Abstract
In multi-junction devices, due to the series connection of junctions, recombination current from the top junctions can be directed to the bottom ones affecting their electrical characteristics. Recently, luminescence coupling effects during External Quantum Efficiency (EQE) measurements at very intense light bias conditions indicated high recombination current flowing towards the bottom junctions of the cells. In an attempt to find the magnitude of coupling current as well as the factors affecting the optical interactions between junctions, excitation and voltage dependent Photoluminescence (PL) measurements of GaInP/GaInAs/Ge have been carried out. An investigation using junctions with different shunt resistances has been conducted to identify the impact of shunts on the coupling current. Furthermore the impact of temperature on the coupling current has been considered. Our results show that a maximum of 2.3% of the recombination current of the top junction is converted to coupling current in the middle junction depending on the devices under examination. The coupling efficiency depends on the shunt resistance of the top junctions as well as on the temperature. Furthermore a physical model of the current limiting junction was built taking into consideration the impact of local ohmic shunts in the solar cell device and used to validate the experimental data taken.
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