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Bandlike and localized defect states in CuInSe2 solar cells
Journal of Physics and Chemistry of Solids 66 (2005) 1855–1857 www.elsevier.com/locate/jpcs
Bandlike and localized defect states in CuInSe2 solar cells Jehad A.M. AbuShama *, S. Johnston, R. Noufi National Renewable Energy Laboratory, Golden, CO 80401, USA
Abstract We used the deep-level transient spectroscopy (DLTS) to investigate the electronic properties of p-type CuInSe2 (CIS) polycrystalline thin-film solar cells. We detected electron (or minority) traps with activation energies ranging from EcK0.1 to EcK0.22 eV (where Ec is the energy of electrons at the conduction band minimum). While varying the filling pulse duration, we observed the gradual increase in the amplitude of the DLTS signal for these states until it apparently saturates at a pulse duration w1 s. Increasing the duration of the filling pulse also results in broadening the DLTS signals and shifting the maximum of these signals towards lower temperature, whereas the high-temperature sides coincide. We also detected a hole (or majority) trap around a temperature of 190 K. Using a model that allows us to distinguish between bandlike states and localized ones based on the dependence of the shape of their DLTS-signal on the filling-pulse duration, we relate the electron trap to bandlike states and the hole trap to localized ones. q 2005 Published by Elsevier Ltd. Keywords: A. Copper indium diselenide; A. CuInSe2; A. Thin films; C. Deep-level transient spectroscopy; D. Electrical properties
1. Introduction CuIn1KxGaxSe2 (with 0%x%1) thin film solar cells have great potential in photovoltaic applications. They are among the most promising candidates for commercially feasible solar cells. They have reached high levels of photovoltaic performance, both on laboratory scale [1–3] and on largearea modules [4]. Recently, new world record total-area efficiencies of 15.0, 19.5 and 10.2% for CdS/CuIn1KxGaxSe2 solar cells have been achieved for xw0 (CuInSe2 or CIS), xw0.28 (CuInGaSe2 or CIGS), and xZ1 (CuGaSe2 or CGS), respectively [1–3]. The many elements of the CuIn1KxGaxSe2 multinary polycrystalline thin film may form different components as dictated by the phase equilibria. Although this multiplicity makes the material complicated, CuIn1KxGaxSe2 nevertheless tolerates defects and impurities by self-adjusting its chemistry and structure [5]. CIS (xZ0) films used in the devices examined in this paper were grown using the threestage process [1–3]. CIS solar cells examined in this paper have a high level of photovoltaic performance. Devices made from these films were analyzed using the deep level transient spectroscopy (DLTS). In p-type CIS, this technique allows for * Corresponding author. Tel.: C1 518 383 4600 x339; fax: C518 383 4900. E-mail addresses: [email protected] (J.A.M. AbuShama), [email protected] (J.A.M. AbuShama).
0022-3697/$ - see front matter q 2005 Published by Elsevier Ltd. doi:10.1016/j.jpcs.2005.09.004
investigation of minority (or electron) traps in the upper half of the band gap and majority (or hole) traps in the lower half of the band gap. In this paper, we report on the electronic properties of CuInSe2 (CIS). Investigating the dependence of the amplitude and the shape of the DLTS signal on the filling time of the traps with charge carriers allows us to identify the nature of the traps, whether bandlike or localized. Increasing the duration of the filling pulse results in broadening the DLTS signals and shifting the maximum of these signals towards lower temperature, whereas the high-temperature sides coincide. Using a model of the electronic states at the extended defects, which allows us to distinguish between bandlike states and localized ones based on the dependence of the shape of their DLTS-signal on the filling-pulse duration, we relate the electron trap to bandlike states and the hole trap to localized ones. 2. Experimental The p-type CIS thin films were deposited by physical vapor deposition using the three-stage growth process [1–3] onto molybdenum-coated, soda-lime glass substrates. The final CIS films were Cu-poor. A 50–60 nm thickness of n-type CdS was deposited using chemical bath deposition (CBD) at a bath temperature of about 65 8C. Undoped and Al-doped ZnO layers were deposited using sputtering in Ar/O2 working gas. A layer of 90-nm thickness of undoped ZnO was deposited from a pure ZnO target, followed by an Al-doped ZnO layer of about
1856
J.A.M. AbuShama et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1855–1857
120 nm thickness deposited from an Al2O3-doped ZnO target. The ZnO bilayer has a sheet resistance of about 65–70 U/ square. Front contacts of Ni/Al grids were deposited by electron beam evaporation. The areas of our cells were defined by mechanical scribing. A 100-nm-thick MgF2 anti-reflection coating layer was then deposited using sputtering.
3. Results and discussion CIS devices were analyzed using deep level transient spectroscopy (DLTS) [6]. The DLTS technique is commonly used to study the nature of the electronic traps and can give the following information: type of the trap (whether minority or majority), activation energy relative to the respective band edge (either conduction or valance band), cross-section and defect concentration. To collect DLTS data, the sample was reverse biased to 0.4 V and then biased to 0 V for deep filling. The filling pulses have durations ranging from 1 ms to 10 s. We used a transient analysis window (or period) of 10 ms. We collected our DLTS data over a temperature range of 20–300 K using a DLTS system made by accent optical technologies. This system is capable of using pulse periods ranging from 16 ms to 4000 s, which allows us (when using long periods) to detect deeper levels without heating the sample above room temperature. Our samples were cooled down to about 20 K using a closed-cycle He-cryostat. Schro¨ter et al. and Riedel et al. [7,8] in their work on Si proposed that electronic states associated with extended defects in semiconductors could be classified as localized or band-like states based on the rate at which the states reach their internal electronic equilibrium (ri). Comparing this rate to the carrier emission rate re and the capture rate rc allows to distinguish between localized states and band-like ones. The states are localized if ri/re, rc, and band-like if ri[re, rc. The authors
[7,8] used computer simulation of DLTS spectra induced by both minority and majority states and demonstrated that these states can be distinguished as bandlike states or localized ones based on the dependence of the shape of their DLTS-signal on the filling-pulse duration. In other words, for bandlike states, increasing the filling-pulse duration results in broadening the DLTS signals and shifting their peaks towards lower temperature, whereas the high-temperature sides coincide. For localized states, the peaks of the DLTS signals stay constant while varying the filling-pulse duration. In this contribution, we use the same criterion, which is the dependence of the shape of the DLTS signal on the fillingpulse duration, to distinguish between bandlike states and localized ones in our CIS solar cells. The largest DLTS peak from the data is identified as a minority trap with an activation energy ranging from EcK0.1 to EcK0.22 eV (where Ec is the energy of electrons at the conduction band minimum) as shown in Fig. 1. We detected this minority state signal, even though we did not use forward bias. This may result from trap filling and emptying that occurs near the CIS/CdS interface. Here, electrons from the n-type CdS layer may be energetic enough to surmount the band bending potential and fill the near-interface traps; however, this is still under investigation. Fig. 1 also shows that the amplitudes of the DLTS signals for the minority traps shift from DCw2 fF at Tw110 K to DCw1.1 pF at Tw90 K. The DLTS signal with amplitude of DCw2 fF corresponds to the smallest filling pulse duration of 20 ms, and the one with amplitude of DCw1.1 pF corresponds to the highest filling pulse duration of 10 s (Fig. 1). Fig. 1 also shows that the peaks of DLTS signals for filling-pulse durations of %200 ms occur at the same temperature (w110 K). A shift and broadening of the DLTS signals can clearly be seen at filling pulses of durations in the range of 20 µs 50 µs 100 µs 200 µs 500 µs 1 ms 2 ms 5 ms 10 ms 20 ms 50 ms 100 ms 200 ms 500 ms 1s 2s 5s 10 s
period=10 ms
1.0 10 s
DLTS Signal (pF)
0.8
0.6
0.4
20 µs
0.2
20 µs 10 s
0.0
50
100
150
200
250
300
Temperature (K) Fig. 1. DLTS data for CIS cells (DC vs. T) using various filling-pulse durations and a period of 10 ms. The samples were reverse biased to 0.4 V and biased to 0 V for deep filling.
J.A.M. AbuShama et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1855–1857
density is w6.5!1013 cmK3. A fit (not shown here) to the data shown in Fig. 2 (defect density vs. filling duration) exhibits a linear dependence of the defect density on the logarithm of the pulse filling duration.
60x1012
Defect Density (cm–3)
1857
50 40 30
4. Conclusion
20 10
10–4
10–3
10–2
10–1
100
101
Filling-pulse duration (s) Fig. 2. Defect density vs. filling-pulse duration for CIS cell. Electron (or minority) traps saturate at a filling-pulse duration of w1 s.
500 ms–1 s. Therefore, we conclude that, for minority (or electron) traps, increasing the filling-pulse durations results in broadening the DLTS signals and shifting their peaks to lower temperatures (Tw105–90 K), whereas the high-temperature sides (Tw110 K) coincide. Based on the criterion mentioned above, we conclude that the minority traps detected in our CIS cells are related to band-like ones, which are spatially extended over an energy range of (EcK0.1) to (EcK0.22) eV (i.e. the energy width of the band is 0.12 eV). Fig. 1 also shows a hole (or majority) trap at about 190 K. The activation energy for this trap is around EvC0.23 eV (where Ev is the energy of the holes at the valence band maximum). The peaks of the DLTS signals for this majority trap are smaller than their minority traps counterparts (Fig. 1), and this results in noisier Arrhenius plots and therefore higher uncertainty in the value of the activation energy. The peaks of the DLTS signals for this trap do not shift with temperature. Based on the criterion mentioned above, we conclude that the majority trap detected in our CIS cells is related to localized states, which are spatially fixed at activation energy of EvC0.23 eV. Fig. 2 shows the defect concentrations for the bandlike minority traps (EcK0.22Ractivation energyREcK0.1 eV). This defect concentration varies from 0.2 to 6.5!1013 cmK3 as we vary the filling-pulse duration from 20 ms to 10 s, respectively. The trap with an activation energy of EcK0.1 eV has a concentration of w0.2!1013 cmK3 (at filling-pulse duration of 20 ms), whereas the trap with an activation energy of EcK0.22 eV has a concentration of w6.5!1013 cmK3 (at filling-pulse duration of 10 s). The filling of these minority traps depends on their location relative to the Fermi level. Therefore, each trap within the band may not be completely filled, even though the DLTS signal has shown saturation. Based on this argument, the defect concentration in Fig. 2 should not be taken as an absolute value, bur rather a lower limit. Fig. 2 also shows that the DLTS signal saturates at pulse durations w1 s. The saturation defect
We used the DLTS technique to investigate the electronic properties of ZnO/CdS/CuInSe2 (CIS)/Mo/SLG polycrystalline thin-film solar cells. We detected electron (or minority) states with activation energies ranging from EcK0.1 to EcK 0.22 eV. We also observed the gradual increase in the amplitude of the DLTS signal until it apparently saturates at a pulse duration of w1 s. We also found that the amplitude of the DLTS signal depends linearly on the logarithm of the filling time. We detected majority traps with activation energy EvC 0.23 eV. For these traps, the amplitudes of the DLTS signals did not shift with temperature. Based on a model that allows us to distinguish between bandlike states and localized ones based on the dependence of the shape of their DLTS-signal on the filling-pulse duration, we relate the electron trap to bandlike states and the hole trap to localized ones. Acknowledgements The authors acknowledge the technical support of J. Dolan and J. Keane. This work is supported by DOE Contract No. DE-AC36-99GO10337. J. AbuShama was supported by NREL subcontract No. KXEA-3-33607-03 to the Colorado School of Mines. References [1] Jehad, A.M. AbuShama, S. Johnston, T. Moriarty, G. Teeter, K. Ramanathan, R. Noufi, Properties of ZnO/CdS/CuInSe2 solar cells with improved performance, Prog. Photovolt. Res. Appl. 12 (2004) 39–45. [2] M.A. Contreras, J. AbuShama, F. Hasoon, D. Young, B. Egaas and R. Noufi, Diode characteristics of state-of-the art ZnO/CdS/Cu(In1-xGax) Se2 solar cells, Prog. Photovol. Res. Appl. 13 (2005) 209–216. [3] Jehad, A. AbuShama, R. Noufi, Steve Johnston, Scott Ward, X. Wu, Improved performance in CuInSe2 and surface-modified CuGaSe2 (CGS) thin film solar cells, Proceedings of the 31st IEEE Photovoltaic Specialists Conference, Lake Buena Vista, Florida, 2005. [4] Jehad, M.A. Green, K. Emery, D.L. King, S. Igari, W. Warta, Prog. Photovolt. Res. Appl. Solar Cell Efficiency Tables (Version 24) 12 (2004) 365–372. [5] Hans-Warner Schock, Rommel Noufi, CIGS-based solar cells for the next millennium, Prog. Photovolt. Res. Appl. 8 (2000) 151–160. [6] D.V. Lang, Deep-level transient spectroscopy: a new method to characterize traps in semiconductors, J. Appl. Phys. 45 (1974) 3023–3032. [7] W. Schro¨ter, J. Kronewitz, U. Gnauert, F. Riedel, M. Seibt, Bandlike and localized states at extended defects in silicon, Phys. Rev. B 52 (1995) 13726–13729. [8] F. Riedel, W. Schro¨ter, Electrical and structural properties of nanoscale NiSi2 precipitates in silicon, Phys. Rev. B 62 (2000) 7150–7156.
Bandlike and localized defect states in CuInSe2 solar cells Jehad A.M. AbuShama *, S. Johnston, R. Noufi National Renewable Energy Laboratory, Golden, CO 80401, USA
Abstract We used the deep-level transient spectroscopy (DLTS) to investigate the electronic properties of p-type CuInSe2 (CIS) polycrystalline thin-film solar cells. We detected electron (or minority) traps with activation energies ranging from EcK0.1 to EcK0.22 eV (where Ec is the energy of electrons at the conduction band minimum). While varying the filling pulse duration, we observed the gradual increase in the amplitude of the DLTS signal for these states until it apparently saturates at a pulse duration w1 s. Increasing the duration of the filling pulse also results in broadening the DLTS signals and shifting the maximum of these signals towards lower temperature, whereas the high-temperature sides coincide. We also detected a hole (or majority) trap around a temperature of 190 K. Using a model that allows us to distinguish between bandlike states and localized ones based on the dependence of the shape of their DLTS-signal on the filling-pulse duration, we relate the electron trap to bandlike states and the hole trap to localized ones. q 2005 Published by Elsevier Ltd. Keywords: A. Copper indium diselenide; A. CuInSe2; A. Thin films; C. Deep-level transient spectroscopy; D. Electrical properties
1. Introduction CuIn1KxGaxSe2 (with 0%x%1) thin film solar cells have great potential in photovoltaic applications. They are among the most promising candidates for commercially feasible solar cells. They have reached high levels of photovoltaic performance, both on laboratory scale [1–3] and on largearea modules [4]. Recently, new world record total-area efficiencies of 15.0, 19.5 and 10.2% for CdS/CuIn1KxGaxSe2 solar cells have been achieved for xw0 (CuInSe2 or CIS), xw0.28 (CuInGaSe2 or CIGS), and xZ1 (CuGaSe2 or CGS), respectively [1–3]. The many elements of the CuIn1KxGaxSe2 multinary polycrystalline thin film may form different components as dictated by the phase equilibria. Although this multiplicity makes the material complicated, CuIn1KxGaxSe2 nevertheless tolerates defects and impurities by self-adjusting its chemistry and structure [5]. CIS (xZ0) films used in the devices examined in this paper were grown using the threestage process [1–3]. CIS solar cells examined in this paper have a high level of photovoltaic performance. Devices made from these films were analyzed using the deep level transient spectroscopy (DLTS). In p-type CIS, this technique allows for * Corresponding author. Tel.: C1 518 383 4600 x339; fax: C518 383 4900. E-mail addresses: [email protected] (J.A.M. AbuShama), [email protected] (J.A.M. AbuShama).
0022-3697/$ - see front matter q 2005 Published by Elsevier Ltd. doi:10.1016/j.jpcs.2005.09.004
investigation of minority (or electron) traps in the upper half of the band gap and majority (or hole) traps in the lower half of the band gap. In this paper, we report on the electronic properties of CuInSe2 (CIS). Investigating the dependence of the amplitude and the shape of the DLTS signal on the filling time of the traps with charge carriers allows us to identify the nature of the traps, whether bandlike or localized. Increasing the duration of the filling pulse results in broadening the DLTS signals and shifting the maximum of these signals towards lower temperature, whereas the high-temperature sides coincide. Using a model of the electronic states at the extended defects, which allows us to distinguish between bandlike states and localized ones based on the dependence of the shape of their DLTS-signal on the filling-pulse duration, we relate the electron trap to bandlike states and the hole trap to localized ones. 2. Experimental The p-type CIS thin films were deposited by physical vapor deposition using the three-stage growth process [1–3] onto molybdenum-coated, soda-lime glass substrates. The final CIS films were Cu-poor. A 50–60 nm thickness of n-type CdS was deposited using chemical bath deposition (CBD) at a bath temperature of about 65 8C. Undoped and Al-doped ZnO layers were deposited using sputtering in Ar/O2 working gas. A layer of 90-nm thickness of undoped ZnO was deposited from a pure ZnO target, followed by an Al-doped ZnO layer of about
1856
J.A.M. AbuShama et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1855–1857
120 nm thickness deposited from an Al2O3-doped ZnO target. The ZnO bilayer has a sheet resistance of about 65–70 U/ square. Front contacts of Ni/Al grids were deposited by electron beam evaporation. The areas of our cells were defined by mechanical scribing. A 100-nm-thick MgF2 anti-reflection coating layer was then deposited using sputtering.
3. Results and discussion CIS devices were analyzed using deep level transient spectroscopy (DLTS) [6]. The DLTS technique is commonly used to study the nature of the electronic traps and can give the following information: type of the trap (whether minority or majority), activation energy relative to the respective band edge (either conduction or valance band), cross-section and defect concentration. To collect DLTS data, the sample was reverse biased to 0.4 V and then biased to 0 V for deep filling. The filling pulses have durations ranging from 1 ms to 10 s. We used a transient analysis window (or period) of 10 ms. We collected our DLTS data over a temperature range of 20–300 K using a DLTS system made by accent optical technologies. This system is capable of using pulse periods ranging from 16 ms to 4000 s, which allows us (when using long periods) to detect deeper levels without heating the sample above room temperature. Our samples were cooled down to about 20 K using a closed-cycle He-cryostat. Schro¨ter et al. and Riedel et al. [7,8] in their work on Si proposed that electronic states associated with extended defects in semiconductors could be classified as localized or band-like states based on the rate at which the states reach their internal electronic equilibrium (ri). Comparing this rate to the carrier emission rate re and the capture rate rc allows to distinguish between localized states and band-like ones. The states are localized if ri/re, rc, and band-like if ri[re, rc. The authors
[7,8] used computer simulation of DLTS spectra induced by both minority and majority states and demonstrated that these states can be distinguished as bandlike states or localized ones based on the dependence of the shape of their DLTS-signal on the filling-pulse duration. In other words, for bandlike states, increasing the filling-pulse duration results in broadening the DLTS signals and shifting their peaks towards lower temperature, whereas the high-temperature sides coincide. For localized states, the peaks of the DLTS signals stay constant while varying the filling-pulse duration. In this contribution, we use the same criterion, which is the dependence of the shape of the DLTS signal on the fillingpulse duration, to distinguish between bandlike states and localized ones in our CIS solar cells. The largest DLTS peak from the data is identified as a minority trap with an activation energy ranging from EcK0.1 to EcK0.22 eV (where Ec is the energy of electrons at the conduction band minimum) as shown in Fig. 1. We detected this minority state signal, even though we did not use forward bias. This may result from trap filling and emptying that occurs near the CIS/CdS interface. Here, electrons from the n-type CdS layer may be energetic enough to surmount the band bending potential and fill the near-interface traps; however, this is still under investigation. Fig. 1 also shows that the amplitudes of the DLTS signals for the minority traps shift from DCw2 fF at Tw110 K to DCw1.1 pF at Tw90 K. The DLTS signal with amplitude of DCw2 fF corresponds to the smallest filling pulse duration of 20 ms, and the one with amplitude of DCw1.1 pF corresponds to the highest filling pulse duration of 10 s (Fig. 1). Fig. 1 also shows that the peaks of DLTS signals for filling-pulse durations of %200 ms occur at the same temperature (w110 K). A shift and broadening of the DLTS signals can clearly be seen at filling pulses of durations in the range of 20 µs 50 µs 100 µs 200 µs 500 µs 1 ms 2 ms 5 ms 10 ms 20 ms 50 ms 100 ms 200 ms 500 ms 1s 2s 5s 10 s
period=10 ms
1.0 10 s
DLTS Signal (pF)
0.8
0.6
0.4
20 µs
0.2
20 µs 10 s
0.0
50
100
150
200
250
300
Temperature (K) Fig. 1. DLTS data for CIS cells (DC vs. T) using various filling-pulse durations and a period of 10 ms. The samples were reverse biased to 0.4 V and biased to 0 V for deep filling.
J.A.M. AbuShama et al. / Journal of Physics and Chemistry of Solids 66 (2005) 1855–1857
density is w6.5!1013 cmK3. A fit (not shown here) to the data shown in Fig. 2 (defect density vs. filling duration) exhibits a linear dependence of the defect density on the logarithm of the pulse filling duration.
60x1012
Defect Density (cm–3)
1857
50 40 30
4. Conclusion
20 10
10–4
10–3
10–2
10–1
100
101
Filling-pulse duration (s) Fig. 2. Defect density vs. filling-pulse duration for CIS cell. Electron (or minority) traps saturate at a filling-pulse duration of w1 s.
500 ms–1 s. Therefore, we conclude that, for minority (or electron) traps, increasing the filling-pulse durations results in broadening the DLTS signals and shifting their peaks to lower temperatures (Tw105–90 K), whereas the high-temperature sides (Tw110 K) coincide. Based on the criterion mentioned above, we conclude that the minority traps detected in our CIS cells are related to band-like ones, which are spatially extended over an energy range of (EcK0.1) to (EcK0.22) eV (i.e. the energy width of the band is 0.12 eV). Fig. 1 also shows a hole (or majority) trap at about 190 K. The activation energy for this trap is around EvC0.23 eV (where Ev is the energy of the holes at the valence band maximum). The peaks of the DLTS signals for this majority trap are smaller than their minority traps counterparts (Fig. 1), and this results in noisier Arrhenius plots and therefore higher uncertainty in the value of the activation energy. The peaks of the DLTS signals for this trap do not shift with temperature. Based on the criterion mentioned above, we conclude that the majority trap detected in our CIS cells is related to localized states, which are spatially fixed at activation energy of EvC0.23 eV. Fig. 2 shows the defect concentrations for the bandlike minority traps (EcK0.22Ractivation energyREcK0.1 eV). This defect concentration varies from 0.2 to 6.5!1013 cmK3 as we vary the filling-pulse duration from 20 ms to 10 s, respectively. The trap with an activation energy of EcK0.1 eV has a concentration of w0.2!1013 cmK3 (at filling-pulse duration of 20 ms), whereas the trap with an activation energy of EcK0.22 eV has a concentration of w6.5!1013 cmK3 (at filling-pulse duration of 10 s). The filling of these minority traps depends on their location relative to the Fermi level. Therefore, each trap within the band may not be completely filled, even though the DLTS signal has shown saturation. Based on this argument, the defect concentration in Fig. 2 should not be taken as an absolute value, bur rather a lower limit. Fig. 2 also shows that the DLTS signal saturates at pulse durations w1 s. The saturation defect
We used the DLTS technique to investigate the electronic properties of ZnO/CdS/CuInSe2 (CIS)/Mo/SLG polycrystalline thin-film solar cells. We detected electron (or minority) states with activation energies ranging from EcK0.1 to EcK 0.22 eV. We also observed the gradual increase in the amplitude of the DLTS signal until it apparently saturates at a pulse duration of w1 s. We also found that the amplitude of the DLTS signal depends linearly on the logarithm of the filling time. We detected majority traps with activation energy EvC 0.23 eV. For these traps, the amplitudes of the DLTS signals did not shift with temperature. Based on a model that allows us to distinguish between bandlike states and localized ones based on the dependence of the shape of their DLTS-signal on the filling-pulse duration, we relate the electron trap to bandlike states and the hole trap to localized ones. Acknowledgements The authors acknowledge the technical support of J. Dolan and J. Keane. This work is supported by DOE Contract No. DE-AC36-99GO10337. J. AbuShama was supported by NREL subcontract No. KXEA-3-33607-03 to the Colorado School of Mines. References [1] Jehad, A.M. AbuShama, S. Johnston, T. Moriarty, G. Teeter, K. Ramanathan, R. Noufi, Properties of ZnO/CdS/CuInSe2 solar cells with improved performance, Prog. Photovolt. Res. Appl. 12 (2004) 39–45. [2] M.A. Contreras, J. AbuShama, F. Hasoon, D. Young, B. Egaas and R. Noufi, Diode characteristics of state-of-the art ZnO/CdS/Cu(In1-xGax) Se2 solar cells, Prog. Photovol. Res. Appl. 13 (2005) 209–216. [3] Jehad, A. AbuShama, R. Noufi, Steve Johnston, Scott Ward, X. Wu, Improved performance in CuInSe2 and surface-modified CuGaSe2 (CGS) thin film solar cells, Proceedings of the 31st IEEE Photovoltaic Specialists Conference, Lake Buena Vista, Florida, 2005. [4] Jehad, M.A. Green, K. Emery, D.L. King, S. Igari, W. Warta, Prog. Photovolt. Res. Appl. Solar Cell Efficiency Tables (Version 24) 12 (2004) 365–372. [5] Hans-Warner Schock, Rommel Noufi, CIGS-based solar cells for the next millennium, Prog. Photovolt. Res. Appl. 8 (2000) 151–160. [6] D.V. Lang, Deep-level transient spectroscopy: a new method to characterize traps in semiconductors, J. Appl. Phys. 45 (1974) 3023–3032. [7] W. Schro¨ter, J. Kronewitz, U. Gnauert, F. Riedel, M. Seibt, Bandlike and localized states at extended defects in silicon, Phys. Rev. B 52 (1995) 13726–13729. [8] F. Riedel, W. Schro¨ter, Electrical and structural properties of nanoscale NiSi2 precipitates in silicon, Phys. Rev. B 62 (2000) 7150–7156.