研究目的
Investigating the effects of the lightest alkali-metal, Li, on CIGS thin-film and device properties compared to the effects of heavy alkali-metals such as K, Rb, and Cs.
研究成果
Elemental Li has minor effects on enhancing the PL peak intensity and photovoltaic performance of CIGS compared to heavier alkali-metals. Li does not significantly affect CIGS surface modification or growth orientation. The study suggests that the minor effect of Li may be due to its small ionic radius and unstable local environment in Se-based CIGS.
研究不足
The study focuses on the effects of Li on CIGS thin-films and devices, but the beneficial effects are found to be relatively small compared to heavier alkali-metals. The mechanism behind the minor effect of Li is not fully understood and requires further investigation.
1：Experimental Design and Method Selection
CIGS films were grown on sputtered-Mo coated substrates by the three-stage process using elemental Cu, In, Ga, and Se Knudsen cell sources in a vacuum chamber. Lithium fluoride (LiF) was supplied during the first stage or after CIGS film growth (postdeposition treatment, PDT) using a LiF Knudsen cell source.
2：Sample Selection and Data Sources
Alkali-containing SLG and alkali-free a-plane sapphire or sintered zirconia substrates were used. The typical CIGS film composition ratios [Cu]/([Ga]+[In]) (CGI) and [Ga]/([Ga]+[In]) (GGI) were about 0.9 and 0.35, respectively.
3：List of Experimental Equipment and Materials
Knudsen cell sources for Cu, In, Ga, Se, LiF, and RbF; electron-probe micro analyzer (EPMA); scanning electron microscopy (SEM); x-ray diffraction (XRD); photoluminescence (PL) and time-resolved PL (TRPL) measurements; secondary ion mass spectrometry (SIMS); capacitance-voltage (C-V), current-voltage (I-V), and external quantum efficiency (EQE) measurements.
4：Experimental Procedures and Operational Workflow
CIGS films were grown using the three-stage process with specific substrate temperatures for each stage. LiF-PDT was performed with a Se-vapor supply at Ts ~ 350℃ for 10 min. CdS buffer-layer was formed by chemical bath deposition. Photovoltaic devices were fabricated using sputter-deposited intrinsic ZnO (i-ZnO) and conductive ZnO:Al layers.
5：Data Analysis Methods
XRD measurements were carried out in the θ-2θ mode using Cu Kα radiation. PL and TRPL measurements were carried out at room temperature. Solar cell parameters were obtained from I-V measurements under 1 sun illumination (AM 1.5 G) at 25℃. NCV values were calculated from C-V measurement results.
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Knudsen cell
Used for the evaporation of elemental Cu, In, Ga, Se, LiF, and RbF during the CIGS film growth process.
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electron-probe micro analyzer
EPMA
Used to determine the composition ratios of CIGS films.
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scanning electron microscopy
SEM
Used to observe the surface morphology of CIGS films.
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x-ray diffraction
XRD
Used to analyze the crystal structure and orientation of CIGS films.
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photoluminescence
PL
Used to measure the PL spectra of CIGS films.
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time-resolved PL
TRPL
Used to measure the lifetime of charge carriers in CIGS films.
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secondary ion mass spectrometry
SIMS
Used to obtain elemental depth profiles of CIGS films.
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capacitance-voltage
C-V
Used to calculate the nominal carrier density (NCV) of CIGS photovoltaic devices.
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current-voltage
I-V
Used to obtain solar cell parameters under 1 sun illumination.
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external quantum efficiency
EQE
Used to measure the quantum efficiency of CIGS photovoltaic devices.
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