The carrier lifetimes obtained from the CdS/CIGS and CIGS layer are summarized as a function of substrate.
Abstract
Among other materials, the p-type Cu(In,Ga)Se2 (CIGS) alloy has attracted attention as the most efficient absorber in thin-film solar cells. The typical CIGS layer is deposited with a polycrystalline structure containing an amount of native defect states, which serve as carrier traps and recombination centers. These defect states in the CIGS layer can be easily changed after deposition of an n-type buffer layer, due to the formation of p-n junctions. To understand the influence of the p-n junction on these defect states, the behavior of photoexcited carriers, from the CIGS absorber to the buffer layer, is considered to be an important issue and is closely related to solar cell performance. In this study, we performed experiments to investigate the ultrafast carrier dynamics of CIGS-based solar cells, using optical pump terahertz (THz) probe (OPTP) spectroscopy, and demonstrated the correlation between solar cell performance and the behavior of photoexcited carrier dynamics.
Keywords
- ultrafast carrier dynamics
- Cu(In
- Ga)Se2
- optical pump terahertz probe spectroscopy
- p-n junction
- defect states
1. Introduction
1.1. Basic properties of a CIGS solar cell
Cu(In,Ga)Se2 (CIGS) is an attractive material for solar cells because of its beneficial qualities, including a high absorption coefficient for visible light, favorable direct bandgap (Eg), and great power conversion efficiency [1–5]. CIGS is comprised of CuInSe2 (CIS)-CuGaSe2 (CGS) and its
A CIGS solar cell with a chemical bath deposited-ZnS (CBD-ZnS) buffer layer is composed of several stacked layers, as indicated in the scanning electron microscopy (SEM) image of Figure 1a . p-Type CIGS layers with a polycrystalline structure approximately 2.2 μm thick were deposited by the conventional coevaporation method of a multistage process on Mo coated soda lime glass (SLG) [10–12]. As the n-type buffer layer, CBD-ZnS with a thickness of ~30 nm was prepared on the CIGS layer by precipitation from an aqueous solution. RF sputtering was then conducted to form an i-ZnO(70 nm)/ITO(150 nm) film as the transparent conducting oxide (TCO) layer. Finally, a current-collecting grid of Ni(50 nm)/Al(3 μm) was deposited by e-beam evaporation, completing the structure of the solar cell described in Figure 1b .
In CIGS-based solar cells, understanding the interfacial reaction at the p-n junction between the p-type CIGS absorber and n-type buffer layer is particularly important. This is because the intrinsic defect states in the CIGS, which experience a metastable redistribution of charge carriers after deposition of the buffer layer, have an influence on device performance [13–15]. Moreover, these defect states and the properties of the p-n junction have a decisive effect on the carrier relaxation dynamics in the CIGS layer, dominating the device performance. Thus, to improve the CIGS efficiency in a solar cell, it is essential to understand the intrinsic characteristics of the native defects in the CIGS, and the p-n junction.
1.2. Optical pump-THz probe spectroscopy
Among various measurement tools, optical pump terahertz (THz) probe (OPTP) spectroscopy was utilized in this experiment to verify the effect of the defect states because it is an extremely sensitive tool for investigating scattering mechanisms and dynamic energy transitions of photoexcited carriers in the region of shorter timescales on the order of ~
To measure the photoexcited carrier dynamics related with defect states, THz-time domain spectroscopy (in short, THz-TDs) measurement is carried out in advance. A fs pulse laser with 800 nm wavelength and pulse duration of 120 fs is generated from a Ti:sapphire regenerative amplifier system (Micra-Legend Elite, Coherent Inc.), which is split into two beams; one is transformed into THz signal, passing <110> ZnTe crystal with a thickness of 1 mm and the other is maintained as a probe beam with a time delay by moving the delay stage. The former of THz signal is transmitted throughout the sample, and then detected by means of an electro-optic (EO) sampling method of 3 mm thick <110> ZnTe nonlinear crystal, which signal is collected with a lock-in amplifier (Stanford Research System, SR830). For OPTP measurement, the maximum point of the transmitted THz signal is verified and probe beam is positioned at this point by fixing the delay stage. The optical pump beam excites the samples with the time delay and transient change of the sample in the 0.2–2.6 THz frequency range was probed by THz probe pulse via EO sampling. The diameter of the pump beam is 3 mm, which is over two times larger than that of the THz probe beam. Thus, the time evolution of the pump-probe signal can be collected by scanning the time delay of the pump pulses with respect to the THz pulse. All OPTP experiments are carried out at room temperature in confined area by purging dry air.
2. Ultrafast carrier dynamics of CIGS solar cell
2.1. Na effect in CIGS solar cell
In CIGS solar cell, Na supply is known to be one of the methods for enhancing solar cell efficiency. Na atoms are typically supplied by utilizing SLG, which is a preferred substrate material for the industrial manufacturing of rigid CIGS-based modules. It is a generally accepted contention that Na atoms diffuse from the SLG into the CIGS layer through a Mo layer during fabrication of solar cell, beneficially affecting the conversion efficiency of the solar cell [20–24]. On the basis of Na effect, we assume that Na diffusion into the CIGS layer may alter the electronic structure and defect states. However, direct evidence of how Na influences the performance of solar cell has not yet been obtained experimentally. We consider the possibility that electronic structure of abundant defect states existed in the CIGS layer can be changed by Na content, which is a crucial issue in efforts to optimize solar cell design.
In this section, to investigate the formation of defect states in the CIGS layer depending on the Na content, the study of ultrafast carrier dynamics was conducted on the CIGS layers grown on two different substrates, borosilicate (BS) and SLG by measuring OPTP spectroscopy. Carrier dynamics related to defect states can be determined by the relaxation times of photoexcited carriers relative to the scattering rate or carrier-trapping at defect states, ranging from hundreds of
2.1.1. The fabrication of CIGS solar cell depending on Na content
CIGS layers of approximately 2.5 μm thick were deposited on Mo coated BS (Na2O: 4 at.%) and SLG (Na2O: 14 at.%), respectively. The Ga/III and Cu/III composition ratios of the CIGS layer were about 0.14 and 0.87 in both cases. As a buffer layer, an n-type CdS layer with a thickness of ~70 nm was grown on the CIGS via CBD method. Typical solar cells (ITO/i-ZnO/CdS/CIGS/Mo) were then fabricated on BS and SLG under identical conditions, and their efficiencies were determined to be 8.5 and 10.9%, respectively. The device performance on SLG was superior to that on BS by ~2.4%, which is ascribed to the diffusion of Na atoms from the SLG. To investigate how the substrates affected defect states, PL and OPTP measurements were conducted on CIGS and CdS/CIGS layers directly grown on BS and SLG without a Mo layer. For OPTP measurement, metal layer such as Mo should be removed because THz probe pulse can be easily absorbed by metal layer due to numerous free carriers.
SEM measurement was performed on each CdS/CIGS samples grown on BS and SLG as shown in Figure 3a, b and c, d, respectively. Approximately 2.5 μm thick CIGS absorber and 70 nm thick CdS buffer layer were similarly deposited on both substrates. We found out that grain size of CIGS grown on SLG is larger than that on BS, which is considered as diffused-Na from the SLG. To verify the existence of the Na content, depth profiles of the elemental constituents in the CdS/CIGS layer grown on BS and SLG were examined by using secondary ion mass spectroscopy (SIMS) as shown in Figure 4 . A substantial Na content diffused up to the CdS layer in the SLG case, but not the BS. SIMS results clearly demonstrate that the SLG can effectively supply Na atoms into the CIGS and the CdS layer as compared to BS.
2.1.2. OPTP spectroscopy results
To measure the photoexcited carrier dynamics, the measurement of THz-TDs is essential. Figure 5 shows the THz pulse spectra transmitted through the CIGS films grown on BS and SLG. After penetration of THz pulse through the samples, the intensity of THz pulse was drastically decreased as compared with the reference THz pulse (noted as “air”).
Based on the results of THz-TDs spectrum, the measurement of OPTP spectroscopy was conducted on the CIGS and CdS/CIGS layer. In this experiment, pump beam of 400 nm is utilized to excite photocarriers from the CdS buffer layer. When an intense
BS/CIGS | BS/CIGS/CdS | SLG/CIGS | SLG/CIGS/CdS | |
---|---|---|---|---|
|
1.7 ps | 9.3 ps | 1.3 ps | 7.6 ps |
|
1150 ps | 520 ps | 1110 ps | 2580 ps |
2.1.3. Ultrafast carrier dynamics at p-n junction of CdS/CIGS depending on Na content
To identify the defect states determining the lifetime of the relaxed carriers, PL measurement was performed with an excitation light of 400 nm, an identical value to the pump beam energy of OPTP measurement. PL peak assigned to the donor-acceptor pair (DAP) transition [26–29]. The DAP transition energy is measured of approximately 0.97 and 0.92 eV in CIGS on SLG and BS, respectively. Using the estimated
Generally, photocarriers excited by high photon energy comply with Fermi-Dirac distributions through carrier-carrier scattering within a few hundred
2.2. Variation of buffer layer in CIGS solar cell
Typical CIGS-based solar cells have a buffer layer between the CIGS absorber layer and the transparent ZnO front electrode, which plays an important role in improving the cell performance. Among various buffer materials, Zn-based materials have been frequently studied because of their beneficial properties, for example, good transparency with large direct
In this section, we fabricated a CIGS solar cell with Zn-based buffer layers grown by three deposition methods, cracker-Zn(O,S), CBD-Zn(O,S), and sputter-Zn(O,S), based on optimized conditions previously determined by our group [42–45]. To extract the correlation between cell efficiency and the interfacial characteristics between the CIGS and buffer layer, OPTP spectroscopy was utilized to investigate the CIGS, and the CIGS with the buffer layer, with respect to carrier trapping times at defect states.
2.2.1. The fabrication of a CIGS solar cell with a Zn-based buffer layer
A CIGS layer of approximately 2.2 μm thick was deposited on a Mo coated SLG. The Ga/III and Cu/III composition ratios of the CIGS layer were measured to be about 0.3 and 0.96, respectively, by X-ray fluorescence. We prepared two vacuum-based buffer layers of cracker-Zn(O,S) and sputter-Zn(O,S) with thicknesses of ~8 and 70 nm, respectively. We also arranged one chemical-based buffer layer of CBD-Zn(O,S) with a thickness of 30 nm. Since the buffer layers were deposited utilizing the optimized conditions from our group, their thicknesses were different.
Typical solar cells were fabricated with the various buffer layers, and the other deposition conditions were identical. The performance of the solar cell showed that the results depend on the buffer types. The best cell efficiency of approximately 13% was obtained for the CIGS solar cell with CBD-Zn(O,S).
Details of the cell structures and their cell performance are provided in Figure 8 . For the OPTP measurement, CIGS without a Mo layer and a Zn-based buffer layer was directly deposited on SLG, and the injected and transmitted optical and THz pulses are simply illustrated in Figure 9 .
2.2.2. OPTP spectroscopy results
For OPTP measurement, THz-TDs were conducted on the samples by comparing the THz pulse before and after transmission through the sample. Figure 10a shows the THz pulse spectra transmitted through the SLG, which is drastically reduced in comparison with the reference THz pulse through “air”. After penetration of the THz pulse through the sample of CIGS and the Zn(O,S) buffer layers deposited on the CIGS, the intensity of the THz pulses were found to be similar to each other, as indicated in Figure 10b . This means that no photocarriers were excited by the THz pulse.
Based on the results of the THz-TDs spectra, OPTP spectroscopy measurements were carried out on the CIGS and Zn(O,S)/CIGS layers. In those experiments, two pump beams of 400 and 800 nm were used to investigate optically photoexcited carrier dynamics along the depth distribution.
Figure 11a and b show the −Δ
Considering the penetration depth of the pump beam into the CIGS film, a wavelength of 400 nm should penetrate the surface of the CIGS less than 50 nm, and the 800 nm beam should reach the near-surface of the CIGS at about 150 nm. Thus, we assume that the charge carrier density acting as long-lived photocarriers is higher at the near-surface than the surface of the CIGS.
To extract carrier lifetimes relative to defect states, we fitted the normalized Δ
In this study, a sharp peak appeared at the near-edge of the normalized Δ
After deposition of the Zn(O,S) buffer layers on the CIGS film, OPTP spectroscopy was also conducted. The photoexcited carrier density and carrier lifetimes at 800 nm were much higher than that at 400 nm, which is a similar tendency to the measured spectra from the CIGS films as shown in Figure 11a . After depositing the buffer layer on the CIGS, the decay curves were extended regardless of the deposition method of buffer layer for both the 400 and 800 nm pump beams.
To clearly express the distinctive features of the carrier lifetimes according to buffer types, we fitted the normalized Δ
Pump beam energy | Lifetimes | CIGS | Cracker | CBD | Sputter |
---|---|---|---|---|---|
400 nm |
|
61.8 | 141 | 119 | - |
|
1552 | 2171 | 2529 | 2212 | |
800 nm |
|
- | - | - | - |
|
1930 | 2611 | 3325 | 2500 |
2.2.3. Ultrafast carrier dynamics of the CIGS/Zn-based buffer layer
To investigate the carrier dynamics related to the defect states along the depth direction in the CIGS and Zn(O,S)/CIGS, 400 and 800 nm pump beam energies were applied. The 400 nm pump beam is sensitive to the surface of the film, and the 800 nm pump beam reflects from the near-surface of the film. The photocarriers in the CIGS film can be excited by both pump beams, whereas those of the Zn(O,S) film cannot be excited by the 800 nm pump beam due to its low energy of 1.55 eV.
To verify the light absorption of the Zn(O,S) buffer layer at the 800 nm wavelength depending on the deposition method, transmittance spectra were measured for various Zn(O,S) films directly grown on SLG, as indicated in Figure 14 . After deposition of the Zn(O,S) film, the wavelength of 800 nm mostly penetrated toward the CIGS film.
In the pure CIGS film, two carrier lifetimes (fast:
Even though there are several defect levels in the
Among the several point defects that exist in the CIGS film, Cu vacancies (
With the injection of the 800 nm pump beam into the CIGS film, photoexcited electrons occur at the near-surface, and not the surface. Thus,
After deposition of the Zn(O,S) buffer layers on the CIGS film, both
In the sputter-Zn(O,S) buffer layer,
With the 800 nm pump beam,
The carrier dynamics model in the Zn(O,S)/CIGS film is illustrated in
Figure 15b
.
3. Conclusion
In summary, we have demonstrated the effectiveness of OPTP measurement for determining the ultrafast carrier dynamics of photocarriers excited from CIGS and buffer layers, depending on substrate type (BS and SLG) and several Zn(O,S) buffer layers (cracker-, CBD-, sputter-Zn(O,S)). By fitting the normalized Δ
Considering the enthalpy of formation energy,
In the second study, the behavior of
Acknowledgments
This work was supported by the “New & Renewable Energy” project of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korean government Ministry Of Trade, Industry, & Energy (20153010011990, 20153000000030). The authors also would like to acknowledge the financial support from the R&D Convergence Program of MSIP (Ministry of Science, ICT and Future Planning) and ISTK (Korea Research Council for Industrial Science and Technology) of the Republic of Korea (Grant B551179-12-01-00).
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