Film Preparation

All films were prepared on two different working electrodes (WE), on indium-doped tin oxide (ITO) substrates, or on home-built interdigitated gold electrodes (IDEs). The IDE is a glass substrate containing four different gold working electrodes prepared by optical lithography. The four working electrodes provide four different sensitivities in the measurements. See Figure S1 in the Supporting Information for more details.

ZnO QD films were drop-casted in air and annealed for 1 h at 60 °C.

CdSe and CdSe/CdS films were made by the layer-by-layer dip-coating procedure in a nitrogen-filled glovebox. The substrate was immersed in a QD dispersion for 30 s and thereafter in a solution containing 0.2 M 1,4-butanedithiol cross-linking ligands in methanol for 30 s; ultimately, it was dipped in methanol to rinse excess ligands.

For P3DT film preparation, 10 mg of P3DT was dissolved in 1 mL of 1,2-dichlorobenzene. The substrate was coated by spin-coating for 60 s at 3000 rpm with a ramp of 1000 rpm/s.

For PCBM films, 50 μL of 3 wt % PCBM chlorobenzene solution was spin-coated for 45 s at 3000 rpm with a ramp of 1000 rpm/s. The substrate was placed on a hot plate at 100 °C for 10 min to ensure the evaporation of chlorobenzene.³⁵

C₆₀ films were prepared by a physical vapor deposition technique using an AJA ATC Orion evaporator at high vacuum (≤1 × 10^–6 mbar). The C₆₀ powder was placed in a crucible and heated to sublimation temperature. The temperature was kept fixed during the evaporation.³⁶