Tunnel Junction Limited Performance Of Ingaasn/Gaas Tandem Solar Cell

From tandem solar cell we expect a wide absorption range due to different bandgaps of the subcells, higher open circuit voltage V-oc and finally better photo conversion efficiency eta than single junction solar cell. The main drawback of it is a current limitation i.e. the short circuit current J(sc) of whole structure is equal to smaller J(sc) of both subcells. However, even two subcells with very good photovoltaic performance do not guarantee the correct operation of tandem solar cell. The tunnel junction plays a crucial role on tandem devices as it has to provide low loss electrical and optical connection between top and bottom subcells.Within this paper we present two subcells: top GaAs based, bottom InGaAsN based with good photovoltaic performance and InGaAs tunnel diode used for InGaAsN/GaAs tandem solar cell construction. We compare reference devices with monolithic tandem cell and conclude about tunnel junction failure based on both dark and illuminated J-V characteristics of the InGaAsN/GaAs final device. Based on detailed analysis of tandem performance we propose two most likely effects responsible for tunnel junction induced deterioration of tandem solar cell performance.