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Substrate-free large gap InGaN solar cells with bottom reflector
Solid-State Electronics 54 (2010) 541–544
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Substrate-free large gap InGaN solar cells with bottom reflector Chia-Lung Tsai a,b,*, Guan-Shan Liu a, Gong-Cheng Fan a, Yu-Sheng Lee a a b
Department of Electronic Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan, Tao-Yuan 333, Taiwan Green Technology Research Center of Chang Gung University, Tao-Yuan, Taiwan
a r t i c l e
i n f o
Article history: Received 26 August 2009 Received in revised form 2 January 2010 Accepted 21 January 2010 Available online 13 February 2010 The review of this paper was arranged by Prof. E. Calleja Keywords: InGaN Solar cells Substrate-free Bottom reflector
a b s t r a c t In this study, we report on the realization of the In0.085Ga0.915N p–i–n solar cell by low-pressure metalorganic vapor phase epitaxy (MOVPE). The w–2h scans of (0 0 0 2) reflection observation indicate good suppression of phase separation for the p–i–n solar cells with a 150-nm-thick i-In0.085Ga0.915N epilayer. The sharp and narrow signals in the photoluminescence spectra of In0.085Ga0.915N provided evidence for high material quality, even through the epilayer thickness exceeded its critical value. Furthermore, the laser lift-off (LLO) technique is used to fabricate the substrate-free thin-film solar cells (TF-SCs) with a bottom reflector. Although the samples have suffered from thermal-gradient-induced damage during LLO process, the fabricated TF-SCs still exhibit a low forward voltage of 3.3 V at 20 mA along with an ideality factor of 3.1. Finally, since unabsorbed photons can be reflected by the bottom reflector, the TF-SCs show a 13.6% increase in short-circuit current density as compared to their counterparts without a bottom reflector. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, much research effort has been focused on renewable energy, such as wind turbines, geothermal power, hydroelectricity, and biomass fuels [1]. In particular, semiconductor p–n junction solar cell is a promising technology as electricity can be generated directly by photovoltaic effect. In order to enhance the photoresponse, it is desirable to fabricate an all-monolithic multi-junction solar cell with the absorption bandgaps matching the full solar spectrum [2]. Geisz et al. have reported a world record of 40.8% efficiency for an inverted metamorphic Ga0.51In0.49P/In0.04Ga0.96As/In0.37Ga0.63As triple-junction solar cell [3]. Alternatively, the InGaN alloy system is also a candidate suited for realization of high-efficiency solar cells because of its high absorption coefficient, carrier mobility, and radiation tolerance [4–6]. With suitable control of the epilayer thickness and In-content, numerical simulations indicate that the conversion efficiency of the InGaN solar cells with a seven-junction design can be increased up to 46% [7]. However, epitaxial growth of InGaN is a challenging process due to the larger lattice and thermal mismatch between InN and GaN. This would give rise to a large miscibility gap at the usual growth temperature (800 °C) and thus obstruct the feasibility of the thick and In-rich InGaN epilayers [8]. Recently, Pantha et al. reported that the single-phase In-rich InGaN alloys
* Corresponding author. Address: Department of Electronic Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan, Tao-Yuan 333, Taiwan. E-mail address: [email protected] (C.-L. Tsai). 0038-1101/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2010.01.012
could be achieved by using metalorganic chemical vapor deposition at a low growth temperature (<730 °C) [9]. In this article, we shall realize the substrate-free InGaN p–i–n solar cells by low-pressure metalorganic vapor phase epitaxy (MOVPE). Experimentally, the laser lift-off (LLO) technique is used to strip the sapphire substrate of the normal solar cell. In order to increase the photogenerated current, a metallic reflector is deposited onto the bottom of the cells [10]. Since unabsorbed photons could be reflected towards the intrinsic absorption region by the metallic reflector, the thin-film solar cells (TF-SCs) exhibit improved photoresponse as compared to that of the normal solar cells. 2. Experimental The InGaN solar cell structures were grown on c-facet sapphire substrates by low-pressure vertical-flow MOVPE. The methyl-organometallics and ammonia (NH3) were used as feedstock gases for the growth, while bicyclopentadienyl magnesium (Cp2Mg) and silane (SiH4) were the p- and n-type dopants, respectively. Before the intrinsic InGaN absorption layer was grown, a 25-nm-thick lowtemperature (570 °C) GaN layer was first deposited. A 2-lm-thick un-doped GaN along with a 500-nm-thick Si-doped GaN were then sequentially grown at 1060 °C. In this study, a low growth temperature of 730 °C was used for the i-InGaN with a 9% In-content and the epilayer thickness was set to 150 nm. Finally, a 50-nm-thick Mg-doped GaN layer was deposited to terminate the growth procedures.
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After the epitaxial growth, the standard processes including photolithographic, metallization, and dry etching techniques were used to fabricate the InGaN p–i–n solar cells with an opening area of 5 10 4 cm2. The completed solar cells with a sapphire substrate but without the bottom reflector are referred as ‘‘normal solar cells” throughout this article. The manufacturing processes used to realize the TF-SCs is briefly described as below: prior to LLO process, the normal solar cells were bonded onto a temporary Si carrier by a high-temperature wax. Theoretically, the threshold laser fluence required to decompose the GaN into Ga and N2 is about 300 mJ/cm2 [11]. Taking the propagation loss and interface reflections into account, the GaN-based epitaxial structure can be easily separated from the sapphire substrate by using a KrF excimer laser (LPX 210, 248-nm, 25 ns) with an energy fluence of 800 mJ/cm2. Fig. 1a shows a cross-sectional SEM image of the thin-film devices after the LLO process. At this stage, the solar cell was mounted with p-side facing down, supported by a temporary holder. Finally, the residual metallic Ga was removed by diluted HCl solution. In order to improve the photoresponse of the TFSCs, a metallic Ag layer with the reflectivity of 96% at a wavelength above 400 nm was coated onto the un-doped GaN layer to act as the bottom reflector. After dicing, the TF-SC chips were mounted
(a)
Wax
Thin-film solar cell
(b)
Light input ITO
p-contact
again onto an AlN submount and the temporary Si carrier was removed by dissolving the wax adhesive. The schematic diagram of a completed TF-SC is shown in Fig. 1b. High-resolution X-ray diffraction (HRXRD) was used to characterize the material quality and structural parameters of the asgrown samples. During the temperature-dependent photoluminescence (PL) measurements, the samples were excited by a 30-mW continuous-wave (cw) He–Cd laser (k = 325 nm) and placed in the Oxford cryostat to vary the operating temperatures from 90 to 300 K. The luminescence spectra observed from the sample were dispersed using a monochromator and detected by a photon multiplier employing a standard lock-in technique. Further, the photovoltaic properties of the fabricated solar cells were evaluated from the illuminated current–voltage (I–V) measurements using an AM 1.5G solar simulator (Yamashita Denso, YSS-50A) with a Keithley 2410 Sourcemeter. 3. Results and discussion The structural properties of the as-grown samples can be clarified by HRXRD. Fig. 2 shows HRXRD w–2h scans of (0 0 0 2) reflection measured from the InGaN p–i–n solar cells. The strongest peak located at 34.56° corresponds to the GaN buffer layer. As shown in Fig. 2, only one intense diffraction peak relative to the GaN (0 0 0 2) reflection can be observed, indicating good suppression of phase separation in the InGaN epilayer. The full widths at half maximum (FWHM) of the h–2h curve is estimated as 24 and 205 arcsec for the GaN and InGaN layers; thereby, providing further evidence of good crystalline quality for the as-grown samples. The In-content of InGaN is evaluated as about 8.5% by using X-ray simulation program. Fig. 3 shows the PL spectra measured at 300 K for the InGaN p– i–n solar cells. The main emission peak at 413 nm (2.99 eV) is in good agreement with the HRXRD analysis for the InGaN with an Incontent of 8.5%. Although the thickness of i-In0.085Ga0.915N epilayer exceeds its critical value, the fabricated solar cells still exhibit a strong and sharp PL peak [12]. The reason for this may be attributed to the presence of the misfit-induced strain between the InGaN layer and the underlying template layer so as to alleviate the phase separation and to ensure reasonable crystalline quality [9]. Furthermore, the broadening of the PL spectra in yellow-band regions (550 nm) is caused by the impurity-related defects within the n-GaN underlayer that might have slightly decreased the photogenerated current [13]. Fig. 4a shows the dark forward I–V characteristics of the TF-SC and the normal solar cell, respectively. The forward voltage at a
8
10
p-GaN n-contact
n-GaN u-GaN Reflector Reflector(Ag) (Ag)
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InGaN p-i-n solar cell
GaN
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10
5
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3
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1
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0
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Fig. 1. (a) Cross-sectional SEM image of the thin-film devices after the LLO process. At this stage, the solar cell was mounted p-side facing down on a temporary holder, which acts as a mechanical support for the thin membranes. (b) Schematic diagram of a substrate-free InGaN thin-film solar cell. The opening window for light incident is about 5 10 4 cm2.
30.0 30.5 31.0 31.5 32.0 32.5 33.0 33.5 34.0 34.5 35.0 35.5 36.0 36.5 37.0 37.5 38.0
2θ (deg) Fig. 2. HRXRD w–2h scans of (0 0 0 2) reflection measured from the InGaN p–i–n solar cells.
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InGaN p-i-n solar cell In0.085Ga0.915N
PL Intensity (arb. units)
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GaN 0.0 350
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Wavelength (nm) Fig. 3. The PL spectra measured at 300 K for the InGaN p–i–n solar cells.
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0.1 0.01
Diode Current (A)
1E-3 1E-4 1E-5
drive current of 20 mA and the series resistance are evaluated as 3.3 V and 6.3 X, 3.0 V and 5.7 X for the TF-SC and the normal solar cell, respectively. It is observed that the ideality factor (g) extracted from the slope of the semi-logarithmic I–V curve of the TF-SC is slightly larger than that of the normal solar cell, i.e., 3.1 versus 2.9. Lee et al. reported that the samples with the LLO process would suffer a higher thermal gradient in the peripheral area of the laser spot, and thus the crystalline quality and optical properties become worse at these regions [14]. Experimentally, similar observation was also made in our devices, resulting in a reduced shunt resistance and an increased value of g. In addition to the existence of a leakage current at the interface of the heterojunctions, the higher value of g (>2) could be attributed to the deep-level-assisted tunneling current caused by the structural defects, Mg–H complexes, and impurity-induced vacancies, etc. [15]. In fact, more defects in the materials imply the minority carrier lifetime is reduced, which will deteriorate device performance significantly [16]. Thereby, optimizing the growth procedure of the epitaxial layers and the LLO processes would be beneficial for the fabrication of the nitride-based TF-SCs. The illuminated I–V characteristics of the TF-SC and the normal solar cell are shown in Fig. 4b, respectively. The photovoltaic characteristics of the fabricated solar cells measured under one sun AM 1.5G spectrum are summarized in Table 1. The open circuit voltages (Voc) of the TF-SC is about 1.79 V, comparable to that of the normal solar cell grown directly on sapphire; however, this value is less than that of the previous reports [4–6]. In order to clarify the homogeneity in InGaN alloy, the temperature-dependent PL measurements were performed. Fig. 5 shows the observed PL spectra as a function of temperature for the InGaN p–i–n solar cells. As increasing the temperature from 90 K, the InGaN epilayers exhibit the blueshift behaviors in PL spectra due to the limited carrier life-
1E-6
Table 1 Comparison of the photoresponse under one sun AM 1.5G spectrum for the TF-SC and the normal solar cell, respectively.
1E-7
TF-SC (η = 3.1) Normal solar cell (η = 2.9)
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0.44 1.80 63 0.50
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InGaN p-i-n solar cell 95 K
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Wavelength (nm) Fig. 4. (a) The dark forward current–voltage (I–V) characteristics of the TF-SC and the normal solar cell, respectively. The diagram is plotted on a semi-log scale. (b) Illuminated I–V characteristics of the TF-SC and the normal solar cell, respectively.
Fig. 5. The PL spectra as a function of temperature for the InGaN p–i–n solar cells. The solid circles are used as a guide to the eyes.
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time for radiative recombination in the potential minima. Further, the increased PL peak energy by filling of the band-tail states reaches a maximum at near 100 K, and then the temperaturedependent bandgap shrinkage occurs till room temperature is reached. Feng et al. also reported that the InGaN alloy would suffer compositional fluctuation even for In mole fraction less than 6% [17]. The formation of the nanometer-scale In-rich clusters in the InGaN layer is detrimental to the cell’s performance because they cause bandgap energy fluctuation and thus results in the decrease of Voc. Besides, carrier recombination through these localized states is also responsible for the reduced short-circuit current density (JSC) [18]. In spite of the photoresponse limited by a low In-content of the thin InGaN epilayer, the TF-SC still exhibits a 13.6% increase in JSC as compared to their counterpart without a bottom reflector due to an increased absorption probability for the unused photons reflected by the bottom metallic reflector [10]. Although the available efficiency is not high (0.57%), the photovoltaic characteristics of the InGaN solar cell with a bottom reflector are indeed superior to those of the normal solar cells. Further improvements in device characteristic might be achievable using strained InGaN/GaN multiple quantum wells to increase the In-content and suppress phase separation in the InGaN layer [19]. Besides, the micro- or nano-textured patterns can be integrated into the GaN buffer layer so as to increase the amount of scattered light from the bottom of the cells [20]. 4. Conclusions We have demonstrated the fabrication and characterization of the substrate-free InGaN p–i–n solar cells with a bottom reflector. Experimentally, the unoptimized LLO process causes an additional laser-induced thermal damage in the fabricated TF-SCs and thus results in an increased ideality factor. In addition to the existence of material defects in epitaxial layers, the inhomogeneity in In0.085Ga0.915N alloy also contributes to the reduced Voc. However,
the TF-SCs still exhibit an enhanced photocurrent density and better conversion efficiency as compared to the normal solar cells without a bottom reflector. Acknowledgement The authors would like to acknowledge the financial support from National Science Council (NSC 98-2221-E-182-060). References [1] Li B, Wang L, Kang B, Wang P, Qiu Y. Sol Energy Mater Sol Cells 2006;90:549. [2] Henry CH. J Appl Phys 1980;51:4494. [3] Geisz JF, Friedman DJ, Ward JS, Duda A, Olavarria WJ, Moriarty TE, et al. Appl Phys Lett 2008;93:123505. [4] Jani O, Ferguson I, Honsberg C, Kurtz S. Appl Phys Lett 2007;91:132117. [5] Zheng X, Horng RH, Wuu DS, Chu MT, Liao WY, Wu MH, et al. Appl Phys Lett 2008;93:261108. [6] Neufeld CJ, Toledo NG, Cruz SC, Iza M, DenBaars SP, Mishra UK. Appl Phys Lett 2008;93:143502. [7] Islam MR, Rayhan MA, Hossain ME, Bhuiyan AG, Islam MR, Yamamoto A. In: Proc of 4th international conference on electrical and computer engineering (ICECE), vol. 241; 2006. p. 19–21. [8] Hu IH, Stringfellow GB. Appl Phys Lett 1996;69:2701. [9] Pantha BN, Li J, Lin JY, Jiang HX. Appl Phys Lett 2008;93:182107. [10] Johnson DC, Ballard I, Barnham KWJ, Bishnell DB, Connolly JP, Lynch MC, et al. Sol Energy Mater Sol Cells 2005;87:169. [11] Chu CF, Lai FI, Chu JT, Yu CC, Lin CF, Kuo HC, et al. J Appl Phys 2004;95:3916. [12] Holec D, Zhang Y, Rao DVS, Kappers MJ, McAleese C, Humphreys CJ. J Appl Phys 2008;104:123514. [13] Jain SC, Willander M, Narayan J, Overstraeten RV. J Appl Phys 2000;87:965. [14] Lee KT, Lee YC, Tu SH, Lin CL, Chen PH, Liu CY, et al. Jpn J Appl Phys 2008;47:930. [15] Cao XA, Stokes EB, Sandvik PM, LeBoeuf SF, Kretchmer J, Walker D. Appl Phys Lett 2002;23:535. [16] Cai XM, Zeng SW, Zhang BP. Appl Phys Lett 2009;95:173504. [17] Feng ZC, Liu W, Chua SJ, Yu JW, Yang CC, Yang TR, et al. Thin Solid Films 2006;498:118. [18] Yam FK, Hassan Z. Superlatt Microstruct 2008;43:1. [19] Dahal R, Pantha B, Li J, Lin JY, Jiang HX. Appl Phys Lett 2009;94:063505. [20] Horng RH, Lin ST, Tsai YL, Chu MT, Liao WY, Wu MH, et al. IEEE Electron Dev Lett 2009;30:724.
Contents lists available at ScienceDirect
Solid-State Electronics journal homepage: www.elsevier.com/locate/sse
Substrate-free large gap InGaN solar cells with bottom reflector Chia-Lung Tsai a,b,*, Guan-Shan Liu a, Gong-Cheng Fan a, Yu-Sheng Lee a a b
Department of Electronic Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan, Tao-Yuan 333, Taiwan Green Technology Research Center of Chang Gung University, Tao-Yuan, Taiwan
a r t i c l e
i n f o
Article history: Received 26 August 2009 Received in revised form 2 January 2010 Accepted 21 January 2010 Available online 13 February 2010 The review of this paper was arranged by Prof. E. Calleja Keywords: InGaN Solar cells Substrate-free Bottom reflector
a b s t r a c t In this study, we report on the realization of the In0.085Ga0.915N p–i–n solar cell by low-pressure metalorganic vapor phase epitaxy (MOVPE). The w–2h scans of (0 0 0 2) reflection observation indicate good suppression of phase separation for the p–i–n solar cells with a 150-nm-thick i-In0.085Ga0.915N epilayer. The sharp and narrow signals in the photoluminescence spectra of In0.085Ga0.915N provided evidence for high material quality, even through the epilayer thickness exceeded its critical value. Furthermore, the laser lift-off (LLO) technique is used to fabricate the substrate-free thin-film solar cells (TF-SCs) with a bottom reflector. Although the samples have suffered from thermal-gradient-induced damage during LLO process, the fabricated TF-SCs still exhibit a low forward voltage of 3.3 V at 20 mA along with an ideality factor of 3.1. Finally, since unabsorbed photons can be reflected by the bottom reflector, the TF-SCs show a 13.6% increase in short-circuit current density as compared to their counterparts without a bottom reflector. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction Nowadays, much research effort has been focused on renewable energy, such as wind turbines, geothermal power, hydroelectricity, and biomass fuels [1]. In particular, semiconductor p–n junction solar cell is a promising technology as electricity can be generated directly by photovoltaic effect. In order to enhance the photoresponse, it is desirable to fabricate an all-monolithic multi-junction solar cell with the absorption bandgaps matching the full solar spectrum [2]. Geisz et al. have reported a world record of 40.8% efficiency for an inverted metamorphic Ga0.51In0.49P/In0.04Ga0.96As/In0.37Ga0.63As triple-junction solar cell [3]. Alternatively, the InGaN alloy system is also a candidate suited for realization of high-efficiency solar cells because of its high absorption coefficient, carrier mobility, and radiation tolerance [4–6]. With suitable control of the epilayer thickness and In-content, numerical simulations indicate that the conversion efficiency of the InGaN solar cells with a seven-junction design can be increased up to 46% [7]. However, epitaxial growth of InGaN is a challenging process due to the larger lattice and thermal mismatch between InN and GaN. This would give rise to a large miscibility gap at the usual growth temperature (800 °C) and thus obstruct the feasibility of the thick and In-rich InGaN epilayers [8]. Recently, Pantha et al. reported that the single-phase In-rich InGaN alloys
* Corresponding author. Address: Department of Electronic Engineering, Chang Gung University, 259 Wen-Hwa 1st Road, Kwei-Shan, Tao-Yuan 333, Taiwan. E-mail address: [email protected] (C.-L. Tsai). 0038-1101/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.sse.2010.01.012
could be achieved by using metalorganic chemical vapor deposition at a low growth temperature (<730 °C) [9]. In this article, we shall realize the substrate-free InGaN p–i–n solar cells by low-pressure metalorganic vapor phase epitaxy (MOVPE). Experimentally, the laser lift-off (LLO) technique is used to strip the sapphire substrate of the normal solar cell. In order to increase the photogenerated current, a metallic reflector is deposited onto the bottom of the cells [10]. Since unabsorbed photons could be reflected towards the intrinsic absorption region by the metallic reflector, the thin-film solar cells (TF-SCs) exhibit improved photoresponse as compared to that of the normal solar cells. 2. Experimental The InGaN solar cell structures were grown on c-facet sapphire substrates by low-pressure vertical-flow MOVPE. The methyl-organometallics and ammonia (NH3) were used as feedstock gases for the growth, while bicyclopentadienyl magnesium (Cp2Mg) and silane (SiH4) were the p- and n-type dopants, respectively. Before the intrinsic InGaN absorption layer was grown, a 25-nm-thick lowtemperature (570 °C) GaN layer was first deposited. A 2-lm-thick un-doped GaN along with a 500-nm-thick Si-doped GaN were then sequentially grown at 1060 °C. In this study, a low growth temperature of 730 °C was used for the i-InGaN with a 9% In-content and the epilayer thickness was set to 150 nm. Finally, a 50-nm-thick Mg-doped GaN layer was deposited to terminate the growth procedures.
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After the epitaxial growth, the standard processes including photolithographic, metallization, and dry etching techniques were used to fabricate the InGaN p–i–n solar cells with an opening area of 5 10 4 cm2. The completed solar cells with a sapphire substrate but without the bottom reflector are referred as ‘‘normal solar cells” throughout this article. The manufacturing processes used to realize the TF-SCs is briefly described as below: prior to LLO process, the normal solar cells were bonded onto a temporary Si carrier by a high-temperature wax. Theoretically, the threshold laser fluence required to decompose the GaN into Ga and N2 is about 300 mJ/cm2 [11]. Taking the propagation loss and interface reflections into account, the GaN-based epitaxial structure can be easily separated from the sapphire substrate by using a KrF excimer laser (LPX 210, 248-nm, 25 ns) with an energy fluence of 800 mJ/cm2. Fig. 1a shows a cross-sectional SEM image of the thin-film devices after the LLO process. At this stage, the solar cell was mounted with p-side facing down, supported by a temporary holder. Finally, the residual metallic Ga was removed by diluted HCl solution. In order to improve the photoresponse of the TFSCs, a metallic Ag layer with the reflectivity of 96% at a wavelength above 400 nm was coated onto the un-doped GaN layer to act as the bottom reflector. After dicing, the TF-SC chips were mounted
(a)
Wax
Thin-film solar cell
(b)
Light input ITO
p-contact
again onto an AlN submount and the temporary Si carrier was removed by dissolving the wax adhesive. The schematic diagram of a completed TF-SC is shown in Fig. 1b. High-resolution X-ray diffraction (HRXRD) was used to characterize the material quality and structural parameters of the asgrown samples. During the temperature-dependent photoluminescence (PL) measurements, the samples were excited by a 30-mW continuous-wave (cw) He–Cd laser (k = 325 nm) and placed in the Oxford cryostat to vary the operating temperatures from 90 to 300 K. The luminescence spectra observed from the sample were dispersed using a monochromator and detected by a photon multiplier employing a standard lock-in technique. Further, the photovoltaic properties of the fabricated solar cells were evaluated from the illuminated current–voltage (I–V) measurements using an AM 1.5G solar simulator (Yamashita Denso, YSS-50A) with a Keithley 2410 Sourcemeter. 3. Results and discussion The structural properties of the as-grown samples can be clarified by HRXRD. Fig. 2 shows HRXRD w–2h scans of (0 0 0 2) reflection measured from the InGaN p–i–n solar cells. The strongest peak located at 34.56° corresponds to the GaN buffer layer. As shown in Fig. 2, only one intense diffraction peak relative to the GaN (0 0 0 2) reflection can be observed, indicating good suppression of phase separation in the InGaN epilayer. The full widths at half maximum (FWHM) of the h–2h curve is estimated as 24 and 205 arcsec for the GaN and InGaN layers; thereby, providing further evidence of good crystalline quality for the as-grown samples. The In-content of InGaN is evaluated as about 8.5% by using X-ray simulation program. Fig. 3 shows the PL spectra measured at 300 K for the InGaN p– i–n solar cells. The main emission peak at 413 nm (2.99 eV) is in good agreement with the HRXRD analysis for the InGaN with an Incontent of 8.5%. Although the thickness of i-In0.085Ga0.915N epilayer exceeds its critical value, the fabricated solar cells still exhibit a strong and sharp PL peak [12]. The reason for this may be attributed to the presence of the misfit-induced strain between the InGaN layer and the underlying template layer so as to alleviate the phase separation and to ensure reasonable crystalline quality [9]. Furthermore, the broadening of the PL spectra in yellow-band regions (550 nm) is caused by the impurity-related defects within the n-GaN underlayer that might have slightly decreased the photogenerated current [13]. Fig. 4a shows the dark forward I–V characteristics of the TF-SC and the normal solar cell, respectively. The forward voltage at a
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p-GaN n-contact
n-GaN u-GaN Reflector Reflector(Ag) (Ag)
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GaN
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In = 8.5% 4
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Fig. 1. (a) Cross-sectional SEM image of the thin-film devices after the LLO process. At this stage, the solar cell was mounted p-side facing down on a temporary holder, which acts as a mechanical support for the thin membranes. (b) Schematic diagram of a substrate-free InGaN thin-film solar cell. The opening window for light incident is about 5 10 4 cm2.
30.0 30.5 31.0 31.5 32.0 32.5 33.0 33.5 34.0 34.5 35.0 35.5 36.0 36.5 37.0 37.5 38.0
2θ (deg) Fig. 2. HRXRD w–2h scans of (0 0 0 2) reflection measured from the InGaN p–i–n solar cells.
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InGaN p-i-n solar cell In0.085Ga0.915N
PL Intensity (arb. units)
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Wavelength (nm) Fig. 3. The PL spectra measured at 300 K for the InGaN p–i–n solar cells.
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Diode Current (A)
1E-3 1E-4 1E-5
drive current of 20 mA and the series resistance are evaluated as 3.3 V and 6.3 X, 3.0 V and 5.7 X for the TF-SC and the normal solar cell, respectively. It is observed that the ideality factor (g) extracted from the slope of the semi-logarithmic I–V curve of the TF-SC is slightly larger than that of the normal solar cell, i.e., 3.1 versus 2.9. Lee et al. reported that the samples with the LLO process would suffer a higher thermal gradient in the peripheral area of the laser spot, and thus the crystalline quality and optical properties become worse at these regions [14]. Experimentally, similar observation was also made in our devices, resulting in a reduced shunt resistance and an increased value of g. In addition to the existence of a leakage current at the interface of the heterojunctions, the higher value of g (>2) could be attributed to the deep-level-assisted tunneling current caused by the structural defects, Mg–H complexes, and impurity-induced vacancies, etc. [15]. In fact, more defects in the materials imply the minority carrier lifetime is reduced, which will deteriorate device performance significantly [16]. Thereby, optimizing the growth procedure of the epitaxial layers and the LLO processes would be beneficial for the fabrication of the nitride-based TF-SCs. The illuminated I–V characteristics of the TF-SC and the normal solar cell are shown in Fig. 4b, respectively. The photovoltaic characteristics of the fabricated solar cells measured under one sun AM 1.5G spectrum are summarized in Table 1. The open circuit voltages (Voc) of the TF-SC is about 1.79 V, comparable to that of the normal solar cell grown directly on sapphire; however, this value is less than that of the previous reports [4–6]. In order to clarify the homogeneity in InGaN alloy, the temperature-dependent PL measurements were performed. Fig. 5 shows the observed PL spectra as a function of temperature for the InGaN p–i–n solar cells. As increasing the temperature from 90 K, the InGaN epilayers exhibit the blueshift behaviors in PL spectra due to the limited carrier life-
1E-6
Table 1 Comparison of the photoresponse under one sun AM 1.5G spectrum for the TF-SC and the normal solar cell, respectively.
1E-7
TF-SC (η = 3.1) Normal solar cell (η = 2.9)
1E-8 1E-9 1E-10 0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
JSC (mA/cm2) Voc (V) Fill factor (%) Efficiency (%)
Normal solar cell
TF-SC
0.44 1.80 63 0.50
0.50 1.79 64 0.57
Voltage (V)
(b)
0.7
2
@ R.T, one sun, AM 1.5 spectrum (100mW/cm )
0.6
InGaN p-i-n solar cell 95 K
100 K
0.5
0.4
0.3
TF-SC Normal solar cell
0.2
120 K
PL Intensity (arb. units)
Current Density (mA/cm 2 )
90 K
140 K
160 K
0.1
200 K 300 K
240 K
0.0 0.0
0.5
1.0
1.5
2.0
2.5
Voltage (V)
370
380
390
400
410
420
430
440
450
460
Wavelength (nm) Fig. 4. (a) The dark forward current–voltage (I–V) characteristics of the TF-SC and the normal solar cell, respectively. The diagram is plotted on a semi-log scale. (b) Illuminated I–V characteristics of the TF-SC and the normal solar cell, respectively.
Fig. 5. The PL spectra as a function of temperature for the InGaN p–i–n solar cells. The solid circles are used as a guide to the eyes.
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time for radiative recombination in the potential minima. Further, the increased PL peak energy by filling of the band-tail states reaches a maximum at near 100 K, and then the temperaturedependent bandgap shrinkage occurs till room temperature is reached. Feng et al. also reported that the InGaN alloy would suffer compositional fluctuation even for In mole fraction less than 6% [17]. The formation of the nanometer-scale In-rich clusters in the InGaN layer is detrimental to the cell’s performance because they cause bandgap energy fluctuation and thus results in the decrease of Voc. Besides, carrier recombination through these localized states is also responsible for the reduced short-circuit current density (JSC) [18]. In spite of the photoresponse limited by a low In-content of the thin InGaN epilayer, the TF-SC still exhibits a 13.6% increase in JSC as compared to their counterpart without a bottom reflector due to an increased absorption probability for the unused photons reflected by the bottom metallic reflector [10]. Although the available efficiency is not high (0.57%), the photovoltaic characteristics of the InGaN solar cell with a bottom reflector are indeed superior to those of the normal solar cells. Further improvements in device characteristic might be achievable using strained InGaN/GaN multiple quantum wells to increase the In-content and suppress phase separation in the InGaN layer [19]. Besides, the micro- or nano-textured patterns can be integrated into the GaN buffer layer so as to increase the amount of scattered light from the bottom of the cells [20]. 4. Conclusions We have demonstrated the fabrication and characterization of the substrate-free InGaN p–i–n solar cells with a bottom reflector. Experimentally, the unoptimized LLO process causes an additional laser-induced thermal damage in the fabricated TF-SCs and thus results in an increased ideality factor. In addition to the existence of material defects in epitaxial layers, the inhomogeneity in In0.085Ga0.915N alloy also contributes to the reduced Voc. However,
the TF-SCs still exhibit an enhanced photocurrent density and better conversion efficiency as compared to the normal solar cells without a bottom reflector. Acknowledgement The authors would like to acknowledge the financial support from National Science Council (NSC 98-2221-E-182-060). References [1] Li B, Wang L, Kang B, Wang P, Qiu Y. Sol Energy Mater Sol Cells 2006;90:549. [2] Henry CH. J Appl Phys 1980;51:4494. [3] Geisz JF, Friedman DJ, Ward JS, Duda A, Olavarria WJ, Moriarty TE, et al. Appl Phys Lett 2008;93:123505. [4] Jani O, Ferguson I, Honsberg C, Kurtz S. Appl Phys Lett 2007;91:132117. [5] Zheng X, Horng RH, Wuu DS, Chu MT, Liao WY, Wu MH, et al. Appl Phys Lett 2008;93:261108. [6] Neufeld CJ, Toledo NG, Cruz SC, Iza M, DenBaars SP, Mishra UK. Appl Phys Lett 2008;93:143502. [7] Islam MR, Rayhan MA, Hossain ME, Bhuiyan AG, Islam MR, Yamamoto A. In: Proc of 4th international conference on electrical and computer engineering (ICECE), vol. 241; 2006. p. 19–21. [8] Hu IH, Stringfellow GB. Appl Phys Lett 1996;69:2701. [9] Pantha BN, Li J, Lin JY, Jiang HX. Appl Phys Lett 2008;93:182107. [10] Johnson DC, Ballard I, Barnham KWJ, Bishnell DB, Connolly JP, Lynch MC, et al. Sol Energy Mater Sol Cells 2005;87:169. [11] Chu CF, Lai FI, Chu JT, Yu CC, Lin CF, Kuo HC, et al. J Appl Phys 2004;95:3916. [12] Holec D, Zhang Y, Rao DVS, Kappers MJ, McAleese C, Humphreys CJ. J Appl Phys 2008;104:123514. [13] Jain SC, Willander M, Narayan J, Overstraeten RV. J Appl Phys 2000;87:965. [14] Lee KT, Lee YC, Tu SH, Lin CL, Chen PH, Liu CY, et al. Jpn J Appl Phys 2008;47:930. [15] Cao XA, Stokes EB, Sandvik PM, LeBoeuf SF, Kretchmer J, Walker D. Appl Phys Lett 2002;23:535. [16] Cai XM, Zeng SW, Zhang BP. Appl Phys Lett 2009;95:173504. [17] Feng ZC, Liu W, Chua SJ, Yu JW, Yang CC, Yang TR, et al. Thin Solid Films 2006;498:118. [18] Yam FK, Hassan Z. Superlatt Microstruct 2008;43:1. [19] Dahal R, Pantha B, Li J, Lin JY, Jiang HX. Appl Phys Lett 2009;94:063505. [20] Horng RH, Lin ST, Tsai YL, Chu MT, Liao WY, Wu MH, et al. IEEE Electron Dev Lett 2009;30:724.