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An intrinsic property of quaternary alloys A 1Ϫy B y C 1Ϫx D x (xϷ1 -3 %) with D being an isovalent trap is reported: a set of discrete band gaps occurs due to substitution of the isovalent trap D on sites with different nearest-neighbor environments. Exemplary, this phenomenon is demonstrated for ͑Ga,In͒͑N,As͒ by experiment and explained by tight-binding supercell calculations. The band gap of this nitrogen-poor alloy is blueshifted by simply moving the nitrogen isovalent traps from Ga-ligand rich sites to In-ligand rich sites, without changing the alloy composition.

Wide-bandgap perovskite solar cells (PSCs) with optimal bandgap (E g ) and high power conversion efficiency (PCE) are key to high-performance perovskite-based tandem photovoltaics. A 2D/3D perovskite heterostructure passivation is employed for double-cation wide-bandgap PSCs with engineered bandgap (1.65 eV ≤ E g ≤ 1.85 eV), which results in improved stabilized PCEs and a strong enhancement in open-circuit voltages of around 45 mV compared to reference devices for all investigated bandgaps. Making use of this strategy, semitransparent PSCs with engineered bandgap are developed, which show stabilized PCEs of up to 25.7% and 25.0% in fourterminal perovskite/c-Si and perovskite/CIGS tandem solar cells, respectively. Moreover, comparable tandem PCEs are observed for a broad range of perovskite bandgaps. For the first time, the robustness of the four-terminal tandem configuration with respect to variations in the perovskite bandgap for two state-of-the-art bottom solar cells is experimentally validated.

The three-dimensional carrier confinement in GaN nanodiscs embedded in GaN/Al x Ga 1−x N nanowires and its effect on their photoluminescence properties is analyzed for Al concentrations between x = 0.08 and 1. Structural analysis by high-resolution transmission electron microscopy reveals the presence of a lateral Al x Ga 1−x N shell due to a composition-dependent lateral growth rate of the barrier material. The structural properties are used as input parameters for three-dimensional numerical simulations of the confinement that show that the presence of the Al x Ga 1−x N shell has to be considered to explain the observed dependence of the emission energy on the Al concentration in the barrier. The simulations reveal that the maximum in the emission energy for x ≈ 30% is assigned to the smallest lateral strain gradient and, consequently, the lowest radial internal electric fields in the nanodiscs. Higher Al concentrations in the barrier cause high radial electric fields that can overcome the exciton binding energy and result in substantially reduced emission intensities. Effects of polarization-induced axial internal electric fields on the photoluminescence characteristics have been investigated using nanowire samples with nanodisc heights ranging between 1.2 and 3.5 nm at different Al concentrations. The influence of the quantum confined Stark effect is significantly reduced compared to GaN/Al x Ga 1−x N quantum-well structures, which is attributed to the formation of misfit dislocations at the heterointerfaces, which weakens the internal electric polarization fields.

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