Protected silver mirror

An Ocean Optics reflection probe with 2 mm focal length was vertically aligned to the LLI. The white light from a tungsten halogen lamp (Ocean Optics) was guided through an optical fibre bundle to the probe and reflected at the LLI back to the probe where a multimode fibre collected the light. The reflected light then propagated to an Andor 163 spectrograph fitted with an iDus charge-coupled device. All the reference for the reflectance measurements were taken from a protected silver mirror (Thorlabs).

Absorption and EQE were measured using a home-built optical setup with a long working distance objective (50×; NA, 0.55). A supercontinuum laser (Fianium) was coupled to a monochromator to produce a tunable, monochromatic light source. A chopper and lock-in detection were used for all measurements. For absorption, the sample reflectance was measured using a National Institute of Standards and Technology (NIST)–calibrated photodetector (Newport 818-ST2-UV/DB) with a beamsplitter. A protected silver mirror (Thorlabs) was used to calibrate the reflectance based on its reported reflectance curve, and a dark background was subtracted from both measurements. For EQE, the current generated by the sample was probed using MiBots and compared with the current collected by the NIST-calibrated photodetector when placed at the sample position, corrected by the photodetector’s responsivity. Absorption and EQE measurements were both corrected by a shading factor of 1.39 that corrects for the shading of the MiBot tips, which was calculated by comparing absorption with and without the tips in place and averaging over the spectral range 450 to 650 nm.