Synthesis And Application Of Alcohol-soluble Interfacial Materials Of Naphthalenediamides In Solar Cells

Polymer solar cells（PSCs）have attracted great attention from scientists because of their obvious advantages such as light weight,low cost,and flexible preparation.Currently,the power conversion efficiency（PCE）of polymer solar cell devices in the laboratory has exceeded19%,but the stability and PCE of the devices still need to be further optimized and enhanced in order to meet the requirements of commercial production.In addition to research on the synthesis of new high-performance materials as active layer materials and morphology optimization,the enhancement of the performance of PSCs through interfacial modification layers is also an important and feasible approach.The contact surface between the electrode and the active layer can be changed by the interface modification layer to form a good ohmic contact,which can reduce the interfacial energy potential barrier and promote carrier transport and collection.Therefore,three alcohol-soluble polymers with self-doping properties were designed and synthesized as cathode interface layers（CILs）for PSCs,which not only enhance the PCE of the devices but also make the preparation process friendly to the environment.The main work is as follows:Firstly,the naphthalenediimine-based alcohol-soluble interfacial material PNDI-FN with a polar group on its fluorene unit was synthesized by a simple reaction and successfully introduced into conventionally structured PSCs as CILs with the device structure of ITO/PEDOT:PSS/PTB7-Th:PC₇₁BM/PNDI-FN/Al.The best PCE of polymer solar cells with the addition of PNDI-FN as CILs reached 8.47%compared to the photovoltaic devices without the addition of interfacial layers（w/o CILs）,which was 134.23%higher compared to the PCE of w/o CILs devices.Mott Schottky curves and ultraviolet photoelectron spectroscopy（UPS）show that PNDI-FN has a decreasing effect on the work function（WF）of the Al electrode,which increases the built-in potential of the device and consequently leads to an increase in the open-circuit voltage(V_OC).In addition,the I-V curves show that the introduction of the n-type backbone leads to a significant increase in the conductivity of PNDI-FN compared with the typical cathodic interface material,PFN,which facilitates electron transport.Then,a new polymer,PNDI-N-F,with a similar structure as PNDI-FN,was designed and synthesized.It has the same number of polar groups compared with PNDI-FN,but the polar group of PNDI-N-F is located on the NDI unit.The strong self-doping properties and the reduction of the Al electrode’s work function were illustrated by electron paramagnetic resonance（EPR）and UPS.The J-V characteristic curves of PNDI-N-F as CILs were tested for different concentrations.The PCE of devices without CILs was 6.31%under the same experimental conditions,and the PCE of PNDI-N-F as CILs devices reached 8.73%,an improvement of 138.35%.Subsequently,the hydrophilicity and surface morphology of PNDI-N-F on the active layer surface were investigated and characterized.Compared with the active layer without the addition of an interfacial layer,the active layer has better hydrophilicity and smoother surface morphology after the addition of CILs,which is favorable for the physical contact with the Al electrode and the active layer and the electron transport.Finally,the naphthalenediamide-based alcohol-soluble interfacial material PNDI-N-FN was designed and synthesized.Compared with PNDI-FN and PNDI-N-F in the previous two chapters,PNDI-N-FN bears polar groups on both the NDI unit and the fluorene unit.The strong self-doping signal and the enhancement of the built-in potential of the device were demonstrated by EPR and Mott Schottky curves.The J-V characteristic curves were tested for different concentrations of PNDI-N-FN as CILs.Under the same experimental conditions,the PCE of the device with PNDI-N-FN as CILs reaches 9.37%,which is a 148.5%improvement compared to the device without CILs.The analysis of the test results for exciton dissociation,dark conductivity,electron mobility,and impedance shows the main reasons for the improved performance of PSCs.This is mainly due to the enhanced built-in electric field of the device caused by the self-doping behavior and the good conductivity of PNDI-N-FN,which improves carrier transport efficiency.