Effects Of Molecular Fluorination And Device Structure On Performance Of Polarization-sensitive Self-assembly Polymer Photodetectors

Polarization detection technology plays an important role in military,environmental monitoring,biomedicine and other fields.Among them,intrinsic polarization-sensitive polymer photodetectors（PDs）have attracted great attention of researchers because it can meet the requirements of miniaturization and integration.Such devices mainly depend on their anisotropic active layers to achieve the polarization detection;however,it is not easy to obtain anisotropic film by aligning the polymer molecules.Compared with the traditional method of orienting polymer film by external mechanical force,the solution-based molecular self-assembly method has the advantages of simple operation,low cost and high success rate,and thus has a good application prospect.Based on the molecular self-assembly preparation method,this work studies the self-assembly anisotropic heterojunction with the fluorinated acceptor molecular structure and the polarization-sensitive PDs with different device structures.In the process of preparing self-assembly bulk heterojunction（BHJ）films,it was found that the introduction of an appropriate amount of fluorine atoms into the molecular structure of polymer acceptors can effectively regulate the molecules interaction,promote the orderly alignment of polymer molecules,and achieve a more uniform distribution of donor-acceptor molecules.Accordingly,the optimal anisotropic polymer blend film obtained a dichroic ratio of more than7.4,and the polarization-sensitive PD based on it presented a high photocurrent dichroic ratio of3.73 under the 660 nm polarized light.Meanwhile,the device also obtained a high responsivity of0.06 A/W and a specific detectivity of 7.8×10¹⁰ Jones（measured under-0.5 V bias）,which were comparable to those of inorganic polarization-sensitive photodetectors.Further measurements showed that the device had a 120.1 d B wide linear dynamic range,a 3.67μs fast response time and a high contrast polarization imaging capability,which can meet the complex application requirements.To overcome the problems of insufficient light absorption in the ultrathin single-layer self-assembly film,we successfully prepared the planar heterojunction（PHJ）and BHJ（double-layer thin-film stack）polymer polarization-sensitive PDs by using the layer-by-layer self-assembly preparation method.Although the PHJ devices showed a low optical response,they obtained a low dark current compared with the BHJ devices at reverse bias voltage.Furthermore,the relationship between the number of laminated BHJ layers and device performance was studied.It was found that although the polarization sensitivity of the BHJ device with 5 self-assembly BHJ layers was reduced,but its responsivity is more than doubled at the 650 nm and increased by dozens of times at 370 nm,compared with the single-layer self-assembly BHJ device.Thanks to the dark current suppression by increasing the film thickness,the device with 5 self-assembly BHJ layers finally achieved a high detectivity of 1.02×10¹² Jones（measured under-0.5 V bias）,which was much higher than that of the device with a single self-assembly BHJ layer with double hole block layers(1.24×10¹¹ Jones).This work combined the fluorination strategy of polymer molecules and the advantages of layer-by-layer preparation of self-assembly films,which effectively improved detection performance of polarization-sensitive polymer PDs and provided a feasible technology for achieving high-performance polarization-sensitive polymer PDs.