Researchers use ‘disorder inorganic nanocrystals’ to set new ultrathin solar cell record

16 Feb 2022

Team from ICFO, UCL, and Imperial (London) say that AgBiS2 nanocrystals are emerging as “star players” for low-cost PV.

An international team of researchers from ICFO, Barcelona, Spain, and University College and Imperial College, both in London, have developed a novel disorder-engineering technique for inorganic solar cells that achieves what they call a “record-breaking power conversion efficiency”.

Although conventional silicon-based solar cells have so far proved to be one of the most efficient systems in generating electricity from sunlight, their fabrication can be expensive and energy demanding, aside from the cells being heavy and bulky.

But the alternative solutions of lower-cost thin film solar cells usually creates the problem of being mainly composed of toxic elements such as lead or cadmium, or containing scarce (expensive) elements such as indium or tellurium.

In the search for new technologies for thin photovoltaic systems, solar cells based on AgBiS₂ nanocrystals have emerged as a “star player in the game”, according to the ICFO-London team, who describe them as “consisting of non-toxic, earth-abundant elements, produced in ambient conditions at low temperatures and with low-cost solution-processing techniques.”

AgBiS₂ can be integrated in ultrathin solar cells and has proven to be very stable, avoiding degradation of the cell over long periods of time. In 2016, research carried out by ICREA Prof. at ICFO Gerasimos Konstantatos fabricated a semiconductor absorber 35nm-thick solar cell based on AgBiS₂ nanocrystals, which were synthesized at relatively low temperatures (100°C; an order of magnitude lower than those required for silicon-based solar cells) and engineered at the nanoscale, through a layer-by-layer deposition process, to achieve an efficiency of ~6%.

Although a promising “green” alternative to silicon, these cells were still not capable of achieving the kind of performance necessary for commercialization. Many studies have investigated ways to improve this performance, which have revealed that the optimal thickness of such semiconductor absorbers is closely linked to the absorption coefficients, thus the goal would be discovering an ultrathin solar cell capable of having a high absorption efficiency, quantum efficiency and performance while reducing cost, weight and manufacturing needs.

But, while aiming for an ultra-thin layered cell, the issue of dealing with light-trapping structures would add cost and complexity to the issue, because the thinner the structure, the more complex it becomes to absorb energy.

Cation disorder

To overcome this challenge, ICFO researchers Yongjie Wang, Ignasi Burgues-Ceballos, in collaboration with Prof David Scanlon from University College London, Prof Aron Walsh from Imperial College London and Seán Kavanagh (UCL & Imperial), led by ICREA Prof. at ICFO Gerasimos Konstantatos, have made a considerable leap forward and achieved a groundbreaking result.

Published in Nature Photonics, their study reports on a completely new approach towards the fabrication of these solar cells based on AgBiS₂ that enables absorption coefficients higher than any other photovoltaic material used to date.

The researchers engineered the layer of nanocrystals in the cell with an unconventional approach called cation disorder engineering. They took the AgBiS₂ nanocrystals and by using a mild annealing process, 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.

By applying different annealing temperatures and achieving different cation distributions in the crystalline arrangement, they were able to show that this semiconducting material exhibits an absorption coefficient 5-10 times greater than any other material currently used in photovoltaic technology and, even more so, across a spectral range that cover from the UV (400nm) to the infrared (1000nm) .

A new surface chemistry was needed for this new material in order to preserve the optoelectronic quality of the nanocrystals upon annealing. Thus the authors made use of mercaptopropionic acid as a passivant ligand that preserved the material quality upon annealing.

‘Atomic disorder currently a hot topic’

To predict the hypotheses of the work, the authors implemented Density Functional Theory calculations that supported the experimental evidence. Seán Kavanagh, a co-first author of the study from UCL and Imperial College, commented, “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.

“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.” With this result, they constructed an ultrathin solution-processed solar cell by depositing the AgBiS2 nanocrystals, layer-by-layer, onto ITO/Glass, the most commonly used transparent conductive oxide substrates, among others.

They coated the devices with a PTAA (Poly triaryl amine) solution and upon illuminating the device under artificial sunlight, they recorded a power conversion efficiency in excess of 9% for a device with a total thickness no more than 100 nm, 10-50 times thinner than current thin film PV technologies and 1000 times thinner that Silicon PV.

One of the champion devices was sent to an accredited Photovoltaic (PV) calibration laboratory in Newport, USA, which certified a conversion efficiency of 8.85% under AM 1.5G full sun illumination. As the ICFO researcher and first author of the study, Yongjie Wang, said, “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.”