Solar cells containing multijunctions (or subcells) perform better than their single-junction counterparts with power conversion efficiencies of around 44% compared with about 29%. As their name suggests, these devices contain two or more junctions (rather than just one), each of which absorb different wavelengths of light from the Sun. For example, the junctions at the front of the cell can be made of a wider bandgap material that harvests high-energy photons while more abundant lower-energy photons can be collected by a smaller-bandgap material situated at the back of the cell.

Most of these cells contain an efficient transparent intermediate layer sandwiched between the junctions. This layer allows photogenerated electrons and holes from neighbouring junctions to pass without recombining.

Lattice matching

The problem is that further improvements to multijunction cells will be difficult because each junction needs to be lattice matched to its neighbour, something that will be challenging to achieve as the number of junctions increases. What is more, with traditional designs, the current outputs from each of the junctions must also be matched, because of their series electrical interconnections.

Although researchers have tried overcoming these problems using techniques such as physical wafer bonding or thick, insulating organic adhesives, with double-sided, multilayer antireflective coatings, none of these approaches have proved particularly successful. For example, the second technique produces multijunction cells that suffer from interface reflections, poor heat flow characteristics and often unfavourable thermomechanical interface stresses at high levels of incoming sunlight.

High-efficiency full spectrum quadruple-junction solar cells

Now, a team from the University of Illinois at Urbana-Champaign and Semprius has put forward a new way to make extremely high efficiency full spectrum quadruple-junction solar cells with four electrical contacts that overcome some of the key limitations of these previous attempts. “We have used our idea to fabricate cells with efficiencies of 43.9% and a module level result of over 36.5%, a value, although not yet validated by external labs, is the best ever recorded for any kind of photovoltaic technology,” team leader John Rogers told nanotechweb.org.

The researchers employed etching techniques to release compound semiconductor solar cells from their epitaxial growth substrates by etching away embedded sacrificial layers. These small, ultrathin cells were then manipulated using soft transfer printing stamps to assemble large arrays of multijunction solar cell stacks. An infrared transparent and refractive-index-matched layer of a chalcogenide glass (arsenic triselenide, As2Se3) serves as a thermally conductive and electrically insulating interface layer in these stacks.

“We used compound semiconductor materials for the top three junctions and germanium for the bottom junction,” explained Scott Burroughs, who is vice-president of technology at Semprius. “We measured the efficiencies of these devices and the efficiencies of the modules that combine these cells with highly focusing optics in both lab settings as well as in natural sunlight outdoors.”

The team says that it is now busy working on making devices with five or six such solar cell junctions, to further improve efficiency.

The present work is reported in Nature Materials doi:10.1038/nmat3946.