According to Geoffrey Landis, scientist at NASA's Glenn Research Center, photovoltaics will not work more than 4 AU from the Sun. This is probably because of the minimum voltage necessary for electrical devices:

Science@NASA article

I would say that solar concentrators would be essential beyond 4 AU, which focuses sunlight from a large area on to a smaller photovoltaic panel. A solar concentrator would also be necessary for growing food in space:

Starship Generations Solar Concentrators for Deep Space

Geoffrey Landis regularly posts on Google groups usenet sci.space.tech, but I do not because it takes 9 hours or more for my posts to show up on the board (it sucks). But here's the link:

Sci.Space.Tech Usenet