Disorder-engineered inorganic nanocrystals set a new efficiency record for ultrathin solar cells

• Presented over roof covering tops and also in solar ranches, silicon-based solar cells are, up until now, among the most efficient systems in generating electrical power from sunlight, however their fabrication can be pricey and power demanding, in addition to being heavy and also large.

The different remedy of lower-cost thin film solar cells likewise brings the caution of being mostly made up of poisonous elements such as lead or cadmium, or consisting of scarce elements such as indium or tellurium.

In the look for new technologies for thin photovoltaic systems, solar cells based on AgBiS2 nanocrystals have actually emerged as a star gamer in the video game, containing safe, earth-abundant elements, produced in ambient problems at low temperatures and also with inexpensive solution-processing techniques. It can be incorporated in ultrathin solar cells and has actually proven to be really steady, preventing deterioration of the cell over extended periods of time.

Back in 2016, research study executed by ICREA Prof. at ICFO Gerasimos Konstantatos produced a semiconductor absorber 35nm thick solar cell based on AgBiS2 nanocrystals, which were synthesized at extremely low temperature levels (100ºC) (an order of magnitude lower than the ones needed for silicon based solar cells) as well as crafted at the nanoscale, through a layer-by-layer deposition procedure, to accomplish an efficiency in the order of ~ 6%. Although a promising environment-friendly option to silicon, these cells were still not with the ability of accomplishing engaging performance pertinent for commercialization.

Thus, numerous research studies looked into methods to enhance their performance and also discovered that the ideal density of these semiconductor absorbers is very closely linked to the absorption coefficients, hence the goal would certainly be to find an ultrathin solar cell with the ability of having a high absorption efficiency, quantum efficiency and ultimate efficiency while lowering price, weight and production. Yet, while aiming for an ultra-thin layered cell, the problem of managing light-trapping frameworks would certainly include expense as well as intricacy to the issue, since the thinner the structure, the extra intricate it becomes to absorb energy.

To overcome this obstacle, 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 also Seán Kavanagh (UCL & Imperial), led by ICREA Prof. at ICFO Gerasimos Konstantatos, have made a significant leap onward as well as accomplished a groundbreaking outcome. Published in Nature Photonics, their research reports on an entirely new approach towards the construction of these solar cells based on AgBiS2 that allows absorption coefficients higher than any other photovoltaic material used to date.

Cation disorder

In their research, the scientists cleverly engineered the layer of nanocrystals in the cell with an unconventional technique called cation disorder design. To do this, they took the AgBiS2 nanocrystals as well as by utilizing a moderate annealing procedure, they had the ability to tune the atomic placements of the cations within the lattice to really force a cation inter-site exchange and also accomplish a homogenous cation circulation. By using different annealing temperatures as well as attaining various cation circulations in the crystalline plan, they had the ability to show that this semiconducting product exhibits an absorption coefficient 5-10 times greater than any other material presently utilized in photovoltaic or pv modern technology as well as, much more so, across a spooky array that cover from the UV (400nm) to the infrared (1000nm). To do this, 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.

To anticipate and validate the hypotheses of the work, the writers implemented Density Functional Theory estimations that sustained the speculative proof. Seán Kavanagh, a co-first author of the research study from UCL and Imperial College, states: "The importance of atomic disorder in emerging inorganic solar cells is currently a hot subject of discussion in the field. Our theoretical examinations of the thermodynamics and optical/ digital effects of cation disorder in AgBiS2 disclosed both the ease of access of cation re-distribution and the strong effect of this on the optoelectronic residential properties. Our calculations revealed that an uniform cation distribution would yield optimum solar cell performance in these disordered products, corroborating the experimental discoveries as a testament of the synergism in between theory as well as 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 typically used transparent conductive oxide substrates, among others. They coated the gadgets with a PTAA (Poly triaryl amine) service as well as upon brightening the gadget under man-made sunlight, they recorded a power conversion effectiveness over of 9% for a device with a complete thickness no more than 100 nm, 10-50 times thinner than current thin film PV technologies and also 1000 times thinner that Silicon PV.

One of the champion devices was sent to an approved Photovoltaic (PV) calibration laboratory in Newport, USA, which accredited a conversion efficiency of 8.85% under AM 1.5 G full sunlight lighting. As the ICFO researcher as well as initial author of the research study, Yongjie Wang, remarks, "While we observed a solid eclipse of our thin films upon moderate annealing due to boosted absorption, it was challenging to produce such thin tools at the start. After realizing control of the procedure as well as optimization of the complete pile consisting of maximizing electron as well as hole transport layers, we finally found an extremely reproducible framework for efficient solar cells with improved security. It is truly interesting to see that 30nm gadget provides such a high short-circuit existing density approximately 27mA/cm2 as well as an efficiency up to9%. "

As ICREA Prof. at ICFO Gerasimos Konstantatos ultimately highlights, "the devices reported in this research set a record amongst low-temperature and solution refined, environmentally friendly inorganic solar cells in terms of security, form element and performance. The engineering of the multinary systems with cation disordered AgBiS2 colloidal nanocrystals has proven to offer an absorption coefficient greater than any other solar material utilized to date, allowing highly reliable very thin absorber solar gadgets. We are delighted with the outcomes and will continue to proceed in this line of research study to exploit their intriguing residential or commercial properties in photovoltaics as well as other optoelectronic devices".