The Morphology And Vertical Phase Of Active Layer In Continuous Spin-coating Bilayer Planar Heterojunction Organic Solar Cells

The conventional bulk heterojunction(BHJ)structure is widely used for fabricating high performance organic solar cells(OSCs)due to the nanometer-scale phase separation of donor/acceptor component.In recent years,the device efficiency of single-BHJ OSCs based on organic donor-acceptor materials has exceeded 18%.On the one hand,the short-circuit current as well as open-circuit voltage both can be improved by synthesizing new donor-acceptor materials,broadening its absorption spectrum and reducing its energy loss.On the other hand,the generation,diffusion and separation of excitons are closely related to the morphology of the active layer,and the performance of the device can be adjusted by optimizing the morphology of the active layer.The elaborate control of the BHJ morphology is difficult to be carried out because the morphology evolution is such a complicated process.The compatibility requirement of materials in same solvent restricts the structural diversity of molecules to some extent.Meanwhile,the nanoscopic interpenetrating donor/acceptor domains reduce the crystallinity of themselves.The bilayer planar heterojunction,by contrast,possesses of complementary advantages that can be an alternative candidate to achieve the device fabrication and produce different vertical stratification in heterojunction films.But the flat contact area limits the charge separation and transmission efficiency.Here the sequential solution processed approach was used to facilitate material diffusion in layers,to prepare a structure with a pseudo-bilayer planar heterojunction(PPHJ)OSCs adjusted the morphology and vertical phase separation structure of the active layer.We first carefully study the morphology of the non-fullerene based PPHJ device in three dimensions.An enhanced polymer crystallinity and vertical rearrangement of donor and acceptor were observed,which enable the pseudo-bilayer devices to equip with a comparable spectral response to BHJ devices.We demonstrate that a freak device architecture(ITO/ZnO/PBDB-T/ITIC/MoO3/Ag)with a PCE of 7%can be obtained from a larger molecular weight of PBDB-T without using extra additives.The results proposed that with appropriate molecular design and vertical phase separation optimization,the performance of solution processed PPHJ based OSCs can be further improved.Subsequently,we selected a strong crystalline donor material PffBT4T-20D,matched with the fullerene acceptor PC71BM,and continued to use solution processing and continuous spin coating to prepare the device structure of ITO/PEDOT:PSS/PffBT4T-20D/PC71BM/PDINO/Ag,and achieve a device efficiency of up to 9.14%.Through the control of the film thickness,a 120 nm double-layer quasi-plane heterojunction film is obtained,which can ensure the effective separation and diffusion of excitons.Without the use of orthogonal solvents,we ensure that the thickness of the donor is not lost,and under the action of the same solvent,the donor film and the acceptor film have better contact,thereby increasing the donor/acceptor interface,and higher efficiency has been achieved.