N-Type Doped Organic Electron Transport Materials And Their Application In Organic Solar Cells

Organic photovoltaic devices have unique advantages such as flexibility,lightweight and solution processing,which meet the needs of roll-to-roll or large-area processing.Therefore,organic devices fabricated with small molecule and polymer materials have broad application prospects and have received extensive attention.As an indispensable part of the organic photovoltaic devices,the electron transport layers can effectively improve the device’s performance by doping modification.Water/alcohol soluble conjugated polyelectrolytes and perylene diimide dye molecules perform as electron transport layers in organic solar cells（OSCs）devices due to their excellent photoelectric properties,but there is still room for improvement.This thesis mainly involves the n-type doping of water/alcohol soluble conjugated polyelectrolytes with anions and the n-type doping of perylene diimide derivatives with N,N’,N’’,N’’’-tetramethylethylenediylidenetetraamine（TDAE）,aiming to study their doping performance and the device’s photovoltaic performance.In chapter 2,I studied the doping effect of anion on the thickness dependence of ETL.The host materials are the wide band gap polyelectrolyte PFN-Br and the narrow band gap polyelectrolyte PNDIT-F3N-Br;the n-type dopants are the alkali metal sodium salts Na₂Ox and Na₂SO₃.The results showed that the Br^–in the polyelectrolyte was replaced by Ox^2–and SO₃^2–when adding alkali metal sodium salts.The incorporation of Ox^2–and SO₃^2–changed the chemical state of the elements in the polyelectrolyte and reduced its work function.Compared with PFN-Br with a wide band gap,Ox^2–and SO₃^2–showed a more obvious doping phenomenon for PNDIT-F3N-Br with a narrow band gap.The introduction of Ox^2–and SO₃^2–improved the charge extraction efficiency,reduced device recombination,improved device performance,and decreased thickness sensitivity.In chapter 3,I further studied the doping mechanism in two aspects.On the one hand,PFNBT-Br with band gap and energy level between PFN-Br and PNDIT-F3N-Br conducted as host materials;on the other hand,how the valent of anions affect the doping process was further researched by using the anions Ox^2–and H₂PO₂^–with similar reducibility.The results showed that both Ox^2–and H₂PO₂^–with strong reducibility could partially replace the Br^–in PFNBT-Br,but the divalent anion Ox^2–had a more significant doping performance.The incorporation of Ox^2–and H₂PO₂^–not only reduced the work function of polyelectrolytes films but also improved the electron mobility.The incorporation of anion effectively decreased the thickness sensitivity of PFNBT-Br:when the thickness of ETL increased from5 nm to 50 nm,the PCE of undoped PFNBT-Br decreased from 15.03%to 7.11%,while the PCE of PFNBT-H₂PO₂ decreased from 15.25%to 9.82%,the PCE of PFNBT-Ox decreased from 15.56%to 13.35%.The work in this chapter provides a new solution to reduce the thickness sensitivity of ETL in future commercial production.In chapter 4,perylene diimide derivatives have good charge transport properties due to theirπ-conjugated plane,which are potential electron transport materials.However,their structures lead to strong intermolecular interactions,causing poor solubility,which limits the solution processing of such molecules.Thus,I developed a method to solve the above problems by reducing the insoluble neutral perylene diimide dye molecules into anions.I conducted electron spin resonance spectroscopy,Fourier transform infrared spectroscopy,absorption spectroscopy,and other tests.The results showed that doping perylene diimide small molecules PDI and i Pr-PDI with TDAE could convert the neutral molecules to charged anions,thereby improving their solubility.Organic solar cells based on doped PDI and i Pr-PDI achieved the PCE of 14.72%and 14.76%,respectively.This work provided a new solution to the solubility problem of perylene diimide small molecules and achieved the highest device performance of perylene diimide derivates in inverted OSCs at that time.In chapter 5,I verify the universality of the reduction method in the previous chapter.NTCDA and PTCDA,as the synthetic precursors of naphthalimide and perylene diimide,also have good charge transport properties.The anhydride functional groups of NTCDA and PTCDA provide more sites to form hydrogen bonds with protonic solvents and theoretically have good solubility in protonic solvents.However,due to the intermolecular interaction of NTCDA and PTCDA being stronger than the hydrogen bond interaction with the solvents,these molecules face similar solubility problems with perylene diimide molecules.Therefore,in this work,TDAE was also used to reduce NTCDA and PTCDA,which were converted into anions by n-type doping,to improve the solubility.At the same time,the doped molecules were applied to OSCs.The OSCs devices based on doped NTCDA and PTCDA achieved the PCE of 14.12%and 15.34%,respectively,and the device performance was further improved.