The silicon-based solar cells, generally placed on rooftops and distributed in solar farms, are one of the most efficient systems in generating electricity from sunlight. But their fabrication can be expensive and energy-demanding, in addition to being heavy and bulky. Current ultra-thin alternatives are mainly composed of toxic elements such as lead or cadmium or containing scarce elements such as indium or tellurium.
Now, a team of researchers from ICFO, University College London, and Imperial College London has devised a novel disorder-engineering technique for inorganic solar cells that achieves record-breaking power conversion efficiency.
The researchers have substantially increased the efficiency of a newer type of solar cells based on AgBiS2 nanocrystals. It is composed of non-toxic, earth-abundant elements, produced in ambient conditions at low temperatures and with low-cost solution-processing techniques. The bismuth-based nanocrystals can be integrated into ultrathin solar cells and have proven to be very stable, avoiding degradation of the cell over long periods of time.
The team cleverly engineered the layers of nanocrystals in the cells with an unconventional approach called cation disorder engineering. By using a mild annealing process, they were able to tune the atomic positions of the cations within the lattice to actually force a cation inter-site exchange and achieve a homogenous cation distribution.
The researchers were able to demonstrate that this semiconducting material exhibits an absorption coefficient 5-10 times greater than any other material currently used in photovoltaic technology by using different annealing temperatures and accomplishing various cation distributions in the crystalline arrangement. This is true even across a spectral range spanning from the UV (400 nm) to the infrared (1000 nm).
Using complex computer modeling, the researchers at UCL found that the even spread of silver and bismuth atoms across the material increased how much light the nanocrystals absorbed, allowing more energy to be generated. The team at ICFO fabricated an ultrathin solution-processed solar cell by depositing the AgBiS2 nanocrystals, layer-by-layer, onto ITO/Glass. They recorded the power conversion efficiency of more than 9% for a device with a total thickness of no more than 100 nm, 10-50 times thinner than current thin-film PV technologies, and 1000 times thinner than Silicon PV. While this may not seem enormous, the maximum physically-possible solar cell efficiency is 30%, so this is a major result.
“The results show how our research, looking at the underlying chemistry and physics of materials, can help in the design of high-performance, low-cost devices and support a green economy,” said Ph.D. researcher Seán Kavanagh, co-author of the paper. “These particular solar cells have made huge leaps in efficiency in less than a decade, from 1-2% to 9%. This gives us confidence that further improvements are possible, and the goal is to further improve efficiency, so it is comparable with silicon-based solar cells.”
Researchers say this level of efficiency could make the technology commercially viable, but more improvement is needed to make the technology as efficient as silicon-based solar cells.
“The devices reported in this study set a record among low-temperature and solution-processed, environmentally friendly inorganic solar cells in terms of stability, form factor, and performance,” highlights co-author Professor Gerasimos Konstantatos of ICFO. “We are thrilled with the results and will continue to proceed in this line of study to exploit their intriguing properties in photovoltaics as well as other optoelectronic devices.”