Photovoltaics or photovoltaic systems

It is correct because the meaning is clear, unambiguous, and concise.

There are already long-established precedents in scientific usage: mechanics, electrostatics, hydraulics, kinetics ... a long list of nouns derived from adjectives, in each case encompassing a large body of knowledge or practice within one word. It is neither necessary nor helpful to replace such terms by noun phrases such as “mechanical systems” or “photovoltaic cells” unless the further discussion of the matter justifies it.