SPV signals are microvolt-to-volt level surface potentials measured through a capacitive coupling. That makes them exquisitely sensitive to sample preparation, and explains why identical materials measured in different labs sometimes disagree. Preparation discipline is half the measurement.
Substrate: conductive and uniform (FTO is standard), cleaned before deposition. The film must be uniform in thickness and coverage, pinholes short the measurement through the substrate, and roughness variations shift the coupling geometry. Define the active area and mask everything else, the SPV signal averages over whatever the electrode sees.
Powders are pressed or spread into uniform layers of controlled thickness on the measurement stage. Particle size distribution and packing density shift both contact and capacitance; prepare the same way every time, and document pressing pressure or spreading protocol. Loose, uneven layers produce signals that vary with every repositioning.
Moisture and adsorbed oxygen alter surface states, changing the SPV response itself. Prepare and measure under stated conditions, and for sensitive materials use controlled atmosphere or vacuum. Two "identical" samples measured at different humidities are not identical samples.
Repeat-measure the same spot, then different spots on the same sample: spot-to-spot spread reveals preparation quality. Only when the sample reproduces itself does the data deserve interpretation. If it does not, fix the preparation, not the interpretation.
See Perfectlight's surface photovoltage systems for measurement guidance and instrument maintenance resources.
