Optoelectronic optimization of graded-bandgap thin-film AlGaAs solar cells

Faiz Ahmad; Akhlesh Lakhtakia; Peter B. Monk

doi:10.1364/AO.381246

Applied Optics
Vol. 59,
Issue 4,
pp. 1018-1027
(2020)
•https://doi.org/10.1364/AO.381246

Optoelectronic optimization of graded-bandgap thin-film AlGaAs solar cells

Faiz Ahmad, Akhlesh Lakhtakia, and Peter B. Monk

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Faiz Ahmad, Akhlesh Lakhtakia, and Peter B. Monk, "Optoelectronic optimization of graded-bandgap thin-film AlGaAs solar cells," Appl. Opt. 59, 1018-1027 (2020)

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- Optical Devices, Sensors, and Detectors
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About this Article
History
- Original Manuscript: October 22, 2019
- Revised Manuscript: December 5, 2019
- Manuscript Accepted: December 6, 2019
- Published: January 28, 2020

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Abstract

An optoelectronic optimization was carried out for an $ {{\rm Al}_\xi }{{\rm Ga}_{1 - \xi }}{\rm As} $ (AlGaAs) solar cell containing (i) an $ n $-AlGaAs absorber layer with a graded bandgap and (ii) a periodically corrugated Ag backreflector combined with localized ohmic Pd–Ge–Au backcontacts. The bandgap of the absorber layer was varied either sinusoidally or linearly. An efficiency of 33.1% with the 2000-nm-thick $ n $-AlGaAs absorber layer is predicted with linearly graded bandgap along with silver backreflector and localized ohmic backcontacts, in comparison to 27.4% efficiency obtained with homogeneous bandgap and a continuous ohmic backcontact. Sinusoidal grading of the bandgap is predicted to enhance the maximum efficiency to 34.5%. Thus, grading the bandgap of the absorber layer, along with a periodically corrugated Ag backreflector and localized ohmic Pd-Ge-Au backcontacts, can help realize ultrathin and high-efficient AlGaAs solar cells for terrestrial applications.

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	$J_{s c}$ (mA ${c m}^{- 2}$ )	$V_{o c}$ (V)	$FF$ (%)	$η$ (%)
Model	29.8	1.081	85.1	27.4
Experiment [18]	29.5	1.045	84.6	26.1

$L_{s}$ (nm)	$L_{x}$ (nm)	$ζ$	$L_{A g}$ (nm)	$J_{s c}$ (mA ${c m}^{- 2}$ )	$V_{o c}$ (V)	$FF$ (%)	$η$ (%)
100	-	1	0	17.3	1.132	84.2	16.5
100	510	0.05	100	19.7	1.093	85.1	18.3
1000	-	1	0	28.3	1.089	85.1	26.3
1000	510	0.05	100	29.4	1.090	85.0	27.3
2000	-	1	0	29.8	1.081	85.1	27.4
2000	510	0.05	100	30.4	1.081	85.1	28.0

$L_{s}$ (nm)	$E_{g, w}$ (eV)	$E_{g, m i n}$ (eV)	$L_{x}$ (nm)	$ζ$	$J_{s c}$ (mA ${c m}^{- 2}$ )	$V_{o c}$ (V)	$FF$ (%)	$η$ (%)
100	2.09	1.424	500	0.05	18.9	1.149	85.1	18.5
200	2.09	1.424	510	0.05	21.6	1.128	85.2	20.7
300	2.09	1.424	502	0.05	24.1	1.124	85.7	23.2
400	2.09	1.424	510	0.05	25.8	1.119	86.5	24.9
500	2.09	1.424	500	0.05	27.0	1.117	86.3	26.1
1000	2.09	1.424	500	0.05	29.2	1.104	87.0	28.1
2000	2.09	1.424	510	0.05	30.2	1.090	87.3	28.8

$L_{s}$ (nm)	$E_{g, w}$ (eV)	$E_{g, m i n}$ (eV)	$E_{g, m a x}$ (eV)	A	$L_{x}$ (nm)	$ζ$	$J_{s c}$ (mA ${c m}^{- 2}$ )	$V_{o c}$ (V)	$FF$ (%)	$η$ (%)
100	1.424	1.424	1.98	0.99	500	0.05	16.8	1.399	89.3	21.0
200	1.424	1.424	1.98	0.99	510	0.05	19.0	1.422	81.9	22.2
300	1.424	1.424	1.98	1.0	502	0.05	19.1	1.441	85.3	23.5
400	1.424	1.424	1.98	0.99	510	0.05	19.8	1.453	86.5	24.9
500	1.424	1.424	1.98	0.98	500	0.05	20.5	1.462	87.1	26.1
1000	1.424	1.424	1.98	0.99	500	0.05	22.7	1.486	88.3	29.8
2000	1.424	1.424	1.98	1.0	500	0.05	24.7	1.507	88.8	33.1

$L_{s}$ (nm)	$E_{g, w}$ (eV)	$E_{g, m i n}$ (eV)	$E_{g, m a x}$ (eV)	A	$α$	$K$	$ψ$	$L_{x}$ (nm)	$ζ$	$J_{s c}$ (mA ${c m}^{- 2}$ )	$V_{o c}$ (V)	$FF$ (%)	$η$ (%)
100	2.09	1.424	1.98	1.0	6	3	0.75	510	0.05	16.1	1.455	90.3	21.2
200	2.09	1.424	1.98	1.0	6	1	0.75	520	0.05	19.2	1.471	80.2	22.6
300	2.06	1.424	1.98	1.0	6	1	0.74	512	0.05	19.7	1.486	80.2	23.5
400	2.09	1.424	1.98	1.0	6	1	0.75	509	0.05	20.2	1.497	82.0	24.8
500	2.08	1.424	1.98	1.0	6	1	0.75	524	0.05	20.8	1.505	83.0	26.0
1000	2.09	1.424	1.98	1.0	6	2	0.75	516	0.05	22.5	1.533	87.8	30.4
2000	2.09	1.424	1.98	0.99	6	3	0.75	550	0.05	24.8	1.556	89.2	34.5

Parameter	Symbol (unit)	AlInP [42, 43]	GaInP [42, 43,46]	${A l}_{ξ} {G a}_{1 - ξ} A s$ [44–46]
Bandgap	$E_{g}$ (eV)	2.35	1.9	$1.424 + 1.247 ξ$ , $0 \leq ξ < 0.45$ ;
				$1.9 + 0.125 ξ + 0.143 ξ^{2}$ , $0.45 \leq ξ \leq 1$
Electron affinity	$χ$ (eV)	3.78	4.1	$4.07 - 1.1 ξ$ , $0 \leq ξ < 0.45$ ;
				$3.64 - 0.14 ξ$ , $0.45 \leq ξ \leq 1$
Doping density	$N_{d}$ ( ${c m}^{- 3}$ )	$2 \times 10^{18}$ (acceptor)	$2 \times 10^{18}$ (donor)	$1 \times 10^{18}$ (acceptor and donor)
Conduction-band	$N_{c}$ ( ${c m}^{- 3}$ )	$2.5 \times 10^{18}$	$6.5 \times 10^{17}$	$2.5 \times 10^{19} (0.063 + 0.083 ξ)^{3 / 2}$ , $0 \leq ξ < 0.45$ ;
Density of states				$2.5 \times 10^{19} (0.85 - 0.14 ξ)^{3 / 2}$ , $0.45 \leq ξ < 1$
Valence-band density of states	$N_{v}$ ( ${c m}^{- 3}$ )	$7 \times 10^{18}$	$1.5 \times 10^{19}$	$2.5 \times 10^{19} (0.51 + 0.25 ξ)^{3 / 2}$
Electron mobility	$μ_{n}$ ( ${c m}^{2} V^{- 1} s^{- 1}$ )	100	500	$8 \times 10^{3} - 2.2 \times 10^{4} ξ + 10^{4} ξ^{2}$ , $0 \leq ξ < 0.45$ ;
				$- 255 + 1160 ξ - 720 ξ^{2}$ , $0.45 \leq ξ \leq 1$
Hole mobility	$μ_{p}$ ( ${c m}^{2} V^{- 1} s^{- 1}$ )	10	30	$370 - 970 ξ + 740 ξ^{2}$
DC relative permittivity	$ε_{d c}$	11.8	11.8	$13.18 - 3.12 ξ$
Defect/trap density	$N_{t}$ ( ${c m}^{- 3}$ )	$10^{17}$	$10^{17}$	$(1 + 9 ξ) \times 10^{15}$
Defect/trap level	$E_{t}$ (eV)	midgap	midgap	0.75 eV below conduction-band energy
Electron capture cross section	$σ_{n}$ ( ${c m}^{2})$	$10^{- 14}$	$10^{- 14}$	$10^{- 16}$
Hole capture cross section	$σ_{p}$ ( ${c m}^{2})$	$10^{- 14}$	$10^{- 14}$	$10^{- 16}$
Radiative recombination coefficient	$R_{B}$ ( ${c m}^{- 3} s^{- 1}$ )	$10^{- 10}$	$10^{- 10}$	$1.8 \times 10^{- 10}$
Electron thermal speed	$v_{t h, n}$ ( $c m s^{- 1}$ )	$10^{7}$	$10^{7}$	$(4.4 - 2.1 ξ) \times 10^{7}$
Hole thermal speed	$v_{t h, p}$ ( $c m s^{- 1}$ )	$10^{7}$	$10^{7}$	$(1.8 - 0.5 ξ) \times 10^{7}$
Auger electron recombination coefficient	$C_{n}$ ( ${c m}^{6} s^{- 1}$ )	$10^{- 30}$	$10^{- 30}$	$10^{- 30}$
Auger hole recombination coefficient	$C_{p}$ ( ${c m}^{6} s^{- 1}$ )	$10^{- 30}$	$10^{- 30}$	$10^{- 30}$

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Applied Optics