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Field-aided collection in GaInP2 top solar cells
ARTICLE IN PRESS
Solar Energy Materials & Solar Cells 80 (2003) 265–272
Field-aided collection in GaInP2 top solar cells M.B. Chena, R.Q. Cuia, Z.W. Zhangb, J.F. Lub, L.X. Wangb, W.Y. Chib, X.B. Xiangc,*, X.B. Liaoc a Shanghai Jiaotong University, Shanghai 200030,China Shanghai Institute of Space Power Sources, Shanghai 200233, China c Laboratory of Semiconductor Material Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China b
Received 30 May 2003
Abstract This paper reports the study on a field-aided collection in p-on-n GaInP2 top cells. The cells were produced by metalorganic vapor phase epitaxy at a low gas pressure. In order to optimize the device configuration, numerical simulations have been performed for the impacts of fieldaided collection on the performance of the top cells. On the basis of the modeling results, a modified p+–p–n–n+ configuration is introduced for GaInP2 top cells. This modification has brought out improved photovoltaic performance of the top cells, with conversion efficiency EFF=14.26% (AM0, 2 2 cm2, 25 C). r 2003 Elsevier B.V. All rights reserved. Keywords: Field-added collection effect; GaInP2; Solar cells
1. Introduction Two-terminal monolithic GaInP2/GaAs tandem cells have gained a great deal of attention and achieved much progress in recent years, since the use of GaInP2 material for the top cells was proposed by Olson et al. in 1990 [1]. The highest conversion efficiency for GaInP2/GaAs tandem solar cells has been obtained with an n-on-p polarity structure, due to the low sheet resistance in the n-type emitter and long diffusion length in the p-type base region [2,3]. However, GaInP2/GaAs tandem cells with a p-on-n polarity have some advantages in achieving an inactive GaAs/Ge *Corresponding author. Tel./fax: +86-10-82304253. E-mail address: [email protected] (X.B. Xiang). 0927-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2003.08.002
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interface for epitaxy growth of GaAs on Ge substrates [4]. In both cases, the prerequisite for preparing high-efficient GaInP2/GaAs tandem cells is to develop a high quality of GaInP2 material and to optimize the device configuration matched to the material quality. Although qualified GaInP2 material and high-efficient top cells have been reported, the quality of GaInP2 material is closely correlated with the deposition technology used, such as the metalorganic vapor phase epitaxy (MOVPE) apparatus, source gases and growth conditions, is still a challenging subject. The motivation for this work is to develop two-terminal, monolithic p-on-n GaInP2/GaAs tandem cells, which would be preferable for our existing protomanufacturing line of single junction GaAs solar cells on Ge substrates with a mean conversion efficiency of 19% (AM0). As the first step we focused on the development of high efficient GaInP2 top cells with a p-on-n polarity. The preliminary results indicated that the photovoltaic performances of GaInP2 top cells with a commonly used p+–n structure were not satisfactory, compared to the reported data in the literature [5,6]. In order to understand what happened in our top cells, a numerical modeling for the illuminated I2V characteristics of the GaInP2 top cells was performed on the basis of accounting for the field-aided collection effect. The calculated results showed that the material quality of the GaInP2 base region used was not good enough, with a product (mt) of mobility and lifetime of the photogenerated carriers being about 2.93 108 cm2/V and a corresponding diffusion length (ld ) of about 300 nm, which is less than the thickness of the GaInP2 base region (500–600 nm). Therefore, we proposed a p+–p–n–n+ structure for the GaInP2 top cells to enhance the field-aided collection effect for the photo-generated carriers, and thus achieved a much improved performance of the top cells.
2. Experimental The GaInP2 top cell samples used were grown on n-type GaAs substrates using a low-pressure (200 mbar) MOVPE technology in a horizontal reactor (Model Aixtron 200-4). The precursors for Group III elements of Ga, Al and In, were trimethylgallium (TMGa), trimethylaluminum (TMAl) and trimethylindium (TMIn). The precursors for Group V elements of As and P were arsine and phosphine. For the n-type dopant source, silane (SiH4) was used. For the p-type dopant sources, diethylzinc (DEZn) and carbon tetrachloride (CCl4) were used. The carrier gas was hydrogen, purified on-line by passing through a palladium-filter. The GaAs substrates doped with Si were misoriented by 2 to (1 1 0) from (1 0 0). The main post-growth technology of GaInP2 cells includes photolithography, evaporation, thermal annealing and selecting etching. The front grids contact was evaporated using TiPdAg, defined by a standard photolithographic method, and the back contact was AuGeNi plus Au. The obscurance of the front grids was approximately 3% of the total cell area (2 2 cm2). The cell perimeter was also defined by a photolithography, and followed by a mesa etching. In order to form an Ohmic contact between the front grids and the p++-GaAs cap layer, an appropriate thermal annealing was performed. The antireflection coating (ARC) on the cell
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surface was composed of double layers of evaporated TiO2 and SiO2 with thickness of 62 and 95 nm, respectively. In the initial stage of the research we designed a p+–n configuration for the GaInP2 top cells, similar to the structure published in Ref. [2]. But after a set of experiments we found that the photovoltaic performances of the GaInP2 top cells with the p+–n structure were not as good as expected. Under the guidance of the numerical modeling results accounting for the field-aided collection effect, we modified the structure of the top cells with a p+–p–n–n+ configuration in order to promote the cell performances. The photovoltaic performances of the cells were measured using a simulator of Spectrolab x-25 as the AM0 spectrum. The light intensity of 135.3 mW/cm2 was calibrated using a GaInP2/GaAs tandem cell sample made in Spectrolab as the reference cell. The external quantum-efficiency (QE) curves were measured at 0 V bias using a monochromator-based system with a metal halide lamp source and a calibrated silicon diode detector.
3. Results and discussion 3.1. p+–n GaInP2 cells The schematic structure of the GaInP2 solar cells with a p+–n configuration used in the experiments is presented in Fig. 1, which comprises a 40 nm thick p+-AlInP window layer with carrier concentration of 1 1018 cm3, a 100 nm thick p+-GaInP2 emitter with carrier concentration of 1 1018 cm3, a 500 nm thick n-GaInP2 base layer with carrier concentration of 1 1017 cm3 and a 50 nm thick n+-GaInP2 BSF layer with carrier concentration of 1 1018 cm3. The typical dark Jd 2V and light Jph 2V curve of the GaInP2 top cell samples with the p+–n configuration are illustrated in Fig. 2 by using dashed and solid line, respectively. The deduced photovoltaic parameters are the open circuit voltage Voc ¼ 1:329 V; short circuit current density Jsc ¼ 13:70 mA/cm2 and fill factor FF ¼ 0:771: As can be seen that the Jsc and FF of this device is obviously lower than that expected. From the comparison of the Jph –V and Jd 2V curve, it is seen that the light p++-GaAs p+-AlInP window p+-GaInP2 emitter n-GaInP2 base n+-GaInP2 BSF n+-GaAs buffer n+ GaAs substrate
40 nm 100 nm 500 nm 50 nm 200 nm
1×1018cm-3 1×1018cm-3 1×1017cm-3 1×1018cm-3 1×1018cm-3
Fig. 1. Schematic cross-section diagram of the tested p+–n GaInP2 top cell.
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25
Current density (mA/cm2)
20 Jd Jph Modeling
15 10 5 0 -5 -10 -15 0.0
0.2
0.4
0.6 0.8 1.0 Voltage (V)
1.2
1.4
1.6
Fig. 2. Comparison of light Jph 2V (solid line) and dark Jd 2V (dashed line) curves of a typical p+–n GaInP2 cell. The open circles denote the modeling data fitting to the light Jph V curve.
Jph 2V curve contains apparently some shunt components, while the dark Jd 2V curve looks not shunted in this linear axis scale. This phenomenon can be explained neither by the ideality factors of a real diode nor the losses of series resistance (Rs ) and shunt resistance (Rsh ). Indeed, the dark Jd 2V curve could be approximated by an empirical expression as [7] Jd ¼ J01 ðexpðqðV Jd Rs Þ=n1 kTÞ 1Þ þ J02 ðexpðqðV Jd Rs Þ=n2 kTÞ 1Þ þ ðV Jd Rs Þ=Rsh
ð1Þ
in which q is the electron charge, k is the Bolztmann constant and T is the temperature in Kelvins. This expression includes two exponential regions characterized by two pre-exponential factors (J01 and J02 ) and two ideality factors (n1 and n2 ), and the items represented by Rs and Rsh losses. However, for the Jph 2V curve the commonly used expression: Jph ¼ Jd 2Jsc is not held, because of the fact that the light Jph 2V curve is not parallel to the dark Jd 2V curve. This means that in our case the superposition principle is no longer suitable and the field-aided collection effect must be considered. 3.2. Modeling results The field-aided collection effect has been successfully employed to enhance photovoltaic performances for solar cells based upon materials with a small minority carriers diffusion length, such as p–i–n-type hydrogenated amorphous silicon (a-Si:H) [8] devices, 1-eV GaInNAs [9] and GaNPAs [10] solar cells. In order to understand the impacts of the field-aided collection on the photovoltaic perfor-
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mances of the GaInP2 top cells, a numerical simulation has been performed for the light Jph 2V curve shown in Fig. 2 by using a field-aided collection model developed by Crandall especially for the case of p–i–n a-Si:H solar cells [8]. This expression reads as follows: Jph ¼ qGlc ð1 expðL=lc ÞÞ;
ð2Þ
where q is the electron charge, G is the generation ratio of the photo-carriers, L is the thickness of the intrinsic absorber layer. This expression contains only one unknown parameter, the collection length lc ¼ lp þ ln ; the sum of the electron drift length (ln ¼ mn tn E) and the hole drift length ðlp ¼ mp tp EÞ; where E denotes the built-in electrical field in the intrinsic absorber layer. Obviously, the collection length lc is a function of the electrical field E and the applied voltage bias V : If E is uniformly distributed in the n-GaInP2 base absorber region with the thickness L ¼ 500 nm (see Fig. 1), and is given by E ¼ ðVi V Þ=L; where Vi is the built-in potential, then lc increases linearly with V decreasing. Under the short circuit condition (V ¼ 0), the photogenerated current density Jph is saturated at the value of Jph ¼ qGL; indicating that all the photoexcited electron–hole pairs are separated by the electric field and collected in the external circuit. Also for simplicity, the value of qGL is supposed to be equal to the Jsc (13.70 mA/cm2) and the Vi is taken as Voc (1.329 V), the lowest limitation of Vi ; thus we obtained the modeling results shown in Fig. 3 as the open circles. It notes that the modeling results fit the light Jph 2V curve quite well with only one modeling parameter mt ¼ 2:93 108 cm2/V. The analysis forenamed relies on a pure field-added collection model for photoexcited carriers. In fact the diffusion process should also play a role. Using Einstein relationship D ¼ kTm=q; where D is the diffusion coefficient, and the above mt value, we estimated the diffusion length ld ¼ ðDtÞ1=2 ; to be about 0.276 mm. This diffusion length is obviously less than the GaInP2 base thickness (L) of the top cells and far deficient for the diffusion collection of photoexcited carriers. Furthermore, the derived ld is the sum of the diffusion length of electrons and holes, among which the diffusion length of minority carriers (holes in our case) should be much less than the derived ld value.
p++-GaAs p+-AlInP window p+-GaInP2 emitter p-GaInP2 emitter n-GaInP2 base n+-GaInP2 BSF n+-GaAs buffer n+ GaAs substrate
40 nm 50 nm 100 nm 500 nm 50 nm 200 nm
1×1018cm-3 1×1018cm-3 2×1015cm-3 1×1016cm-3 1×1018cm-3 1×1018cm-3
Fig. 3. Schematic cross-section diagram of the GaInP2 cells with a p+–p–n–n+ structure.
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As for the assumption of uniform electrical field distribution, it is a questioning issue. It is well known that the electrical field set up by the built-in potential in a p+–n junction concentrates in the space charge region. For the doping data shown in Fig. 1 the width of the space charge region (W ) is estimated to be about 0.129 mm, according to the depletion approach with the dielectric constant e=11.4 for GaInP2 material. This value of W is much less than the thickness of the GaInP2 absorber layer, therefore if one would extend the space charge region to cover the whole base region (B500 nm), the doping concentration in the GaInP2 absorber layer should be decreased from 1 1017 cm3 to at least 6 1015 cm3. 3.3. p+–p–n–n+ GaInP2 cells Based upon the modeling results we were aware of that our GaInP2 material quality is not good enough to have a larger product of mobility and lifetime and a larger diffusion length. The reason for this may be correlated with the gases purity and the growth-related defects. In order to explore an optimized devise configuration on the basis of the present material quality, we proposed a p+–p–n–n+ structure for the GaInP2 top cells, instead of the p+–n structure. The proposed p+–p–n–n+ structure could play a twofold role, one is to enlarge the width of the space charge regions and thus to enhance the field-aided collection for the photoexcited carriers, and the other is to decrease the defect density of the n-GaInP2 absorber layer and thus to increase the minority carrier lifetime. The second effect was reported by Katahashi [11]. They observed that the reduced majority-carrier concentration leads to an improvement in the minority-carrier lifetime in AlGaAs solar cells with p+– p–n–n+ structure. The schematic diagram of the modified GaInP2 cells with a p+–p–n–n+ structure is shown in Fig. 3. The 500 nm thick n-GaInP2 absorber layer was not intentionally doped, with a background n-type concentration on the order of 1 1016 cm3. The 100 nm thick p-GaInP2 emitter was compensated to carrier concentration of 2 1015 cm3 by Zn doping, as measured by electric-chemical capacitance–voltage (CV) measurements. The p+-GaInP2 emitter layer of 50 nm thickness and the p+-AlInP window layer of 40 nm thickness were doped by C with carrier concentration of 1 1018 cm3. The n+-GaInP BSF layer was doped by Si with carrier concentration of 1 1018 cm3. It is worthnoting that we used Si instead of Se, as the n+-type dopant, to decrease the memory effect, and used C instead of Zn, as the p+-type dopant, to suppress the diffusion effect. The resultant illuminated I2V curve of GaInP2 top solar cells with the p+–p– n –n+ structure is given in Fig. 4. The conversion efficiency for this device measured under one sun, AM0, 25 C on an area of 2 2 cm2 is 14.26% with the photovoltaic parameters: Voc ¼ 1:364 V; Jsc ¼ 16:504 mA/cm2 and FF ¼ 0:857: As can be seen that the photovoltaic parameters, especially Jsc and FF; are much improved by this p+–p–n–n+ structure. The improved Jsc was confirmed by the external QE curve of the p+–p–n–n+ structure, as shown in Fig. 5, in comparison with that of the p+–n structure. Under AM0 spectrum the integrated photocurrent density at
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80 Voc = 1.364V Isc = 66.015mA FF = 0.857 Eff = 14.26% AM0, 4 cm2
60
Current (mA)
40 20 0 -20 -40 -60
-80 -0.2 0.0
0.2
0.4
0.6 0.8 1.0 Voltage (V)
1.2
1.4
1.6
Fig. 4. Illuminated J2V curve of the modified p+–p–n–n+ GaInP2 solar cells, with Voc ¼ 1:364 V, Jsc ¼ 16:504 mA/cm2, FF ¼ 0:857 and EFF ¼ 14:26% under AM0, 135.3 mW/cm2, 25 C on an area of 2 2 cm2.
100
p+p-n-n+ cell
EQE (%)
80
p+n cell
60
40
20
0 300
400
500 600 700 Wavelength (nm)
800
900
Fig. 5. External QE of a typical GaInP2 solar cell with a p+–p–n–n+ structure along with that of a p+– n junction cell.
wavelengths ranging from 370 to 720 nm is 15.78 mA/cm2 for the p+–p–n–n+ structure cell, while 13.12 mA/cm2 for the p+–n junction cell. At meantime the modeling results for the light I2V curve in Fig. 4 using Eq. (2) indicate that the product of mobility and lifetime (mt) and the diffusion length (ld ) are also improved to 5.58 108 cm2/V and 0.381 mm by the p+–p–n–n+ structure. It is worth noting that the diffusion length is still less than the thickness of the GaInP2 absorber layer (0.5 mm) and also deficient for diffusing collection of
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the photoexcited carriers. Therefore, the resultant much improved photovoltaic performances should be attributed mainly to the enhancement of the field-added collection effect.
4. Conclusion The effects of a field-added collection on the photovoltaic performances of GaInP2 top cells have been simulated. A modified GaInP2 top cells with a p+–p–n–n+ configuration, rather than p+–n structure, were proposed for matching the GaInP2 material quality used. The resultant GaInP2 top cells have much improved conversion efficiency of 14.26% (AM0, 25 C, 2 2 cm2) with the corresponding photovoltaic parameters: Voc ¼ 1:364 V, Jsc ¼ 16:504 mA/cm2 and FF ¼ 0:857: These results provide a good basis for further development of high efficient GaInP2/GaAs tandem solar cells in our laboratory.
Acknowledgements This work was supported by the National Natural Science Foundation of China. The authors would like to thank Xiangwu Wang and Xiao Li in Nanjing Electronic Devices Institute for the help in MOVPE growth.
References [1] J.M. Olson, S.R. Kurtz, A.E. Kibbler, P. Faine, Appl. Phys. Lett. 56 (1990) 623–625. [2] K.A. Bertness, S.R. Kurtz, D.J. Freidman, A.E. Kibbler, C. Kramer, J.M. Olson, Appl. Phys. Lett. 65 (1994) 989–991. [3] T. Takamoto, E. Ikeda, H. Kurita, Appl. Phys. Lett. 70 (1997) 381–383. [4] P.R. Sharps, M.L. Timmons, Y.C.M. Yeh, C.L. Chu, Proceeding of the First World Conference and Exhibition on Photovoltaic Energy Conversion, IEEE, New York, 1994, pp. 1725–1728. [5] R.Ft. King, N.H. Karam, J.H. Ermer, M. Haddad, P. Colter, T. Isshiki, H. Yoon, H.L. Cotal, D.E. Joslin, D.D. Krut, R. Sudharsanan, K. Edmondson, B.T. Cavicchi, D.R. Lillington, Proceeding of the 28th IEEE Photovoltaic Specialists Conference, IEEE, New York, 2000, pp. 998–1001. [6] P.K. Chiang, C.L. Chu, Y.C.M. Yeh, P. Iles, G. Chen, J. Wei, P. Tsung, J. Olbinski, J, Krogen, S. Halbe, S. hemthong, F. Ho, Proceeding of the 28th IEEE Photovoltaic Specialists Conference, IEEE, New York, 2000, pp. 1002–1005. [7] M.D. Archer, R. Hill, Clean Electricity from Photovoltaics, Imperial College Press, Singapore, 2001. [8] R.S. Crandall, J. Appl. Phys. 54 (1983) 7176–7186. [9] D.J. Friedman, J.F. Geisz, S.R. Kurtz, J.M. Olson, Proceeding of the 2nd World Conference and Exhibition on Photovoltaic Energy Conversion, IEEE, New York, 1998, pp. 1725–1728. [10] J.F. Geisz, D.J. Friedman, S. Kurtz, Proceeding of the 29th IEEE PV Specialists Conference, IEEE, New York, 2002, pp. 864–867. [11] K. Takahashi, Y. Minagawa, S. Yamada, T. Unno, Solar Energy Mater. Solar Cells 66 (2001) 525–532.
Solar Energy Materials & Solar Cells 80 (2003) 265–272
Field-aided collection in GaInP2 top solar cells M.B. Chena, R.Q. Cuia, Z.W. Zhangb, J.F. Lub, L.X. Wangb, W.Y. Chib, X.B. Xiangc,*, X.B. Liaoc a Shanghai Jiaotong University, Shanghai 200030,China Shanghai Institute of Space Power Sources, Shanghai 200233, China c Laboratory of Semiconductor Material Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China b
Received 30 May 2003
Abstract This paper reports the study on a field-aided collection in p-on-n GaInP2 top cells. The cells were produced by metalorganic vapor phase epitaxy at a low gas pressure. In order to optimize the device configuration, numerical simulations have been performed for the impacts of fieldaided collection on the performance of the top cells. On the basis of the modeling results, a modified p+–p–n–n+ configuration is introduced for GaInP2 top cells. This modification has brought out improved photovoltaic performance of the top cells, with conversion efficiency EFF=14.26% (AM0, 2 2 cm2, 25 C). r 2003 Elsevier B.V. All rights reserved. Keywords: Field-added collection effect; GaInP2; Solar cells
1. Introduction Two-terminal monolithic GaInP2/GaAs tandem cells have gained a great deal of attention and achieved much progress in recent years, since the use of GaInP2 material for the top cells was proposed by Olson et al. in 1990 [1]. The highest conversion efficiency for GaInP2/GaAs tandem solar cells has been obtained with an n-on-p polarity structure, due to the low sheet resistance in the n-type emitter and long diffusion length in the p-type base region [2,3]. However, GaInP2/GaAs tandem cells with a p-on-n polarity have some advantages in achieving an inactive GaAs/Ge *Corresponding author. Tel./fax: +86-10-82304253. E-mail address: [email protected] (X.B. Xiang). 0927-0248/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2003.08.002
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interface for epitaxy growth of GaAs on Ge substrates [4]. In both cases, the prerequisite for preparing high-efficient GaInP2/GaAs tandem cells is to develop a high quality of GaInP2 material and to optimize the device configuration matched to the material quality. Although qualified GaInP2 material and high-efficient top cells have been reported, the quality of GaInP2 material is closely correlated with the deposition technology used, such as the metalorganic vapor phase epitaxy (MOVPE) apparatus, source gases and growth conditions, is still a challenging subject. The motivation for this work is to develop two-terminal, monolithic p-on-n GaInP2/GaAs tandem cells, which would be preferable for our existing protomanufacturing line of single junction GaAs solar cells on Ge substrates with a mean conversion efficiency of 19% (AM0). As the first step we focused on the development of high efficient GaInP2 top cells with a p-on-n polarity. The preliminary results indicated that the photovoltaic performances of GaInP2 top cells with a commonly used p+–n structure were not satisfactory, compared to the reported data in the literature [5,6]. In order to understand what happened in our top cells, a numerical modeling for the illuminated I2V characteristics of the GaInP2 top cells was performed on the basis of accounting for the field-aided collection effect. The calculated results showed that the material quality of the GaInP2 base region used was not good enough, with a product (mt) of mobility and lifetime of the photogenerated carriers being about 2.93 108 cm2/V and a corresponding diffusion length (ld ) of about 300 nm, which is less than the thickness of the GaInP2 base region (500–600 nm). Therefore, we proposed a p+–p–n–n+ structure for the GaInP2 top cells to enhance the field-aided collection effect for the photo-generated carriers, and thus achieved a much improved performance of the top cells.
2. Experimental The GaInP2 top cell samples used were grown on n-type GaAs substrates using a low-pressure (200 mbar) MOVPE technology in a horizontal reactor (Model Aixtron 200-4). The precursors for Group III elements of Ga, Al and In, were trimethylgallium (TMGa), trimethylaluminum (TMAl) and trimethylindium (TMIn). The precursors for Group V elements of As and P were arsine and phosphine. For the n-type dopant source, silane (SiH4) was used. For the p-type dopant sources, diethylzinc (DEZn) and carbon tetrachloride (CCl4) were used. The carrier gas was hydrogen, purified on-line by passing through a palladium-filter. The GaAs substrates doped with Si were misoriented by 2 to (1 1 0) from (1 0 0). The main post-growth technology of GaInP2 cells includes photolithography, evaporation, thermal annealing and selecting etching. The front grids contact was evaporated using TiPdAg, defined by a standard photolithographic method, and the back contact was AuGeNi plus Au. The obscurance of the front grids was approximately 3% of the total cell area (2 2 cm2). The cell perimeter was also defined by a photolithography, and followed by a mesa etching. In order to form an Ohmic contact between the front grids and the p++-GaAs cap layer, an appropriate thermal annealing was performed. The antireflection coating (ARC) on the cell
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surface was composed of double layers of evaporated TiO2 and SiO2 with thickness of 62 and 95 nm, respectively. In the initial stage of the research we designed a p+–n configuration for the GaInP2 top cells, similar to the structure published in Ref. [2]. But after a set of experiments we found that the photovoltaic performances of the GaInP2 top cells with the p+–n structure were not as good as expected. Under the guidance of the numerical modeling results accounting for the field-aided collection effect, we modified the structure of the top cells with a p+–p–n–n+ configuration in order to promote the cell performances. The photovoltaic performances of the cells were measured using a simulator of Spectrolab x-25 as the AM0 spectrum. The light intensity of 135.3 mW/cm2 was calibrated using a GaInP2/GaAs tandem cell sample made in Spectrolab as the reference cell. The external quantum-efficiency (QE) curves were measured at 0 V bias using a monochromator-based system with a metal halide lamp source and a calibrated silicon diode detector.
3. Results and discussion 3.1. p+–n GaInP2 cells The schematic structure of the GaInP2 solar cells with a p+–n configuration used in the experiments is presented in Fig. 1, which comprises a 40 nm thick p+-AlInP window layer with carrier concentration of 1 1018 cm3, a 100 nm thick p+-GaInP2 emitter with carrier concentration of 1 1018 cm3, a 500 nm thick n-GaInP2 base layer with carrier concentration of 1 1017 cm3 and a 50 nm thick n+-GaInP2 BSF layer with carrier concentration of 1 1018 cm3. The typical dark Jd 2V and light Jph 2V curve of the GaInP2 top cell samples with the p+–n configuration are illustrated in Fig. 2 by using dashed and solid line, respectively. The deduced photovoltaic parameters are the open circuit voltage Voc ¼ 1:329 V; short circuit current density Jsc ¼ 13:70 mA/cm2 and fill factor FF ¼ 0:771: As can be seen that the Jsc and FF of this device is obviously lower than that expected. From the comparison of the Jph –V and Jd 2V curve, it is seen that the light p++-GaAs p+-AlInP window p+-GaInP2 emitter n-GaInP2 base n+-GaInP2 BSF n+-GaAs buffer n+ GaAs substrate
40 nm 100 nm 500 nm 50 nm 200 nm
1×1018cm-3 1×1018cm-3 1×1017cm-3 1×1018cm-3 1×1018cm-3
Fig. 1. Schematic cross-section diagram of the tested p+–n GaInP2 top cell.
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25
Current density (mA/cm2)
20 Jd Jph Modeling
15 10 5 0 -5 -10 -15 0.0
0.2
0.4
0.6 0.8 1.0 Voltage (V)
1.2
1.4
1.6
Fig. 2. Comparison of light Jph 2V (solid line) and dark Jd 2V (dashed line) curves of a typical p+–n GaInP2 cell. The open circles denote the modeling data fitting to the light Jph V curve.
Jph 2V curve contains apparently some shunt components, while the dark Jd 2V curve looks not shunted in this linear axis scale. This phenomenon can be explained neither by the ideality factors of a real diode nor the losses of series resistance (Rs ) and shunt resistance (Rsh ). Indeed, the dark Jd 2V curve could be approximated by an empirical expression as [7] Jd ¼ J01 ðexpðqðV Jd Rs Þ=n1 kTÞ 1Þ þ J02 ðexpðqðV Jd Rs Þ=n2 kTÞ 1Þ þ ðV Jd Rs Þ=Rsh
ð1Þ
in which q is the electron charge, k is the Bolztmann constant and T is the temperature in Kelvins. This expression includes two exponential regions characterized by two pre-exponential factors (J01 and J02 ) and two ideality factors (n1 and n2 ), and the items represented by Rs and Rsh losses. However, for the Jph 2V curve the commonly used expression: Jph ¼ Jd 2Jsc is not held, because of the fact that the light Jph 2V curve is not parallel to the dark Jd 2V curve. This means that in our case the superposition principle is no longer suitable and the field-aided collection effect must be considered. 3.2. Modeling results The field-aided collection effect has been successfully employed to enhance photovoltaic performances for solar cells based upon materials with a small minority carriers diffusion length, such as p–i–n-type hydrogenated amorphous silicon (a-Si:H) [8] devices, 1-eV GaInNAs [9] and GaNPAs [10] solar cells. In order to understand the impacts of the field-aided collection on the photovoltaic perfor-
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mances of the GaInP2 top cells, a numerical simulation has been performed for the light Jph 2V curve shown in Fig. 2 by using a field-aided collection model developed by Crandall especially for the case of p–i–n a-Si:H solar cells [8]. This expression reads as follows: Jph ¼ qGlc ð1 expðL=lc ÞÞ;
ð2Þ
where q is the electron charge, G is the generation ratio of the photo-carriers, L is the thickness of the intrinsic absorber layer. This expression contains only one unknown parameter, the collection length lc ¼ lp þ ln ; the sum of the electron drift length (ln ¼ mn tn E) and the hole drift length ðlp ¼ mp tp EÞ; where E denotes the built-in electrical field in the intrinsic absorber layer. Obviously, the collection length lc is a function of the electrical field E and the applied voltage bias V : If E is uniformly distributed in the n-GaInP2 base absorber region with the thickness L ¼ 500 nm (see Fig. 1), and is given by E ¼ ðVi V Þ=L; where Vi is the built-in potential, then lc increases linearly with V decreasing. Under the short circuit condition (V ¼ 0), the photogenerated current density Jph is saturated at the value of Jph ¼ qGL; indicating that all the photoexcited electron–hole pairs are separated by the electric field and collected in the external circuit. Also for simplicity, the value of qGL is supposed to be equal to the Jsc (13.70 mA/cm2) and the Vi is taken as Voc (1.329 V), the lowest limitation of Vi ; thus we obtained the modeling results shown in Fig. 3 as the open circles. It notes that the modeling results fit the light Jph 2V curve quite well with only one modeling parameter mt ¼ 2:93 108 cm2/V. The analysis forenamed relies on a pure field-added collection model for photoexcited carriers. In fact the diffusion process should also play a role. Using Einstein relationship D ¼ kTm=q; where D is the diffusion coefficient, and the above mt value, we estimated the diffusion length ld ¼ ðDtÞ1=2 ; to be about 0.276 mm. This diffusion length is obviously less than the GaInP2 base thickness (L) of the top cells and far deficient for the diffusion collection of photoexcited carriers. Furthermore, the derived ld is the sum of the diffusion length of electrons and holes, among which the diffusion length of minority carriers (holes in our case) should be much less than the derived ld value.
p++-GaAs p+-AlInP window p+-GaInP2 emitter p-GaInP2 emitter n-GaInP2 base n+-GaInP2 BSF n+-GaAs buffer n+ GaAs substrate
40 nm 50 nm 100 nm 500 nm 50 nm 200 nm
1×1018cm-3 1×1018cm-3 2×1015cm-3 1×1016cm-3 1×1018cm-3 1×1018cm-3
Fig. 3. Schematic cross-section diagram of the GaInP2 cells with a p+–p–n–n+ structure.
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As for the assumption of uniform electrical field distribution, it is a questioning issue. It is well known that the electrical field set up by the built-in potential in a p+–n junction concentrates in the space charge region. For the doping data shown in Fig. 1 the width of the space charge region (W ) is estimated to be about 0.129 mm, according to the depletion approach with the dielectric constant e=11.4 for GaInP2 material. This value of W is much less than the thickness of the GaInP2 absorber layer, therefore if one would extend the space charge region to cover the whole base region (B500 nm), the doping concentration in the GaInP2 absorber layer should be decreased from 1 1017 cm3 to at least 6 1015 cm3. 3.3. p+–p–n–n+ GaInP2 cells Based upon the modeling results we were aware of that our GaInP2 material quality is not good enough to have a larger product of mobility and lifetime and a larger diffusion length. The reason for this may be correlated with the gases purity and the growth-related defects. In order to explore an optimized devise configuration on the basis of the present material quality, we proposed a p+–p–n–n+ structure for the GaInP2 top cells, instead of the p+–n structure. The proposed p+–p–n–n+ structure could play a twofold role, one is to enlarge the width of the space charge regions and thus to enhance the field-aided collection for the photoexcited carriers, and the other is to decrease the defect density of the n-GaInP2 absorber layer and thus to increase the minority carrier lifetime. The second effect was reported by Katahashi [11]. They observed that the reduced majority-carrier concentration leads to an improvement in the minority-carrier lifetime in AlGaAs solar cells with p+– p–n–n+ structure. The schematic diagram of the modified GaInP2 cells with a p+–p–n–n+ structure is shown in Fig. 3. The 500 nm thick n-GaInP2 absorber layer was not intentionally doped, with a background n-type concentration on the order of 1 1016 cm3. The 100 nm thick p-GaInP2 emitter was compensated to carrier concentration of 2 1015 cm3 by Zn doping, as measured by electric-chemical capacitance–voltage (CV) measurements. The p+-GaInP2 emitter layer of 50 nm thickness and the p+-AlInP window layer of 40 nm thickness were doped by C with carrier concentration of 1 1018 cm3. The n+-GaInP BSF layer was doped by Si with carrier concentration of 1 1018 cm3. It is worthnoting that we used Si instead of Se, as the n+-type dopant, to decrease the memory effect, and used C instead of Zn, as the p+-type dopant, to suppress the diffusion effect. The resultant illuminated I2V curve of GaInP2 top solar cells with the p+–p– n –n+ structure is given in Fig. 4. The conversion efficiency for this device measured under one sun, AM0, 25 C on an area of 2 2 cm2 is 14.26% with the photovoltaic parameters: Voc ¼ 1:364 V; Jsc ¼ 16:504 mA/cm2 and FF ¼ 0:857: As can be seen that the photovoltaic parameters, especially Jsc and FF; are much improved by this p+–p–n–n+ structure. The improved Jsc was confirmed by the external QE curve of the p+–p–n–n+ structure, as shown in Fig. 5, in comparison with that of the p+–n structure. Under AM0 spectrum the integrated photocurrent density at
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80 Voc = 1.364V Isc = 66.015mA FF = 0.857 Eff = 14.26% AM0, 4 cm2
60
Current (mA)
40 20 0 -20 -40 -60
-80 -0.2 0.0
0.2
0.4
0.6 0.8 1.0 Voltage (V)
1.2
1.4
1.6
Fig. 4. Illuminated J2V curve of the modified p+–p–n–n+ GaInP2 solar cells, with Voc ¼ 1:364 V, Jsc ¼ 16:504 mA/cm2, FF ¼ 0:857 and EFF ¼ 14:26% under AM0, 135.3 mW/cm2, 25 C on an area of 2 2 cm2.
100
p+p-n-n+ cell
EQE (%)
80
p+n cell
60
40
20
0 300
400
500 600 700 Wavelength (nm)
800
900
Fig. 5. External QE of a typical GaInP2 solar cell with a p+–p–n–n+ structure along with that of a p+– n junction cell.
wavelengths ranging from 370 to 720 nm is 15.78 mA/cm2 for the p+–p–n–n+ structure cell, while 13.12 mA/cm2 for the p+–n junction cell. At meantime the modeling results for the light I2V curve in Fig. 4 using Eq. (2) indicate that the product of mobility and lifetime (mt) and the diffusion length (ld ) are also improved to 5.58 108 cm2/V and 0.381 mm by the p+–p–n–n+ structure. It is worth noting that the diffusion length is still less than the thickness of the GaInP2 absorber layer (0.5 mm) and also deficient for diffusing collection of
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the photoexcited carriers. Therefore, the resultant much improved photovoltaic performances should be attributed mainly to the enhancement of the field-added collection effect.
4. Conclusion The effects of a field-added collection on the photovoltaic performances of GaInP2 top cells have been simulated. A modified GaInP2 top cells with a p+–p–n–n+ configuration, rather than p+–n structure, were proposed for matching the GaInP2 material quality used. The resultant GaInP2 top cells have much improved conversion efficiency of 14.26% (AM0, 25 C, 2 2 cm2) with the corresponding photovoltaic parameters: Voc ¼ 1:364 V, Jsc ¼ 16:504 mA/cm2 and FF ¼ 0:857: These results provide a good basis for further development of high efficient GaInP2/GaAs tandem solar cells in our laboratory.
Acknowledgements This work was supported by the National Natural Science Foundation of China. The authors would like to thank Xiangwu Wang and Xiao Li in Nanjing Electronic Devices Institute for the help in MOVPE growth.
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