Application Of Surfactant-encapsulated Polyoxometalate Complexs As Cathode Interlayer In Polymer Solar Cells

Energy crisis is threatening human people with the depletion of fossil energy and environmental pollution.We are eager to find clean and renewable energy sources.Against this background,polymer solar cells?PSCs?have attracted great attention because of their solution-processing,light weight,flexible processing,and high permeability.At present,low power conversion efficiency?PCE?is one of biggest challenge towarding the commercialization of PSCs.In general,PSCs consist of an active layer,conductive electrodes,and interlayers.Interlayers added between the electrode and the active layer play an important role in improving the performance of PSCs because they can improve the contact between the electrode and the active layer,thereby reducing the potential barrier of charge collection or charge extraction.In recent years,many efforts have been made to develop novel interlayer materials,particularly cathode interlayer?CIL?materials.An ideal CIL would lower the electrode work function,have solubility properties orthogonal to those of the photoactive layer,exhibit good film-forming properties,transport electrons selectivity,possess large electron affinity,and exhibit long-term stability.So far,the CIL material that can meet all the above requirements is quite scarce.At present,water/alcohol-soluble CIL materials successfully used in PSCs mainly consist of conjugated polymers and small molecules.However it is difficult to massively utilize the materials for low cost and large-area roll to roll production of the PSCs because their molecules containing complicated?conjugated units require difficult and complicated synthetic procedures resulting in a high cost in terms of production and environment.To overcome their shortcomings,we developed a new class of high yieldandenvironment-friendlyalcohol-solubleCILmaterials,namely surfactant-encapsulated polyoxometalate complexs?SEPCs?.They are made of the organic?positively charged quaternary ammonium salt?and inorganic?negatively charged polyoxometalate?moieties bound together by electrostatic interactions.The SEPCs are synthesized by replacing the cation of polyoxometalate cluster with cation of surfactant in an aqueous solution,and their yields exceed 99%.In Chapter 2,in order to optimize SEPCs as CIL,we chose TBA-SiW₁₂containing four tetra-n-alkyl ammonium and DTA-SiW₁₂ containing a long alkyl chain ammonium as CIL to fabricate PSCs,respectively.As a result,the device with TBA-SiW₁₂ exhibited a higher PCE?8.5%?.Atomic force microscopy?AFM?and Scanning electron microscopy?SEM?measurements indicated that TBA-SiW₁₂ on the active layer not only smooth the surface of active layer and enhanced surface potential of active layer,but also may affect the light field of the solar light in the device through the formation of the hexagon cluster.Those results further clarified that the device incorporating TOA-SiW₁₂ can give rise to higher built-in potential,charge carrier density and mobility,better charge carrier extraction and smaller bimolecular recombination rate than DTA-SiW₁₂.In Chapter 3,we undertake the work of the previous chapter and systematically studied the effect of alkyl chain length in{[CH₃?CH₂?_n-1]₄N}₄[SiW₁₂O₄₀](abbreviated as TA-SiW₁₂,n=2,4,8,10)as CIL on the performance of the PSCs.The PSCs with the four different TA-Si W₁₂s exhibit different device performance.The highest PCE?9.15%?was obtained in PTB7:PC₇₁BM based PSCs with TOA-SiW₁₂?n=8?due to the highest open circuit voltage(V_OC),short circuit current(J_SC)and fill factor?FF?.Combined measurements of Capacitance-Voltage?C-V?characteristics,charge carrier mobility and J_ph–V_eff characteristics demonstrated that incorporating TOA-SiW₁₂ can give rise to higher built-in potential,charge carrier density and mobility,and better charge carrier extraction than other TA-SiW₁₂s?n=2,4,10?in the PSCs.AFM images presented only TOA-SiW₁₂2 forms homogenous,closely packed and well-distributed grain clusters with a quasi-periodic structure,which rationalizes the higher J_SC and FF of the PSC with TOA-SiW₁₂.In Chapter 4,we systematically clarified that TOA-SiW₁₂ can be a novel CIL material to replace the classical CIL materials,such as LiF and PDINO.In additional,the PCE of more than 10%is obtained in PSCs with TOA-SiW₁₂.At the same time,using TOA-SiW₁₂ as a model,UV/X-ray photoelectron spectroscopy?XPS/UPS?were applied to study the mechanism of SEPCs as CIL.Our findings suggest that TOA-SiW₁₂ as a good CIL not only decreased the WF of cathode,but also could be n-doped upon contact with the metal cathodes?Al and Ag?.In Chapter 5,we developed another SEPC,namely K₂[(C₈H₁₇)₄N]₅[PW₁₁O₃₉]?PW-11?,as ZnO modifier to form a co-interlayer to fabricated inverted structure of PSCs?i-PSCs?with a PCE of>9%(ITO/Zn O/PW-11/PTB7:PC₇₁BM/MoO₃/Al).Combined XPS/UPS measurements with C-AFM measurements demonstrated that PW-11 deposited on Zn O through chemical bonds not only reduced the work function of ZnO but also increased its conductivity.These results are beneficial to reduce the recombination loss of carriers at the surface of ZnO and increase the extraction efficiency of electrons.At the same time,fluorescence spectroscopy?PL?showed that electrons were transferred from the PTB7 to PW-11,which is conducive to improve the separation rate of excitons in i-PSCs.In addition,i-PSCs with PW-11 as CIL was also fabricated and obtained a PCE of 8.00%.These results indicate PW-11 is a promising ZnO modifier and a CIL for i-PSCs.