Study On Organic Semiconductors Based Photodetectors

Photodetectors made of solution-processed organic semiconductors have recently emerged as candidates for the next-generation light detecting.They combine ease of processing,integratable,compatibility with flexible substrates and good performance.Although a significant progress has been made in the field of organic photodetectors?OPDs?,their performance regarding the detectivity,color selectivity and energy losses are still inferior to their inorganic counterparts.In this thesis,we explored ways to improve the overall performance of OPDs by synergistic combining the materials and device engineering.Additionally,the characters of photo-responses and charge transport properties were studied,which provided a better understanding of OPDs fabrication,facilitating the progress toward the realization of next generation high-performance and multifunctional optoelectronics.1 OPDs consisting of a polymer active layer of poly?N-vinyl carbazole??PVK?:[6,6]-phenyl-C71-butyric acid methyl ester(PC₇₁BM)were fabricated,and the photodetection properties of OPDs were systematically investigated by optimizing the active layer and structure engineering.Through thickness modulation and thermal annealing,the ultraviolet detectivity of OPD was improved with an optimized value of1.07×10¹² Jones under 350 nm-ultraviolet?UV?light illumination?0.6 mW/cm²?.In addition,by inserting an 8 nm-TAPC layer as the electron blocking layer,a remarkable reduction in dark current was achieved with the maximum D^*value of 5.59×10¹² Jones.Lastly,we introduced a double-layer photodetector with zinc oxide?ZnO?/poly?N,N'-bis-4-butylphenyl-N,N'-bisphenyl?benzidine?poly-TPD?interconnecting layer.The device was able to double the photocurrent and reduce the dark current,leading to a remarkably enhanced in detectivity by 15.6 folds.2 Three thermally activated delayed fluorescence?TADF?materials with strong UV absorption were firstly utilized as the electron acceptors in solution-processed OPDs.The best comprehensive detectivity could reach to 1.09×10¹² Jones under 0.5mW/cm² 350 nm UV light at-9 V.Additionly,by using a ultra-thin silver layer as the electrode,visibly semitransparent OPDs with an average transmittance of 43%were obtained,and the detectivity could reach to 2.09×10¹¹ Jones under the same testing conditions.Moreover,by utilizing the molybdenum oxide?MoO_x?/silver?Ag?/MoO_x?MAM?structure as the bottom transparent electrode,high performance indium-tin-oxid?ITO?-free semitransparent OPDs have been realized.The champion device exhibited a high transmittance of 46.7%,and a peak response located at 380 nm.3 PNNT and PNNTH have been prepared as polymeric electron acceptors for OPDs.By tuning the thickness of the active layer,the resultant performance and the charge transport properties of the all-polymer photodetectors were systematic studied.Firstly,the response spectra of OPDs with layer-by-layer?bilayer?structures varied from broadband to narrowband,which was due to the filter effect of PNNT layer.The broadband response showed the visible spectral region of 300 nm-750 nm,while the narrowband responses located at 450 nm and 780 nm,respectivity.Secondly,by tuning the thickness of BHJ from 150 nm to 2?m,spectrally selective all-polymer photodetectors with broad and narrow band responses were realized.The OPDs with 2?m-thickness had a narrowband D^*of 1.61×10¹¹ Jones at 770 nm.Lastly,the bimolecular recombination losses and optical losses were found to be remarkably similar for the two systems,and our observation of the higher light current for bilayer devices was shown to result from the higher charge generation yield due the smaller first-order recombination losses.4 Graphene was developed as the transparent conducting electrode?TCE?for OPDs to meet the requirements for the development of flexible devices.We used a non-polymeric molecular benzodithiophene-terthiophene-rhodanine?BQR?as donor and[6,6]-phenyl C71 butyric acid methyl ester as acceptor to make the bulk heterojunction.The effect of MoO_x were investgated,along with the power loss mechanism within the devices.By utilizing the thermally evaporated MoO_x interlayer,the graphene anode showed a remarkably reduced sheet resistance,smoother surface and increased work function,which leveled the sharp difference in various parameters for ITO and graphene TCEs.In comparison,we found the limited photoresponse in flexible devices were mainly due to the optical losses and less charge generation,although the bimolecular recombination has no function for the loss mechanism.