Photovoltaic devices, or solar cells, are a means of generating electricity from sunlight in an environmentally friendly manner without emissions. Among the various types of solar cells, organic photovoltaics (OPVs) are known as lightweight, transparent, thin and flexible solutions.
OPVs generally consist of a charge donor layer, a charge acceptor layer, and two electrodes for charge extraction. Applications in transparent electronics demand at least one of the electrodes to be transparent, and the most commonly used material for that purpose is indium tin oxide (ITO). Although ITO has excellent electrical and optical properties, it is not very abundant and is brittle, hence incompatible with the emerging technology of flexible electronics.
Graphene is considered as a viable alternative to ITO that is both made from an abundant material, flexible, and equally good or better in regards to the optoelectronic properties. However, integration of graphene into the OPV fabrication process is still in development. Now, as a step along the way to using graphene as a transparent electrode in OPVs, researchers have capped a thin layer of ITO with graphene and used it as an electrode in an organic solar cell. Furthermore the work, recently published in Thin Solid Films, shows that the graphene layer enables tuning the work function of the electrode to better match the Fermi level of the organic donor component, which results in more efficient photovoltaic operation.
Research teams from the Helmholtz Zentrum in Berlin, Germany, and Graphenea in Spain, prepared ITO substrates and transfered high quality CVD graphene on top of them. The transfer was performed with a standard recipe that involves a polymer transfer support which is later removed in acetone. The study shows that some polymer residue is inevitable on the final device, although cleaning is much more efficient on graphene than on bare ITO.
Image: Graphene-enhanced organic photovoltaic device
In order to produce a working OPV device, the ITO/graphene heterostructure is used as a growth substrate for a hole transport layer (HTL), onto which a photoactive material is deposited (ZnPc:C60). The hole transport layer plays a crucial role in device performance optimization by tuning the work function of the graphene to match the Fermi level in ZnPc:C60. The team studied several different metal-based HTLs and found that MoOx does the job best for this type of OPV device, raising the work function to 5.2 eV, which eliminates a potential barrier between the HTL and the ZnPc donor component.
Photovoltaic measurements showed a strong current response to light, with a power conversion efficiency (PCE) of 1.48%. It is demonstrated that work-function tuning with MoOx significantly raises the PCE, making the solar cell more efficient. Apart from enhancing the functionality of these OPV devices, graphene is expected to provide long-term chemical stability of ITO, preventing degradation and making the devices more technologically usable.