An international team of researchers from ICFO, Imperial College London, and University College London has claimed to have developed a new disorder-engineering technique for inorganic solar cells that achieved a record-breaking power conversion efficiency.

The researchers said silicon-based solar cells are one of the most efficient in generating power from sunlight. However, their fabrication is expensive and energy demanding. The alternative solution of lower-cost thin-film solar cells is also mainly developed from toxic elements such as lead or cadmium or contains scarce elements like indium or tellurium.

On the other hand, solar cells based on AgBiS2 nanocrystals include non-toxic, earth-abundant elements produced in ambient conditions at low temperatures and with low-cost solution-processing techniques. It can be integrated into ultrathin solar cells and proved to be very stable, avoiding cell degradation over long periods. However, these cells were not capable of achieving performance relevant for commercialization.

The researchers said several studies found that the optimal thickness of these semiconductors’ absorbers is closely linked to the absorption coefficients. Therefore, the aim was to find an ultrathin solar cell capable of having a high absorption efficiency, quantum efficiency, and ultimate performance while reducing the weight and costs of solar cells.

To develop an inorganic solar cell, the researchers engineered the layer of nanocrystals in the cell with an unconventional approach called cation (positively charged ion) disorder engineering. They took the AgBiS2 nanocrystals and used a mild annealing process to engineer the layer of nanocrystals in the cell. With this, they were able to tune the atomic positions of the cations within the lattice to force a cation inter-site exchange and achieve a homogenous cation distribution.

They applied different annealing temperatures to achieve different cation distributions in the crystalline arrangement. The researcher discovered that the semiconducting material exhibits an absorption coefficient five-ten times higher than any other material used in photovoltaic technology.

The researchers required new surface chemistry to preserve the optoelectronic quality of the nanocrystals upon annealing. They utilized mercaptopropionic acid as a passivant ligand that preserved the material quality upon annealing.

Seán Kavanagh, a co-first author of the study from UCL and Imperial College, said: “Our theoretical investigations of the thermodynamics and optical/electronic effects of cation disorder in AgBiS2 revealed both the accessibility of cation re-distribution and the strong impact of this on the optoelectronic properties. Our calculations revealed that a homogeneous cation distribution would yield optimal solar cell performance in these disordered materials, corroborating the experimental discoveries as a testament of the synergism between theory and experiment.”

Thereafter, the researchers developed an ultrathin solution-processed solar cell by depositing the AgBiS2 nanocrystals layer-by-layer onto the glass. They coated the devices with poly triarylamine (PTTA) solutions. The power conversion efficiency of new solar cells was more than 9% while illuminating the device under artificial sunlight. The new solar cell has a total thickness of less than 100 nm, 10-50 times thinner than thin-film photovoltaic technologies, and 1,000 times thinner than silicon solar cells.

Earlier, researchers from the Hasselt University Interuniversity Microelectronics Center, VITO, EnergyVille, and PERCISTAND consortium claimed to have achieved 25% efficiency using a thin-film solar cell.