Proceedings of nanoGe Fall Meeting 2018 (NFM18)
DOI: https://doi.org/10.29363/nanoge.nfm.2018.188
Publication date: 6th July 2018
Recent research and progress in organic photovoltaic (OPV) repeatedly insist on the importance of the molecular organization of the compounds forming the active bulk-heterojunction (BHJ) blends. The morphology of the blend has been to tremendously affect both the charge transfer at the donor-acceptor interface and the carrier transport to the electrodes. And still, for each material combination, much remains to be understood to fully assess its specific and ultimate morphology. For this purpose, high resolution characterization methods are of primary interest to locally depict tthe photovoltaic process. Conductive Atomic Force Microscopy (C-AFM) and Kelvin Probe Force Microscopy (KPFM) have already proven to be of significant help to yield nanoscale two-dimensional mapping of electrical properties. C-AFM and related PeakForce TUNA emerged as powerful technique to electrically evidence phase separation in blends. An additional key feature lies in local I-V curve providing useful information about the charge transport mechanisms within the materials forming the blends. Quantitative measurements leading to local determination of hole mobility have already been reported. It appears that upon illumination the technique has shown to be sensitive to photocurrent. With photoconductive-AFM (pc-AFM), a dedicated external calibrated module has been recently introduced allowing full quantitative mapping of photovoltaic mechanisms.
In this lecture, we will carefully analyze, as a model sample, BHJ made of poly(3-hexylthiophene) (P3HT) and fullerene derivative (PCBM). While photocurrent is determined at 0 V DC bias, additional parameters including the open-voltage, the fill factor and the resistances can be obtained spanning the DC bias between the probe and the sample back electrode.(KPFM is also used in this work to delineate phase separation and potential variations at interfaces. Upon illumination, photovoltage can also be evidenced. With the additional external calibrated illumination module, mapping of photovoltage in BHJ blends can be obtained, opening the doors of local characterization of charge transfer at donor-acceptor interfaces, where crucial processes are occurring in photovoltaic devices.
In fine, we will also address the photovoltaic properties of other systems such hybrid TiO2 nanopillars : conjugated polymers.
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