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Effects of junction parameters on Cu(In,Ga)Se2 solar cells
ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 69 (2008) 779–783 www.elsevier.com/locate/jpcs
Effects of junction parameters on Cu(In,Ga)Se2 solar cells Chia-Hua Huang Department of Electrical Engineering, National Dong Hwa University, Shou-Feng, Hualien 97401, Taiwan
Abstract The performance of Cu(In,Ga)Se2 (CIGS) solar cells has been modeled and numerically simulated with the emphasis on the effects of junction properties by using the AMPS-1D device simulation tool. The impacts of the inverted surface defect layers or the Cd-doped interface layers on the performance of CIGS solar cells are examined. The device physics and performance parameters of the CIGS solar cells with different junction parameters are analyzed. The results suggest that the open-circuit voltage and fill factor of CIGS solar cells are improved by diminishing the recombination around the junction region, due to the surface band gap widening with a valence band offset and the high Cd doping concentration around the interface. r 2007 Elsevier Ltd. All rights reserved. Keywords: D. Defects
1. Introduction Copper indium gallium diselenide (CuIn1xGaxSe2 or CIGS) films are considered as the most promising thin-film materials for achieving the goals of low-cost and highperformance solar cells. The best CIGS thin-film solar cell has reached a conversion efficiency of 19.5% [1]. However, further optimization of the device performance could be substantially accelerated by a better understanding of fundamental issues. In the past, several different junction models to explain the observed device behaviors of the CIGS solar cells have been proposed with a certain level of discrepancy. So far, the exact nature of the electronic junction of the CIGS solar cells still remains an unsettled issue. In order to understand the electrical junction formation as well as how the junction parameters affect the device performance of CIGS solar cells, the impacts of the cell structures based on the proposed junctionformation models are studied by using the numerical simulations. The device physics and performance parameters of cells with various junction models are valuated and discussed. Moreover, the simulation results and the typically observed device characteristics of CIGS cells are compared and analyzed. In addition, the dependence of Tel.: +886 3 8634077; fax: +886 3 8634060.
E-mail address: [email protected] 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.07.118
performance parameters for the CIGS cell on the carrier density as well as thickness of the Cu-poor surface defect layer and other junction parameters are investigated. 2. Device model The CIGS solar-cell structure used in the simulation consists of several layers including the top contact, bottom contact, intrinsic ZnO layer, CdS buffer layer, highrecombination interface, surface defect layer, and the CIGS absorber. The computer simulation tool, analysis of microelectronic and photonic structures (AMPS)-1D [2], is employed by specifying the semiconductor parameters in each defined layer of the cell structure as input parameters in the simulation. To proceed with the simulation, the material parameters employed as the inputs are selected based on the reported literature values or constrained to reasonable ranges. The key semiconductor properties of the intrinsic ZnO, CdS, and CIGS layers as the input parameters for the simulations are given in Table 1. Although no attempt was made to match the simulation results with the experimental data, the purpose is to analyze the trend in the performance of CIGS cells versus the junction parameters intended to study. The total thickness of the absorber layer is maintained at 2 mm for all cases. An interfacial layer with a high density of effective recombination centers is placed at the metallurgical
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C.-H. Huang / Journal of Physics and Chemistry of Solids 69 (2008) 779–783 -3.0
Table 1 Semiconductor properties of the intrinsic ZnO, CdS, and CIGS layers as the input parameters for the simulations Layer properties
ZnO
CdS
CIGS
Dielectric constant es/e0 me (cm2/V s) mh (cm2/V s) Carrier density, n or p (cm3) NC (cm3) NV (cm3) Thickness (mm) Eg (eV)
9 50 5 n: 5 1017 1 1019 1 1019 0.055 3.3
10 10 1 n: 6 1016 1 1019 1 1019 0.03 2.4
13.6 300 30 p: 8 1016 3 1018 1.5 1019 2 1.16
Cu(In,Ga)Se2 -4.0
EC
buffer Eg
me, electron mobility; mh, hole mobility; NC , effective density of states in conduction band; NV, effective density of states in valence band; and Eg, band gap energy.
EF
Energy (eV)
-5.0 EV -6.0 surface defect layer -7.0
-8.0
junction between the CIGS surface layer and CdS layer. A deep level defect is placed in the middle of the band gap as an effective recombination center for the interface, CdS layer, and the space charge region (SCR) in the CIGS absorber.
ZnO -9.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
Position (µm)
3. Results and discussion
Fig. 1. Energy band diagram of a typical CIGS solar cell under equilibrium condition.
Schmid et al. have demonstrated the existence of an Inrich n-type surface layer, which was identified as an ordered defect chalcopyrite (ODC) and tentatively assigned the stoichiometry CuIn2Se3.5 or CuIn3Se5 [3]. The electrical characteristics of low hole mobilities (mnp10 cm2/V s), high resistivity (105–106 O cm), and low carrier densities (1011–1012 cm3) were proposed for the defect chalcopyrite Cu(In1xGax)3Se5 materials for 14x40 [4]. In contrast, some works revealed that the compound of In-rich surface layer might not be the ODC [5,6]. The existence of ODC on the CIGS films has not been confirmed by the structural characterizations. No evidence of ODC structure observed on the device-quality CIGS films was reported [7]. Thus, a more general term ‘surface defect layer’ is used to describe this thin n-type layer on the surface of the as-deposited CIGS films. The schematic energy-band diagram of a typical CIGS/CdS/ZnO solar cell under the equilibrium condition is illustrated in Fig. 1. Most of the CIGS absorbers employed for the highperformance solar cells are deposited by the three-stage deposition process [8]. During the third stage of this deposition process, only the additional atoms of In, Ga, and Se are supplied to deposit on substrates resulting in the final Cu-poor composition as expected. The SIMS depth profiles of the as-deposited films indicated that the surface layer is slightly more Cu-poor than the bulk regions [7]. In addition, around the surface region the high Ga ratio intentionally introduced by the three-stage process forms a surface field in the devices. The contribution to this offstoichiometry includes the segregation of Cu vacancies and defects of dislocations in the surface regions of the films [7]. Due to the slightly different composition from the bulk regions, the surface region of the CIGS films is n-type,
and thus with the inverted surface layer the CIGS solar cells are considered as the buried-homojunction devices [7]. The Cu-deficient region at the CIGS films surface with a high density of dislocations was estimated to have a thickness in the range of 100–300 nm [9,10]. The effects of thickness of the inverted surface defect layer on the performance of CIGS solar cells are investigated. The results are given in Fig. 2. Basically, the performance of the cells with a thin inverted surface layer is superior to that of the cells without an inverted surface layer. The conversion efficiency of the cells increases with the increased thickness of the inverted surface layer but starts to drop when the thickness of the inverted layer is greater than 150 nm due to the properties of low carrier densities and high densities of recombination centers of the inverted layer. With the existence of the weakly n-type surface layer in the CIGS cells, the beneficial effects include the reduction of recombination rate and enhancement of the carrier collection efficiency by shifting the electrical junction away from the high-recombination hetero-interface between the CdS and the inverted surface layer. Moreover, with the wider band-gap of the inverted layer than that of the CIGS layer existing between the CdS and CIGS layer, the recombination rate at the hetero-interface region is further decreased due to the enlargement of valence band offset created by the increased recombination barrier between the Fermi level and the valence band maximum. Thus, both the short-circuit density (Jsc) and open-circuit voltage (Voc) of CIGS solar cells are improved. Although the weakly n-type surface layer can benefit the performance of CIGS cells as explained above, the thicker inverted layer with material
ARTICLE IN PRESS C.-H. Huang / Journal of Physics and Chemistry of Solids 69 (2008) 779–783
17
(%)
16 15 14
VOC (mV)
13 640 630 620
JSC (mA/cm2)
35 34 33 32
F.F. (%)
80 H
76
H
HH 72 H
H
H
H H H
68
H
64 0
100
200 300 400 500 600 Thickness of the inverted surface layer
700
Fig. 2. Performance parameters of CIGS (Eg ¼ 1.16 eV) solar cells with the inverted surface layer in different thicknesses.
Table 2 Dependence of valence band offset due to the surface band gap widening on the performance of CIGS solar cells DEV (eV)
Efficiency (%)
VOC (mV)
F.F. (%)
JSC (mA cm2)
0 0.05 0.1 0.14 0.2 0.25 0.3
13.7 14.8 15.6 16.1 16.4 16.5 16.5
564 594 612 621 629 632 633
71.4 73.2 74.9 75.9 76.6 76.8 76.7
34.1 34.1 34.1 34.1 34.1 34.1 34.0
properties such as low carrier density, high resistivity, low carrier mobility, and high density of recombination centers could further worsen the device performance. With the intentional Cu-depletion and non-uniform Ga distribution in the surface region, the band gap widening around the surface region of the CIGS films was reported recently [11]. The band gap energy of surface region is at least 0.1 eV wider than that of CIGS bulk regions [11]. With the consideration of n-type surface layer, the surfacelayer band gap widening could result in a valence band offset around the subinterface between the surface layers and bulk regions of CIGS films. The impacts of this valence band offset on the performance of CIGS cells are carried out. As shown in Table 2, the valence band offset critically improves the device performance by enhancing both the open-circuit voltage (VOC) and fill factor (F.F.) of devices. Clearly the interface recombination rate is lowered due to
781
the valence band offset. The effect of valence band offset on the short-circuit density (JSC) is not significant at all. The other viewpoint for the CIGS junction model is that the Fermi level pinning occurs due to the electronic states at the CIGS surface, resulting from the interface states and/or Cd doping. With the dangling bond of missing Se at the surface of CIGS films, the shallow surface donors of positively charged vacancies VSe result in the band bending of CIGS at the surface, and hence the n-type inversion occurs. The other possibility for the creation of band bending in CIGS solar cells is that the Cd doping in the shallow surface of CIGS films result in the n-type surface layer. A CdS buffer layer is typically deposited on the top of CIGS absorber layers as a buffer layer. Initially, the CdS buffer layers were considered as an n-type partner for the p-type CIGS films to form the electrical junction. However, some results have indicated that the real junction may not exist around the interface between the CdS and CIGS layers. So far, the CdS buffer layers are still typically deposited by the chemical bath deposition (CBD) process for the CIGS solar-cell technology. Therefore, due to the low-temperature process the Cd ions are not expected to diffuse into the CIGS films. However, the existence of Cd in the very surface layer of CIGS films has been observed [12,13]. The surface defect layers of CIGS films are copper deficient with Cu vacancies. The Cd ions are considered to diffuse into the first few atomic layers of CIGS films and to occupy the sites of Cu vacancies. The doping of Cd in the CIGS films is known to turn the conductivity type of CIGS films into n-type. And hence, the thin Cd-doped CIGS surface layer becomes n-type while the bulk regions of the CIGS films are still p-type. The electrical junction of the CIGS solar cells might thus be formed. The thickness of the Cd-doped surface layer is reported from about a few atomic layers to 100 A˚ [12–14]. The dependence of Cd-doping concentration on the performance of CIGS solar cells is investigated. In the simulations, the thickness of the Cd-doped surface layer is considered as 3 nm. As shown in Fig. 3, obviously the CIGS solar cells with the Cd doping concentration of 1022 cm3 has a better performance than those with low doping concentration. The high Cd doping concentration could enhance the band bending around the junction and increase the energy barrier in the valence band at the interface between CdS and CIGS layers. Due to this sufficiently high barrier, the hole concentration at the interface is lowered and the Fermi level around the interface region is close to the conduction band edge. The interface recombination is suppressed, and the open-circuit voltage (VOC) and the fill factor (FF) are considerably improved. The short-circuit densities (JSC) of both the low-doping and high-doping concentration CIGS solar cells remain constant for a wide range of interface defect density, and are nearly identical except for the high interface defect density of 1019 cm3. The effects of both the high-doping n-type surface layer and the electrical junction shifted away from the hetero-interface between the CdS and CIGS layers lead to the high performance of
ARTICLE IN PRESS C.-H. Huang / Journal of Physics and Chemistry of Solids 69 (2008) 779–783
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Cd doping density 1022 cm-3
18
Cd doping density 1016 cm-3
0.7
16 0.6
12 10
0.5
8
VOC (V)
Efficiency (%)
14
6 0.4
4
40
80
35
70
30
60
25
50
20
40
15
30 20
10 10
Fill factor (%)
JSC (mA/cm2)
2
14
10
15
10
16
10
17
10
18
10
19
10
20
10
15
10
16
10
17
10
18
10
19
10
20
Interface defect density (cm-3)
Fig. 3. Dependence of device performance on the interface defect density for two different interface carrier concentrations (1022 and 1016 cm3).
CIGS solar cells. The interface defect density has almost no effect on the performance of the CIGS solar cells with the high Cd doping concentration. Contrarily, the high interface defect density deteriorates the performance of CIGS solar cells with a low Cd doping concentration due to a lack of the sufficiently high barrier to suppress the interface recombination. Using the thickness of the high-doping surface layer as a variable in the simulation, no significant difference of device performance for the thickness from 3 to 10 nm is found.
considered as the critical points to attain the high-efficiency CIGS solar cells. Additionally, the effects of interface properties in the surface defect layer and the CIGS/CdS interface layer on the performance parameters of CIGS solar cells are given. Acknowledgment The author gratefully acknowledges the use of AMPS1D developed by Dr. S.J. Fonash of the Pennsylvania State University.
4. Summary and conclusions Device modeling and numerical simulations of the CIGS solar cells with consideration of different junction formation and various junction parameters have been carried out using the AMPS-1D program. A detailed analysis of effects of the inverted surface defect layers or the Cd-doped interface layers has been presented. The benefits of conductive inversion in the surface defect layers for the buried-homojunction CIGS solar cells are obvious. Both the surface band gap widening with a valence band offset and the high Cd doping concentration around the interface are capable of diminishing the recombination rate around the junction region and, therefore, the open-circuit voltage and fill factor of CIGS solar cells are improved. Thus, the efficiency of devices is enhanced. These two factors are
References [1] M.A. Contreras, K. Ramanathan, J. AbuShama, F. Hasoon, D.L. Young, B. Egaas, R. Noufi, Prog. Photovolt. 13 (2005) 209–216. [2] H. Zhu, A.K. Kalkan, J. Hou, S.J. Fonash, AIP Conf. Proc. 462 (1999) 309–314. [3] D. Schmid, M. Ruckh, F. Grunwald, H.W. Schock, J. Appl. Phys. 73 (1993) 2902–2909. [4] M.A. Contreras, H. Wiesner, D. Niles, K. Ramanathan, R. Matson, J. Tuttle, J. Keane, R. Noufi, in: Conference Record of the 25th IEEE Photovoltaic Specialists Conference, 1996, pp. 809–812. [5] R. Herberhoz, U. Rau, H.W. Schock, T. Haalboom, T. Go¨decke, F. Ernst, C. Beilharz, K.W. Benz, D. Cahen, Eur. Phys. J. Appl. Phys. 6 (1999) 131–139. [6] J. Guillemoles, T. Haalboom, T. Go¨decke, F. Ernst, D. Cahen, Mater. Res. Soc. Symp. Proc. 485 (1998) 127–132.
ARTICLE IN PRESS C.-H. Huang / Journal of Physics and Chemistry of Solids 69 (2008) 779–783 [7] Y. Yan, K.M. Jones, J. Abushama, M. Young, S. Asher, M.M. Al-Jassim, R. Noufi, Appl. Phys. Lett. 81 (2002) 1008–1111. [8] M.A. Contreras, B. Egaas, K. Ramanathan, J. Hiltner, A. Swartzlander, F. Hansoon, R. Noufi, Prog. Photovoltaics: Res. Appl. 7 (1999) 311. [9] F.S. Hasoon, Y. Yan, H. Althani, K.M. Jones, H.R. Moutinho, J. Alleman, M.M. Al-Jassim, R. Noufi, Thin Solid Films 387 (2001) 1–5. [10] F.S. Hasoon, H.A. Al-Thani, K.M. Jones, Y. Yan, H.R. Moutinho, M. Young, S.E. Asher, J. Alleman, R. Bhattacharya, K. Ramanathan,
[11] [12] [13] [14]
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J. Keane, M.M. Al-Jassim, Mater. Res. Soc. Symp. Proc. 668 (2001) H6.8.1-7. M.J. Romero, K.M. Jones, J. AbuShama, Y. Yan, M.M. Al-Jassim, R. Noufi, Appl. Phys. Lett. 83 (2003) 4731–4733. T. Nakada, A. Kunioka, Appl. Phys. Lett. 74 (1999) 2444–2446. D. Liao, A. Rockett, J. Appl. Phys. 93 (2003) 9380–9382. A. Rockett, D. Liao, J.T. Heath, J.D. Cohen, Y.M. Strzhemechny, L.J. Brillson, K. Ramanthan, W.N. Shafarman, Thin Solid Films 431–432 (2003) 301–306.
Journal of Physics and Chemistry of Solids 69 (2008) 779–783 www.elsevier.com/locate/jpcs
Effects of junction parameters on Cu(In,Ga)Se2 solar cells Chia-Hua Huang Department of Electrical Engineering, National Dong Hwa University, Shou-Feng, Hualien 97401, Taiwan
Abstract The performance of Cu(In,Ga)Se2 (CIGS) solar cells has been modeled and numerically simulated with the emphasis on the effects of junction properties by using the AMPS-1D device simulation tool. The impacts of the inverted surface defect layers or the Cd-doped interface layers on the performance of CIGS solar cells are examined. The device physics and performance parameters of the CIGS solar cells with different junction parameters are analyzed. The results suggest that the open-circuit voltage and fill factor of CIGS solar cells are improved by diminishing the recombination around the junction region, due to the surface band gap widening with a valence band offset and the high Cd doping concentration around the interface. r 2007 Elsevier Ltd. All rights reserved. Keywords: D. Defects
1. Introduction Copper indium gallium diselenide (CuIn1xGaxSe2 or CIGS) films are considered as the most promising thin-film materials for achieving the goals of low-cost and highperformance solar cells. The best CIGS thin-film solar cell has reached a conversion efficiency of 19.5% [1]. However, further optimization of the device performance could be substantially accelerated by a better understanding of fundamental issues. In the past, several different junction models to explain the observed device behaviors of the CIGS solar cells have been proposed with a certain level of discrepancy. So far, the exact nature of the electronic junction of the CIGS solar cells still remains an unsettled issue. In order to understand the electrical junction formation as well as how the junction parameters affect the device performance of CIGS solar cells, the impacts of the cell structures based on the proposed junctionformation models are studied by using the numerical simulations. The device physics and performance parameters of cells with various junction models are valuated and discussed. Moreover, the simulation results and the typically observed device characteristics of CIGS cells are compared and analyzed. In addition, the dependence of Tel.: +886 3 8634077; fax: +886 3 8634060.
E-mail address: [email protected] 0022-3697/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.jpcs.2007.07.118
performance parameters for the CIGS cell on the carrier density as well as thickness of the Cu-poor surface defect layer and other junction parameters are investigated. 2. Device model The CIGS solar-cell structure used in the simulation consists of several layers including the top contact, bottom contact, intrinsic ZnO layer, CdS buffer layer, highrecombination interface, surface defect layer, and the CIGS absorber. The computer simulation tool, analysis of microelectronic and photonic structures (AMPS)-1D [2], is employed by specifying the semiconductor parameters in each defined layer of the cell structure as input parameters in the simulation. To proceed with the simulation, the material parameters employed as the inputs are selected based on the reported literature values or constrained to reasonable ranges. The key semiconductor properties of the intrinsic ZnO, CdS, and CIGS layers as the input parameters for the simulations are given in Table 1. Although no attempt was made to match the simulation results with the experimental data, the purpose is to analyze the trend in the performance of CIGS cells versus the junction parameters intended to study. The total thickness of the absorber layer is maintained at 2 mm for all cases. An interfacial layer with a high density of effective recombination centers is placed at the metallurgical
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Table 1 Semiconductor properties of the intrinsic ZnO, CdS, and CIGS layers as the input parameters for the simulations Layer properties
ZnO
CdS
CIGS
Dielectric constant es/e0 me (cm2/V s) mh (cm2/V s) Carrier density, n or p (cm3) NC (cm3) NV (cm3) Thickness (mm) Eg (eV)
9 50 5 n: 5 1017 1 1019 1 1019 0.055 3.3
10 10 1 n: 6 1016 1 1019 1 1019 0.03 2.4
13.6 300 30 p: 8 1016 3 1018 1.5 1019 2 1.16
Cu(In,Ga)Se2 -4.0
EC
buffer Eg
me, electron mobility; mh, hole mobility; NC , effective density of states in conduction band; NV, effective density of states in valence band; and Eg, band gap energy.
EF
Energy (eV)
-5.0 EV -6.0 surface defect layer -7.0
-8.0
junction between the CIGS surface layer and CdS layer. A deep level defect is placed in the middle of the band gap as an effective recombination center for the interface, CdS layer, and the space charge region (SCR) in the CIGS absorber.
ZnO -9.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
Position (µm)
3. Results and discussion
Fig. 1. Energy band diagram of a typical CIGS solar cell under equilibrium condition.
Schmid et al. have demonstrated the existence of an Inrich n-type surface layer, which was identified as an ordered defect chalcopyrite (ODC) and tentatively assigned the stoichiometry CuIn2Se3.5 or CuIn3Se5 [3]. The electrical characteristics of low hole mobilities (mnp10 cm2/V s), high resistivity (105–106 O cm), and low carrier densities (1011–1012 cm3) were proposed for the defect chalcopyrite Cu(In1xGax)3Se5 materials for 14x40 [4]. In contrast, some works revealed that the compound of In-rich surface layer might not be the ODC [5,6]. The existence of ODC on the CIGS films has not been confirmed by the structural characterizations. No evidence of ODC structure observed on the device-quality CIGS films was reported [7]. Thus, a more general term ‘surface defect layer’ is used to describe this thin n-type layer on the surface of the as-deposited CIGS films. The schematic energy-band diagram of a typical CIGS/CdS/ZnO solar cell under the equilibrium condition is illustrated in Fig. 1. Most of the CIGS absorbers employed for the highperformance solar cells are deposited by the three-stage deposition process [8]. During the third stage of this deposition process, only the additional atoms of In, Ga, and Se are supplied to deposit on substrates resulting in the final Cu-poor composition as expected. The SIMS depth profiles of the as-deposited films indicated that the surface layer is slightly more Cu-poor than the bulk regions [7]. In addition, around the surface region the high Ga ratio intentionally introduced by the three-stage process forms a surface field in the devices. The contribution to this offstoichiometry includes the segregation of Cu vacancies and defects of dislocations in the surface regions of the films [7]. Due to the slightly different composition from the bulk regions, the surface region of the CIGS films is n-type,
and thus with the inverted surface layer the CIGS solar cells are considered as the buried-homojunction devices [7]. The Cu-deficient region at the CIGS films surface with a high density of dislocations was estimated to have a thickness in the range of 100–300 nm [9,10]. The effects of thickness of the inverted surface defect layer on the performance of CIGS solar cells are investigated. The results are given in Fig. 2. Basically, the performance of the cells with a thin inverted surface layer is superior to that of the cells without an inverted surface layer. The conversion efficiency of the cells increases with the increased thickness of the inverted surface layer but starts to drop when the thickness of the inverted layer is greater than 150 nm due to the properties of low carrier densities and high densities of recombination centers of the inverted layer. With the existence of the weakly n-type surface layer in the CIGS cells, the beneficial effects include the reduction of recombination rate and enhancement of the carrier collection efficiency by shifting the electrical junction away from the high-recombination hetero-interface between the CdS and the inverted surface layer. Moreover, with the wider band-gap of the inverted layer than that of the CIGS layer existing between the CdS and CIGS layer, the recombination rate at the hetero-interface region is further decreased due to the enlargement of valence band offset created by the increased recombination barrier between the Fermi level and the valence band maximum. Thus, both the short-circuit density (Jsc) and open-circuit voltage (Voc) of CIGS solar cells are improved. Although the weakly n-type surface layer can benefit the performance of CIGS cells as explained above, the thicker inverted layer with material
ARTICLE IN PRESS C.-H. Huang / Journal of Physics and Chemistry of Solids 69 (2008) 779–783
17
(%)
16 15 14
VOC (mV)
13 640 630 620
JSC (mA/cm2)
35 34 33 32
F.F. (%)
80 H
76
H
HH 72 H
H
H
H H H
68
H
64 0
100
200 300 400 500 600 Thickness of the inverted surface layer
700
Fig. 2. Performance parameters of CIGS (Eg ¼ 1.16 eV) solar cells with the inverted surface layer in different thicknesses.
Table 2 Dependence of valence band offset due to the surface band gap widening on the performance of CIGS solar cells DEV (eV)
Efficiency (%)
VOC (mV)
F.F. (%)
JSC (mA cm2)
0 0.05 0.1 0.14 0.2 0.25 0.3
13.7 14.8 15.6 16.1 16.4 16.5 16.5
564 594 612 621 629 632 633
71.4 73.2 74.9 75.9 76.6 76.8 76.7
34.1 34.1 34.1 34.1 34.1 34.1 34.0
properties such as low carrier density, high resistivity, low carrier mobility, and high density of recombination centers could further worsen the device performance. With the intentional Cu-depletion and non-uniform Ga distribution in the surface region, the band gap widening around the surface region of the CIGS films was reported recently [11]. The band gap energy of surface region is at least 0.1 eV wider than that of CIGS bulk regions [11]. With the consideration of n-type surface layer, the surfacelayer band gap widening could result in a valence band offset around the subinterface between the surface layers and bulk regions of CIGS films. The impacts of this valence band offset on the performance of CIGS cells are carried out. As shown in Table 2, the valence band offset critically improves the device performance by enhancing both the open-circuit voltage (VOC) and fill factor (F.F.) of devices. Clearly the interface recombination rate is lowered due to
781
the valence band offset. The effect of valence band offset on the short-circuit density (JSC) is not significant at all. The other viewpoint for the CIGS junction model is that the Fermi level pinning occurs due to the electronic states at the CIGS surface, resulting from the interface states and/or Cd doping. With the dangling bond of missing Se at the surface of CIGS films, the shallow surface donors of positively charged vacancies VSe result in the band bending of CIGS at the surface, and hence the n-type inversion occurs. The other possibility for the creation of band bending in CIGS solar cells is that the Cd doping in the shallow surface of CIGS films result in the n-type surface layer. A CdS buffer layer is typically deposited on the top of CIGS absorber layers as a buffer layer. Initially, the CdS buffer layers were considered as an n-type partner for the p-type CIGS films to form the electrical junction. However, some results have indicated that the real junction may not exist around the interface between the CdS and CIGS layers. So far, the CdS buffer layers are still typically deposited by the chemical bath deposition (CBD) process for the CIGS solar-cell technology. Therefore, due to the low-temperature process the Cd ions are not expected to diffuse into the CIGS films. However, the existence of Cd in the very surface layer of CIGS films has been observed [12,13]. The surface defect layers of CIGS films are copper deficient with Cu vacancies. The Cd ions are considered to diffuse into the first few atomic layers of CIGS films and to occupy the sites of Cu vacancies. The doping of Cd in the CIGS films is known to turn the conductivity type of CIGS films into n-type. And hence, the thin Cd-doped CIGS surface layer becomes n-type while the bulk regions of the CIGS films are still p-type. The electrical junction of the CIGS solar cells might thus be formed. The thickness of the Cd-doped surface layer is reported from about a few atomic layers to 100 A˚ [12–14]. The dependence of Cd-doping concentration on the performance of CIGS solar cells is investigated. In the simulations, the thickness of the Cd-doped surface layer is considered as 3 nm. As shown in Fig. 3, obviously the CIGS solar cells with the Cd doping concentration of 1022 cm3 has a better performance than those with low doping concentration. The high Cd doping concentration could enhance the band bending around the junction and increase the energy barrier in the valence band at the interface between CdS and CIGS layers. Due to this sufficiently high barrier, the hole concentration at the interface is lowered and the Fermi level around the interface region is close to the conduction band edge. The interface recombination is suppressed, and the open-circuit voltage (VOC) and the fill factor (FF) are considerably improved. The short-circuit densities (JSC) of both the low-doping and high-doping concentration CIGS solar cells remain constant for a wide range of interface defect density, and are nearly identical except for the high interface defect density of 1019 cm3. The effects of both the high-doping n-type surface layer and the electrical junction shifted away from the hetero-interface between the CdS and CIGS layers lead to the high performance of
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Cd doping density 1022 cm-3
18
Cd doping density 1016 cm-3
0.7
16 0.6
12 10
0.5
8
VOC (V)
Efficiency (%)
14
6 0.4
4
40
80
35
70
30
60
25
50
20
40
15
30 20
10 10
Fill factor (%)
JSC (mA/cm2)
2
14
10
15
10
16
10
17
10
18
10
19
10
20
10
15
10
16
10
17
10
18
10
19
10
20
Interface defect density (cm-3)
Fig. 3. Dependence of device performance on the interface defect density for two different interface carrier concentrations (1022 and 1016 cm3).
CIGS solar cells. The interface defect density has almost no effect on the performance of the CIGS solar cells with the high Cd doping concentration. Contrarily, the high interface defect density deteriorates the performance of CIGS solar cells with a low Cd doping concentration due to a lack of the sufficiently high barrier to suppress the interface recombination. Using the thickness of the high-doping surface layer as a variable in the simulation, no significant difference of device performance for the thickness from 3 to 10 nm is found.
considered as the critical points to attain the high-efficiency CIGS solar cells. Additionally, the effects of interface properties in the surface defect layer and the CIGS/CdS interface layer on the performance parameters of CIGS solar cells are given. Acknowledgment The author gratefully acknowledges the use of AMPS1D developed by Dr. S.J. Fonash of the Pennsylvania State University.
4. Summary and conclusions Device modeling and numerical simulations of the CIGS solar cells with consideration of different junction formation and various junction parameters have been carried out using the AMPS-1D program. A detailed analysis of effects of the inverted surface defect layers or the Cd-doped interface layers has been presented. The benefits of conductive inversion in the surface defect layers for the buried-homojunction CIGS solar cells are obvious. Both the surface band gap widening with a valence band offset and the high Cd doping concentration around the interface are capable of diminishing the recombination rate around the junction region and, therefore, the open-circuit voltage and fill factor of CIGS solar cells are improved. Thus, the efficiency of devices is enhanced. These two factors are
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