Study On Energy Levels Regulation Of Electron Transport Interface In Polymer Solar Cells

Solar cells,as the most effective means of utilizing solar energy,have attracted wide attention and aroused the hot research of scholars all over the world.Among them,polymer solar cells(PSCs)receive more attention because of their huge number of advantages.After nearly twenty years,the power conversion efficiency(PCE)of PSCs have exceeded 15%,while still suffering from the lower PCE compared with the silicon solar cells used for commercial applications,which is the main reason that restricts the development of PSCs.Thus,how to improve the PCE of device has become the most important issue.Under the dual pressures of energy shortage and environmental pollution,a series of effective methods have proposed,such as synthesizing new donor/acceptor materials,using appropriate charge transport layer,optimizing device structure and so on.It is well known that the synthesis of new materials need take a long experimental period,and the optimization of device structure often requires complex experimental techniques.Therefore,it will be a simple and effective way to improve the PCE of devices by optimizing the interface layer,which can obviously improve the charge transport characteristics in PSCs.In PSCs,charge extraction,transport and collection play important roles in working process of device,thus,the charge transport materials commonly used can not always meet the various requirements of device design.At this time,the composite or hybrid interface layer will attract wide attention and become the research hotspot.In this work,the interface contact characteristics between active layer and electron transport layer was modulated by incorporating interface dipoles,constructing semiconductor-metal nanojunction and using organic-inorganic hybrid interface layer.It can modulate the interface energy level and reduce the electron transport barrier,which facilitates the electron transport and decreases the interface charge recombination,ultimately achieving the enhanced PCE of devices.The research on optimization and modification of electron transport layer are carried out,and the main research contents include:To improve the charge extraction capacity,the electron transport layer with lower enough work function(WF)is necessary,thus the electron transport interface characteristics can be modulated and optimized by the interface modification,which is shown in Chapter 2.Conjugated polymer electrolyte poly[(9,9-bis(3?-(N,N-dimethylamion)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl)-fluorene](PFN)was spin-coated onto the TiO2 layer fabricated with method of sol-gel.The incorporation of interface dipoles can induce a vacuum-level shift to reduce the surface WF of TiO2,which is beneficial for facilitated electron transfer from the acceptor of active layer to TiO2 electron transport layer and suppressed undesired interface charge recombination.At the same time,the introduction of PFN layer can also regulate the active layer system and increase the internal electric field between the donor and the acceptor,which promote the exciton dissociation at the donor/acceptor interface,contributing to the improved open-circuit voltage(Voc).In addition,the thickness of PFN la yer is optimized because of the low conductivity of PFN.When the concentration of PFN solution is 0.5 mg/m L,the device based on the PCDTBT:PC71BM active layer achieves the maximum performance.In C hapter 3,the electron transport characteristics of devic es are improved by constructing semiconductor-metal nanojunctuion.The annealing-free ZnO nanoparticles were synthesized by solution method and treated as the electon transport layer.It can be found that there are two critical problems for ZnO.O n the one hand,the energy levels of ZnO can not form the barrier-less charge transport with active layer,which will form the electron transport barrier and aggravate the interface charge recombination.On the other hand,the defect states on the surface and inter nal of ZnO will act as the electron wells to trap electrons,decreasing the charge transport and collection efficiency.In order to overcome the drawbacks of ZnO,the ultra-thin Al layer was thermally evaporated onto the ZnO layer to construct the semicond uctor-metal nanojunction.By constructing the ZnO-Al nanojunction,the electron of Al will be transferred to ZnO due to the existence of Fermi level difference between them,eliminating the charge transport barrier by reducing the WF of ZnO layer and pre-filling the interface defect states.In a word,the construction of ZnO-Al nanojunction simultaneously solves the problems of energy level mismatch and high defect states density between ZnO and active layer.These two improvements have been proved by electron injection devices and photoluminescence measurements.Meanwhile,the construction of semiconductor-metal nanojunction can further improve the potential of ZnO in PSCs and other photoelectric conversion devices.In C hapter 4,to improve the electron tra nsport layer and interfacial charge transport characteristics,the organic-inorganic hybrid system is formed.The modification of inorganic electron transport layer by polymer electrolyte film can realize the interfacial energy level regulation,but the co nductivity of polymer electrolyte material is poor,so the device performance is sensitive to its film thickness.To overcome this shortcoming,polymer electrolyte material polyethyleneimine(PEI)as the dopant is introduced into the ZnO electron transport layer to form ZnO :PEI organic-inorganic hybrid electron transport layer.Although the introduction of PEI will inevitably affect the conductivity of the ZnO layer,the reduced WF of the hybrid electron transport layer can enhance its charge extraction abi lity,which is beneficial for the improvement of short-circuit current density(Jsc)and PCE.At the same time,it can be found that the post-annealing treatment for the completed device can lead to anode buffer layer diffusion to the active layer,which promotes the hole transport at anode side and decreases interface charge recombination by improving the interface contact,thus further improving the device performance.Meanwhile,it is proved that this method has universal applicability for improving the performance of devices basaed on thermally evaporated molybdenum trioxide(MoO3)or tungsten trioxide(WO3)hole transport layer.Finally,the Jsc,Voc and fill factor(FF)of PSC s based on PTB7:PC71 BM active layer are simultaneously improved,achieving a PCE of 9.90%.In this thesis,the effect of interface characteristics at cathode side on device performance was mainly investigated,and the charge transport characteristics between active layer and charge transport layer was optimized by interface modification and doping,thus achieving barrier-less charge transfer between the adjacent interfaces and decreasing the undesired interface charge recombination.It is also proved that optimizing the charge transport layer and adjusting the interface characteristics are important in improving the PCE of PSCs.