Modification Of Cathode Interlayer And Aggregation Of The Active Layer In Organic Solar Cells

At present,with the rapid consumption of non-renewable energy such as coal,oil and other fossil fuel,which has caused more and more serious environmental pollution and other problems,the development of renewable new clean energy technology has become an important way to solve the energy crisis and protect the ecological environment in the field of energy.In recent decades,organic solar cells?OSCs?have been extensively concerned by researchers because of their intrinsic merits of high energy conversion efficiency,light weight,low cost and large area for roll-to-roll processing,which is considered as a promising technology to convert directly from sunlight into electricity.In recent years,with the continuous synthesis of new materials?including donor materials,acceptor materials and interface materials,etc?,and the continuous optimization of device fabrication?including new device structure,aggregation regulation,etc?,the energy conversion efficiency of OSCs has been greatly improved,laying a good foundation for the further commercialization of this technology.This paper focuses on the frontier research field of OSCs.Several key factors affecting the device performance are systematically studied,including the modification of the cathode interlayer and the aggregation structure of the photoactive layer.In the following three aspects,some important research results have been obtained.First of all,in view of the critical problem that the traditional cathode modified material zinc oxide?Zn O?has low conductivity,which limits the photocurrent extraction efficiency in the photovoltaic devices,a kind of cathode modified material with photoconductivity characteristics was synthesized and applied in the devices,which effectively improves the power conversion efficiency?PCE?of the devices.The organic-inorganic hybrid photoconductive interlayer?Zn O:PBI-SO₃H?with high conductivity was successfully fabricated by doping the water-soluble dye molecules perylene bisimide?PBI-SO₃H?into the precursor aqueous solution of Zn O.The interlayer annealed at 150?can be prepared by using environmental friendly and pollution-free aqueous solution.Therefore,it can be processed on flexible substrate to lay a foundation for the preparation of flexible modules in the future.The results show that PBI-SO₃H combined with Zn O via ion bond at low doping concentration?1wt%?,and PBI-SO₃H is monodisperse in Zn O matrix.The hole transport ability of the hybrid material is reduced,but the performance of electron mobility is significantly improved,which effectively improves the charge selective at the cathode interface,suppresses the current recombination loss and improves the carrier lifetime in the the photovoltaic devices.The hybrid photoconductive interlayer?Zn O:PBI-SO₃H?is used in photovoltaic devices?PM6:Y6 as the active layer of the device?,and the PCE value of the device is as high as 15.4%.Due to the high conductivity of the Zn O:PBI-SO₃H interlayer,the device performance only decreases by 5%when the thickness of the interlayer is increased from 30nm to 90nm.Furthermore,based on the characteristic that the hybrid material can be fabricated into a film at a lower temperature,a flexible OSCs with a PCE of 13.5%has been successfully fabricated on a flexible substrate.The experimental results show that this kind of environment-friendly and low temperature processable hybrid interlayer has certain application potential in the future industrial production of OSCs.Then,a special cathode interlayer modification material was designed and synthesized to improve the performance of OSCs,in view of the problem that the recombination centers of photogenerated carriers formed by surface defects of Zn O film destroys the charge selection characteristics of the film and causes the loss of photocurrent of OSCs.The organic-inorganic cathode bilayer was successfully fabricated by spin coating the naphthalimide derivative molecules?NDI-TPA?on Zn O surface and annealing treatment.The results show that the two kinds of interface materials cling tightly together by Zn-N chemical bond,which has good solvent resistance.The surface modification improves the contact characteristics between the cathode interlayer and the active layer,and reduces the linear resistance of the device.Ultroviolet Photoelectron Spectrometer shows that the modified Zn O film has lower work function,which is conducive to improve the built-in electric field of the OSCs to facilitate the carrier extraction.The lower valence band level enhances the energy barrier of hole transport,thus enhancing the hole-block ability of the film.Zn O/NDI-TPA is introduced as cathode interlayer of the OSCs and PM6:Y6 or PTB7:PC₇₁BM is used as the active layer,16%and 9.1%PCE are obtained respectively.Due to the LUMO energy level of PC₇₁BM align to the conduction band?CB?of Zn O/NDI-TPA,the energy barrier needed to overcome in electron transmission in smaller.Therefore,compared with Zn O as cathode interlayer,the PCE of the device based on PTB7:PC₇₁BM as active layer is increased by 22%,while that based on PM6:Y6 is only increased by 6%.This provides a new design idea and method for the surface modification of Zn O film.Finally,due to the self aggregation of fullerene receptor material,it is difficult for PC₇₁BM to uniformly disperse in the network structure formed by the donor material,which seriously results affects the OSCs efficiency and stability.Therefore,the active layer doped with J-3 molecules whic can combine with PC₇₁BM molecule through?-?interaction to inhibit the self-aggregation of PC₇₁BM and make it evenly dispersed in the donor material.The optimized phase separation morphology improvs the balance of electron-hole migration rate in the active layer,thus reducing the space charge accumulation inside the OSCs.The saturation photocurrent measurement shows that the number of photogenerated excitons in the doped active layer did not change significantly,but the separation and collection efficiency of photogenerated exciton was remarkably improved.Introduction of J-3 molecule reduces the recombination in the OSCs and increases the carrier lifetime and extraction rate,which greatly improves the short-circuit current density and filling factor.The stability test shows that the active layer structure can be stable for a long time,which indicates that the self-aggregation ability of PC₇₁BM can be effectively restrained through the introduction of J-3 molecule,thus avoiding the damage of the mophology of active layer and seriously affecting the overall performance of the device.Therefore,our results clearly demonstrate the great potential of this new dopant in improving the efficiency and stability of the OSCs.