Why haven’t PV/T hybrids taken off?

Solar PV can recover only about 20% of the energy falling on the solar panels.

What if that could be doubled or even better?

That's the promise solar PV/thermal hybrid systems offer. By recovering the heat of the sunlight falling on the solar panels,, and using that heat for useful purposes (typically low temperature heating applications), PV/T hybrids indeed look good as a concept and on paper. They look even better when used with concentrated solar PV (CPV) systems that generated a significant amount of heat.

But they have not taken off the ground, in spite of research being done on these systems for over a decade.

One of the reasons could be because of the non-progress of the CPV technology - it has proved too expensive and even more so with the dramatic reductions in the price of standard solar PV systems.

The other research dimension in the PV/T domain - conversion of the heat recovered from PV systems to power through thermoelectric technologies has proved a non-started as well, but here it could be more because the technology in itself hasn't worked on any scale worth mentioning (here’s one the latest of these efforts - https://bit.ly/3B1EFw8 ).

All in all, PV/T hybrids seem to be consigned to research labs, at least for now.