A group of international scientists from ICFO, University College London, and Imperial College London, reported a novel disorder-engineering technique for inorganic solar cells that achieves a record-breaking power conversion efficiency.
Silicon-based solar cells, which can be seen on rooftops and in solar farms, are among the most advanced equipment for obtaining electricity from sunlight. However, their production is costly and energy-intensive, not to mention hefty and large.
The alternate solution of affordable thin-film solar cells comes with the drawback of being mostly made of harmful materials like lead or cadmium or consisting of rare elements like tellurium or indium.
Solar cells based on AgBiS2 nanocrystals have evolved as a star player in the hunt for new technologies for thin photovoltaic systems. It is comprised of non-toxic, earth-abundant materials, synthesized in ambient settings at low temperatures and with affordable solution-processing procedures.
It could be incorporated in ultrathin solar cells and has been demonstrated to be much more resilient, thereby preventing deterioration of the cell over extended periods.
In 2016, ICREA Professor at ICFO Gerasimos Konstantatos developed a semiconductor absorber measuring a thickness of 35 nm and solar cell based on AgBiS2 nanocrystals, which were synthesized at very low temperatures (100 oC) (an order of magnitude below those essential for silicon-based solar cells) and designed at the nanoscale, through a layer-by-layer deposition process.
This helps to obtain effectiveness in the order of approximately 6%. Despite being an efficient and promising replacement to silicon, these cells were unable to achieve attractive functionality that was commercially viable.
As a result, many studies have looked into ways to enhance their efficiency and discovered that the optimal thickness of these semiconductor absorbers is strongly linked to the absorption coefficients.
So, the primary objective would be to develop an ultrathin solar cell with high absorption efficiency, quantum efficiency and ultimate performance while lowering costs, weight and industrial production.
However, despite striving for an ultra-thin layered cell, coping with light-trapping structures would add cost and complexity to the problem, as the thinner the structure gets, the more difficult it is to absorb energy.
ICFO researchers Yongjie Wang and Ignasi Burgues-Ceballos, in collaboration with Professor David Scanlon from University College London, Professor Aron Walsh from Imperial College London, and Seán Kavanagh (UCL and Imperial), headed by ICREA Prof. at ICFO Gerasimos Konstantatos, have made a significant leap forward and accomplished a pioneering output to overcome this challenge.
The work explains an entirely new technique for the production of these solar cells based on AgBiS2, which allows for better absorption coefficients compared to other photovoltaic materials previously used. The study was published in the journal Nature Photonics.
The researchers used an unconventional method termed cation disorder engineering to intelligently create the layer of nanocrystals in the cell. They did this by taking AgBiS2 nanocrystals and tuning the atomic locations of the cations within the lattice using a light annealing technique to force a cation inter-site exchange and establish homogeneous cation dispersion.
The researchers were able to demonstrate that this semiconducting material has an absorption coefficient that is 5–10 times stronger than any other material currently in use 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).
Novel surface chemistry was required for this new material to retain the optoelectronic quality of the nanocrystals upon annealing. As a result, the authors used mercaptopropionic acid as a passivant ligand to retain the quality of materials while it was annealed.
The authors used Density Functional Theory calculations to anticipate and validate the work’s hypotheses, which were supported by experimental evidence.
The importance of atomic disorder in emerging inorganic solar cells is currently a hot topic of discussion in the field. 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.
Seán Kavanagh, Study Co-First Author, University College London
“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,” added Kavanagh.
Consequently, scientists were able to create an ultrathin solution-processed solar cell by layering AgBiS2 nanocrystals over ITO/Glass, one of the most regularly used transparent conductive oxide substrates.
Researchers covered the devices with a PTAA (Poly triaryl amine) solution and measured a power conversion efficiency of more than 9% for a device with a total thickness measuring less than 100 nm, which is 10–50 times thinner compared to the current thin-film PV technologies and 1000 times thinner than Silicon PV.
One of the champion devices was transported to a qualified Photovoltaic (PV) calibration laboratory in Newport, USA, which confirmed 8.85% conversion efficiency under AM 1.5 G complete solar illuminations.
While we noticed a strong darkening of our thin films upon mild annealing due to increased absorption, it was challenging to fabricate such thin devices at the beginning. After grasping control of the process and optimization of the full stack including optimizing electron and whole transport layers, we finally found a highly reproducible structure for efficient solar cells with improved stability.
Yongjie Wang, Study First Author and Researcher, ICFO
“It is really exciting to see that 30 nm device gives such a high short-circuit current density up to 27 mA/cm2 and efficiency up to 9%,” added Wang.
As ICREA Professor at ICFO Gerasimos Konstantatos stated, “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.”
Professor at ICFO Gerasimos Konstantatos added, “The engineering of the multinary systems with cation disordered AgBiS2 colloidal nanocrystals has proven to offer an absorption coefficient higher than any other photovoltaic material used to date, enabling highly efficient extremely thin absorber photovoltaic devices.”
“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,” concluded Konstantatos.
The study received partial funding from the Joan Ribas Araquistain Foundation (FJRA), and funding from European competitive funds, like the European Research Council (ERC), among others.
Wang, Y., et al. (2022) Cation disorder engineering yields AgBiS2nanocrystals with enhanced optical absorption for efficient ultrathin solar cells. Nature Photonics. doi.org/10.1038/s41566-021-00950-4.