Cathode Interface Modification Of Self-assembled Diblock Conjugated Polyelectrolytes And Hyperbranched Small Molecules For Polymer Solar Cells

Bulk heterojunction?BHJ?polymer cells?PSCs?have attracted enormous attention due to their various advantages,such as low-cost,flexibility and large-scales roll-to-roll fabrication.Recently,the power conversion efficiency?PCE?of PSCs has been boosted to over 13 %.However,several scientific issues and challenges,poor charge carrier separation,transfer,and collection,limited its application in large-scale commercial production.In this case,good interfacial contact and suitable morphologies of the interlayer and active layer are very critical.To solve these problems,an efficient interfacial layer should be designed,which can tune the interfacial barrier and improve the morphology of the active layer simultaneously.Conjugated polyelectrolytes?CPEs?contain a delocalized ? conjugated backbone and functional polar ionic side chain.The CPEs do not intermix with the hydrophobic active layer owing to the orthogonal solubility.Meanwhile,the electrostatic interaction between the interfaces of CPEs and metal electrode could produce an aligned interfacial dipole assembly,which can decrease the work function?WF?of cathode electrodes,facilitate the electron extraction and collection,and enhance the PCE of the device.Because of the immiscibility and different crystallinity of two different blocks backbone,diblock conjugated polymers can self-assemble into ordered nanostructure spontaneously.The diblock CPEs?DBCPEs?could be obtained by combining diblock conjugated polymers with CPEs at the molecular level,which can form ordered nanostructure,decrease the WF and interfacial barrier and realize water/alcohol environment-friendly fabrication.Two novel water/alcohol DBCPEs were synthesized based on fluorene and thiophene?PFEO-b-PTNBr?,and fluorene and carbazole?PFEO-b-PCNBr?with functionalized polar group,respectively.The DBCPEs were used as electron transport layers?ETLs?in PSCs,which not only can tune the WF of ZnO,but also can simultaneously improve the interfacial compatibility.Moreover,the self-assembled PFEO-b-PCNBr and PFEO-b-PTNBr can act as templates to further induce the upper active layer to form nanofiber and wide nanowire morphology,facilitating the electron extraction and transportation.Consequently,compared to the device based on Zn O with a PCE of 3.0 %,devices based on poly?3-hexylthiophene??P3HT?:?6,6?-phenyl-C61 butyric acid methyl ester?PC61BM?with ZnO/PFEO-b-PCNBr and Zn O/PFEO-b-PTNBr as electron transport layers?ETLs?showed improved PCE of 3.6 % and 3.8 %,respectively.Due to the larger interfacial dipole moment and better morphology,Zn O/PFEO-b-PTNBr exhibited better performance than ZnO/PFEO-b-PCNBr.Delightfully,the device based on ZnO/PFEO-b-PTNBr ETL and PBDB-T:ITIC active layer yields a PCE of 10.8 %.Then,the structure of the superior performance CPE PFEO-b-PTNBr was optimized by replacing the side chain terminal quaternary ammonium?PFEO-b-PTNBr?with the larger size imidazolium cationic?PFEO-b-PTImBr?.PFEO-b-PTNBr and PFEO-b-PTImBr were used as ETLs for PSCs,the effects of different terminal cationic groups of PFEO-b-PTNBr and PFEO-b-PTImBr on the WF of ITO and morphology of the upper active layer have been systematically studied.The spontaneous self-assembled DBCPEs not only can create ohmic contact between ITO electrode and the active layer,but also can act as a scolopendra-like template to partially induce a favorable face-on orientation of the active layer.Surprisingly,one n-type self-doping behavior was observed in two DBCPEs based on p-type conjugated backbone by electron paramagnetic resonance?EPR?measurement.Moreover,a more prominent EPR intensity was detected in PFEO-b-PTImBr relative to PFEO-b-PTNBr.Consequently,the performance of devices based on P3HT:PC61BM active layer are dramatically enhanced by inserting DBCPEs as ETLs,showing the PCE of 3.1 % for PFEO-b-PTNBr and 3.5 % for PFEO-b-PTImBr,respectively.Notably,the inverted devices based on PTB7-Th:PC71BM with ZnO/CPEs as ETLs improved the PCE to 9.0 % for PFEO-b-PTNBr and 9.4 % for PFEO-b-PTImBr.Notably,both ZnO/PFEO-b-PTNBr and ZnO/PFEO-b-PTImBr based PTB7-Th:PC71BM devices exhibit superior PCEs than those of the classical published and commonly used PFN?8.4 %?and PFNBr?8.4 %?ETLs based devices.Owing to the intrinsic advantages of high repeatability,easy purification and well defined structures,small molecule electrolytes?SMEs?have attracted tremendous interests.One novel hyperbranched SME PNSO₃ Na was synthesized through a simple one-step reaction and applied as ETL in PSCs.One mole PNSO₃ Na contains seven moles butyl sulfonate sodium.In this case,hyperbranched SMEs PNSO₃ Na organic interfacial layer can improve the interface contact and tune the interfacial energy alignment,giving rise to the enhancement of the PSCs device performance.Compared to the device with bare ITO,the device based on P3HT:PC61BM active layer with PNSO₃ Na ETL enhanced the PCE from 0.8 % to 3.7 %.Moreover,the device using ZnO/PNSO₃ Na as ETL based on PBDB-T:ITIC active layer obtained a PCE of 10.9 %.Two new perylene diimides small molecule ETLs were synthesized with tetraethylenepentamine?PDIPN?and butyl sulfonate sodium tetraethylenepentamine?PDIPNSO₃Na?as branched chain by combining the advantages of SMEs with that of high electron affinity material.Due to the polar branched chain,both PDIPN and PDIPNSO₃ Na ETLs can form interfacial dipole and improve the interfacial contact.Both PDIPN and PDIPNSO₃ Na exhibited EPR signal originating from the polaron because the intrinsic high electron affinity of perylene diimides,indicating the n-type self-doping.Owing to the higher negative charge density of tetraethylenepentamine in PDIPN,a stronger EPR intensity was observed in PDIPN.As a result,the device based on PDIPN ETL showed enhanced photovoltaic performance than that of the device based on PDIPNSO₃ Na.Above all,these water/alcohol soluble DBCPEs and hyperbranched small molecule ETLs can not only improve the interfacial contact between the cathode and the organic active layer,but also produce a better energy alignment in the device,which can optimize PSCs device and provide valuable guidance for the fabrication of printable and large scale PSCs.