Poly(sodium 4-styrenseulfonate)-modified Monolayer Graphene And Its Application In Organic Photovoltaic Cells

As one of quasi-two-dimensional(2D)materials,graphene is a honeycomb single atom structure consisting of sp2 hybridized monolayer carbon atoms.It has been extensively studied in the field of optoelectronic devices due to its unique optical,electronic and mechanical properties.In this thesis,graphene as a novel transparent electrode is applied into organic photovoltaic device(OPV)to replace common ITO anode.In this thesis,graphene sheets were synthesized with a chemical vapor deposition(CVD)process and then transferred onto glass substrates by empolying polymethyl methacrylate(PMMA)as a supporting layer and then removing copper substrate by wet etching.After that,the as-transferred graphene sheets were applied as anode into poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate]:[6,6]-phenyl-C71-butyric acid methyl ester(PTB7-Th:PC71BM)-based bulk heterojunction OPVs.However,the power conversion efficiency(PCE)values in these OPVs based on graphene electrodes were far lower than the counterparts with ITO electrodes,which was mainly attributed to low work function,high sheet resistance and intrinsic hydrophobic properties of graphene films.In order to improve the performance of the device,monolayer graphene anode was modified with poly(sodium 4-styrenseulfonate)(PSS).The PSS coating on the graphene induced a p-type doping to graphene film accompanied with an increased work function and a decreased contact angle.With an optimal PSS concentration of 5×10-4 wt%,the PCE was significant enhanced from 3.42% in the control OPV without PSS layer to 5.35%,with a corresponding enhancement factor of 56.4%.Further research on transmittivity,energy level,contact angle,Raman spectroscopy,and electrochemical impedance spectroscopy demonstrated the increased work function of graphene and recombination sheet of excitons/carriers significantly improved device’s performances.In order to further improve device performance,a small molecule 1,2,5-thiadiazole-fused 12-ring polyaromatic hydrocarbon(DNNBT-C12)with highly contorted conformation was further introduced into the active layer,forming a ternary blend polymer solar cell.With a doping concentration of 2 wt%,the short-circuit current density,fill factor and PCE were improved from 12.27 mA/cm2,36.4% and 3.42% in the counterparts to 16.15 mA/cm2,47.8% and 6.29%,corresponding to enhancement factors of 31.6%,31.3% and 83.9%.Research on the absorption spectra,grazing incidence wide-angle X-ray scattering,and electrochemical impedance spectroscopy showed that the PCE was majorly increased due to an improved light absorption and a larger recombination resistance in the active layer.