Advances in Flexible and Lightweight III-V Multijunction Solar Cells for High Power Density Applications

Resumen

We are developing flexible and lightweight III-V multijuntion solar cells for high power density applications such as unmanned aerial vehicles (drones), High Altitude Pseudo Satellites (HAPS) and high altitude stratospheric balloons for satellite-like communications, cheap and widespread internet connectivity, etc. We focus our research on two different types of III-V solar cells covering the range from 1 kW/kg to 3 kW/kg (considering the weight of the solar cell itself and not that of the module). Our approach to achieve a goal of 3 kW/kg is based on 3J inverted metamorphic solar cells. We have recently developed GaInP/GaAs/GaInAs 3J IMM solar cells with a power density of 5.7 kW/kg under 1xAM1.5g for small areas. Demonstrating large area devices (tens of square cm) is the next challenge to tackle. In addition, we are developing lower-cost designs, targeting 1 kW/kg, based on GaInP/Ga(In)As/Ge 3J lattice matched solar cells. In this paper we present the advances in the modelling and manufacturing of this last type of solar cell. Modelling indicates that there are multiple efficiency-thickness combinations reaching the 1 kW/kg target. For example in the case of AM1.5g, a solar thickness of 46 microns requires an efficiency of 30% which is a demanding value. However, if the thickness is reduced to 40 microns, a more easily achievable efficiency of 27% would be required. We have demonstrated a process for thinning the Ge substrates reaching thicknesses as low as 14 microns with highly homogeneous thickness profiles. Preliminary results show that the 3J thinned solar cell exhibits an efficiency which is the 91% of the efficiency achieved by the non-thinned cell.