  Organic photodetectors (OPDs) possess bright prospects in applications of
medical imaging and wearable electronics due to the advantages such as low
cost, good biocompatibility, and good flexibility. Photomultiplication OPDs
(PM-OPDs) enabled by the trap-assisted carrier tunneling injection effect
exhibit external quantum efficiencies far greater than unity, thus the acquired
responsivities are extremely high. However, the reported PM-OPDs with high
responsivity performances are all accompanied by high dark currents due to the
introduction of carrier traps, which inevitably results in inferior
detectivities. In this work, we modify a P3HT:PCBM donor-rich PM-OPD by
introducing an atomically thin Al2O3 interfacial layer through the ALD
technique, obtaining a high responsivity of 8294 A/W and high detectivity of
6.76*10^14 Jones, simultaneously, both of which are among the highest reported
for bulk heterojunction PM-OPDs. Ascribed to the introduction of the atomically
thin Al2O3 layer, the metal-insulator-semiconductor (MIS) tunneling junction is
formed, which brings forward a suppressed dark current along with an increased
amounts of holes tunneling under forward bias. Meanwhile, the weak light
detection limit of the modified PM-OPD within the linear response range reaches
the level of nW/cm2. Based on the proposed PM-OPD, a proof-of-concept image
sensor with 26*26 pixels is demonstrated, which can respond to both ultraviolet
light and visible light. The PM-OPD based sensor arrays can find broad
applications for medical imaging, wearable electronics, etc.
