A GaAs/GaInP dual junction solar cell grown by molecular beam epitaxy

Pan Dai; Shulong Lu; Lian Ji; Wei He; Lifeng Bian; Hui Yang; M. Arimochi; H. Yoshida; S. Uchida; M. Ikeda

doi:10.1088/1674-4926/34/10/104006

J. Semicond. > 2013, Volume 34 > Issue 10 > 104006

SEMICONDUCTOR DEVICES

A GaAs/GaInP dual junction solar cell grown by molecular beam epitaxy

Pan Dai^{1, 3}, Shulong Lu^1,, Lian Ji¹, Wei He¹, Lifeng Bian¹, Hui Yang¹, M. Arimochi², H. Yoshida², S. Uchida² and M. Ikeda²

+ Author Affiliations

Corresponding author: Lu Shulong, sllu2008@sinano.ac.cn

DOI: 10.1088/1674-4926/34/10/104006

Abstract: We report the recent result of GaAs/GaInP dual-junction solar cells grown by all solid-state molecular-beam-epitaxy (MBE). The device structure consists of a GaIn_0.48P homojunction grown epitaxially upon a GaAs homojunction, with an interconnected GaAs tunnel junction. A photovoltaic conversion efficiency of 27% under the AM1.5 globe light intensity is realized for a GaAs/GaInP dual-junction solar cell, while the efficiencies of 26% and 16.6% are reached for a GaAs bottom cell and a GaInP top cell, respectively. The energy loss mechanism of our GaAs/GaInP tandem dual-junction solar cells is discussed. It is demonstrated that the MBE-grown phosphide-containing Ⅲ-Ⅴ compound semiconductor solar cell is very promising for achieving high energy conversion efficiency.

Key words: molecular beam epitaxy, Ⅲ-Ⅴ semiconductor PN junction, solar cell

1. Introduction

Tandem solar cells, which consist of several sub-cells with different energy band gap energies, enable conversion efficiency to be increased. As one suitable candidate for the top cell material, GaInP has been proved to be the best option owing to its good material quality and appropriate band gap energy^[1-4]. From the modeling at a given band gap energy of 1.42 eV of GaAs, a potential efficiency as high as 36% of GaAs/GaInP dual-junction at AM 1.5G light intensity is expected^[5]. For GaInP/GaAs dual-junction solar cells (SCs), since 1997 a record efficiency of 30.2% has not been beaten, though plenty of novel concepts have been employed to improve the efficiency. The metal-organic chemical vapor deposition (MOCVD) technique is generally used for epitaxial growth. As one of the most important epitaxial techniques, molecular-beam-epitaxy (MBE) has shown its unique advantages for basic research^{[6, 7]} However, the performance of the earlier GaAs SCs grown by MBE is worse than those obtained by MOCVD growth because of its low growth temperature and the presence of isolated defects^{[8, 9]}. With the successful development of control and operation of the phosphorous source, high quality phosphide-related material has been obtained by MBE growth^{[10, 11]}. However, research about solar cells using MBE is scarce. Very recently, we reported that MBE-grown phosphide-contained Ⅲ-Ⅴ compound semiconductor solar cells can be quite comparable to those grown by MOCVD^[12]. In this paper, we report the initial result of GaInP/GaAs dual-junction SCs with an efficiency of 27% grown by MBE. The energy loss mechanism of our GaAs/GaInP tandem dual-junction SC is also discussed on the basis of the optical and electrical characteristic of devices.

2. Experiment

The epitaxial growth of the SC material was performed in a Veeco GEN20A dual-chamber all solid-state MBE machine with a valved phosphorous cracker cell and a valved arsenic cracker cell. The typical growth rate of GaAs and GaInP is 1 $\mu$ m/h, which is evidenced by X-ray diffraction (XRD) measurements and reflection high energy electron diffraction (RHEED) oscillations. The typical growth temperature of GaAs is 580 ℃, and silicon and berylium are used as the n and p type doping sources, respectively. For the phosphide-related material growth, a moderate temperature of 470-490 ℃ is used. During the GaInP growth, the RHEED image shows a 2 $\times$ 1 surface reconstruction. The beam equivalent pressure of phosphor is 1 $\times$ 10^-5 Torr with the Ⅴ/Ⅲ ratio of 50. The room temperature photoluminescence (PL) peak with an emission energy of 1.875 eV of GaInP epilayers is observed, which have a negligible lattice mismatch of less than 5 $\times$ 10^-4 with GaAs substrate. A single exponential decay curve together with a decay time independent of the detecting energy indicates that the emission originates mostly from the disordered structure in the GaInP film^[13]. After growth, the device structures are then processed following the standard Ⅲ-Ⅴ solar cell device technique. The cell size is 5.0 $\times$ 5.0 mm². The metal in the front grid is the AuGe/Ni/Au. A shadowing area of 2.1% of total area is used in the mask design. An anti-reflecting coating (ARC) layer of Si₃N₄/SiO₂ is deposited on the devices with an average loss of less than 5%. The photovoltaic current-voltage ( $I - V$ ) characteristics are measured under AM 1.5G standard solar spectrum illumination.

Figure 1 shows the scanning electronic microscopy (SEM) image in the cross section of our GaAs/GaInP dual-junction solar cell, and the right part shows the corresponding layers within the structure. The GaAs bottom cell consists of a p+-GaInP back surface field (BSF) layer, a p-GaAs base layer, an n+-GaAs emitter, and an n+-GaInP window layer. The GaInP top cell consists of a p+-AlGaInP/p+-GaInP BSF layer, a p-GaInP base layer, an n+-GaInP emitter and an n-AlInP window layer. A clear interface between the As and P can be observed, indicating a good switch of V-group As and P. The GaInP top cell and the GaAs bottom cell are electrically and optically connected by a GaAs tunnel junction.

Figure 1. Scanning electronic microscopy image of a GaAs/GaInP on solar cell.

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3. Results and discussion

Figure 2 shows the current-voltage ${\text{(}}I{\text{ }} - {\text{ }}V{\text{)}}$ characteristic of a GaAs/GaInP dual-junction SC. Under AM 1.5G illumination, the GaAs/GaInP dual-junction SC has a photovoltaic convention efficiency of 27% with the open circuit voltage $({V_{{\text{oc}}}})$ of 2.312 V, a short-circuit current density $({J_{{\text{sc}}}})$ of 13.6 mA/cm2 and a fill factor (FF) of 86%. For comparison, the $I$ - $V$ characteristics of GaAs and GaInP single junction SCs with the same structures as in a tandem device are also presented. The GaAs bottom cell has an efficiency of 26% with the ${V_{{\text{oc}}}}$ of 1.04 V, a ${J_{{\text{sc}}}}$ 29.1 mA/cm2, and an FF of 86%. The GaInP top cell has an efficiency of 16.6% with a ${V_{{\text{oc}}}}$ of 1.377 V, a ${J_{{\text{sc}}}}$ value of 13.6 mA/cm2, and an FF of 88%. Figure 3 shows the external quantum efficiency (EQE) of the dual-junction SC. The overlap of the two curves of GaAs and GaInP implies the response of both the bottom and the top cells. For the GaAs bottom cell, an average value of 90% is obtained. For the GaInP cell, the ${J_{{\text{sc}}}}$ calculated by means of the convolution of the EQE value obtained for the device with the AM 1.5G illumination shows a value of 13.8 mA/cm², which is almost the same as the measured value from the $I$ - $V$ characteristics.

Figure 2. The current–voltage (I–V) characteristics of GaAs, GaInP and GaAs/GaInP solar cells

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Figure 3. External quantum efficiency (EQE) of the dual-junction solar cell.

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The operation of all the solar cell devices includes the processes of photon absorption, carrier separation, transport and collection, therefore many parameters may affect the performance improvement of the device. If only taking into account the material quality, the most important point is to lower the recombination, including the radiative and nonradiative recombination of the material, interface recombination and to increase the carrier mobility. Benefitting from high purity material growth, high performances of the GaAs and GaInP single junction SCs have been obtained. It is important to note that the performance of our GaAs single-junction is comparable to the best efficiency reported at the end of 2010^[14]. In addition, the GaInP single junction SC also reaches a high level. However, the GaAs/GaInP tandem dual-junction SC is almost 3% lower than the best reported value. The short circuit current of the dual-junction solar cell is almost equal to the GaInP single junction. While the open voltage of the tandem SC is only 2.31 V, which is about 100 meV less than the sum of the single top GaInP (1.37 V) and bottom GaAs (1.045 V) SCs. The lower than expected efficiency is due to the decreased open voltage. In contrast to GaAs and GaInP single junction SCs, the performance of the tunnel junction connected to the top and bottom cells is significant to the tandem multi-junction SC. Since the device structure within the dual-junction is almost identical to the respective cell of the top and bottom ones, it is reasonable to blame the decrease of the total voltage drop resulting from the energy loss of the incorporation of the tunnel junction. Figure 4 presents the $J$ - $V$ curve of a separated GaAs/GaAs tunnel junction used in the tandem SC. It is noted that at the working point of current density of 13.6 mA/cm², the voltage drop upon the tunnel junction is less than 10 meV. This voltage drop above the tunnel junction is much less than the 100 meV mentioned above. Therefore, it may not be the main reason for the decreased open voltage of the tandem SC.

Figure 4. The current–voltage (I–V) characteristics of the GaAs tunnel junction

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In the device, an AlGaInP layer is used as the back surface field layer of the GaInP SC, and it plays also a role as the barrier of the tunnel junction. Normally, a highly doped p-n junction is needed to form a tunnel junction. In our case, a highly-doped beryllium GaAs layer (4 $\times$ 10¹⁹ cm^-3) is used for the p-type GaAs forming the tunnel junction. The diffusion of the highly-doped beryllium from the GaAs layer (4 $\times$ 10¹⁹ cm^-3) within the tunnel junction to the BSF AlGaInP layer is inevitable and will strongly degrade its material quality. Beryllium has a larger diffusion coefficient than carbon, which is usually used in MOCVD growth. Figure 5 presents the SIMS profiles of the device structures near the GaInP top cell. The concentration of the beryllium within the GaInP cell is gradually decreased from the AlGaInP. The thickness of GaInP layer where Be concentration decreases gradually is about 200 nm. A significant Beryllium inter-diffusion at the interface will worsen the function of the AlGaInP layer and increase the interface recombination between the GaInP base layer and the AlGaInP layer. It is well known that the open voltage will decrease with increasing recombination rate. As a result, a smaller open voltage is observed for the dual-junction. An optimized growth condition and optimized device structure, such as a dual-layer BSF design, are needed to decrease the beryllium diffusion.

Figure 5. The SIMS profiles of the device structures near the GaInP top cell.

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4. Summary

In summary, owing to the high purity and high material quality grown by MBE, the high performance of a GaInP/GaAs dual-junction SC with an efficiency of 27% has been obtained. The energy loss mechanism of our GaAs/GaInP tandem dual-junction solar cells is discussed. It is demonstrated that the MBE-grown phosphide-contained Ⅲ-Ⅴ compound semiconductor solar cell is very promising for achieving high energy conversion efficiency.

References

[1]	Bertness K A, Kurtz S R, Friedman D J, et al. 29.5%-efficient GaInP/GaAs tandem solar cells. Appl Phys Lett, 1994, 65:989 doi: 10.1063/1.112171
[2]	Takamoto T, Ikeda E, Kurita H, et al. Over 30% efficient InGaP/GaAs tandem solar cells. Appl Phys Lett, 1997, 70:331
[3]	Garcia I, Rey-Stolle I, Galiana B, et al. A 32.6% efficient lattice-matched dual-junction solar cell working at 1000 suns. Appl Phys Lett, 2009, 94:053509 doi: 10.1063/1.3078817
[4]	King R R, Law D C, Edmondson K M, et al. 40% efficient metamorphic GaInP/InGaAs/Ge multijunction solar cells. Appl Phys Lett, 2007, 90:183516 doi: 10.1063/1.2734507
[5]	Kurtz S R, Olson J M, Kibbler A. Modeling of tow-junction, series-connected tandem solar cells using top cell thickness as an adjustable parameter. J Appl Phys, 1990, 68:1890 doi: 10.1063/1.347177
[6]	Ragay F W, Leys M R, Nouwens P A M, et al. A MBE-grown high-efficiency GaAs solar cell with a directly deposited aluminum front contact. IEEE Electron Device Lett, 1992, 13:618 doi: 10.1109/55.192863
[7]	Chuang H L, Klausmeier-Brown M E, Melloch M R, et al. Effective minority-carrier hole confinement of Si-doped, n⁺-n GaAs homojunction barriers. J Appl Phys, 1989, 66:273 doi: 10.1063/1.343868
[8]	Tobin S P, Vernon C B, Wojtczuk S J, et al. Assessment of MOCVD-and MBE-grown GaAs for high-efficiency solar cell applications. IEEE Trans Electron Devices, 1990, 37:469 doi: 10.1109/16.46385
[9]	Leinonan P, Pessa M, Haapamaa J, et al. Advances in production MBE grown GaInP/GaAs cascade solar cells. IEEE Xplore-Photovoltaic Specialists Conference, 2000, DOI10.1109/PVSC.2000.916098
[10]	Wicks G W, Koch M W, Varriano J A, et al. Use of a valved, solid phosphorus source for the growth of Ga_0.5In_0.5P and Al_0.5In_0.5P by molecular beam epitaxy. Appl Phys Lett, 1991, 59:342 doi: 10.1063/1.105590
[11]	Yoon S F, Mah K W, Zheng H Q. Effects of ⅤⅢ ratio on the properties of InGaP grown by a valved phosphorous cracker cell in solid source MBE. Jpn J Appl Phys, 1999, 38:5740 doi: 10.1143/JJAP.38.5740
[12]	Lu S L, Ji L, He W, et al. High-efficiency GaAs and GaInP solar cells grown by all solid-state molecular-beam-epitaxy. Nanoscale Res Lett, 2011, 6:576 doi: 10.1186/1556-276X-6-576
[13]	He W, Lu S L, Dong J R, et al. Structural and optical properties of GaInP grown on germanium by metal-organic chemical vapour deposition. Appl Phys Lett, 2010, 97:121909 doi: 10.1063/1.3492854
[14]	Bauhuis G J, Mulder P, Haverkamp E J, et al. 26.1% thin-film GaAs solar cell using epitaxial lift-off. Solar Energy Materials and Solar Cells, 2009, 93:1488 doi: 10.1016/j.solmat.2009.03.027

Fig. 1. Scanning electronic microscopy image of a GaAs/GaInP on solar cell.

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Fig. 2. The current–voltage (I–V) characteristics of GaAs, GaInP and GaAs/GaInP solar cells

DownLoad: Full-Size Img PowerPoint

Fig. 3. External quantum efficiency (EQE) of the dual-junction solar cell.

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Fig. 4. The current–voltage (I–V) characteristics of the GaAs tunnel junction

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Fig. 5. The SIMS profiles of the device structures near the GaInP top cell.

DownLoad: Full-Size Img PowerPoint

[1]	Bertness K A, Kurtz S R, Friedman D J, et al. 29.5%-efficient GaInP/GaAs tandem solar cells. Appl Phys Lett, 1994, 65:989 doi: 10.1063/1.112171
[2]	Takamoto T, Ikeda E, Kurita H, et al. Over 30% efficient InGaP/GaAs tandem solar cells. Appl Phys Lett, 1997, 70:331
[3]	Garcia I, Rey-Stolle I, Galiana B, et al. A 32.6% efficient lattice-matched dual-junction solar cell working at 1000 suns. Appl Phys Lett, 2009, 94:053509 doi: 10.1063/1.3078817
[4]	King R R, Law D C, Edmondson K M, et al. 40% efficient metamorphic GaInP/InGaAs/Ge multijunction solar cells. Appl Phys Lett, 2007, 90:183516 doi: 10.1063/1.2734507
[5]	Kurtz S R, Olson J M, Kibbler A. Modeling of tow-junction, series-connected tandem solar cells using top cell thickness as an adjustable parameter. J Appl Phys, 1990, 68:1890 doi: 10.1063/1.347177
[6]	Ragay F W, Leys M R, Nouwens P A M, et al. A MBE-grown high-efficiency GaAs solar cell with a directly deposited aluminum front contact. IEEE Electron Device Lett, 1992, 13:618 doi: 10.1109/55.192863
[7]	Chuang H L, Klausmeier-Brown M E, Melloch M R, et al. Effective minority-carrier hole confinement of Si-doped, n⁺-n GaAs homojunction barriers. J Appl Phys, 1989, 66:273 doi: 10.1063/1.343868
[8]	Tobin S P, Vernon C B, Wojtczuk S J, et al. Assessment of MOCVD-and MBE-grown GaAs for high-efficiency solar cell applications. IEEE Trans Electron Devices, 1990, 37:469 doi: 10.1109/16.46385
[9]	Leinonan P, Pessa M, Haapamaa J, et al. Advances in production MBE grown GaInP/GaAs cascade solar cells. IEEE Xplore-Photovoltaic Specialists Conference, 2000, DOI10.1109/PVSC.2000.916098
[10]	Wicks G W, Koch M W, Varriano J A, et al. Use of a valved, solid phosphorus source for the growth of Ga_0.5In_0.5P and Al_0.5In_0.5P by molecular beam epitaxy. Appl Phys Lett, 1991, 59:342 doi: 10.1063/1.105590
[11]	Yoon S F, Mah K W, Zheng H Q. Effects of ⅤⅢ ratio on the properties of InGaP grown by a valved phosphorous cracker cell in solid source MBE. Jpn J Appl Phys, 1999, 38:5740 doi: 10.1143/JJAP.38.5740
[12]	Lu S L, Ji L, He W, et al. High-efficiency GaAs and GaInP solar cells grown by all solid-state molecular-beam-epitaxy. Nanoscale Res Lett, 2011, 6:576 doi: 10.1186/1556-276X-6-576
[13]	He W, Lu S L, Dong J R, et al. Structural and optical properties of GaInP grown on germanium by metal-organic chemical vapour deposition. Appl Phys Lett, 2010, 97:121909 doi: 10.1063/1.3492854
[14]	Bauhuis G J, Mulder P, Haverkamp E J, et al. 26.1% thin-film GaAs solar cell using epitaxial lift-off. Solar Energy Materials and Solar Cells, 2009, 93:1488 doi: 10.1016/j.solmat.2009.03.027

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P Dai, S L Lu, L Ji, W He, L F Bian, H Yang, M. Arimochi, H. Yoshida, S. Uchida, M. Ikeda. A GaAs/GaInP dual junction solar cell grown by molecular beam epitaxy[J]. J. Semicond., 2013, 34(10): 104006. doi: 10.1088/1674-4926/34/10/104006.

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Received: 01 March 2013 Revised: 19 March 2013 Online: Published: 01 October 2013

Article Navigation > Journal of Semiconductors > 2013 > 34(10): 104006

Pan Dai, Shulong Lu, Lian Ji, Wei He, Lifeng Bian, Hui Yang, M. Arimochi, H. Yoshida, S. Uchida, M. Ikeda. A GaAs/GaInP dual junction solar cell grown by molecular beam epitaxy[J]. Journal of Semiconductors, 2013, 34(10): 104006. doi: 10.1088/1674-4926/34/10/104006 ****P Dai, S L Lu, L Ji, W He, L F Bian, H Yang, M. Arimochi, H. Yoshida, S. Uchida, M. Ikeda. A GaAs/GaInP dual junction solar cell grown by molecular beam epitaxy[J]. J. Semicond., 2013, 34(10): 104006. doi: 10.1088/1674-4926/34/10/104006.

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A GaAs/GaInP dual junction solar cell grown by molecular beam epitaxy

DOI: 10.1088/1674-4926/34/10/104006

1.
Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China
2.
Advanced Material Laboratories, Sony Corporation, Atsugi Tec. 4-14-1 Asahi-cho, Atsugi-shi, Kanagawa, 243-0014, Japan
3.
University of Chinese Academy of Sciences, Beijing 100049, China

Funds:

the SINANO-SONY Joint Program, China Y1AAQ11002

the National Natural Science Foundation of China 61176128

Project supported by the National Natural Science Foundation of China (No. 61176128) and the SINANO-SONY Joint Program, China (No. Y1AAQ11002)

More Information

Corresponding author: Lu Shulong, sllu2008@sinano.ac.cn
Received Date: 2013-03-01
Revised Date: 2013-03-19
Published Date: 2013-10-01

Abstract

Abstract

We report the recent result of GaAs/GaInP dual-junction solar cells grown by all solid-state molecular-beam-epitaxy (MBE). The device structure consists of a GaIn_0.48P homojunction grown epitaxially upon a GaAs homojunction, with an interconnected GaAs tunnel junction. A photovoltaic conversion efficiency of 27% under the AM1.5 globe light intensity is realized for a GaAs/GaInP dual-junction solar cell, while the efficiencies of 26% and 16.6% are reached for a GaAs bottom cell and a GaInP top cell, respectively. The energy loss mechanism of our GaAs/GaInP tandem dual-junction solar cells is discussed. It is demonstrated that the MBE-grown phosphide-containing Ⅲ-Ⅴ compound semiconductor solar cell is very promising for achieving high energy conversion efficiency.
- molecular beam epitaxy,
- Ⅲ-Ⅴ semiconductor PN junction,
- solar cell

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1. Introduction

Tandem solar cells, which consist of several sub-cells with different energy band gap energies, enable conversion efficiency to be increased. As one suitable candidate for the top cell material, GaInP has been proved to be the best option owing to its good material quality and appropriate band gap energy^[1-4]. From the modeling at a given band gap energy of 1.42 eV of GaAs, a potential efficiency as high as 36% of GaAs/GaInP dual-junction at AM 1.5G light intensity is expected^[5]. For GaInP/GaAs dual-junction solar cells (SCs), since 1997 a record efficiency of 30.2% has not been beaten, though plenty of novel concepts have been employed to improve the efficiency. The metal-organic chemical vapor deposition (MOCVD) technique is generally used for epitaxial growth. As one of the most important epitaxial techniques, molecular-beam-epitaxy (MBE) has shown its unique advantages for basic research^{[6, 7]} However, the performance of the earlier GaAs SCs grown by MBE is worse than those obtained by MOCVD growth because of its low growth temperature and the presence of isolated defects^{[8, 9]}. With the successful development of control and operation of the phosphorous source, high quality phosphide-related material has been obtained by MBE growth^{[10, 11]}. However, research about solar cells using MBE is scarce. Very recently, we reported that MBE-grown phosphide-contained Ⅲ-Ⅴ compound semiconductor solar cells can be quite comparable to those grown by MOCVD^[12]. In this paper, we report the initial result of GaInP/GaAs dual-junction SCs with an efficiency of 27% grown by MBE. The energy loss mechanism of our GaAs/GaInP tandem dual-junction SC is also discussed on the basis of the optical and electrical characteristic of devices.

2. Experiment

The epitaxial growth of the SC material was performed in a Veeco GEN20A dual-chamber all solid-state MBE machine with a valved phosphorous cracker cell and a valved arsenic cracker cell. The typical growth rate of GaAs and GaInP is 1 $\mu$ m/h, which is evidenced by X-ray diffraction (XRD) measurements and reflection high energy electron diffraction (RHEED) oscillations. The typical growth temperature of GaAs is 580 ℃, and silicon and berylium are used as the n and p type doping sources, respectively. For the phosphide-related material growth, a moderate temperature of 470-490 ℃ is used. During the GaInP growth, the RHEED image shows a 2 $\times$ 1 surface reconstruction. The beam equivalent pressure of phosphor is 1 $\times$ 10^-5 Torr with the Ⅴ/Ⅲ ratio of 50. The room temperature photoluminescence (PL) peak with an emission energy of 1.875 eV of GaInP epilayers is observed, which have a negligible lattice mismatch of less than 5 $\times$ 10^-4 with GaAs substrate. A single exponential decay curve together with a decay time independent of the detecting energy indicates that the emission originates mostly from the disordered structure in the GaInP film^[13]. After growth, the device structures are then processed following the standard Ⅲ-Ⅴ solar cell device technique. The cell size is 5.0 $\times$ 5.0 mm². The metal in the front grid is the AuGe/Ni/Au. A shadowing area of 2.1% of total area is used in the mask design. An anti-reflecting coating (ARC) layer of Si₃N₄/SiO₂ is deposited on the devices with an average loss of less than 5%. The photovoltaic current-voltage ( $I - V$ ) characteristics are measured under AM 1.5G standard solar spectrum illumination.

Figure 1 shows the scanning electronic microscopy (SEM) image in the cross section of our GaAs/GaInP dual-junction solar cell, and the right part shows the corresponding layers within the structure. The GaAs bottom cell consists of a p+-GaInP back surface field (BSF) layer, a p-GaAs base layer, an n+-GaAs emitter, and an n+-GaInP window layer. The GaInP top cell consists of a p+-AlGaInP/p+-GaInP BSF layer, a p-GaInP base layer, an n+-GaInP emitter and an n-AlInP window layer. A clear interface between the As and P can be observed, indicating a good switch of V-group As and P. The GaInP top cell and the GaAs bottom cell are electrically and optically connected by a GaAs tunnel junction.

Figure 1. Scanning electronic microscopy image of a GaAs/GaInP on solar cell.

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3. Results and discussion

Figure 2 shows the current-voltage ${\text{(}}I{\text{ }} - {\text{ }}V{\text{)}}$ characteristic of a GaAs/GaInP dual-junction SC. Under AM 1.5G illumination, the GaAs/GaInP dual-junction SC has a photovoltaic convention efficiency of 27% with the open circuit voltage $({V_{{\text{oc}}}})$ of 2.312 V, a short-circuit current density $({J_{{\text{sc}}}})$ of 13.6 mA/cm2 and a fill factor (FF) of 86%. For comparison, the $I$ - $V$ characteristics of GaAs and GaInP single junction SCs with the same structures as in a tandem device are also presented. The GaAs bottom cell has an efficiency of 26% with the ${V_{{\text{oc}}}}$ of 1.04 V, a ${J_{{\text{sc}}}}$ 29.1 mA/cm2, and an FF of 86%. The GaInP top cell has an efficiency of 16.6% with a ${V_{{\text{oc}}}}$ of 1.377 V, a ${J_{{\text{sc}}}}$ value of 13.6 mA/cm2, and an FF of 88%. Figure 3 shows the external quantum efficiency (EQE) of the dual-junction SC. The overlap of the two curves of GaAs and GaInP implies the response of both the bottom and the top cells. For the GaAs bottom cell, an average value of 90% is obtained. For the GaInP cell, the ${J_{{\text{sc}}}}$ calculated by means of the convolution of the EQE value obtained for the device with the AM 1.5G illumination shows a value of 13.8 mA/cm², which is almost the same as the measured value from the $I$ - $V$ characteristics.

Figure 2. The current–voltage (I–V) characteristics of GaAs, GaInP and GaAs/GaInP solar cells

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Figure 3. External quantum efficiency (EQE) of the dual-junction solar cell.

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The operation of all the solar cell devices includes the processes of photon absorption, carrier separation, transport and collection, therefore many parameters may affect the performance improvement of the device. If only taking into account the material quality, the most important point is to lower the recombination, including the radiative and nonradiative recombination of the material, interface recombination and to increase the carrier mobility. Benefitting from high purity material growth, high performances of the GaAs and GaInP single junction SCs have been obtained. It is important to note that the performance of our GaAs single-junction is comparable to the best efficiency reported at the end of 2010^[14]. In addition, the GaInP single junction SC also reaches a high level. However, the GaAs/GaInP tandem dual-junction SC is almost 3% lower than the best reported value. The short circuit current of the dual-junction solar cell is almost equal to the GaInP single junction. While the open voltage of the tandem SC is only 2.31 V, which is about 100 meV less than the sum of the single top GaInP (1.37 V) and bottom GaAs (1.045 V) SCs. The lower than expected efficiency is due to the decreased open voltage. In contrast to GaAs and GaInP single junction SCs, the performance of the tunnel junction connected to the top and bottom cells is significant to the tandem multi-junction SC. Since the device structure within the dual-junction is almost identical to the respective cell of the top and bottom ones, it is reasonable to blame the decrease of the total voltage drop resulting from the energy loss of the incorporation of the tunnel junction. Figure 4 presents the $J$ - $V$ curve of a separated GaAs/GaAs tunnel junction used in the tandem SC. It is noted that at the working point of current density of 13.6 mA/cm², the voltage drop upon the tunnel junction is less than 10 meV. This voltage drop above the tunnel junction is much less than the 100 meV mentioned above. Therefore, it may not be the main reason for the decreased open voltage of the tandem SC.

Figure 4. The current–voltage (I–V) characteristics of the GaAs tunnel junction

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In the device, an AlGaInP layer is used as the back surface field layer of the GaInP SC, and it plays also a role as the barrier of the tunnel junction. Normally, a highly doped p-n junction is needed to form a tunnel junction. In our case, a highly-doped beryllium GaAs layer (4 $\times$ 10¹⁹ cm^-3) is used for the p-type GaAs forming the tunnel junction. The diffusion of the highly-doped beryllium from the GaAs layer (4 $\times$ 10¹⁹ cm^-3) within the tunnel junction to the BSF AlGaInP layer is inevitable and will strongly degrade its material quality. Beryllium has a larger diffusion coefficient than carbon, which is usually used in MOCVD growth. Figure 5 presents the SIMS profiles of the device structures near the GaInP top cell. The concentration of the beryllium within the GaInP cell is gradually decreased from the AlGaInP. The thickness of GaInP layer where Be concentration decreases gradually is about 200 nm. A significant Beryllium inter-diffusion at the interface will worsen the function of the AlGaInP layer and increase the interface recombination between the GaInP base layer and the AlGaInP layer. It is well known that the open voltage will decrease with increasing recombination rate. As a result, a smaller open voltage is observed for the dual-junction. An optimized growth condition and optimized device structure, such as a dual-layer BSF design, are needed to decrease the beryllium diffusion.

Figure 5. The SIMS profiles of the device structures near the GaInP top cell.

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4. Summary

In summary, owing to the high purity and high material quality grown by MBE, the high performance of a GaInP/GaAs dual-junction SC with an efficiency of 27% has been obtained. The energy loss mechanism of our GaAs/GaInP tandem dual-junction solar cells is discussed. It is demonstrated that the MBE-grown phosphide-contained Ⅲ-Ⅴ compound semiconductor solar cell is very promising for achieving high energy conversion efficiency.

References(14)

References

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