Morphology Control Of Active Layer In Small Molecule Solar Cells Based On BTR Electron Donor

In the past several decades,the structure of organic solar cell devices has evolved from the Schottky heterojunction with very low efficiency at the beginning to the bulk heterojunction（BHJ）,the evolution of device structures reflects a deeper understanding of the underlying mechanisms.Most of the materials in the active layer of organic solar cells in current research consist of a mixture of several materials that can provide and receive electrons.When these blends of electron donor and electron acceptor are spin-coated from organic solvent to substrate to form thin films,different sizes of phase separation regions will be formed spontaneously due to the different material properties of the system,which will affect the photovoltaic performance of the device.With the research of organic solar cell molecular materials and the optimization of preparation process,the photovoltaic performance of organic solar cells has been rapidly improved.Today’s most efficient single-cell organic solar cells consist of a polymer that provides electrons and a non-fullerene that receives them,but batch variation and low reproducibility due to the polydispersity of molecular weight of polymer materials have prevented the commercialization of this technology.All-small-molecule organic solar cells are considered to be commercially viable because of the well-defined chemical structure of the donor and acceptor in their active layer.However,the spin-coating of a single solvent usually leads to inappropriate phase separation of BHJ blends and low molecular crystallization order,which limits the efficiency of the device,and the whole small molecule system is no exception.Nanoscale phase separation of active layer is the key factors to achieve the ideal exciton effective dissociation and charge carrier effective transmission,active layer of thin film morphology have a direct impact on the performance of organic photovoltaic solar cells,regulating the activity of layer morphology with small molecules induced good separation between donor and acceptor has not been very good to solve.In order to research and solve the similar problem,based on the device with active layer materials on all small molecules,this paper studied the impact of adjusting the morphology of active layer by thermal annealing,solvent vapor annealing,adding the third component and other methods to improve the photovoltaic performance of organic solar cell devices.The research content of this paper will be described from the following two parts:（1）Single organic solar cells were prepared using nematic liquid crystal small molecule BTR and narrow band-gap small molecule Y6 as donor and acceptor,respectively.The impact of the photovoltaic performance of the system devices with different annealing processes were investigated by thermal annealing,solvent annealing,and thermal annealing combined with solvent annealing.Results show that for the BTR:Y6 system,The impact of device performance with only solvent annealing treatment is not obvious,but we can see the performance of device with thermal annealing process has significantly increased with the thermal annealing treatment for 10 min at 110℃temperatures,and can be optimized to get the best performance of device when the solvent annealing combined with thermal annealing process.The effectiveness of different annealing treatments on the morphology control of the active layer of organic solar cells was further verified.In particular,the continuous annealing process combined with solvent annealing and thermal annealing in this paper can be used as a reference for the morphology control of the active layer of organic solar cells in other systems.（2）We prepare a series of ternary organic solar cell by nematic phase liquid crystal molecules BTR as a donor,narrow band gap of small molecules Y6 and PC₇₁BM fullerene derivatives as acceptor.Here,we added the third component PC₇₁BM after optimization of annealing process based on BTR:Y6,expect to add PC₇₁BM components can widen the spectral absorption and change the energy cascade at the same time,morphology regulation of active layer to improve charge carrier transport,to improve the performance of organic solar cell.The impact of the device structure and performance with PC₇₁BM added was researched.The results showed that the absorption range of the blended film became wider with the addition of PC₇₁BM as the third component,and the energy level cascade was conducive to charge transfer,thus increasing the EQE value.Optimized ternary organic solar cell active layer blend membrane formed with appropriate separation of fiber interpenetrating network structure,and the main performance parameters of the device has a certain degree of ascension optimized ternary organic solar cells,and the photoelectric conversion efficiency（PCE）of optimized ternary solar cell significantly increased to 11.82%compared with binary solar cell.The impact of the addition of the third component on the morphology regulation of the active layer of organic solar cell was further demonstrated.Particularly,the synergistic effect of two different systems of acceptor in this paper is of great significance in the preparation of organic solar cells.