1,1-Bis[(di-4-tolylamino)phenyl]cyclohexane for fast response organic photodetectors with high external efficiency and low leakage current

Fast response organic photodetectors were developed based on a high mobility hole-transporting organic semiconductor 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC). The active layer was constructed by mixing TAPC with a high electron mobility C70 and sandwiched between TAPC and C70. This kind of planar-mixed heterojunction structure significantly enhanced the external quantum efficiency (EQE) and greatly suppressed the leakage current. When the TAPC concentration was 45% and the reverse bias voltage was -6 V, a high performance organic photodetector with a bandwidth exceeding 30 MHz was realized. Simultaneously, the device also showed a peak EQE value approaching 60%. When the TAPC concentration was 30% and the reverse bias voltage was -3 V, the dark current density of the photodetector was as low as 3 pA mm(-2). Hence, a high detectivity organic photodetector with a maximum value of up to 2.5 x 10(13) Jones was achieved with a corresponding EQE of 56%. The device also exhibited a constant responsivity over 6 orders. Given that the bandwidth and EQE of photodetectors are dependent on the mixing ratio of TAPC and C70, the performances can be adjusted according to the requirements of the specific applications. Relevant theory models were used to demonstrate the limitation factors in the bandwidth of organic photodetectors. Our study clearly reveals that the utilization of high mobility organic materials and the design of suitable device structures is necessary in constructing high bandwidth organic photodetectors with high external efficiency and low leakage current.