1.25-eV GaAsSbN/Ge Double-Junction Solar Cell Grown by Metalorganic Vapor Phase Epitaxy for High Efficiency Multijunction Solar Cell Application

Abstract

Dilute-nitride-antimonide materials grown by metalorganic vapor phase epitaxy (MOVPE) with bandgap energies of 1.25 eV have been integrated into solar cell structures employing a Ge bottom cell on Ge substrate. Single homo-and heterojunction solar cells employing narrow bandgap GaAsSbN (E g ∼ 1.25 eV) are grown normally lattice-matched on a GaAs substrate, using MOVPE. Homojunction solar cell structures were realized by employing GaAsSbN material with low carbon background concentration and Si doping to form a p/n junction. External quantum efficiency measurements in the range (870 nm-1000 nm) reveal that the efficiency of the homojunction solar cell is significantly improved over that of the heterojunction structure. The GaAsSbN homojunction cell was integrated with a Ge single-junction bottom cell on Ge substrate. Under AM1.5 direct illumination, the fabricated GaAsSbN (1.24 eV)/Ge double-junction solar cell with a 600-nm-thick GaAsSbN base layer exhibits J sc , V o c , FF, and efficiency values of 11.59 mA/cm 2 , 0.83 V, 72.58%, and 7% with anti-reflection coating (ARC), respectively.

Structures based on GaAs(Sb)(N) semiconductor alloys for high efficiency multi-junction solar cells

“…Some of them (cells from refs. [77,78,82]) have values lower than those obtained here, but they have window layer and anti-reflective coating [78,82] or, at least, a window layer [77]. Therefore, although no reference GaAs cell is studied in those papers, it is reasonable to think that the W OC value of an equivalent GaAs cell would be lower than ours (see values reported in [239], for example).…”

“…Three different temperatures, 750, 800 and 850 ºC, were employed to anneal different pieces of each structure. These temperatures are within the range that has been found to improve the quality of dilute nitride materials [56,188,191,196,197] and, particularly, GaAs 1-x-y Sb x N y [72,78]. PL measurements before and after RTA in the very same sample pieces were used to test the three RTA temperatures.…”

“…However, the beneficial effect of increasing the absorbing layer in dilute nitride p-i-n solar cells has been already demonstrated. EQE and J SC of GaAs 1-x-y Sb x N y and Ga 1-x In x As 1-y N y :Sb solar cells have been shown to increase with the active layer thickness from 0.6 µm to 1 µm in as-grown structures [78,238]. The effect is also noticeable for larger thicknesses in annealed solar cells: though increasing the Ga 1-x In x As 1-y N y :Sb layer thickness from 1 µm to 3 µm results in as-grown solar cells in a reduction of the J SC due to recombination loses, after RTA the J SC increases for the 2 and 3 µm solar cells compared to the 1 µm one because of the improved field-assisted carrier collection [56].…”

“…Indeed, the band structure of GaAs 1-x-y Sb x N y can be modeled by a double BAC (DBAC) model, which is a combination of a CB BAC model for GaAs 1-y N y and a VB BAC model for GaAs 1-x Sb x [68,69]. Moreover, the The GaAs 1-x-y Sb x N y alloy has been recently applied to solar cell technology, both as a thick layer [52,[71][72][73][74][75][76][77][78][79][80][81][82][83][84] and as a capping layer over InAs quantum dots [85,86]. Nevertheless, the obtained solar cell performance is not satisfactory up to now.…”

“…Nevertheless, this could be in principle solved by making thicker absorber layers. Indeed, the EQE or J SC of similar GaAs 1-x-y Sb x N y and Ga 1-x In x As 1-y N y :Sb solar cells has been shown to increase with the active layer thickness up to 1 μm in as-grown structures [78,238], and even up to 3 μm in annealed solar cells [56].…”