Pb perovskite solar cells have been causing a storm – reaching 17% efficiency from a standing start only three years ago. They are made from cheap raw materials, are atmospherically stable, and easy to make over large areas.
Although the amount of lead in each cell is tiny, the presence of the toxic metal could be a barrier to commercialisation.
“To our knowledge, this is the first time tin perovskites have been used as the active layer in a solar cell,” Oxford physicist Nakita Noel told Electronics Weekly.
“We wanted to try and replace the Pb with something similar but non-toxic. Tin is safe, cheap and abundant, and has been reported in perovskites before, but not in a solar cell, so we decided to see if it would work,” she said.
The cells work like other solar cells – absorbed photons create electron-hole pairs which have to be separated or they uselessly re-combine.
The absorber is the tin perovskite which, unfortunately, has poor electron diffusion length.
To counter this, the crystals are grown in a sensitising structure – a layer of highly porous (‘mesoporous’ with 10-20nm pores) TiO2 which acts as an electron-selective transporter.
When pairs are created, electrons get injected into the intimately-mingled TiO2, while the more frisky holes can diffuse all the way out to an adjacent hole transport layer, made from an organic compound called ‘spiro-OMeTAD’.
This structure is delivering 6% efficiency, but Noel sees no reason this should not be brought up to the 17% already demonstrated with Pb perovskites, and on to around 20%.
Holding back tin perovskite is that the perovskite structure requires tin (or Pb) to be in the 2+ oxidation state, while tin would prefer to be in its 4+ state – a transition promoted by moisture, and causing the early demise of the prototype tin perovskite cells when exposed to the atmosphere.
“We need the 2+ oxidation state to maintain charge balance, and the 2+ to 4+ transition makes excess positive charge which p-dopes the perovskyte,” said Noel. “This self-doping is detrimental and decreases diffusion length. One of the reasons Pb perovskite works so well is its long diffusion length.”
However, once the oxidation can be controlled, the background concentration of holes will decrease and the tin perovskite diffusion length will increase to that of Pb perovskite, giving equal performance – and there might be even more performance because the tin material has a smaller bandgap.
Can the 2+ to 4+ transition be halted?
“We have some promising avenues,” said Noel.
Noel also works on Pb perovskite cells. How stable are these?
Pb perovskite is not impervious to degeneration but, “Pb is not unstable in its oxidation state, not unstable at all. All of our Pb materials are tested in atmospheric conditions. The problem we have is with tin”, she said.
The work is published in the journal Energy & Environmental Science in a paper ‘Lead-free organic-inorganic tin halide perovskites for photovoltaic applications’.
To Noel’s knowledge, the only other reference to tin perovskite in solar cells was as a hole-transport material, not the active material, in a dye-sensitised cell made by Northwestern University in 2011.