The traditional photodetection technologies are limited by the single-mode spectral photodetection capability. A bias-switchable spectral response organic photodetector (OPD) offers an attractive option for applications in environmental pollution detection, bioimaging process, wellness, and security monitoring in two distinct bands.

Recently, the Department of Electrical and Electronic Engineering Associate Professor Dan Luo at Southern University of Science and Technology (SUSTech) and the Department of Physics Professor Furong Zhu at Hong Kong Baptist University (HKBU), have published their research progress on dual-mode photodetection in Science Advances (IF: 12.804), titled “Near-infrared and visible light dual-mode organic photodetectors”.

The thin-film OPDs are a promising alternative optical detecting technology to conventional inorganic counterparts. They offer additional advantages such as having a solution-processable fabrication process, which also leads to substantial cost-benefits, thereby creating next-generation solution-processable, flexible, and low-cost photodetectors. Realizing spectral tunable OPDs with high detectivity in two distinct bands, particularly over the wavelength range from NIR and visible light, is very attractive for a plethora of applications in environmental pollution detection, bioimaging process, wellness, security monitoring.

The group has proposed a dual-mode OPD, comprising a tri-layer organic stack of visible light absorber/optical spacer/NIR light absorber. The tri-layer PM OPD gives rise to the remarkable dual-mode light detection phenomena, exhibiting a NIR response operated at reverse bias and a visible response at forward bias. The dual-mode OPD comprises of a solution-processed 300 nm thick binary blend P3HT:PC70BM (100:1) visible light-absorbing layer, a 320 nm thick pristine P3HT optical spacer layer and a 500 nm thick ternary blend P3HT:PTB7-Th:PC70BM (70:30:1) NIR light-absorbing layer. The bias-switchable NIR and visible light responses in the dual-mode OPD are closely associated with two factors: (1) different distributions of the photo-generated electrons in the presence of the NIR and visible light, and (2) the bias polarity dependent charge injection behaviors. In the presence of NIR light, the tunneling hole injection occurs at the NIR light absorber/cathode interface in the OPD operated at reverse bias, due to the band bending that is induced by the high density of NIR light-generated electrons. In the presence of visible light, the tunneling hole injection takes place at the anode/visible light absorber interface in the OPD at forward bias, due to the band bending that is caused by the high density of visible light-generated electrons. A high responsivity of >10 A/W was obtained for the dual-mode OPD at two distinct short and long wavelengths. The dual-mode OPD developed in this work has a simultaneous high specific detectivity (D*) of ~10^13 Jones in both NIR and visible light bands.

Figure: The dual-mode OPD analysis of (A) photoresponsivity spectrum under reverse biases, (B) photoresponsivity spectrum under forward biases, (C) profile of charge generation in the OPD devices.

The SUSTech-HKBU joint supervised postgraduate student Zhaojue Lan is the first author of the article. Zhaojue Lan obtained a bachelor’s degree from the Department of Electrical and Electronic Engineering, SUSTech, in 2016, and then carried on the joint-PhD studies. Late last year, he received the Journal of Materials Chemistry A Poster Prize from the 11^th Asian Conference on Organic Electronics (ACOE-2019) for the poster presentation titled “Near-infrared and visible light dual-mode organic photodetectors”.

Zhaojue Lan received poster prize in ACOE-2019

Article link: https://advances.sciencemag.org/content/6/5/eaaw8065