR linewidth provides dynamical information of carriers through the motional narrowing effect. 20,27,31 In the case of C 8 -BTBT thin film transistors (TFTs), for example, we have demonstrated the presence of mobile carriers within crystalline grains even at 4 K, where the output current of the device cannot be observed due to the trapping at grain boundaries.…”

Non-contact, non-destructive, quantitative probing of interfacial trap sites for charge carrier transport at semiconductor-insulator boundary

“…[1][2][3][4] However, analytical techniques for evaluating such interfacial carrier transport phenomena are still limited, and this remains a challenging issue. Unique approaches that have been reported include the use of electron spin resonance (ESR) to monitor the field-induced charge carriers in metalinsulator-semiconductor (MIS) devices, [5][6][7][8][9] where detailed carrier dynamics such as the charge carrier concentration, 7 the correlation between mobility and motion narrowing effects, 8 and the distribution of trapped carriers 9 have been discussed. We have recently reported a technique, referred to as field-induced time-resolved microwave conductivity (FI-TRMC at interfaces), which combines field-induced charge carrier generation and microwave-based probing of charge carrier motion.…”

Microscopic Signature of Metallic State in Semicrystalline Conjugated Polymers Doped with Fluoroalkylsilane Molecules

“…In order to elucidate the electronic states of doped organic semiconductors, ESR spectroscopy is one of the most sensitive microscopic tools for both π ‐conjugated polymers and small molecules . Indeed, ESR measurements of electric‐field‐induced carriers can sensitively distinguish spin‐1/2 polarons and singlet bipolarons (or polaron pairs) .…”