A new record at 44.7% total conversion efficiency by a consortium of German and French researchers at Fraunhofer ISE, Soitec, CEA-Leti, and the Helmholtz Center Berlin has demonstrated the viability of an untraditional design for high-efficiency solar cells. The breakthrough was achieved using “direct semiconductor bonding” (DSB), a new technique that represents a significant advance over traditional solar cell design.
The most efficient solar cells today use a “multi-junction” (MJ) design. MJ cells “divide and conquer” solar radiation. A given semiconductor material (formed into a diode or “junction”) is optimized to convert a particular segment of the sun’s spectrum. Light that falls outside of this “sweet-spot” is either converted to electricity at less-than-optimal rates (because the wavelength is too short) or is not converted at all (because the wavelength is too long). In both cases conversion efficiency suffers. By stacking different semiconductors, a MJ cell lets the top-most junction handle the shortest wavelength (highest energy) light, while passing longer-wavelength (lower energy) light to a junction below. This configuration allows each junction to optimally convert each segment of the spectrum, thereby achieving the best total conversion efficiency.
Until now, there were only two kinds of MJ cells: lattice-matched (LM) and metamorphic (MM). LM cells use a constant size crystal structure throughout the whole device, allowing each junction to be produced with very few defects. This approach, however, prevents each junction from being optimized with the solar spectrum. The MM design relaxes this constraint, varying the crystal structure throughout the device allowing the MJ cell to be ideally tailored to the sun’s light. Mixing different crystal structures, however, increases material defects, leading to reduced performance of the “mismatched” junctions.
Previous record-holding cells fabricated by Solar Junction (44 %) and Sharp (44.4%) used the LM and MM designs, respectively, indicating the reigning technologies and DSB are quite competitive. DSB allows groups of dissimilar semiconductors to be joined together in a single device without introducing extra defects. While this seems to avoid the inherent trade-off between LM and MM designs, significant challenges have often involved achieving low-resistance bonds with high-optical quality and high device yield. This result indicates that competition at the top of the solar efficiency charts has heated up again.