Degradation of Ge subcells by thermal load during the growth of multijunction solar cells

Abstract

Germanium solar cells are used as bottom subcells in many multijunction solar cell designs. The question remains whether the thermal load originated by the growth of the upper layers of the multijunction solar cell structure affects the Ge subcell performance. Here, we report and analyze the performance degradation of the Ge subcell due to such thermal load in lattice‐matched GaInP/Ga(In)As/Ge triple‐junction solar cells. Specifically, we have detected a quantum efficiency loss in the wavelength region corresponding to the emitter layer (which accounts for up to 20% loss in equivalent JSC) and up to 55 mV loss in VOC of the Ge subcell as compared with analogous devices grown as single‐junction Ge solar cells on the same type of substrates. We prove experimentally that there is no direct correlation between the loss in VOC and the doping level of the base. Our simulations show that both the JSC and VOC losses are consistent with a degradation of the minority carrier properties at the emitter, in particular at the initial nanometers of the emitter next to the emitter/window heterointerface. In addition, we also rule out the gradual emitter profile shape as the origin of the degradation observed. Our findings underscore the potential to obtain higher efficiencies in Ge‐based multijunction solar cells if strategies to mitigate the impact of the thermal load are taken into consideration.

Development of the Lattice Matched GaInP/GaInAs/Ge Triple Junction Solar Cell with an Efficiency Over 40%

“…This study has confirmed that the V OC obtained in Ge cells grown as single junction devices is significantly higher than that of a Ge BC in a 3JSC. This V OC loss can be as high as 55 mV [19,20] together with a drop in short-circuit current density in the BC of ~2.3 mA/cm 2 (due to thicker emitters as a result of a deeper III-V elements diffusion during the thermal load). Simulations confirm that all these losses can be explained by the degradation in the minority carrier properties at the emitter region, especially close to the GaInP/Ge interface [19].…”

“…This V OC loss can be as high as 55 mV [19,20] together with a drop in short-circuit current density in the BC of ~2.3 mA/cm 2 (due to thicker emitters as a result of a deeper III-V elements diffusion during the thermal load). Simulations confirm that all these losses can be explained by the degradation in the minority carrier properties at the emitter region, especially close to the GaInP/Ge interface [19]. With the aim of alleviating such thermal load, an emphasis was put on the design of new nucleation routines (modifying the two first semiconductor layers) with lower thermal load.…”

Ge virtual substrates for high efficiency III-V solar cells: applications, potential and challenges

“…With this, the short circuit currents (Jsc) attained in these Ge subcells are typically above 50% higher than needed to be current matched to the other junctions. The open circuit voltage (Voc) in these cells is mostly defined by the emitter properties and the bulk recombination properties in the base [7].…”

“…These new characteristics of the substrate that must be used to form the 3 rd junction raise questions about the ability to achieve enough photocurrent in this junction or mitigating the effects on the Voc of the highly recombining back interface. In fact, while the emitter properties typically limit the Voc in the Ge subcell of triple-junction solar cells [7], [12], this may not be the case for Ge/Si substrates. In this context, we present here an analysis of the potential of a Ge/Si virtual substrate to function as the bottom cell in a lattice-matched GaInP/Ga(In)As/Ge triple-junction solar cell.…”

“…As a last note, the calculations presented here are based on realistic Ge SJSC properties of as-grown devices, with no further structures grown on top. We know that our MOVPE process for full 3JSC degrades the Ge bottom cell significantly, affecting both the carrier collection efficiency and recombination currents [7], [14]. However, we decided to do the study using the parameters of as-grown Ge SJSC to be more general since other authors do not report carrier collection and Jsc losses during the growth of the 3JSC.…”