Application And Working Mechanism Of Surfactant-encapsulated Polyoxometalate Complexs As Cathode Interlayer In Inverted Polymer Solar Cells

Polymer solar cells have attracted much attention due to their advantages such as light weight,wild range of materials,low cost,flexibility,simple solution processing,and great potential for large size roll-to-roll production.In this thesis,we first utilized four surfactant-encapsulated polyoxometalate complexs?SEPCs?as cathode interlayers?CILs?to improve the device performance of inverted polymer solar cells?i-PSCs?based on fullerene acceptor.Then an optimal CIL material among them was applied for enhancing the device performance of non-fullerene i-PSCs.Finally working mechanism of the SEPC as a CIL in the i-PSCs was investigated.In Chapter2,SiW-12([(C₈H₁₇)₄N]₄[SiW₁₂O₄₀]),SiW-11(K₃[(C₈H₁₇)₄N]₅[SiW₁₁O₃₉]),PW-12([(C₈H₁₇)₄N]₃[PW₁₂O₄₀]),PW-11(K₂[(C₈H₁₇)₄N]₅[PW₁₁O₃₉])have been used as the CILs in the i-PSCs based on PTB7:PC₇₁BM.All the i-PSCs with four SEPCs as the CILs exhibited better device performance than the device without the CIL.Power conversion efficiencies?PCEs?of the four i-PSCs are 5.44%?SiW-12?,6.38%?SiW-11?,5.85%?PW-12?and 8%?PW-11?which is even higher than the PCE?7.4%?of the device with ZnO as the CIL.Capacitance-voltage?C-V?characteristics,photocurrent density-effective voltage characteristics and carrier mobility measurements show that the i-PSC with PW-11 as a CIL has a higher built-in voltage and carrier density,better electron extraction,and higher electron mobility.Water contact angle measurements and atomic force microscope?AFM?images show that theITO/PW-11 surface is more hydrophobic and more uniform than other SEPCs film on ITO,which leads to a a better interface contact with the active layer.Ultraviolet photoelectron spectroscopy?UPS?results present that the introduction of PW-11 between ITO and the active layer can reduce the electron extraction barrier?EEB?between PC₇₁BM and ITO from 0.49 eV to 0.15eV,which is much lower than the EEBs of other SEPCs as the CILs and even lower than the EEB of ZnO as the CIL?0.18 eV?.Stability of i-PSCs containing PW-11 is slightly better than the device with ZnO as well.All the findings suggest that ts show that,the performance of PW-11 as the CIL of i-PSCs is significantly better than other SEPCs,indicating that t is a promising CIL material to replace ZnO.In Chapter 3,we applied PW-11 as the CIL in non-fullerene i-PSCs.Incorporating PW-11 in the PSCs based on PBDB-T:ITIC can significantly improve the PCE of the devices.,The PCE of the devices with PW-11 can reach 9.44%after exposed to ultraviolet?UV?light for 5 minutes,which is lower than the PCE?9.91%?of the devices with ZnO as the CILs mainly due to lower open circuit voltage(V_oc)and fill factor?FF?than the the devices with ZnO.The built-in potential of the device with PW-11derived from the the C-V characteristic curve is also lower than the built-in potential of the device containing ZnO.To explain this phenomenon,we used UPS to study the effect of annealing treatment on the ITO/PW-11 work function and the EEB at ITO/PW-11/ITIC interface.The results show that annealing increases the ITO/PW-11 work function from 4.02 eV to 4.2 eV,and EEB at ITO/PW-11/ITIC increases from 0.16 eV to 0.20 eV.At the ITO/ZnO/ITIC interface,the EEB is 0.17 eV.Therefore,annealing treatment of the non-fullerene active layer during the device preparation destroyed PW-11 stacking by self-assembly of on ITO,resulting in a poor contact at the ITO/PW-11/ITIC interface.The dark current density-voltage curve,photocurrent density-effective voltage curve,and electron mobility measurements also prove that the charge extraction ability and electron mobility of the i-PSCs containing PW-11 are slightly lower than ZnO-containing devices due to annealing treatment of the active layer.In Chapter 4,we use UPS and X-ray photoelectron spectroscopy?XPS?to study the working mechanism of PW-11 as a CIL in the i-PSCs.We found that UV light treatment can reduce the work function of ITO and ITO/PW-11.XPS data demonstrated that UV light can effectively reduce the oxygen deficiency of ITO in ITO and ITO/PW-11.From the UPS and XPS results,we conclude that the PW-11stacking by self-assembly on the ITO surface is that the organic part of the molecule is far away from the ITO surface,and the inorganic part is chemically adsorbed on the ITO.The introduction of UV light and organic active layers further strengthens this arrangement.Such a self-assembly way of CIL at the cathode interface and the direction of the interface dipole are very beneficial for electron extraction and obtaining high V_oc.