NiO nanoparticles in P3HT:PCBM Bulk Heterojunction Solar Cells
Abstract:
With the amount of energy reaching the earth as sunlight being so large that only a small fraction would be enough to satisfy the world’s energy consumption, solar cells have the potential of playing an important role in the electricity generation of the future. The high production costs of the currently commercial silicon-based solar cells have stimulated the development of alternative solar cells. Among these, polymer solar cells with their low fabrication cost, mechanical flexibility and light weight are particularly promising. One problem with polymer solar cells is their short life time. To prevent electron leakage from the active layer to the ITO anode, a thin layer of PEDOT:PSS is often deposited between the active layer and the anode to serve as a hole transporting layer (HTL). The problem is that PEDOT:PSS is acidic and can degrade the ITO and thereby reduce the lifetime of the polymer solar cell. P-type NiO, with its high band gap and low electron affinity, is a promising alternative for PEDOT:PSS.
In this thesis work, polymer solar cells are fabricated in which NiO nanoparticles are deposited onto the ITO surface by spincoating as HTL. For comparison, polymer solar cells with PEDOT:PSS as HTL are fabricated. Polymer solar cells with NiO nanoparticles dispersed inside the active layer are also fabricated.
AFM is used to see how the ITO surface structure is affected when spincoated with the NiO nanoparticles. The NiO nanoparticles are characterized by SEM, TEM, XPS, absorption spectroscopy and their work function is measured by a Kelvin Probe.
The solar cells with NiO as HTL worked but did not perform as well as the solar cells with PEDOT:PSS as HTL. AFM studies showed that after spincoating with the NiO nanoparticles, the ITO surface is covered and as smooth as when covered with PEDOT:PSS. The solar cells with NiO nanoparticles dispersed in the active layer displayed similar performance as solar cells without NiO with the exception of one device showing a 30% increase in short circuit current density. However, the solar cells with PEDOT:PSS as HTL displayed a rather large variation in performance so more devices would have to be fabricated to really know if the added NiO was the reason for the increase in JSC. The NiO nanoparticles have a size of 50-100 nm and are polycrystalline. The band gap was determined to be 3.6 eV and the work function 5.0 eV.
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