A TCAD-based modeling of GaN/InGaN/Si solar cells

Abstract

Theoretical efficiency potential of GaN/InGaN/cSi tandem solar cells is investigated using two-dimensional numerical computer simulation (i.e. technology-based computer aided design tool: TCAD). With double-junction GaN/InGaN/cSi tandem design, a conversion efficiency of 27% is achieved using a 1.0 μm In 0.5 Ga 0.5 N absorber of top cell over crystalline silicon (cSi) bottom cell. This efficiency is further improved to 29.0% with grading of the In x Ga 1−x N absorber layer close to the top heterointerface (p + -GaN/n − -In x Ga 1−x N) of the solar cell. A maximum conversion efficiency is obtained when the band discontinuity ratio (i.e. E C : E V ) is set to 0.65:0.35. While efficiency remains approximately constant with moderate n-doping (up to 5 × 10 16 cm −3 ) in the top InGaN absorber layer, sensitivity of the efficiency to the interface trap density and trap cross-section (when traps are located only at the heterointerfaces) shows degraded behavior with increasing trap density and trap cross-section. A temperature coefficient for open-circuit voltage (efficiency) of −0.15 (−1.72 × 10 −3 • C −1 ), −0.09 (−0.95 × 10 −3 • C −1 ) and −0.2 (−2.38 × 10 −3 • C −1 )%/ • C for single heterojunction (SHJ), double-heterojunction (DHJ) and tandem-graded design is predicted from the numerical simulations.

Theoretical study of c-GaN/GaAs single heterojunction solar cells

“…Although the different results presented in I-V curves, it was possible to observe similar behavior within each group; in both, as the thickness of the intrinsic layer was increased, the voltage was decreased and current density increased, which coincides with some previously works reported for photovoltaic devices based on GaN [14]. However, although the increase in current density peaked around 32 mA/cm 2 for both groups, a marked difference was observed in the increase of current density for heterostructures with i-GaAs layer thicknesses lower than 400 nm for the reduced thickness of p-GaAs.…”

“…The results obtained in this theoretical study show that to locate the space charge within the intrinsic region, it is required a thickness between 1 and 1.5 microns for i-GaAs, which allows to assume that the intrinsic layer thickness must be at most the order of the absorption length, about 1.2 microns according to what was reported by Nawaz & Ahmad [14]; also, it was observed that increasing excessively the thickness of i-GaAs layer negatively impacts V oc , because the intensity of the internal electric field is reduced and there is an increase in possibilities of recombination of carriers. Therefore, this allows justify why for thicknesses greater than the intrinsic layer, the efficiency peaks that could be attributed to absorption maximum and photogeneration, which subsequently decreases when after that peak only increases recombination and it impacts negatively in device performance.…”

“…For nGaN/i-GaAs interface, no discontinuities were observed between the valence band (Ec) and conduction band (Ev) in the three heterostructures; this can be attributed to the proximity of electron affinity for GaN and GaAs, as this prevents the bending of the bands at the interface of the heterojunction and allows a good fit between them, which contributes to good electrical conduction within the heterostructure; this feature observed in the GaN/GaAs heterojunctions can be considered an advantage over some other GaN-based heterostructures previously reported where there have been marked discontinuities in their energy bands [14][15][16]. Current-voltage (I-V) curves were obtained and grouped by p-GaAs layer thickness; Figure 3 shows I-V curves for different heterostructures with thickness of 200 nm (3a) and 350 µm (3b) for p-GaAs layer, while n-GaN layer is fixed at 300 nm and i-GaAs is varied at different thickness.…”

“…Beside this, a set of physical models was implemented to obtain a realistic prediction of electrical behavior on simulated devices, including bandgap narrowing effect, Shockley-Read-Hall recombination model (SRH), Auger recombination model, Fermi-Dirac statistics and modified Caughey-Thomas model for mobility, similar to others reports [13][14][15][16]. Photovoltaics characteristics were measured under AM1.5 spectrum with 1 sun concentration.…”

Theoretical study on the effect of graded InyGa1−yN layer on p-GaN/InyGa1−yN/n-GaN p-i-n solar cell

“…The photon energy-dependent optical constantsrefractive index, n(E), and extinction coefficient, k(E), of In y Ga 1Ày N material shown below are prerequisite for In y Ga 1Ày N/GaN solar cell modeling [21,22],…”