MBE-grown InGaP/GaAs/InGaAsP triple junction solar cells fabricated by advanced bonding technique

Takeyoshi Sugaya1, Kikuo Makita1, Hidenori Mizuno1, Akihiro Takeda2, Toru Mochizuki2, Ryuji Oshima1, Koji Matsubara1, Yoshinobu Okano2, and Shigeru Niki1

1National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba 305-8568, Japan

2Tokyo City University, 1-28-1 Tamazutsumi Setagaya, Tokyo 158-8557, Japan

Abstract — We report mechanically stacked InGaP (1.9 eV) /GaAs (1.42 eV) /InGaAsP (1.0 eV) triple junction solar cells grown using solid source molecular beam epitaxy (SS-MBE). High quality InGaP/GaAs tandem top and InGaAsP bottom cells are connected by advanced bonding technique using Pd nanoparticle arrays. The InGaAsP bottom cell has a high open circuit voltage (Voc) of 0.49 V, which indicates that high quality InGaAsP solar cells can be fabricated using SS-MBE. A fabricated triple junction solar cell has a high efficiency of 25.6% with high Voc of 2.66 V. We fabricate InGaAsP solar cells and InGaP/GaAs/InGaAsP triple junction solar cells by SS-MBE for the first time.

Index Terms — molecular beam epitaxy, triple junction solar cells, InGaP, InGaAsP, advanced bonding technique.

I. INTRODUCTION

Multijunction solar cells have been studied to realize solar cells with ultra high efficiency. A fabrication method for multijunction solar cells is a monolithic epitaxial growth technique on GaAs or Ge substrates. InGaP/GaAs/Ge lattice-matched triple junction solar cells have been commercialized for space and concentrator applications. InGaP/GaAs/InGaAs triple junction solar cells grown monolithically on GaAs substrates have high efficiencies of 37.7 and 44.4% under AM1.5 and concentrator conditions, respectively [1]. This triple-junction solar cell utilizes a metamorphic buffer growth, which is the very difficult technique and restrict the versatilities in materials choices and cell combinations. Another fabrication method for multijunction solar cells is semiconductor direct bonding technique between two different substrates. 4- and 5-junction solar cells have been realized by using direct bonding technique between GaAs and InP wafers, by which GaAs-based 2- and 3-junction and InP-based 2-junction solar cells are connected. 4- and 5-junction solar cells have the highest reported efficiencies of 44.7 and 38.8 % under concentrator [2] and AM1.5 [3] conditions, respectively. Above multijunction structures are grown by metal organic chemical vapor deposition (MOCVD) because materials are needed that include phosphorus such as InGaP and InGaAsP. Solid-source molecular beam epitaxy (SS-MBE) has the potential to grow high quality InGaP-based materials, because it proceeds under an ultra-high vacuum condition and uses ultra-high purity metal sources. However, there have been few reports on InGaP and multijunction solar

cells fabricated using SS-MBE because the phosphorus based materials are very difficult to grow. Although InGaP/GaAs tandem solar cells have been fabricated using gas-source MBE, expensive gas sources are needed [4] Recently, InGaP/GaAs/GaInNAsSb multijunction solar cells with a high efficiency of 44% and grown using MBE have been achieved under concentrator conditions [5]. However, the growth technique and procedure have not been described in detail.

In our earlier work, we reported a technique for growing InGaP material using solid source MBE [6-10]. We reported the characteristics of InGaP and InGaP/GaAs tandem solar cells [6, 7]. The highest performance of InGaP solar cells was obtained at a growth temperature of 480°C and a growth rate of 1.0 μm/h. The performance of InGaP/GaAs tandem solar cells is greatly changed by changing the carrier concentration in a tunnel junction between the top InGaP and bottom GaAs cells. We also reported In0.48Ga0.52P-based InGaAs quantum dot (QD) superlattices [8, 9] and solar cells [10], in which both the short-circuit current density (Jsc) and the conversion efficiency increase as the number of InGaAs QD layers increases. We have also proposed a new semiconductor bonding technology for mechanically stacked multi-junction solar cells by using conductive nanoparticle alignment [11]. This technique is very attractive for interconnecting different kinds of solar cells. In this paper, we report the characteristics of InGaAsP solar cells fabricated using SS-MBE. Moreover, we report mechanically stacked InGaP (1.9 eV) /GaAs (1.42 eV) /InGaAsP (1.0 eV) triple junction solar cells. GaAs-based InGaP/GaAs top and InP-based InGaAsP bottom cells grown by SS-MBE are mechanically stacked for the first time.

YII. EXPERIMENTS

We grew In0.775Ga0.225As0.489P0.511 layers with the energy gap of 1.0 eV on GaAs (100) substrates at growth rates 1.0 μm/h. A 400 nm GaAs buffer layer was grown at 570°C prior to the In0.775Ga0.225As0.489P0.511 layer growth. The growth temperature of In0.775Ga0.225As0.489P0.511 layer was varied from 380 to 420°C. The In and Ga fluxes were constant at 1.0 × 10-6 and 1.7 × 10-7 Torr, respectively. The P/As2 flux ratio was constant at 3.7 during the InGaAsP growth. Photoluminescence (PL) measurement was performed to study the optical properties of the InGaAsP films.

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Fig. 1. Schematic layer structure of an InGaAsP solar cell grown on an n+-InP (001) substrate.

For solar cell applications, InGaAsP p-n junctions were

grown on a p+-InP substrate. The detailed sample structures are shown in Fig. 1. The growth temperature and the growth rate were 400°C and 1μm/h, respectively. The p+-InGaAsP layer thickness was 2 μm and the n+-InP window layer was grown. The n+-InGaAs was grown on the InP window as an electrode contact layer. After the growth, the front electrode was formed using photolithography and a lift-off technique. AuGe/Ni/Au and Ti/Au were used for the front and back electrodes, respectively. The p+-InGaAs contact layer was removed by chemical etching using the front electrode as an etching mask. Anti-reflection coating (ARC) was not employed.

Triple junction solar cells were fabricated using advanced bonding technique [11]. InGaP/GaAs double junction top cells were grown [7] and separated from GaAs substrates by the selective etching called epitaxial lift-off technique. The InGaP/GaAs top cells were connected to InGaAsP bottom cells on the InP substrates through Pd nanoparticle allays as shown in Fig. 2. A conductive Pd nanoparticle alignment was formed on the InGaAsP bottom cell through the use of self-assembled block copolymer (polystylene-block-poly-2-vinylpyridine). The optical absorption loss at the interface was very small because of the extremely thin nanoparticle thickness (~10 nm) and low surface coverage (< 12 %). Therefore, low bonding resistance (~2 Ωcm2) as well as low interfacial optical loss (< 2 %) were possible.

The performance of the InGaP solar cells was measured under standard conditions of AM1.5G, 100 mW/cm2, and 25°C. The external quantum efficiency (EQE) of the solar cells was measured directly under a constant photon irradiation of 1014/cm2.

Fig. 2. Schematic layer structure of an InGaP/GaAs/InGaAsP triple junction solar cell fabricated using Pd nanoparticle arrays.

III. RESULTS AND DISCUSSION

Figure 3 shows the PL spectra of 500 nm-thick InGaAsP epitaxial films measured at room temperature. The growth temperature of the InGaAsP layers was varied from 380 to 420°C. The InGaAsP layer grown at 400°C has a stronger and narrower PL peak. The PL peak energy was 1.00 eV, which corresponds to the In0.775Ga0.225As0.489P0.511 bulk transition lattice-matched to the InP substrates. The slight deviation in PL peaks of the InGaAsP films is due to the fluctuations of P and As fluxes. The high quality InGaAsP films lattice-matched to InP can be grown by the precise control of group-V fluxes using solid-source MBE.

Fig. 3. PL spectra of InGaAsP epitaxial layers with a growth thickness of 500 nm measured at room temperature. The growth temperature was varied from 380 to 420°C.

InP sub.

p+ InP buffer

p+ InGaAsP

n + - InGaAsP :200 nm

p+ - InP :200 nm

AuGe / Ni / Au

Ti / Au

p+ - InGaAsP :200 nm

p- InGaAsP p - InGaAsP :2.0 μm

n+- InGaAsP

n+-InP window n + - InP :50 nm

n + - InGaAs :50 nm

Growth temperature : 400

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InGaAsP bottom : 2 μm1.0 eV

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Back metal contact

GaAs middle : 2.5 μm1.42 eV

Front metal contact

InGaP top : 0.6 μm1.9 eV

Pd nanoparticle

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Fig. 4. EQE spectrum of an InGaAsP solar cell grown at 400°C. Fig. 5. I-V curve of an InGaAsP solar cell grown at 400°C. Figure 4 and 5 show EQE spectrum and I-V curve of an

InGaAsP solar cell grown at 400°C. The EQE spectrum shows the fabrication of an InGaAsP solar cell with the energy gap of 1.0 eV. The InGaAsP solar cell has high open circuit voltage (Voc) of 0.49 V with the conversion efficiency of 7.0 % without ARC, which indicates that the high quality InGaAsP solar cells can be fabricated by using SS-MBE.

An InGaP/GaAs tandem top cell is connected to abovementioned InGaAsP bottom cell by using advanced bonding technique with Pd nanoparticle arrays. Figure 6 shows the I-V characteristics of the fabricated triple junction solar cell with the top tandem and bottom cells, recorded

under AM1.5 solar spectrum illumination. The device parameters of each cell are also summarized. The excellent Voc of 2.66 V is nearly equal to the sum of those of the top and bottom cells, and high fill factor (FF; 0.791) is observed with the triple junction cell. These results indicate the possibility of achieving high-efficiency multijunction solar cells grown by solid-source MBE. Moreover, these results suggested that our bonding method is highly useful for heterogeneous cell combinations of multijunction solar cells.

Fig. 6. I-V characteristics of a triple junction solar cell with its top tandem and bottom cells. The device parameters of each cell are also summarized.

IV. CONCLUSION

In conclusion, we have reported the SS-MBE growth conditions for realizing high quality InGaAsP epitaxial films. The PL measurements indicate that a growth temperature of 400 ºC is suitable for high quality InGaAsP epitaxial growth. The InGaAsP solar cell has an efficiency of 7.0 % with a high Voc of 0.49 V. The InGaAsP solar cell is used as a bottom cell of InGaP/GaAs/InGaAsP triple junction solar cells fabricated using Pd nanoparticle arrays. A triple junction solar cell has a high efficiency of 25.6% with high Voc of 2.66 V, which indicates that SS-MBE has the potential to grow high quality multijunction solar cells including phosphorus based materials such as InGaP and InGaAsP.

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η : 7.0 %Voc : 0.49 VJsc : 24.0 mA/cm2

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Isc(mA/cm2) Voc (V) F. F. η (%)

3J SC 12.1 2.66 0.791 25.6Bottom 24.0 0.49 0.595 7.0Top cell 12.1 2.20 0.794 21.1

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ACKNOWLEDGEMENT

This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) under the Ministry of Economy, Trade and Industry (METI).

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