Study On The Dark Current Performance And Application Of Non-fullerene-based Organic Photodetectors

Photodetectors or photodiodes,which can convert optical signals into electrical signals,are widely used for optical signal detection and measurement.As an emerging candidate for the next generation photodetection,organic photodetectors（OPDs）possess many unique advantages in terms of lightweight,low-cost,solution-processable,compatible with flexible substrates,easy to prepare in large areas,and wide spectral response range.Furthermore,their application scenarios are well complementary to those traditional photodetectors based on inorganic semiconductors,thus have attracted widespread attention from industry and academia in recent years.Research on the performance of organic photodetectors have made tremendous progress over the last decade.Benefiting from the rapid development of organic optoelectronic materials,studies on organic photodetectors have heightened the research activities regarding on improving their sensitivity and broadening the spectral response range through molecular design and device structure design.In addition,research on OPDs is still in early laboratory stage and there is a long way to go to reach a mature level for mass production.At the same time,in order to further develop high performance OPDs,enhanced understanding of the origin of noise in OPDs is in urgent need,while effective methods for noise suppression need to be explored.Apart from these issues,their practical application scenarios of OPDs are still unclear,which deserve more investigation to verify.In response to the above scientific questions and the issues that limit their appliacations,the first section of this thesis focuses on the fabrication of self-filtering narrowband organic photodetector and its application for visible-infrared ray optical communication system via a homemade setup.It was found that shot noise is still inevitable in the system.Subsequently,the latter two research efforts focused on the origin and suppression methods of the dark current,which contributes to the shot noise.In the framework of topological defect,the effect of trap density and its distribution on the current–volatge characteristics of the devices is obtained to provide a theoretical basis for our understanding of the impact of traps on charge transport.In the context of dark current characterisation,the trap density of states is suppressed to realize the low noise-high responsitivity OPDs.In Chapter 2 we propose a method to fabricate self-filtering narrowband photodetector and design an optical communication system to verify its selective-detection function.As the spatial distribution of photogenerated carriers in organic photodetectors is modulated by the thickness and optical properties of the film,a self-filtering organic photodetector can be achieved based on this concept,with a peak responsivity of 0.24 A/W and full width half maximum（FWHM）of 47 nm.When integrating the narrowband photodetector as a light-to-current conversion element in the receiver module of the system,the narrowband response function has been verified by changing the illumination wavelength of the optical transmitter.Our demonstration provides a successful platform for the use of organic photodetectors in optical communication applications.Chapter 3 presents a comprehensive study of“charge injection”in OPDs by using a low bandgap small molecule ITIC and its fluorinated derivatives with varying energy levels as electron acceptors,considering influence of various device and material physical parameters on the effective barrier for injection.As compared to the devices based on fluorinated acceptors,the devices from non-fluorinated acceptors exhibit similar external quantum efficiency but lower dark current density,hence a higher specific detectivity（D*）.By combining the analysis of effective injection barrier,trap states,activation energy,it is found that the dark current is closed related to the charge injection process from the electrode to the organic semiconductor layer via sub-bandgap traps.Based on the above understanding combined with experimental results,a modified model that gives a correct description of the effective injection barrier under reverse bias is proposed to explain the lower dark current density in the ITIC-based OPD,that is,fluorinated end groups affect the dark current by affecting the effective charge injection barrier,on which the dependence of dark current is consistent with a quantitative description based on Shockley equationChapter 4 shows that leakage current in organic photodetectors can be effectively reduced to an intrinsic lower limit by using four composite hole blocking layers（HBLs）.The best device shows an ultralow dark current density down to 0.18 n A cm^-2,which translates to high specific detectivity（D^*）over 1×10¹³ Jones in broad response range from 340 to 1100 nm.The origin of reduced dark current density is qualitatively correlated to the reduced traps density according to our calculation model.The intrinsic current of the device（including intrinsic recombination current due to thermal radiation and non-radiative recombination current due to the presence of defects and non-radiative transiton processes）was related to the radiative transition under electrically injection（electroluminescence）,pointing to the intrinsic dark current-limited detectivity of OPDs.