Ternary Alloy And Pseudo-planar Heterojunction Strategy For Nonfullerene Organic Solar Cells

Organic solar cell（OSC）is an established and alternative method that efficiently converts light energy into electrical energy.Solution-based organic photovoltaics（OPVs）have attracted attention in the scientific community due to many obvious advantages,such as light weight,convenient transfer,flexible,translucent,color modules,and rapid roll-to-roll manufacturing.So far,the power conversion efficiency（PCE）of OPV has exceeded 18%.Adding the third component to the binary system to prepare ternary OSCs is an effective way to improve the PCE of solar cells,but most bulk heterojunction（BHJ）ternary organic solar cells are manufactured by simply mixing the three components.Although the traditional BHJ active layer has more donor/acceptor（D/A）contact interfaces,it is difficult to control the vertical phase separation gradient.In addition,there may be some incompatible third component island regions inside,resulting in the recombination of charges and the reduction of efficiency.Therefore,it is extremely important to select the appropriate third component and to reasonably optimize the vertical phase separation of the active layers.In response to these problems,this article intends to prepare a pseudo-planar heterojunction（PPHJ）structure by sequential spin-coating method.The goal of this method is to improve the vertical phase separation of the active layers and reasonably select the third components to reduce the generation of islands and reduce the probability of charge recombination,which is helpful for improving the PCE and stability of organic solar cells.Firstly,the wide band gap donor PM6 and two non-fullerene acceptors（IT-4F and F8IC）with very similar structures are used to manufacture BHJ ternary organic solar cells.The ultra-narrow band gap material F8IC（E_g=1.27 e V）was used as the third component to achieve an excellent PCE of 13.8%.On this basis,we developed a method of manufacturing ternary solar cells by sequential spin-coating of donors and acceptors.The donor and acceptor mixture were spin-coated into a film in turn,and PPHJ ternary OSCs were obtained.The deep X-ray photoelectron spectroscopy（DXPS）test proved that the vertical component distribution of the active layer was reasonably controlled,and finally realized a PCE of 14.2%.At the same time,through a series of in-depth exploration,including contact angle test（WCA）,atomic force microscope test（AFM）,transmission electron microscope test（TEM）,differential scanning calorimetry（DSC）and grazing incidence X-ray diffraction（GIXRD）,etc.The results proved that the state of the alloyed acceptor is likely to be formed between the two acceptors.This work shows that non-fullerene alloyed acceptors may have great potential for realizing high-efficiency PPHJ ternary organic solar cells.In addition,it was found in research work that the PM6:F8IC binary system can still achieve a maximum short-circuit current(J_sc)of 23.0 m A/cm² and a PCE of 12.2%under the condition of close to zero HOMO energy level driving force.Since the energy level can be continuously adjusted in organic semiconductor alloyed acceptor materials based on F8IC:IT-4F and F8IC:Y6,the third component of the alloy can be used as an energy level regulator.Reasonable regulation of the energy level driving force between donor and acceptors increases the hole transport rate of the active layers,therefore,the performance and PCE of the ternary device were significantly improved and both achieve a high PCE of 13.8%,J_sc increased to 24.4 and 25.2 m A/cm²,respectively.In addition,the PPHJ devices based on alloyed acceptor prepared by sequential spin-coating method further improved the photovoltaic performance.This work puts forward the concept of an energy level regulator and proves that the ternary alloy strategy has unique advantages and huge research prospects in continuously adjusting the driving force of the energy level,which can provide a new idea for deepening the understanding of the charge transfer state of OSCs.