A photovoltaic (PV) power generation system comprising an array of PV cell modules arranged in strings connected via secondary stage power efficiency optimizers to a central inverter is provided. In at least one of the strings, sunlight receiver assemblies (including the PV cells) of the PV cell modules are provided each with a corresponding primary stage or integrated power efficiency optimizer to adjust the output voltage and current of the PV cell. The PV cell modules can, but need not include optical concentrators.