The mechanism of generation of high open-circuit photovoltages (V(oc)s) of 0.62-0.63 V for n-Si (similar to 1 Omega cm) electrodes modified with colloidal Pt particles (4 nm in diameter) is investigated by measurements of minority-carrier (hole) diffusion length (L(p)) and temperature dependence of V-oc Langmuir-Blodgett (LB) layers of colloidal Pt particles are used to control the Pt density on n-Si. The L(p) value is determined to be 200 mu m, irrespective of whether n-Si is modified with Pt or not. The temperature dependences of V(oc)s at 203-298 K have been explained well by our previously proposed model. It is shown that heat treatments of the Pt-modified n-Si electrodes increase the area and the width of the direct Pt-Si contacts and thus decrease V-oc, but minority-carrier controlled (ideal) solar cells are obtained if the electrodes are prepared under appropriate conditions.