Vacuum flash helps perovskite solar cells beat 20 per cent efficiency

A team at the École Polytechnqie Fédéral de Lausanne (EPFL) led by organic solar cell pioneer Michael Grätzel has grown perovskite crystals whose efficiency exceeds 20 per cent. Perovskites are crystals with a structure similar to a calcium titanium oxide salt found naturally in Russia; they can be grown under mild conditions without the need for vacuum and high temperatures associated with silicon film manufacture.

Grätzel’s team has developed a method for making a perovskite film that incorporates a low-pressure process they refer to a ‘vacuum flash’. The process starts with a solution of caesium-containing metal halide salts coated onto a glass cell treated so that it conducts electricity. Addition of caesium improves the thermal stability of the perovskite, making it more suitable for commercial application.

Normally, this solution is let to dry so that the salts crystallise out; but this tends to produce an irregular film of crystals with mall grain sizes; these have low photovoltaic efficiencies. The EPFL team has had success with spinning the glass cells while the solution is still wet; this removes some of the excess liquid, giving a flatter crystal film with larger grain sizes. This is accompanied by dripping a liquid known as an ‘anti-solvent’ onto the cell, which encourages crystallisation; unfortunately, the crystals produced tend to be impure, and the anti-solvents are toxic.