Using room-temperature surface photovoltage spectroscopy, we have characterized several 30-layer stacked self-assembled InAs/GaAs quantum dot (QD) structures with different spacer layer (SL) thicknesses. Signals from every relevant portion of the samples, including QDs, wetting layer, and GaAs barrier have been observed. The strain-induced field for thinner SL is responsible for a significant modification of the band structure, possibly resulting in the appearance of an additional excited state lying higher than the second excited QD state. A peculiar feature below the fundamental transition is tentatively attributed to the optical absorption from uncoupled dots of which the density is significantly lower than that of vertically coupled ones. The spectra show blueshifted features with a decrease of the SL thickness, indicating that the materials intermixing between InAs QDs and GaAs SL are strongly driven by strain.