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Effects of Ga content on Cu(In,Ga)Se2 solar cells studied by numerical modeling
ARTICLE IN PRESS
Journal of Physics and Chemistry of Solids 69 (2008) 330–334 www.elsevier.com/locate/jpcs
Effects of Ga content on Cu(In,Ga)Se2 solar cells studied by numerical modeling Chia-Hua Huang Department of Electrical Engineering, National Dong Hwa University, Shou-Feng, Hualien 97401, Taiwan
Abstract The band-gap energy of the CuIn1xGaxSe2 (CIGS) films is varied in the range from 1.04 to 1.68 eV with the corresponding Ga content in the CIGS films from x ¼ 0 to 1. With the addition of Ga in the CuInSe2 films for the CIGS-based solar cells, the high open-circuit voltage is achieved, and hence the performance of the CIGS solar cells is improved. However, the performance of the CIGS solar cells with the high Ga content falls short of the expectations. The simulation results suggest that the defect density in the surface layers and/or the bulk defect density of the CIGS films are the critical factors responsible for the limitation of the performance for the CIGS solar cells with the high Ga content. Due to the surface band-gap widening, the conduction band offset between the surface region and bulk region of the CIGS films is formed. The simulation results indicate that the open-circuit voltage of the CIGS solar cells increases with the increase of the conduction band offset. The band-gap grading in the space charge region and near the back surface region with the spatial gradient distribution of the Ga content significantly improves the performance of the CIGS solar cells. r 2007 Elsevier Ltd. All rights reserved. Keywords: D. Defects
1. Introduction The best thin-film CuIn1xGaxSe2 (CIGS) solar cells have reached the confirmed conversion efficiency of 19.5% for the solar spectrum of AM1.5G [1] and the efficiency of 21.5% under a concentrated illumination of 14 suns [2]. The key advantage of the CIGS-based compounds is that the material system has the tunable band-gap energy and lattice parameters by alloying the CuInSe2 (CIS) compounds with Ga and/or S to better match the solar spectrum. The addition of Ga in the CIS films is considered as the critical factor to remarkably improve the performance of the CIGS-based solar cells, resulting in the increase of band-gap for the CIGS films as well as allowing the band-gap engineering for the cell structures. Practically for the CIGS solar cells with the high Ga content, not only the short-circuit current density decreases but also the increase of the open-circuit voltage is not proportional to the increase of the Ga content as anticipated. In order to identify the responsible mechanisms for the limitation 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.2007.07.093
the performance for the CIGS solar cells with the high Ga content, a study of the numerical modeling and device simulations for the variation of the Ga content in the CIGS solar cells is conducted.
2. Simulation of Cu(In,Ga)Se2 solar cells The computer simulation tool, Analysis of Microelectronic and Photonic Structures (AMPS-1D) [3], is employed for the solar-cell simulations. The simulations are conducted by specifying the material parameters in each defined layer of the device structures as the input parameters. Within the reasonable ranges, the material parameters selected from the reported literature are employed for the simulations. In the simulations, several layers including the top contact, bottom contact, intrinsic ZnO layer, CdS buffer layer, high-recombination interface, surface defect layer on the top of the CIGS film, and the CIGS absorber construct the CIGS solar-cell structures. With the incorporation of the different Ga contents in the CIGS films, the physical properties of the CIGS films including the band-gap energy primarily shifting the
ARTICLE IN PRESS C.-H. Huang / Journal of Physics and Chemistry of Solids 69 (2008) 330–334
layer is employed in the cell structures for the simulations. As illustrated in Fig. 1, the open-circuit voltage of the devices increases with the increased Ga content in the CIGS films. However, the open-circuit voltage does not increase proportionally with the Ga content in the CIGS solar cells. The short-circuit current density decreases with the increased Ga content of the CIGS films due to the lower absorption coefficients and a lack of absorption in the long-wavelength portion of the spectrum. The simulation results have a good agreement with the theoretical estimation to achieve the best performance of the typical photovoltaic devices with the optimal band-gap energy in the range from 1.4 to 1.5 eV for the solar spectrum of AM1.5G. However, experimentally the efficiency of CuIn1xGaxSe2 solar cells increases with the increase of Ga content only in the range of x ¼ 00.3. Not only the shortcurrent density decreases but also the open-circuit voltage does not increase as expected for the CIGS solar cells with the high Ga content. For the high-performance CIGS solar cells, the CIGS films typically have the Ga ratio of around x ¼ 0.3 corresponding to the band-gap energy of around 1.15 eV. For the Ga ratio of x exceeding 0.3, the overall performance of the CIGS solar cells begins to drop down. So far, the attempts to use the wide band-gap CIGS materials for the solar modules are not completely successful. The responsible mechanisms for the limitation of the performance for the CIGS solar cells with the high Ga contents are not well understood. The failure to dope the surface of the CIGS films sufficiently n-type was proposed to account for the limitation of the performance
position of the conduction band minimum [4], hole concentration [5], bulk defect densities [6], absorption coefficients, and electron affinities are varied accordingly with the Ga contents in the CIGS films. The roomtemperature mobility of the CIGS films was found to remain nearly constant while varying the Ga content over a wide range [5]. In order to particularly study the effects of band-gap grading on the performance of the CIGS solar cells, the material parameters employed in the simulations are kept unchanged except that the band-gap energies, electron affinities, and optical absorption coefficients are adjusted to the corresponding Ga mole fraction in the CIGS films. The total thickness of the absorber layer is maintained at 2 mm. It is not intended to fit the simulation results with the experimental data. The trend in the performance of the CIGS solar cells versus the defect densities, various Ga contents, and Ga grading profiles are analyzed. 3. Results and discussion 3.1. Impacts of Ga contents on the CIGS solar cells Alloying Ga in the CIS films to form the wide band-gap materials of CIGS is anticipated to improve the opencircuit voltage of the CIGS solar cells mainly due to the enlargement of the band-gap energy for the absorber layers. With a uniform band-gap profile, the effects of the Ga mole ratios on the performance of the CIGS solar cells are investigated. A uniform band-gap profile of the CIGS 20
1
16 0.6 14
VOC (V)
0.8
0.4
12 40
85
35
80
30
75
25
70
Fill factor (%)
Efficiency (%)
18
JSC (mA/cm2)
331
65
20 1
1.2
1.4
1.6
1.2
1.4
1.6
Band gap energy (eV)
Fig. 1. Dependence of the device performance on the band-gap energy of the uniform band-gap CIGS solar cells.
ARTICLE IN PRESS C.-H. Huang / Journal of Physics and Chemistry of Solids 69 (2008) 330–334
for the CIGS solar cells with the high Ga contents [7]. Additionally, the presence of the low-resistance Cu2xSe compounds on the surface layer of the as-deposited CIGS films with the high Ga contents were suggested to be responsible for the poor performance [8]. From the simulation results in this study, no significant effects of the surface-layer doping density ranging from 1013 to 1020 cm3 on the device performance are obtained for the CIGS solar cells with the high Ga contents. Moreover, it was suggested that a Gaussian defect band centered at 0.8 eV from the valence band edge in the high Ga-content CIGS films played an important role in determining the performance of the CIGS solar cells because of this defect band near the mid-gap of the CIGS films acting as an efficient recombination center [9,10]. To evaluate the impacts of the defect parameters on the CIGS solar cells with the high Ga contents, the defect densities and defect levels near the interface or in the bulk region of the CIGS films are used as the input variables for the device simulations. The uniform band-gap profile with the bandgap energy of 1.5 eV is employed in the simulations for the cell structures. The simulation results show that the effects of position of the defect energy levels are not as critical as the impacts of the defect densities on the performance of the high Ga-content CIGS solar cells. However, the midgap defect concentrations in either the surface layer or the bulk region of the CIGS films varying from 1014 to 1018 cm3 affect the performance of the CIGS solar cells dramatically, as illustrated in Fig. 2. The increase of the Bulk defect density
surface defect density or the bulk defect density deteriorates the open-circuit voltage of the CIGS solar cells without significantly affecting the short-circuit current density except for the value of the bulk defect density up to 1018 cm3, and hence the efficiency of the cells decreases. The results conclude that the surface defect density and/or the bulk defect density are the key factors in limiting the overall performance of the high Ga-content CIGS solar cells. 3.2. Impacts of spatial Ga distribution on the CIGS solar cells With the incorporation of the high Ga content in the CIGS layers near the region of the bottom Mo contacts, a quasi-electrical field [11] can be established in the back region of the CIGS films, and thus the back surface field (BSF) is created in the CIGS solar cells. The photogenerated minority carriers away from the space charge region (SCR) count on the diffusion mechanism to be collected for the contribution to the current. Therefore, the performance of the CIGS solar cells with the BSF is expected to improve by reducing the back surface recombination and increasing the effective minority carrier diffusion length, resulting in an efficient carrier collection. Both the open-circuit voltage and short-circuit current density of the devices with the BSF are supposed to be enhanced. The thickness and band-gap energy in the back regions of the CIGS films are used as variables varied from Surface defect density 1.1
25
1
Efficiency (%)
20
0.9 0.8
15
0.7 10
0.6
VOC (V)
332
0.5
5
0.4
0 30
JSC(mA/cm2)
25
80
20
70
15
60
10 5
50
0
40
Fill factor (%)
90
1013 1014 1015 1016 1017 1018 1019 1014 1015 1016 1017 1018 1019
Defect density (cm-3)
Fig. 2. Effects of the surface defect density or bulk defect density on the performance parameters of the CIGS (Eg ¼ 1.5 eV) solar cells.
ARTICLE IN PRESS C.-H. Huang / Journal of Physics and Chemistry of Solids 69 (2008) 330–334
400 to 1500 nm and from 1.16 to 1.6 eV in the device simulations, respectively. Comparing the performance of the cells having the BSF with that of the cells without the BSF, both the open-circuit voltage and the short-circuit current density of the solar cells with the BSF are improved simultaneously, and hence the conversion efficiency also increases. From the simulation results, the suitable values of thickness and the band-gap energy in the back region of the CIGS films to establish an effective BSF in the CIGS solar cells are around 600 nm and 1.35 eV, respectively. The surface band-gap widening and the inverted surface layer resulting in the n-type conductivity around the surface region of the CIGS films have been reported recently [12,13]. It was indicated that the band-gap energy of the surface region is at least 0.1 eV wider than that of the CIGS bulk region [13]. The resulting valence band offset between the surface region and the bulk region of the CIGS layers increases the energy barrier in the valence band and suppresses the interface recombination. Therefore, the performance of CIGS solar cells is improved due to the valence band offset. The non-uniform Ga distribution around the surface region could also contribute to the effects of the surface-layer band-gap widening in the CIGS films and result in a conduction band offset between the surface region and bulk region of the CIGS films. The effects of this conduction band offset on the performance of the CIGS solar cells are investigated. The valence band offset between the surface region and the bulk region of the CIGS layers is assumed to be 0.1 eV for the simulations. In addition, a uniform band-gap profile with the band-gap energy of 1.16 eV is employed for the CIGS layers. As shown in Table 1, the open-circuit voltage of the CIGS solar cells increases with the increase of the conduction band offset while the short-circuit current density remains almost the same. Obviously, the conduction band offset suppresses the interface recombination rate resulting in an increase in the open-circuit voltage. Further increasing the conduction band offset would impede the minority carrier transportation. Thus, when the value of the conduction band offset is 0.1 eV, the fill factor of the devices decreases, and hence the efficiency of the solar cells drops. By varying the Ga content spatially within the absorber layers of the CIGS solar cells, the technique of the bandgap engineering can be applied in the CIGS solar cells. The Table 1 The dependence of the conduction band offset due to the surface band-gap widening on the performance of the CIGS solar cells (assuming the valence band offset DEV ¼ 0.1 eV) DEC (eV)
Eg (eV)
Efficiency (%)
VOC (mV)
FF (%)
JSC (mA/cm2)
0.05 0 0.05 0.1
1.21 1.26 1.31 1.36
15.2 15.6 15.7 15.5
603 612 615 617
73.9 74.9 74.9 73.7
34.0 34.1 34.1 34.1
Eg: the band-gap energy of the surface layers of the CIGS films.
333
Table 2 The performance parameters of the CIGS solar cells with various bandgap profiles Band-gap profile
Efficiency (%)
VOC (mV)
FF (%)
JSC (mA/cm2)
Uniform band-gap Front surface grading Back surface grading Double grading
15.7 17.5 16.9 18.9
656 689 660 701
74.6 79.1 74.5 78.7
32.0 32.0 34.4 34.1
simulation results of the CIGS solar cells with various band-gap profiles including the uniform band-gap, front surface grading, back surface grading, and double grading are shown in Table 2. In the simulations, the CIGS solar cells with a uniform band-gap profile have the band-gap energy of 1.2 eV. The front surface grading is implemented by the increase of the band-gap energy in the SCR with the intentional addition of high Ga content in the SCR of the CIGS films. The recombination rate in the SCR, which limits the open-circuit voltage of the CIGS solar cells, is reduced by increasing the barrier height via the increase of the band-gap energy in the SCR without a significant loss in the short-circuit current density. The back surface grading mainly improves the carrier collection resulting in an enhancement of the short-circuit current density. The CIGS solar cells with the double grading band-gap profile incorporating both the front surface grading and back surface grading achieve the best performance by reducing the carrier recombination in the SCR and increasing the carrier collection, and hence both the open-circuit voltage and the short-circuit density are dramatically improved. 4. Summary and conclusions The simulation results show that the optimal band-gap energy to match the solar spectrum AM1.5G for the CIGS solar cells is around 1.5 eV. However, experimentally the performance of CIGS solar cells with high Ga content is deteriorated, mostly resulting from that the open-circuit voltage does not increase with the increase of the Ga content. Form the simulation results, the defect densities in the surface layers and/or bulk defect density of the CIGS films have a profound effect on the open-circuit voltage of the CIGS solar cells. The high defect densities in the surface and bulk region of the CIGS films decrease the open-circuit voltage and the overall device performance. The conduction band offset between the surface region and bulk region of the CIGS films resulting from the surface band-gap widening reduces the interface recombination rate and thus enhances the open-circuit voltage of CIGS solar cells. The effects of various Ga contents and various spatial Ga distributions within the absorber layers including the front surface grading, back surface grading, and double grading are investigated. With the gradient distribution of the Ga content in the SCR and back region of the CIGS absorber layers, the overall performance of the
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CIGS solar cells could be dramatically improved due to the beneficial effects of reducing the recombination rate in the SCR and enhancing the effective carrier collection. The CIGS solar cells with the double grading profiles give the best performance. Acknowledgment The author gratefully acknowledges the use of AMPS1D developed by Dr. S.J. Fonash of the Pennsylvania State University. 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] J.S. Ward, K. Ramanathan, F.S. Hasoon, T.J. Coutts, J. Keane, M.A. Contreras, T. Moriarty, R. Noufi, Prog. Photovolt. 10 (2002) 41–46.
[3] H. Zhu, A.K. Kalkan, J. Hou, S.J. Fonash, AIP Conf. Proc. 462 (1999) 309–314. [4] S.H. Wei, A. Zunger, J. Appl. Phys. 78 (1995) 3846–3856. [5] D.J. Schroeder, J.L. Herberholz, A.A. Rockett, in: Proceedings of the 11th International Conference on Ternary and Multinary Compounds, 1999, pp. 749–752. [6] G. Henna, A. Jasenek, U. Rau, H.W. Schock, Thin Solid Films 387 (2001) 71–73. [7] D. Liao, A. Rockett, Mater. Res. Soc. Symp. Proc. 763 (2003) 243–248. [8] H. Miyazaki, R. Mikami, A. Yamada, M. Konagai, J. Phys. Chem. Solids 64 (2003) 2055–2058. [9] J.T. Heath, J.D. Cohen, W.N. Shafarman, D.X. Liao, A. Rockett, Appl. Phys. Lett. 80 (2002) 4540–4542. [10] J.T. Heath, J.D. Cohen, W.N. Shafarman, Mater. Res. Soc. Symp. Proc. 763 (2003) 429–440. [11] H. Kroemer, RCA Rev. 18 (1957) 332. [12] Y. Yan, K.M. Jones, J. Abushama, M. Young, S. Asher, M.M. Al-Jassim, R. Noufi, Appl. Phys. Lett. 81 (2002) 1008–1111. [13] M.J. Romero, K.M. Jones, J. AbuShama, Y. Yan, M.M. Al-Jassim, R. Noufi, Appl. Phys. Lett. 83 (2003) 4731–4733.
Journal of Physics and Chemistry of Solids 69 (2008) 330–334 www.elsevier.com/locate/jpcs
Effects of Ga content on Cu(In,Ga)Se2 solar cells studied by numerical modeling Chia-Hua Huang Department of Electrical Engineering, National Dong Hwa University, Shou-Feng, Hualien 97401, Taiwan
Abstract The band-gap energy of the CuIn1xGaxSe2 (CIGS) films is varied in the range from 1.04 to 1.68 eV with the corresponding Ga content in the CIGS films from x ¼ 0 to 1. With the addition of Ga in the CuInSe2 films for the CIGS-based solar cells, the high open-circuit voltage is achieved, and hence the performance of the CIGS solar cells is improved. However, the performance of the CIGS solar cells with the high Ga content falls short of the expectations. The simulation results suggest that the defect density in the surface layers and/or the bulk defect density of the CIGS films are the critical factors responsible for the limitation of the performance for the CIGS solar cells with the high Ga content. Due to the surface band-gap widening, the conduction band offset between the surface region and bulk region of the CIGS films is formed. The simulation results indicate that the open-circuit voltage of the CIGS solar cells increases with the increase of the conduction band offset. The band-gap grading in the space charge region and near the back surface region with the spatial gradient distribution of the Ga content significantly improves the performance of the CIGS solar cells. r 2007 Elsevier Ltd. All rights reserved. Keywords: D. Defects
1. Introduction The best thin-film CuIn1xGaxSe2 (CIGS) solar cells have reached the confirmed conversion efficiency of 19.5% for the solar spectrum of AM1.5G [1] and the efficiency of 21.5% under a concentrated illumination of 14 suns [2]. The key advantage of the CIGS-based compounds is that the material system has the tunable band-gap energy and lattice parameters by alloying the CuInSe2 (CIS) compounds with Ga and/or S to better match the solar spectrum. The addition of Ga in the CIS films is considered as the critical factor to remarkably improve the performance of the CIGS-based solar cells, resulting in the increase of band-gap for the CIGS films as well as allowing the band-gap engineering for the cell structures. Practically for the CIGS solar cells with the high Ga content, not only the short-circuit current density decreases but also the increase of the open-circuit voltage is not proportional to the increase of the Ga content as anticipated. In order to identify the responsible mechanisms for the limitation 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.2007.07.093
the performance for the CIGS solar cells with the high Ga content, a study of the numerical modeling and device simulations for the variation of the Ga content in the CIGS solar cells is conducted.
2. Simulation of Cu(In,Ga)Se2 solar cells The computer simulation tool, Analysis of Microelectronic and Photonic Structures (AMPS-1D) [3], is employed for the solar-cell simulations. The simulations are conducted by specifying the material parameters in each defined layer of the device structures as the input parameters. Within the reasonable ranges, the material parameters selected from the reported literature are employed for the simulations. In the simulations, several layers including the top contact, bottom contact, intrinsic ZnO layer, CdS buffer layer, high-recombination interface, surface defect layer on the top of the CIGS film, and the CIGS absorber construct the CIGS solar-cell structures. With the incorporation of the different Ga contents in the CIGS films, the physical properties of the CIGS films including the band-gap energy primarily shifting the
ARTICLE IN PRESS C.-H. Huang / Journal of Physics and Chemistry of Solids 69 (2008) 330–334
layer is employed in the cell structures for the simulations. As illustrated in Fig. 1, the open-circuit voltage of the devices increases with the increased Ga content in the CIGS films. However, the open-circuit voltage does not increase proportionally with the Ga content in the CIGS solar cells. The short-circuit current density decreases with the increased Ga content of the CIGS films due to the lower absorption coefficients and a lack of absorption in the long-wavelength portion of the spectrum. The simulation results have a good agreement with the theoretical estimation to achieve the best performance of the typical photovoltaic devices with the optimal band-gap energy in the range from 1.4 to 1.5 eV for the solar spectrum of AM1.5G. However, experimentally the efficiency of CuIn1xGaxSe2 solar cells increases with the increase of Ga content only in the range of x ¼ 00.3. Not only the shortcurrent density decreases but also the open-circuit voltage does not increase as expected for the CIGS solar cells with the high Ga content. For the high-performance CIGS solar cells, the CIGS films typically have the Ga ratio of around x ¼ 0.3 corresponding to the band-gap energy of around 1.15 eV. For the Ga ratio of x exceeding 0.3, the overall performance of the CIGS solar cells begins to drop down. So far, the attempts to use the wide band-gap CIGS materials for the solar modules are not completely successful. The responsible mechanisms for the limitation of the performance for the CIGS solar cells with the high Ga contents are not well understood. The failure to dope the surface of the CIGS films sufficiently n-type was proposed to account for the limitation of the performance
position of the conduction band minimum [4], hole concentration [5], bulk defect densities [6], absorption coefficients, and electron affinities are varied accordingly with the Ga contents in the CIGS films. The roomtemperature mobility of the CIGS films was found to remain nearly constant while varying the Ga content over a wide range [5]. In order to particularly study the effects of band-gap grading on the performance of the CIGS solar cells, the material parameters employed in the simulations are kept unchanged except that the band-gap energies, electron affinities, and optical absorption coefficients are adjusted to the corresponding Ga mole fraction in the CIGS films. The total thickness of the absorber layer is maintained at 2 mm. It is not intended to fit the simulation results with the experimental data. The trend in the performance of the CIGS solar cells versus the defect densities, various Ga contents, and Ga grading profiles are analyzed. 3. Results and discussion 3.1. Impacts of Ga contents on the CIGS solar cells Alloying Ga in the CIS films to form the wide band-gap materials of CIGS is anticipated to improve the opencircuit voltage of the CIGS solar cells mainly due to the enlargement of the band-gap energy for the absorber layers. With a uniform band-gap profile, the effects of the Ga mole ratios on the performance of the CIGS solar cells are investigated. A uniform band-gap profile of the CIGS 20
1
16 0.6 14
VOC (V)
0.8
0.4
12 40
85
35
80
30
75
25
70
Fill factor (%)
Efficiency (%)
18
JSC (mA/cm2)
331
65
20 1
1.2
1.4
1.6
1.2
1.4
1.6
Band gap energy (eV)
Fig. 1. Dependence of the device performance on the band-gap energy of the uniform band-gap CIGS solar cells.
ARTICLE IN PRESS C.-H. Huang / Journal of Physics and Chemistry of Solids 69 (2008) 330–334
for the CIGS solar cells with the high Ga contents [7]. Additionally, the presence of the low-resistance Cu2xSe compounds on the surface layer of the as-deposited CIGS films with the high Ga contents were suggested to be responsible for the poor performance [8]. From the simulation results in this study, no significant effects of the surface-layer doping density ranging from 1013 to 1020 cm3 on the device performance are obtained for the CIGS solar cells with the high Ga contents. Moreover, it was suggested that a Gaussian defect band centered at 0.8 eV from the valence band edge in the high Ga-content CIGS films played an important role in determining the performance of the CIGS solar cells because of this defect band near the mid-gap of the CIGS films acting as an efficient recombination center [9,10]. To evaluate the impacts of the defect parameters on the CIGS solar cells with the high Ga contents, the defect densities and defect levels near the interface or in the bulk region of the CIGS films are used as the input variables for the device simulations. The uniform band-gap profile with the bandgap energy of 1.5 eV is employed in the simulations for the cell structures. The simulation results show that the effects of position of the defect energy levels are not as critical as the impacts of the defect densities on the performance of the high Ga-content CIGS solar cells. However, the midgap defect concentrations in either the surface layer or the bulk region of the CIGS films varying from 1014 to 1018 cm3 affect the performance of the CIGS solar cells dramatically, as illustrated in Fig. 2. The increase of the Bulk defect density
surface defect density or the bulk defect density deteriorates the open-circuit voltage of the CIGS solar cells without significantly affecting the short-circuit current density except for the value of the bulk defect density up to 1018 cm3, and hence the efficiency of the cells decreases. The results conclude that the surface defect density and/or the bulk defect density are the key factors in limiting the overall performance of the high Ga-content CIGS solar cells. 3.2. Impacts of spatial Ga distribution on the CIGS solar cells With the incorporation of the high Ga content in the CIGS layers near the region of the bottom Mo contacts, a quasi-electrical field [11] can be established in the back region of the CIGS films, and thus the back surface field (BSF) is created in the CIGS solar cells. The photogenerated minority carriers away from the space charge region (SCR) count on the diffusion mechanism to be collected for the contribution to the current. Therefore, the performance of the CIGS solar cells with the BSF is expected to improve by reducing the back surface recombination and increasing the effective minority carrier diffusion length, resulting in an efficient carrier collection. Both the open-circuit voltage and short-circuit current density of the devices with the BSF are supposed to be enhanced. The thickness and band-gap energy in the back regions of the CIGS films are used as variables varied from Surface defect density 1.1
25
1
Efficiency (%)
20
0.9 0.8
15
0.7 10
0.6
VOC (V)
332
0.5
5
0.4
0 30
JSC(mA/cm2)
25
80
20
70
15
60
10 5
50
0
40
Fill factor (%)
90
1013 1014 1015 1016 1017 1018 1019 1014 1015 1016 1017 1018 1019
Defect density (cm-3)
Fig. 2. Effects of the surface defect density or bulk defect density on the performance parameters of the CIGS (Eg ¼ 1.5 eV) solar cells.
ARTICLE IN PRESS C.-H. Huang / Journal of Physics and Chemistry of Solids 69 (2008) 330–334
400 to 1500 nm and from 1.16 to 1.6 eV in the device simulations, respectively. Comparing the performance of the cells having the BSF with that of the cells without the BSF, both the open-circuit voltage and the short-circuit current density of the solar cells with the BSF are improved simultaneously, and hence the conversion efficiency also increases. From the simulation results, the suitable values of thickness and the band-gap energy in the back region of the CIGS films to establish an effective BSF in the CIGS solar cells are around 600 nm and 1.35 eV, respectively. The surface band-gap widening and the inverted surface layer resulting in the n-type conductivity around the surface region of the CIGS films have been reported recently [12,13]. It was indicated that the band-gap energy of the surface region is at least 0.1 eV wider than that of the CIGS bulk region [13]. The resulting valence band offset between the surface region and the bulk region of the CIGS layers increases the energy barrier in the valence band and suppresses the interface recombination. Therefore, the performance of CIGS solar cells is improved due to the valence band offset. The non-uniform Ga distribution around the surface region could also contribute to the effects of the surface-layer band-gap widening in the CIGS films and result in a conduction band offset between the surface region and bulk region of the CIGS films. The effects of this conduction band offset on the performance of the CIGS solar cells are investigated. The valence band offset between the surface region and the bulk region of the CIGS layers is assumed to be 0.1 eV for the simulations. In addition, a uniform band-gap profile with the band-gap energy of 1.16 eV is employed for the CIGS layers. As shown in Table 1, the open-circuit voltage of the CIGS solar cells increases with the increase of the conduction band offset while the short-circuit current density remains almost the same. Obviously, the conduction band offset suppresses the interface recombination rate resulting in an increase in the open-circuit voltage. Further increasing the conduction band offset would impede the minority carrier transportation. Thus, when the value of the conduction band offset is 0.1 eV, the fill factor of the devices decreases, and hence the efficiency of the solar cells drops. By varying the Ga content spatially within the absorber layers of the CIGS solar cells, the technique of the bandgap engineering can be applied in the CIGS solar cells. The Table 1 The dependence of the conduction band offset due to the surface band-gap widening on the performance of the CIGS solar cells (assuming the valence band offset DEV ¼ 0.1 eV) DEC (eV)
Eg (eV)
Efficiency (%)
VOC (mV)
FF (%)
JSC (mA/cm2)
0.05 0 0.05 0.1
1.21 1.26 1.31 1.36
15.2 15.6 15.7 15.5
603 612 615 617
73.9 74.9 74.9 73.7
34.0 34.1 34.1 34.1
Eg: the band-gap energy of the surface layers of the CIGS films.
333
Table 2 The performance parameters of the CIGS solar cells with various bandgap profiles Band-gap profile
Efficiency (%)
VOC (mV)
FF (%)
JSC (mA/cm2)
Uniform band-gap Front surface grading Back surface grading Double grading
15.7 17.5 16.9 18.9
656 689 660 701
74.6 79.1 74.5 78.7
32.0 32.0 34.4 34.1
simulation results of the CIGS solar cells with various band-gap profiles including the uniform band-gap, front surface grading, back surface grading, and double grading are shown in Table 2. In the simulations, the CIGS solar cells with a uniform band-gap profile have the band-gap energy of 1.2 eV. The front surface grading is implemented by the increase of the band-gap energy in the SCR with the intentional addition of high Ga content in the SCR of the CIGS films. The recombination rate in the SCR, which limits the open-circuit voltage of the CIGS solar cells, is reduced by increasing the barrier height via the increase of the band-gap energy in the SCR without a significant loss in the short-circuit current density. The back surface grading mainly improves the carrier collection resulting in an enhancement of the short-circuit current density. The CIGS solar cells with the double grading band-gap profile incorporating both the front surface grading and back surface grading achieve the best performance by reducing the carrier recombination in the SCR and increasing the carrier collection, and hence both the open-circuit voltage and the short-circuit density are dramatically improved. 4. Summary and conclusions The simulation results show that the optimal band-gap energy to match the solar spectrum AM1.5G for the CIGS solar cells is around 1.5 eV. However, experimentally the performance of CIGS solar cells with high Ga content is deteriorated, mostly resulting from that the open-circuit voltage does not increase with the increase of the Ga content. Form the simulation results, the defect densities in the surface layers and/or bulk defect density of the CIGS films have a profound effect on the open-circuit voltage of the CIGS solar cells. The high defect densities in the surface and bulk region of the CIGS films decrease the open-circuit voltage and the overall device performance. The conduction band offset between the surface region and bulk region of the CIGS films resulting from the surface band-gap widening reduces the interface recombination rate and thus enhances the open-circuit voltage of CIGS solar cells. The effects of various Ga contents and various spatial Ga distributions within the absorber layers including the front surface grading, back surface grading, and double grading are investigated. With the gradient distribution of the Ga content in the SCR and back region of the CIGS absorber layers, the overall performance of the
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CIGS solar cells could be dramatically improved due to the beneficial effects of reducing the recombination rate in the SCR and enhancing the effective carrier collection. The CIGS solar cells with the double grading profiles give the best performance. Acknowledgment The author gratefully acknowledges the use of AMPS1D developed by Dr. S.J. Fonash of the Pennsylvania State University. 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] J.S. Ward, K. Ramanathan, F.S. Hasoon, T.J. Coutts, J. Keane, M.A. Contreras, T. Moriarty, R. Noufi, Prog. Photovolt. 10 (2002) 41–46.
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