Photovoltaics: Towards 50 percent efficiency

A record-breaking photovoltaic efficiency comes from bonding a pair of dual-junction solar cells with different lattice constants.

BY OLIVER HÖHN FROM FRAUNHOFER ISE

What's needed to realise exceptionally high efficiencies with solar cells? A significant degree of concentration is a given, alongside the use of multi-junction solar cells, with history attesting that the best results tend to come when these devices are produced by MOCVD. Allied to these key attributes, the fabrication of best-in-class devices demands great designs, excellent expertise, and thorough process technology.

The success story of multi-junction solar cells begins decades ago, with the deployment of these devices in space. Satellites have been powered by solar cells for many years, and for this application a high efficiency is incredibly valuable. Due to this, III-V multi-junction solar cells have had a stranglehold on this market since the 1990s, when they replaced their silicon siblings. Since then efficiencies have increased, but the standard product for space applications is still the triple-junction solar cell, produced on a germanium substrate that also provides the lowest sub-cell.

To bring this technology from space to earth, it is essential that the expensive III-V solar cell is used far more effectively. This can be accomplished by placing high-efficiency, multi-junction III-V cells in concentrating photovoltaic (CPV) systems. In this case, the expensive large-area semiconductor is replaced by a lens, which focuses sunlight onto a very small solar cell, thereby reducing the size of the III-V material that’s required by a factor of 500-1000, depending on the concentration of the lenses.

The benefits of concentration are not limited to slashing the outlay on multi-junction cells. In addition, the increase in concentration enhances the efficiency of the solar cells by a thermodynamic effect. The increase in current density leads to a logarithmic increase in voltage – this is a fundamental consequence of diode characteristics – and as the power that’s generated by the solar cell is the product of its voltage and current, there is an increase in efficiency under concentration.

Over the past ten years or so, efforts to increase solar cell efficiency have included the introduction of more junctions, with devices featuring up to six of them. Efforts in this direction highlight the limitations of this approach, with monolithic epitaxial growth requiring lattice-matching of an increasing number of sub-cells, each made from III-Vs differing in content or composition. This requirement restricts the range of band gaps that can be employed within one monolithic device.

In general, two approaches have been pursued to overcome this limitation. One option, arguably the less radical of the two, is to introduce metamorphic buffers that gradually change the lattice constant, thus opening the door to a broader range of band gaps. However, there’s a penalty to pay for grading the lattice constant: material quality suffers, dragging down solar cell performance.

At Fraunhofer ISE, our team adopted a different approach. In our case, we grow lattice-matched top junctions with high band gaps on a GaAs substrate; and we use an InP substrate for lattice-matched growth of the bottom junctions, which have lower band gaps (see Figure 1). These two devices, which have totally different lattice constants, are monolithically stacked on one another, using surface-activated direct wafer bonding. By taking this approach, we overcome the limitations in cell performance that stem from a metamorphic buffer.

Over the last few years we have engaged in friendly competition with a team at NREL for the world record in solar cell efficiency. Back in 2015 we gained the lead, in partnership with Soitec and CEA, with a wafer-bonded four-junction solar cell with an efficiency of 46 percent. We held the record until 2019, when NREL reported a metamorphic six-junction device with an efficiency of 47.1 percent. Then last year we announced that we had improved on this value, realising a record for photovoltaic efficiency of 47.6 percent, through a BMWK-funded project entitled ‘50 Prozent’ (03EE1060).

While regaining the record is a triumph in itself, we are particularly pleased that we have accomplished this with a device with just four junctions. We attribute this success to the game-changing direct wafer bond, allied to the excellent design and realisation of a pair of dual-junction lattice-matched ternary and quaternary solar cells on InP and GaAs. We shall soon detail the magic of the wafer bond, but before we get to that, we’ll discuss the cell design in more detail.