Polyoxometalate Complexes As Electrode Interlayers In Organic Solar Cells

Organic solar cells(OSCs)have great application potential for building-integrated photovoltaics,smart greenhouses,mobile devices,wearable devices and wireless sensor networks of Io Ts(Internet of Things)etc.due to their numerous advantages such as low cost,light weight,flexibility,semitransparency and large-area production by roll-to-roll technology.Recently many breakthroughs of OSCs have been made after considerable efforts for nearly 30 years.Power conversion efficiencies(PCEs)of single-junction and tandem OSCs with small areas have exceeded 19%and 20%,respectively,indicating that OSCs are not far from the practical application.Therefore,the large-area production of OSCs has attracted more and more attention.However,cathode interface layer(CIL)materials suitable for large-area roll-to-roll production are severely lacking,which greatly limits the transformation of OSCs from laboratory samples to mass-produced commercial products.It is necessary to develop environmentally friendly,inexpensive,and sufficiently conductive CIL materials for both basic research and practical applications of OSCs.So a good solution is to select organic-inorganic hybrids with both low cost and high performance as printable CIL materials.Surfactant encapsulated polyoxometalate complexes(SEPCs)are a class of environmentally friendly,low-cost organic-inorganic hybrids,including inorganic polyoxometalates(POMs)anion and alkyl ammonium salt cations through electrostatic interaction.SEPCs as the CILs have effectively improved the efficiency of fullerene OSCs due to their good alcohol solubility and film-forming properties,highly tunable structure,and strong electron accepting ability.Therefore,SEPCs are a kind of promising and potential CIL materials.However,the applications and working mechanisms of SEPCs in non-fullerene OSCs(NF-OSCs)have been not explored yet.Moreover it is necessary to study the effect of different POMs in SEPCs on the device performance of OSCs due to the diverse structures and types of POMs though the effects of the organic alkyl chain length of SEPCs on the performance of the fullerene OSCs have been investigated.In this thesis,a series of SEPCs have been utilized as the CILs in the NF-OSCs and their working mechanism has been revealed as well.In addition,we have extended the application of giant clusters{Mo₁₃₂}and their derivatives to the anode interlayer(AIL)of OSCs and organic electroluminescent diodes(OLEDs).In chapter 2,an alcohol-soluble,environmentally friendly and low-cost TOASi W₁₂([(C₈H₁₇)₄N]₄[Si W₁₂O₄₀])was applied as the CIL in the NF-OSCs.The device performance and working mechanism of TOASi W₁₂ were investigated.As a result,the PCEs of PM6:Y6 and PM6:BTP-BO-4Cl based NF-OSCs incorporating TOASi W₁₂ as the CIL were enhanced to 16.14%and 17.04%,respectively.Combined characterizations of the photocurrent density versus effective voltage(J_ph-V_eff),capacitance versus voltage(C-V)and electron mobility(?_e)demonstrated that TOASi W₁₂ as the CIL effectively promoted exciton dissociation,charge-carrier extraction,built-in potential,charge-carrier density and electron mobility in the NF-OSCs.Futher more,the NF-OSCs with TOASi W₁₂ exhibited better stability in air due to the hydrophobic alkyl chains around TOASi W₁₂ surface that prevented penetration of moisture.More importantly,the performances of the devices with TOASi W₁₂ were insensitive to the TOASi W₁₂ thickness from 3 to 33 nm,which indicated that TOASi W₁₂ is a potential material to the preparation of large area OSCs by roll to roll.Finally,TOASi W₁₂ exhibited excellent universality for various active layers and different cathodes in NF-OSCs.These findings suggest that TOASi W₁₂ is a promising,competitive CIL for NF-OSCs.In chapter 3,three SEPCs with different keggin structure POMs anion were synthesis.The effects of the structure POMs anion on the performance of OSCs were researched.The applications of SEPCs were expanded in OSCs.The POMs anion were encapsulated by(C₈H₁₇)₄NBr,and TOAPW₁₁(K₂[(C₈H₁₇)₄N]₅[PW₁₁O₃₉]),TOAPW₁₁V([(C₈H₁₇)₄N]₄[PW₁₁VO₄₀])and TOAPMo₁₀V₂([(C₈H₁₇)₄N]₅[PMo₁₀V₂O₄₀])were gained.The PCEs of PM6:Y6 based OSCs with Al as cathode and TOAPW₁₁,TOAPW₁₁V and TOAPMo₁₀V₂ as the CIL were increased to 16.11%,15.85%and16.09%,respectively.The PCEs of PM6:Y6 based OSCs with Ag as cathode and TOAPW₁₁,TOAPW₁₁V and TOAPMo₁₀V₂ as the CIL were enhanced to 16.30%,16.44%and 16.54%,respectively.Results of ultraviolet photoelectron spectroscopy(UPS)and X-ray photoelectron spectroscopy(XPS)indicated that TOAPW₁₁,TOAPW₁₁V and TOAPMo₁₀V₂ can decrease the work function of the cathode,reduce the electron extraction barrier from the active layer to the cathode and be n-doped by Al cathode.Therefore,SEPCs with keggin POMs are a good class of CIL materials,which provide a guidance of the CIL material for the synthesis of SEPCs with other POMs structures.In chapter 4,the SEPC containing keplerate POM({Mo₁₃₂}),{Mo₁₃₂}-TOA was synthesized.Firstly,{Mo₁₃₂}-TOA as the CIL was applied in the PM6:Y6 based OSCs and was compared with PFN-Br and TOASi W₁₂.The PCE of the OSCs with{Mo₁₃₂}-TOA was higher than that of OSCs with PFN-Br or TOASi W₁₂ due to the increased short circuit current(J_SC),fill factor(FF)and open circuit voltage(V_OC),simultaneously.Combined characterization of the J_ph-V_eff,C-V,?_e and UPS illustrated that{Mo₁₃₂}-TOA as the CIL can effectively promote exciton dissociation,charge-carrier extraction,built-in potential,charge-carrier density,electron mobility and reduced electron extraction barrier from the active layer to the cathode,which leading to the increase of J_SC,FF and V_OC.Then,the performances of large area OSCs with{Mo₁₃₂}-TOA as the CIL were researched.The maximum PCEs of PM6:Y6 based OSCs were 14.48%,11.63%and 10.54%,respectively,when the effective area of the OSCs increased to 1 cm²,10.8cm² and 216.0 cm².It indicated that{Mo₁₃₂}-TOA was a potential CIL to fabricated large-area OSCs.Finally,the universality of{Mo₁₃₂}-TOA as the CIL for various active layers and cathode in OSCs and flexible OSCs were researched.In particular,PM6:L8-BO based OSCs incorporating{Mo₁₃₂}-TOA as the CIL,the PCE can reach to 18.11%.Therefore,{Mo₁₃₂}-TOA is an effective,universal,suitable to large-area prepared CIL material.In chapter 5,we further expanded the application of giant clusters{Mo₁₃₂}and{Mo₁₃₂}-TOA.The applications of{Mo₁₃₂}and{Mo₁₃₂}-TOA as the electrode interface in OSCs and OLEDs were investigated.{Mo₁₃₂}can increase the work function of the electrode,while{Mo₁₃₂}-TOA can reduce the work function of the electrode by UPS.So{Mo₁₃₂}can work as the AIL and{Mo₁₃₂}-TOA can work as the CIL in OSCs and OLEDs.PCE of the OSC with PEDOT:PSS as the AIL and{Mo₁₃₂}-TOA as the CIL reached to 16.71%.The PCE of the OSC with{Mo₁₃₂}as the AIL and{Mo₁₃₂}-TOA as the CIL can exceede 15%.More importantly,the OSC with{Mo₁₃₂}as the AIL and{Mo₁₃₂}-TOA as the CIL exhibited better stability.The current efficiency and power efficiency of Super Yellow based OLED with{Mo₁₃₂}as the AIL and{Mo₁₃₂}-TOA as the CIL were 14.96 cd/A and 13.08 lm/W,respectively.These results suggest that{Mo₁₃₂}and{Mo₁₃₂}-TOA are promising electrode interlayers in OSCs and OLEDs.