The carrier lifetimes obtained from the CdS/CIGS and CIGS layer are summarized as a function of substrate.
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.
- ultrafast carrier dynamics
- optical pump terahertz probe spectroscopy
- p-n junction
- defect states
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
||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|
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 ( ) can be easily formed due to their low formation energy, creating a shallow acceptor level . Thus, the defect level can have a dominant role in the trapping of photoexcited electrons. (In the OPTP experiments, we only considered photoexcited electrons, not holes, due to the higher mobility of photoexcited electrons as compared to photoexcited holes.) Considering that photocarriers are primarily excited in the surface region for the 400 nm, the trapping time of the surface defect states is relatively faster than the bulk defects of . Based on surface defects and , we assumed that and correspond to the trapping times of photoexcited electrons in the surface defect states, and the defects entirely distributed from surface to bulk, respectively.
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, reflecting the CIGS surface is not detected, because the photoexcited electrons are dominantly captured at bulk defect states before being trapped at the surface defect states. The lower the defect density, the more that carrier lifetime is increased. Here, is increased as compared to that for the 400 nm pump beam, which means that the bulk defect density, , decreases along the depth direction, resulting in a rise in . The carrier dynamics model in the pure CIGS film is illustrated in Figure 15a .
After deposition of the Zn(O,S) buffer layers on the CIGS film, both
were increased for the 400 nm pump beam, excluding the sputter-Zn(O,S) buffer layer. The increase in
is attributed to the surface curing effect, which reduces surface defect states after the deposition of the buffer layer. Since a thin-buffer layer does not entirely cover the rough surface of the CIGS film,
is slightly increased up to ~140
In the sputter-Zn(O,S) buffer layer, is dissipated because of its thick-film thickness of 70 nm, which relieves the surface scattering effect, as compared with the other buffer layers. In general, Zn atoms of the buffer layer have been known to diffuse toward the CIGS film. The diffused Zn atoms can occupy the sites, forming a Zn+ charge state as a donor [46, 47]. Thus, Zn substitution to , , causes a reduction in the bulk defect states available to trap photoexcited electrons, resulting in a rise of .
With the 800 nm pump beam, also disappeared, like the CIGS film, implying surface defect states were not detected. The values also increased, which is ascribed to the combination of increased in the surface region of the CIGS, and decreased bulk defect density, .
The carrier dynamics model in the Zn(O,S)/CIGS film is illustrated in Figure 15b . showed the highest values for both pump beams of 400 and 800 nm in the CBD-Zn(O,S) buffer layer grown on CIGS, as compared with the others. From these results, we carefully suggest that solar cell efficiency can be enhanced when carrier lifetime reflecting bulk defect density is prolonged.
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,
can be easily formed in CIGS, which has an important influence on carrier lifetimes. In the first study, we found that a deep complex defect level of (
), designated “
In the second study, the behavior of defect states was determined to play a dominant role. Based on the geometric characteristics of the CIGS layer, we defined the two types of defect states, and , which correspond to the trapping times of photoexcited electrons in the surface defect states at the CIGS surface, and the defects, which are entirely distributed in the CIGS bulk, respectively. From the fitted and values, we discovered that the defect states decrease in the depth direction of the CIGS, and Zn substitution to , , causes a decrease in bulk defect states available to trap photoexcited electrons, resulting in a rise of .
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).
Andrew M. Gabor, John R. Tuttle, David S. Albin, Miguel A. Contreras, Rommel Noufi, and Allen M. Hermann: High‐efficiency CuInxGa1−xSe2 solar cells made from (Inx,Ga1−x)2Se3 precursor films 65: 198–200. DOI: 10.1063/1.112670
Clas Persson, Yu-Jun Zhao, Stephan Lany, and Alex Zunger: n-type doping of CuInSe2 and CuGaSe2 72: 035211. DOI: 10.1103/PhysRevB.72.035211
Ingrid Repins, Miguel A. Contreras, Brian Egaas, Clay DeHart, John Scharf, Craig L. Perkins, Bobby To, and Rommel Noufi: 19.9%‐efficient ZnO/CdS/CuInGaSe2 solar cell with 81.2% fill factor 16: 235–239. DOI: 10.1002/pip.822
Philip Jackson, Dimitrios Hariskos, Erwin Lotter, Stefan Paetel, Roland Wuerz, Richard Menner, Wiltraud Wischmann, and Michael Powalla: New world record efficiency for Cu(In,Ga)Se2 thin‐film solar cells beyond 20% 19: 894–897. DOI: 10.1002/pip.1078
Philip Jackson, Roland Wuerz, Dimitrios Hariskos, Erwin Lotter, Wolfram Witte, and Michael Powalla: Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6%
B Ohnesorge, R Weigand, G Bacher, A Forchel, W Riedl, and FH Karg: Minority-carrier lifetime and efficiency of Cu(In,Ga)Se2 solar cells 73: 1224–1226. DOI: 10.1063/1.122134
Michael Hafemeister, Susanne Siebentritt, Jürgen Albert, Martha Ch Lux-Steiner, and Sascha Sadewasser: Large neutral barrier at grain boundaries in chalcopyrite thin films 104: 196602. DOI: 10.1103/PhysRevLett.104.196602
Harry Mönig, Y Smith, Raquel Caballero, Christian Kaufmann, Iver Lauermann, Martha ch. Lux-Steiner, and Sascha Sadewasser: Direct evidence for a reduced density of deep level defects at grain boundaries of Cu(In,Ga)Se2 thin films 105: 116802. DOI: 10.1103/PhysRevLett.105.116802
Shengbai Zhang, Su-Huai Wei, Alex Zunger, and H Katayama-Yoshida: Defect physics of the CuInSe2 chalcopyrite semiconductor 57: 9642. DOI: 10.1103/PhysRevB.57.9642
Yong-Duck Chung, Dae-Hyung Cho, Won-Seok Han, Nae-Man Park, Kyu-Seok Lee, and Jeha Kim: Incorporation of Cu in Cu(In,Ga)Se2-based thin-film solar cells 57: 1826–1830. DOI: 10.3938/jkps.57.1826
Dae-Hyung Cho, Yong-Duck Chung, Kyu-Seok Lee, Nae-Man Park, Kyung-Hyun Kim, Hae-Won Choi, and Jeha Kim: Influence of growth temperature of transparent conducting oxide layer on Cu(In,Ga)Se2 thin-film solar cells 520: 2115–2118. DOI: 10.1016/j.tsf.2011.08.083
Jae‐Hyung Wi, Woo‐Jung Lee, Dae‐Hyung Cho, Won Seok Han, Jae Ho Yun, and Yong‐Duck Chung: Characteristics of temperature and wavelength dependence of CuInSe2 thin‐film solar cell with sputtered Zn(O,S) and CdS buffer layers 211: 2172–2176. DOI: 10.1002/pssa.201431232
Katsumi Kushiya and Osamu Yamase: Stabilization of PN heterojunction between Cu(InGa)Se2 thin-film absorber and ZnO window with Zn(O,S,OH)x buffer 39: 2577. DOI: 10.1143/JJAP.39.2577
Taizo Kobayashi, Hiroshi Yamaguchi, and Tokio Nakada: Effects of combined heat and light soaking on device performance of Cu(In,Ga)Se2 solar cells with ZnS(O,OH) buffer layer 22: 115–121. DOI: 10.1002/pip.2339
Negar Naghavi, Solange Temgoua, Thibaud Hildebrandt, Jean François Guillemoles, and Daniel Lincot: Impact of oxygen concentration during the deposition of window layers on lowering the metastability effects in Cu(In,Ga)Se2/CBD Zn(S,O) based solar cell 23: 1820–1827. DOI: 10.1002/pip.2626
Rohit P. Prasankumar, Prashanth C. Upadhya, and Antoinette J. Taylor: Ultrafast carrier dynamics in semiconductor nanowires 246: 1973–1995. DOI: 10.1002/pssb.200945128
Priti Tiwana, Patrick Parkinson, Michael B. Johnston, Henry J. Snaith, and Laura M. Herz: Ultrafast terahertz conductivity dynamics in mesoporous TiO2: influence of dye sensitization and surface treatment in solid-state dye-sensitized solar cells 114: 1365–1371. DOI: 10.1021/jp908760r
Ronald Ulbricht, Euan Hendry, Jie Shan, Tony F. Heinz, and Mischa Bonn: Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy 83: 543. DOI: 10.1103/RevModPhys.83.543
Woo-Jung Lee, Dae-Hyung Cho, Jae-Hyung Wi, Won Seok Han, Yong-Duck Chung, Jaehun Park, Jung Min Bae, and Mann-Ho Cho: Na-dependent ultrafast carrier dynamics of CdS/Cu(In,Ga)Se2 measured by optical pump-terahertz probe spectroscopy 119: 20231–20236. DOI: 10.1021/acs.jpcc.5b02282
Leeor Kronik, David Cahen, and Hans Werner Schock: Effects of sodium on polycrystalline Cu(In,Ga)Se2 and its solar cell performance 10: 31–36. DOI: 10.1002/(SICI)1521-4095(199801)10:1<31::AID-ADMA31>3.0.CO;2-3
Dominik Rudmann, Antonio F. da Cunha, Marc Kaelin, Fe Kurdesau, Hans Zogg, Ayodhya N. Tiwari, and Gerhard Bilger: Efficiency enhancement of Cu(In,Ga)Se2 solar cells due to post-deposition Na incorporation 84: 1129–1131. DOI: 10.1063/1.1646758
Shogo Ishizuka, Akimasa Yamada, Muhammad Monirul Islam, Hajime Shibata, Paul Fons, Takeaki Sakurai, Katsuhiro Akimoto, and Shigeru Niki: Na-induced variations in the structural, optical, and electrical properties of Cu(In,Ga)Se2 thin films 106: 034908. DOI: 10.1063/1.3190528
Dae-Hyung Cho, Kyu-Seok Lee, Yong-Duck Chung, Ju-Hee Kim, Soo-Jeong Park, and Jeha Kim: Electronic effect of Na on Cu(In,Ga)Se2 solar cells 101: 023901. DOI: 10.1063/1.4733679
Woo-Jung Lee, Dae-Hyung Cho, Jae-Hyung Wi, Won Seok Han, Jeha Kim, and Yong-Duck Chung: Na effect on flexible Cu(In,Ga)Se2 photovoltaic cell depending on diffusion barriers (SiOx, i-ZnO) on stainless steel 147: 783–787. DOI: 10.1016/j.matchemphys.2014.06.021
Andreas Othonos: Probing ultrafast carrier and phonon dynamics in semiconductors 83: 1789–1830. DOI: 10.1063/1.367411
Ingo Dirnstorfer, Mt. Wagner, Detlev M. Hofmann, MD Lampert, Franz Karg, and Bruno K. Meyer: Characterization of CuIn(Ga)Se2 thin films 168: 163–175. DOI: 10.1002/(SICI)1521-396X(199807)168:1<163::AID-PSSA163>3.0.CO;2-T
Rajmund Bacewicz, P. Zuk, and R. Trykozko: Photoluminescence study of ZnO/CdS/Cu(In,Ga)Se2 solar cells 11: 277–280.
Sho Shirakata, Katsuhiko Ohkubo, Yasuyuki Ishii, and Tokio Nakada: Effects of CdS buffer layers on photoluminescence properties of Cu(In,Ga)Se2 solar cells 93: 988–992. DOI: 10.1016/j.solmat.2008.11.043
Young Min Shin, Chang Soo Lee, Dong Hyeop Shin, Young Min Ko, Essam A Al-Ammar, Hyuck Sang Kwon, and Byung Tae Ahn: Characterization of Cu(In,Ga)Se2 solar cells grown on Na-free glass with an NaF layer on a Mo film 2: P248–P252. DOI: 10.1149/2.002306jss
Su-Huai Wei, Shengbai Zhang, and Alex Zunger: Effects of Ga addition to CuInSe2 on its electronic, structural, and defect properties 72: 3199–3201. DOI: 10.1063/1.121548
S Zotta, Karl Leo, Martin Ruckh, and Hans Warner Schock: Photoluminescence of polycrystalline CuInSe2 thin films 68: 1144–1146. DOI: 10.1063/1.115704
Shih-Chen Chen, Yu-Kuang Liao, Hsueh-Ju Chen, Chia-Hsiang Chen, Chih-Huang Lai, Yu-Lun Chueh, Hao-Chung Kuo, Kaung-Hsiung Wu, Jenh-Yih Juang, and Shun-Jen Cheng: Ultrafast carrier dynamics in Cu(In,Ga)Se2 thin films probed by femtosecond pump-probe spectroscopy 20: 12675–12681. DOI: 10.1364/OE.20.012675
Makoto Okano, Yutaro Takabayashi, Takeaki Sakurai, Katsuhiro Akimoto, Hajime Shibata, Shigeru Niki, and Yoshihiko Kanemitsu: Slow intraband relaxation and localization of photogenerated carriers in CuIn1−xGaxSe2 thin films: Evidence for the existence of long-lived high-energy carriers 89: 195203. DOI: 10.1103/PhysRevB.89.195203
Stephan Lany and Alex Zunger: Intrinsic DX Centers in Ternary Chalcopyrite Semiconductors 100: 016401. DOI: 10.1103/PhysRevLett.100.016401
Yu-Jun Zhao, Clas Persson, Stephan Lany, and Alex Zunger: Why can CuInSe2 be readily equilibrium-doped n-type but the wider-gap CuGaSe2 cannot? Applied Physics Letters. 2004; 85: 5860–5862. DOI: 10.1063/1.1830074
Stephan Lany, Yu-Jun Zhao, Clas Persson, and Alex Zunger: Halogen n-type doping of chalcopyrite semiconductors 86: 42109–42109. DOI: 10.1063/1.1854218
Tokio Nakada, Masayuki Mizutani, Y Hagiwara, and Akio Kunioka: High-efficiency Cu(In,Ga)Se2 thin-film solar cells with a CBD-ZnS buffer layer 67: 255–260. DOI: 10.1016/S0927-0248(00)00289-0
Tokio Nakada and Masayuki Mizutani: 18% efficiency Cd-free Cu(In,Ga)Se2 thin-film solar cells fabricated using chemical bath deposition (CBD)-ZnS buffer layers 41: L165.
Negar Naghavi, Stefani Spiering, Michael Powalla, Bruno Cavana, and Daniel Lincot: High‐efficiency copper indium gallium diselenide (CIGS) solar cells with indium sulfide buffer layers deposited by atomic layer chemical vapor deposition (ALCVD) 11: 437–443. DOI: 10.1002/pip.50
Muhammad Monirul Islam, Shogo Ishizuka, Akimasa Yamada, Keiichiro Sakurai, Shigeru Niki, Takeaki Sakurai, and Katsuhiro Akimoto: CIGS solar cell with MBE-grown ZnS buffer layer 93: 970–972. DOI: 10.1016/j.solmat.2008.11.047
Theresa Magorian Friedlmeier, Philip Jackson, Andreas Bauer, Dimitrios Hariskos, Oliver Kiowski, Roland Wuerz, and Michael Powalla: Improved photocurrent in Cu (In,Ga)Se2 solar cells: from 20.8% to 21.7% efficiency with CdS buffer and 21.0% Cd-free 5: 1487–1491. DOI: 10.1109/JPHOTOV.2015.2458039
Dae-Hyung Cho, Woo-Jung Lee, Sang-Woo Park, Jae-Hyung Wi, Won Seok Han, Jeha Kim, Mann-Ho Cho, Dongseop Kim, and Yong-Duck Chung: Non-toxically enhanced sulfur reaction for formation of chalcogenide thin films using a thermal cracker 2: 14593–14599. DOI: 10.1039/c4ta02507e
Jae-Hyung Wi, Tae Gun Kim, Jeong Won Kim, Woo-Jung Lee, Dae-Hyung Cho, Won Seok Han, and Yong-Duck Chung: Photovoltaic performance and interface behaviors of Cu(In,Ga)Se2 solar cells with a sputtered-Zn(O,S) buffer layer by high-temperature annealing 7: 17425–17432. DOI: 10.1021/acsami.5b04815
Dae-Hyung Cho, Woo-Jung Lee, Jae-Hyung Wi, Won Seok Han, Tae Gun Kim, Jeong Won Kim, and Yong-Duck Chung: Interface analysis of Cu(In,Ga)Se2 and ZnS formed using sulfur thermal cracker 38: 265–271. DOI: 10.4218/etrij.16.2515.0031
Woo-Jung Lee, Hye-Jung Yu, Jae-Hyung Wi, Dae-Hyung Cho, Won Seok Han, Jisu Yoo, Yeonjin Yi, Jung-Hoon Song, and Yong-Duck Chung: Behavior of photocarriers in the light-induced metastable state in the p-n heterojunction of a Cu(In,Ga)Se2 solar cell with CBD-ZnS buffer layer. ACS Applied Materials & Interfaces. 2016; 8:22151–22158. DOI: 10.1021/acsami.6b05005
Tokio Nakada, Tomoyuki Kume, Takahiro Mise, and Akio Kunioka: Superstrate-type Cu(In,Ga)Se2 thin film solar cells with ZnO buffer layers 37: L499.
Chang-Soo Lee, Suncheul Kim, Essam A Al-Ammar, HyuckSang Kwon, and Byung Tae Ahn: Effects of Zn diffusion from (Zn,Mg)O buffer to CIGS film on the performance of Cd-Free Cu(In,Ga)Se2 solar cells 3: Q99–Q103. DOI: 10.1149/2.003406jss