Imaging Charge Transfer State Excitations in Polymer/Fullerene Solar Cells with Time-Resolved Electrostatic Force Microscopy.
We demonstrate nanoscale imaging of charge transfer state photoexcitations in polymer/fullerene bulk heterojunction solar cells using time-resolved electrostatic force microscopy (trEFM). We compare local trEFM charging rates and external quantum efficiencies (EQE) for both above-gap and below-gap excitation of the model system poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM). We show that the local trEFM charging rate correlates with device EQE for both above-gap and below-gap photoexcitation, demonstrating that EFM methods have sufficient sensitivity to detect the low EQEs associated with CT state formation, a result that could be useful for probing weak subgap excitations in nanostructured materials such as quantum dot and organometal halide perovskite solar cells. Further, we use trEFM to map spatial variations in EQE arising from subgap CT excitation in organic photovoltaics (OPVs) and find that the local distribution of photocurrent arising from these states is nearly identical to the spatial variation in EQE from above-gap singlet excitation. These results are consistent with recent work showing that both above-gap and below-gap excitation have similar internal quantum efficiency.
Department of Chemistry, University of Washington, Seattle, Washington 98195, United States.
Phillip A
PA
Cox
Department of Chemistry, University of Washington, Seattle, Washington 98195, United States.
Micah S
MS
Glaz
Department of Chemistry, University of Washington, Seattle, Washington 98195, United States.
Jeffrey S
JS
Harrison
Department of Chemistry, University of Washington, Seattle, Washington 98195, United States.
Samuel R
SR
Peurifoy
Department of Chemistry, University of Washington, Seattle, Washington 98195, United States.
David C
DC
Coffey
Department of Chemistry, University of Washington, Seattle, Washington 98195, United States.
David S
DS
Ginger
atomic force microscopy; charge transfer state; photovoltaics; polymer solar cells; time-resolved electrostatic force microscopy