Modification of large grainp‐CdTe by Ru is shown to reduce the sub‐band‐gap response and increase minority‐carrier diffusion length from 0.67 to 0.92 &mgr;m. Contact potential difference (CPD) measurements onn‐ andp‐CdTe show shifts in surface Fermi level in opposite directions corresponding to increase in barrier height in each case. The amphoteric nature of Ru on CdTe surfaces depending on conductivity type is thus inferred. The magnitudes of the changes in CPD are approximately equal to the increase of open circuit voltageVocobserved in photoelectro‐chemical cells.