PEIE Buffer Layer Based Polymer Photoconductive Detectors

A photodetector is a device for detecting and measuring the properties of light via the photoelectric effect.Compared to the inorganic photodetectors,the organic photodetectors(OPDs)are emerging devices due to their potential to realize large area and flexible electronics by cost-effective printing methods.Among the various organic photodetectors,the organic photoconductive detectors get more and more attention recently due to their unique advantages,such as the high EQE(above 100%),the simple architecture and so on.Although the device performance increases continuously recently,the organic photoconductive detector also has their own issues,such as the high dark current density,the low linear dynamic range(LDR),the complicated fabrication processes,the toxic materials,and so on.Therefore,how to get an organic photoconductive detector with high performance in every figure of merit becomes a major issue need to be solved in this field.A key to improve the performance of photodetectors is to lower the device’s dark current.To date,most organic photoconductive detectors utilize a bulk heterojunction(BHJ)structure,which can be simply prepared by the one-step solution process of the polymer(donor):fullerene(acceptor)blend.Although,the BHJ structure can dramatically improve the exciton dissociation efficiency,it also leads to an undesirable high dark current,which greatly degrades the device performances.Furthermore,in the BHJ devices,both holes and electrons can inject from either low work function cathode or high work function anode because the donor domains and the acceptor domains are in contact with the same electrode.Therefore,first of all,the water soluble PEIE is used as the electrode buffer layer with a one-step and low temperature solution-process to shift the work function of ITO electrode in the P3HT:PC61BM based photoconductive detector.It can decrease the device’s dark current significantly.The maximum EQE of 3250%in the visible wavelength is obtained at-1 V under 370 nm illumination with the intensity of 0.14 μW.The detectivity is 1.04 × 1012 Jones.The rise time is 78 μs and the fall time is 87 μs.The effect of the PEIE modified ITO electrode on the polymer photoconductive detector is investigated by the C-V curves,transient photocurrent and so on.However,compared with the over 100 dB LDR of the inorganic photodetectors,the 55 dB LDR of the PEIE device is obviously need to be improved.Practically,the presence of the severe variation in photocurrent over light intensity can be a limiting factor for the proposed devices as well.To achieve the better performance of the PEIE device,especially the LDR.We synthesized ligand-free anatase TiO2 nanocrystals with the clean surface and excellent electron extraction.It could disperse in water which also is the solvent of PEIE.By introducing the trap states between the TiO2 nanocrystals and the photoactive layer(P3HT:PC61BM),we demonstrate a high performance solution-processed broadband OPD in the visible wavelength based on the P3HT:PC6iBM BHJ.The OPD shows the high EQE of 113%and the high detectivity of 1.9 × 1012 Jones at-1 V.The device also exhibits a respectable response speed with a 34 μs rise time and a 26 μs fall time.Then,with introducing the TiO2 into the PEIE layer,we further improve the performance of the PEIE device.The response speed improves from 78 μs to 42 μs for the rise time and from 87 μs to 32 μs for the fall time,and the LDR achieves 103 dB which is almost twice as much as the PEIE device(55 dB).However,because of the TiO2 interlayer,the resulting EQE is quite lower than that of the PEIE device.To achieve the high photodetector performances in almost every figure of merit,including the response speed,EQE,LDR and so on.A P3HT:PC61BM BHJ film is used as the photo-active layer with a PEIE modified ITO electrode.By adjusting the PC61BM:P3HT weight ratio in the device,the interfaces between the donor and acceptor can be dramatically varied:Among these devices,the devices has the EQE from 1024%to 12000%,the LDR from 55 dB to 79 dB,the detectivity from 9.17 μ1011 Jones to 2.59 × 1012 Jones and a fast response time of 24 μs(rise time),78 μs(fall time)at around-0.8 V bias.With the help of transmission electron microscope images(TEM),light-assisted capacitance-voltage(C-V),and transient photocurrent measurements,the fullerene concentration effect in the device has been investigated.