High sensitivity and fast response sol-gel ZnO electrode buffer layer based organic photodetectors with large linear dynamic range at low operating voltage

In this work, we demonstrate high sensitive organic photodetectors (OPDs) by incorporating the common sol-gel processed ZnO film between the ITO electrode and the polymer: fullerene active layer. The device requires a low negative working voltage to exhibit fast response and broad linear dynamic range (LDR), and the external quantum efficiency (EQE) of the devices can exceed 100%. The sol-gel ZnO layer can efficiently block the external charge injection from the ITO electrode to the active layer, which dramatically reduces the dark current density. Two kind devices (ITO/ZnO/P3HT: PC61BM/Al and ITO/ZnO/PTB7: PC71BM/Ag) are demonstrated and their dark current densities are effectively reduced from 0.07 mA/cm(2) to 8.16x10(-5) mA/cm(2) and from 0.75 mA/cm(2) to 6.50x10(-5) mA/cm(2) at -0.5 V, respectively. Although the two kind devices show the comparable characteristics, the transient photocurrent and capacitance-voltage (C-V) measurements indicate the two different working mechanisms related. Under illumination, in the P3HT: PC61BM based devices, some surface defect states of ZnO can be occupied and the photogenerated charges accumulate around the interface between the ZnO layer and the active layer, which contribute a tunneling injection under the applied reverse bias. While in the PTB7: PC71BM based devices, the photogenerated charges rapidly transport to the corresponding electrodes and then be collected under reverse bias. The different working mechanisms lead to the various device performances. The OPDs based on P3HT: PC61BM exhibit a higher EQE of more than 200% (460 nm - 600 nm) at -0.5 V, while the PTB7: PC71BM based devices show a more than 1.0x10(15) Jones detectivity (390 nm 730 nm) and achieve the over 110 dB linear dynamic range (LDR) under the different monochromatic illuminations at -0.5 V.