OSA's Digital Library

Optics Express

Optics Express

  • Editor: C. Martijin de Sterke
  • Vol. 19, Iss. 7 — Mar. 28, 2011
  • pp: 6609–6615
« Show journal navigation

Increasing surface band gap of Cu(In,Ga)Se2 thin films by post depositing an In-Ga-Se thin layer

Xiao-Hui Tan, Sheng-Lin Ye, and Xu Liu  »View Author Affiliations


Optics Express, Vol. 19, Issue 7, pp. 6609-6615 (2011)
http://dx.doi.org/10.1364/OE.19.006609


View Full Text Article

Acrobat PDF (1110 KB)





Browse Journals / Lookup Meetings

Browse by Journal and Year


   


Lookup Conference Papers

Close Browse Journals / Lookup Meetings

Article Tools

Share
Citations

Abstract

We have developed a simple approach to fabricate wide band gap surface layer for Cu(In,Ga)Se2 (CIGS) thin film. The Cu depleted surface layer was reconstructed by an In-Ga-Se post deposition treatment at different temperatures, which was monitored by a light controlling method. A desirable Cu concentration in surface layer has been achieved after depositing a 80nm thick In-Ga-Se layer at 400°C and the corresponding device performance is remarkably improved compared with device without surface modification. Additionally, the excess Cu2- x Se phase on the surface could also be eliminated by this method in case of high Cu/(In+Ga).

© 2011 OSA

1. Introduction

Solar cells and modules based on polycrystalline Cu(In,Ga)Se2 (CIGS) thin films are considered to be a leading candidate for low-cost power generation. The performance of high-efficiency CIGS solar cells is limited mainly by moderate open circuit voltage (Voc). The Voc can be increased by increasing band gap of the CIGS absorber, although this gives a corresponding photocurrent loss. Analysis and modeling of CIGS device has shown that the Shockley-Read-Hall recombination in space charge region of the absorber layer limits the open circuit voltage [1

1. F. Engelhardt, M. Schmidt, Th. Meyer, O. Seifert, J. Parisi, and U. Rau, “Metastable electrical transport in Cu(In,Ga)Se2 thin films and ZnO/CdS/Cu(In,Ga)Se2 heterostructures,” Phys. Lett. A 245(5), 489–493 (1998). [CrossRef]

,2

2. S. H. Wei and A. Zunger, “Band offsets and optical bowings of chalcopyrites and Zn-based II-VI alloys,” J. Appl. Phys. 78(6), 3846–3856 (1995). [CrossRef]

]. It has also been experimentally proved that an elevated defect density in the near surface region contributes to the recombination [3

3. A. Rockett, D. Liao, J. T. Heath, J. D. Cohen, Y. M. Strzhemechny, L. J. Brillson, K. Ramanathan, and W. N. Shafarman, “Near surface defect distributions in Cu(In,Ga)Se2,” Thin Solid Films 431, 301–306 (2003). [CrossRef]

]. Numerical simulation conducted by Rau and Turcu has led to conclusion that the grading of the valence band close to the absorber surface provides a transport barrier for holes which reduces the density of holes at the interface and hence the interface recombination [4

4. U. Rau and M. Turcu, “Role of surface band gap widening in Cu(In,Ga)(Se,S)2 thin-films for the photovoltaic performance of ZnO/CdS/Cu(In,Ga)(Se,S)2 heterojunction solar cells,” Mater. Res. Soc. Symp. Proc. 763, 335–340 (2003).

,5

5. M. Turcu, O. Pakma, and U. Rau, “Interdependence of absorber composition and recombination mechanism in Cu(In, Ga)(Se,S)2 heterojunction solar cells,” Appl. Phys. Lett. 80(14), 2598–2600 (2002). [CrossRef]

]. Therefore incorporating a surface layer of wide band gap, accommodated by a valence-band offset, is an effective way to increase Voc without loss of photocurrent. Numerous researches on surface band gap engineering have been carried on. A grading structure of CIGS thin film has been achieved by varying Ga/In atomic ratio near the surface [6

6. T. Dullweber, G. Hanna, W. Shams-Kolahi, A. Schwartzlander, M. A. Contreas, R. Noufi, and H. W. Schock, “Study of the effect of gallium grading in Cu(In,Ga)Se2,” Thin Solid Films 361(1-2), 478–481 (2000). [CrossRef]

]. Grading the band gap of space charge region by surface sulfurization is more effective in comparison because more of the band gap increase is due to lowering of valence band in case of S incorporation. However, the sulfurization treatment using toxic H2S gas [7

7. T. Nakada, H. Ohbo, T. Watanabe, H. Nakazawa, M. Matsui, and A. Kunioka, “Improved Cu(In,Ga)(S,Se)2 thin film solar cells by surface sulfurization,” Sol. Energy Mater. Sol. Cells 49(1–4), 285–290 (1997). [CrossRef]

,8

8. U. P. Singh, W. N. Shafarman, and R. W. Birkmire, “Surface sulfurization studies of Cu(InGa)Se2 thin film,” Sol. Energy Mater. Sol. Cells 90(5), 623–630 (2006). [CrossRef]

] or difficultly controlled S vapor [9

9. D. Ohashi, T. Nakada, and A. Kunioka, “Improved CIGS thin-film solar cells by surface sulfurization using In2S3 and sulfur vapor,” Sol. Energy Mater. Sol. Cells 67(1–4), 261–265 (2001). [CrossRef]

] is not preferred in industry application. Mokel et al. found an unusually large band gap at the free surfaces of Cu-poor grown CIS thin films [10

10. M. Morkel, L. Weinhardt, B. Lohmuller, C. Heske, E. Umbach, W. Riedl, S. Zweigart, and F. Karg, “Flat conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction,” Appl. Phys. Lett. 79(21), 4482–4484 (2001). [CrossRef]

]. Direct evidences of surface band gap widening in CIGS thin films have also been reported [11

11. M. J. Romero, K. M. Jones, J. AbuShama, Y. Yan, M. M. Al-Jassim, and R. Noufi, “Surface-layer band gap widending in Cu(In,Ga)Se2 thin films,” Appl. Phys. Lett. 83(23), 4731–4733 (2003). [CrossRef]

,12

12. S.-H. Han, F. S. Hasoon, A. M. Hermann, and D. H. Levi, “Spectroscopy evidence for a surface layer in CuInSe2:Cu deficiency,” Appl. Phys. Lett. 91(2), 021904 (2007). [CrossRef]

]. Those observations have led to the assumption that a Cu-depleted chalcopyrite phase segregation, the so called ordered vacancy compound (ODC), occurs at the surfaces of the films [13

13. D. Schmid, M. Ruckh, F. Grunwald, and H. W. Schock, “Chalcopyrite defect chalcopyrite heterojunctions on the basis of CuInSe2,” J. Appl. Phys. 73(6), 2902–2909 (1993). [CrossRef]

]. Those surfaces were considered type inversion, which leads to a shift of the regime p=n into the absorber and hence away from the defect rich CdS/CIGS interface resulting in a reduced recombination rate [4

4. U. Rau and M. Turcu, “Role of surface band gap widening in Cu(In,Ga)(Se,S)2 thin-films for the photovoltaic performance of ZnO/CdS/Cu(In,Ga)(Se,S)2 heterojunction solar cells,” Mater. Res. Soc. Symp. Proc. 763, 335–340 (2003).

]. However, a high-quality ODC surface layer with conduction band alignment at CIGS/CdS interface [10

10. M. Morkel, L. Weinhardt, B. Lohmuller, C. Heske, E. Umbach, W. Riedl, S. Zweigart, and F. Karg, “Flat conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction,” Appl. Phys. Lett. 79(21), 4482–4484 (2001). [CrossRef]

] is difficult to be spontaneously obtained by using the regular three stage physical vapor deposition process especially in case of a relatively high process temperature or relatively low Se vapor pressure [14

14. S. Nishiwaki, N. Kohara, T. Negami, H. Miyake, and T. Wada, “Microstructure of Cu(In,Ga)Se2 Films deposited in Low Se Vapor Pressure,” Jpn. J. Appl. Phys. 38(Part 1, No. 5A), 2888–2892 (1999). [CrossRef]

]. Commonly, a KCN solution is used to modify the surfaces of CIGS thin films to obtain Cu-depleted surface layers [15

15. A. Darga, D. Mencaragila, Z. Djebbour, A. Migan Dubois, J. F. Guillemoles, J. P. Connolly, O. Roussel, D. Lincot, B. Canava, and A. Etcheberry, “Two step wet surface treatment influence on the electronic properties of Cu(In,Ga)Se2 solar cells,” Thin Solid Films 517(7), 2550–2553 (2009). [CrossRef]

].

In the present study, we have proposed a simple method to fabricate the wide band gap surface layer by reconstructing the Cu-depleted absorber surface. A light controlling system is firstly used to monitor the surface layers of CIGS thin films. Effects of surface layer fabricated by our approach on properties of CIGS thin films and devices have been investigated in details.

2. Experiment

The CdS buffer layer was deposited onto the CIGS absorber by chemical bath deposition (CBD) technique using a CdI2 (1.4×10−3M)-thiouea (0.14M)-ammonia (1M) aqueous solution. Approximately 20nm thick CdS layers were grown using a room temperature solution whose temperature rose to 80°C after one CBD run. Thick buffer layer were obtained by repeating this CBD process. CIGS solar cells were then completed by RF sputtering of i-ZnO/ZnO:Al front contacts, printing of Ag contact grids and mechanical scribing. There are 10×15 cells prepared on the substrate, and each one’s active area is 0.46 cm2. No anti-reflection coating was applied. The current density–voltage (J-V) measurements were performed under the standard 1.5 AM spectrum with 100mW/cm2 at 25°C. Average values of cells on the substrate were used for performance evaluation. Cells with very atypical characteristics were excluded from statistics.

3. Results and discusions

3.1 Monitoring details of PDT

Mono wavelength of 1100nm was used to monitor the thickness of IGS layer due to strong absorption of IGS material at visible spectrum. The monitoring glass was kept at 300°C to drive off Se. Reflective light monitoring mode was adopted in order to avoid contaminating photoelectric sensor by Se. Additionally, since refractive index of IGS thin film strongly depends on the Ga/(In+Ga) atomic ratio, deposition rates of In and Ga were regulated by crystal oscillator before PDT respectively based on compositional measurement by XRF and finally the Ga/(In+Ga) of IGS thin film was controlled around 0.4. Firstly, a thick IGS layer was deposited onto the monitoring glass. As shown in Fig. 1(a)
Fig. 1 (a) Monitoring curve of IGS layer. (b) The refractive index of IGS material with Ga/(In+Ga)~0.4. (c) cross-sectional SEM image of IGS layer and the thickness measured by SEM is 1.455μm.
, the light monitoring curve contains 18 peaks, and the peak width extends along with increase in deposition time. It is mainly because the anti-evaporation of Se on the chamber wall decreases the deposition rates, which may affect the Ga/(In+Ga) simultaneously as well. Figure 1(b) shows the optical constants of as deposited IGS layer which were determined by spectrometric measurements. The thickness of IGS layer calculated by software is 1437nm based on dielectric functions obtained from the transmission and reflection spectra. The thickness of IGS thin film measured by SEM is 1455nm as shown in Fig. 1(c), which basically agrees with the calculated result, thus the compositional ununiformity in depth could be negligible. After calibration by SEM result, the thickness of IGS layer corresponding to one peak in the light monitoring curve is around 80nm with errors within 1nm, suggesting a precise controlling for thickness of IGS layer. It is convenient to control the thickness of IGS layer for PDT at half peak positions and peak positions.

3.2 SEM results

Figure 2
Fig. 2 Cross-sectional SEM images of (a) IGS_0nm, (b) IGS500°C_80nm, (c) IGS400°C_80nm, (d) IGS300°C_80nm, (e) IGS300°C_320nm and (f) surface SEM image of IGS500°C_320nm.
shows the cross sectional and surface SEM image of CIGS thin films with PDT at different temperatures. It is observed that there is no clear difference on structural properties between IGS400°C_80nm, IGS500°C_80nm and IGS_0nm as shown in Figs. 2(a), 2(b) and 2(c). The post deposited IGS thin films are probably transformed into chalcopyrite phase due to Cu diffusion at relatively high T sub above 400°C, thus a clear surface layer could not be observed. While the IGS300°C_80nm shows fine grains on the surface and the IGS300°C_320nm clearly exhibits a double-layer structure consisting of large (bottom) and small (upper) crystal grains, which implies poor lattice interdiffusion at T sub of 300°C. According to the Cu-In-Se phase diagram [17

17. U.-C. Boehnke and G. Kühn, “Phase relations in the ternary system Cu-In-Se,” J. Mater. Sci. 22(5), 1635–1641 (1987). [CrossRef]

], those small grains formed at 300°C should be mostly in In2Se3 or Ga2Se3 phase. Obviously, the small grains would bring more surface defect states and influence the formation of p-n junction during chemical bath deposition (CBD) [18

18. T. Nakada and A. Kunioka, “Direct evidence of Cd diffusion into Cu(In,Ga)Se2 thin films during chemical-bath deposition process of CdS films,” Appl. Phys. Lett. 74(17), 2444–2446 (1999). [CrossRef]

]. Figure 2(f) shows the surface SEM image of IGS500°C_320nm. The triangular-shaped grains suggest the (112) plane of a chalcopyrite structure, indicating the surface material of IGS500°C_320nm is chalcopyrite phase.

3.3 Depth profiles analysis

Figure 3
Fig. 3 SIMS depth profile of IGS_0nm.
shows the SIMS depth profile of IGS_0nm. It is observed that the upper region (close to front surface) of thin film presents higher Cu intensity, which is probably resulted by excess Cu2- xSe formed on the surface during deposition stage 2. The Cu concentration distribution is not beneficial to the spontaneous formation of the Cu-poor surface layer during stage 3 since the In and Ga has higher activity in case of high Cu concentration.

In order to investigate the effects of PDT at different temperatures on Cu concentration in surface layer, AES depth measurements were performed on IGS400°C_80nm, IGS500°C_80nm and IGS_0nm. As can be seen in Fig. 4
Fig. 4 The AES compositional depth profiles of IGS_0nm, IGS500°C_80nm and IGS400°C_80nm.
, the Cu/(In+Ga) in the near surface region of IGS_0nm is around 0.7, whereas the Cu/(In+Ga) in surface layer of IGS400°C_80nm decreases to around 0.4 which is close to the CIGS β phase (Cu(In,Ga)3Se5) in stoichiometry [13

13. D. Schmid, M. Ruckh, F. Grunwald, and H. W. Schock, “Chalcopyrite defect chalcopyrite heterojunctions on the basis of CuInSe2,” J. Appl. Phys. 73(6), 2902–2909 (1993). [CrossRef]

], suggesting a desirable Cu depleted surface layer. It is noted that Cu concentration near the surface of IGS500°C_80nm remains high, revealing that the T sub is a key factor for formation of Cu depleted surface layer. Although the Cu concentration in surface layer could be decreased by post-depositing a thicker IGS layer, the triangle-hole structure of Cu-poor chalcopyrite phase formed on the surface would lead to more surface defects as shown in Fig. 2(f). Therefore, the Cu concentration in the surface layer could be optimized by post depositing a IGS thin layer with proper thickness at different temperatures to obtain surface layer with large band gap, conduction band alignment at CIGS/CdS interface and fewer surface defect structures.

3.4 Device performance

The photovoltaic performance parameters of 3 sets of CIGS devices with PDT at 300°C, 400°C and 500°C are summarized in Fig. 5
Fig. 5 Device parameters of CIGS solar cells with PDT at 300°C, 400°C, 500°C and solar cells with surface sulfurization treatment (maked with “★”).
. As the thickness of IGS increases systematically, the IGS400°C set cells are greatly improved in performance and reach the best performance at 80nm. In particular, the fill factor (FF) and Voc of IGS400°C_80nm are enhanced remarkably, confirming that the performance was improved by surface band gap widening [4

4. U. Rau and M. Turcu, “Role of surface band gap widening in Cu(In,Ga)(Se,S)2 thin-films for the photovoltaic performance of ZnO/CdS/Cu(In,Ga)(Se,S)2 heterojunction solar cells,” Mater. Res. Soc. Symp. Proc. 763, 335–340 (2003).

]. As the thickness of IGS layer continues to increase, performance of IGS400°C cells become deteriorated. It is mainly because of too low Cu concentration and increased defect states in surface layer due to excess IGS post deposited. However, the performance of IGS400°C cells shows a benign tolerance to excess thickness of IGS layer. It is noted that the performance of IGS400°C_160nm is still comparable with that of IGS_0nm. For IGS500°C cells, the improvement of performance due to PDT is not clear. The AES result has shown that the wide band gap surface layer is not successfully fabricated by PDT at 500°C due to strong lattice diffusion [19

19. J. S. Park, Z. Dong, S. Kim, and J. H. Perepezko, “CuInSe2 phase formation during Cu2Se/In2Se3 interdiffusion reaction,” J. Appl. Phys. 87(8), 3683–3690 (2000). [CrossRef]

], which agrees well with the device performance results. It is observed that the device parameters decrease after PDT at 300°C, which is probably due to the excess defect states brought by small grains on the surface as shown in Fig. 2(d). Additionally, CIGS thin film was surface sulfurized by annealing in S vapor at 520°C. Performance of CIGS solar cells with sulfurization treatment is shown for comparison with that of IGS400°C_80nm in Fig. 5. The comparable efficiencies confirm that the PDT could be a successful alternative to surface sulfurization.

4. Conclusion

The surface band gap of CIGS thin film has been optimized by varying Cu concentration in surface layer using PDT. Precise monitoring of thickness of IGS layer by light controlling method plays important roles in investigating the effects of PDT on CIGS thin films and devices quantitatively. The SEM results and depth profiles analysis suggest that both substrate temperature and thickness of IGS layer are important to fabricate high-quality Cu-depleted surface layer. The device parameters are greatly improved by optimizing the Cu concentration in surface layer, further more the Cu2-xSe phase on the surface could be eliminated by PDT in case of high Cu/(In+Ga), which could not be done by other methods for increasing surface band gap such as Ga grading or surface sulfurization. Additionally, the PDT is easy to be implemented in CIGS deposition process. Thus a simple and effective method for grading surface band gap of CIGS thin films is suggested.

References and links

1.

F. Engelhardt, M. Schmidt, Th. Meyer, O. Seifert, J. Parisi, and U. Rau, “Metastable electrical transport in Cu(In,Ga)Se2 thin films and ZnO/CdS/Cu(In,Ga)Se2 heterostructures,” Phys. Lett. A 245(5), 489–493 (1998). [CrossRef]

2.

S. H. Wei and A. Zunger, “Band offsets and optical bowings of chalcopyrites and Zn-based II-VI alloys,” J. Appl. Phys. 78(6), 3846–3856 (1995). [CrossRef]

3.

A. Rockett, D. Liao, J. T. Heath, J. D. Cohen, Y. M. Strzhemechny, L. J. Brillson, K. Ramanathan, and W. N. Shafarman, “Near surface defect distributions in Cu(In,Ga)Se2,” Thin Solid Films 431, 301–306 (2003). [CrossRef]

4.

U. Rau and M. Turcu, “Role of surface band gap widening in Cu(In,Ga)(Se,S)2 thin-films for the photovoltaic performance of ZnO/CdS/Cu(In,Ga)(Se,S)2 heterojunction solar cells,” Mater. Res. Soc. Symp. Proc. 763, 335–340 (2003).

5.

M. Turcu, O. Pakma, and U. Rau, “Interdependence of absorber composition and recombination mechanism in Cu(In, Ga)(Se,S)2 heterojunction solar cells,” Appl. Phys. Lett. 80(14), 2598–2600 (2002). [CrossRef]

6.

T. Dullweber, G. Hanna, W. Shams-Kolahi, A. Schwartzlander, M. A. Contreas, R. Noufi, and H. W. Schock, “Study of the effect of gallium grading in Cu(In,Ga)Se2,” Thin Solid Films 361(1-2), 478–481 (2000). [CrossRef]

7.

T. Nakada, H. Ohbo, T. Watanabe, H. Nakazawa, M. Matsui, and A. Kunioka, “Improved Cu(In,Ga)(S,Se)2 thin film solar cells by surface sulfurization,” Sol. Energy Mater. Sol. Cells 49(1–4), 285–290 (1997). [CrossRef]

8.

U. P. Singh, W. N. Shafarman, and R. W. Birkmire, “Surface sulfurization studies of Cu(InGa)Se2 thin film,” Sol. Energy Mater. Sol. Cells 90(5), 623–630 (2006). [CrossRef]

9.

D. Ohashi, T. Nakada, and A. Kunioka, “Improved CIGS thin-film solar cells by surface sulfurization using In2S3 and sulfur vapor,” Sol. Energy Mater. Sol. Cells 67(1–4), 261–265 (2001). [CrossRef]

10.

M. Morkel, L. Weinhardt, B. Lohmuller, C. Heske, E. Umbach, W. Riedl, S. Zweigart, and F. Karg, “Flat conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction,” Appl. Phys. Lett. 79(21), 4482–4484 (2001). [CrossRef]

11.

M. J. Romero, K. M. Jones, J. AbuShama, Y. Yan, M. M. Al-Jassim, and R. Noufi, “Surface-layer band gap widending in Cu(In,Ga)Se2 thin films,” Appl. Phys. Lett. 83(23), 4731–4733 (2003). [CrossRef]

12.

S.-H. Han, F. S. Hasoon, A. M. Hermann, and D. H. Levi, “Spectroscopy evidence for a surface layer in CuInSe2:Cu deficiency,” Appl. Phys. Lett. 91(2), 021904 (2007). [CrossRef]

13.

D. Schmid, M. Ruckh, F. Grunwald, and H. W. Schock, “Chalcopyrite defect chalcopyrite heterojunctions on the basis of CuInSe2,” J. Appl. Phys. 73(6), 2902–2909 (1993). [CrossRef]

14.

S. Nishiwaki, N. Kohara, T. Negami, H. Miyake, and T. Wada, “Microstructure of Cu(In,Ga)Se2 Films deposited in Low Se Vapor Pressure,” Jpn. J. Appl. Phys. 38(Part 1, No. 5A), 2888–2892 (1999). [CrossRef]

15.

A. Darga, D. Mencaragila, Z. Djebbour, A. Migan Dubois, J. F. Guillemoles, J. P. Connolly, O. Roussel, D. Lincot, B. Canava, and A. Etcheberry, “Two step wet surface treatment influence on the electronic properties of Cu(In,Ga)Se2 solar cells,” Thin Solid Films 517(7), 2550–2553 (2009). [CrossRef]

16.

X. H. Tan, S. L. Ye, B. Fan, K. Tang, and X. Liu, “Effects of Na incorporated at different periods of deposition on Cu(In,Ga)Se2 films,” Appl. Opt. 49(16), 3071–3074 (2010). [CrossRef] [PubMed]

17.

U.-C. Boehnke and G. Kühn, “Phase relations in the ternary system Cu-In-Se,” J. Mater. Sci. 22(5), 1635–1641 (1987). [CrossRef]

18.

T. Nakada and A. Kunioka, “Direct evidence of Cd diffusion into Cu(In,Ga)Se2 thin films during chemical-bath deposition process of CdS films,” Appl. Phys. Lett. 74(17), 2444–2446 (1999). [CrossRef]

19.

J. S. Park, Z. Dong, S. Kim, and J. H. Perepezko, “CuInSe2 phase formation during Cu2Se/In2Se3 interdiffusion reaction,” J. Appl. Phys. 87(8), 3683–3690 (2000). [CrossRef]

OCIS Codes
(040.5350) Detectors : Photovoltaic
(310.1860) Thin films : Deposition and fabrication
(310.3840) Thin films : Materials and process characterization
(350.6050) Other areas of optics : Solar energy

ToC Category:
Thin Films

History
Original Manuscript: December 23, 2010
Revised Manuscript: March 6, 2011
Manuscript Accepted: March 7, 2011
Published: March 23, 2011

Citation
Xiao-Hui Tan, Sheng-Lin Ye, and Xu Liu, "Increasing surface band gap of Cu(In,Ga)Se2 thin films by post depositing an In-Ga-Se thin layer," Opt. Express 19, 6609-6615 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-7-6609


Sort:  Author  |  Year  |  Journal  |  Reset  

References

  1. F. Engelhardt, M. Schmidt, Th. Meyer, O. Seifert, J. Parisi, and U. Rau, “Metastable electrical transport in Cu(In,Ga)Se2 thin films and ZnO/CdS/Cu(In,Ga)Se2 heterostructures,” Phys. Lett. A 245(5), 489–493 (1998). [CrossRef]
  2. S. H. Wei and A. Zunger, “Band offsets and optical bowings of chalcopyrites and Zn-based II-VI alloys,” J. Appl. Phys. 78(6), 3846–3856 (1995). [CrossRef]
  3. A. Rockett, D. Liao, J. T. Heath, J. D. Cohen, Y. M. Strzhemechny, L. J. Brillson, K. Ramanathan, and W. N. Shafarman, “Near surface defect distributions in Cu(In,Ga)Se2,” Thin Solid Films 431, 301–306 (2003). [CrossRef]
  4. U. Rau and M. Turcu, “Role of surface band gap widening in Cu(In,Ga)(Se,S)2 thin-films for the photovoltaic performance of ZnO/CdS/Cu(In,Ga)(Se,S)2 heterojunction solar cells,” Mater. Res. Soc. Symp. Proc. 763, 335–340 (2003).
  5. M. Turcu, O. Pakma, and U. Rau, “Interdependence of absorber composition and recombination mechanism in Cu(In, Ga)(Se,S)2 heterojunction solar cells,” Appl. Phys. Lett. 80(14), 2598–2600 (2002). [CrossRef]
  6. T. Dullweber, G. Hanna, W. Shams-Kolahi, A. Schwartzlander, M. A. Contreas, R. Noufi, and H. W. Schock, “Study of the effect of gallium grading in Cu(In,Ga)Se2,” Thin Solid Films 361(1-2), 478–481 (2000). [CrossRef]
  7. T. Nakada, H. Ohbo, T. Watanabe, H. Nakazawa, M. Matsui, and A. Kunioka, “Improved Cu(In,Ga)(S,Se)2 thin film solar cells by surface sulfurization,” Sol. Energy Mater. Sol. Cells 49(1–4), 285–290 (1997). [CrossRef]
  8. U. P. Singh, W. N. Shafarman, and R. W. Birkmire, “Surface sulfurization studies of Cu(InGa)Se2 thin film,” Sol. Energy Mater. Sol. Cells 90(5), 623–630 (2006). [CrossRef]
  9. D. Ohashi, T. Nakada, and A. Kunioka, “Improved CIGS thin-film solar cells by surface sulfurization using In2S3 and sulfur vapor,” Sol. Energy Mater. Sol. Cells 67(1–4), 261–265 (2001). [CrossRef]
  10. M. Morkel, L. Weinhardt, B. Lohmuller, C. Heske, E. Umbach, W. Riedl, S. Zweigart, and F. Karg, “Flat conduction-band alignment at the CdS/CuInSe2 thin-film solar-cell heterojunction,” Appl. Phys. Lett. 79(21), 4482–4484 (2001). [CrossRef]
  11. M. J. Romero, K. M. Jones, J. AbuShama, Y. Yan, M. M. Al-Jassim, and R. Noufi, “Surface-layer band gap widending in Cu(In,Ga)Se2 thin films,” Appl. Phys. Lett. 83(23), 4731–4733 (2003). [CrossRef]
  12. S.-H. Han, F. S. Hasoon, A. M. Hermann, and D. H. Levi, “Spectroscopy evidence for a surface layer in CuInSe2:Cu deficiency,” Appl. Phys. Lett. 91(2), 021904 (2007). [CrossRef]
  13. D. Schmid, M. Ruckh, F. Grunwald, and H. W. Schock, “Chalcopyrite defect chalcopyrite heterojunctions on the basis of CuInSe2,” J. Appl. Phys. 73(6), 2902–2909 (1993). [CrossRef]
  14. S. Nishiwaki, N. Kohara, T. Negami, H. Miyake, and T. Wada, “Microstructure of Cu(In,Ga)Se2 Films deposited in Low Se Vapor Pressure,” Jpn. J. Appl. Phys. 38(Part 1, No. 5A), 2888–2892 (1999). [CrossRef]
  15. A. Darga, D. Mencaragila, Z. Djebbour, A. Migan Dubois, J. F. Guillemoles, J. P. Connolly, O. Roussel, D. Lincot, B. Canava, and A. Etcheberry, “Two step wet surface treatment influence on the electronic properties of Cu(In,Ga)Se2 solar cells,” Thin Solid Films 517(7), 2550–2553 (2009). [CrossRef]
  16. X. H. Tan, S. L. Ye, B. Fan, K. Tang, and X. Liu, “Effects of Na incorporated at different periods of deposition on Cu(In,Ga)Se2 films,” Appl. Opt. 49(16), 3071–3074 (2010). [CrossRef] [PubMed]
  17. U.-C. Boehnke and G. Kühn, “Phase relations in the ternary system Cu-In-Se,” J. Mater. Sci. 22(5), 1635–1641 (1987). [CrossRef]
  18. T. Nakada and A. Kunioka, “Direct evidence of Cd diffusion into Cu(In,Ga)Se2 thin films during chemical-bath deposition process of CdS films,” Appl. Phys. Lett. 74(17), 2444–2446 (1999). [CrossRef]
  19. J. S. Park, Z. Dong, S. Kim, and J. H. Perepezko, “CuInSe2 phase formation during Cu2Se/In2Se3 interdiffusion reaction,” J. Appl. Phys. 87(8), 3683–3690 (2000). [CrossRef]

Cited By

Alert me when this paper is cited

OSA is able to provide readers links to articles that cite this paper by participating in CrossRef's Cited-By Linking service. CrossRef includes content from more than 3000 publishers and societies. In addition to listing OSA journal articles that cite this paper, citing articles from other participating publishers will also be listed.


« Previous Article  |  Next Article »

OSA is a member of CrossRef.

CrossCheck Deposited