For modules based on crystalline silicon cells, which have ~95% market share, the only rare element they presently use is silver. Silver is a component of screen-printed pastes used to form electrical contacts on cells. Silver can be replaced by base metals such as aluminum, copper, nickel, tin, and combinations thereof. So far most manufacturers find that the simplicity of processes using screen printed silver pastes outweighs the high material cost of silver. But that's an economic trade-off rather than a hard technical limit.
Beware of silver bulls proclaiming that silver consumption will "inevitably" grow alongside PV manufacturing volume. I think that the transition away from silver will be more like a ratchet than a floating level. If silver prices rise enough to spur high volume PV manufacturers to implement more complex metallization schemes using base metals, they're not going to switch back to silver pastes even after silver prices fall again.
Beware of silver bulls proclaiming that silver consumption will "inevitably" grow alongside PV manufacturing volume. I think that the transition away from silver will be more like a ratchet than a floating level. If silver prices rise enough to spur high volume PV manufacturers to implement more complex metallization schemes using base metals, they're not going to switch back to silver pastes even after silver prices fall again.