Optical scattering modeling of etched ZnO:Al superstrates and device simulation studies of a-Si:H solar cells with different texture morphologies

Xia Yan; Weimin Li; Armin G. Aberle; Selvaraj Venkataraj

doi:10.1364/AO.55.006718

Applied Optics
Vol. 55,
Issue 24,
pp. 6718-6726
(2016)
•https://doi.org/10.1364/AO.55.006718

Optical scattering modeling of etched ZnO:Al superstrates and device simulation studies of a-Si:H solar cells with different texture morphologies

Xia Yan, Weimin Li, Armin G. Aberle, and Selvaraj Venkataraj

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Xia Yan, Weimin Li, Armin G. Aberle, and Selvaraj Venkataraj, "Optical scattering modeling of etched ZnO:Al superstrates and device simulation studies of a-Si:H solar cells with different texture morphologies," Appl. Opt. 55, 6718-6726 (2016)

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Abstract

Transparent conductive oxide (TCO) materials have been widely used as the front electrodes of thin-film amorphous silicon (a-Si:H) solar cells. To improve the performance of solar cells, textured front TCO is required as the optical layer which effectively scatters the incoming light and thus enhances the photon absorption within the device. One promising TCO material is aluminum-doped zinc oxide (AZO), which is most commonly prepared by magnetron sputtering. After deposition, sputtered AZO films are typically wet-chemically etched using diluted hydrochloric (HCl) or hydrofluoric (HF) acid to obtain rough surface morphologies. In this paper, we report the effects of a textured AZO front electrode on the performance of a-Si:H solar cells based on optical scattering modeling and electrical device simulations, involving four different AZO surface morphologies. The simulated light scattering behaviors indicate that a better textured surface not only scatters more light, but also allows more light get transmitted into the absorber ( $\sim 90 %$ of visible light), due to greatly reduced front reflection by the rough surface. Device simulation results show that the two-step AZO texturing process should give improved a-Si:H solar cell performance, with an enhanced short-circuit current density of $16.5 mA / {cm}^{2}$ , which leads to a high photovoltaic (PV) efficiency of 9.9%.

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Sample	Power (kW)	Pressure (mbar)	Temperature (°C)	Thickness (nm)	Etchant Solution	Etching Time (s)
As-grown	2.0	$3 \times 10^{- 3}$	350	650	–	–
HF Textured	2.0	$3 \times 10^{- 3}$	350	650	1% HF	20
HCl Textured	2.0	$3 \times 10^{- 3}$	350	650	0.5% HCl	20
Two-step Textured	2.0	$3 \times 10^{- 3}$	350	650	0.5% HCl/1% HF	20/20

Structure
	$p$ -layer	$i$ -layer	$n$ -layer
$d (m)$	$15 \times 10^{- 9}$	$300 \times 10^{- 9}$	$20 \times 10^{- 9}$
Contact
$R_{series} (Ω {cm}^{2})$	4
$R_{shunt} (Ω {cm}^{2})$	4000
Transport, Doping and Extended States
	$p$ -layer	$i$ -layer	$n$ -layer
$E_{μ} (eV)$	1.90	1.75	1.73
$ε_{r}$	11.9	11.9	11.9
$χ (eV)$	4.08	4.13	4.10
$N_{v} (m^{- 3})$	$4.6 \times 10^{26}$	$4.6 \times 10^{26}$	$4.6 \times 10^{26}$
$N_{c} (m^{- 3})$	$4.6 \times 10^{26}$	$4.6 \times 10^{26}$	$4.6 \times 10^{26}$
$μ_{n} (m^{2} / Vs)$	$1 \times 10^{- 3}$	$1 \times 10^{- 2}$	$1 \times 10^{- 3}$
$μ_{p} (m^{2} / Vs)$	$1 \times 10^{- 4}$	$1 \times 10^{- 3}$	$1 \times 10^{- 4}$
$E_{a} (eV)$	0.55	−	0.29
Band Tail States
	$p$ -layer	$i$ -layer	$n$ -layer
$E_{v 0} (meV)$	85	47	85
$N_{v 0} (m^{- 3})$	$4.6 \times 10^{27}$	$3.0 \times 10^{25}$	$4.6 \times 10^{27}$
$σ_{p}^{0} (m^{- 3} / s)$	$7 \times 10^{- 16}$	$7 \times 10^{- 16}$	$7 \times 10^{- 16}$
$σ_{n}^{+} (m^{- 3} / s)$	$7 \times 10^{- 15}$	$7 \times 10^{- 15}$	$7 \times 10^{- 15}$
$E_{c 0} (meV)$	51	28	51
$N_{c 0} (m^{- 3})$	$4.6 \times 10^{27}$	$3.0 \times 10^{25}$	$4.6 \times 10^{27}$
$σ_{n}^{0} (m^{- 3} / s)$	$7 \times 10^{- 16}$	$7 \times 10^{- 16}$	$7 \times 10^{- 16}$
$σ_{p}^{-} (m^{- 3} / s)$	$7 \times 10^{- 15}$	$7 \times 10^{- 15}$	$7 \times 10^{- 15}$
Dangling Bond States
	$p$ -layer	$i$ -layer	$n$ -layer
$U (eV)$	0.2	0.2	0.2
$N_{db} (m^{- 3})$	$1.35 \times 10^{25}$	$1.9 \times 10^{22}$	$2.7 \times 10^{24}$
$E_{db} (eV)$	1.27	1.27	1.24
$σ_{p}^{0} (m^{- 3} / s)$	$1 \times 10^{- 15}$	$1 \times 10^{- 15}$	$1 \times 10^{- 15}$
$σ_{n}^{0} (m^{- 3} / s)$	$1 \times 10^{- 15}$	$1 \times 10^{- 15}$	$1 \times 10^{- 15}$
$σ_{p}^{-} (m^{- 3} / s)$	$1 \times 10^{- 14}$	$1 \times 10^{- 14}$	$1 \times 10^{- 14}$
$σ_{n}^{+} (m^{- 3} / s)$	$1 \times 10^{- 14}$	$1 \times 10^{- 14}$	$1 \times 10^{- 14}$

		Equivalent Photocurrent Density ( $J_{eqv}, mA / {cm}^{2}$ )
Layer	Thickness (nm)	As-grown	HF Textured	HCl Textured	Two-step Textured
Front AZO	650	1.46	1.90	2.67	2.71
$p$ -layer	15	1.11	1.48	1.80	2.05
$i$ -layer	300	11.75	12.99	14.23	14.65
$n$ -layer	20	0.40	0.58	0.74	0.84
Back AZO	100	0.11	0.30	0.63	0.74
Back Al	100	1.10	1.80	2.88	2.51
Reflection at front side		10.48	6.73	3.82	3.32
Yablonovitch limit ( $4 n^{2}$ )		23.2
Shockley–Queisser limit		26.8

Sample	Power (kW)	Pressure (mbar)	Temperature (°C)	Thickness (nm)	Etchant Solution	Etching Time (s)
As-grown	2.0	$3 \times 10^{- 3}$	350	650	–	–
HF Textured	2.0	$3 \times 10^{- 3}$	350	650	1% HF	20
HCl Textured	2.0	$3 \times 10^{- 3}$	350	650	0.5% HCl	20
Two-step Textured	2.0	$3 \times 10^{- 3}$	350	650	0.5% HCl/1% HF	20/20

Structure
	$p$ -layer	$i$ -layer	$n$ -layer
$d (m)$	$15 \times 10^{- 9}$	$300 \times 10^{- 9}$	$20 \times 10^{- 9}$
Contact
$R_{series} (Ω {cm}^{2})$	4
$R_{shunt} (Ω {cm}^{2})$	4000
Transport, Doping and Extended States
	$p$ -layer	$i$ -layer	$n$ -layer
$E_{μ} (eV)$	1.90	1.75	1.73
$ε_{r}$	11.9	11.9	11.9
$χ (eV)$	4.08	4.13	4.10
$N_{v} (m^{- 3})$	$4.6 \times 10^{26}$	$4.6 \times 10^{26}$	$4.6 \times 10^{26}$
$N_{c} (m^{- 3})$	$4.6 \times 10^{26}$	$4.6 \times 10^{26}$	$4.6 \times 10^{26}$
$μ_{n} (m^{2} / Vs)$	$1 \times 10^{- 3}$	$1 \times 10^{- 2}$	$1 \times 10^{- 3}$
$μ_{p} (m^{2} / Vs)$	$1 \times 10^{- 4}$	$1 \times 10^{- 3}$	$1 \times 10^{- 4}$
$E_{a} (eV)$	0.55	−	0.29
Band Tail States
	$p$ -layer	$i$ -layer	$n$ -layer
$E_{v 0} (meV)$	85	47	85
$N_{v 0} (m^{- 3})$	$4.6 \times 10^{27}$	$3.0 \times 10^{25}$	$4.6 \times 10^{27}$
$σ_{p}^{0} (m^{- 3} / s)$	$7 \times 10^{- 16}$	$7 \times 10^{- 16}$	$7 \times 10^{- 16}$
$σ_{n}^{+} (m^{- 3} / s)$	$7 \times 10^{- 15}$	$7 \times 10^{- 15}$	$7 \times 10^{- 15}$
$E_{c 0} (meV)$	51	28	51
$N_{c 0} (m^{- 3})$	$4.6 \times 10^{27}$	$3.0 \times 10^{25}$	$4.6 \times 10^{27}$
$σ_{n}^{0} (m^{- 3} / s)$	$7 \times 10^{- 16}$	$7 \times 10^{- 16}$	$7 \times 10^{- 16}$
$σ_{p}^{-} (m^{- 3} / s)$	$7 \times 10^{- 15}$	$7 \times 10^{- 15}$	$7 \times 10^{- 15}$
Dangling Bond States
	$p$ -layer	$i$ -layer	$n$ -layer
$U (eV)$	0.2	0.2	0.2
$N_{db} (m^{- 3})$	$1.35 \times 10^{25}$	$1.9 \times 10^{22}$	$2.7 \times 10^{24}$
$E_{db} (eV)$	1.27	1.27	1.24
$σ_{p}^{0} (m^{- 3} / s)$	$1 \times 10^{- 15}$	$1 \times 10^{- 15}$	$1 \times 10^{- 15}$
$σ_{n}^{0} (m^{- 3} / s)$	$1 \times 10^{- 15}$	$1 \times 10^{- 15}$	$1 \times 10^{- 15}$
$σ_{p}^{-} (m^{- 3} / s)$	$1 \times 10^{- 14}$	$1 \times 10^{- 14}$	$1 \times 10^{- 14}$
$σ_{n}^{+} (m^{- 3} / s)$	$1 \times 10^{- 14}$	$1 \times 10^{- 14}$	$1 \times 10^{- 14}$

Abstract

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Applied Optics