4‑Terminal Tandem Photovoltaic Cell Using Two Layers of PTB7:PC₇₁BM for Optimal Light Absorption

A 4-terminal architecture is proposed in which two thin active layers (<100 nm) of PTB7:PC₇₁BM are deposited on a two-sided ITO covered glass substrate. By modeling the electric field distribution inside the multilayer structure and applying an inverse solving problem procedure, we designed an optimal device architecture tailored to extract the highest photocurrent possible. By adopting such a 4-terminal configuration, we numerically demonstrated that even when the two subcells use identical absorber materials, the performance of the 4-terminal device may overcome the performance of the best equivalent single-junction device. In an experimental implementation of such a 4-terminal device, we demonstrate the viability of the approach and find a very good match with the trend of the numerical predictions

Anisotropic-Strain-Induced Band Gap Engineering in Nanowire-Based Quantum Dots

Tuning light emission in bulk and quantum structures by strain constitutes a complementary method to engineer functional properties of semiconductors. Here, we demonstrate the tuning of light emission of GaAs nanowires and their quantum dots up to 115 meV by applying strain through an oxide envelope. We prove that the strain is highly anisotropic and clearly results in a component along the NW longitudinal axis, showing good agreement with the equations of uniaxial stress. We further demonstrate that the strain strongly depends on the oxide thickness, the oxide intrinsic strain, and the oxide microstructure. We also show that ensemble measurements are fully consistent with characterizations at the single-NW level, further elucidating the general character of the findings. This work provides the basic elements for strain-induced band gap engineering and opens new avenues in applications where a band-edge shift is necessary