Organic Solar Cells Based On Solution Processed Graphene

In recent years, due to the energy crisis on our planet, organic solar cells （OSCs）with high power conversion efficiency and low cost have been extensively studied.During the past decade, OSCs develop rapidly with power conversion efficiencyapproaching9%. But there still remain following challenges:First, most of the OSCs used toxic organic solvents, such as toluene, chloroformand chlorobenzene etc. during film-forming process. And the majority of OSCsgenerally utilized fullerene derivatives, as the electron accepting component in theactive layer. The required purity of these fullerene derivatives significantly increasesthe number of purification steps, and therefore, the amount of generated waste, whichmakes the fabrication process complicated and thus the material expensive. This is inconflict with the prospect of developing clean and environmentally friendly energysource at low cost.Secondly, most of the OSCs employ PEDOT: PSS or MoO₃to be the electrodemodification layer, which can facilitate the hole collection within the device. However,PEDOT: PSS is usually deposited from highly acidic aqueous suspensions that areknown to corrode ITO at elevated temperatures and can also introduce water into theactive layer, degrading the device performance and stability. And the mismatchbetween the work function of ITO and MoO₃will result in an energy barrier for holecollection, which will limit the short circuit current density of the device.According to the challenges mentioned above, we are trying to find ways to solvethe problem in this thesis.1. Aqueous-dispersible graphene （a-dG） sheets were fabricated by reducing thegraphite oxide （GO） nanosheets in water in the presence of poly[3-（potassium-6-hexanoate） thiophene-2,5-diyl]（P3KT） through noncovalentfunctionalization.“Green” polymer solar cells with the structure of ITO/PEDOT: PSS/P3KT: a-dG/ZnO/Al, where P3KT acts as donor, aqueous-dispersiblenoncovalent functionalized graphene sheets as acceptor have been fabricatedusing water as the solvent for the active layer. The short-circuit current （Jsc）, opencircuit voltage （Voc） and power conversion efficiency （PCE） of the devices withP3KT: a-dG composite film as the active layer increased compared with those ofthe devices with P3KT pure film as the active layer, which indicated that a-dG canbe used as the electron acceptor in polymer solar cells. Our work suggests apotential application of environmental friendly green polymer solar cell at lowcost.2. We introduce a layer of solution processed graphene between ITO electrode andPEDOT: PSS or MoO₃to avoid the corrosion and at the same time increase thestability of the device. Because of the high conductivity of graphene, the electricalproperties of the interfaces improved significantly. Combined with the energylevel alignment near the electrode, the collection of holes became more facilitated.Moreover, graphene, originated from graphite powder, could be solutionprocessed, which demonstrate a promising way for simple and low-cost electrodemodification.