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Title: Nanoscale insight into the p-n junction of alkali-incorporated Cu(In,Ga)Se 2 solar cells

Abstract

The effects of alkali diffusion and post-deposition treatment in three-stage processed Cu(In,Ga)Se 2 solar cells are examined by using atom probe tomography and electrical property measurements. Cells, for which the substrate was treated at 650 °C to induce alkali diffusion from the substrate prior to absorber deposition, exhibited high open-circuit voltage (758 mV) and efficiency (18.2%) and also exhibited a 50 to 100-nm-thick ordered vacancy compound layer at the metallurgical junction. Surprisingly, these high-temperature samples exhibited higher concentrations of K at the junction (1.8 at.%) than post-deposition treatment samples (0.4 at.%). A model that uses Ga/(Ga + In) and Cu/(Ga + In) profiles to predict bandgaps (+/-17.9 meV) of 22 Cu(In,Ga)Se2 solar cells reported in literature was discussed and ultimately used to predict band properties at the nanoscale by using atom probe tomography data. The high-temperature samples exhibited a greater drop in the valence band maximum (200 meV) due to a lower Cu/(Ga + In) ratio than the post-deposition treatment samples. There was an anticorrelation of K concentrations and Cu/(Ga + In) ratios for all samples, regardless of processing conditions. In conclusion, changes in elemental profiles at the active junctions correlate well with the electrical behaviour of these devices.

Authors:
ORCiD logo [1];  [2]; ORCiD logo [2];  [3];  [3]
  1. National Renewable Energy Lab. (NREL), Golden, CO (United States); Colorado School of Mines, Golden, CO (United States)
  2. National Renewable Energy Lab. (NREL), Golden, CO (United States)
  3. Colorado School of Mines, Golden, CO (United States)
Publication Date:
Research Org.:
National Renewable Energy Lab. (NREL), Golden, CO (United States)
Sponsoring Org.:
USDOE Office of Energy Efficiency and Renewable Energy (EERE)
OSTI Identifier:
1375623
Alternate Identifier(s):
OSTI ID: 1374459
Report Number(s):
NREL/JA-5K00-68439
Journal ID: ISSN 1062-7995
Grant/Contract Number:
AC36-08GO28308
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Progress in Photovoltaics
Additional Journal Information:
Journal Volume: 25; Journal Issue: 9; Journal ID: ISSN 1062-7995
Publisher:
Wiley
Country of Publication:
United States
Language:
English
Subject:
14 SOLAR ENERGY; 77 NANOSCIENCE AND NANOTECHNOLOGY; bandgap; alkali; chalcogenides; atom probe tomography

Citation Formats

Stokes, Adam, Al-Jassim, Mowafak, Norman, Andrew, Diercks, David, and Gorman, Brian. Nanoscale insight into the p-n junction of alkali-incorporated Cu(In,Ga)Se2 solar cells. United States: N. p., 2017. Web. doi:10.1002/pip.2883.
Stokes, Adam, Al-Jassim, Mowafak, Norman, Andrew, Diercks, David, & Gorman, Brian. Nanoscale insight into the p-n junction of alkali-incorporated Cu(In,Ga)Se2 solar cells. United States. doi:10.1002/pip.2883.
Stokes, Adam, Al-Jassim, Mowafak, Norman, Andrew, Diercks, David, and Gorman, Brian. Wed . "Nanoscale insight into the p-n junction of alkali-incorporated Cu(In,Ga)Se2 solar cells". United States. doi:10.1002/pip.2883. https://www.osti.gov/servlets/purl/1375623.
@article{osti_1375623,
title = {Nanoscale insight into the p-n junction of alkali-incorporated Cu(In,Ga)Se2 solar cells},
author = {Stokes, Adam and Al-Jassim, Mowafak and Norman, Andrew and Diercks, David and Gorman, Brian},
abstractNote = {The effects of alkali diffusion and post-deposition treatment in three-stage processed Cu(In,Ga)Se2 solar cells are examined by using atom probe tomography and electrical property measurements. Cells, for which the substrate was treated at 650 °C to induce alkali diffusion from the substrate prior to absorber deposition, exhibited high open-circuit voltage (758 mV) and efficiency (18.2%) and also exhibited a 50 to 100-nm-thick ordered vacancy compound layer at the metallurgical junction. Surprisingly, these high-temperature samples exhibited higher concentrations of K at the junction (1.8 at.%) than post-deposition treatment samples (0.4 at.%). A model that uses Ga/(Ga + In) and Cu/(Ga + In) profiles to predict bandgaps (+/-17.9 meV) of 22 Cu(In,Ga)Se2 solar cells reported in literature was discussed and ultimately used to predict band properties at the nanoscale by using atom probe tomography data. The high-temperature samples exhibited a greater drop in the valence band maximum (200 meV) due to a lower Cu/(Ga + In) ratio than the post-deposition treatment samples. There was an anticorrelation of K concentrations and Cu/(Ga + In) ratios for all samples, regardless of processing conditions. In conclusion, changes in elemental profiles at the active junctions correlate well with the electrical behaviour of these devices.},
doi = {10.1002/pip.2883},
journal = {Progress in Photovoltaics},
number = 9,
volume = 25,
place = {United States},
year = {Wed Apr 05 00:00:00 EDT 2017},
month = {Wed Apr 05 00:00:00 EDT 2017}
}

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  • A comparative study with focusing on carrier recombination properties in Cu{sub 2}ZnSn(S,Se){sub 4} (CZTSSe) and the CuInGaSe{sub 2} (CIGS) solar cells has been carried out. For this purpose, electroluminescence (EL) and also bias-dependent time resolved photoluminescence (TRPL) using femtosecond (fs) laser source were performed. For the similar forward current density, the EL-intensity of the CZTSSe sample was obtained significantly lower than that of the CIGS sample. Primarily, it can be attributed to the existence of excess amount of non-radiative recombination center in the CZTSSe, and/or CZTSSe/CdS interface comparing to that of CIGS sample. In case of CIGS sample, TRPL decaymore » time was found to increase with the application of forward-bias. This can be attributed to the reduced charge separation rate resulting from the reduced electric-field at the junction. However, in CZTSSe sample, TRPL decay time has been found almost independent under the forward and reverse-bias conditions. This phenomenon indicates that the charge recombination rate strongly dominates over the charge separation rate across the junction of the CZTSSe sample. Finally, temperature dependent V{sub OC} suggests that interface related recombination in the CZTSSe solar cell structure might be one of the major factors that affect EL-intensity and also, TRPL decay curves.« less
  • The precise control of alkali-metal concentrations in Cu(In,Ga)Se 2 (CIGS) solar cells via post deposition treatment (PDT) has recently attracted attention. When PDT is performed at an elevated temperature, an accompanying annealing effect is expected. Here, we investigate how thermal annealing affects the redistribution of alkali metals in CIGS solar cells on glass substrates and the properties of the solar cells. In addition, we investigate the origin of non-homogeneous alkali-metal depth profiles that are typical of CIGS grown using a three-stage process. In particular, we use secondary-ion mass spectrometry measurements of the ion concentration as a function of distance frommore » the CIGS surface to investigate the impact of thermal annealing on the distribution of alkali metals (Na, Ka, and Rb) and constituent elements (Ga and In) in the CIGS absorbers. We find that the depth profiles of the alkali metals strongly reflect the density of sites that tend to accommodate alkali metals, i.e., vacancies. Annealing at elevated temperature caused a redistribution of the alkali metals. The thermal-diffusion kinetics of alkali metals depends strongly on the species involved. We introduced low flux potassium fluoride (KF) to study a side effect of KF-PDT, i.e., Na removal from CIGS, separately from its predominant effects such as surface modification. When sufficient amounts of Na are supplied from the soda lime glass via annealing at an elevated temperature, the negative effect was not apparent. Conversely, when the Na supply was not sufficient, it caused a deterioration of the photovoltaic properties.« less
  • Our group studied the effects of conduction band offset of window/Cu(In,Ga)Se{sub 2} (CIGS) layers on CIGS-based solar cell performance. To control the conduction band offset, we considered the use of a window layer of Zn{sub 1{minus}x}Mg{sub x}O thin film with a controllable band gap as an alternative to the conventional window layer using CdS film. From the measurement of valence band offset between Zn{sub 1{minus}x}Mg{sub x}O/CIGS layers and the band gap of each layer, we confirmed that the conduction band offset of Zn{sub 1{minus}x}Mg{sub x}O/CIGS layers could be controlled by changing the Mg content of the Zn{sub 1{minus}x}Mg{sub x}O film.more » The CIGS-based solar cells prepared for this study consisted of an ITO/Zn{sub 1{minus}x}Mg{sub x}O/CIGS/Mo/soda-lime glass structure. When the conduction band minimum of Zn{sub 1{minus}x}Mg{sub x}O was higher than that of CIGS, the performance of CIGS-based solar cells with a Zn{sub 1{minus}x}Mg{sub x}O window layer was equivalent to that of CIGS-based solar cells with CdS window layers. We confirmed that the control of the conduction band offset of the window/CIGS layers decreases the majority carrier recombination via the Zn{sub 1{minus}x}Mg{sub x}O/CIGS interface defects. {copyright} 2001 American Institute of Physics.« less
  • Al/sub 0.2/Ga/sub 0.8/As p/sup +/-n junction solar cells were fabricated by molecular beam epitaxy (MBE) and the relationship between cell properties and growth conditions was examined. It was found that growth temperature strongly influenced the minority carrier diffusion length in cell layers. At a growth temperature of 700/sup 0/C, minority carrier diffusion length was much improved and a high conversion efficiency of 12.9% (1 sun AM1.5, for an active area) was obtained.