The authors’ previous research [2] developed the novel structure shown in Figure 1(a) to simulate a HE GaAs cell from [4] [8] [9]. Though not a perfect representation, the model was useful for examining various design parameters. Extensive research was conducted to ensure that the simulation accurately reproduced experimental results before the GaAs Back-surface Alternating-Contact (GaAs- BAC) cell shown in Figure 1(b) was derived. All variables were held constant for

the new design while only the emitter and associates electrical contacts were moved to the back-surface. The small, but significant design change improved optical and electrical performance such that model η improved from 28.8% to 30.3%; open-circuit voltage V_OC improved from 1.12 V to 1.13 V; short-circuit current density J_SC improved from 29.7 mA/cm² to 30.1 mA/cm²; and FF improved from 86.5% to 88.8%. To further improve cell η, the emitter was removed from the GaAs-BAC cell model to produce the novel design shown in Figure 2(a). FF and η improved slightly to produce the output characteristics shown in Figure 2(b).

The purpose of this research is to optimize the thin-film GaAs-EBAC cell model from [2] in order to further approach theoretical η. We again utilize Silvaco® ATLAS software to alter design variables, predict electrical characteristics, and simulate the transport of charge carriers through the cell structure [10]. This research represents the final stage before prototype development.

GaAs-EBAC cell terrestrial performance is simulated at 300˚K under an AM1.5G solar spectrum. Back-surface reflectivity, absorption layer thickness and absorption layer doping concentration are varied to examine impacts on model performance and maximize η.

Reflectivity of the bottom contacts contributes directly to photon recycling, which contributes to a higher effective minority carrier lifetime [5] [6] [7] [8] [9]. Radiative recombination is modeled in ATLAS as

where B is the intrinsic radiative recombination coefficient, E_Fn - E_Fp is the energy difference between electron-hole-pair (EHP) quasi-Fermi levels, k is Boltz- mann’s constant, and T is the operating temperature. When reflectivity is varied from 90% to 99%, the model indicates a positive correlation with J_SC and V_OC, and no correlation with FF as shown in Table 1.

Absorption layer thickness contributes to EHP generation in the cell. Maximum thickness should not exceed minority carrier diffusion length in order to ensure carrier capture at the electrical contacts. Spectral generation rate g is defined as

*Baseline design parameter setting.

where R is the front-surface reflectivity, λ is the spectral wavelength, η' is the internal quantum efficiency, ϕ is the photon flux, d is the depth (thickness) of the cell and α is the absorption coefficient. When absorption layer thickness is varied from 1 μm to 1.6 μm, the model indicates a positive correlation with J_SC and a negative correlation with V_OC and FF as shown in Table 1.

Absorption layer doping is the intentional distribution of impurities within a semiconductor’s crystal lattice to increase the density of majority carriers-either electrons or holes. Increased doping generally improves V_OC (i.e. the splitting of quasi-Fermi levels) in non-degenerately doped materials and negatively impacts- minority carrier mobility and lifetime; therefore, optimization is required. When doping is varied from 2 × 10¹⁷ cm⁻³ to 2 × 10¹⁸ cm⁻³, the model indicates a positive correlation with V_OC and FF, and a negative correlation with J_SC as shown in Table 1.

Complex cell designs often require innovative methods (i.e. genetic algorithms, Monte-Carlo simulation, etc.) to optimize the design; however, the simplicity of the GaAs-EBAC cell model permits an iterative approach to achieve the best design variable combination and minimize the risk of converging on a local maximum. Optimization produces a maximum η of 31% when back-sur- face reflectivity ≈ 99%, absorption layer thickness ≈ 1.2 μm and doping concentration ≈ 8 × 10¹⁷ cm⁻³. Decreasing back-surface reflectivity to a conservative value of 96% reduces η only slightly to 30.9%.

GaAs-EBAC cell space performance is simulated at 350˚K under an AM0 solar spectrum. The design is well-suited for space operation due to the intrinsic radiation hardness of GaAs [11]; the superior temperature coefficient of GaAs (as compared to silicon) [12] [13] [14]; the high packing density of the back-contact design (W/m²) [15]; and the high power density (W/kg) of HE thin-film cells [16].

Design parameters from section 2.1 are varied again with results shown in Table 2. Optimization produces a maximum η of 25% when back-surface reflectivity ≈ 99%, absorption layer thickness ≈ 1.0 μm and doping concentration ≈ 8 × 10¹⁷ cm⁻³. Decreasing back-surface reflectivity to 96% reduces η slightly to 24.9%. The lower η under the AM0 spectrum (compared to AM1.5G) is attributed to high temperature operation which has a negative impact on V_OC: ap- proximately −1.4 mV/˚K [13]. A temperature coefficient adjustment was calculated for a HE triple-junction cell using the manufacturer’s specification sheet [17], which yielded η of 26.8% at 350˚K under an AM0 spectrum. Thus, our model indicates that conceding just 1.8% η can dramatically reduce design complexity, which could improve system reliability and decrease manufacturing cost.