First demonstration of a three terminals-quadruple junction InGaP/InGaAs/Ge/T/Ge solar cell - all MOVPE grown-exploiting the transistor effect

Three-junction (TJ) GaInP/GaInAs/Ge devices have been the most commercially available solar cells both for space and terrestrial concentrator photovoltaic (CPV) applications since the year 2000. The current mismatch and large amount of unusable sunlight absorbed by the Ge sub-cell, already brought more than 25 years ago to the theoretical proposal of combining epitaxial germanium and III-V compounds to realize multi-junction (MJ) solar cells where near infrared spectrum could be divided between two Ge sub-cells rather than one. However, since III-Vs act as dopants for those of group IV and vice versa, such MJ structures would have required the utilization of two different deposition apparatus to be realized. Thus, so far, the "cross contamination" among III-V and IV elements has hindered a technological path commercially competitive for such devices. With this contribution we present the first demonstration of a three terminal (T) - quadruple junction (QJ) InGaP/GaAs/Ge/T/Ge device – with all layers sequentially deposited in the same metal organic vapor phase epitaxy (MOVPE) growth chamber. The QJ device has also a new architecture which exploits the "transistor effect" to transfer power from the top Ge junction to the bottom one. The current mismatch is almost eliminated in InGaP/GaAs/Ge TJ top part, while an extraordinary increase in the external quantum efficiency of the connected bottom Ge part is obtained. A simple theoretical model is first applied to DJ Ge/T/Ge cells to explain the advantage of Ge upright homojunctions transistor structure. Then, the performances of QJ InGaP/GaAs/Ge/T/Ge and TJ InGaP/GaAs/Ge functional devices are compared. The obtained results pave the way to realize more advanced 5J - lattice matched - solar cells adopting III-Vs and epitaxial germanium and can be generalized to reduce current mismatch in MJ solar cells enlarging the possibilities to combine materials with different energy gap.