Photodetectors Based On Solution-processed Organic Semiconductors Towards A Dual-band Vertically-stacked Device Architecture

Organic photodetectors have attracted the attention of a growing number of researchers due to their advantages such as ease of fabrication through solution-based methods,low cost,tunable spectra range,and light weight.This thesis studies two types of photodetectors based on solution-processible organic semiconductors,one sensitive in the 385-505 nm wavelength range and the other in the 400-600 nm wavelength range,with potential for blue and green light detection.Further,this thesis stacks them vertically and explores the performance of the blue and green dual-band stacked devices.This thesis first presents the development and characterization of organic photodiodes based on a blend of a polymer semiconductor called F8T2 and a smallmolecule semiconductor called PC61BM.The blue-light-absorbing donor material F8T2 and the weakly-absorbing electron acceptor PC61BM are mixed in a ratio of 1:1 to obtain the F8T2:PC61BM photodetector with high photoresponse in the blue light region.The device gives an EQE of 45.8%at 455 nm under the-3 V reverse bias.Without an external bias,the specific detectivity of the photodetector reaches 3.3×1012 Jones,and the response time is below 6 μs.Aiming to develop photodetectors also responsive in the green wavelength range,we investigated organic photodetectors with photocurrent gain based on MEH-PPV,a green-and blue-absorbing polymer,and PC71BM,a small-molecule acceptor.MEH-PPV blended with a small amount of PC71BM(4%wt)is used as the active layer for sandwichtype Al|MEH-PPV:PC71BM|ITO devices.Given the small amount of PC71BM,the corresponding domains do not form a continuous path,thereby acting as electron traps.Indeed,when a large bias voltage is applied across the device,a photocurrent gain is observed,with the apparent EQE being greater than 100%.This can be rationalized as arising from band bending near the aluminum electrode caused by electron trapping,which leads to hole tunneling into the MEH-PPV.The response of the photodetector covers the blue and green wavelength range,When the active layer is annealed at 150℃,the EQE peak of the device at 560 nm under-30 V bias is greater than 1200%,the specific detectivity reaches 2.2×1012 Jones,and the response time is approximately 1 ms.Given that the photocurrent gain of the MEH-PPV-based devices could offset part of the optical losses arising from stacking,we therefore build on the F8T2:PC61BM and MEH-PPV:PC71BM photodetectors to realize a dual-band stacked device structure,with the F8T2:PC61BM device at the bottom of the stack.First,the electrodes between the F8T2:PC61BM and MEH-PPV:PC71BM are optimized.To minimize optical losses,ultrathin electrodes are used.A transparent PMMA is used as an isolation layer between the upper and bottom devices to avoid solvent damage to the active layer.The preliminary steady-state photocurrent performance characterization of the dual-band stacked device is carried out,giving a proof-of-concept demonstration of this approach.In this thesis,the organic photodetector is prepared by the solution method to realize the dual-band spectrally selective response of the vertically stacked device structure.The vertically stacked device realizes the color separation response in the blue and green wavelength bands,respectively.While requiring further performance optimization,our findings show that solution-based organic photodetector stacking is viable,which provides the basis for future efforts in spectrally-selective solution-based organic photodetectors for color imaging.