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Comparison of amorphous silicon solar cell performance following light and high-energy electron-beam induced degradation
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Journal of Non-Crystalline Solids 354 (2008) 2464–2467 www.elsevier.com/locate/jnoncrysol
Comparison of amorphous silicon solar cell performance following light and high-energy electron-beam induced degradation R.A.C.M.M. van Swaaij *, A. Klaver Delft University of Technology, DIMES-ECTM, P.O. Box 5053, 2600 GB Delft, The Netherlands Available online 1 February 2008
Abstract In this article a comparison is reported between amorphous silicon (a-Si:H) solar cells that have been degraded using light soaking and 1 MeV electron-beam irradiation. Solar cells were degraded in open- and short-circuit condition, with the aim to change the recombination profile in the cell. For light-soaked solar cells a clear difference is found between open- and short-circuit conditions. Under open-circuit condition the solar cells degrade much more, which is explained by a much higher recombination rate under illumination in this case. These recombination events are believed to initiate defect formation. The performance of thin solar cells degrades less, as expected. For solar cells degraded under electron-beam irradiation no difference is found between open- and short-circuit conditions. Therefore we think that during electron-beam irradiation defect creation is not initiated by recombination events, but by energy transfer during collisions. The fill factor of thin solar cells degrades more after electron-beam irradiation. This effect is ascribed to a significant increase of the activation energy of the doped layers after irradiation. Ó 2007 Elsevier B.V. All rights reserved. PACS: 84.60.Jt; 85.30.De; 81.05.Gc Keywords: Amorphous silicon; Solar cells; Recombination; Electron irradiation
1. Introduction Hydrogenated amorphous silicon (a-Si:H) solar cells have great potential for space applications, because they can be produced inexpensively, are lightweight, and are relatively radiation hard. In a space environment large part of the solar cell performance degradation is due to high-energy charged-particle irradiation [1–3]. On the other hand, for terrestrial application the degradation of a-Si:H solar cells is usually ascribed to the Staebler–Wronski effect [4]. Some workers have reported that the change in a-Si:H material properties as a result of charged-particle irradiation is similar to light-induced degradation [5,6], implying that the solar cell performance is affected in a similar way. In this article we will make a comparison between a-Si:H solar cells *
Corresponding author. Tel.: +31 15 2787259; fax: +31 15 2622163. E-mail address: [email protected] (R.A.C.M.M. van Swaaij). 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.09.025
that have been subjected to 1 MeV electron-beam irradiation and cells that have been light soaked. We will show that there are distinct differences between the two degradation methods, which have important implications for the process initiating the defect creation. This article is organized as follows. We will first describe the experimental details concerning the degradation of the solar cells. Then we will present and discuss the results of the experiments. Finally, we end with some concluding remarks. 2. Experimental details For the degradation experiments described in this article we used single-junction a-Si:H solar cells. For the lightsoaking experiments solar cells deposited on Asahi U-type substrates are used, with the following structure: 9 nm thick p-doped a-SiC:H, 15 nm thick a-SiC:H buffer layer, a-Si:H i-layer, 15 nm thick n-doped a-Si:H, and 300 nm
R.A.C.M.M. van Swaaij, A. Klaver / Journal of Non-Crystalline Solids 354 (2008) 2464–2467
3. Results In Fig. 1 the results of the fill factor, FF, and short-current density, Jsc, are displayed of solar cells after light soaking under open- and short-circuit condition (open and closed symbols, respectively). From Fig. 1(a) it is clear that the FF decreases as a function of the illumination time and that this decease is faster for the 900 nm thick cell than for the 300 nm thick cell. This thickness dependence has been reported before by several groups e.g., [7,8]. In addition to the above observation, the FF decreases substantially faster of solar cells that have been degraded under opencircuit condition than of cells degraded under short-circuit condition. The dependence of light-induced performance degradation on applied bias was already reported before [9,10]. The Jsc variation with illumination time is depicted in Fig. 1(b). Several interesting features can be observed. Of course a larger drop is seen for the 900 nm thick cell, which is consistent with the decrease of the FF. Also in this figure a larger decrease is observed for the solar cells degraded under open-circuit condition, however, initially
0.7
FF (-)
0.6
0.5
0.4
0.3 16
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Jsc (mA/cm )
thick Al layer. Cells subjected to electron-beam irradiation were deposited on radiation-tolerant CMZ glass. The structure of the cells was as follows: 0.5 mm thick CMZ glass, 700 nm thick texture-etched ZnO:Al front contact, 5 nm thick p-type lc-Si:H, 9 nm thick p-type a-SiC:H, 15 nm thick intrinsic a-SiC:H, an intrinsic a-Si:H layer with varying thickness, 15 nm thick n-type a-Si:H, and as back contact 100 nm Ag covered with 200 nm Al. The i-layer thickness was varied between 300 and 900 nm. The area of all solar cells is 4 mm 4 mm. For the light-soaking experiments a semiconductor laser was used with a wavelength of 635 nm. The power density was 3.4 kW/m2. During the entire light-soaking experiment the same two cells were used: one cell was degraded under short-circuit conditions and the other under open-voltage conditions. The cells were short-circuited by connecting them to a current meter and a contact resistance of less the 4 X was achieved. The temperature of the solar cell during light soaking was measured to be about 35 °C. For irradiation experiments solar cells were subjected to a 1 MeV electron-beam, under both open- and short-circuit conditions. The fluence was varied between 5 1015 and 2 1016 electrons/cm2, using a beam current of 1.5 1012 electrons/cm2s. Half of the cells were short-circuited by applying silver paste between the front and back contacts. For each short-circuited cell, the resistance between both contacts was measured to be less than 5 X before and after the irradiation. After irradiation the connections were removed by cleaning with iso-propanol and no extra shunting due to traces of silver paste was found. The external parameters of the cells are obtained from the current density versus voltage (J–V) curve measured under AM 1.5 illumination (Oriel AM1.5 solar simulator). The quantum efficiency (QE) was measured using a bias of 0 V and no bias light was used.
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open circuit, 300 nm short circuit, 300 nm open circuit, 900 nm short circuit, 900 nm
12
10
0.0 1
10
100
illumination time (min) Fig. 1. (a) The fill factor, FF, and (b) short-circuit current density, Jsc, as a function of the illumination time. The results of solar cells degraded under open-circuit and short-circuit condition are shown by open and closed symbols, respectively.
the Jsc decrease appears to be independent of the circuiting condition. A similar effect is observed for the 300 nm thick cell, although in this case the sharper decrease of the opencircuited cell occurs later. For solar cells with i-layer thickness in between the values displayed in Fig. 1 the same is observed. The FF and the Jsc of solar cells subjected to 1 MeV electron-beam irradiation are shown in Fig. 2 as a function of the fluence. Within the investigated fluence range a much larger drop can be seen in the FF than for light soaking. In contrast to the solar cells subjected to light soaking, the FF of the thinnest solar cell decreases more than that of the thickest solar cell, resulting in very similar values for a fluence of 2 1016 electrons/cm2. The reduction of the Jsc following electron-beam irradiation shown in Fig. 2(b) is much larger than for light soaking. Although we have used radiation tolerant CMZ glass, part of this drop may be explained by the increase in absorption of the glass. Simulations have shown that the Jsc reduction due to the increased absorption in the CMZ glass is about 1.4 mA/ cm2. In Fig. 2(b) we see that now the change in Jsc is larger for the thicker cell, similar to what was observed for the light-soaked solar cells. However, the most striking observation in Fig. 2 is that for both FF and Jsc no clear
R.A.C.M.M. van Swaaij, A. Klaver / Journal of Non-Crystalline Solids 354 (2008) 2464–2467
open circuit, 300 nm short circuit, 300 nm open circuit, 900 nm short circuit, 900 nm
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Fig. 3. The quantum efficiency of 900 nm thick solar cell following light soaking. The filled and open symbols represent the curves of cells degraded under short- (SC) and open-circuit (OC) condition, respectively.
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fluence (10 electrons/cm2) Fig. 2. (a) The fill factor, FF, and (b) short-circuit current density, Jsc, as a function of the electron-beam fluence. The results of solar cells degraded under open-circuit and short-circuit condition are shown by open and closed symbols, respectively.
difference is found between solar cells irradiated under open- and short-circuit conditions. Herbst et al. [11] reported earlier that they did not find a difference in conversion-efficiency reduction between a-Si:H solar cells biased in open- or short-circuit condition after irradiation with 20 keV electrons, though no specific details on the external parameters were given. The response of the solar cells was further analyzed by QE measurements and some results are shown in Figs. 3 and 4. Fig. 3 compares the QE of a 900 nm thick solar cell degraded by light soaking under open- and short-circuit condition. Obviously, the QE of cells degraded under short-circuit condition decreases much slower than under open-circuit condition. This observation is in agreement with the results found for the Jsc degradation. Although the degradation rate depends very much on the circuit condition, the shape of the QE is similar, irrespective under which circuit condition the cell has been degraded. For instance, the cell light soaked under open-circuit condition for 5 min is comparable within 5% over the entire wavelength range to the cell degraded under short-circuit condition for 80 min. For longer light-soaking times the QE shows a narrow peak around 600 nm.
quantum efficiency
Jsc (mA/cm2)
600
wavelength (nm)
As-deposited 15 1 x 10 e/cm2 15 5 x 10 e/cm2 16 1 x 10 e/cm2 16 2 x 10 e/cm2
0.8 0.6 0.4 0.2 0.0
400
500
600
700
800
900
1000
wavelength (nm) Fig. 4. The quantum efficiency of 900 nm thick solar cell following 1 MeV electron-beam irradiation. In this case no difference was observed between degradation under open- and short-circuit condition.
The QE of the solar cells used for the irradiation measurements shows some differences when compared to light soaked cells. In Fig. 4 the QE of 900 nm thick solar cells following 1 MeV electron irradiation are plotted. The overall response of the as-deposited solar cell is lower than that of the as-deposited solar used in the light soaking experiment. This difference is obviously due to the different structure that was required. Although the solar cell structure is different, the variation of QE with irradiation fluence is similar to the QE of light degraded solar cells. However, in this case no difference was observed between the degradation of the QE under open- and short-circuit condition. Also in this case the QE peaks around 600 nm. 4. Discussion It is generally accepted that solar cell performance degradation as a result of light soaking is explained by defect creation. This defect-creation process is still under debate, but it
R.A.C.M.M. van Swaaij, A. Klaver / Journal of Non-Crystalline Solids 354 (2008) 2464–2467
is believed that recombination events initiate this process (e.g., see Ref. [12]). The larger FF degradation of the 900 nm thick solar cell is due to a higher recombination rate, which then leads to a higher defect-creation rate in this cell. The higher recombination rate is the result of lower internal electric field strength in the thicker cell. Similarly, the degradation under open-circuit condition can be explained also by the increased recombination rate. The internal electric field collapses in an illuminated solar cell operating under open-circuit condition, thereby increasing the recombination rate and therefore the defect-creation rate. The measurement results presented above show that there is a clear difference between light soaking and electron-beam irradiation, which we think is due to a difference in initiating the defect-creation process. As explained above, defect creation during light soaking is initiated by recombination events in the material, leading to clear differences between degradation under open- and short-circuit condition as in this way the total recombination rate in the device is altered significantly. After electron-beam irradiation, however, such differences between degradation under open- and short-circuit conditions are not found, implying that defect creation in that case is not initiated by recombination events in contradiction with the conclusion of Yelon et al. [5]. They concluded that electron–hole pairs are created almost instantaneously in a tube at about every nanometer along the electron track and due to the high carrier density the lifetime of the electron–hole pairs is very short (10 ps). As a result of the recombination, mobile hydrogen is excited out of Si–H bonds, which may lead to defect formation as described by the hydrogen-collision model. We estimate that in our irradiation experiments pairs are created about every 15 nm and therefore we find a lifetime in excess of 100 ps [13]. This time is more or less equivalent to the time required for irradiation induced electrons to drift across a 300 nm thick a-Si:H solar cell. We therefore argue that if defect creation during irradiation is triggered by recombination this should lead to a clear difference between open-circuit and short-circuit degradation, which we do not observe. We suggest instead that during irradiation defect creation is initiated by energy transfer from the electrons to the amorphous silicon network during collision events. In that case defects are created directly at the position where the collision event occurs, similar to the conclusion of Herbst et al. [11], either by breaking a weak bond or a Si–H bond. We observed a larger FF degradation for the thin solar cell following electron-beam degradation. We think that this higher degradation rate is due to changes in the properties of the doped layers. In other experiments (not shown here) we found that the activation energy of the p- and ndoped layers in the solar cell can change by as much as 0.2 eV after electron-beam irradiation. This change in activation energy particularly affects the FF of thin solar cells, as was confirmed by computer simulations [14].
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5. Conclusions We have shown that there are significant differences between the degradation of a-Si:H solar cells by light soaking and 1 MeV electron-beam degradation. The performance of solar cells degraded under open-circuit condition during light soaking degrades much faster than under short-circuit condition. Under open-circuit condition the recombination rate is much higher in the solar cell, leading to a higher defect-creation rate. For solar cells subjected to electron-beam irradiation no difference is found between open- and short-circuit degradation. We conclude that defect creation in the solar cells during electron-beam irradiation is not initiated by recombination events, but by energy transfer during collision events. Further, we found that the FF of thin solar cells degrades faster after electron-beam irradiation. We ascribe this effect to the decrease of the activation energy of the doped layers. Acknowledgements We thank Martijn Tijssen for the deposition of the solar cells, Kasper Zwetsloot for the technical assistance, and Marinus Hom for the electron-beam irradiation. This work was supported by Technology Foundation STW and SenterNovem. References [1] J.R. Srour, G.J. Vendura, D.H. Lo, C.M.C. Toporow, M. Dooley, R.P. Nakano, E.E. King, IEEE Trans. Nucl. Sci. 45 (1998) 2624. [2] J.E. Granata, T.D. Sahlstrom, P.E. Hausgen, S.R. Messenger, R.J. Walters, J.R. Lorentzen, in: Proceedings of 31st IEEE-PVSC, 2005, p. 607. [3] K.R. Lord, M.R. Walters, J.R. Woodyard, in: Proceedings of 23rd IEEE-PVSC, 1993, p. 1448. [4] D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292. [5] A. Yelon, H. Fritzsche, H.M. Branz, J. Non-Cryst. Solids 266–269 (2000) 437. [6] P. Danesh, B. Pantchev, E. Vlaikova, Nucl. Instr. Meth. Phys. Res. B 239 (2005) 370. [7] P. Chaudhuri, S. Ray, A.K. Batabyal, A.K. Barua, Solar Cells 31 (1991) 13. [8] B. Rech, H. Wagner, Appl. Phys. A 69 (1999) 155. [9] D.L. Staebler, R.S. Crandall, R. Williams, Appl. Phys. Lett. 39 (1981) 733. [10] L. Yang, L. Chen, J.Y. Hou, Y.M. Li, in: M.J. Thompson, Y. Hamakawa, P.G. LeComber, A. Madan, E.A. Schiff (Eds.), Materials Research Society Symposium Proceedings, Amorphous Silicon Technology, April 27–May 1, San Francisco, USA, 1992, vol. 258, 2001, p. 365. [11] W. Herbst, J. Dudel, A. Scholz, B. Schro¨der, H. Oechsner, Sol. Energy Mater. Sol. Cells 37 (1995) 55. [12] H.M. Branz, Phys. Rev. B 59 (1999) 5498. [13] E.A. Schiff, J. Non-Cryst. Solids 190 (1995) 1. [14] A. Klaver, Irradiation-induced degradation of amorphous silicon solar cells in space, PhD thesis, Delft University of Technology, 2007.
Journal of Non-Crystalline Solids 354 (2008) 2464–2467 www.elsevier.com/locate/jnoncrysol
Comparison of amorphous silicon solar cell performance following light and high-energy electron-beam induced degradation R.A.C.M.M. van Swaaij *, A. Klaver Delft University of Technology, DIMES-ECTM, P.O. Box 5053, 2600 GB Delft, The Netherlands Available online 1 February 2008
Abstract In this article a comparison is reported between amorphous silicon (a-Si:H) solar cells that have been degraded using light soaking and 1 MeV electron-beam irradiation. Solar cells were degraded in open- and short-circuit condition, with the aim to change the recombination profile in the cell. For light-soaked solar cells a clear difference is found between open- and short-circuit conditions. Under open-circuit condition the solar cells degrade much more, which is explained by a much higher recombination rate under illumination in this case. These recombination events are believed to initiate defect formation. The performance of thin solar cells degrades less, as expected. For solar cells degraded under electron-beam irradiation no difference is found between open- and short-circuit conditions. Therefore we think that during electron-beam irradiation defect creation is not initiated by recombination events, but by energy transfer during collisions. The fill factor of thin solar cells degrades more after electron-beam irradiation. This effect is ascribed to a significant increase of the activation energy of the doped layers after irradiation. Ó 2007 Elsevier B.V. All rights reserved. PACS: 84.60.Jt; 85.30.De; 81.05.Gc Keywords: Amorphous silicon; Solar cells; Recombination; Electron irradiation
1. Introduction Hydrogenated amorphous silicon (a-Si:H) solar cells have great potential for space applications, because they can be produced inexpensively, are lightweight, and are relatively radiation hard. In a space environment large part of the solar cell performance degradation is due to high-energy charged-particle irradiation [1–3]. On the other hand, for terrestrial application the degradation of a-Si:H solar cells is usually ascribed to the Staebler–Wronski effect [4]. Some workers have reported that the change in a-Si:H material properties as a result of charged-particle irradiation is similar to light-induced degradation [5,6], implying that the solar cell performance is affected in a similar way. In this article we will make a comparison between a-Si:H solar cells *
Corresponding author. Tel.: +31 15 2787259; fax: +31 15 2622163. E-mail address: [email protected] (R.A.C.M.M. van Swaaij). 0022-3093/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jnoncrysol.2007.09.025
that have been subjected to 1 MeV electron-beam irradiation and cells that have been light soaked. We will show that there are distinct differences between the two degradation methods, which have important implications for the process initiating the defect creation. This article is organized as follows. We will first describe the experimental details concerning the degradation of the solar cells. Then we will present and discuss the results of the experiments. Finally, we end with some concluding remarks. 2. Experimental details For the degradation experiments described in this article we used single-junction a-Si:H solar cells. For the lightsoaking experiments solar cells deposited on Asahi U-type substrates are used, with the following structure: 9 nm thick p-doped a-SiC:H, 15 nm thick a-SiC:H buffer layer, a-Si:H i-layer, 15 nm thick n-doped a-Si:H, and 300 nm
R.A.C.M.M. van Swaaij, A. Klaver / Journal of Non-Crystalline Solids 354 (2008) 2464–2467
3. Results In Fig. 1 the results of the fill factor, FF, and short-current density, Jsc, are displayed of solar cells after light soaking under open- and short-circuit condition (open and closed symbols, respectively). From Fig. 1(a) it is clear that the FF decreases as a function of the illumination time and that this decease is faster for the 900 nm thick cell than for the 300 nm thick cell. This thickness dependence has been reported before by several groups e.g., [7,8]. In addition to the above observation, the FF decreases substantially faster of solar cells that have been degraded under opencircuit condition than of cells degraded under short-circuit condition. The dependence of light-induced performance degradation on applied bias was already reported before [9,10]. The Jsc variation with illumination time is depicted in Fig. 1(b). Several interesting features can be observed. Of course a larger drop is seen for the 900 nm thick cell, which is consistent with the decrease of the FF. Also in this figure a larger decrease is observed for the solar cells degraded under open-circuit condition, however, initially
0.7
FF (-)
0.6
0.5
0.4
0.3 16
14 2
Jsc (mA/cm )
thick Al layer. Cells subjected to electron-beam irradiation were deposited on radiation-tolerant CMZ glass. The structure of the cells was as follows: 0.5 mm thick CMZ glass, 700 nm thick texture-etched ZnO:Al front contact, 5 nm thick p-type lc-Si:H, 9 nm thick p-type a-SiC:H, 15 nm thick intrinsic a-SiC:H, an intrinsic a-Si:H layer with varying thickness, 15 nm thick n-type a-Si:H, and as back contact 100 nm Ag covered with 200 nm Al. The i-layer thickness was varied between 300 and 900 nm. The area of all solar cells is 4 mm 4 mm. For the light-soaking experiments a semiconductor laser was used with a wavelength of 635 nm. The power density was 3.4 kW/m2. During the entire light-soaking experiment the same two cells were used: one cell was degraded under short-circuit conditions and the other under open-voltage conditions. The cells were short-circuited by connecting them to a current meter and a contact resistance of less the 4 X was achieved. The temperature of the solar cell during light soaking was measured to be about 35 °C. For irradiation experiments solar cells were subjected to a 1 MeV electron-beam, under both open- and short-circuit conditions. The fluence was varied between 5 1015 and 2 1016 electrons/cm2, using a beam current of 1.5 1012 electrons/cm2s. Half of the cells were short-circuited by applying silver paste between the front and back contacts. For each short-circuited cell, the resistance between both contacts was measured to be less than 5 X before and after the irradiation. After irradiation the connections were removed by cleaning with iso-propanol and no extra shunting due to traces of silver paste was found. The external parameters of the cells are obtained from the current density versus voltage (J–V) curve measured under AM 1.5 illumination (Oriel AM1.5 solar simulator). The quantum efficiency (QE) was measured using a bias of 0 V and no bias light was used.
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open circuit, 300 nm short circuit, 300 nm open circuit, 900 nm short circuit, 900 nm
12
10
0.0 1
10
100
illumination time (min) Fig. 1. (a) The fill factor, FF, and (b) short-circuit current density, Jsc, as a function of the illumination time. The results of solar cells degraded under open-circuit and short-circuit condition are shown by open and closed symbols, respectively.
the Jsc decrease appears to be independent of the circuiting condition. A similar effect is observed for the 300 nm thick cell, although in this case the sharper decrease of the opencircuited cell occurs later. For solar cells with i-layer thickness in between the values displayed in Fig. 1 the same is observed. The FF and the Jsc of solar cells subjected to 1 MeV electron-beam irradiation are shown in Fig. 2 as a function of the fluence. Within the investigated fluence range a much larger drop can be seen in the FF than for light soaking. In contrast to the solar cells subjected to light soaking, the FF of the thinnest solar cell decreases more than that of the thickest solar cell, resulting in very similar values for a fluence of 2 1016 electrons/cm2. The reduction of the Jsc following electron-beam irradiation shown in Fig. 2(b) is much larger than for light soaking. Although we have used radiation tolerant CMZ glass, part of this drop may be explained by the increase in absorption of the glass. Simulations have shown that the Jsc reduction due to the increased absorption in the CMZ glass is about 1.4 mA/ cm2. In Fig. 2(b) we see that now the change in Jsc is larger for the thicker cell, similar to what was observed for the light-soaked solar cells. However, the most striking observation in Fig. 2 is that for both FF and Jsc no clear
R.A.C.M.M. van Swaaij, A. Klaver / Journal of Non-Crystalline Solids 354 (2008) 2464–2467
open circuit, 300 nm short circuit, 300 nm open circuit, 900 nm short circuit, 900 nm
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Fig. 3. The quantum efficiency of 900 nm thick solar cell following light soaking. The filled and open symbols represent the curves of cells degraded under short- (SC) and open-circuit (OC) condition, respectively.
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fluence (10 electrons/cm2) Fig. 2. (a) The fill factor, FF, and (b) short-circuit current density, Jsc, as a function of the electron-beam fluence. The results of solar cells degraded under open-circuit and short-circuit condition are shown by open and closed symbols, respectively.
difference is found between solar cells irradiated under open- and short-circuit conditions. Herbst et al. [11] reported earlier that they did not find a difference in conversion-efficiency reduction between a-Si:H solar cells biased in open- or short-circuit condition after irradiation with 20 keV electrons, though no specific details on the external parameters were given. The response of the solar cells was further analyzed by QE measurements and some results are shown in Figs. 3 and 4. Fig. 3 compares the QE of a 900 nm thick solar cell degraded by light soaking under open- and short-circuit condition. Obviously, the QE of cells degraded under short-circuit condition decreases much slower than under open-circuit condition. This observation is in agreement with the results found for the Jsc degradation. Although the degradation rate depends very much on the circuit condition, the shape of the QE is similar, irrespective under which circuit condition the cell has been degraded. For instance, the cell light soaked under open-circuit condition for 5 min is comparable within 5% over the entire wavelength range to the cell degraded under short-circuit condition for 80 min. For longer light-soaking times the QE shows a narrow peak around 600 nm.
quantum efficiency
Jsc (mA/cm2)
600
wavelength (nm)
As-deposited 15 1 x 10 e/cm2 15 5 x 10 e/cm2 16 1 x 10 e/cm2 16 2 x 10 e/cm2
0.8 0.6 0.4 0.2 0.0
400
500
600
700
800
900
1000
wavelength (nm) Fig. 4. The quantum efficiency of 900 nm thick solar cell following 1 MeV electron-beam irradiation. In this case no difference was observed between degradation under open- and short-circuit condition.
The QE of the solar cells used for the irradiation measurements shows some differences when compared to light soaked cells. In Fig. 4 the QE of 900 nm thick solar cells following 1 MeV electron irradiation are plotted. The overall response of the as-deposited solar cell is lower than that of the as-deposited solar used in the light soaking experiment. This difference is obviously due to the different structure that was required. Although the solar cell structure is different, the variation of QE with irradiation fluence is similar to the QE of light degraded solar cells. However, in this case no difference was observed between the degradation of the QE under open- and short-circuit condition. Also in this case the QE peaks around 600 nm. 4. Discussion It is generally accepted that solar cell performance degradation as a result of light soaking is explained by defect creation. This defect-creation process is still under debate, but it
R.A.C.M.M. van Swaaij, A. Klaver / Journal of Non-Crystalline Solids 354 (2008) 2464–2467
is believed that recombination events initiate this process (e.g., see Ref. [12]). The larger FF degradation of the 900 nm thick solar cell is due to a higher recombination rate, which then leads to a higher defect-creation rate in this cell. The higher recombination rate is the result of lower internal electric field strength in the thicker cell. Similarly, the degradation under open-circuit condition can be explained also by the increased recombination rate. The internal electric field collapses in an illuminated solar cell operating under open-circuit condition, thereby increasing the recombination rate and therefore the defect-creation rate. The measurement results presented above show that there is a clear difference between light soaking and electron-beam irradiation, which we think is due to a difference in initiating the defect-creation process. As explained above, defect creation during light soaking is initiated by recombination events in the material, leading to clear differences between degradation under open- and short-circuit condition as in this way the total recombination rate in the device is altered significantly. After electron-beam irradiation, however, such differences between degradation under open- and short-circuit conditions are not found, implying that defect creation in that case is not initiated by recombination events in contradiction with the conclusion of Yelon et al. [5]. They concluded that electron–hole pairs are created almost instantaneously in a tube at about every nanometer along the electron track and due to the high carrier density the lifetime of the electron–hole pairs is very short (10 ps). As a result of the recombination, mobile hydrogen is excited out of Si–H bonds, which may lead to defect formation as described by the hydrogen-collision model. We estimate that in our irradiation experiments pairs are created about every 15 nm and therefore we find a lifetime in excess of 100 ps [13]. This time is more or less equivalent to the time required for irradiation induced electrons to drift across a 300 nm thick a-Si:H solar cell. We therefore argue that if defect creation during irradiation is triggered by recombination this should lead to a clear difference between open-circuit and short-circuit degradation, which we do not observe. We suggest instead that during irradiation defect creation is initiated by energy transfer from the electrons to the amorphous silicon network during collision events. In that case defects are created directly at the position where the collision event occurs, similar to the conclusion of Herbst et al. [11], either by breaking a weak bond or a Si–H bond. We observed a larger FF degradation for the thin solar cell following electron-beam degradation. We think that this higher degradation rate is due to changes in the properties of the doped layers. In other experiments (not shown here) we found that the activation energy of the p- and ndoped layers in the solar cell can change by as much as 0.2 eV after electron-beam irradiation. This change in activation energy particularly affects the FF of thin solar cells, as was confirmed by computer simulations [14].
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5. Conclusions We have shown that there are significant differences between the degradation of a-Si:H solar cells by light soaking and 1 MeV electron-beam degradation. The performance of solar cells degraded under open-circuit condition during light soaking degrades much faster than under short-circuit condition. Under open-circuit condition the recombination rate is much higher in the solar cell, leading to a higher defect-creation rate. For solar cells subjected to electron-beam irradiation no difference is found between open- and short-circuit degradation. We conclude that defect creation in the solar cells during electron-beam irradiation is not initiated by recombination events, but by energy transfer during collision events. Further, we found that the FF of thin solar cells degrades faster after electron-beam irradiation. We ascribe this effect to the decrease of the activation energy of the doped layers. Acknowledgements We thank Martijn Tijssen for the deposition of the solar cells, Kasper Zwetsloot for the technical assistance, and Marinus Hom for the electron-beam irradiation. This work was supported by Technology Foundation STW and SenterNovem. References [1] J.R. Srour, G.J. Vendura, D.H. Lo, C.M.C. Toporow, M. Dooley, R.P. Nakano, E.E. King, IEEE Trans. Nucl. Sci. 45 (1998) 2624. [2] J.E. Granata, T.D. Sahlstrom, P.E. Hausgen, S.R. Messenger, R.J. Walters, J.R. Lorentzen, in: Proceedings of 31st IEEE-PVSC, 2005, p. 607. [3] K.R. Lord, M.R. Walters, J.R. Woodyard, in: Proceedings of 23rd IEEE-PVSC, 1993, p. 1448. [4] D.L. Staebler, C.R. Wronski, Appl. Phys. Lett. 31 (1977) 292. [5] A. Yelon, H. Fritzsche, H.M. Branz, J. Non-Cryst. Solids 266–269 (2000) 437. [6] P. Danesh, B. Pantchev, E. Vlaikova, Nucl. Instr. Meth. Phys. Res. B 239 (2005) 370. [7] P. Chaudhuri, S. Ray, A.K. Batabyal, A.K. Barua, Solar Cells 31 (1991) 13. [8] B. Rech, H. Wagner, Appl. Phys. A 69 (1999) 155. [9] D.L. Staebler, R.S. Crandall, R. Williams, Appl. Phys. Lett. 39 (1981) 733. [10] L. Yang, L. Chen, J.Y. Hou, Y.M. Li, in: M.J. Thompson, Y. Hamakawa, P.G. LeComber, A. Madan, E.A. Schiff (Eds.), Materials Research Society Symposium Proceedings, Amorphous Silicon Technology, April 27–May 1, San Francisco, USA, 1992, vol. 258, 2001, p. 365. [11] W. Herbst, J. Dudel, A. Scholz, B. Schro¨der, H. Oechsner, Sol. Energy Mater. Sol. Cells 37 (1995) 55. [12] H.M. Branz, Phys. Rev. B 59 (1999) 5498. [13] E.A. Schiff, J. Non-Cryst. Solids 190 (1995) 1. [14] A. Klaver, Irradiation-induced degradation of amorphous silicon solar cells in space, PhD thesis, Delft University of Technology, 2007.