Triple-junction solar cells from III-V compound semiconductors have thus far delivered the highest solar-electric conversion efficiencies. Increasing the number of junctions generally offers the potential to reach even higher efficiencies, but material quality and the choice of bandgap energies turn out to be even more importance than the number of junctions. Several four-junction solar cell architectures with optimum bandgap combination are found for lattice-mismatched III-V semiconductors as high bandgap materials predominantly possess smaller lattice constant than low bandgap materials. Direct wafer bonding offers a new opportunity to combine such mismatched materials through a permanent, electrically conductive and optically transparent interface. In this work, a GaAs-based top tandem solar cell structure was bonded to an InP-based bottom tandem cell with a difference in lattice constant of 3.7%. The result is a GaInP/GaAs//GaInAsP/GaInAs four-junction solar cell with a new record efficiency of 44.7% at 297-times concentration of the AM1.5d (ASTM G173-03) spectrum. This work demonstrates a successful pathway for reaching highest conversion efficiencies with III-V multi-junction solar cells having four and in the future even more junctions.

We demonstrate the self-assembled creation of a novel type of strain-free semiconductor quantum dot (QD) by local droplet etching (LDE) with Al to form nanoholes in AlGaAs or AlAs surfaces and subsequent filling with GaAs. Since the holes are filled with a precisely defined filling level, we achieve ultrauniform LDE QD ensembles with extremely narrow photoluminescence (PL) linewidth of less than 10 meV. The PL peaks agree with a slightly anisotropic parabolic potential. Small QDs reveal indications for transitions between electron and hole states with different quantization numbers. For large QDs, a very small fine-structure splitting is observed.

Photoluminescence from GaAs nanodisks fabricated by using combination of neutral beam etching and atomic hydrogen-assisted molecular beam epitaxy regrowth Independent wavelength and density control of uniform GaAs/AlGaAs quantum dots grown by infilling selfassembled nanoholes

Experimental results of the local droplet etching technique for the self-assembled formation of nanoholes and quantum rings on semiconductor surfaces are discussed. Dependent on the sample design and the process parameters, filling of nanoholes in AlGaAs generates strain-free GaAs quantum dots with either broadband optical emission or sharp photoluminescence (PL) lines. Broadband emission is found for samples with completely filled flat holes, which have a very broad depth distribution. On the other hand, partly filling of deep holes yield highly uniform quantum dots with very sharp PL lines.

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