Field modulation in Na-incorporated Cu(In,Ga)Se2 (CIGS) polycrystalline films influenced by alloy-hardening and pair-annihilation probabilities
- Yonkil Jeong†1,
- Chae-Woong Kim†2,
- Dong-Won Park1,
- Seung Chul Jung2,
- Jongjin Lee1 and
- Hee-Sang Shim1Email author
© Jeong et al; licensee Springer. 2011
Received: 12 August 2011
Accepted: 7 November 2011
Published: 7 November 2011
The influence of Na on Cu(In,Ga)Se2 (CIGS) solar cells was investigated. A gradient profile of the Na in the CIGS absorber layer can induce an electric field modulation and significantly strengthen the back surface field effect. This field modulation originates from a grain growth model introduced by a combination of alloy-hardening and pair-annihilation probabilities, wherein the Cu supply and Na diffusion together screen the driving force of the grain boundary motion (GBM) by alloy hardening, which indicates a specific GBM pinning by Cu and Na. The pair annihilation between the ubiquitously evolving GBMs has a coincident probability with the alloy-hardening event.
PACS: 88. 40. H-, 81. 10. Aj, 81. 40. Cd,
KeywordsCu(In,Ga)Se2 solar cells grain growth model alloy hardening pair-annihilation
Thin film solar cells are promising candidates for power generation and other integrated photovoltaic applications, as part of an effort to develop new renewable energy technologies [1, 2]. Specifically, chalcopyrite semiconductor systems, such as Cu(In,Ga)Se2 (CIGS), have attracted a great deal of interest as potential absorber materials for thin film solar cells. In recent years, the CIGS solar cells have demonstrated efficiencies of greater than 20% using three-stage co-evaporation methods . One of the common methods for improving the performance of CIGS solar cells uses soda-lime glass (SLG) substrates, in which the amount of Na incorporated into the CIGS absorber layer is on the order of 0.1 at.% [4, 5]. Several models have been proposed that explain the effect of Na on device performance. Wang et al. reported that the carrier concentration in the CIGS absorber layer increases due to a reduction in the amount of compensating (In,Ga)Cu defects due to the substitution with NaCu[6–8]. In contrast, Herberholz et al. suggest that the existence range of α-CuInSe2 widens due to the incorporation of 0.1 at.% of Na, which suppresses the formation of the β-phase . Rockett suggests that Na incorporation leads to an increase in the grain size and the lowest energy surfaces, such as Se-terminated (112) surfaces, due to an increase in the atomic mobility during CIGS growth and at grain boundaries [10, 11]. Based on ab initio calculations, Persson and Zunger demonstrate that NaCu defects or NaInSe2 phases at grain boundaries decrease the valence-band (VB) maximum due to a lack of Na d-electron states, which is similar to the case of (2V Cu + InCu) neutral defect complexes at grain boundaries [12, 13]. However, the dominant phenomenon is an increase in the output voltage from the perspective of CIGS device physics, and an increase in the grain size from a crystallographic perspective. These increases are being recognized as far more acceptable explanations for the improvement in performance.
In this paper, we discuss how an increase in the fill factor is caused by the back surface field (BSF) effect from the perspective of CIGS device physics, and a structural change in the grain size from a crystallographic approach using a combination of alloy-hardening and pair-annihilation events.
Fabrication of CIGS absorber film and device
CTE and chemical composition of soda-lime glass and Corning glass substrates
Chemical contents (%)
Na 2 O
8.4 (in the range of 25°C to approximately 513°C)
4.2 (in the range of 25°C to approximately 671°C)
The microstructure of the CIGS absorber layers was investigated using transmission electron microscopy (TEM, JEOL, Tokyo, Japan) operated at an acceleration voltage of 200 keV. The samples were prepared using a dual focused ion beam (FIB) system. Depth profiling of the chemical composition in both device structures was performed with a secondary ion mass spectrometer (SIMS, in a Cameca IMS 4f system, CAMECA SAS, Gennevilliers Cedex, France) using an impact energy of 7.5 keV and a 200 nA O2 + beam and detecting MCs + complexes (M = 63Cu, 115In, 69 Ga, 80Se, 23Na, and98Mo). The solar cell efficiencies were measured and recorded using a Keithley 4300 source meter under 100-mW/cm2 irradiation (Oriel® Sol3A™, 450-W solar simulator equipped with an AM 1.5-G filter; Oriel Instruments, Irvine, CA, USA) and an incident-photon-to-electron conversion efficiency measurement system for the wavelength range of 300 to 1,200 nm (QEX7, PV Measurements Inc., Boulder, Colorado, USA).
Results and discussion
Microstructure and photovoltaic performances
Parameters obtained from illuminated J-V curves.
J sc(mA cm-2)
Grain growth model
We fabricated Na-restricted and Na-incorporated CIGS solar cells to investigate the influence of Na on the device performances. The enhancement in the output voltage and photocurrent density for the Na-incorporated device was negligible, while the FF revealed a remarkable increase. This finding could be attributed to the strengthening of the BSF by the energy-level pinning in the bottom region of the CIGS absorber layer. The energy-level pinning originates from the proposed grain growth model wherein the Cu supply and the Na diffusion both contribute to the grain growth by a combination of alloy-hardening and pair-annihilation events that occur between grain boundaries.
grain boundary motion
back surface field
coefficient of thermal expansion
transmission electron microscopy
focused ion beam
secondary ion mass spectrometer
external quantum efficiency
- V oc :
- J sc :
This work was supported by the Core Technology Development Program for the Next-generation Solar Cells of Research Institute for Solar and Sustainable Energies (RISE), GIST.
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