Researchers from an Oxford-Berlin (Helmholtz-Zentrum) collaboration reported that an ultimate efficiency of 30% should be attainable with perovskite-silicon tandem solar cells. They discovered a structurally stable perovskite composition with its band gap tuned to an optimum value of 1.75 eV.
Tandem solar cells work by absorbing the high energy photons (visible light) in a top cell which generates a high voltage, and the lower energy photons (Infra red) in a rear cell, which generates a lower voltage. This increases the theoretical maximum efficiency by about 50% in comparison to a standalone silicon cell. To maximize efficiency, the amount of light absorbed in the top cell has to precisely match the light absorbed in the rear cell. However, the band gap of ~1.6eV of the standard perovskite material is too small to fully exploit the efficiency potential of this technology.
The scientists in this collaboration have shown that an ultimate efficiency of 30% should be attainable with such tandem cells. They designed a tandem cell, in a configuration where the perovskite and the silicon cell are mechanically stacked and contacted separately. The HZB team contributed the silicon cell. The Oxford group systematically varied the chemical composition of the perovskite layer, and with a precise mix of ions, discovered a structurally stable perovskite with its band gap tuned to an optimum value of 1.75 electron volts. At the same time, they increased the chemical and thermal stability of the material significantly.
Currently, cells with these sorts of efficiencies can be achieved, they're just too expensive for widespread deployment; But if a perovskite layer could be added at a relatively low cost, it could shift the economics of installing photovoltaic hardware. The panels would be a bit more expensive, but they would produce a lot more electricity for the same installment costs.