ESR studies of ambipolar charge carriers in metal–insulator–semiconductor diodes of regioregular poly(3-hexylthiophene)/PCBM composites

We have applied field-induced electron spin resonance (FI-ESR) method to organic metal–insulator–semiconductor (MIS) diodes using regioregular poly(3-hexylthiophene) (RR-P3HT) and 6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) composites as the active layer, Al₂O₃ as the gate insulator and gold (Au) or aluminum (Al) as the top electrode. In the case of Al electrode, clear FI-ESR signals are observed with g ～ 2.002 and 1.999 for negative and positive gate voltage (V_G), respectively, demonstrating that these signals with former and the latter g-values correspond to field-induced positive polarons of RR-P3HT and PCBM radical anions, respectively. Both RR-P3HT and PCBM signals monotonically increase as |V_G| increases. The threshold voltages (V_th) for field-induced ambipolar charge carriers coincide well with those of capacitance measurements with hysteresis behaviors. On the other hand, in the case of Au electrode, only positive polarons signals are clearly observed and PCBM signals are nearly undetected. These different behaviors of FI-ESR signals between Al and Au top electrodes are discussed in relation to the difference of work functions.