Uv Detective Materials And Heterojunction Devices Based On Organic Semiconductors With Wide Bandgap

Ultraviolet (UV) detection technology has lots of applications in both civil and military fields. Currently, commercial semiconductor UV photodetectors are mainly based on inorganic materials. However, their processing is rather complicated and the cost is high as well, which is not beneficial for large-scale application. By contrast, organic semiconductors have advantages of low cost, excellent processibility, mechanical flexibility and large-area scalability. Besides, the bandgap of organic semiconductors can be easily tuned by structure design to realize UV selective absorption, making them promising materials for UV detection. On the other hand, though organic semiconductors have made a great progress in the past two decades, the relationship between structure and performance under UV radiation is a problem remained to be solved in the frontier of material science, which is of great academic significance. This thesis reviews the progress in organic UV detective materials and devices. It's found that new materials in this field are still limited and research on physical phenomena or mechanism is also rare. In view of such status, the following research work has been carried out in this thesis.ZnO nanoparticles were fabricated by wet chemical method. Then hybrid UV photodetectors with bulk heterojunction (BHJ) structure were demonstrated by spin-coating employing blends of poly (9,9-dihexylfluorene) (PFH) and ZnO nanoparticles as the active layers. The effect of ZnO content on the device performance was investigated. Under UV radiation of 1 mW/cm2 at 365 nm. the device with 33 wt% ZnO exhibits a photovoltaic efficiency of 1.77%. At -1 V, a responsivity of 94 mA/W and a photo-to-dark current ratio of 3 orders of magnitude can be obtained. Besides, by using PFH/ZnO bilayer films as active layers, both normal and inverted planar heterojunction (PHJ) devices were also fabricated via spin-coating, and their photo-to-dark current ratios under the same radiation reach 3 orders of magnitude as well.Phenomenon of persistent photoconduction (PPC) was found in the above-mentioned hybrid devices after UV excitation. In view of this, the effect of atmosphere and bias on the transient photoresponse behavior of the devices was systematically investigated. It's found that the presence of defects in ZnO is the physical cause of PPC and by aging devices in air or applying normal PHJ device configuration, rapid response can be achieved.Two wide-bandgap molecules (CFC and FCF) based on carbazole and triarylamine substituted fluorene were designed and synthesized. The photophysical property, thermal property, electrochemical property and solution processibility of the molecules were investigated. The results indicate that both molecules exhibit absorption below 400 nm and also possess excellent thermal stability and solution processibility. ZnO nanoparticles capped with dodecylamine were then fabricated and dispersed in the solution of the synthesized molecules. By spin-coating, hybrid films were obtained and applied as active layers for UV photodetectors with BHJ structure. Under UV radiation of 1 mW/cm2 at 365 nm, a photo-to-dark current ratio of 3 orders of magnitude can be obtained and rapid response can be realized as well. The spectral response for devices based on CFC:ZnO and FCF:ZnO hybrid films covers 300～402 nm and 300～385 nm, respectively. So those devices may be applied as visible-blind near-ultraviolet (NUV) photodetectors.By applying naphthalene diimide (NDI-BA) and PFH as acceptor and donor, respectively, organic UV photodetectors with BHJ structure were fabricated by spin-coating. The effect of annealing temperature and blend ratio on the film morphology and device performance was investigated. Under UV radiation of 1 mW/cm2 at 365 nm, the optimized device exhibits a photo-to-dark current ratio of more than 2 orders of magnitude and a responsivity of 224 mA/W can be obtained under -4 V. PHJ devices employing PFH/NDI-BA bilayer films as active layers were also demonstrated, and the photovoltaic efficiency under 1 mW/cm2 365 nm UV radiation reached 3.3%.Wide-bandgap organic acceptor (NSN) with strong deep-ultraviolet (DUV) absorption was synthesized. Then four materials (PFH, m-MTDATA, PFP and PVK) with different bandgaps were applied as donors and respectively combined with NSN to construct UV photodetectors with PHJ structure. The study indicates that by using different donors, the response cutoff can be tuned between 370 and 425 nm. If semitransparent Al electrodes are applied for the incidence of light, the response region can be extended to DUV region. Organic filters based on dyes were further fabricated and used for blocking NUV in the incident light. With those filters. DUV selective detection was finally realized in PFH/NSN and PVK/NSN heterojunction devices.In summary, these results expand the research scope of organic semiconductors, enrich their theoretical contents, and provide important basis for deepening both the fundamental and practical research of organic UV detective materials and devices.