- Email: [email protected]
Effect of p-μc-Si1−xOx:H layer on performance of hetero-junction microcrystalline silicon solar cells
Current Applied Physics 10 (2010) S357–S360
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Effect of p-lc-Si1xOx:H layer on performance of hetero-junction microcrystalline silicon solar cells Taweewat Krajangsang a,*, Ihsanul Afdi Yunaz a, Shinsuke Miyajima a, Makoto Konagai a,b a b
Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1, S9-9, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Photovoltaics Research Center (PVREC), Tokyo Institute of Technology, 2-12-1, S9-9, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
a r t i c l e
i n f o
Article history: Received 2 November 2009 Received in revised form 12 January 2010 Accepted 16 February 2010 Available online 19 February 2010 Keywords: Hetero-junction solar cells Hydrogenated microcrystalline silicon oxide Light intensity AMPS-1D
a b s t r a c t A theoretical analysis using Analysis of Microelectronic and Photonic Structures (AMPS-1D) has been performed to investigate how the widegap p-lc-Si1xOx:H influences the hetero-junction lc-Si:H solar cells. We observed that the open-circuit voltage (Voc) depends on the bandgap of p-layer. Using wide bandgap p-layer can reduce recombination at p-layer and p/i interface. Moreover, we also have studied the effect of light intensity on the performance of hetero-junction lc-Si:H solar cells. From simulation result, it was confirmed that the Voc logarithmically increases with increasing the light intensity. Besides, we also observed that the p-layer bandgap strongly influences the light-intensity dependence of hetero-junction lc-Si:H solar cells. The enhancement of Voc (DVoc) with increasing light intensity improves as the bandgap of p-layer is increased. Therefore, widegap p-lc-Si1xOx:H is promising for use as window layer in hetero-junction lc-Si:H solar cells. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydrogenated microcrystalline silicon (lc-Si:H) has been widely applied as thin film solar cell materials, particularly as the bottom cell material in a-Si:H/lc-Si:H tandem solar cells. To optimize the tandem solar cells, optimizations of single-junction lc-Si:H solar cells have been performed. In general, the efficiency of lc-Si:H solar cells depends strongly on the properties of the i-layer, e.g. the contamination, defect density, and crystalline volume fraction which should be close to lc-Si:H/a:Si:H transition [1,2]. Therefore, many researchers mainly focus on the properties of i-layer. Although the improvement of i-layer properties is an important issue for the development of lc-Si:H solar cells, research on the p/i interface is also important since p/i interface must have a significant impact on the solar cell performance when the i-layer quality is very good [3]. It is well known that hetero-junction p/i interface is most important to obtain high efficiency in amorphous silicon solar cells [4]. However, homo-junction structure is generally employed in lc-Si:H solar cells. Thus, for further improvement of the performance of lc-Si:H solar cells, effect of p/i hetero-junction should be investigated. Wide bandgap hydrogenated microcrystalline silicon oxide p-type (p-lc-Si1xOx:H), which can be a mixture of SiO microcrystallites and Si microcrystallites, a mixture of amorphous SiO and Si microcrystallites, or a mixture of amorphous SiO and SiO micro-
* Corresponding author. Tel.: +81 080 5478 0556; fax: +81 3 5734 2897. E-mail address: [email protected] (T. Krajangsang). 1567-1739/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.02.016
crystallites, is promising material for use as p-layer in p–i–n structure silicon-based solar cells due to the low absorption coefficient and high conductivity [5,6]. In a previous work, we also have employed p-lc-Si1xOx:H as a window layer of hetero-junction crystalline silicon solar cells [7]. However, application of widegap p-lcSi1xOx:H in hetero-junction lc-Si:H has not been reported yet. In this study, we investigated the effect of hetero-junction p/i interface on the performance of lc-Si:H solar cells by using device simulations. The theoretical study was conducted using a onedimensional device simulator called AMPS-1D to examine how the widegap p-lc-Si1xOx:H layer affects the efficiency of heterojunction lc-Si:H solar cells. We also discuss the effect of p-layer defect density on the performance of hetero-junction lc-Si:H solar cells. Moreover, the performance of hetero-junction lc-Si:H solar cells under various light intensity was also studied. 2. Simulation model The theoretical study was carried out using Analysis of Microelectronic and Photonic Structures (AMPS-1D), a one-dimensional device simulator [8]. The calculations carried out for the analysis were based on Poison’s equation and electron and hole continuity equations approach to analyze the transport behavior of semiconductor electronic and optoelectronic device structure including solar cells. Firstly, we studied how the bandgap and defect density of p-layer affect the performance of hetero-junction microcrystalline silicon solar cells. Secondly, in order to analyze the light-intensity dependence of the hetero-junction microcrystalline silicon solar
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cells, the concentration ratio of sunlight was varied from 0.01 to 50 suns. The structure of the calculated lc-Si:H solar cells was glass substrate/transparent conducting oxide (TCO)/p-lc-Si1xOx:H/i-lcSi:H/n-lc-Si:H/aluminum. The band diagram of the typical calculated cell is shown in Fig. 1. Material parameters used in the simulation were general parameters based on various measurement results and references [9,10]. The main parameters used in the simulation are shown in Table 1. In this calculation, a light trapping effect of TCO front contact was not considered. We only simply assumed that there is no reflection for light impinging on front surfaces and there are no absorption losses at the front TCO. We assumed that the TCO/p-surface bend bending was 0.15 eV and assumed that the surface recombination speeds of electrons and hole at TCO/p interface were both 1 107 cm/s. We also assumed that the back contact (aluminum) has a reflection coefficient of 0.8. In this simulation, a model of the continuous density of states (DOS) was applied. For the density of localized states in the mobility gap of p-, i-, and n-layers, it has been assumed that there were both acceptor-like states and donor-like states. Both of these acceptor and donor-like states consisted of tail states and midgap defect states (dangling bonds). The tail states were approximated by exponential distributions, whereas the midgap defects were modeled using two Gaussian distributions.
The thickness of doping layers was set to be 20 nm. The acceptor concentration of p-layer was 1 1018 cm3. The defect density of p-layer was changed from 1 1016 cm3 to 5 1017 cm3. The thickness and defect density of lc-Si:H i-layer were 1.5 lm and 2 1015 cm3, respectively. In this simulation p-layer was assumed to be highly doped, which is the same as experimental data, thus the acceptor concentration was assumed to be high (greater than about 1017 cm3). Generally, such high carrier concentration leads to a bandgap narrowing in silicon material [11,12]. Therefore, the bandgap of p-layer was varied from 0.9 eV to 1.4 eV. Since the distribution of the band offsets between various p-lc-Si1xOx:H has been argued and is still not clear yet, to simplify the calculation, the band offsets of the conduction band (DEc) and valence band (DEv) were assumed to be same value as follows,
DEc ¼ DEv ¼ ðEgp Egi Þ=2;
ð1Þ
where Egp and Egi are the bandgap of p- and i-layer, respectively as shown in Fig. 1. 3. Results and discussion 3.1. Effect of bandgap and defect density of p-layer on performance of lc-Si:H solar cells p-lc-Si1xOx:H film is widely used for the a-Si:H and lc-Si:H solar cells as window layer. When the CO2 is introduced to deposit p-lc-Si1xOx:H for wider bandgap which increases the oxygen-rich phase in material [5], alternative effects can be expected. In order to investigate how the p-lc-Si1xOx:H wide bandgap influences the photovoltaic performance, the effect of p-layer bandgap on lc-Si:H solar cell was then studied. Fig. 2 shows solar cell performances as a function of p-layer bandgap. The bandgap of conventional p-lc-Si:H was assumed to
Fig. 1. Energy band diagram of calculated lc-Si:H solar cell in thermodynamic equilibrium.
Table 1 Material parameters used for AMPS-1D. p-Layer Thickness (nm) Acceptor/donor concentration (cm3) Energy bandgap (eV) Gaussian defect states density (cm3) Peak energy of Gaussian acceptor-like states (above Ev) (eV) Peak energy of Gaussian donor-like states (below Ec) (eV) Characteristic energy of acceptor-/donor-like tail states (eV) Electron affinity (eV)
lc-Si:H
n-Layer n-lc-Si:H
20 1 1018/0
1500 0/1 1015
20 0/1 1019
0.9–1.4 1 1016–5 1017
1.1 2 1015
1.1 2 1016
0.3
0.3
0.3
0.3
0.3
0.3
0.01/0.02
0.01/0.02
0.01/0.02
3.75–4.0
3.9
3.9
The concentration ratio of sunlight 0.01–50 suns
i-Layer
Fig. 2. Solar cell performances as a function of p-layer bandgap at defect density of 1 1016 cm3, 1 1017 cm3 and 5 1017 cm3.
T. Krajangsang et al. / Current Applied Physics 10 (2010) S357–S360
be lower than 1.1 eV due to effect of bandgap narrowing by highly boron doping, whereas bandgap of p-lc-Si1xOx:H was assumed to be wider than 1.1 eV. It can be observed that the Voc depends on the bandgap of p-layer. The Voc increases with increasing the p-layer bandgap from 0.9 to 1.4 eV. The short-circuit current (Jsc) remarkably increases by increasing the bandgap from 0.9 to 1.0 eV, and then tends to slightly increase or saturate. The FF slightly decreases with increasing bandgap from 1.0 eV to 1.4 eV and drops at bandgap of 0.9 eV. In order to know the reasons for above results, the recombination rates of solar cell were investigated. Fig. 3 shows the calculated recombination rate of the solar cells. From Fig. 2, we observed that the Voc increases with the use of wide bandgap p-layer because the potential barrier at p/i interface increases which suppresses the electron back-diffusion from i-layer to p-layer. The suppression of the back-diffusion results in the low recombination in the p-layer or at the p/i interface, as can be seen in Fig. 3. Next, the effect of p-layer defect density on the performance of lc-Si:H with various p-layer bandgap were analyzed. Normally, ptype microcrystalline silicon (lc-Si:H) films are deposited by the plasma enhanced chemical vapor deposition (PECVD) technique from a mixture of SiH4, H2 and B2H6 gases. In general, boron doping influences the conductivity, but a too much boron doping deteriorates film crystallinity and optical transparency due to an increase in the defect density in the material. In this study, p-layer defect density was varied from 1 1016 cm3 to 5 1017 cm3 and acceptor concentration was 1 1018 cm3. As can be seen from Fig. 2, it can be observed that the tendency of each PV parameter with increasing p-layer bandgap depends on the defect density of p-layer. For example, for p-layer with high defect density of 5 1017 cm3, the Voc largely increases from 0.45 V to 0.54 V with increasing p-layer bandgap from 0.9 to 1.2 eV, and then tends to slightly increase or saturate. The increase rate of Voc for defect density of 5 1017 cm3 was larger than those of 1 1016 cm3 and 1 1017 cm3. Therefore, widegap p-layer can reduce effect of defect density in p-layer on the solar cell performance. 3.2. Performance of hetero-junction lc-Si:H solar cells under various light intensity In order to investigate how the device structure influences the photovoltaic performance of solar cells under various concentration ratios of sunlight, the effect of p-layer bandgap on lc-Si:H solar cells were studied. In this case, the defect density and accepter
Fig. 3. Recombination rate of lc-Si:H thin film solar cell with p-layer defect density 5 1017 cm3 as a function of p-layer bandgap.
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Fig. 4. Calculated PV parameter of lc-Si:H solar cells with various p-layer bandgap as a function of light intensity.
concentration of p-layer were 5 1017 cm3 and 1 1018 cm3, respectively. The thickness and defect density of i-layer were 1.5 lm and 2 1015 cm3, respectively. Fig. 4 shows the calculated PV parameter of lc-Si:H solar cells with various p-layer bandgap as a function of light intensity. It should be noted that the values of short-circuit current (Jsc) given in Fig. 4 was divided by the light intensity. The short-circuit current divided by the light intensity slightly decreases with increasing the light intensity. From quantum efficiency (QE) calculation results, it was observed that long wavelength region slightly decreases with increasing light intensity. Besides, we also can see that the FF decreases with increasing light intensity to more than 10 suns. From simulation results, we found that the recombination rate at i-layer increases with increasing light intensity which results slight drops in Jsc and FF. It is well known that the charge carrier injection increases under a high light concentration condition. The increase of charge carrier injection would lead to an increase in the charged states in the midgap defect states and tail states. Due to changes in the carrier distribution and thus in the electric field, the band bending in i-layer would occur. As results, the recombination rate at the i-layer increases, therefore Jsc and FF drops with increasing light intensity. Furthermore, from Fig. 4, one also can see that the Voc logarithmically increases with increasing the light intensity. Moreover, it is interesting to note that the p-layer bandgap strongly influences the light-intensity dependence of Voc of hetero-junction lc-Si:H solar cells. In case of hetero-junction lc-Si:H solar cells with widegap p-layer of 1.3 eV, the Voc increases from 0.39 V at 0.01 sun to 0.66 V at 50 suns. On the other hand, the conventional lc-Si:H solar cells with bandgap narrowing in player of 1.0 eV, the Voc increases from 0.37 V to 0.60 V. The enhancement of Voc (DVoc) with increasing light intensity improves as the bandgap of p-layer is increased. Thus, widegap p-
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layer is beneficial not only for 1 sun condition, but also in higher light concentration. 4. Conclusions In this paper, we have presented a numerical study using AMPS1D to comprehend how the widegap p-layer influences the heterojunction lc-Si:H solar cells. From the simulation results, we observed that the open-circuit voltage depends on the bandgap of p-layer. A wide bandgap can reduce the recombination at p-layer and p/i interface even in the p-layer with high defect density. Therefore, a widegap (1.3 eV) p-layer is effective to improve the Voc of the solar cells. Furthermore, we also have studied the effect of light intensity on the performance of hetero-junction lc-Si:H solar cells. We confirmed that the Voc logarithmically increases with increasing the light intensity. Besides, we also observed that the p-layer bandgap strongly influences the lightintensity dependence of hetero-junction lc-Si:H solar cells. The enhancement of Voc (DVoc) with increasing light intensity improves as the bandgap of p-layer is increased. Thus, widegap p-layer is beneficial not only for 1 sun condition, but also in higher light concentration. Therefore, widegap p-lc-Si1xOx:H is promising for use as window layer in hetero-junction lc-Si:H solar cells.
Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under Ministry of Economy, Trade and Industry (METI), Japan. References [1] M. Kondo, T. Matsui, Y. Nasuno, H. Sonobe, S. Shimizu, Thin Solid Films 501 (2006) 243–246. [2] A.V. Shah, J. Meier, E. Vallat-Sauvain, N. Wyrsch, U. Kroll, C. Droz, U. Graf, Sol. Energy Mater. Sol. Cells 78 (2003) 469–491. [3] F. Finger, Y. Mai, S. Klein, R. Carius, Thin Solid Films 516 (2008) 728–732. [4] K. Chikusa, K. Takemoto, T. Itoh, N. Yoshida, S. Nonomura, Thin Solid Films 430 (2003) 245–248. [5] H. Watanabe, K. Haga, T. Lohner, J. Non-Cryst. Solids 164–166 (1993) 1085. [6] P. Sichanugrist, T. Sasaki, A. Asano, Y. Ichikawa, H. Sakai, Sol. Energy Mater. Sol. Cells 34 (1994) 415. [7] J. Sritharathikhun, F. Jiang, S. Miyajima, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 48 (2009) 101603. [8] H. Zhu, A.K. Kalkan, J. Hou, S.J. Fonash, AIP Conf. Proc. 462 (1999) 309. [9] Y. Ide, Y. Saito, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 43 (2004) 2419. [10] I.A. Yunaz, K. Sriprapha, S. Hiza, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 46 (2007) 1398. [11] J.W. Slotboom, H.C. de Graaff, Solid-State Electron. 19 (1976) 857. [12] H. Chena, M.H. Gullanara, W.Z. Shen, J. Cryst. Growth 260 (2004) 91–101.
Contents lists available at ScienceDirect
Current Applied Physics journal homepage: www.elsevier.com/locate/cap
Effect of p-lc-Si1xOx:H layer on performance of hetero-junction microcrystalline silicon solar cells Taweewat Krajangsang a,*, Ihsanul Afdi Yunaz a, Shinsuke Miyajima a, Makoto Konagai a,b a b
Department of Physical Electronics, Tokyo Institute of Technology, 2-12-1, S9-9, O-okayama, Meguro-ku, Tokyo 152-8552, Japan Photovoltaics Research Center (PVREC), Tokyo Institute of Technology, 2-12-1, S9-9, O-okayama, Meguro-ku, Tokyo 152-8552, Japan
a r t i c l e
i n f o
Article history: Received 2 November 2009 Received in revised form 12 January 2010 Accepted 16 February 2010 Available online 19 February 2010 Keywords: Hetero-junction solar cells Hydrogenated microcrystalline silicon oxide Light intensity AMPS-1D
a b s t r a c t A theoretical analysis using Analysis of Microelectronic and Photonic Structures (AMPS-1D) has been performed to investigate how the widegap p-lc-Si1xOx:H influences the hetero-junction lc-Si:H solar cells. We observed that the open-circuit voltage (Voc) depends on the bandgap of p-layer. Using wide bandgap p-layer can reduce recombination at p-layer and p/i interface. Moreover, we also have studied the effect of light intensity on the performance of hetero-junction lc-Si:H solar cells. From simulation result, it was confirmed that the Voc logarithmically increases with increasing the light intensity. Besides, we also observed that the p-layer bandgap strongly influences the light-intensity dependence of hetero-junction lc-Si:H solar cells. The enhancement of Voc (DVoc) with increasing light intensity improves as the bandgap of p-layer is increased. Therefore, widegap p-lc-Si1xOx:H is promising for use as window layer in hetero-junction lc-Si:H solar cells. Ó 2010 Elsevier B.V. All rights reserved.
1. Introduction Hydrogenated microcrystalline silicon (lc-Si:H) has been widely applied as thin film solar cell materials, particularly as the bottom cell material in a-Si:H/lc-Si:H tandem solar cells. To optimize the tandem solar cells, optimizations of single-junction lc-Si:H solar cells have been performed. In general, the efficiency of lc-Si:H solar cells depends strongly on the properties of the i-layer, e.g. the contamination, defect density, and crystalline volume fraction which should be close to lc-Si:H/a:Si:H transition [1,2]. Therefore, many researchers mainly focus on the properties of i-layer. Although the improvement of i-layer properties is an important issue for the development of lc-Si:H solar cells, research on the p/i interface is also important since p/i interface must have a significant impact on the solar cell performance when the i-layer quality is very good [3]. It is well known that hetero-junction p/i interface is most important to obtain high efficiency in amorphous silicon solar cells [4]. However, homo-junction structure is generally employed in lc-Si:H solar cells. Thus, for further improvement of the performance of lc-Si:H solar cells, effect of p/i hetero-junction should be investigated. Wide bandgap hydrogenated microcrystalline silicon oxide p-type (p-lc-Si1xOx:H), which can be a mixture of SiO microcrystallites and Si microcrystallites, a mixture of amorphous SiO and Si microcrystallites, or a mixture of amorphous SiO and SiO micro-
* Corresponding author. Tel.: +81 080 5478 0556; fax: +81 3 5734 2897. E-mail address: [email protected] (T. Krajangsang). 1567-1739/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2010.02.016
crystallites, is promising material for use as p-layer in p–i–n structure silicon-based solar cells due to the low absorption coefficient and high conductivity [5,6]. In a previous work, we also have employed p-lc-Si1xOx:H as a window layer of hetero-junction crystalline silicon solar cells [7]. However, application of widegap p-lcSi1xOx:H in hetero-junction lc-Si:H has not been reported yet. In this study, we investigated the effect of hetero-junction p/i interface on the performance of lc-Si:H solar cells by using device simulations. The theoretical study was conducted using a onedimensional device simulator called AMPS-1D to examine how the widegap p-lc-Si1xOx:H layer affects the efficiency of heterojunction lc-Si:H solar cells. We also discuss the effect of p-layer defect density on the performance of hetero-junction lc-Si:H solar cells. Moreover, the performance of hetero-junction lc-Si:H solar cells under various light intensity was also studied. 2. Simulation model The theoretical study was carried out using Analysis of Microelectronic and Photonic Structures (AMPS-1D), a one-dimensional device simulator [8]. The calculations carried out for the analysis were based on Poison’s equation and electron and hole continuity equations approach to analyze the transport behavior of semiconductor electronic and optoelectronic device structure including solar cells. Firstly, we studied how the bandgap and defect density of p-layer affect the performance of hetero-junction microcrystalline silicon solar cells. Secondly, in order to analyze the light-intensity dependence of the hetero-junction microcrystalline silicon solar
S358
T. Krajangsang et al. / Current Applied Physics 10 (2010) S357–S360
cells, the concentration ratio of sunlight was varied from 0.01 to 50 suns. The structure of the calculated lc-Si:H solar cells was glass substrate/transparent conducting oxide (TCO)/p-lc-Si1xOx:H/i-lcSi:H/n-lc-Si:H/aluminum. The band diagram of the typical calculated cell is shown in Fig. 1. Material parameters used in the simulation were general parameters based on various measurement results and references [9,10]. The main parameters used in the simulation are shown in Table 1. In this calculation, a light trapping effect of TCO front contact was not considered. We only simply assumed that there is no reflection for light impinging on front surfaces and there are no absorption losses at the front TCO. We assumed that the TCO/p-surface bend bending was 0.15 eV and assumed that the surface recombination speeds of electrons and hole at TCO/p interface were both 1 107 cm/s. We also assumed that the back contact (aluminum) has a reflection coefficient of 0.8. In this simulation, a model of the continuous density of states (DOS) was applied. For the density of localized states in the mobility gap of p-, i-, and n-layers, it has been assumed that there were both acceptor-like states and donor-like states. Both of these acceptor and donor-like states consisted of tail states and midgap defect states (dangling bonds). The tail states were approximated by exponential distributions, whereas the midgap defects were modeled using two Gaussian distributions.
The thickness of doping layers was set to be 20 nm. The acceptor concentration of p-layer was 1 1018 cm3. The defect density of p-layer was changed from 1 1016 cm3 to 5 1017 cm3. The thickness and defect density of lc-Si:H i-layer were 1.5 lm and 2 1015 cm3, respectively. In this simulation p-layer was assumed to be highly doped, which is the same as experimental data, thus the acceptor concentration was assumed to be high (greater than about 1017 cm3). Generally, such high carrier concentration leads to a bandgap narrowing in silicon material [11,12]. Therefore, the bandgap of p-layer was varied from 0.9 eV to 1.4 eV. Since the distribution of the band offsets between various p-lc-Si1xOx:H has been argued and is still not clear yet, to simplify the calculation, the band offsets of the conduction band (DEc) and valence band (DEv) were assumed to be same value as follows,
DEc ¼ DEv ¼ ðEgp Egi Þ=2;
ð1Þ
where Egp and Egi are the bandgap of p- and i-layer, respectively as shown in Fig. 1. 3. Results and discussion 3.1. Effect of bandgap and defect density of p-layer on performance of lc-Si:H solar cells p-lc-Si1xOx:H film is widely used for the a-Si:H and lc-Si:H solar cells as window layer. When the CO2 is introduced to deposit p-lc-Si1xOx:H for wider bandgap which increases the oxygen-rich phase in material [5], alternative effects can be expected. In order to investigate how the p-lc-Si1xOx:H wide bandgap influences the photovoltaic performance, the effect of p-layer bandgap on lc-Si:H solar cell was then studied. Fig. 2 shows solar cell performances as a function of p-layer bandgap. The bandgap of conventional p-lc-Si:H was assumed to
Fig. 1. Energy band diagram of calculated lc-Si:H solar cell in thermodynamic equilibrium.
Table 1 Material parameters used for AMPS-1D. p-Layer Thickness (nm) Acceptor/donor concentration (cm3) Energy bandgap (eV) Gaussian defect states density (cm3) Peak energy of Gaussian acceptor-like states (above Ev) (eV) Peak energy of Gaussian donor-like states (below Ec) (eV) Characteristic energy of acceptor-/donor-like tail states (eV) Electron affinity (eV)
lc-Si:H
n-Layer n-lc-Si:H
20 1 1018/0
1500 0/1 1015
20 0/1 1019
0.9–1.4 1 1016–5 1017
1.1 2 1015
1.1 2 1016
0.3
0.3
0.3
0.3
0.3
0.3
0.01/0.02
0.01/0.02
0.01/0.02
3.75–4.0
3.9
3.9
The concentration ratio of sunlight 0.01–50 suns
i-Layer
Fig. 2. Solar cell performances as a function of p-layer bandgap at defect density of 1 1016 cm3, 1 1017 cm3 and 5 1017 cm3.
T. Krajangsang et al. / Current Applied Physics 10 (2010) S357–S360
be lower than 1.1 eV due to effect of bandgap narrowing by highly boron doping, whereas bandgap of p-lc-Si1xOx:H was assumed to be wider than 1.1 eV. It can be observed that the Voc depends on the bandgap of p-layer. The Voc increases with increasing the p-layer bandgap from 0.9 to 1.4 eV. The short-circuit current (Jsc) remarkably increases by increasing the bandgap from 0.9 to 1.0 eV, and then tends to slightly increase or saturate. The FF slightly decreases with increasing bandgap from 1.0 eV to 1.4 eV and drops at bandgap of 0.9 eV. In order to know the reasons for above results, the recombination rates of solar cell were investigated. Fig. 3 shows the calculated recombination rate of the solar cells. From Fig. 2, we observed that the Voc increases with the use of wide bandgap p-layer because the potential barrier at p/i interface increases which suppresses the electron back-diffusion from i-layer to p-layer. The suppression of the back-diffusion results in the low recombination in the p-layer or at the p/i interface, as can be seen in Fig. 3. Next, the effect of p-layer defect density on the performance of lc-Si:H with various p-layer bandgap were analyzed. Normally, ptype microcrystalline silicon (lc-Si:H) films are deposited by the plasma enhanced chemical vapor deposition (PECVD) technique from a mixture of SiH4, H2 and B2H6 gases. In general, boron doping influences the conductivity, but a too much boron doping deteriorates film crystallinity and optical transparency due to an increase in the defect density in the material. In this study, p-layer defect density was varied from 1 1016 cm3 to 5 1017 cm3 and acceptor concentration was 1 1018 cm3. As can be seen from Fig. 2, it can be observed that the tendency of each PV parameter with increasing p-layer bandgap depends on the defect density of p-layer. For example, for p-layer with high defect density of 5 1017 cm3, the Voc largely increases from 0.45 V to 0.54 V with increasing p-layer bandgap from 0.9 to 1.2 eV, and then tends to slightly increase or saturate. The increase rate of Voc for defect density of 5 1017 cm3 was larger than those of 1 1016 cm3 and 1 1017 cm3. Therefore, widegap p-layer can reduce effect of defect density in p-layer on the solar cell performance. 3.2. Performance of hetero-junction lc-Si:H solar cells under various light intensity In order to investigate how the device structure influences the photovoltaic performance of solar cells under various concentration ratios of sunlight, the effect of p-layer bandgap on lc-Si:H solar cells were studied. In this case, the defect density and accepter
Fig. 3. Recombination rate of lc-Si:H thin film solar cell with p-layer defect density 5 1017 cm3 as a function of p-layer bandgap.
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Fig. 4. Calculated PV parameter of lc-Si:H solar cells with various p-layer bandgap as a function of light intensity.
concentration of p-layer were 5 1017 cm3 and 1 1018 cm3, respectively. The thickness and defect density of i-layer were 1.5 lm and 2 1015 cm3, respectively. Fig. 4 shows the calculated PV parameter of lc-Si:H solar cells with various p-layer bandgap as a function of light intensity. It should be noted that the values of short-circuit current (Jsc) given in Fig. 4 was divided by the light intensity. The short-circuit current divided by the light intensity slightly decreases with increasing the light intensity. From quantum efficiency (QE) calculation results, it was observed that long wavelength region slightly decreases with increasing light intensity. Besides, we also can see that the FF decreases with increasing light intensity to more than 10 suns. From simulation results, we found that the recombination rate at i-layer increases with increasing light intensity which results slight drops in Jsc and FF. It is well known that the charge carrier injection increases under a high light concentration condition. The increase of charge carrier injection would lead to an increase in the charged states in the midgap defect states and tail states. Due to changes in the carrier distribution and thus in the electric field, the band bending in i-layer would occur. As results, the recombination rate at the i-layer increases, therefore Jsc and FF drops with increasing light intensity. Furthermore, from Fig. 4, one also can see that the Voc logarithmically increases with increasing the light intensity. Moreover, it is interesting to note that the p-layer bandgap strongly influences the light-intensity dependence of Voc of hetero-junction lc-Si:H solar cells. In case of hetero-junction lc-Si:H solar cells with widegap p-layer of 1.3 eV, the Voc increases from 0.39 V at 0.01 sun to 0.66 V at 50 suns. On the other hand, the conventional lc-Si:H solar cells with bandgap narrowing in player of 1.0 eV, the Voc increases from 0.37 V to 0.60 V. The enhancement of Voc (DVoc) with increasing light intensity improves as the bandgap of p-layer is increased. Thus, widegap p-
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layer is beneficial not only for 1 sun condition, but also in higher light concentration. 4. Conclusions In this paper, we have presented a numerical study using AMPS1D to comprehend how the widegap p-layer influences the heterojunction lc-Si:H solar cells. From the simulation results, we observed that the open-circuit voltage depends on the bandgap of p-layer. A wide bandgap can reduce the recombination at p-layer and p/i interface even in the p-layer with high defect density. Therefore, a widegap (1.3 eV) p-layer is effective to improve the Voc of the solar cells. Furthermore, we also have studied the effect of light intensity on the performance of hetero-junction lc-Si:H solar cells. We confirmed that the Voc logarithmically increases with increasing the light intensity. Besides, we also observed that the p-layer bandgap strongly influences the lightintensity dependence of hetero-junction lc-Si:H solar cells. The enhancement of Voc (DVoc) with increasing light intensity improves as the bandgap of p-layer is increased. Thus, widegap p-layer is beneficial not only for 1 sun condition, but also in higher light concentration. Therefore, widegap p-lc-Si1xOx:H is promising for use as window layer in hetero-junction lc-Si:H solar cells.
Acknowledgement This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under Ministry of Economy, Trade and Industry (METI), Japan. References [1] M. Kondo, T. Matsui, Y. Nasuno, H. Sonobe, S. Shimizu, Thin Solid Films 501 (2006) 243–246. [2] A.V. Shah, J. Meier, E. Vallat-Sauvain, N. Wyrsch, U. Kroll, C. Droz, U. Graf, Sol. Energy Mater. Sol. Cells 78 (2003) 469–491. [3] F. Finger, Y. Mai, S. Klein, R. Carius, Thin Solid Films 516 (2008) 728–732. [4] K. Chikusa, K. Takemoto, T. Itoh, N. Yoshida, S. Nonomura, Thin Solid Films 430 (2003) 245–248. [5] H. Watanabe, K. Haga, T. Lohner, J. Non-Cryst. Solids 164–166 (1993) 1085. [6] P. Sichanugrist, T. Sasaki, A. Asano, Y. Ichikawa, H. Sakai, Sol. Energy Mater. Sol. Cells 34 (1994) 415. [7] J. Sritharathikhun, F. Jiang, S. Miyajima, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 48 (2009) 101603. [8] H. Zhu, A.K. Kalkan, J. Hou, S.J. Fonash, AIP Conf. Proc. 462 (1999) 309. [9] Y. Ide, Y. Saito, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 43 (2004) 2419. [10] I.A. Yunaz, K. Sriprapha, S. Hiza, A. Yamada, M. Konagai, Jpn. J. Appl. Phys. 46 (2007) 1398. [11] J.W. Slotboom, H.C. de Graaff, Solid-State Electron. 19 (1976) 857. [12] H. Chena, M.H. Gullanara, W.Z. Shen, J. Cryst. Growth 260 (2004) 91–101.