Photovoltaic Mode Ultraviolet Organic Photodetectors with High On/Off Ratio and Fast Response

Ultraviolet (UV) organic photodetectors (OPDs) operated in the photovoltaic mode, achieving a very high on/off ratio of 10 5 and a fast response time of 20 ns (a decay time of 888 ns), are demonstrated in this work. Light‐induced tuning of the barrier height at the two interfaces of the carrier‐extraction layer and light‐induced tuning of series resistance are proposed to obtain the high on/off ratio. Fast response of the device is demonstrated through the ultrafast trap filling process. The high on/off ratio and fast response are simultaneously realized by introducing TiO 2 nanocrystals as the electron‐extraction layer of OPDs. In addition, the good on/off ratio can be maintained in a wide concentration range of 2,2′‐(1,3‐phenylene)bis[5‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazole] (OXD‐7) from 25% to 3.75% in the N,N′ ‐bis(naphthalen‐1‐yl)‐ N,N′ ‐bis(phenyl)benzidine (NPB):OXD‐7 active layer and the corresponding OPDs can also achieve a fast response time simultaneously, good for its practical application. With the optimized device structure, a large detectivity of over 10 12 Jones covering the UV‐A region (320–400 nm) and a high linear dynamic range of 100 dB are obtained. Additionally, stability of TiO 2 ‐based OPDs, which is not well studied but an important parameter for OPDs, is experimentally investigated.