Effect Of Electrode Modification On The Performance Of Organic Photodetectors

Low dark current density and high photocurrent density play the key role in determining the overall performance of organic photodetectors(OPDs).Although,the research of photodiode(PD)type OPDs is more common so far since its working mechanism is similar with the organic photovoltaic devices,the photomultiplication(PM)type OPDs are receiving more attention due to their high external quantum efficiency(EQE)and photoresponse.Recently,the PM type OPDs with high EQE have been successfully realized with the help of trap-assisted or blocking-layer assisted charge carrier tunneling injection.However,they are usually facing the deficiencies of complex preparation process,slow response speed,high working voltage,toxicity,and so on.Therefore,it is an important research direction to realize the full optimization performance OPDs under low working voltage.In this work,by introducing the electrode buffer layer to the bulk heterojunction device,the high performance OPDs are achieved and the related working mechanisms are investigated.The dissertation comprises of three main parts.1.We demonstrate high sensitive PM OPDs by applying the common sol-gel processed ZnO film to the OPDs based on P3HT:PC6iBM and PTB7:PC71BM active layers,and the device structure is ITO/ZnO/active layer/Al.(1)The results indicate that the sol-gel ZnO can efficiently block the external charge injection and thus obviously reduce the dark current density.Under illumination,some surface defect states of ZnO can be occupied and the photogenerated charges accumulate around the interface between the ZnO layer and the active layer under reverse bias,which creates a tunnel injection and is helpful to obtain high photocurrent density.(2)By introducing the ZnO buffer layer,the dark current density is effectively reduced and the photocurrent density is obviously improved.The OPDs based on P3HT:PC6iBM show a high Ion/Ioff ratio of 2.04 × 105,which is 2.55 × 103 times compared with the control device.Meanwhile,the devices show an EQE of more than 200%(460 nm-600 nm),a detectivity of more than 1.0 × 1012 Jones(400 nm-630 nm),and a linear dynamic range over 100 dB under the different monochromatic illuminations at-0.5 V.Similarly,for the OPDs based on PTB7:PC71BM,the Ion/Ioffratio increases from 93.97 to 1.25 × 103.The OPDs have an EQE of more than 2000%(350 nm-750 nm)and the corresponding detectivity approaches 1.0 × 1013 Jones at-0.5 V.2.We demonstrate high sensitive PM OPDs by applying the water/alcohol soluble conjugated polymers(PFN)or soluble alkali salts(CS2CO3)to the OPDs based on the P3HT:PC61BM or PTB7:PC71BM active layers,and the device structure is ITO/PFN(Cs2CO3)/active layer/Al.(1)A dipole layer is formed between the electrode and the active layer due to the insertion of PFN(CS2CO3)electrode buffer layer.The PFN(CS2CO3)dipole layer can efficiently block the external charge injection and thus obviously reduce the dark current density.Under illumination,the photogenerated charges accumulate around the interface between the ZnO layer and the active layer due to the PFN(CS2CO3)dipole layer under reverse bias,which creates a tunnel injection and is helpful to obtain high photocurrent density.(2)For the OPDs based on P3HT:PC61BM with the PFN electrode buffer layer,the dark current density decreases from 0.07 mA/cm2 to 1.92 × 105 mA/cm2,and the devices show a high Ion/Ioff ratio of 1.93 × 105.The OPDs have an EQE of 208.11%and the corresponding responsivity and detectivity are 0.92 A/W and 9.10 × 1012 Jones(550 nm)at-0.5 V,respectively.Meanwhile,the OPDs based on P3HT:PC61BM with the Cs2CO3 electrode buffer layer shows a high Ion/Ioffratio of 1.02 × 105,an EQE of 503.62%and the corresponding responsivity and detectivity are 2.27 A/W and 3.97 × 1011 Jones(560 nm)at-0.5 V,respectively.(3)For the OPDs based on PTB7:PC71BM with the PFN electrode buffer layer,the Ion/Ioff ratio increases from 93.97 to 1.61 × 104.The OPDs have an EQE of more than 100%(350 nm-690 nm)and an approximate 1.00 × 1014 Jones detectivity(350 nm-730 nm)at-0.5 V.3.Based on the P3HT:PC61BM and PTB7:PC71BM devices with the above mentioned ITO electrode buffer layers(ZnO,PFN,Cs2CO3),we study the effect of metal electrode on the OPDs.The results indicate that the metal electrode(Ag,Au,Al)can obviously influence the performance of the OPDs.(1)The results indicate the OPDs show two kinds of working modes due to the different metal electrode work function.For the Ag(Au)top metal electrode device,the photogenerated charges rapidly transport to the corresponding electrodes and then to be collected under reverse bias,which belongs to the PD-type OPDs.However,for the Al top metal electrode device,the photogenerated charges accumulate around the interface between the electrode buffer layer and the active layer,leading to a tunneling charge injection under reverse bias,which is contributed to the more than 100%EQE.This device belongs to the PM-type OPDs.(2)The OPDs with two different working modes both exhibit large Ion/Ioff,fast response,and low working voltage.Meanwhile,these two type OPDs show their unique strengths in our devices.The PD-type OPDs have the higher detectivity and the wider linear dynamic range,while the PM-type OPDs possess more than 100%EQE and the better responsivity.The results indicate that the different working mode can be achieved by tuning the work function,thus we can obtain the corresponding optimized performance parameters.