In continuity with the previous research, the development activity of multi-junction (MJ) cells based on III-V and IV compounds, with new cell architectures has been carried out. A three-terminal (3T), stepped, 2J Ge cell with a ring third terminal has been developed, in which the two junctions can interact through a transistor effect. Layers of InGaP and InGaAs have been deposited on top of the 2J Ge/Ge structure, to simulate the optical behavior that the two Ge junctions would have in the 4J InGaP/InGaAs/Ge/Ge structure.

The 2J-Ge-3T structure, never realized before, presents peculiar characteristics that have been studied by simulation, performed with an “ad hoc” software, and an intense structural characterization activity with HRXRD, SEM, AFM and ECV. To carry out the electro-optic characterization of the 3T MJ device, the quantum efficiency and current-voltage characterization equipment, for the dark and illuminated (under solar simulator) measurements have been specially adapted. It has been ascertained that the transistor effect can determine a current density that is even 100% higher than that produced by light.

The good functioning of the 2JGe devices made it possible to confirm that it is possible to control the doping of the Ge junctions when they are deposited in the same growth chamber used for the deposition of the III-V elements. For the manufacturing of the 2J Ge-3T device, both the lithographic and the chemical processes have been developed, till to the deposition of the metals and the antireflective coatings, up to the individualization of the devices and their integration into a proper support to facilitate optical-electrical measurements.

Furthermore, activities concerning the simulations of SJ (Al)InGaP and Ge structures, as well as of innovative anti-reflective coatings with gradual refractive index, subsequently deposited by combining the Magnetron Sputtering deposition technique with the Glancing angle deposition one, have been carried out. The simulations of the Ge SJ cells have been validated by experimental EQE measurements, while the coatings simulations by reflectance measurements. Both measurements allowed performing a preliminary validation of the optical properties of the used materials. To further reduce reflection losses, a metal front grid with triangular rather than rectangular profile has been designed.

The report concludes with a preliminary study on the plasma etcher, to obtain vertical etching profiles necessary for an advanced realization of the 3T MJ device. The results obtained will allow continuing the research towards the realization of the InGaP/InGaAs/Ge/Ge QJ device, and also open the path to the development of new type of MJ devices that allow combining semiconductor materials differently than so far proposed.

The Report is available on the Italian site