The Study On Device Engineering Of Non-fullerene Organic Photovoltaic Cells

Organic photovoltaic（OPV）cells based on organic compounds as photoactive layer materials are one of the most promising next-generation photovoltaic technologies in the utilization of clean and renewable energy.Compared with crystalline silicon solar cells,the solution-processability of OPV cells makes it suitable for large-scale production by roll-to-roll method at room temperature.Therefore,it is promising for low-cost and large-area manufacturing on flexible substrates.Thanks to the research in the design of new materials,the optimization of morphology,and the understanding of charge generation mechanisms,the power conversion efficiency（PCE）of OPV cells has exceeded 18%recently.For OPV cells,high efficiency,high stability and low cost are the basis for its commercialization.Although high-performance OPV cells continue to emerge,there are few reports of high stability and low cost.Therefore,it is very important for OPV cells to improve stability and achieve large-area fabrication.Based on these problems for OPV commercialization,including efficiency,cost,and stability,this thesis has carried out research on the comparison of photovoltaic performance with the similar materials,eco-compatible solvents for processing large-area devices,and stability.The details are as follows:1.With the development of non-fullerene acceptors（NFAs）,many studies have shown that NFAs can still achieve higher efficiency at lower energy loss(E_loss)than fullerene-based acceptors.In order to investigate the effect of E_loss on the performance of NFA OPV devices,two NFAs,IE-4F and IE-4Cl,were designed and synthesized based on IEIC.Six devices were prepared by blending them with two polymer donor PBDB-T and PBDB-TF,which have the same molecular backbone.Due to the electron-withdrawing effect of fluorine and chlorine atoms,these five materials have different absorption spectra and energy levels.However,the six blend films have similar morphologies and electron transport properties for their similar molecular structures.Therefore,we studied the photovoltaic performance of these six devices to understand the impact of E_loss on this system.2.In the design of active layer materials for OPV commercialization,it is not only necessary to consider improving the device performance,but important to develop large-area device manufacturing technology processed with eco-compatible solvents.We obtained an NFA,BTP-4F-12,with better photovoltaic performance and processability by modifying the commonly used NFA Y6 with simple alkyl chain modification（i.e.replacing the original 2-ethylhexyl with 2-butyloctyl on its side chain）.Although the alkyl chain of the material is longer,the lamellar stacking was increased after film-forming for its better solubility.Thus,the device showed better charge transfer performance.After blending with the polymer donor PBDB-TF with chloroform as the processing solvent,the efficiency of OPV cell was up to 16.4%.Because of the limited solubility of the polymer donor PBDB-TF,the newly synthesized BTP-4F-12 was blended with the polymer donor PBDB-TF-T1 with good solubility,and all the devices processed by tetrahydrofuran,o-xylene,or 1,2,4-tritoluene achieved high PCEs.Among them,the device processed by tetrahydrofuran showed the comparative PCE value with chloroform as the processing solvent.Furthermore,a device with an active area of1.0 cm² was fabricated by blade-coating,and the efficiency of the device was as high as 14.4%.3.Improving the PCE and stability is a basic requirement for OPV commercialization.With the development of NFAs,there is a lot of literature on high-efficiency devices in recent years.However,there are few high-efficiency devices with excellent stability.In order to improve the stability of the device,we modified Zn O with a cross-linkable fullerene derivative PCBSD.C-PCBSD can not only fill and passivate the defects,but also restrain the self-aggregation of Zn O.In addition,c-PCBSD also prevented the photocatalysis of Zn O on NFAs.When c-PCBSD was used as the third component of the active layer,it can not only increase the absorption of photons,but also fix the morphology to improve the stability of devices.Finally,the stability of the device is significantly improved after the two-step modification.