  Non-fullerene acceptors based on perylenediimides (PDIs) have garnered
significant interest as an alternative to fullerene acceptors in organic
photovoltaics (OPVs), but their charge transport phenomena are not well
understood, especially in bulk heterojunctions (BHJs). Here, we investigate
charge transport and current fluctuations by performing correlated
low-frequency noise and impedance spectroscopy measurements on two BHJ OPV
systems, one employing a fullerene acceptor and the other employing a dimeric
PDI acceptor. In the dark, these measurements reveal that PDI-based OPVs have a
greater degree of recombination in comparison to fullerene-based OPVs.
Furthermore, for the first time in organic solar cells, 1/f noise data are fit
to the Kleinpenning model to reveal underlying current fluctuations in
different transport regimes. Under illumination, 1/f noise increases by
approximately four orders of magnitude for the fullerene-based OPVs and three
orders of magnitude for the PDI-based OPVs. An inverse correlation is also
observed between noise spectral density and power conversion efficiency.
Overall, these results show that low-frequency noise spectroscopy is an
effective in-situ diagnostic tool to assess charge transport in emerging
photovoltaic materials, thereby providing quantitative guidance for the design
of next-generation solar cell materials and technologies.
