Research Of High-Performance Organic Solar Cells Based On The Layer-by-Layer Spin-Coating Method

As a promising novel photovoltaic technology,organic solar cells(OSCs)have received widespread attention in recent years,with the advantages of low cost,light weight,environmental protection,as well as the possibility of preparing flexible devices.The use of layer-by-layer spin-coating method to construct the active layer is an effective strategy for fabricating high-performance OSCs.This method does not involve the doping ratio between donor and acceptor,less dependent on the solvent concentration and easy to form a vertical morphology structure that is conducive to charge transport and collection.Furthermore,layer-by-layer OSCs have significant advantages in device stability and reproducible fabrication.However,the following problems still exist with the development of layer-by-layer spin-coating devices.Firstly,how to avoid the erosive washout of the upper solvent on the lower layer during the continuous spin-coating process is the primary issue in fabricating high-performance devices.Secondly,the precise control of the active layer morphology still requires in-depth research.In addition,most of the reported sequential spin-coating OSCs are prepared using halogen solvents,but the use of halogen solvents pollutes the environment and is harmful to human health,which is not favorable to the future practical application of devices.To address the above problems,this dissertation started from the morphological control of the active layer,introduced the dual-solvent strategy,the additive strategy and the doping layer strategy,precisely regulated the distribution of components within the active layer and deeply analysed the effect of different strategies on devices performance,to realize highefficiency OSCs based on layer-by-layer spin-coating method.In addition,from the perspective of environmental protection,the dissertation utilised sequential deposition method with non-halogen solvent,providing a new idea for the fabrication of highperformance environmentally friendly devices.The main research and innovative work of this dissertation are as follows:1.Taking advantage of the difference in volatility of solvents and their variation in solubility to the materials,high performance OSCs based on the layer-by-layer spincoating method were obtained by using an innovative dual-solvent strategy to balance the solubility and crystallisation of the polymer as well as to individually optimize the morphology of the donor layer.After the dual-solvent treatment,D18-Cl(CB+CF)/Y6 and D18-Cl(CB+THF)/Y6-based bilayer devices achieved power conversion efficiency(PCE)of 17.33% and 17.73%,respectively.Compared with the control devices based on D18-Cl(CB)/Y6 system(16.38%),the performance was significant improvement.It is due to the apply of dual solvents enhanced the crystallization of the donor layer and attenuated the effect on itself when spin coating the upper solution,while allowing the deposited Y6 molecules to penetrate uniformly into the D18-Cl framework,improving the amorphous morphology of the active layer.In addition,the efficiency of the dual-solvents treated bilayer devices remained above 90% of the initial value after 2500 hours of storage in a glove box.Further,the method was introduced into D18-Cl/N3 system to verify the strategy effectiveness,and the dual-solvent-processed bilayer OSCs also showed excellent performance.2.Based on the advantage of layer-by-layer tunability of bilayer devices,the additive strategy was utilized rationally to target the morphology of both the donor layer and the acceptor layer through different mechanisms.With the corresponding synergistic effect,the more optimal vertical component distribution structure was formed,further enhancing the efficiency of OSCs based on D18-Cl/Y6 system.Firstly,the additives n-octane and 1-fluoronaphthalene(FN)were used to respectively modulate the donor and acceptor layer.On the one hand,the addition of n-octane to D18-Cl induced an orderly arrangement of the donor molecules,reduced the surface roughness and suppressed the over-penetration of Y6.The efficiency of the corresponding D18-Cl+/Y6 devices was up to 17.70%.On the other hand,by adding FN to the Y6 solution,the acceptor crystallisation was increased,which in turn improved the charge transport.The corresponding D18-Cl/Y6+ devices obtained a PCE of 17.39%.On the basis of the above optimization,the dual additives strategy with modifying both the donor and the acceptor further precisely modulated the active layer and optimized the vertical structure,which facilitated charge transport and collection.The D18-Cl+/Y6+ devices after the introduction of the dual additives ultimately achieved a PCE of up to 18.16%.3.For achieveing eco-friendly OSCs,green layer-by-layer devices based on PM6/BO-4Cl system were constructed using the non-halogen solvent o-xylene.On this basis,an innovative doping layer strategy was proposed,whereby the solid additive 4-Aminobenzoic acid(PABA)or 3,4-Diaminobenzoicacid(DABA),which is not easily soluble in o-xylene,was dissolved in methanol and used as the doping layer between the donor layer and the acceptor layer.Finally,high performance OSCs with the stacked structure were prepared by layer-by-layer spin-coating method.The devices based on PM6/PABA/BO-4Cl and PM6/DABA/BO-4Cl system obtained an optimized PCE of17.46% and 17.17% respectively.This sequential spin-coating method overcomed the solubility limitation of the solid additives and avoided the corrosion problem of the upper solvent layer on the lower film,while improving the material distribution in active layer,effectively regulating the thin-film microscopic morphology and promoting carrier balanced transport.4.Taking advantage of the ternary strategy to broaden absorption spectrum and promote energy level complementarity,high-performance green bilayer OSCs based on PM6/BO-4Cl: L8-BO system have been realized by combining the ternary strategy with the layer-by-layer spin-coating method,together with the use of non-halogen solvents.The introduction of the third component,L8-BO,into the acceptor layer promoted the formation of an alloy model between the acceptors,and further regulated donor and acceptor component distribution on the basis of the vertical morphology structure in layer-by-layer devices.Ultimately,PM6/BO-4Cl: L8-BO-based bilayer OSCs achieved a PCE of up to 17.89%,which is one of the highest efficiencies reported so far for nonhalogen solvent treated OSCs.Compared to the ternary heterojunction devices based on the same system(17.00%),the bilayer devices avoided the problem of complex disorder in the morphology and exhibited good storage stability while improving performance.Therefore,the layer-by-layer spin-coating method is an effective way to achieve highperformance ternary OSCs.Combined with the use of non-halogen solvents,it offered a new option for the development of energy technology and the future commercialisation of OSCs.