Quantum well solar cells, as a promising approach for next-generation photovoltaic technology, have received great attention in the last few years. Recent developments in materials growth and device structures of quantum wells have opened up new avenues for the incorporation of quantum well structures in next-generation III/V multi-junction solar cells. In this paper, the advantages and challenges of growing quantum wells in the unintentionally doped (i) region of p-i-n solar cells are reviewed. We focus on the recent progress in 1.1–1.3 eV strain-balanced InGaAs/GaAsP, 1.6–1.8 eV strain-balanced and lattice-matched InGaAsP/InGaP, and >2.1 eV strained InGaN/GaN quantum well solar cells, including optimization of the quantum well growth conditions and improving the solar cell structure. For each material system, the challenges associated with materials growth and device performance such as critical layer thickness constraints, strain balance, bandgap tunability, and carrier transport limitations, are discussed. The performance of each quantum well solar cell is compared with bulk absorber operating in the same bandgap range, with the advantages of each being highlighted. The effect of the unintentional background doping on carrier collection (by drift) is presented through modeling and recent experimental results. The recent strategies to enhance the electric field distribution across the quantum well region are reviewed. The potential of incorporating quantum well structures in next-generation multi-junction devices is also discussed.

Tunnel Junctions, as addressed in this review, are conductive, optically transparent semiconductor layers used to join different semiconductor materials in order to increase overall device efficiency. The first monolithic multi-junction solar cell was grown in 1980 at NCSU and utilized an AlGaAs/AlGaAs tunnel junction. In the last 4 decades both the development and analysis of tunnel junction structures and their application to multi-junction solar cells has resulted in significant performance gains. In this review we will first make note of significant studies of III-V tunnel junction materials and performance, then discuss their incorporation into cells and modeling of their characteristics. A Recent study implicating thermally activated compensation of highly doped semiconductors by native defects rather than dopant diffusion in tunnel junction thermal degradation will be discussed. AlGaAs/InGaP tunnel junctions, showing both high current capability and high transparency (high bandgap), are the current standard for space applications. Of significant note is a variant of this structure containing a quantum well interface showing the best performance to date. This has been studied by several groups and will be discussed at length in order to show a path to future improvements.

We have developed a technique for defect reduction in GaN epitaxial films grown on sapphire substrates. This technique relies on the generation of high densities of embedded microvoids (∼108/cm2), a few microns long and less than a micron in diameter. These voids are located near the sapphire substrate, where high densities of dislocations are present. Network of embedded voids offer free surfaces that act as dislocation sinks or termination sites for the dislocations generated at the GaN/sapphire interface. Both transmission electron and atomic force microscopy results confirm the uniform reduction of the dislocation density by two orders of magnitude.

Multiple quantum wells (MQW) lattice matched to GaAs consisting of In_0.14Ga_0.76As wells balanced with GaAs_0.24P_0.76 barriers have been used to extend the absorption of GaAs subcells to longer wavelengths for use in an InGaP/GaAs/Ge triple-junction photovoltaic cell. Thin barriers with high-phosphorus composition are capable of balancing the strain from the InGaAs wells; thus, creating conditions to allow for thicker wells and for carrier tunneling to dominate transport across the structure. As a result, a larger percentage of the depletion region is occupied by InGaAs quantum wells that absorb wavelengths beyond 875 nm and the indium composition is not limited by thermionic emission requirements. Measurements at elevated temperatures and reverse bias suggest that a thermally assisted tunneling mechanism is responsible for transport through the barriers.