Mapping Local Photocurrents in Polymer/Fullerene Solar Cells with Photoconductive Atomic Force Microscopy

TLDR

Organic solar cell performance is highly dependent on film morphology, yet directly linking local structures to device output remains difficult. This study demonstrates that photoconductive AFM can map local photocurrents with 20‑nm resolution in MDMO‑PPV/PCBM blend cells prepared from different solvents. Photocurrent maps were acquired under short‑circuit conditions and at various applied biases to reveal spatial variations. The maps show pronounced short‑circuit current differences between topographically identical regions, across domains and larger scales, indicating that morphology control could further enhance device performance.

Abstract

The performance of organic solar cells is highly dependent on film morphology. However, directly correlating local film structures with device performance remains challenging. We demonstrate that photoconductive atomic force microscopy (pcAFM) can be used to map local photocurrents with 20 nm resolution in donor/acceptor blend solar cells of the conjugated polymer poly[2-methoxy-5-(3',7'-dimethyloctyl-oxy)-1,4-phenylene vinylene] (MDMO-PPV) with the fullerene (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) spin-coated from various solvents. We present photocurrent maps under short-circuit conditions (zero applied bias) as well as under various applied voltages. We find significant variation in the short-circuit current between regions that appear identical in AFM topography. These variations occur from one domain to another as well as on larger length scales incorporating multiple domains. These results suggest that the performance of polymer-fullerene blends can still be improved through better control of morphology.