The Theoretical And Experimental Study On Photo-induced Charge Separation In Organic Semiconductors

In recent years,organic semiconductors have been widely used in solar cells,field effect transistors,electroluminescent devices,photodetectors,and so on,which due to the unique photoelectric properties.Among them,the mechanism of photo-induced charge separation and its related applications based on organic semiconductors have been a hot topic in the field of organic optoelectronics at home and abroad,especially in the aspects of improving the performance and developing new functions of organic optoelectronic devices.This paper has carried on the research to this field on the above two aspects,the concrete research content and the conclusion are as follows:1.In previous studies,it's found that on the basis of the general organic photodetector structure of cathode electrode/working layer/anode electrode,when ionic liquid is inserted into the working layer and electrode,an electric double layer will be formed at the interface in contact with it.The resulting displacement current makes the incident light can still produce a relatively large photocurrent through the non-overlapping region of the upper and lower electrodes in organic photodetector,which lets the opaque electrodes can also be put into use and get rid of the dependence on transparent electrodes such as indium tin oxide.However,ionic liquids not only increase the output of photocurrent,but also bring loss.In order to study the influence of ionic liquids on device loss,some kinds of ionic liquids have been studied in this paper.Firstly,a sandwich structure of electrode/ionic liquid/electrode is constructed.By changing the voltage between electrodes,and changing the offset to realize the change of ionic liquids' effective distance between electrodes,the loss of ionic liquids with the bias voltage and effective distance are studied.Finally,through the admittance and capacitance two indicators to carry on the comparative analysis.Furthermore,it provides a direction for the selection of ionic liquids in photodetectors.For the detection of AC signals by photodetectors,the admittance reflects the efficiency of photocurrent,and the capacitance represents the response rate.According to the analysis of experimental data,at the first time,bias has little effect on the trend of admittance and capacitance varying with frequency.Secondly,for the same ionic liquid,the admittance is only related to the offset,and decreases with the increase of the effective distance.Thirdly,for different ionic liquids,when bias,offset and frequency are constant,the radius of admittance and cation is inversely proportional.If the device focuses on the efficiency of photocurrent,the ionic liquid with smaller radius of cation should be better.Finally,as far as capacitance is concerned,the capacitance will decrease with the increase of cation radius.If the device requires rapid response,it is appropriate to select ionic liquids with a larger cation radius.2.The commonly charge separation methods are P-N junction charge separation and charge separation induced by Schottky barrier.In this paper,we have fabricated a device with structure of ITO/PEDOT:PSS/ZnPc/LiPc/Au,and found that it both have photocurrent output under 532 nm and 1550 nm laser irradiation,but this phenomenon can't be explained by the above two kinds of charge separation methods.Besides,through the construction of different device structures and the corresponding experimental verification,it is found that the photocurrent generated by the device comes from the charge separation between ZnPc and LiPc,that is,the unpaired electrons excited in the LiPc attract unexcited pairs of electrons in the ZnPc,resulting in the transfer of electrons from ZnPc to LiPc and photocurrent generated in the device.This finding opens a new path for photoinduced charge separation.