Recent technological advancement has seen the development and utilization of photodetectors in different applications including biological detection and image sensing. Organic photodetectors (OPDs), in particular, have attracted significant attention owing to the numerous desirable properties and technological advantages of organic semiconductors, i.e., adjustable energy levels, mechanical flexibility, large-area fabrication, solution process, and low cost. The photodiode type OPDs obeying photovoltaic effect exhibit less than 100% external quantum efficiency (EQE), thus researchers have been looking for better alternatives and have identified photomultiplication (PM) type OPDs as a promising direction. PM phenomenon is desirable for sensitive photodetectors with strong detection capability of weak light, which can further simplify the photo-detection systems without pre-amplifier circuit. The working mechanism of PM type OPDs with EQE above 100% is charge tunneling injection from external circuit assisted by interfacial traps. Although PM type OPDs can possess high performance, these photodetectors can exclusively operate under single spectral response model and consequently exhibit single response spectrum, limiting their broader applications.
Recently, researchers Jianli Miao and Professor Fujun Zhang et al. from Beijing Jiaotong University developed a multifunctional photomultiplication type OPDs with broadband and narrowband response ability. Their main objective was to realize tunable spectral response that mainly depend on the bias polarities as well as to determine the spectral response range and external quantum efficiency values under reverse and forward bias conditions. This novel PM type OPDs were designed with with the structure of ITO/PFN-OX/P3HT:PTB7-Th:PC61BM (100-x:x:1, wt/wt/wt)/Al (Figure 1a), which can work well at forward and reverse bias and exhibits tunable spectral response range. The optimal OPDs with 3 um P3HT:PTB7-Th:PC61BM (40:60:1) as active layer exhibit broadband spectral response range of 350-800 nm under forward bias (Figure 1b) and narrowband spectral response range of 750-850 nm with a full width half maximum (FWHM) of approximately 40 nm under reverse bias (Figure 1c). Interesting, the optimal OPDs exhibit PM phenomenon under bi-directional bias, resulting from hole tunneling injection from external circuit assisted by interfacial traps. In addition, these PM types OPDs exhibit rather low dark current, achieving the acceptable level for a high-speed receiver design. The study is the first to successfully develop an efficient method to fabricate multifunctional PM type OPDs with broadband and narrowband response ability. It enables alteration of the bias polarity to desirable spectral response range.
Broadband photodetectors are predominantly applied in multicolor light detection and imaging. While narrowband photodetectors with the ability of spectral discrimination are predominantly applied in specific wavelength light detection, such as biological detection, intelligent monitoring and security systems. From the reliable experimental results obtained, the multifunctional OPDs integrating broadband and narrowband detection ability and exhibiting PM phenomenon would possess wide potentials application, especially in integrated optoelectronic systems.
Miao, J., Zhang, F., Du, M., Wang, W., & Fang, Y. (2018). Photomultiplication Type Organic Photodetectors with Broadband and Narrowband Response Ability. Advanced Optical Materials, 6(8), 1800001.Go To Advanced Optical Materials