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Changes in the characteristics of CuInGaSe2 solar cells under light irradiation and during recovery: degradation analysis by the feeble light measuring method
Microelectronics Reliability 42 (2002) 219–223 www.elsevier.com/locate/microrel
Changes in the characteristics of CuInGaSe2 solar cells under light irradiation and during recovery: degradation analysis by the feeble light measuring method Takeshi Yanagisawa *, Takeshi Kojima, Tadamasa Koyanagi, Kiyoshi Takahisa, Kuniomi Nakamura Institute of Energy Electronics, National Institute of Advanced Industrial Science and Technology, 1-1-1, Umezono, Tsukuba-shi, Ibaraki 305-8568, Japan Received 8 May 2001; received in revised form 26 June 2001
Abstract Changes in the characteristics of CuInGaSe2 solar cells in response to light irradiation were investigated. Then these changes, which suggest long-term degradation, were clarified using the measurement technique by feeble light. The thinfilm cell of this type is considered to be ‘‘ever stable’’. A stable result over the short term was also obtained in the light accelerated test of 2-SUN performed in this experiment. On the other hand, it was found that the characteristics measured with feeble light show a remarkable change over time. As a result of measuring at 0:065–105 mW/cm2 light intensity, the change rate of cell output power was so intense the measurement light was weak. This finding reflects the increase in an internal defect and suggests a possibility that light irradiation exerts the influence on long-term cell performance. Moreover, by measuring with feeble light, we found that the changed output recovers by reverse voltage application. The phenomenon of recovery up on comparatively low reverse voltage can be considered as an application for maintaining stability. Ó 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Use of the solar cell as a source of clean energy is expected to see large-scale application. Various materials for and structures for cells are being developed. At present, use of cells of utilizing single-crystal and polycrystal silicon is on the increase. However, widespread use of these cells presents problems, such as security of material supply for the future and manufacturing costs, etc. To address these problems, cells of the thin-film type were developed [1,2]. However, cells of this type are still at the developmental stage, and their long-term stability is not yet clear. For example, the thin-film amorphous silicone cell undergoes light degradation [3,4]. Given that a solar cell is exposed to the natural environment
*
Corresponding author. E-mail address: [email protected] (T. Yanagisawa).
and potentially severe weather conditions, an exact reliability evaluation is demanded. It is likely that a method for prediction of product lifetime and an acceleration test method will be established in the future. At present, the evaluation research is that clarifies degradation and the failure factor estimated experimentally, and gives feedback that can contribute to the development of improved cells. Of the thin-film cells under development, the CuInGaSe2 (CIGS) solar cell is expected to have a high conversion efficiency, and the stability of performance has also so far been excellent. Conversion efficiency has demonstrated a tendency to stabilize or improve in short-term reliability laboratory tests [5]. At present, an efficiency over 18% can be achieved [6], and the thin film type solar cell is promising in this regard. However, generation of new defects can occur as the result of an internal defect, and cells of this type have comparatively large internal defects; generation of new defects were observed to be generated by light irradiation [7–10].
0026-2714/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 0 1 ) 0 0 1 3 4 - 2
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T. Yanagisawa et al. / Microelectronics Reliability 42 (2002) 219–223
These defects may exert an influence on long-term stability of cell performance. In an attempt to solve this problem, we investigated changes in various characteristics of CIGS solar cells under light irradiation. We found the phenomenon that the characteristics of light-degraded cells recovered with application of reverse voltage. These findings were based on observations of solar cell photo characteristics measured using a feeble light source. We consider that this measuring method will be useful as one of the evaluation parameters of optical devices using solar cells.
2. Evaluation by measurement using feeble light Various evaluation parameters are used for analysis of the degradation of optical devices including solar cells. The sensitivity measurement under feeble light may be able to predict not only degradation in the practical use condition but also future degradation. This is due to the fact that slight degradation in the optical device is detectable. One factor of degradation in an optical device is growth of the internal defect. Generally there is good correlation between increase in an internal defect, and a reverse saturation current. However, the defective state cannot be thoroughly ascertained only by observation of reverse saturation current. The measuring method using a feeble white light and monochrome light is convenient to obtain detailed information on changes in characteristics. This experiment is an example of the behavior of CIGS solar cell observed by this technique. 3. Sample cell and experimental procedure The structure of the sample cell used for the test is the comb-shaped Au electrode/ZnO/n-CdS/p-CuInGaSe2 /Mo electrode/glass substrate. The light receiving area is 1.8 cm2 , and the light incident ZnO side. Two kinds of tests were performed: under light irradiation and the conditions of (I), and under applied reverse voltage and the conditions of (II) continuously. Moreover, the test by applied constant reverse voltage was performed under the condition of (III). (I) Light irradiation test: environment at 20°C, 2-SUN (accelerated light intensity of 200 mW/cm2 ), 230 h. (II) Reverse voltage (VR ) application test: after light irradiation test, dark state at 30°C,
The following measurement was performed before a test and at the appropriate time interval within the tests. (A) The photo I–V characteristics are measured at 12 points at white-light intensity (LW ) in a range of 0:065–105 mW/cm2 (Halogen light source, type of lamp: PHILIPS FCRA1/215). (B) Photo I–V characteristics are measured at feeble monochromatic-light (fixed photon number: 2 1013 s) at a 25 nm interval in the 375–1200 nm wavelength region. (A) was prepared combining a neutral density (ND) filter and (B) use of a monochrome meter. From measurement data of the photo I–V characteristics, the maximum output power (Pmax ), short-circuit current (Isc ), open circuit voltage (Voc ) and fill factor (FF) were calculated. The I–V characteristics and the junction capacitance are measured in the dark state.
4. Results and discussion 4.1. Change of the maximum output power (Pmax ) over time Fig. 1 shows change of Pmax by light irradiation test (test-I). In this experiment, given that light intensity (LW ) of the measurement is fixed, the change of Pmax is equivalent to the change of conversion efficiency. The plotting curves shown in the figure represent measurements by LW intensity of 0.065 and 105 mW/cm2 , respectively. The plot data was normalized with the initial value. In the measurement by LW ¼ 105 mW/cm2 (about 1-SUN), the changes of Pmax by tests of light irradiation were not observed. Because evaluations of cell performance are usually based on measurement at the level of 100 mW/cm2 , evaluation of stability is typically based solely on this data. There is report that Conversion efficiency has increase in early stage under light irra-
ðThe step applied methodÞ 0:5 V; 20 h ! 1 V; 20 h ! 2 V; 20 h ! 3 V; 20 h:
(III) Reverse voltage (VR ) application test: after light irradiation test, dark state at 30°C, ðThe constant applied methodÞ
1 V; 63 h:
Fig. 1. Changes of Pmax by light irradiation tests (test-I).
T. Yanagisawa et al. / Microelectronics Reliability 42 (2002) 219–223
diation [5]. Under certain conditions, it may be advantageous that performance improves short term. However, this finding suggests the simultaneous presence of an unstable factor. The structure of CIGS type solar cells, which are multi-element semiconductors and form a hetero-junction with CdS/ZnO, are complicated in comparison with silicone cells of a single crystal type. On the other hand, in measurement by feeble light of 0.065 mW/cm2 , we found that Pmax decreased remarkably under the light irradiation test. Pmax decreases about 65% over the short term after light irradiation at 50 h, after which, the rate of decrease slows. The cells for which Pmax increased temporarily by measurement at 105 mW/cm2 intensity showed a remarkable change almost similar to that shown in Fig. 1 by feeble light measurement. Usually, evaluation of a solar cell is performed by the measurement in the light intensity of 100 mW/cm2 . Probably, the slight change of the characteristic in the initial stage of generation of an internal defect is undetectable in the measurement under the intensity of this level. However, the measurement by feeble light can observe few defects, and the remarkable change observed by measurement may indicate future instability. The observation using the measuring method with feeble light will be useful for analysis of degradation. 4.2. Dependence on light intensity and changes in this dependence Fig. 2 shows changes in the LW dependence of Pmax , Voc , and FF in the light irradiation test. As a result of measuring by light intensity of 0:065–105 mW/cm2 , the decline rate of each parameter became notable when measurement light was weak. Isc hardly changed. The
221
change in Pmax mainly derives from changes in Voc and FF, which correspond with changes in Pmax . One of the factors, which affect FF, is series resistance. The series resistance was about 4 X, and no clear change was observed. The LW dependence of initial Voc is expressed with Voc ¼ 0:035 ln LW þ 0:35. The slant of dependence line increased greatly in the feeble light side. The value of Voc after 230 h under light irradiation declined to about 50% of the initial value in measurement at 0.065 mW/cm2 intensity. However, change of Voc is hardly observed by measurement at light intensity of 100 mW/ cm2 . The LW dependence of change rate (DVoc ) to initial value of Voc can be approximately expressed by DVoc / L0:4 W . In the case of the CIGS cell used this experiment, the change in Voc notably observed by measurement with feeble light can be explained by the following equation. Voc ¼ nkT =q lnð1 þ IL =I0 Þ
ð1Þ
Here, IL is the photo current, and I0 is the reverse saturation current. Supposing IL is much larger than I0 , the change of I0 is hardly to be indicated by change of Voc . However, when IL is a remarkable small level by feeble light measurement, change of I0 becomes being easy to be observed as change of Voc . The increase of I0 reflects an increase in an internal defect of the cell. The result of light irradiation test, I0 obtained from the dark I–V characteristic was increase as shown in Fig. 3. That is, a decline of Voc reflects this result, in which I0 is increased by growth of a defect. Similarly, diode factor (n) was also increased, from 1.9 to 2.6, by the light irradiation. Generally, in single crystal devices of small size, it may change to the value of 1 to 2 with generation of the internal defects. However, it has value larger than 2 in the
Fig. 2. Changes in the LW dependence of Pmax , Voc and FF.
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T. Yanagisawa et al. / Microelectronics Reliability 42 (2002) 219–223
Fig. 3. Changes in the dark I–V characteristics.
diodes of the large area like a solar cell. The change of the n value corresponds to increase of the recombination current by internal defective generation. The increase in the internal defect by light irradiation was confirmed by changes in junction capacitance, decreases of the electroluminescence change of the photo response by light of a long wavelength smaller than the energy of the band gap, etc. However, the measurement by feeble light has the advantage of enabling observation of the behavior of the photo characteristics of a solar cell directly. Based on these results, degradation does not appear in measurement on a 1-SUN level, though the decline of Pmax over time as observed by feeble light measurement suggests the possibility of negative influence of maintenance on cell performance over the long term. Further, this method may allow for prediction of performance degradation in the field under 1-SUN. 4.3. Measurement by feeble monochromatic light The evaluation data of light incident to the thickness direction of the cell can be obtained by measuring the photo I–V characteristics with a feeble monochromatic light. This reflects the difference in the invasion length of the light and the film thickness. Fig. 4 shows the wavelength dependability of Pmax , which were calculated from the result of measuring the photo I–V characteristics in the 375–1200 nm wavelength region. Measurement was performed at intervals of a 25 nm wavelength. By light irradiation and applied reverse voltage, Pmax was decreased and recovered over all wavelength regions, as shown in Fig. 4(a). The rate of decrease is almost the same as that in the range of 550–1100 nm wavelength, shown in Fig. 4(b). However, a large decrease was relatively observed in the shortwavelength side from 550 nm and in the long-wavelength side from 1100 nm. That is, these wavelength regions indicate the behavior in the layer near the win-
Fig. 4. Changes in the wavelength dependability of Pmax .
dow and the deepest layer in the thickness of a cell. Establishing a correspondence between these measurement results and the structural characteristics of a cell may enable detailed analysis of the cause of the decrease. 4.4. Recovery of Pmax by application of reverse voltage (VR ) Moreover, when measuring with feeble light, we found that Pmax , which decreased under light irradiation, was recovered by application of the reverse voltage VR . However, because a decrease of Pmax is hardly observed by measurement at LW ¼ 105 mW/cm2 intensity, under that condition recovery is likewise not observed. Fig. 5 shows the change of Pmax , to which VR was applied in the state of dark (test-II), following light irradiation. The rate of recovery reached about 60% by application of the step voltage to 1 V, 20 h. However, in contrast, the recovery effect was suppressed by applying voltage be-
Fig. 5. Changes of Pmax , to which VR was applied in the state of dark (test-II), following light irradiation.
T. Yanagisawa et al. / Microelectronics Reliability 42 (2002) 219–223
Fig. 6. Recovery characteristics by applied reverse voltage tests (test-III) with the passage of time.
yond VR ¼ 2 V. We consider that in these circumstances another factor that educes Pmax is acting. It is considered that the recovery phenomenon, which occurs in response to applied reverse voltage, is a restoration process of the defect generated with light irradiation. The characteristics of the amorphous silicon cell that has been degraded with light irradiation recovered at a comparatively low temperature of 100°C or less [4]. However, it is likely that the recovery phenomenon of CIGS cell occurs only by applied reverse voltage; it is probably not a thermal effect. CIGS semiconductor has mismatch to composition of the element. This mismatch is also a defect structurally, and may be the unstable factor. Fig. 6 shows the results of investigating the recovery characteristics by applied reverse voltage (1 V) with the passage of time, following light irradiation. Each Pmax , Voc , and FF was recovered to some extent in its early stages, and tended to be saturated along the way. The rate of recovery of Pmax was about 75% after 63 h. It is as yet unclear whether recovery has completely reversibility. If a 100% recovery is assumed, the time constant of recovery of Pmax is estimated as 10–15 h. Moreover, I0 recovered to about 100%, and its recovery was the most rapid of all parameters investigated herein. It was found that junction capacitance has a time constant of recovery at about 18 h. That is, a feature of the CIGS type solar cell is that the recovery effect appears for a comparatively short time by applied reverse (low) voltage. 5. Conclusions From the measurement using feeble light, we showed that generation of the internal defect, which may affect a
223
cell performance, could be notably observed at an early stage. In this report, we have not yet considered the information that might obtained from measurement data by using this method fully. However, this measurement method can be used as one evaluation of general optical devices. We found that the photo characteristics and the diode characteristics of CIGS type solar cell changed under light irradiation, and that these changes were recovered by applied reverse voltage. Changes in these characteristics may enable detection and prediction of signs of long-term degradation of cell performance. Moreover, the phenomenon of recovery up on low reverse voltage can could contribute to maintenance of stability.
References [1] Takamoto T, Ikeda E, Kurita H. Over 30% efficient InGaP/GaAs tandem solar cells. Appl Phys Lett 1997; 70:381. [2] Zhu F, Kohara H, Fuyuki T, Matsunami H. Structure and optical properties of polycrystalline silicon thin films deposited by the plasma enhanced chemical vapour deposition method. Jpn J Appl Phys 1996;35:3321. [3] Lee Y, Jiao L, Liu H, Lu Z, Collins RW, Wronski CR. Light-induced changes in hydrogen-diluted a-Si:H materials and solar cells: a new perspective on selfconsistent analysis. Solar Energy Mater Solar Cells 1997; 49:149. [4] Yanagisawa T. Temperature dependence of light-induced degradation in a-Si solar cells. Solar Cells 1987;22: 125. [5] Willett D, Kuriyagawa S. The effects of sweep rate, voltage bias and light soaking on the measurement of cis-based solar cell characteristics. Proc 23th IEEE Photovoltaic Specialist Conf, Kentucky, May 1993. p. 379. [6] Contreras MA, Egaas B, Ramanathan K, Hiltner J, Swartzlander A, Hasoon F, Noufi R. Progress toward 20% efficiency in Cu(In,Ga)Se2 polycrystalline thin-film solar cells. Prog Photovoltaics: Res Appl 1999;7(4):311. [7] Walter T, Herberholz R, Mueller C, Schock HW. Determination of defect distribution from admittance measurements and application to Cu(In,Ga)Se2 based heterojunctions. J Appl Phys 1996;80:4411. [8] Igalson M, Stolt L. Dlts spectra of thin film photovoltaic devices based on Cu(In,Ga)Se2 . Proc 14th European photovoltaic solar energy conf, Barcelona, June 1997. p. 2153. [9] Yanagisawa T, Kojima T, Koyanagi T, Takahisa K, Nakamura K. Behaviour of CuInGaSe2 solar cells under light irradiation. Electron Lett 2000;36(19):1659. [10] Rockett A. The electronic effects of point defects in Cu(Inx Ga1x )Se2 . Thin Solid Films 2000;361–362:330.
Changes in the characteristics of CuInGaSe2 solar cells under light irradiation and during recovery: degradation analysis by the feeble light measuring method Takeshi Yanagisawa *, Takeshi Kojima, Tadamasa Koyanagi, Kiyoshi Takahisa, Kuniomi Nakamura Institute of Energy Electronics, National Institute of Advanced Industrial Science and Technology, 1-1-1, Umezono, Tsukuba-shi, Ibaraki 305-8568, Japan Received 8 May 2001; received in revised form 26 June 2001
Abstract Changes in the characteristics of CuInGaSe2 solar cells in response to light irradiation were investigated. Then these changes, which suggest long-term degradation, were clarified using the measurement technique by feeble light. The thinfilm cell of this type is considered to be ‘‘ever stable’’. A stable result over the short term was also obtained in the light accelerated test of 2-SUN performed in this experiment. On the other hand, it was found that the characteristics measured with feeble light show a remarkable change over time. As a result of measuring at 0:065–105 mW/cm2 light intensity, the change rate of cell output power was so intense the measurement light was weak. This finding reflects the increase in an internal defect and suggests a possibility that light irradiation exerts the influence on long-term cell performance. Moreover, by measuring with feeble light, we found that the changed output recovers by reverse voltage application. The phenomenon of recovery up on comparatively low reverse voltage can be considered as an application for maintaining stability. Ó 2002 Elsevier Science Ltd. All rights reserved.
1. Introduction Use of the solar cell as a source of clean energy is expected to see large-scale application. Various materials for and structures for cells are being developed. At present, use of cells of utilizing single-crystal and polycrystal silicon is on the increase. However, widespread use of these cells presents problems, such as security of material supply for the future and manufacturing costs, etc. To address these problems, cells of the thin-film type were developed [1,2]. However, cells of this type are still at the developmental stage, and their long-term stability is not yet clear. For example, the thin-film amorphous silicone cell undergoes light degradation [3,4]. Given that a solar cell is exposed to the natural environment
*
Corresponding author. E-mail address: [email protected] (T. Yanagisawa).
and potentially severe weather conditions, an exact reliability evaluation is demanded. It is likely that a method for prediction of product lifetime and an acceleration test method will be established in the future. At present, the evaluation research is that clarifies degradation and the failure factor estimated experimentally, and gives feedback that can contribute to the development of improved cells. Of the thin-film cells under development, the CuInGaSe2 (CIGS) solar cell is expected to have a high conversion efficiency, and the stability of performance has also so far been excellent. Conversion efficiency has demonstrated a tendency to stabilize or improve in short-term reliability laboratory tests [5]. At present, an efficiency over 18% can be achieved [6], and the thin film type solar cell is promising in this regard. However, generation of new defects can occur as the result of an internal defect, and cells of this type have comparatively large internal defects; generation of new defects were observed to be generated by light irradiation [7–10].
0026-2714/02/$ - see front matter Ó 2002 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 6 - 2 7 1 4 ( 0 1 ) 0 0 1 3 4 - 2
220
T. Yanagisawa et al. / Microelectronics Reliability 42 (2002) 219–223
These defects may exert an influence on long-term stability of cell performance. In an attempt to solve this problem, we investigated changes in various characteristics of CIGS solar cells under light irradiation. We found the phenomenon that the characteristics of light-degraded cells recovered with application of reverse voltage. These findings were based on observations of solar cell photo characteristics measured using a feeble light source. We consider that this measuring method will be useful as one of the evaluation parameters of optical devices using solar cells.
2. Evaluation by measurement using feeble light Various evaluation parameters are used for analysis of the degradation of optical devices including solar cells. The sensitivity measurement under feeble light may be able to predict not only degradation in the practical use condition but also future degradation. This is due to the fact that slight degradation in the optical device is detectable. One factor of degradation in an optical device is growth of the internal defect. Generally there is good correlation between increase in an internal defect, and a reverse saturation current. However, the defective state cannot be thoroughly ascertained only by observation of reverse saturation current. The measuring method using a feeble white light and monochrome light is convenient to obtain detailed information on changes in characteristics. This experiment is an example of the behavior of CIGS solar cell observed by this technique. 3. Sample cell and experimental procedure The structure of the sample cell used for the test is the comb-shaped Au electrode/ZnO/n-CdS/p-CuInGaSe2 /Mo electrode/glass substrate. The light receiving area is 1.8 cm2 , and the light incident ZnO side. Two kinds of tests were performed: under light irradiation and the conditions of (I), and under applied reverse voltage and the conditions of (II) continuously. Moreover, the test by applied constant reverse voltage was performed under the condition of (III). (I) Light irradiation test: environment at 20°C, 2-SUN (accelerated light intensity of 200 mW/cm2 ), 230 h. (II) Reverse voltage (VR ) application test: after light irradiation test, dark state at 30°C,
The following measurement was performed before a test and at the appropriate time interval within the tests. (A) The photo I–V characteristics are measured at 12 points at white-light intensity (LW ) in a range of 0:065–105 mW/cm2 (Halogen light source, type of lamp: PHILIPS FCRA1/215). (B) Photo I–V characteristics are measured at feeble monochromatic-light (fixed photon number: 2 1013 s) at a 25 nm interval in the 375–1200 nm wavelength region. (A) was prepared combining a neutral density (ND) filter and (B) use of a monochrome meter. From measurement data of the photo I–V characteristics, the maximum output power (Pmax ), short-circuit current (Isc ), open circuit voltage (Voc ) and fill factor (FF) were calculated. The I–V characteristics and the junction capacitance are measured in the dark state.
4. Results and discussion 4.1. Change of the maximum output power (Pmax ) over time Fig. 1 shows change of Pmax by light irradiation test (test-I). In this experiment, given that light intensity (LW ) of the measurement is fixed, the change of Pmax is equivalent to the change of conversion efficiency. The plotting curves shown in the figure represent measurements by LW intensity of 0.065 and 105 mW/cm2 , respectively. The plot data was normalized with the initial value. In the measurement by LW ¼ 105 mW/cm2 (about 1-SUN), the changes of Pmax by tests of light irradiation were not observed. Because evaluations of cell performance are usually based on measurement at the level of 100 mW/cm2 , evaluation of stability is typically based solely on this data. There is report that Conversion efficiency has increase in early stage under light irra-
ðThe step applied methodÞ 0:5 V; 20 h ! 1 V; 20 h ! 2 V; 20 h ! 3 V; 20 h:
(III) Reverse voltage (VR ) application test: after light irradiation test, dark state at 30°C, ðThe constant applied methodÞ
1 V; 63 h:
Fig. 1. Changes of Pmax by light irradiation tests (test-I).
T. Yanagisawa et al. / Microelectronics Reliability 42 (2002) 219–223
diation [5]. Under certain conditions, it may be advantageous that performance improves short term. However, this finding suggests the simultaneous presence of an unstable factor. The structure of CIGS type solar cells, which are multi-element semiconductors and form a hetero-junction with CdS/ZnO, are complicated in comparison with silicone cells of a single crystal type. On the other hand, in measurement by feeble light of 0.065 mW/cm2 , we found that Pmax decreased remarkably under the light irradiation test. Pmax decreases about 65% over the short term after light irradiation at 50 h, after which, the rate of decrease slows. The cells for which Pmax increased temporarily by measurement at 105 mW/cm2 intensity showed a remarkable change almost similar to that shown in Fig. 1 by feeble light measurement. Usually, evaluation of a solar cell is performed by the measurement in the light intensity of 100 mW/cm2 . Probably, the slight change of the characteristic in the initial stage of generation of an internal defect is undetectable in the measurement under the intensity of this level. However, the measurement by feeble light can observe few defects, and the remarkable change observed by measurement may indicate future instability. The observation using the measuring method with feeble light will be useful for analysis of degradation. 4.2. Dependence on light intensity and changes in this dependence Fig. 2 shows changes in the LW dependence of Pmax , Voc , and FF in the light irradiation test. As a result of measuring by light intensity of 0:065–105 mW/cm2 , the decline rate of each parameter became notable when measurement light was weak. Isc hardly changed. The
221
change in Pmax mainly derives from changes in Voc and FF, which correspond with changes in Pmax . One of the factors, which affect FF, is series resistance. The series resistance was about 4 X, and no clear change was observed. The LW dependence of initial Voc is expressed with Voc ¼ 0:035 ln LW þ 0:35. The slant of dependence line increased greatly in the feeble light side. The value of Voc after 230 h under light irradiation declined to about 50% of the initial value in measurement at 0.065 mW/cm2 intensity. However, change of Voc is hardly observed by measurement at light intensity of 100 mW/ cm2 . The LW dependence of change rate (DVoc ) to initial value of Voc can be approximately expressed by DVoc / L0:4 W . In the case of the CIGS cell used this experiment, the change in Voc notably observed by measurement with feeble light can be explained by the following equation. Voc ¼ nkT =q lnð1 þ IL =I0 Þ
ð1Þ
Here, IL is the photo current, and I0 is the reverse saturation current. Supposing IL is much larger than I0 , the change of I0 is hardly to be indicated by change of Voc . However, when IL is a remarkable small level by feeble light measurement, change of I0 becomes being easy to be observed as change of Voc . The increase of I0 reflects an increase in an internal defect of the cell. The result of light irradiation test, I0 obtained from the dark I–V characteristic was increase as shown in Fig. 3. That is, a decline of Voc reflects this result, in which I0 is increased by growth of a defect. Similarly, diode factor (n) was also increased, from 1.9 to 2.6, by the light irradiation. Generally, in single crystal devices of small size, it may change to the value of 1 to 2 with generation of the internal defects. However, it has value larger than 2 in the
Fig. 2. Changes in the LW dependence of Pmax , Voc and FF.
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T. Yanagisawa et al. / Microelectronics Reliability 42 (2002) 219–223
Fig. 3. Changes in the dark I–V characteristics.
diodes of the large area like a solar cell. The change of the n value corresponds to increase of the recombination current by internal defective generation. The increase in the internal defect by light irradiation was confirmed by changes in junction capacitance, decreases of the electroluminescence change of the photo response by light of a long wavelength smaller than the energy of the band gap, etc. However, the measurement by feeble light has the advantage of enabling observation of the behavior of the photo characteristics of a solar cell directly. Based on these results, degradation does not appear in measurement on a 1-SUN level, though the decline of Pmax over time as observed by feeble light measurement suggests the possibility of negative influence of maintenance on cell performance over the long term. Further, this method may allow for prediction of performance degradation in the field under 1-SUN. 4.3. Measurement by feeble monochromatic light The evaluation data of light incident to the thickness direction of the cell can be obtained by measuring the photo I–V characteristics with a feeble monochromatic light. This reflects the difference in the invasion length of the light and the film thickness. Fig. 4 shows the wavelength dependability of Pmax , which were calculated from the result of measuring the photo I–V characteristics in the 375–1200 nm wavelength region. Measurement was performed at intervals of a 25 nm wavelength. By light irradiation and applied reverse voltage, Pmax was decreased and recovered over all wavelength regions, as shown in Fig. 4(a). The rate of decrease is almost the same as that in the range of 550–1100 nm wavelength, shown in Fig. 4(b). However, a large decrease was relatively observed in the shortwavelength side from 550 nm and in the long-wavelength side from 1100 nm. That is, these wavelength regions indicate the behavior in the layer near the win-
Fig. 4. Changes in the wavelength dependability of Pmax .
dow and the deepest layer in the thickness of a cell. Establishing a correspondence between these measurement results and the structural characteristics of a cell may enable detailed analysis of the cause of the decrease. 4.4. Recovery of Pmax by application of reverse voltage (VR ) Moreover, when measuring with feeble light, we found that Pmax , which decreased under light irradiation, was recovered by application of the reverse voltage VR . However, because a decrease of Pmax is hardly observed by measurement at LW ¼ 105 mW/cm2 intensity, under that condition recovery is likewise not observed. Fig. 5 shows the change of Pmax , to which VR was applied in the state of dark (test-II), following light irradiation. The rate of recovery reached about 60% by application of the step voltage to 1 V, 20 h. However, in contrast, the recovery effect was suppressed by applying voltage be-
Fig. 5. Changes of Pmax , to which VR was applied in the state of dark (test-II), following light irradiation.
T. Yanagisawa et al. / Microelectronics Reliability 42 (2002) 219–223
Fig. 6. Recovery characteristics by applied reverse voltage tests (test-III) with the passage of time.
yond VR ¼ 2 V. We consider that in these circumstances another factor that educes Pmax is acting. It is considered that the recovery phenomenon, which occurs in response to applied reverse voltage, is a restoration process of the defect generated with light irradiation. The characteristics of the amorphous silicon cell that has been degraded with light irradiation recovered at a comparatively low temperature of 100°C or less [4]. However, it is likely that the recovery phenomenon of CIGS cell occurs only by applied reverse voltage; it is probably not a thermal effect. CIGS semiconductor has mismatch to composition of the element. This mismatch is also a defect structurally, and may be the unstable factor. Fig. 6 shows the results of investigating the recovery characteristics by applied reverse voltage (1 V) with the passage of time, following light irradiation. Each Pmax , Voc , and FF was recovered to some extent in its early stages, and tended to be saturated along the way. The rate of recovery of Pmax was about 75% after 63 h. It is as yet unclear whether recovery has completely reversibility. If a 100% recovery is assumed, the time constant of recovery of Pmax is estimated as 10–15 h. Moreover, I0 recovered to about 100%, and its recovery was the most rapid of all parameters investigated herein. It was found that junction capacitance has a time constant of recovery at about 18 h. That is, a feature of the CIGS type solar cell is that the recovery effect appears for a comparatively short time by applied reverse (low) voltage. 5. Conclusions From the measurement using feeble light, we showed that generation of the internal defect, which may affect a
223
cell performance, could be notably observed at an early stage. In this report, we have not yet considered the information that might obtained from measurement data by using this method fully. However, this measurement method can be used as one evaluation of general optical devices. We found that the photo characteristics and the diode characteristics of CIGS type solar cell changed under light irradiation, and that these changes were recovered by applied reverse voltage. Changes in these characteristics may enable detection and prediction of signs of long-term degradation of cell performance. Moreover, the phenomenon of recovery up on low reverse voltage can could contribute to maintenance of stability.
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