We appreciate the opportunity to answer questions about tip electrodes and their effects on measured current, and the validity of the thermalization length l 0 in estimating efficiency. Kirk and Cardwell challenge our interpretation by proposing 1 that the carriers might be collected within the whole slab, rather than just over the thermalization length, based on our findings that 25 nm and 5 μm radii tips yielded similar currents. We respond with the following three arguments.

First, we emphasize that our experimental result involving an array of 24 tips supports our claims. Measurements of power conversion efficiency were performed both using individual nanoscale tips and a hexagonal array of 24 equally spaced indium tin oxide nanoscale tip electrodes over a device area of≈ 1× 1 μm 2 connected over a planar area (Fig. 4 and Methods in ref. 2). If the single tip were collecting current over a very large area, then the array of 24 closely spaced tips would collect the same total current as the single tip. However, we find that the multi-tip array collects several times as much current per unit illumination, invalidating this hypothesis and confirming that the optimal distance between tip-electrode neighbours is close to l 0.