Vertical Stratification Engineering For Organic Bulk-Heterojunction Devices

In recent years,the photoelectric conversion efficiency（PCE）of single junction organic bulk-heterojunction（BHJ）photovoltaics have already reached to 14%.The evolution of organic solar cells（OPVs）performance was principally attributed to novel donor and acceptor combination.Through broadening the absorption spectra and reducing the energy loss of donor and acceptor molecules,the short circuit current and open voltage can be improved.However,the internal quantum efficiency was generally lower than 90%and external quantum efficiency was lower than 80%,which obstructed the further enhancement in photocurrent.Meanwhile,the morphology of active layer closely correlates with the exciton generation,transmission and dissociation.Thus,the external quantum efficiency and final device performance can be manipulated through morphology optimization.Among the multitude of fundamental considerations in film fabrication,vertical stratification,which originates from spontaneous phase segregation of the donor and acceptor in the vertical dimension during the film drying process,is determined by both thermodynamic and kinetic parameters.The vertical stratification will directly affect the ultimate PCE of devices.Here,we altered the vertical stratification of active layer through adjusting the surface energy of interfacial materials and cooperating with sequential spin-coating method.Firstly,we inquired the effect of ionic liquids on the conductivity and work function of PEDOT:PSS film,and explored the way to obtain high qualified interfacial materials.The surface work function of modified PEDOT:PSS film dramatically declined with the increase of ionic liquids doping density.Meanwhile,it was found that high in-plane conductivity of interlayer can reduce the shunt resistance of devices,which in turn enlarged the effective area of device and caused errors in efficiency calculation.Subsequently,a series of chlorinated or fluorine-substituted aromatic self-assembled small molecule materials based on carboxyl and sulfonic acid groups were introduced to replace the traditional PEDOT:PSS hole transport layer to directly modify the ITO electrode.Through X-ray photoelectron spectroscopy,it was found that the C-Cl bond on the small molecules containing chlorine substituents spontaneously breaks and is grafted onto the ITO substrate to form M-Cl bonds,thereby synergistically producing triple-dipole interactions with the self-assembled molecules.The work function of the electrode was improved and finally applied to the organic solar cell device based on PTB7-Th:PC₇₁BM BHJ film,achieving a photoelectric conversion efficiency of more than 9%.Furthermore,by using self-assembled small molecules to regulate the surface energy of the interface layer of the organic solar cell,active layer with different vertical phase separation structures was generated.We establish a connection between the material surface energy,absorption and vertical stratification which can then be linked to photovoltaic conversion characteristics.Through assessing the performance of temporary,artificial vertically stratified layers created by the sequential casting of the individual components to form a multilayered structure,optimal vertical stratification can be achieved.Under the manipulation of vertical stratification of non-fullerene acceptor-based pseudo planar heterojunction device,the EQE,photocurrent and thus the efficiency of traditional bilayer device can be further enhanced.