Hysteresis loops on voltage-current characteristics and optical responses of PEDOT:PSS/ZnO nanorods/ZnO:Ga heterostructure

Volage (V)-current (I) curves of the poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)/ZnO nanorods (NRs)/ZnO:Ga (GZO) heterostructure exhibited a rectification behavior with hysteresis loops both in forward voltage (V_F) and reverse voltage (V_R) regions. The ultraviolet (UV) light irradiation of 360 nm led to the increase in the reverse current (I_R), i.e. generation of photocurrent (PC), and the decrease in the hysteresis loop area in the V_F region. In dark, the 1/V vs. ln (I/V²) plot revealed that the dominant carrier transport mechanisms in the low- and the high-V_F regions are the direct tunneling and the Fowler-Nordheim (F-N) tunneling through the dipole layer formed at the interface between the PEDOT:PSS and ZnO NRs layers, respectively. On the contrary, the carrier transport in the middle-V_F region in dark was dominated by the bulk-limited mechanisms, such as the space-charge-limited (SCL) conduction controlled by the single shallow traps, the trap-free SCL conduction, and the trap-filled-limit conduction controlled by the traps with Gaussian distribution. The UV light irradiation changed the carrier transport mechanism in the middle-V_F region to the trap-free SCL conduction. The resistive switching related to the hysteresis loop was found to be caused by the trapping and detrapping of the injected carriers at the traps. In dark, the maximum forward current was increased by the repetition of the V_F sweep of 0 V → 5 V → 0 V, but decreased by the repetition of the V_R sweep of 0 V → -5V → 0 V, indicating the possibility of the resistive memory. At the low V_Rs, PC spectra showed a main peak at 350 nm, which is corresponding to slightly higher photon energy than the bandgap energy of ZnO. Moreover, the existence of the tail extending into the bandgap was also observed on the PC spectra. From the time response of PC, the depth of the trap state was estimated to be 0.64–0.77 eV.