Anticorrelation between Local Photoluminescence and Photocurrent Suggests Variability in Contact to Active Layer in Perovskite Solar Cells

We use high-resolution, spatially resolved, laser beam induced current, confocal photoluminescence, and photoconductive atomic force microscopy (pcAFM) measurements to correlate local solar cell performance with spatially heterogeneous local material properties in methylammonium lead triiodide (CH3NH3PbI3) perovskite solar cells. We find that, for this material and device architecture, the photocurrent heterogeneity measured via pcAFM on devices missing a top selective contact with traditional Au-coated tips is significantly larger than the photocurrent heterogeneity observed in full devices with both electron- and hole-selective extraction layers, indicating that extraction barriers at the Au/perovskite interface are ameliorated by deposition of the organic charge extraction layer. Nevertheless, in completed, efficient device structures (PCE ≈ 16%) with state-of-the-art nickel oxide and [6,6]-phenyl-C61-butyric acid (PCBM) methyl ester contacts, we observe that the local photoluminescence (PL) is weakly anticorrelated with local photocurrent at both short-circuit and open-circuit conditions. We determine that the contact materials are fairly homogeneous; thus the heterogeneity stems from the perovskite itself. We suggest a cause for the anticorrelation as being related to local carrier extraction heterogeneity. However, we find that the contacts are still the dominating source of losses in these devices, which minimizes the impact of the material heterogeneity on device performance at present. These results suggest that further steps to prevent recombination losses at the interfaces are needed to help perovskite-based cells approach theoretical efficiency limits; only at this point will material heterogeneity become crucial.