Flexible Self-Powered Ultraviolet Photodetectors Based On ZnO Nanoheterojunctions

Ultraviolet(UV)photodetectors(PDs),with the function of converting UV light to electrical signals,are key components in optoelectronic devices and have been widely used in flame sensing,optical communication and environmental monitoring.Conventional UV PDs are mainly fabricated based on inorganic semiconductor films on rigid substrates prepared by high temperature growth methods.UV PDs based on the crystalline inorganic semiconductor films suffer from some drawbacks including inherent brittleness and rigidity,expensive and rigorous preparation technology,limiting their applications in portable and wearable electronics.In contrast,flexible UV PDs,with the advantages of light-weight,good portability,high flexibility and low manufacturing cost,has recently been widely studied for their potential application in electronic skin,smart textiles and wearable devices.However,most of the reported flexible UV PDs need to be driven by external power source,leading to the increased size,weight and circuitry complexity,as well as reduced lifetime due to the generation of internal heat in the presence of an external bias voltage.Hence,the design and fabrication of flexible self-powered PDs are of great significance for the development of of the next-generation portable and wearable optoelectronic system.Zinc oxide(ZnO)with wide direct bandgap(3.4 e V)and large exaction binding energy(60 me V)is a very promising material for UV PDs due to its high UV light absorption,low cost and good biocompatibility.Different types of ZnO-based flexible self-powered UV PDs driven by p–n homojunction,heterojunction,and Schottky junction have been fabricated.Among these devices,ZnO-based p-n heterojunctions devices with easy fabrication process and low response times are promsing candidates for self-powered UV PDs.Various p-type materials such as graphite,Cd S,Cu Zn S,P3HT,PEDOT:PSS and PVK are composited with ZnO to produce p-n heterojunctions for the fabrication of self-powered UV PDs.However,most of these devices are based on ZnO thin-film p-n heterojunctions,leading to the unsatisfactory device performance,such as low response speed and bad flexibility.Therefore,it is highly desired to fabricate high-performance self-powered UV PDs with high flexibility and fast response speed.In this paper,we prepare novel UV detecting materials of CuO/ZnO and rGO/ZnO nanoheterostructures,and then fabricate flexible self-powered UV PDs with fast response time,large on/off current ratio and high flexibility based on CuO/ZnO and rGO/ZnO nanoheterostructures.The main results of this paper is presented as follows:(1)The CuO/ZnO NWs heterojunction active material is prepared by the successive growth of CuO NWs and ZnO NWs on the Cu NWs substrates.By changing the growth time of the ZnO NWs layer,the density and length of ZnO NWs and then the interface contact area of CuO/ZnO heterojunction can be effectively regulated.By using silver nanowires(Ag NWs)conductive films as electrodes,we fabricate flexible transparent self-powered UV PDs based on CuO/ZnO NWs heterojunction active layer with different ZnO NWs growth time.The as-fabricated device with ZnO NWs growth time of 3h exhibits a high photocurrent/dark current ratio(I_on/I_off)of 392and fast photoresponse times with rise time(t_rise)of 16 ms and decay time(t_decay)of 17 ms at 0 V bias.Moreover,the as-fabricated flexible self-powered UV PDs has a high transparency of 80%and excellent mechanical stability.After 10000 bending cycles,the device can still remain good photoresponse performance,with t_rise=41 ms and t_decay=47 ms.(2)Reduced graphene oxide(rGO)nanosheet films and ZnO NWs layers are successfully deposited on cotton yarn fiber to prepare fiber-shaped rGO/ZnO nanoheterojunctions by capillarity-driven assembly and hydrothermal processes,repestively,for the farication of flexible self-powered fiber-shaped UV PDs.By changing the ZnO seed solution,the density of ZnO seeds and then the the density of the growth ZnO NWs in fiber-shaped rGO/ZnO nanoheterojunctions can be effectively adjusted.Moreover,flexible self-powered fiber-shaped UV PDs are fabricated by using rGO/ZnO nanoheterostructure as active layer and Ag NWs conductive yarns as electrodes.The optimized performance of flexible self-powered fiber-shaped UV PDs is obtained by using rGO/ZnO nanoheterostructure active layer of seed concentration of 7 m M,with I_on/I_off=289,t_rise=88ms and t_decay=135 ms.In addition,this device exhabites excellent flexibility,remaining good photoresponse performance after 10000 bending cycles.