Investigation of carrier escape and recombination dynamics in GaAsN/GaAs superlattice and resonantly coupled quantum well solar cells

Abstract

III-V multijunction devices that incorporate a dilute nitride 1-1.2 eV bottom cell have already demonstrated conversion efficiencies of about 44% under high sunlight concentration (942 Suns). However, the poor minority carrier properties of dilute nitride have, thus far, prevented the full realization of the practical potential for tandem configuration (>40% 1 sun, and >50% at 500X and above). To overcome this shortcoming, our group, over the past years, have focused on dilute nitride-based devices where the degraded minority carrier diffusion length has a minimal impact on the device performance. We have shown that the incorporation of resonantly coupled GaAsN/GaP multi-quantum wells in the intrinsic region of p-i-n GaAs cells allows both a significant sub-GaAs-bandgap photon harvesting while maintaining a high open circuit voltage. Here, in order to gain a better understanding of photo-generated carrier escape and recombination mechanisms in these devices and further optimize the performance, we examine optical and electrical properties of such devices using various characterization techniques such as: photoluminescence (PL), modulated photo-reflectance (PR), photo-current (PC) as well as current-voltage (IV) measurements under dark or illuminated conditions. The temperature dependent analysis enables us to modulate and freezes carrier thermalization phenomena, while simultaneous measurement of photogenerated carrier extraction (SR) and recombination’s (PL) as a function of the applied load (bias) enables a close correlation between the evolution of I-V characteristics and the physics at play. Next, typical temperature and bias dependent activation energies reveal interesting details about carrier escape, intra-cells coupling and recombination sequences.