Spanish researchers have developed an intermediate band (IB) solar cell using gallium phosphide and titanium, which has the potential to achieve an energy conversion efficiency of up to 60%.
Yeah, like everything the challenge is to get from the Lab to production. Perovskite solar cells, another type of solar cells that show great theoretical promise, have issues with long-term stability. Solar cells need to survive in tough conditions for many years to be useful. Here I would also wonder about the relative scarcity of gallium being a limiting factor.