It loves the sunlight! 

Our little cicada immediately pipes up when it's in the sun. It turns out we only needed to make one small modification: the bleeder circuit with a 560Ω in parallel with the buzzer did the trick!

Unfortunately, we spent some time chasing our tails by working on it at night in artificial light which was not intended for the solar cells. We spend an evening experimenting with much bigger panels which were supposed to be able to charge in artificial light, but we had no luck.

So we concluded that this was an epic sculpture and we learnt lots.

But the next morning, as soon as the cicada encountered sunlight it sang for us!