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Optics Express

Optics Express

  • Editor: C. Martijn de Sterke
  • Vol. 20, Iss. 12 — Jun. 4, 2012
  • pp: 12675–12681
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Ultrafast carrier dynamics in Cu(In,Ga)Se2 thin films probed by femtosecond pump-probe spectroscopy

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, Shun-Jen Cheng, Tung-Po Hsieh, and Takayoshi Kobayashi  »View Author Affiliations


Optics Express, Vol. 20, Issue 12, pp. 12675-12681 (2012)
http://dx.doi.org/10.1364/OE.20.012675


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Abstract

Ultrafast carrier dynamics in Cu(In,Ga)Se2 films are investigated using femtosecond pump-probe spectroscopy. Samples prepared by direct sputtering and co-evaporation processes, which exhibited remarkably different crystalline structures and free carrier densities, were found to result in substantially different carrier relaxation and recombination mechanisms. For the sputtered CIGS films, electron-electron scattering and Auger recombination was observed, whereas for the co-evaporated CIGS films, bandgap renormalization accompanied by band filling effect and hot phonon relaxation was observed. The lifetime of defect-related recombination in the co-evaporated CIGS films is much longer than that in the direct-sputtered CIGS films, reflecting a better quality with higher energy conversion efficiency of the former.

© 2012 OSA

1. Introduction

2. Experiments

The CIGS thin films investigated in this study were prepared by direct sputtering without post-selenization (sputtered CIGS) and conventional three-stage co-evaporation (co-evaporated CIGS) processes. A post-treatment utilizing KCN solution was performed to remove the Cu segregation and secondary phases in both samples.

For OPOP measurements, a commercial cavity-dumped Ti:sapphire laser system providing short pulses (~100 fs) with repetition rate of 5 MHz and wavelength of 800 nm (hv = 1.55 eV) was used. The power of the pump beam was varied from 10 to 60 mW and was focused at a diameter of about 35 μm. Thus, the corresponding pump fluence was ranging from 0.21 to 1.25 mJ/cm2 while the probe fluence was fixed at 20 μJ/cm2. The pump pulses were modulated at 2 KHz with chopper. A mechanical delay stage was used to vary the time delay between the pump and probe pulses. The transient reflectivity change ΔR/R of the probe beam was measured as a function of the pump-probe delay time. The small reflected signals were detected and fed into a lock-in amplifier.

3. I-V characteristics and SEM images

Figures 1(a)
Fig. 1 I-V characteristics of CIGS solar cells prepared by (a) direct sputtering and (b) co-evaporation processes. Insets show the corresponding SEM images of the two samples.
and 1(b) show the typical I-V characteristics (IVCs) of the sputtered and co-evaporated CIGS samples, respectively. From the IVCs, the obtained VOC (open-circuit voltage), JSC (short-circuit current density), filling factor (FF), and solar efficiency (η) for the sputtered and co-evaporated CIGS samples are 0.50 V and 0.66 V, 24.58 mA/cm2 and 25.81 mA/cm2, 62% and 72%, and 7.61% and 12.35%, respectively. The results are summarized in Table 1

Table 1. The photovoltaic parameters for direct sputtered and co-evaporated CIGS samples

table-icon
View This Table
. A lower VOC for the sputtered CIGS is attributed to a smaller bandgap comparing to that of the co-evaporated CIGS, which has been confirmed by photoluminescence spectra (not shown here). Therefore, it was anticipated that the sputtered CIGS films should exhibit a higher JSC than the co-evaporated CIGS films. Nevertheless, an opposite trend was observed. Namely, the JSC of the sputtered CIGS is slightly lower than that of the co-evaporated CIGS. One of the possible reasons might be due to a higher defect concentration in the p-n junction of the sputtered CIGS that suppresses the JSC. The insets show the corresponding SEM images of both CIGS absorption layers. It is evident that the two films display very different grain morphologies, with a columnar structure for the sputtered CIGS thin film while that for the co-evaporated CIGS films exhibits a coaxial grain structure. We suspect that columnar grain boundaries of the former may have played an important role in suppressing the JSC. In addition, the lower FF exhibited in the sputtered CIGS comparing to that of the co-evaporated CIGS reflects a higher shunt leakage, which is also attributable to the columnar grain structure of the former. It appears that the columnar grain structure may have resulted in a higher concentration of recombination centers for the sputtered CIGS solar cells, which, in turn, yields a lower efficiency of 7.61% as compared to 12.35% obtained for the co-evaporated CIGS solar cells.

4. Optical pump-optical probe measurements

In order to discuss the mechanism of the fast relaxation process for the sputtered CIGS, the fast relaxation time (τfast) versus total carrier density was plotted and displayed in Fig. 3(a)
Fig. 3 (a) The extracted carrier cooling lifetime (τfast) in a short delay time under different carrier densities in the direct sputtered CIGS film. (b) The 1/τslow extracted from the slower component in the biexponential decay function used to fit the reflectivity transient of the direct sputtered CIGS thin films.
. Here we assume that the total carrier density is approximately equal to the photoexcited carrier density since the sputtered CIGS has a carrier density in the range of 1018 ~1019 cm−3 measured by Hall effect measurement, which is smaller than that of photoexcited carriers. For example, a total carrier density of 4.19 × 1019 cm−3 could be generated under a pump fluence of 0.21 mJ/cm2. As shown in Fig. 3(a), τfast decreases from ~7.49 to ~3.76 ps as the total carrier density increases from ~4.19 × 1019 cm−3 to ~2.51 × 1020 cm−3 and reaches the plateau value of ~3.76 ps at the total carrier density larger than ~1.50 × 1020 cm−3. One of the hot carrier relaxation channels at high electron concentration is that the hot electron energy loss through LO phonon emissions, which is called the hot phonon effect and the screening effect [9

9. A. Othonos, “Probing ultrafast carrier and phonon dynamics in semiconductors,” J. Appl. Phys. 83(4), 1789–1830 (1998). [CrossRef]

]. In this case, the electron relaxation time increases as the excitation density increases. However, our measurement results obviously do not follow this expectation. This anomalous carrier density dependence is similar to those results reported by Tsai et al. [10

10. T. R. Tsai, C. F. Chang, and S. Gwo, “Ultrafast hot electron relaxation time anomaly in InN epitaxial films,” Appl. Phys. Lett. 90(25), 252111 (2007). [CrossRef]

], wherein the hot electron relaxation process in InN and GaAs has been attributed to electron-electron scattering [11

11. J. A. Kash, “Carrier-carrier scattering: An experimental comparison of bulk GaAs and GaAs/AlxGa1-xAs quantum wells,” Phys. Rev. B Condens. Matter 48(24), 18336–18339 (1993). [CrossRef] [PubMed]

,12

12. D. W. Snoke, “Density dependence of electron scattering at low density,” Phys. Rev. B Condens. Matter 50(16), 11583–11591 (1994). [CrossRef] [PubMed]

]. Therefore, we may conclude that the dominant fast electron relaxation process in the sputtered CIGS films is the electron-electron scattering.

5. Conclusion

Acknowledgments

References and links

1.

P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, and M. Powalla, “New world record efficiency for Cu(In,Ga)Se2 thin-film solar cells beyond 20%,” Prog. Photovolt. Res. Appl. 19(7), 894–897 (2011). [CrossRef]

2.

M. Ganchev, J. Kois, M. Kaelin, S. Bereznev, E. Tzvetkova, O. Volobujeva, N. Stratieva, and A. Tiwari, “Preparation of Cu(In,Ga)Se2 layers by selenization of electrodeposited Cu–In–Ga precursors,” Thin Solid Films 511–512, 325–327 (2006). [CrossRef]

3.

G. M. Hanket, W. N. Shafarman, B. E. McCandless, and R. W. Birkmire, “Incongruent reaction of Cu–(InGa) intermetallic precursors in H2Se and H2S,” J. Appl. Phys. 102(7), 074922 (2007). [CrossRef]

4.

V. Alberts, J. Titus, and R. W. Birkmire, “Material and device properties of single-phase Cu(In,Ga)(Se,S)2 alloys prepared by selenizationy/sulfurization of metallic alloys,” Thin Solid Films 451–452, 207–211 (2004). [CrossRef]

5.

S. Chaisitsak, A. Yamada, and M. Konagai, “Preferred orientation control of Cu(In1-xGax)Se2 (x ≈ 0.28) thin films and its influence on solar cell characteristics,” Jpn. J. Appl. Phys. 41(Part 1, No. 2A), 507–513 (2002). [CrossRef]

6.

C. H. Liu, C. H. Chen, S. Y. Chen, Y. T. Yen, W. C. Kuo, Y. K. Liao, J. Y. Juang, H. C. Kuo, C. H. Lai, L. J. Chen, and Y. L. Chueh, “Large scale single-crystal Cu(In,Ga)Se2 nanotip arrays for high efficiency solar cell,” Nano Lett. 11(10), 4443–4448 (2011). [CrossRef] [PubMed]

7.

M. Nishitani, T. Negami, N. Kohara, and T. Wada, “Analysis of transient photocurrents in Cu(In,Ga)Se2 thin film solar cells,” J. Appl. Phys. 82(7), 3572–3575 (1997). [CrossRef]

8.

B. Ohnesorge, R. Weigand, G. Bacher, A. Forchel, W. Riedl, and F. H. Karg, “Minority-carrier lifetime and efficiency of Cu(In,Ga)Se2 solar cells,” Appl. Phys. Lett. 73(9), 1224–1226 (1998). [CrossRef]

9.

A. Othonos, “Probing ultrafast carrier and phonon dynamics in semiconductors,” J. Appl. Phys. 83(4), 1789–1830 (1998). [CrossRef]

10.

T. R. Tsai, C. F. Chang, and S. Gwo, “Ultrafast hot electron relaxation time anomaly in InN epitaxial films,” Appl. Phys. Lett. 90(25), 252111 (2007). [CrossRef]

11.

J. A. Kash, “Carrier-carrier scattering: An experimental comparison of bulk GaAs and GaAs/AlxGa1-xAs quantum wells,” Phys. Rev. B Condens. Matter 48(24), 18336–18339 (1993). [CrossRef] [PubMed]

12.

D. W. Snoke, “Density dependence of electron scattering at low density,” Phys. Rev. B Condens. Matter 50(16), 11583–11591 (1994). [CrossRef] [PubMed]

13.

A. Haug, “Carrier density dependence of Auger recombination,” Solid-State Electron. 21(11-12), 1281–1284 (1978). [CrossRef]

14.

T. Korn, A. Franke-Wiekhorst, S. Schnüll, and I. Wilke, “Characterization of nanometer As-clusters in low-temperature grown GaAs by transient reflectivity measurements,” J. Appl. Phys. 91(4), 2333–2336 (2002). [CrossRef]

15.

R. Ascázubi, I. Wilke, S. Cho, H. Lu, and W. J. Schaff, “Ultrafast recombination in Si-doped InN,” Appl. Phys. Lett. 88(11), 112111 (2006). [CrossRef]

OCIS Codes
(040.5350) Detectors : Photovoltaic
(300.6530) Spectroscopy : Spectroscopy, ultrafast

ToC Category:
Solar Energy

History
Original Manuscript: March 16, 2012
Revised Manuscript: April 26, 2012
Manuscript Accepted: May 9, 2012
Published: May 21, 2012

Citation
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, Shun-Jen Cheng, Tung-Po Hsieh, and Takayoshi Kobayashi, "Ultrafast carrier dynamics in Cu(In,Ga)Se2 thin films probed by femtosecond pump-probe spectroscopy," Opt. Express 20, 12675-12681 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-12-12675


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References

  1. P. Jackson, D. Hariskos, E. Lotter, S. Paetel, R. Wuerz, R. Menner, W. Wischmann, and M. Powalla, “New world record efficiency for Cu(In,Ga)Se2 thin-film solar cells beyond 20%,” Prog. Photovolt. Res. Appl.19(7), 894–897 (2011). [CrossRef]
  2. M. Ganchev, J. Kois, M. Kaelin, S. Bereznev, E. Tzvetkova, O. Volobujeva, N. Stratieva, and A. Tiwari, “Preparation of Cu(In,Ga)Se2 layers by selenization of electrodeposited Cu–In–Ga precursors,” Thin Solid Films511–512, 325–327 (2006). [CrossRef]
  3. G. M. Hanket, W. N. Shafarman, B. E. McCandless, and R. W. Birkmire, “Incongruent reaction of Cu–(InGa) intermetallic precursors in H2Se and H2S,” J. Appl. Phys.102(7), 074922 (2007). [CrossRef]
  4. V. Alberts, J. Titus, and R. W. Birkmire, “Material and device properties of single-phase Cu(In,Ga)(Se,S)2 alloys prepared by selenizationy/sulfurization of metallic alloys,” Thin Solid Films451–452, 207–211 (2004). [CrossRef]
  5. S. Chaisitsak, A. Yamada, and M. Konagai, “Preferred orientation control of Cu(In1-xGax)Se2 (x ≈ 0.28) thin films and its influence on solar cell characteristics,” Jpn. J. Appl. Phys.41(Part 1, No. 2A), 507–513 (2002). [CrossRef]
  6. C. H. Liu, C. H. Chen, S. Y. Chen, Y. T. Yen, W. C. Kuo, Y. K. Liao, J. Y. Juang, H. C. Kuo, C. H. Lai, L. J. Chen, and Y. L. Chueh, “Large scale single-crystal Cu(In,Ga)Se2 nanotip arrays for high efficiency solar cell,” Nano Lett.11(10), 4443–4448 (2011). [CrossRef] [PubMed]
  7. M. Nishitani, T. Negami, N. Kohara, and T. Wada, “Analysis of transient photocurrents in Cu(In,Ga)Se2 thin film solar cells,” J. Appl. Phys.82(7), 3572–3575 (1997). [CrossRef]
  8. B. Ohnesorge, R. Weigand, G. Bacher, A. Forchel, W. Riedl, and F. H. Karg, “Minority-carrier lifetime and efficiency of Cu(In,Ga)Se2 solar cells,” Appl. Phys. Lett.73(9), 1224–1226 (1998). [CrossRef]
  9. A. Othonos, “Probing ultrafast carrier and phonon dynamics in semiconductors,” J. Appl. Phys.83(4), 1789–1830 (1998). [CrossRef]
  10. T. R. Tsai, C. F. Chang, and S. Gwo, “Ultrafast hot electron relaxation time anomaly in InN epitaxial films,” Appl. Phys. Lett.90(25), 252111 (2007). [CrossRef]
  11. J. A. Kash, “Carrier-carrier scattering: An experimental comparison of bulk GaAs and GaAs/AlxGa1-xAs quantum wells,” Phys. Rev. B Condens. Matter48(24), 18336–18339 (1993). [CrossRef] [PubMed]
  12. D. W. Snoke, “Density dependence of electron scattering at low density,” Phys. Rev. B Condens. Matter50(16), 11583–11591 (1994). [CrossRef] [PubMed]
  13. A. Haug, “Carrier density dependence of Auger recombination,” Solid-State Electron.21(11-12), 1281–1284 (1978). [CrossRef]
  14. T. Korn, A. Franke-Wiekhorst, S. Schnüll, and I. Wilke, “Characterization of nanometer As-clusters in low-temperature grown GaAs by transient reflectivity measurements,” J. Appl. Phys.91(4), 2333–2336 (2002). [CrossRef]
  15. R. Ascázubi, I. Wilke, S. Cho, H. Lu, and W. J. Schaff, “Ultrafast recombination in Si-doped InN,” Appl. Phys. Lett.88(11), 112111 (2006). [CrossRef]

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