Efficiency improvement in InGaN-based solar cells by indium tin oxide nano dots covered with ITO films

doi:10.1364/OE.20.00A991

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Efficiency improvement in InGaN-based solar cells by indium tin oxide nano dots covered with ITO films

Dong-Ju Seo, Jae-Phil Shim, Sang-Bae Choi, Tae Hoon Seo, Eun-Kyung Suh, and Dong-Seon Lee »View Author Affiliations

Optics Express, Vol. 20, Issue S6, pp. A991-A996 (2012)
http://dx.doi.org/10.1364/OE.20.00A991

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– Abstract
1. Introduction
2. Experiment
3. Theory
4. Results and discussion
5. Conclusion
– References and links

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Abstract

InGaN based MQW solar cells have been fabricated with 4 different transparent top electrode structures: (1)- ITO 200 nm, (2)-ITO nano dots only, (3)-ITO nano dots on ITO 50 nm and (4)-ITO nano dots on ITO 100 nm. The solar cell with the ITO 50 nm on ITO nano dots under AM 1.5 conditions showed the best results: 2.3 V for V_oc, 0.69 mA/cm² for J_sc, 41.8% for peak EQE, and 0.91% for conversion efficiency. Efficiency improvement was possible due to the decreased reflectance achieved by the ITO nano dots covered with an ITO film with optimized thickness.

1. Introduction

Indium Gallium Nitride (InGaN) material is now widely used in various optoelectronic devices including light-emitting diodes (LEDs) [1

S. Nakamura, M. Senoh, N. Iwasa, and S. I. Nagahama, “High power InGaN single quantum well structure blue and violet light emitting diodes,” Appl. Phys. Lett. 67(13), 1868–1870 (1995). [CrossRef]

] and laser diodes (LDs) [2

S. Nakamura, M. Senoh, S. I. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and K. Chocho, “Continuous-wave operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substrates,” Appl. Phys. Lett. 72(16), 2014–2016 (1998). [CrossRef]

]. It is also being studied increasingly as a prospective material for solar cells [3

O. Jani, I. Ferguson, C. Honsberg, and S. Kurtz, “Design and characterization of GaN/InGaN solar cells,” Appl. Phys. Lett. 91(13), 132117 (2007). [CrossRef]

–5

E. Matioli, C. Neufeld, M. Iza, S. C. Cruz, A. A. Al-Heji, X. Chen, R. M. Farrell, St. Keller, St. DenBaars, U. Mishra, S. Nakamura, J. Speck, and C. Weisbuch, “High internal and external quantum efficiency InGaN/GaN solar cells,” Appl. Phys. Lett. 98(2), 021102 (2011). [CrossRef]

]. One of the merits for solar cell applications is that the band gap energy can be engineered from 0.7eV for InN to 3.4eV for GaN by varying the indium composition, which covers most of the solar spectrum from UV to IR [6

J. Wu, W. Walukiewicz, K. M. Yu, J. W. Ager, E. E. Haller, H. Lu, and W. J. Schaff, “Small band gap bowing in In_1-xGa_xN alloys,” Appl. Phys. Lett. 80(25), 4741–4743 (2002). [CrossRef]

]. Despite this potential advantage, conversion efficiency of InGaN based solar cells is still very low. It is essential for better cell efficiency to improve not only the crystalline quality of the epitaxial layers but also the fabrication of the solar cells. Fabrication includes transparent top electrodes and surface texturing which will improve the carrier extraction. Surface texturing is one of the most employed methods to enhance the extraction efficiency in LED fabrication and can be formed on a p-GaN surface [7

S. I. Na, D. S. Han, S. S. Kim, J. H. Lim, J. Y. Kim, and S. J. Park, “Surface texturing of p-GaN layer for efficient GaN LED by maskless selective etching,” Phys. Status Solidi 2(7 c), 2916–2919 (2005). [CrossRef]

]^, on a N-face of GaN [8

T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. Denbaars, and S. Nakamura, “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Appl. Phys. Lett. 84(6), 855–857 (2004). [CrossRef]

], and even on an indium tin oxide (ITO) layer [9

R. H. Horng, C. C. Yang, J. Y. Wu, S. H. Huang, C. E. Lee, and D. S. Wuu, “GaN based light emitting diodes with indium tin oxide texturing window layers using natural lithography,” Appl. Phys. Lett. 86(22), 221101 (2005). [CrossRef]

–11

S. J. Chang, C. F. Shen, W. S. Chen, C. T. Kuo, T. K. Ko, S. C. Shei, and J. K. Sheu, “Nitride based light emitting diodes with indium tin oxide electrode patterned by imprint lithography,” Appl. Phys. Lett. 91(1), 013504 (2007). [CrossRef]

]. Surface texturing methods have also been adopted in InGaN-based solar cells [12

R. H. Horng, S. T. Lin, Y. L. Tsai, M. T. Chu, W. Y. Liao, M. H. Wu, R. M. Lin, and Y. C. Lu, “Improved conversion efficiency of GaN/InGaN thin-film solar cells,” IEEE Electron Device Lett. 30(7), 724–726 (2009). [CrossRef]

,13

S. Y. Bae, J. P. Shim, D. S. Lee, S. R. Jeon, and G. Namkoong, “Improved photovoltaic effects of a vertical-type InGaN/GaN multiple quantum well solar cell,” Jpn. J. Appl. Phys. 50(9), 092301 (2011). [CrossRef]

] and proved to enhance the efficiency. Because, however, texturing by direct etching of p-GaN was known to induce damage [14

X. A. Cao, S. J. Pearton, A. P. Zhang, G. T. Dang, F. Ren, R. J. Shul, L. Zhang, R. Hickman, and J. M. Van Hove, “Electrical effects of plasma damage in p-GaN,” Appl. Phys. Lett. 75(17), 2569–2571 (1999). [CrossRef]

] and resulted in degraded electrical properties, texturing has been studied widely on the ITO layers. ITO is used for visible LEDs as a current spreading layer [15

Y. C. Lin, S. J. Chang, Y. K. Su, T. Y. Tsai, C. S. Chang, S. C. Shei, C. W. Kuo, and S. C. Chen, “InGaN/GaN light emitting diodes with Ni/Au, Ni/ITO and ITO p-type contacts,” Solid-State Electron. 47(5), 849–853 (2003). [CrossRef]

] due to its high transparency in the visible spectrum and low electrical resistivity. For similar reasons, the ITO has also been used in InGaN-based solar cells [16

J. P. Shim, S. R. Jeon, Y. K. Jeong, and D. S. Lee, “Improved efficiency by using transparent contact layers in InGaN based p-i-n solar cells,” IEEE Electron Device Lett. 31(10), 1140–1142 (2010). [CrossRef]

]. It is well-known that ITO thickness optimization is important in LED applications since the reflectance is fluctuated by the variation of the ITO thickness resulting in reduced light extraction at the target wavelength [17

H. Kim, J. S. Horwitz, G. Kushto, A. Pique´, Z. H. Kafafi, C. M. Gilmore, and D. B. Chrisey, “Effect of film thickness on the properties of indium tin oxide thin films,” J. Appl. Phys. 88(10), 6021–6025 (2000). [CrossRef]

]. In solar cell applications, the ITO thickness should also be optimized since its variation also causes reflectance fluctuation [18

J. P. Shim, M. H. Choe, S. R. Jeon, D. J. Seo, T. H. Lee, and D. S. Lee, “InGaN-based p-i-n solar cells with graphene electrodes,” Appl. Phys. Express 4(5), 052302 (2011). [CrossRef]

] resulting in reduced absorption of sunlight. Reflectance of the ITO can be further reduced by combining the thickness optimization with the ITO texturing. ITO texturing by wet etching or dry etching, however, was also revealed to increase series resistance of the ITO film. Moreover, those wet- and dry-etched ITO films turned out to be just rough surfaces without any control over film thickness [19

S. M. Huang, Y. Yao, C. Jin, Z. Sun, and Z. J. Dong, “Enhancement of the light output of GaN-based light-emitting diodes using surface-textured indium-tin-oxide transparent ohmic contacts,” Displays 29(3), 254–259 (2008). [CrossRef]

,20

D. S. Leem, T. H. Lee, and T. Y. Seong, “Enhancement of the light output of GaN-based light-emitting diodes with surface-patterned ITO electrodes by maskless wet-etching,” Solid-State Electron. 51(5), 793–796 (2007). [CrossRef]

]. In this work, we report a new way of texturing by deposition of thickness-optimized ITO films on the ITO nano dots, so that the films can have minimum reflectance as well as no electrical degradation originated from the ITO etching process.

2. Experiment

The solar cell structures were grown using metal organic chemical vapor deposition (MOCVD). Trimethylgallium, ammonia and trimethylindium were used as Ga, N, and In sources, respectively. Si doped 2.2 um n-GaN was grown at 1040 °C on an undoped GaN/Al₂O₃ template. Multi quantum well (MQW) pairs of In_0.27Ga_0.73N/GaN layers were grown at 810 °C, and a Mg-doped 100 nm p-GaN layer were grown at 1020 °C sequentially. Doping concentration of p-GaN and n-GaN was 10¹⁷ cm⁻³ and 10¹⁸ cm⁻³, respectively. 1 mm 1 mm solar cell devices with the ITO film on the ITO nano dots were fabricated by a sequential process as shown in Fig. 1(a) . ITO 100 nm is deposited by an electron beam evaporator and is annealed in an ambient with a mixture of N₂ and O₂ at 600 °C for 60s in a rapid thermal annealing (RTA) chamber, etched by a diluted HCl (HCl: H₂O = 1: 3) for 5 s to form the ITO nano dots on the p-GaN surface, and annealed at 600 °C for 5 min again. The nano dots formed this way are shown in Fig. 1(b). The ITO nano dots are about 80 nm in diameter and cover about 30% of the surface. Next, the 2nd ITO film is deposited as a current spreading layer on the ITO nano dots to form a network. Then it is annealed at 600 °C for 5 min. ITO 50 nm and 100 nm were deposited for comparison as shown in Fig. 1(c) and 1(d) and we can see that the surfaces are as rough as the ITO nano dots. Finally, Cr/Au (30 nm/300 nm) is deposited for metal contact.

Fig. 1 (Color online) (a) Schematic representation of the key steps for fabricating In_0.27Ga_0.73N/GaN MQW based Solar Cells with the ITO films on the ITO nano dots. SEM images of the (b) ITO nano dots only, (c) ITO 50 nm on the ITO nano dots and (d) ITO 100 nm on the ITO nano dots.

3. Theory

The thickness of the ITO film is very important and is controlled by adjusting the deposition time. Reflectance can be calculated and minimized at the target wavelength by varying the ITO thickness according to the following reflectance equation at normal incidence.

R = \frac{n_{1}^{2} {(n_{0} - n_{s})}^{2} \cos^{2} δ + {(n_{0} n_{s} - n_{1}^{2})}^{2} \sin^{2} δ}{n_{1}^{2} {(n_{0} + n_{s})}^{2} \cos^{2} δ + {(n_{0} n_{s} + n_{1}^{2})}^{2} \sin^{2} δ}

(1)

δ = \frac{2 π}{λ_{0}} (n_{1} t)

(2)

n_s, n₁ and n₀ are the refractive indices of each materials as shown in the inset in Fig. 2(a) . δ is the phase difference between the light reflected on the n_s material (red arrows in the inset of Fig. 2(a)) and the light reflected on the n₁ material (blue arrows). t is thickness of the n₁ film and λ₀ is the wavelength of light at normal incidence. In this case, constructive or destructive interference occurs between lights with different phases. That makes reflectance fluctuation at the thin film dependent on the thickness of the sandwiched film between the n_s and n₀ material. This phenomenon is caused by the interference between the reflected lights from the ITO and the GaN surface. Figure 2(a) shows the reflectance of the ITO on GaN. The lines with filled markers are measured data, and unfilled markers are simulated data by finite-difference time-domain (FDTD) method using Eq. (1). Simulated reflectance fluctuation tendency is very close to the measured data and the small deviation is thought to come from the surface morphology, absorption and thickness variation by the E-beam evaporator. Contour plot in Fig. 2(b) was obtained as a result and we can see that the 50 nm ITO has a lower reflectance than the 100 nm ITO on GaN.

Fig. 2 (Color online) (a) Measured reflectance data in ITO on GaN compared with calculated data for ITO 50nm, 100nm and 200nm by the FDTD method, respectively. The inset shows a schematic illustration of reflected light at the thin films. (b) Contour plot of the variation of reflectance spectra as a function of the ITO thickness on GaN substrate.

Based on the previous simulation, structures having ITO nano dots covered by ITO films were made and reflectance was measured on them at normal incidence. Results are shown in Fig. 3 for 4 different structures and are drawn on the right side. In the same plot, the ITO 50 nm and 100 nm without nano dots are also plotted for comparison. It clearly shows that (3)-ITO 50 nm on the ITO nano dots has the lowest reflectance and (4)-ITO 100 nm on the ITO nano dots has the next lowest reflectance over the entire wavelength range. Not only the lower reflectance of the ITO 50 nm, but also the surface roughening due to the nano dots [21

T. H. Seo, K. J. Lee, T. S. Oh, Y. S. Lee, H. Jeong, A. H. Park, H. Kim, Y. R. Choi, E. K. Suh, T. V. Cuong, V. H. Pham, J. S. Chung, and E. J. Kim, “Graphene network on indium tin oxide nanodot nodes for transparent and current spreading electrode in InGaN/GaN light emitting diode,” Appl. Phys. Lett. 98(25), 251114 (2011). [CrossRef]

] contribute to the lowest reflectance of the ITO 50 nm on the ITO nano dots.

Fig. 3 (Color online) Measured reflectance spectra of the fabricated structures at normal incidence and schematic representation of fabricated structures.

4. Results and discussion

Then, in order to verify the enhancement through reducing the reflectance, solar cells were fabricated with the (1)-200 nm ITO film, (2)-ITO nano dots only, (3)-ITO 50 nm on the ITO nano dots and (4)-ITO 100 nm on the ITO nano dots, respectively. Figure 4(a) shows current density versus voltage (J-V) characteristics of the samples under AM 1.5 illumination. The obtained open-circuit voltages (V_oc) are approximately 2.3 V for all samples and a large V_oc value had identical characteristics as the InGaN solar cells that were in previous publications [22

B. R. Jampana, A. G. Melton, M. Jamil, N. N. Faleev, R. L. Opila, I. T. Ferguson, and C. B. Honsberg, “Design and realization of wide-band-gap(~2.67ev) InGaN p-n junction solar cell,” IEEE Electron Device Lett. 31(1), 32–34 (2010). [CrossRef]

,23

J. K. Sheu, C. C. Yang, S. J. Tu, K. H. Chang, M. L. Lee, W. C. Lai, and L. C. Peng, “Demonstration of GaN-based solar cells with GaN/InGaN superlattice absorption layers,” IEEE Electron Device Lett. 30(3), 225–227 (2009). [CrossRef]

]. Fill factors (F.F.) are 58.2%, 58.5%, 58.1% and 58.1%, corresponding to (1)-200 nm ITO film, (2)-ITO nano dots only, (3)-ITO 50 nm on the ITO nano dots and (4)-ITO 100 nm on the ITO nano dots, respectively. F.F. of these samples are very similar in spite of the degraded curvature in the J-V curve, which is known to come from doping variation and/or polarization in the InGaN MQW structure [24

C. J. Neufeld, S. C. Cruz, R. M. Farrell, M. Iza, J. R. Lang, S. Keller, S. Nakamura, S. P. Denbaars, J. S. Speck, and U. K. Mishra, “Effect of doping and polarization on carrier collection in InGaN quantum well solar cells,” Appl. Phys. Lett. 98(24), 243507 (2011). [CrossRef]

]. While the V_oc and the F.F. are almost the same for all samples, we can see a considerable difference in the short-circuit current density (J_sc) which is influenced by the light absorption. J_sc values were 0.56 mA/cm² for (1)-ITO 200 nm (ref.), 0.53 mA/cm² for (2)-ITO nano dots only, 0.69 mA/cm² for (3)-ITO 50 nm on the ITO nano dots and 0.63 mA/cm² for (4)-ITO 100 nm on the ITO nano dots. The sample with the ITO 100 nm on the ITO nano dots showed ~12% improvement in J_sc compared to the ref. and ITO 50 nm on the ITO nano dots showed a 22% improvement as shown in Fig. 4(a). It indicates that the samples with the ITO film on the ITO nano dots have higher sunlight absorption.

Fig. 4 (Color online) (a) J-V curves and (b) EQE characteristics for Solar Cells with planar ITO 200 nm (ref.), ITO nano dots only, ITO 50 nm on the ITO nano dots and ITO 100 nm on the ITO nano dots for transparent current spreading layer, respectively.

External quantum efficiency (EQE) curves in Fig. 4(b) also surely show an enhancement of carrier extraction due to increased sunlight absorption. The peak intensity of the EQE is approximately 5.19% higher in the sample with ITO 100 nm on the ITO nano dots, and 14.05% higher in the sample with ITO 50 nm on the ITO nano dots than the conventional ITO film sample. It is due to the reduced reflectance coming from the surface roughness and is consistent with the result in Fig. 3 that ITO 50 nm has approximately a 10% lower reflectance than ITO 100 nm on GaN. All the measured solar cell parameters are summarized and compared in Table 1 . Cell efficiency is highest, 0.91%, in the sample with ITO 50 nm on the ITO nano dots and 21.76% higher than the reference.

Table 1 Solar Cell Parameters

Sample name	V_oc (V)	F.F. (%)	J_sc (mA/cm²)	Peak EQE (%)	Efficiency (%)
(1)-ITO 200 nm (ref.)	2.3	58.2	0.56	36.7	0.74
(2)-ITO nano dots	2.3	58.5	0.53	32.6	0.71
(3)-ITO 50 nm on ITO nano dots	2.3	58.1	0.69	41.8	0.91
(4)-ITO 100 nm on ITO nano dots	2.3	58.1	0.63	38.6	0.82

Figure 5 shows dark current versus voltage (I-V) curves of the 4 samples and the inset shows electroluminescence (EL) at 20 mA. We can see that the sample with nano dots only has higher series resistance than the others and this is due to lack of current spreading. This also resulted in highly reduced EL intensity as in the Fig. 5 inset. While the voltage at 20mA was 3.2 V for the ITO dots only sample, all the other samples having ITO films showed almost the same value, 2.9 V. This indicates that the electrical properties such as resistivity were recovered to the same level as the ITO 200 nm after the 2nd deposition of the ITO film. EL spectra intensity at I = 20 mA was enhanced by 18% in ITO 100 nm on the ITO nano dots, and 40% in ITO 50 nm on the ITO nano dots compared to the planar ITO 200 nm sample. The EL enhancement comes from the surface roughening effect by the ITO nano dots resulting in better light extraction, and at the same time results in the current spreading being maintained by the 2nd ITO deposition. This result shows that the ITO 50 nm on the ITO nano dots can also act very nicely as a top electrode with better extraction efficiency in LEDs.

Fig. 5 (Color online) Dark I-V curve for Solar Cells with (1)-200 nm ITO film only, (2)-ITO nano dots only, (3)-ITO 50 nm on the ITO nano dots and (4)-ITO 100 nm on the ITO nano dots, respectively. The inset shows typical EL spectra at 20 mA.

5. Conclusion

In conclusion, InGaN-based MQW solar cells have been fabricated with 4 different transparent top electrode structures to enhance the power conversion efficiency by reducing reflectance. The solar cell with the ITO 50 nm on the ITO nano dots under the AM 1.5 condition showed the following superior results: 2.3 V for Voc, 0.69 A/cm² for Jsc, 41.8% for peak EQE, and 0.91% for conversion efficiency. Efficiency improvement was possible due to the decreased reflectance achieved by the ITO nano dots covered with a thickness optimized ITO film. This study indicates that the thickness optimized ITO film on the ITO nano dots can improve the power conversion efficiency without electrical degradations in solar cells.

Acknowledgments

This work was supported by the Ministry of Education, the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST, No. 20120002256).

References and links

1.	S. Nakamura, M. Senoh, N. Iwasa, and S. I. Nagahama, “High power InGaN single quantum well structure blue and violet light emitting diodes,” Appl. Phys. Lett. 67(13), 1868–1870 (1995). [CrossRef]
2.	S. Nakamura, M. Senoh, S. I. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and K. Chocho, “Continuous-wave operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substrates,” Appl. Phys. Lett. 72(16), 2014–2016 (1998). [CrossRef]
3.	O. Jani, I. Ferguson, C. Honsberg, and S. Kurtz, “Design and characterization of GaN/InGaN solar cells,” Appl. Phys. Lett. 91(13), 132117 (2007). [CrossRef]
4.	C. J. Neufeld, N. G. Toledo, S. C. Cruz, M. Iza, S. P. DenBaars, and U. K. Mishra, “High quantum efficiency InGaN/GaN solar cells with 2.95 eV band gap,” Appl. Phys. Lett. 93(14), 143502 (2008). [CrossRef]
5.	E. Matioli, C. Neufeld, M. Iza, S. C. Cruz, A. A. Al-Heji, X. Chen, R. M. Farrell, St. Keller, St. DenBaars, U. Mishra, S. Nakamura, J. Speck, and C. Weisbuch, “High internal and external quantum efficiency InGaN/GaN solar cells,” Appl. Phys. Lett. 98(2), 021102 (2011). [CrossRef]
6.	J. Wu, W. Walukiewicz, K. M. Yu, J. W. Ager, E. E. Haller, H. Lu, and W. J. Schaff, “Small band gap bowing in In_1-xGa_xN alloys,” Appl. Phys. Lett. 80(25), 4741–4743 (2002). [CrossRef]
7.	S. I. Na, D. S. Han, S. S. Kim, J. H. Lim, J. Y. Kim, and S. J. Park, “Surface texturing of p-GaN layer for efficient GaN LED by maskless selective etching,” Phys. Status Solidi 2(7 c), 2916–2919 (2005). [CrossRef]
8.	T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. Denbaars, and S. Nakamura, “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Appl. Phys. Lett. 84(6), 855–857 (2004). [CrossRef]
9.	R. H. Horng, C. C. Yang, J. Y. Wu, S. H. Huang, C. E. Lee, and D. S. Wuu, “GaN based light emitting diodes with indium tin oxide texturing window layers using natural lithography,” Appl. Phys. Lett. 86(22), 221101 (2005). [CrossRef]
10.	D. S. Leem, J. H. Cho, C. S. Sone, Y. J. Park, and T. Y. Seong, “Light-output enhancement of GaN-based light-emitting diodes by using hole-patterned transparent indium tin oxide electrodes,” J. Appl. Phys. 98(7), 076107 (2005). [CrossRef]
11.	S. J. Chang, C. F. Shen, W. S. Chen, C. T. Kuo, T. K. Ko, S. C. Shei, and J. K. Sheu, “Nitride based light emitting diodes with indium tin oxide electrode patterned by imprint lithography,” Appl. Phys. Lett. 91(1), 013504 (2007). [CrossRef]
12.	R. H. Horng, S. T. Lin, Y. L. Tsai, M. T. Chu, W. Y. Liao, M. H. Wu, R. M. Lin, and Y. C. Lu, “Improved conversion efficiency of GaN/InGaN thin-film solar cells,” IEEE Electron Device Lett. 30(7), 724–726 (2009). [CrossRef]
13.	S. Y. Bae, J. P. Shim, D. S. Lee, S. R. Jeon, and G. Namkoong, “Improved photovoltaic effects of a vertical-type InGaN/GaN multiple quantum well solar cell,” Jpn. J. Appl. Phys. 50(9), 092301 (2011). [CrossRef]
14.	X. A. Cao, S. J. Pearton, A. P. Zhang, G. T. Dang, F. Ren, R. J. Shul, L. Zhang, R. Hickman, and J. M. Van Hove, “Electrical effects of plasma damage in p-GaN,” Appl. Phys. Lett. 75(17), 2569–2571 (1999). [CrossRef]
15.	Y. C. Lin, S. J. Chang, Y. K. Su, T. Y. Tsai, C. S. Chang, S. C. Shei, C. W. Kuo, and S. C. Chen, “InGaN/GaN light emitting diodes with Ni/Au, Ni/ITO and ITO p-type contacts,” Solid-State Electron. 47(5), 849–853 (2003). [CrossRef]
16.	J. P. Shim, S. R. Jeon, Y. K. Jeong, and D. S. Lee, “Improved efficiency by using transparent contact layers in InGaN based p-i-n solar cells,” IEEE Electron Device Lett. 31(10), 1140–1142 (2010). [CrossRef]
17.	H. Kim, J. S. Horwitz, G. Kushto, A. Pique´, Z. H. Kafafi, C. M. Gilmore, and D. B. Chrisey, “Effect of film thickness on the properties of indium tin oxide thin films,” J. Appl. Phys. 88(10), 6021–6025 (2000). [CrossRef]
18.	J. P. Shim, M. H. Choe, S. R. Jeon, D. J. Seo, T. H. Lee, and D. S. Lee, “InGaN-based p-i-n solar cells with graphene electrodes,” Appl. Phys. Express 4(5), 052302 (2011). [CrossRef]
19.	S. M. Huang, Y. Yao, C. Jin, Z. Sun, and Z. J. Dong, “Enhancement of the light output of GaN-based light-emitting diodes using surface-textured indium-tin-oxide transparent ohmic contacts,” Displays 29(3), 254–259 (2008). [CrossRef]
20.	D. S. Leem, T. H. Lee, and T. Y. Seong, “Enhancement of the light output of GaN-based light-emitting diodes with surface-patterned ITO electrodes by maskless wet-etching,” Solid-State Electron. 51(5), 793–796 (2007). [CrossRef]
21.	T. H. Seo, K. J. Lee, T. S. Oh, Y. S. Lee, H. Jeong, A. H. Park, H. Kim, Y. R. Choi, E. K. Suh, T. V. Cuong, V. H. Pham, J. S. Chung, and E. J. Kim, “Graphene network on indium tin oxide nanodot nodes for transparent and current spreading electrode in InGaN/GaN light emitting diode,” Appl. Phys. Lett. 98(25), 251114 (2011). [CrossRef]
22.	B. R. Jampana, A. G. Melton, M. Jamil, N. N. Faleev, R. L. Opila, I. T. Ferguson, and C. B. Honsberg, “Design and realization of wide-band-gap(~2.67ev) InGaN p-n junction solar cell,” IEEE Electron Device Lett. 31(1), 32–34 (2010). [CrossRef]
23.	J. K. Sheu, C. C. Yang, S. J. Tu, K. H. Chang, M. L. Lee, W. C. Lai, and L. C. Peng, “Demonstration of GaN-based solar cells with GaN/InGaN superlattice absorption layers,” IEEE Electron Device Lett. 30(3), 225–227 (2009). [CrossRef]
24.	C. J. Neufeld, S. C. Cruz, R. M. Farrell, M. Iza, J. R. Lang, S. Keller, S. Nakamura, S. P. Denbaars, J. S. Speck, and U. K. Mishra, “Effect of doping and polarization on carrier collection in InGaN quantum well solar cells,” Appl. Phys. Lett. 98(24), 243507 (2011). [CrossRef]

OCIS Codes
(230.3670) Optical devices : Light-emitting diodes
(310.6860) Thin films : Thin films, optical properties
(350.6050) Other areas of optics : Solar energy
(220.4241) Optical design and fabrication : Nanostructure fabrication
(310.6628) Thin films : Subwavelength structures, nanostructures
(240.6645) Optics at surfaces : Surface differential reflectance

ToC Category:
Photovoltaics

History
Original Manuscript: July 26, 2012
Revised Manuscript: September 3, 2012
Manuscript Accepted: October 7, 2012
Published: November 1, 2012

Citation
Dong-Ju Seo, Jae-Phil Shim, Sang-Bae Choi, Tae Hoon Seo, Eun-Kyung Suh, and Dong-Seon Lee, "Efficiency improvement in InGaN-based solar cells by indium tin oxide nano dots covered with ITO films," Opt. Express 20, A991-A996 (2012)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-20-S6-A991

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References

S. Nakamura, M. Senoh, N. Iwasa, and S. I. Nagahama, “High power InGaN single quantum well structure blue and violet light emitting diodes,” Appl. Phys. Lett.67(13), 1868–1870 (1995). [CrossRef]
S. Nakamura, M. Senoh, S. I. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and K. Chocho, “Continuous-wave operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substrates,” Appl. Phys. Lett.72(16), 2014–2016 (1998). [CrossRef]
O. Jani, I. Ferguson, C. Honsberg, and S. Kurtz, “Design and characterization of GaN/InGaN solar cells,” Appl. Phys. Lett.91(13), 132117 (2007). [CrossRef]
C. J. Neufeld, N. G. Toledo, S. C. Cruz, M. Iza, S. P. DenBaars, and U. K. Mishra, “High quantum efficiency InGaN/GaN solar cells with 2.95 eV band gap,” Appl. Phys. Lett.93(14), 143502 (2008). [CrossRef]
E. Matioli, C. Neufeld, M. Iza, S. C. Cruz, A. A. Al-Heji, X. Chen, R. M. Farrell, St. Keller, St. DenBaars, U. Mishra, S. Nakamura, J. Speck, and C. Weisbuch, “High internal and external quantum efficiency InGaN/GaN solar cells,” Appl. Phys. Lett.98(2), 021102 (2011). [CrossRef]
J. Wu, W. Walukiewicz, K. M. Yu, J. W. Ager, E. E. Haller, H. Lu, and W. J. Schaff, “Small band gap bowing in In1-xGaxN alloys,” Appl. Phys. Lett.80(25), 4741–4743 (2002). [CrossRef]
S. I. Na, D. S. Han, S. S. Kim, J. H. Lim, J. Y. Kim, and S. J. Park, “Surface texturing of p-GaN layer for efficient GaN LED by maskless selective etching,” Phys. Status Solidi2(7c), 2916–2919 (2005). [CrossRef]
T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. Denbaars, and S. Nakamura, “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Appl. Phys. Lett.84(6), 855–857 (2004). [CrossRef]
R. H. Horng, C. C. Yang, J. Y. Wu, S. H. Huang, C. E. Lee, and D. S. Wuu, “GaN based light emitting diodes with indium tin oxide texturing window layers using natural lithography,” Appl. Phys. Lett.86(22), 221101 (2005). [CrossRef]
D. S. Leem, J. H. Cho, C. S. Sone, Y. J. Park, and T. Y. Seong, “Light-output enhancement of GaN-based light-emitting diodes by using hole-patterned transparent indium tin oxide electrodes,” J. Appl. Phys.98(7), 076107 (2005). [CrossRef]
S. J. Chang, C. F. Shen, W. S. Chen, C. T. Kuo, T. K. Ko, S. C. Shei, and J. K. Sheu, “Nitride based light emitting diodes with indium tin oxide electrode patterned by imprint lithography,” Appl. Phys. Lett.91(1), 013504 (2007). [CrossRef]
R. H. Horng, S. T. Lin, Y. L. Tsai, M. T. Chu, W. Y. Liao, M. H. Wu, R. M. Lin, and Y. C. Lu, “Improved conversion efficiency of GaN/InGaN thin-film solar cells,” IEEE Electron Device Lett.30(7), 724–726 (2009). [CrossRef]
S. Y. Bae, J. P. Shim, D. S. Lee, S. R. Jeon, and G. Namkoong, “Improved photovoltaic effects of a vertical-type InGaN/GaN multiple quantum well solar cell,” Jpn. J. Appl. Phys.50(9), 092301 (2011). [CrossRef]
X. A. Cao, S. J. Pearton, A. P. Zhang, G. T. Dang, F. Ren, R. J. Shul, L. Zhang, R. Hickman, and J. M. Van Hove, “Electrical effects of plasma damage in p-GaN,” Appl. Phys. Lett.75(17), 2569–2571 (1999). [CrossRef]
Y. C. Lin, S. J. Chang, Y. K. Su, T. Y. Tsai, C. S. Chang, S. C. Shei, C. W. Kuo, and S. C. Chen, “InGaN/GaN light emitting diodes with Ni/Au, Ni/ITO and ITO p-type contacts,” Solid-State Electron.47(5), 849–853 (2003). [CrossRef]
J. P. Shim, S. R. Jeon, Y. K. Jeong, and D. S. Lee, “Improved efficiency by using transparent contact layers in InGaN based p-i-n solar cells,” IEEE Electron Device Lett.31(10), 1140–1142 (2010). [CrossRef]
H. Kim, J. S. Horwitz, G. Kushto, A. Pique´, Z. H. Kafafi, C. M. Gilmore, and D. B. Chrisey, “Effect of film thickness on the properties of indium tin oxide thin films,” J. Appl. Phys.88(10), 6021–6025 (2000). [CrossRef]
J. P. Shim, M. H. Choe, S. R. Jeon, D. J. Seo, T. H. Lee, and D. S. Lee, “InGaN-based p-i-n solar cells with graphene electrodes,” Appl. Phys. Express4(5), 052302 (2011). [CrossRef]
S. M. Huang, Y. Yao, C. Jin, Z. Sun, and Z. J. Dong, “Enhancement of the light output of GaN-based light-emitting diodes using surface-textured indium-tin-oxide transparent ohmic contacts,” Displays29(3), 254–259 (2008). [CrossRef]
D. S. Leem, T. H. Lee, and T. Y. Seong, “Enhancement of the light output of GaN-based light-emitting diodes with surface-patterned ITO electrodes by maskless wet-etching,” Solid-State Electron.51(5), 793–796 (2007). [CrossRef]
T. H. Seo, K. J. Lee, T. S. Oh, Y. S. Lee, H. Jeong, A. H. Park, H. Kim, Y. R. Choi, E. K. Suh, T. V. Cuong, V. H. Pham, J. S. Chung, and E. J. Kim, “Graphene network on indium tin oxide nanodot nodes for transparent and current spreading electrode in InGaN/GaN light emitting diode,” Appl. Phys. Lett.98(25), 251114 (2011). [CrossRef]
B. R. Jampana, A. G. Melton, M. Jamil, N. N. Faleev, R. L. Opila, I. T. Ferguson, and C. B. Honsberg, “Design and realization of wide-band-gap(~2.67ev) InGaN p-n junction solar cell,” IEEE Electron Device Lett.31(1), 32–34 (2010). [CrossRef]
J. K. Sheu, C. C. Yang, S. J. Tu, K. H. Chang, M. L. Lee, W. C. Lai, and L. C. Peng, “Demonstration of GaN-based solar cells with GaN/InGaN superlattice absorption layers,” IEEE Electron Device Lett.30(3), 225–227 (2009). [CrossRef]
C. J. Neufeld, S. C. Cruz, R. M. Farrell, M. Iza, J. R. Lang, S. Keller, S. Nakamura, S. P. Denbaars, J. S. Speck, and U. K. Mishra, “Effect of doping and polarization on carrier collection in InGaN quantum well solar cells,” Appl. Phys. Lett.98(24), 243507 (2011). [CrossRef]

Ager, J. W.
Al-Heji, A. A.
Bae, S. Y.
Cao, X. A.
Chang, C. S.
Chang, K. H.
Chang, S. J.
Chen, S. C.
Chen, W. S.
Chen, X.
Cho, J. H.
Chocho, K.
Choe, M. H.
Choi, Y. R.
Chrisey, D. B.
Chu, M. T.
Chung, J. S.
Cruz, S. C.
Cuong, T. V.
Dang, G. T.
Denbaars, S. P.
DenBaars, St.
Dong, Z. J.
Faleev, N. N.
Farrell, R. M.
Ferguson, I.
Ferguson, I. T.
Fujii, T.
Gao, Y.
Gilmore, C. M.
Haller, E. E.
Han, D. S.
Hickman, R.
Honsberg, C.
Honsberg, C. B.
Horng, R. H.
Horwitz, J. S.
Hu, E. L.
Huang, S. H.
Huang, S. M.
Iwasa, N.
Iza, M.
Jamil, M.
Jampana, B. R.
Jani, O.
Jeon, S. R.
Jeong, H.
Jeong, Y. K.
Jin, C.
Kafafi, Z. H.
Keller, S.
Keller, St.
Kim, E. J.
Kim, H.
Kim, J. Y.
Kim, S. S.
Kiyoku, H.
Ko, T. K.
Kozaki, T.
Kuo, C. T.
Kuo, C. W.
Kurtz, S.
Kushto, G.
Lai, W. C.
Lang, J. R.
Lee, C. E.
Lee, D. S.
Lee, K. J.
Lee, M. L.
Lee, T. H.
Lee, Y. S.
Leem, D. S.
Liao, W. Y.
Lim, J. H.
Lin, R. M.
Lin, S. T.
Lin, Y. C.
Lu, H.
Lu, Y. C.
Matioli, E.
Matsushita, T.
Melton, A. G.
Mishra, U.
Mishra, U. K.
Na, S. I.
Nagahama, S. I.
Nakamura, S.
Namkoong, G.
Neufeld, C.
Neufeld, C. J.
Oh, T. S.
Opila, R. L.
Park, A. H.
Park, S. J.
Park, Y. J.
Pearton, S. J.
Peng, L. C.
Pham, V. H.
Pique´, A.
Ren, F.
Sano, M.
Schaff, W. J.
Senoh, M.
Seo, D. J.
Seo, T. H.
Seong, T. Y.
Sharma, R.
Shei, S. C.
Shen, C. F.
Sheu, J. K.
Shim, J. P.
Shul, R. J.
Sone, C. S.
Speck, J.
Speck, J. S.
Su, Y. K.
Sugimoto, Y.
Suh, E. K.
Sun, Z.
Toledo, N. G.
Tsai, T. Y.
Tsai, Y. L.
Tu, S. J.
Umemoto, H.
Van Hove, J. M.
Walukiewicz, W.
Weisbuch, C.
Wu, J.
Wu, J. Y.
Wu, M. H.
Wuu, D. S.
Yamada, T.
Yang, C. C.
Yao, Y.
Yu, K. M.
Zhang, A. P.
Zhang, L.

Appl. Phys. Express

J. P. Shim, M. H. Choe, S. R. Jeon, D. J. Seo, T. H. Lee, and D. S. Lee, “InGaN-based p-i-n solar cells with graphene electrodes,” Appl. Phys. Express4(5), 052302 (2011). [CrossRef]

Appl. Phys. Lett.

T. H. Seo, K. J. Lee, T. S. Oh, Y. S. Lee, H. Jeong, A. H. Park, H. Kim, Y. R. Choi, E. K. Suh, T. V. Cuong, V. H. Pham, J. S. Chung, and E. J. Kim, “Graphene network on indium tin oxide nanodot nodes for transparent and current spreading electrode in InGaN/GaN light emitting diode,” Appl. Phys. Lett.98(25), 251114 (2011). [CrossRef]
X. A. Cao, S. J. Pearton, A. P. Zhang, G. T. Dang, F. Ren, R. J. Shul, L. Zhang, R. Hickman, and J. M. Van Hove, “Electrical effects of plasma damage in p-GaN,” Appl. Phys. Lett.75(17), 2569–2571 (1999). [CrossRef]
S. Nakamura, M. Senoh, N. Iwasa, and S. I. Nagahama, “High power InGaN single quantum well structure blue and violet light emitting diodes,” Appl. Phys. Lett.67(13), 1868–1870 (1995). [CrossRef]
S. Nakamura, M. Senoh, S. I. Nagahama, N. Iwasa, T. Yamada, T. Matsushita, H. Kiyoku, Y. Sugimoto, T. Kozaki, H. Umemoto, M. Sano, and K. Chocho, “Continuous-wave operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substrates,” Appl. Phys. Lett.72(16), 2014–2016 (1998). [CrossRef]
O. Jani, I. Ferguson, C. Honsberg, and S. Kurtz, “Design and characterization of GaN/InGaN solar cells,” Appl. Phys. Lett.91(13), 132117 (2007). [CrossRef]
C. J. Neufeld, N. G. Toledo, S. C. Cruz, M. Iza, S. P. DenBaars, and U. K. Mishra, “High quantum efficiency InGaN/GaN solar cells with 2.95 eV band gap,” Appl. Phys. Lett.93(14), 143502 (2008). [CrossRef]
E. Matioli, C. Neufeld, M. Iza, S. C. Cruz, A. A. Al-Heji, X. Chen, R. M. Farrell, St. Keller, St. DenBaars, U. Mishra, S. Nakamura, J. Speck, and C. Weisbuch, “High internal and external quantum efficiency InGaN/GaN solar cells,” Appl. Phys. Lett.98(2), 021102 (2011). [CrossRef]
J. Wu, W. Walukiewicz, K. M. Yu, J. W. Ager, E. E. Haller, H. Lu, and W. J. Schaff, “Small band gap bowing in In1-xGaxN alloys,” Appl. Phys. Lett.80(25), 4741–4743 (2002). [CrossRef]
T. Fujii, Y. Gao, R. Sharma, E. L. Hu, S. P. Denbaars, and S. Nakamura, “Increase in the extraction efficiency of GaN-based light-emitting diodes via surface roughening,” Appl. Phys. Lett.84(6), 855–857 (2004). [CrossRef]
R. H. Horng, C. C. Yang, J. Y. Wu, S. H. Huang, C. E. Lee, and D. S. Wuu, “GaN based light emitting diodes with indium tin oxide texturing window layers using natural lithography,” Appl. Phys. Lett.86(22), 221101 (2005). [CrossRef]
S. J. Chang, C. F. Shen, W. S. Chen, C. T. Kuo, T. K. Ko, S. C. Shei, and J. K. Sheu, “Nitride based light emitting diodes with indium tin oxide electrode patterned by imprint lithography,” Appl. Phys. Lett.91(1), 013504 (2007). [CrossRef]
C. J. Neufeld, S. C. Cruz, R. M. Farrell, M. Iza, J. R. Lang, S. Keller, S. Nakamura, S. P. Denbaars, J. S. Speck, and U. K. Mishra, “Effect of doping and polarization on carrier collection in InGaN quantum well solar cells,” Appl. Phys. Lett.98(24), 243507 (2011). [CrossRef]

Displays

S. M. Huang, Y. Yao, C. Jin, Z. Sun, and Z. J. Dong, “Enhancement of the light output of GaN-based light-emitting diodes using surface-textured indium-tin-oxide transparent ohmic contacts,” Displays29(3), 254–259 (2008). [CrossRef]

IEEE Electron Device Lett.

B. R. Jampana, A. G. Melton, M. Jamil, N. N. Faleev, R. L. Opila, I. T. Ferguson, and C. B. Honsberg, “Design and realization of wide-band-gap(~2.67ev) InGaN p-n junction solar cell,” IEEE Electron Device Lett.31(1), 32–34 (2010). [CrossRef]
J. K. Sheu, C. C. Yang, S. J. Tu, K. H. Chang, M. L. Lee, W. C. Lai, and L. C. Peng, “Demonstration of GaN-based solar cells with GaN/InGaN superlattice absorption layers,” IEEE Electron Device Lett.30(3), 225–227 (2009). [CrossRef]
J. P. Shim, S. R. Jeon, Y. K. Jeong, and D. S. Lee, “Improved efficiency by using transparent contact layers in InGaN based p-i-n solar cells,” IEEE Electron Device Lett.31(10), 1140–1142 (2010). [CrossRef]
R. H. Horng, S. T. Lin, Y. L. Tsai, M. T. Chu, W. Y. Liao, M. H. Wu, R. M. Lin, and Y. C. Lu, “Improved conversion efficiency of GaN/InGaN thin-film solar cells,” IEEE Electron Device Lett.30(7), 724–726 (2009). [CrossRef]

J. Appl. Phys.

D. S. Leem, J. H. Cho, C. S. Sone, Y. J. Park, and T. Y. Seong, “Light-output enhancement of GaN-based light-emitting diodes by using hole-patterned transparent indium tin oxide electrodes,” J. Appl. Phys.98(7), 076107 (2005). [CrossRef]
H. Kim, J. S. Horwitz, G. Kushto, A. Pique´, Z. H. Kafafi, C. M. Gilmore, and D. B. Chrisey, “Effect of film thickness on the properties of indium tin oxide thin films,” J. Appl. Phys.88(10), 6021–6025 (2000). [CrossRef]

Jpn. J. Appl. Phys.

S. Y. Bae, J. P. Shim, D. S. Lee, S. R. Jeon, and G. Namkoong, “Improved photovoltaic effects of a vertical-type InGaN/GaN multiple quantum well solar cell,” Jpn. J. Appl. Phys.50(9), 092301 (2011). [CrossRef]

Phys. Status Solidi

S. I. Na, D. S. Han, S. S. Kim, J. H. Lim, J. Y. Kim, and S. J. Park, “Surface texturing of p-GaN layer for efficient GaN LED by maskless selective etching,” Phys. Status Solidi2(7c), 2916–2919 (2005). [CrossRef]

Solid-State Electron.

Y. C. Lin, S. J. Chang, Y. K. Su, T. Y. Tsai, C. S. Chang, S. C. Shei, C. W. Kuo, and S. C. Chen, “InGaN/GaN light emitting diodes with Ni/Au, Ni/ITO and ITO p-type contacts,” Solid-State Electron.47(5), 849–853 (2003). [CrossRef]
D. S. Leem, T. H. Lee, and T. Y. Seong, “Enhancement of the light output of GaN-based light-emitting diodes with surface-patterned ITO electrodes by maskless wet-etching,” Solid-State Electron.51(5), 793–796 (2007). [CrossRef]

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