This research builds upon the authors’ previous work that introduced and modeled a novel Gallium-Arsenide, Emitterless, Back-surface Alternating Contact (GaAs-EBAC) thin-film solar cell to achieve >30% power conversion efficiency. Key design parameters are optimized under an Air-Mass (AM) 1.5 spectrum to improve performance and approach the 33.5% theoretical efficiency limit. A second optimization is performed under an AM0 spectrum to examine the cell’s potential for space applications. This research demonstrates the feasibility and potential of a new thin-film solar cell design for terrestrial and space applications. Results suggest that the straight-forward design may be an inexpensive alternative to multi-junction solar cells.
The renowned British mathematician George Box once quipped, “Essentially, all models are wrong, but some are useful” [
At the time of this publication, power conversion efficiency η for a single-ab- sorption-layer (i.e. single p-n junction) solar cell remains at 28.8%: well below the ~33.5% theoretical limit [
The authors’ previous research [
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 VOC improved from 1.12 V to 1.13 V; short-circuit current density JSC 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
The purpose of this research is to optimize the thin-film GaAs-EBAC cell model from [
In this section we examine the impact of back-surface reflectivity, absorption layer thickness and absorption layer doping concentration on cell η in order to optimize performance under Air-Mass 1.5 Global (AM1.5G) and AM0 solar spectrums.
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 [
where B is the intrinsic radiative recombination coefficient, EFn - EFp 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 JSC and VOC, and no correlation with FF as shown in
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
Back-Surface Reflectivity (%) | 90* | 93 | 96 | 99 |
---|---|---|---|---|
VOC (V) JSC (mA/cm2) FF (%) η (%) | 1.13 30.1 89.1 30.4 | 1.13 30.2 89.1 30.4 | 1.14 30.2 89.1 30.5 | 1.15 30.38 89.1 30.5 |
Absorption Layer Thickness (μm) | 1.0 | 1.2* | 1.4 | 1.6 |
VOC (V) JSC (mA/cm2) FF (%) η (%) | 1.14 29.7 89.2 30.2 | 1.13 30.1 89.1 30.4 | 1.13 30.2 89.1 30.4 | 1.13 30.2 89.1 30.3 |
Absorption Layer Doping (cm−3) | 2 × 1017* | 5 × 1017 | 8 × 1017 | 2 × 1018 |
VOC (V) JSC (mA/cm2) FF (%) η (%) | 1.13 30.1 89.1 30.4 | 1.15 30.0 89.3 30.7 | 1.16 29.8 89.3 30.8 | 1.16 28.5 89.3 30.1 |
*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 JSC and a negative correlation with VOC and FF as shown in
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 VOC (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 VOC and FF, and a negative correlation with JSC as shown in
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 [
Design parameters from section 2.1 are varied again with results shown in
In this work, parameters were optimized for a GaAs-EBAC cell model [
Back-Surface Reflectivity (%) | 90* | 93 | 96 | 99 |
---|---|---|---|---|
VOC (V) JSC (mA/cm2) FF (%) η (%) | 1.07 35.6 87.3 24.4 | 1.07 35.6 87.3 24.5 | 1.07 35.7 87.3 24.5 | 1.07 35.7 87.3 24.5 |
Absorption Layer Thickness (μm) | 1.0 | 1.2* | 1.4 | 1.6 |
VOC (V) JSC (mA/cm2) FF (%) η (%) | 1.08 35.2 87.3 24.3 | 1.07 35.6 87.3 24.4 | 1.07 35.6 87.2 24.4 | 1.07 35.6 87.2 24.3 |
Absorption Layer Doping (cm-3) | 2 × 1017* | 5 × 1017 | 8 × 1017 | 2 × 1018 |
VOC (V) JSC (mA/cm2) FF (%) η (%) | 1.07 35.6 87.3 24.4 | 1.09 35.4 87.4 24.7 | 1.10 35.1 87.5 24.8 | 1.13 33.3 87.6 24.1 |
*Baseline design parameter setting.
1Patents pending.
Results suggest that the novel GaAs-EBAC cell design has record-setting potential for terrestrial applications and offers a good alternative to multi-junction cells for space applications. In fact, the model produced η within 1.8% of a leading HE triple-junction cell [
Future research will investigate the effects of random texturing on the front- and-back surfaces to exceed 98% photon internal reflection as absorption layer thickness is reduced to less than a spectral wavelength. Additionally, a prototype will be developed to experimentally verify the GaAs-EBAC cell design.
Patent applications have been filed for ideas presented in this paper [
O’Connor, J.E. and Michael, S. (2017) Optimizing a Single- Absorption-Layer Thin-Film Solar Cell Model to Achieve 31% Efficiency. Journal of Materials Science and Chemical Engineering, 5, 54-60. http://dx.doi.org/10.4236/msce.2017.51008