Both space and terrestrial applications need high efficiency solar cells.  The conversion efficiency of a practical PV cell is mainly limited by hot electron thermalization, carrier nonradiative recombination in the p-n junction, and Joule heating via the series resistance of the entire device (R).  MJ solar cells are ideal for ultrahigh efficiency requirements as the increased number of junctions minimizes hot electron thermalization and reduces the overall current (I) to lower the Joule heating, which is proportional to I²R.  Therefore, MJ solar cells currently hold the record for conversion efficiency: 31% under 1 sun illumination and 40.7% under concentrated 240 sun illumination.  However, for conventional, state-of-the-art 3-junction solar cells using the GaInP/(In)GaAs/Ge material system, it is challenging to find lattice-matched materials for additional junctions, which hinders the continued increase in conversion efficiency.  It is necessary now to develop a revolutionary approach for providing a substantial improvement in solar cell conversion efficiency.  Ultrahigh efficiency solar cells must overcome several challenges:

• Lattice-matched materials must be used to eliminate misfit dislocations that reduce material quality and conversion efficiency. 
• Direct bandgap materials must be used to minimize the thickness of the junction region and the series resistance. 
• Spontaneous emission coupling between adjacent junctions made of direct bandgap semiconductors must be taken into account for the optimal cell designs. 
• Minimization of series resistance of the contacts, the tunnel diodes, the contact layers, and the substrate becomes essential.

A new approach proposed by Dr. Yong-Hang Zhang and his group offers a novel, high performance MJ solar cell design that uses lattice-matched II/VI (ZnCdMg)(SeTe) and III/V (AlGa)(AsSb) direct bandgap materials grown on GaSb and InAs substrates.  Both of the material systems have been studied in detail for different applications, providing a very broad knowledge base for this program.  The figure shown above gives the energy bandgaps of various II/VI and III/V alloys as a function of the lattice constant.  Note that the zinc blende II/VI and III/V quaternary alloys are perfectly lattice-matched to either GaSb or InAs and cover the entire solar spectrum from 3.0 eV down to 0.4 eV and below.  These material systems possess direct bandgaps at almost all of the energies of interest. The main innovation of the proposed approach is to design junctions with optimal bandgap energies while maintaining the lattice-match condition, and so create solar cells with a large number of junctions.  This novel design addresses all the challenges for ultrahigh efficiency MJ solar cells mentioned above and has great potential for breakthroughs in conversion efficiency and thermal management.  Our modeling, using the commercial Silvaco software package, shows that energy conversion efficiencies of 44% under 1 sun and 55% under 1000 suns can be practically achieved with a 5-junction cell design.

For more information, please, contact, professor  Yong-Hang Zhang and visit the web sites of the MBE Optoelectronics group and the Center for Nanophotonics