Research Laboratory news

Aenert. Research Laboratory news
Solar cells and solar panels are commonly used to harness the energy of the sun and convert it to electricity in an environmentally-friendly manner. However, despite their widespread application their photovoltaic energy conversion efficiency is more or less limited to 29.4 percent. To somewhat improve this limitation, they can be coated with additional materials to create “multijunction” solar cells. In these cells, multiple light absorption layers are stacked on top of each other, so that each layer effectively absorbs a specific part of the color spectrum of sunlight, which can strongly enhance the cell efficiency.

Now (2023), a team of researchers at the Fraunhofer Institute for Solar Energy Research ISE and NWO-Institute AMOLF (Amsterdam) have designed a multijunction solar cell reaching an efficiency of 36.1 percent. This is the highest efficiency ever reported for a solar cell based on silicon. The research project was funded through the Fraunhofer ICON program. The Fraunhofer team specialises in the fabrication of ultra-high efficiency solar cells based on silicon and III-V semiconductors such as GaInP or GaAs.

The new cell combines a “silicon TOPCon” solar cell, a newly-developed high-efficiency cell design, with two semiconductor layers consisting of gallium indium phosphide (GaInP) and Gallium Indium Arsenide Phosphide (GaInAsP). The layer stack is also coated with a specially designed metal/polymer nanocoating. The back reflector improves the ability of the cell to trap light inside the solar cell, which made it possible to increase the efficiency for the first time beyond 36 percent.

Image: Schematic layer stack of the III–V//Si triple-junction solar cell design including a double-layer antireflection coating (ARC), a highly doped n-GaAs cap layer below the contacts, a GaInP-rear-heterojunction top cell, a GaInAsP homojunction middle cell, a silicon bottom cell with tunnel-oxide passivating contacts (TOPCon) and a nanostructured diffractive rear-side grating for light path enhancement



Source: Patrick Schygulla1, Ralph Müller1, Oliver Höhn1, Michael Schachtner1, David Chojniak1, Andrea Cordaro², Stefan Tabernig², Benedikt Bläsi1, Albert Polman², Gerald Siefer1, David Lackner1, and Frank Dimroth/ Wafer-bonded two-terminal III-V//Si triple-junction solar cell with power conversion efficiency of 36.1 % at AM1.5g/ Progress in Photovoltaics Research and Applications, November 2021/ DOI:10.1002/pip.3503/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

Multijunction solar cells are seen as a major player in boosting the application of solar cells. In 2023, scientists created an industrially mask and plate process for front metallization of III–V-based solar cells which could replace expensive photolithography. The metal contacts were designed by nickel (Ni) electroplating directly onto the front of solar cell using a precisely structured mask. Through inkjet printing narrow openings ware made into the solar cell’s front side thus making it ready for subsequent electroplating. The width of the resulting Ni contacts was as low as (10.5 ± 0.8) µm with sharp edges and homogenous shape. The 4 cm²-sized champion III–V-on-silicon triple-junction solar cell with mask and plate front metallization was able to reach a certified conversion efficiency η of (31.6 ± 1.1) % (AM1.5 g spectrum).

Image: Photograph of a III–V//Si wafer with inkjet-printed plating resist/mask on the front side. The mask is structured to realize twelve front grids for separate 4 cm²-sized solar cells. These exhibit two busbars at opposing edges and 22 fingers positioned rectangularly in between a pair of busbars. The mask opening width for the fingers varies from 15 µm (two left and two right cells) over 25 µm (four cells in second column from the left) to 35 µm (four cells in third column from the left). Additionally, a microscope image is shown in a green box. It highlights the transition area where a finger opening meets a busbar opening in the mask



Source: Jörg Schube, Oliver Höhn, Patrick Schygulla, Ralph Müller/ Mask and plate: a scalable front metallization with low-cost potential for III–V-based tandem solar cells enabling 31.6 % conversion efficiency/ Scientific Reports 13(1), September 2023/ DOI:10.1038/s41598-023-42407-4/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

Also in 2023, scientists analysed tensile strain relaxation and composition control of As-rich GaAs1−xPx/(100)GaAs heterostructures with the help of double-crystal X-ray diffraction and field emission scanning electron microscopy. They found that thin (80–150 nm) GaAs1−xPx epilayers were partially relaxed through a network of misfit dislocations along the sample in plane directions. They also compared values of the residual lattice strain as a function of epilayer thickness with predictions from the equilibrium (Matthews–Blakeslee) and energy balance models. The epilayers were found to relax at a slower rate than expected based on the equilibrium model, which was ascribed to the existence of an energy barrier to the nucleation of new dislocations. Studying of GaAs1−xPx composition as a function of the V-group precursors ratio in the vapor during growth allowed the researchers to determinate the As/P anion segregation coefficient. P-incorporation into nearly pseudomorphic heterostructures was shown to be kinetically activated, with an activation energy EA = 1.41±0.04 eV over the entire alloy compositional range.

Image: Plan-view FESEM micrographs of Sample C recorded by using the microscope (a) SE and (b) BSE current signals. A short-faceted trench (FT) indicated by the arrow is observed in (a) and better visualized in the magnified micrograph shown in the inset. The same FT is observed as a few-micron long and narrow black segment in (b). White markers in the micrographs represent 4 μm



Source: Paola Prete, Daniele Calabriso, Emiliano Burresi, Leander Tapfer/ Lattice Strain Relaxation and Compositional Control in As-Rich GaAsP/(100)GaAs Heterostructures Grown by MOVPE/ Materials 16(12):4254,  June 2023/ DOI:10.3390/ma16124254/ Open Access This is an Open Access article is distributed under the terms of the Creative Commons Attribution 4.0 International (CC BY 4.0)

The new multijunction solar cell design has many benefits: The multijunction cell is cheaper to fabricate than ultra-high efficiency solar cells or conventional silicon solar cells, which reach efficiencies of up to 27 percent. Its high efficiency is a great benefit for applications where the available space is limited and a large amount of solar power must be generated within a small area. Possible fields of application include solar-powered electric cars, consumer products, and drones, for example. The new light management design is also applicable in other types of solar cells, such as silicon-perovskite multijunction solar cells, for example.

It was a great achievement that the researchers in both teams were able to combine the best processes available to jointly realise a new efficiency record for a silicon-based multijunction solar cell. For this result not only the new back reflector from AMOLF was responsible but also the improved GaInAsP middle cell from Fraunhofer contributed to this outstanding result.

By the Editorial Board