Design,Synthesis,And Optoelectronic Properties Of Hydrophilic Cathode Interface Polymers And Small-Molecule Donors

Energy crisis is a big problem that threatens the human life and society. Seeking new, pollution-free, and renewable energies has become an important research areafor scientists. Among various technologies for renewable energies,organic/polymer photovoltaic cells(OPVs) have been regarded as a good candidate because of the unique advantages, including mechanical flexibility, light weight, solution-processingtechniques for large area, and potentially low cost. Currently, developments of new efficient donor and acceptor materials and optimization of device fabrication technology are the effective ways to improve power conversion efficiency(PCE) of OPVs.In this thesis, two main parts were included. The first part is the synthesis and development of new hydrophilic conjugated polymers for cathode modifications in inverted OPVs. The second part includes the development of new small-molecule donors and their device performances in OPVs.In chapter 2,two new hydrophilic conjugated polymers, PmP-NOH and PmP36F-NOH, with polar diethanolamine on the side chains and poly(meta-phenylene) and poly(metaphenylene-alt-3,6-fluorene) as the main chain, respectively, are successfully synthesized. The films of PmP-NOH and PmP36F-NOH show absorption edges below 350 nm. Theiroptical bandgaps are the largest ones so far reported for hydrophilicconjugated polymers. The polymers possess deep-lying HOMO levelsof about –6.1 eV. Inserting PmP-NOH andPmP36F-NOH as a cathode interlayer in inverted OPVs with a PTB7: PC71 BM blend as the active layer, high PCEs of 8.58% and 8.33%, respectively, were achieved.In chapter 3,zwitterionic conjugated polyelectrolyte PFNPO with betaine-type phosphonatewas developed, which possesses similar optical and electrochemistryproperties to PFN. The introduction of the zwitterionic groups not only endowed the resulting polymers with excellent solubility in polar solvents but also suppressed the movement of ions. It was found that PFNPO could significantlydecrease the work function of ITOand consequently facilitate the electron collection at the ITO electrode. A high PCE of 8.04% was achieved when using PFNPO as the interface layer in invented OPVs based on PTB7:PC71BM active layer.In chapter 4, four solution-processed small molecules with ethynylene bridges were developed. The introduction of the ethynylenebridges can bring several possible benefits:(1) for a chemistry view, the four small molecules were prepared via Sonogashira coupling reactions, in which toxic stannyl intermediates or potential hazards lithiation reactions could be avoided;(2)weak electron-withdrawing nature due to the sp hybridization can lower the highest occupied molecular orbital(HOMO) energy levels, thus enhancing the Voc;(3) the ethynylene bridgesmay resultin planar conformations due to its cylinder-like π-electron density, thus promotingthe intermolecular π-π stacking andfacilitating the intermolecular charge transport.A high Voc of 0.98 V was achieved inOPVs with N2200 as a polymer acceptor and RDN-E-BDTor 2CN-RDN-E-BDT as a donor. For DPP-E-BDT andDPP-E-BDT-T, high Vocvalues of 0.9V were obtained. The results demonstrate that introducing electron-withdrawing ethynylene bridges in some small-molecule donors would be an effectivemethod to elevate Voc. Moreover, highPCEs of4.19% and 7.12% were achieved forDPP-E-BDT and DPP-E-BDT-T, respectively, and the efficiencies were obviously higher than those of DPP-BDT donors without the ethynylene bridges.In chapter 5, two small-molecule donors, T-DPP-BDT and RDN-T-DPP-BDT, with elongated conjugation skeletons were successfullysynthesized. The resulting molecules show obviously red-shifted absorptions. A PCE of 4.46% was achieved when using T-DPP-BDT: PC71 BM as the active layer.