Cooperative Assembly Of Donor/Acceptor And Surface-assembly Induced Orientation For Controlling The Interface Morphology And Improving Photovoltaic Performance Of Polymer Solar Cells

Due to the advantage of low cost, light weight, the adjustability and flexibility ofdevice structure, the potential for roll-to-roll large area production, Organic polymersolar cells nowadays attract the eye of more and more researchers. Narrow band gappolymers, whose absorbance range, solubility and film-forming can be controlled bythe improved structure, have become the most efficient kind of organic solar cells.Design and optimization of device structure can also achieve the goal of improvingthe device efficiency and stability. Organic solar cells cannot be commercializedmainly because the mediocre photovoltaic efficiency and stability. This article isabout the two problems, we hope to improve the photoelectric conversion efficiencyand stability through organic methods.On one hand，Two ter(ethylene oxide)(TEO) functionalized copolymer donorand fullerene acceptor, namely poly{4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-alt-4,6-thieno[3,4-b]thiophene-2-carboxylic acid2-[2-(2-methoxy-ethoxy)-ethoxy]-ethyl ester}(PBDTT-TT-TEO) and [6,6]-phenyl C61butyric acid2-[2-(2-methoxy-ethoxy)-ethoxy]-ethyl ester (PCB-TEO), respectively,are explored to manipulate the self-assembly nanoscale morphology and enhancestability of photoactive layer in polymer solar cells. poly{4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b’]dithiophene-alt-4,6-thieno[3,4-b] thiophen-2-yl-2-ethylhexan-1-one}(PBDTT-TT) is also synthesized for the comparison. Comparedwith the copolymer PBDTT-TT, TEO side chains induce PBDTT-TT-TEO with moreordered molecular packing, leading to the PCE of device improved from3.6%forPBDTT-TT:PCBM to4.1%for PBDTT-TT-TEO:PCBM. Directly blendingPBDTT-TT-TEO with PCB-TEO as the active layer does not afford high performanceof device due to the discontinuous morphology of blend film caused by the poorsolubility of the PCB-TEO. However, precisely controlling the loadings of PCB-TEOin PBDTT-TT-TEO:PCBM blend greatly promotes the PCE of device. Because TEOmodified fullerene can serve as an effective compatibilizer to well manipulate themiscibility between the polymer donor and acceptor and achieve favorable heterojunction morphology by cooperative assembly effect. The device with5%loading of PCB-TEO achieves the highest PCE of4.8%, with a Jscof12.18mA/cm2,a Vocof0.722V and a FF of55.0%, approximately33%improvement in PCE overPBDTT-TT:PCBM device. In addition, PCB-TEO compatibilizer located at theinterface of donor and acceptor strengths the interaction of PBDTT-TT-TEO withPCBM, consequently improving the morphological and device stability.Compared with the normal-structure polymer solar cells, inverted devices havebetter stability. We have designed a new type of small molecule4’-{(7-Octenyl)-oxyl}-[1,1’-biphenyl]-4-ol (LB-V)containing a double bond at the end of the liquidcrystal to modify the electronic transport layer (ETL) ZnO, and to improve theefficiency and stability of the device. Using ZnO as ETL directly, the device based onPBDTT-TT-TEO: PCBM can get a3.5%PCE; the surface of ZnO has beenmodified through hydrogen bonding interaction and the PCE can reach3.8%byintroducing the small molecule; after annealing and ultraviolet (UV) treatment, themore ordered arrangement can be acquired and the morphology of ZnO has beenmore stable, achieving the optimal PCE of4.2%. The stability of the buffer layermorphology and the overall stability of the device are improved with annealing andultraviolet treatment as well.