perovskite-on-silicon tandem cell

A single-layer solar cell can only ever convert about a third of the sunlight hitting it, a ceiling set by physics rather than engineering. The reason is that one material absorbs one slice of the spectrum well: photons with too little energy pass through unused, and photons with too much give up their surplus as heat.

A tandem cell answers that by stacking. A perovskite layer sits on top of a conventional silicon one. The perovskite absorbs the higher-energy part of the spectrum, and the light it does not use passes through to the silicon below, which absorbs the lower-energy part. Two materials with different absorption thresholds therefore harvest more of the same sunlight than either could alone, which is how such a device gets past the single-layer limit. The US Department of Energy reports perovskite-on-silicon tandems reaching efficiencies of almost 34 percent.

Perovskite here names a crystal structure rather than one substance, which is part of the appeal: the family can be tuned and can be made in thin layers at low temperature, so a perovskite layer can in principle be added to silicon manufacturing that already exists. The open question is not efficiency but endurance. Perovskites have historically degraded under heat, moisture and light far faster than silicon, which is why a headline efficiency figure and a commercial product remain some distance apart.