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  • Editor: Bernard Kippelen
  • Vol. 19, Iss. S3 — May. 9, 2011
  • pp: A258–A269
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Glancing angle deposited ITO films for efficiency enhancement of a-Si:H/μc-Si:H tandem thin film solar cells

Jung Woo Leem and Jae Su Yu  »View Author Affiliations


Optics Express, Vol. 19, Issue S3, pp. A258-A269 (2011)
http://dx.doi.org/10.1364/OE.19.00A258


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Abstract

Indium tin oxide (ITO) thin films with relatively high transparency and low absorption are prepared by glancing angle deposition (GLAD) method and their effect on the device performance of a-Si:H/μc-Si:H tandem thin film solar cells is theoretically investigated by applying the experimentally measured physical data of the fabricated films to the simulation parameters. The GLAD of ITO produces inclined porous columnar nanostructures due to the atomic shadowing effect. With increasing the incident flux angle, the columns are increasingly inclined, thus resulting in the improved transmission property as well as the decrease of the refractive index and extinction coefficient because of enhanced porosity within the film. Furthermore, the antireflection characteristics are improved over a wide wavelength range of 300-1100 nm. For a-Si:H/μc-Si:H tandem thin film solar cell structure incorporated with the 0° ITO/80° ITO bi-layer structure, the conversion efficiency (η) of 13.6% is obtained from simulation under AM1.5g illumination, indicating an efficiency improvement compared to the device with the 0° ITO/0° ITO bi-layer structure (i.e. η = 12.58%).

© 2011 OSA

1. Introduction

There has been great interest in the wide bandgap semiconductor of indium tin oxide (ITO) as a transparent conducting electrode for displays, electroluminescence devices, optical sensors, and photovoltaic devices because of its high optical transmittance in the visible region, low electrical resistivity, and stable chemical property [1

1. G. S. Chae, “A modified transparent conducting oxide for flat panel displays only,” Jpn. J. Appl. Phys. 40(Part 1, No. 3A), 1282–1286 (2001). [CrossRef]

3

3. S. Y. Lien, B. R. Wu, J. C. Liu, and D. S. Wuu, “Fabrication and characteristics of n-Si/c-Si/p-Si heterojunction solar cells using hot-wire CVD,” Thin Solid Films 516(5), 747–750 (2008). [CrossRef]

]. For the fabrication of ITO thin films, a variety of methods including e-beam evaporation, chemical vapor deposition, sputtering, sol-gel process, and spray pyrolysis have been employed [4

4. J. George and C. S. Menon, “Electrical and optical properties of electron beam evaporated ITO thin films,” Surf. Coat. Tech. 132(1), 45–48 (2000). [CrossRef]

8

8. E. Benamar, M. Rami, C. Messaoudi, D. Sayah, and A. Ennaoui, “Structural, optical and electrical properties of indium tin oxide thin films prepared by spray pyrolysis,” Sol. Energy Mater. Sol. Cells 56(2), 125–139 (1998).

]. Recently, the glancing angle deposition (GLAD) which allows for control of morphological properties of nanocolumnar films has attracted much attention for device applications [9

9. J. K. Kim, T. Gessmann, E. F. Schubert, J. Q. Xi, H. Luo, J. Cho, C. Sone, and Y. Park, “GaInN light-emitting diode with conductive omnidirectional reflector having a low-refractive-index indium-tin oxide layer,” Appl. Phys. Lett. 88(1), 013501 (2006). [CrossRef]

11

11. J. J. Steele, J. P. Gospodyn, J. C. Sit, and M. J. Brett, “Impact of morphology on high-speed humidity sensor performance,” IEEE Sens. J. 6(1), 24–27 (2006). [CrossRef]

]. Among them, the e-beam evaporation would be desirable for use in the GLAD due to the strong shadowing property. Since the first report of the GLAD in 1886 [12

12. A. Kundt, “Ueber doppelbrechung des lichtes in metallschichten, welche durch zerstäuben einer kathode hergestellt sind,” Ann. Phys. Chem. 263(1), 59–71 (1886). [CrossRef]

], the technique has been demonstrated to deposit porous and sculptured thin films in various materials such as MgF2, SiO2, Si, TiO2, ITO, etc. [13

13. K. Robbie, L. J. Friedrich, S. K. Dew, T. Smy, and M. J. Brett, “Fabrication of thin films with highly porous microstructures,” J. Vac. Sci. Technol. A 13(3), 1032–1035 (1995). [CrossRef]

16

16. Y. Zhong, Y. C. Shin, C. M. Kim, B. G. Lee, E. H. Kim, Y. J. Park, K. M. A. Sobahan, C. K. Hwangbo, Y. P. Lee, and T. G. Kim, “Optical and electrical properties of indium tin oxide thin films with tilted and spiral microstructures prepared by oblique angle deposition,” J. Mater. Res. 23(9), 2500–2505 (2008). [CrossRef]

]. The low-refractive-index (low-n) films prepared by GLAD are very promising for antireflective coatings, reflectors, and optical microresonators to enhance the device performance [17

17. P. Yu, C. H. Chang, C. H. Chiu, C. S. Yang, J. C. Yu, H. C. Kuo, S. H. Hsu, and Y. C. Chang, “Efficiency enhancement of GaAs photovoltaics employing antireflective indium tin oxide nanocolums,” Adv. Mater. (Deerfield Beach Fla.) 21(16), 1618–1621 (2009). [CrossRef]

19

19. Q. Xu, V. R. Almeida, R. R. Panepucci, and M. Lipson, “Experimental demonstration of guiding and confining light in nanometer-size low-refractive-index material,” Opt. Lett. 29(14), 1626–1628 (2004). [CrossRef] [PubMed]

].

On the other hand, the enhancement in broadband light absorption or trapping, which is crucial especially in thin film solar cells, can improve the cell efficiency. In a-Si:H/μc-Si:H tandem thin film solar cells, it is necessary to match the current flowing through the top and bottom cells since the series connection of the tandem structure can limit the performance of a solar cell [20

20. H. Keppner, J. Meier, P. Torres, D. Fischer, and A. Shah, “Microcrystalline silicon and micromorph tandem solar cells,” Appl. Phys., A Mater. Sci. Process. 69(2), 169–177 (1999). [CrossRef]

]. Additionally, transparent conducting oxide (TCO) films with a high transparency as well as a good electrical conductivity, e.g. ITO, are required as a transparent electrode layer. The morphology of ITO films may affect the performance of solar cells since their optical and electrical characteristics strongly depend on the morphological properties. Although there are some studies on the GLAD ITO films [15

15. K. M. Krause, M. T. Taschuk, K. D. Harris, D. A. Rider, N. G. Wakefield, J. C. Sit, J. M. Buriak, M. Thommes, and M. J. Brett, “Surface area characterization of obliquely deposited metal oxide nanostructured thin films,” Langmuir 26(6), 4368–4376 (2010). [CrossRef]

,16

16. Y. Zhong, Y. C. Shin, C. M. Kim, B. G. Lee, E. H. Kim, Y. J. Park, K. M. A. Sobahan, C. K. Hwangbo, Y. P. Lee, and T. G. Kim, “Optical and electrical properties of indium tin oxide thin films with tilted and spiral microstructures prepared by oblique angle deposition,” J. Mater. Res. 23(9), 2500–2505 (2008). [CrossRef]

], very little work has been reported on the use of low-n ITO films in solar cell structures [17

17. P. Yu, C. H. Chang, C. H. Chiu, C. S. Yang, J. C. Yu, H. C. Kuo, S. H. Hsu, and Y. C. Chang, “Efficiency enhancement of GaAs photovoltaics employing antireflective indium tin oxide nanocolums,” Adv. Mater. (Deerfield Beach Fla.) 21(16), 1618–1621 (2009). [CrossRef]

]. Also, numerical device simulation of multi-junction solar cells is required to analyze theoretically their characteristics. The solar cell structures have been often studied using the Silvaco ATLAS device simulator [21

21. S. Michael and A. Bates, “The design and optimization of advanced multijunction solar cells using the Silvaco ATLAS software package,” Sol. Energy Mater. Sol. Cells 87(1-4), 785–794 (2005). [CrossRef]

23

23. S. T. Chang, M. Tang, R. Y. He, W. C. Wang, Z. Pei, and C. Y. Kung, “TCAD simulation of hydrogenated amorphous silicon-carbon/microcrystalline-silicon/hydrogenated amorphous silicon-germanium PIN solar cells,” Thin Solid Films 518(6), S250–S254 (2010). [CrossRef]

]. In this work, we investigated the effect of GLAD ITO films as a TCO layer, which were fabricated at different incident flux angles by using e-beam evaporator, on the cell efficiency of a-Si:H/μc-Si:H tandem thin film solar cells via the current matching. The experimentally obtained physical data of the fabricated GLAD ITO films were applied to the material parameters for the tandem solar cell simulation.

2. Experimental details and design of solar cell structures

2.1. GLAD of ITO films

2.2. Modeling of a-Si:H/μc-Si:H tandem thin film solar cell structures

For numerical modeling and simulation, the experimentally reported a-Si:H/μc-Si:H tandem thin film solar cell structure was employed [24

24. S. Y. Myong, K. Sriprapha, S. Miyajima, M. Konagai, and A. Yamada, “High efficiency protocrystalline silicon/microcrystalline silicon tandem cell with zinc oxide intermediate layer,” Appl. Phys. Lett. 90(26), 263509 (2007). [CrossRef]

]. First, we theoretically performed the optimization of a-Si:H/μc-Si:H tandem thin film solar cells via the current matching between the top and bottom cells under 1-sun AM1.5g (air mass 1.5 global, 100 mW/cm2) of ASTM standard spectrum with the incident light source angle of 90° from the solar cell top layer [25

25. NREL’s Renewable Resource Data Center, http://rredc.nrel.gov/solar/spectra, Accessed 30 Nov. (2010).

]. Then, the simulations of the optimized a-Si:H/μc-Si:H tandem thin film solar cells incorporated with GLAD ITO films as a TCO layer were performed by applying the experimentally measured ITO film results to the simulation parameters. Except for the fabricated ITO layer, the material parameters of the cell structure used in simulation were referred from Silvaco ATLAS [26

26. ATLAS User's Manual, Silvaco international, June (2008).

]. The refractive index and extinction coefficient of constituent materials, i.e. important parameters for the simulation of solar cells, were referred from the refs [27

27. S. Nitta, S. Itoh, M. Tanaka, T. Endo, and A. Hatano, “Optical properties of a-Si:H and a-SixCl1-x:H films prepared by glow-discharge deposition,” Sol. Energy Mater. 8(1-3), 249–257 (1982). [CrossRef]

29

29. SOPRA, http://www.sopra-sa.com, Accessed 1 Dec. (2010).

]. From the simulation results, the device characteristics of the solar cells were obtained.

3. Results and discussion

3.1. Characteristics of GLAD ITO films

Figure 2
Fig. 2 XRD patterns of the ITO films deposited on glass substrate for different incident flux angles.
shows the XRD patterns of the ITO films deposited on glass substrate for different incident flux angles. The diffraction peaks, which are related to the cubic structure of the ITO with preferred orientations in the (211), (222), (400), (444), and (622) planes, were observed at around 2θ = 21.8°, 30.86°, 35.68°, 51.22°, and 60.78°, respectively, for θα = 0°. The positions and shapes of the XRD peaks of GLAD ITO films remain almost the same except for the width of the peaks. As the incident flux angle was increased, the measured diffraction peaks become lower and broader with a slight increase in the full-width at half maximum (FWHM) of the (222) diffraction peak. The FWHM value of the (222) peaks was increased from 0.2° to 0.34° with increasing the incident flux angle, indicating that the degree of crystallinity in the ITO film is gradually decreased for larger inclination angle. This can be explained by the fact that the diffusion of deposited atoms is disturbed even in the post-annealing process due to the shadowing effect during the GLAD. In GLAD, the c-axis of films is deflected from the substrate normal toward the direction of the incident angle. The grain size can be determined from the FWHM of the dominant XRD peak using the well-known Scherrer formula [30

30. M. I. Mendelson, “Average grain size in polycrystalline ceramics,” J. Am. Ceram. Soc. 52(8), 443–446 (1969). [CrossRef]

]. The estimated grain size of the fabricated ITO films was reduced from 41.2 nm to 24.2 nm with increasing the incident flux angle.

Figure 3
Fig. 3 Top-view and cross-sectional SEM images of the deposited ITO films on Si substrate at incident flux angles of (i) θα = 0°, (ii) θα = 40°, (iii) θα = 60°, and (iv) θα = 80°, respectively.
shows the top-view and cross-sectional SEM images of the deposited ITO films on Si substrate at incident flux angles of (i) θα = 0°, (ii) θα = 40°, (iii) θα = 60°, and (iv) θα = 80°, respectively. The thicknesses of deposited ITO films were 200 nm, 195 nm, 182 nm, and 156 nm at incident flux angles of θα = 0°, 40°, 60°, and 80°, respectively. Thus, the deposition rate of ITO films was decreased from 6.67 nm/min at θα = 0° to 5.17 nm/min at θα = 80°. This is caused by the inclined deposition tendency of the film due to the self-shadowing effect and the angular distribution of the deposition rate which follows the cosine-like distribution for the GLAD [31

31. Y. Sato, K. Yanagisawa, N. Oka, S. I. Nakamura, and Y. Shigesato, “Sputter deposition of Al-doped ZnO films with various incident angles,” J. Vac. Sci. Technol. A 27(5), 1166–1171 (2009). [CrossRef]

]. The void spacing between nanocolumns was also increased with increasing the incident flux angle. From the morphological changes, the effective surface area of ITO films is clearly dependent on the tilted angle [32

32. M. Suzuki, T. Ito, and Y. Taga, “Photocatalysis of sculptured thin films of TiO2,” Appl. Phys. Lett. 78(25), 3968–3970 (2001). [CrossRef]

]. As shown in Fig. 3, the increasingly inclined columnar nanostructure of ITO films was observed because of the enhanced self-shadowing effect and limited atom mobility for larger θα during the deposition process. The measured column angles of ITO films were θβ = 14.6 o, 28.2 o, and 45.8° for θα = 40 o, 60 o, and 80 o, respectively, which are smaller than those expected by the tangent rule for small θα or the cosine rule for large θα [33

33. J. M. Nieuwenhuizen and H. B. Haanstra, “Microfractography of thin films,” Philips Tech. Rev. 27, 87–91 (1966).

,34

34. R. N. Tait, T. Smy, and M. J. Brett, “Modelling and characterization of columnar growth in evaporated films,” Thin Solid Films 226(2), 196–201 (1993). [CrossRef]

]. There exists the difference between the values estimated by the empirical equations and the experimentally measured results for the column inclination angles due to the difference in the deposition condition.

Figure 4
Fig. 4 Measured (a) refractive index and (b) extinction coefficient of the GLAD ITO films on Si substrate in the wavelength range of 350-1100 nm. The inset of (a) shows the measured refractive index and the estimated relative porosity of the GLAD ITO films versus incident flux angle at 633 nm. The inset of (b) shows the (α)2 versus plots of GLAD ITO films.
shows the measured (a) refractive index and (b) extinction coefficient of the GLAD ITO films on Si substrate in the wavelength range of 350-1100 nm. The measured refractive index and the estimated relative porosity of the GLAD ITO films as a function of incident flux angle at a wavelength of 633 nm are shown in the inset of Fig. 4(a). As the θα was increased, the refractive index of ITO films was decreased, rapidly above θα = 60°. The refractive index of ITO film was decreased from 2.19 to 1.35 at 350 nm of wavelength and from 1.76 to 1.28 at 1100 nm when the θα was changed from 0° to 80°. This is attributed to the increased porosity within the inclined columnar films by the GLAD. The porosity of the GLAD ITO films can be evaluated from a well-known relationship [9

9. J. K. Kim, T. Gessmann, E. F. Schubert, J. Q. Xi, H. Luo, J. Cho, C. Sone, and Y. Park, “GaInN light-emitting diode with conductive omnidirectional reflector having a low-refractive-index indium-tin oxide layer,” Appl. Phys. Lett. 88(1), 013501 (2006). [CrossRef]

] of the refractive index of the ITO film at θα = 0°, the effective refractive index of the GLAD ITO film (i.e. θα = 40°, 60°, and 80°), and the volume fraction of the air, relative to the deposited film at θα = 0°. The refractive index values of ITO films were determined from the spectroscopic ellipsometry measurements. We assume that the normal deposited ITO film at θα = 0° (i.e. 0° ITO film) has zero porosity. The relative porosity of the GLAD ITO films at λ = 633 nm was increased from 8.03% at θα = 40° to 63.37% at θα = 80°. The extinction coefficient was also decreased with increasing the incident flux angle as shown in Fig. 4(b). This implies that the scattering loss in the porous ITO films is relatively small compared to the intrinsic material absorption. From the extinction coefficient, the optical energy bandgap (Eg) of the films can be estimated. The inset of Fig. 4(b) shows the (α)2 versus plots of GLAD ITO films. The absorption coefficient (α) was evaluated using α = 4πk/λ and (αhν)2 = A(-E g) [35

35. E. Çetinörgü, S. Goldsmith, and R. L. Boxman, “Air annealing effects on the optical properties of ZnO–SnO2 thin films deposited by a filtered vacuum arc deposition system,” Semicond. Sci. Technol. 21(3), 364–369 (2006). [CrossRef]

], where k is the extinction coefficient of the film, λ is the wavelength, A is a constant that depends on the material, and is the photon energy. For GLAD ITO films, the Eg was increased from 3.93 eV at θα = 0° to 4.1 eV at θα = 80°. In this case, the blueshift in the absorption edge was observed because the oxygen content within the GLAD ITO films was increased.

For solar cell applications, the GLAD ITO films were formed on the 0° ITO film because only porous films exhibit a seriously high resistivity and sheet resistance characteristics caused by the enhanced void [16

16. Y. Zhong, Y. C. Shin, C. M. Kim, B. G. Lee, E. H. Kim, Y. J. Park, K. M. A. Sobahan, C. K. Hwangbo, Y. P. Lee, and T. G. Kim, “Optical and electrical properties of indium tin oxide thin films with tilted and spiral microstructures prepared by oblique angle deposition,” J. Mater. Res. 23(9), 2500–2505 (2008). [CrossRef]

]. Figure 6
Fig. 6 (a) Resistivity and sheet resistance and (b) measured reflectance spectra of the 0° ITO (200 nm)/GLAD ITO bi-layer structures as a function of incident flux angle. The insets of (a) show the SEM images (upper) of the 0° ITO/0° ITO and 0° ITO/80° ITO bi-layer structures, respectively, and the carrier concentration and Hall mobility (lower) of the corresponding films at different incident flux angles. The insets of (b) show the simulation model (left) and calculated reflectance spectra (right) of the corresponding film structures.
shows (a) the resistivity and sheet resistance and (b) the measured reflectance spectra of the 0° ITO (200 nm)/GLAD ITO bi-layer structures as a function of incident flux angle. The thicknesses of GLAD ITO films were similar to those of Fig. 3. The insets of Fig. 6(a) shows the SEM images (upper) of the 0° ITO/0° ITO and 0° ITO/80° ITO bi-layer structures, respectively, and the carrier concentration and Hall mobility (lower) of the corresponding films at different incident flux angles. The resistivity and sheet resistance were increased from 4.94 × 10−4 Ω-cm and 12.46 Ω/sq for 0° ITO/0° ITO to 2.06 × 10−3 Ω-cm and 58.91 Ω/sq for 0° ITO/80° ITO, respectively, due to the increased porosity in the inclined structure as the incident flux angle was increased. For the same reason, the carrier concentration was decreased from 3.76 × 1020 cm−3 for 0° ITO/0° ITO to 1.37 × 1020 cm−3 for 0° ITO/80° ITO. With increasing the incident flux angle, the Hall mobility was decreased from 33.5 cm2/V-s for 0° ITO/0° ITO to 18.7 cm2/V-s for 0° ITO/80° ITO. This may be attributed to the increased grain boundary potential because the electron scattering was increased due to the smaller grain size for more inclined columnar film [37

37. X. Xiao, G. Dong, J. Shao, H. He, and Z. Fan, “Optical and electrical properties of SnO2:Sb thin films deposited by oblique angle deposition,” Appl. Surf. Sci. 256(6), 1636–1640 (2010). [CrossRef]

]. As expected, for the 0° ITO/GLAD ITO bi-layer structures, the electrical properties are not much degraded compared to the 0° ITO/0° ITO bi-layer structure, exhibiting the resistivity and sheet resistance values less than 2.06 × 10−3 Ω-cm and 58.91 Ω/sq, respectively.

As shown in Fig. 6(b), the strong oscillations in reflectance spectra are resulting from the light interference effect as mentioned above, indicating high reflectance maxima of > 30%. The average reflectance was decreased from 17.1% for 0° ITO/0° ITO to 14.6% for 0° ITO/80° ITO. This reason is because the refractive index is gradually changed from air (nair = 1) to Si (nSi = 3.89) via the 0° ITO/GLAD ITO bi-layer structure as shown in Fig. 4(a). For the 0° ITO/80° ITO bi-layer structure, the maxima and number of the oscillation in reflectance spectrum were significantly reduced compare to the 0° ITO/0° ITO bi-layer structure, indicating the maximum value of ~26.4%. The simulation model (right) and calculated reflectance spectra (left) of the 0° ITO/GLAD ITO bi-layer structures are shown in the insets of Fig. 6(b). The calculated results were reasonably consistent with the measured results though there is a slight discrepancy at some wavelengths. For the 0° ITO/80° ITO bi-layer structure, the average reflectance of about 13.1% with the maxima of < 25.6% was obtained from the simulation results.

3.2. Simulation of a-Si:H/μc-Si:H tandem thin film solar cells

The external quantum efficiency (EQE) spectra of the optimized a-Si:H/μc-Si:H tandem thin film solar cells with the 0° ITO/GLAD ITO bi-layer structures as a TCO layer under AM1.5g illumination are shown in Fig. 8(b). The integrated EQE spectrum with solar spectrum enables an estimation of the Jsc. The solar cell with the 0° ITO/80° ITO bi-layer structure exhibited overall highest EQE spectrum for both the top and bottom cells. At wavelengths of 400-640 nm, most of the light are absorbed and converted by the top a-Si:H cell, while the light absorption occurs at wavelengths of 640-1100 nm by the bottom μc-Si:H cell. This can be also confirmed by the photogeneration rate, which is generated by the incident light, in the solar cell structure. The photogeneration rates of optimized a-Si:H/μc-Si:H tandem thin film solar cells with the 0° ITO/80° ITO bi-layer structure for incident light of 400 nm and 800 nm wavelengths under AM1.5g illumination are shown in the inset of Fig. 8(a). As shown in EQE spectra, at a wavelength of 400 nm, the photogeneration occurs only in the top a-Si:H cell region. However, at 800 nm, the photogeneration occurs only in the bottom μc-Si:H cell region because of the transparent a-Si:H (1.84 eV) layer to the incident light. The parameters of a-Si:H/μc-Si:H tandem thin film solar cells with 0° ITO/GLAD ITO bi-layer structures at different incident flux angles under AM1.5g illumination are summarized in Table 2

Table 2. Parameters of a-Si:H/μc-Si:H Tandem Thin Film Solar Cells with 0° ITO/GLAD ITO Bi-Layer Structures at Different Incident Flux Angles under AM1.5g Illumination

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.

4. Conclusion

The porous, nanocolumnar ITO films were fabricated by e-beam evaporator using the GLAD method. By using the experimentally measured physical data of the fabricated GLAD ITO films as the simulation parameters, their effect on the device performance of optimized a-Si:H/μc-Si:H tandem thin film solar cells was theoretically investigated. With increasing the incident flux angle (θα), the inclined nanocolumnar ITO films exhibited lower refractive index and extinction coefficient due to the increase of porosity within the film by the shadowing effect during the GLAD process. The optical transmission characteristics were improved in the wavelength range of 280-1100 nm. The electrical properties of the 0° ITO/GLAD ITO bi-layer structures were not seriously degraded. Additionally, the GLAD ITO films enhanced the antireflection property by suppressing the surface reflection. From simulation results, for optimized a-Si:H/μc-Si:H tandem thin film solar cell structures with the 0° ITO/GLAD ITO bi-layer structures, the conversion efficiency was improved from 12.58% at 0° ITO/0° ITO to 13.6% at 0° ITO/80° ITO under AM1.5g illumination. These results may provide a better understanding of Si-based thin film solar cells with the GLAD low-n TCO layer.

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (No. 2010-0016930 and 2010-0025071).

References and links

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E. Benamar, M. Rami, C. Messaoudi, D. Sayah, and A. Ennaoui, “Structural, optical and electrical properties of indium tin oxide thin films prepared by spray pyrolysis,” Sol. Energy Mater. Sol. Cells 56(2), 125–139 (1998).

9.

J. K. Kim, T. Gessmann, E. F. Schubert, J. Q. Xi, H. Luo, J. Cho, C. Sone, and Y. Park, “GaInN light-emitting diode with conductive omnidirectional reflector having a low-refractive-index indium-tin oxide layer,” Appl. Phys. Lett. 88(1), 013501 (2006). [CrossRef]

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A. Kundt, “Ueber doppelbrechung des lichtes in metallschichten, welche durch zerstäuben einer kathode hergestellt sind,” Ann. Phys. Chem. 263(1), 59–71 (1886). [CrossRef]

13.

K. Robbie, L. J. Friedrich, S. K. Dew, T. Smy, and M. J. Brett, “Fabrication of thin films with highly porous microstructures,” J. Vac. Sci. Technol. A 13(3), 1032–1035 (1995). [CrossRef]

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K. M. Krause, M. T. Taschuk, K. D. Harris, D. A. Rider, N. G. Wakefield, J. C. Sit, J. M. Buriak, M. Thommes, and M. J. Brett, “Surface area characterization of obliquely deposited metal oxide nanostructured thin films,” Langmuir 26(6), 4368–4376 (2010). [CrossRef]

16.

Y. Zhong, Y. C. Shin, C. M. Kim, B. G. Lee, E. H. Kim, Y. J. Park, K. M. A. Sobahan, C. K. Hwangbo, Y. P. Lee, and T. G. Kim, “Optical and electrical properties of indium tin oxide thin films with tilted and spiral microstructures prepared by oblique angle deposition,” J. Mater. Res. 23(9), 2500–2505 (2008). [CrossRef]

17.

P. Yu, C. H. Chang, C. H. Chiu, C. S. Yang, J. C. Yu, H. C. Kuo, S. H. Hsu, and Y. C. Chang, “Efficiency enhancement of GaAs photovoltaics employing antireflective indium tin oxide nanocolums,” Adv. Mater. (Deerfield Beach Fla.) 21(16), 1618–1621 (2009). [CrossRef]

18.

M. F. Schubert, J. Q. Xi, J. K. Kim, and E. F. Schubert, “Distributed Bragg reflector consisting of high- and low-refractive-index thin film layers made of the same material,” Appl. Phys. Lett. 90(14), 141115 (2007). [CrossRef]

19.

Q. Xu, V. R. Almeida, R. R. Panepucci, and M. Lipson, “Experimental demonstration of guiding and confining light in nanometer-size low-refractive-index material,” Opt. Lett. 29(14), 1626–1628 (2004). [CrossRef] [PubMed]

20.

H. Keppner, J. Meier, P. Torres, D. Fischer, and A. Shah, “Microcrystalline silicon and micromorph tandem solar cells,” Appl. Phys., A Mater. Sci. Process. 69(2), 169–177 (1999). [CrossRef]

21.

S. Michael and A. Bates, “The design and optimization of advanced multijunction solar cells using the Silvaco ATLAS software package,” Sol. Energy Mater. Sol. Cells 87(1-4), 785–794 (2005). [CrossRef]

22.

M. Baudrit and C. Algora, “Theoretical optimization of GaInP/GaAs dual-junction solar cell: Toward a 36% efficiency at 1000 suns,” Phys. Status Solidi 207(2), 474–478 (2010) (a). [CrossRef]

23.

S. T. Chang, M. Tang, R. Y. He, W. C. Wang, Z. Pei, and C. Y. Kung, “TCAD simulation of hydrogenated amorphous silicon-carbon/microcrystalline-silicon/hydrogenated amorphous silicon-germanium PIN solar cells,” Thin Solid Films 518(6), S250–S254 (2010). [CrossRef]

24.

S. Y. Myong, K. Sriprapha, S. Miyajima, M. Konagai, and A. Yamada, “High efficiency protocrystalline silicon/microcrystalline silicon tandem cell with zinc oxide intermediate layer,” Appl. Phys. Lett. 90(26), 263509 (2007). [CrossRef]

25.

NREL’s Renewable Resource Data Center, http://rredc.nrel.gov/solar/spectra, Accessed 30 Nov. (2010).

26.

ATLAS User's Manual, Silvaco international, June (2008).

27.

S. Nitta, S. Itoh, M. Tanaka, T. Endo, and A. Hatano, “Optical properties of a-Si:H and a-SixCl1-x:H films prepared by glow-discharge deposition,” Sol. Energy Mater. 8(1-3), 249–257 (1982). [CrossRef]

28.

M. Zeman, R. A. C. M. M. van Swaaij, J. W. Metselaar, and R. E. I. Schropp, “Optical modeling of a-Si:H solar cells with rough interfaces: Effect of back contact and interface roughness,” J. Appl. Phys. 88(11), 6436–6443 (2000). [CrossRef]

29.

SOPRA, http://www.sopra-sa.com, Accessed 1 Dec. (2010).

30.

M. I. Mendelson, “Average grain size in polycrystalline ceramics,” J. Am. Ceram. Soc. 52(8), 443–446 (1969). [CrossRef]

31.

Y. Sato, K. Yanagisawa, N. Oka, S. I. Nakamura, and Y. Shigesato, “Sputter deposition of Al-doped ZnO films with various incident angles,” J. Vac. Sci. Technol. A 27(5), 1166–1171 (2009). [CrossRef]

32.

M. Suzuki, T. Ito, and Y. Taga, “Photocatalysis of sculptured thin films of TiO2,” Appl. Phys. Lett. 78(25), 3968–3970 (2001). [CrossRef]

33.

J. M. Nieuwenhuizen and H. B. Haanstra, “Microfractography of thin films,” Philips Tech. Rev. 27, 87–91 (1966).

34.

R. N. Tait, T. Smy, and M. J. Brett, “Modelling and characterization of columnar growth in evaporated films,” Thin Solid Films 226(2), 196–201 (1993). [CrossRef]

35.

E. Çetinörgü, S. Goldsmith, and R. L. Boxman, “Air annealing effects on the optical properties of ZnO–SnO2 thin films deposited by a filtered vacuum arc deposition system,” Semicond. Sci. Technol. 21(3), 364–369 (2006). [CrossRef]

36.

M. G. Moharam, “Coupled-wave analysis of two-dimensional gratings,” Proc. SPIE 883, 8–11 (1988).

37.

X. Xiao, G. Dong, J. Shao, H. He, and Z. Fan, “Optical and electrical properties of SnO2:Sb thin films deposited by oblique angle deposition,” Appl. Surf. Sci. 256(6), 1636–1640 (2010). [CrossRef]

38.

J. W. Leem, Y. T. Lee, and J. S. Yu, “Optimum design of InGaP/GaAs dual-junction solar cells with different tunnel diodes,” Opt. Quantum Electron. 41(8), 605–612 (2009). [CrossRef]

39.

B. Sang, K. Dairiki, A. Yamada, and M. Konagai, “High-efficiency amorphous silicon solar cells with ZnO as front contact,” Jpn. J. Appl. Phys. 38(Part 1, No. 9A), 4983–4988 (1999). [CrossRef]

40.

Y. Huang, S. Dai, S. Chen, C. Zhang, Y. Sui, S. Xiao, and L. Hu, “Theoretical modeling of the series resistance effect on dye-sensitized solar cell performance,” Appl. Phys. Lett. 95(24), 243503 (2009). [CrossRef]

OCIS Codes
(040.5350) Detectors : Photovoltaic
(310.1210) Thin films : Antireflection coatings
(310.1860) Thin films : Deposition and fabrication
(220.4241) Optical design and fabrication : Nanostructure fabrication
(310.7005) Thin films : Transparent conductive coatings

ToC Category:
Photovoltaics

History
Original Manuscript: January 5, 2011
Revised Manuscript: March 5, 2011
Manuscript Accepted: March 9, 2011
Published: March 30, 2011

Citation
Jung Woo Leem and Jae Su Yu, "Glancing angle deposited ITO films for efficiency enhancement of a-Si:H/μc-Si:H tandem thin film solar cells," Opt. Express 19, A258-A269 (2011)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-19-S3-A258


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References

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  14. D. X. Ye, T. Karabacak, R. C. Picu, G. C. Wang, and T. M. Lu, “Uniform Si nanostructures grown by oblique angle deposition with substrate swing rotation,” Nanotechnology 16(9), 1717–1723 (2005). [CrossRef]
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  16. Y. Zhong, Y. C. Shin, C. M. Kim, B. G. Lee, E. H. Kim, Y. J. Park, K. M. A. Sobahan, C. K. Hwangbo, Y. P. Lee, and T. G. Kim, “Optical and electrical properties of indium tin oxide thin films with tilted and spiral microstructures prepared by oblique angle deposition,” J. Mater. Res. 23(9), 2500–2505 (2008). [CrossRef]
  17. P. Yu, C. H. Chang, C. H. Chiu, C. S. Yang, J. C. Yu, H. C. Kuo, S. H. Hsu, and Y. C. Chang, “Efficiency enhancement of GaAs photovoltaics employing antireflective indium tin oxide nanocolums,” Adv. Mater. (Deerfield Beach Fla.) 21(16), 1618–1621 (2009). [CrossRef]
  18. M. F. Schubert, J. Q. Xi, J. K. Kim, and E. F. Schubert, “Distributed Bragg reflector consisting of high- and low-refractive-index thin film layers made of the same material,” Appl. Phys. Lett. 90(14), 141115 (2007). [CrossRef]
  19. Q. Xu, V. R. Almeida, R. R. Panepucci, and M. Lipson, “Experimental demonstration of guiding and confining light in nanometer-size low-refractive-index material,” Opt. Lett. 29(14), 1626–1628 (2004). [CrossRef] [PubMed]
  20. H. Keppner, J. Meier, P. Torres, D. Fischer, and A. Shah, “Microcrystalline silicon and micromorph tandem solar cells,” Appl. Phys., A Mater. Sci. Process. 69(2), 169–177 (1999). [CrossRef]
  21. S. Michael and A. Bates, “The design and optimization of advanced multijunction solar cells using the Silvaco ATLAS software package,” Sol. Energy Mater. Sol. Cells 87(1-4), 785–794 (2005). [CrossRef]
  22. M. Baudrit and C. Algora, “Theoretical optimization of GaInP/GaAs dual-junction solar cell: Toward a 36% efficiency at 1000 suns,” Phys. Status Solidi 207(2), 474–478 (2010) (a). [CrossRef]
  23. S. T. Chang, M. Tang, R. Y. He, W. C. Wang, Z. Pei, and C. Y. Kung, “TCAD simulation of hydrogenated amorphous silicon-carbon/microcrystalline-silicon/hydrogenated amorphous silicon-germanium PIN solar cells,” Thin Solid Films 518(6), S250–S254 (2010). [CrossRef]
  24. S. Y. Myong, K. Sriprapha, S. Miyajima, M. Konagai, and A. Yamada, “High efficiency protocrystalline silicon/microcrystalline silicon tandem cell with zinc oxide intermediate layer,” Appl. Phys. Lett. 90(26), 263509 (2007). [CrossRef]
  25. NREL’s Renewable Resource Data Center, http://rredc.nrel.gov/solar/spectra , Accessed 30 Nov. (2010).
  26. ATLAS User's Manual, Silvaco international, June (2008).
  27. S. Nitta, S. Itoh, M. Tanaka, T. Endo, and A. Hatano, “Optical properties of a-Si:H and a-SixCl1-x:H films prepared by glow-discharge deposition,” Sol. Energy Mater. 8(1-3), 249–257 (1982). [CrossRef]
  28. M. Zeman, R. A. C. M. M. van Swaaij, J. W. Metselaar, and R. E. I. Schropp, “Optical modeling of a-Si:H solar cells with rough interfaces: Effect of back contact and interface roughness,” J. Appl. Phys. 88(11), 6436–6443 (2000). [CrossRef]
  29. SOPRA, http://www.sopra-sa.com , Accessed 1 Dec. (2010).
  30. M. I. Mendelson, “Average grain size in polycrystalline ceramics,” J. Am. Ceram. Soc. 52(8), 443–446 (1969). [CrossRef]
  31. Y. Sato, K. Yanagisawa, N. Oka, S. I. Nakamura, and Y. Shigesato, “Sputter deposition of Al-doped ZnO films with various incident angles,” J. Vac. Sci. Technol. A 27(5), 1166–1171 (2009). [CrossRef]
  32. M. Suzuki, T. Ito, and Y. Taga, “Photocatalysis of sculptured thin films of TiO2,” Appl. Phys. Lett. 78(25), 3968–3970 (2001). [CrossRef]
  33. J. M. Nieuwenhuizen and H. B. Haanstra, “Microfractography of thin films,” Philips Tech. Rev. 27, 87–91 (1966).
  34. R. N. Tait, T. Smy, and M. J. Brett, “Modelling and characterization of columnar growth in evaporated films,” Thin Solid Films 226(2), 196–201 (1993). [CrossRef]
  35. E. Çetinörgü, S. Goldsmith, and R. L. Boxman, “Air annealing effects on the optical properties of ZnO–SnO2 thin films deposited by a filtered vacuum arc deposition system,” Semicond. Sci. Technol. 21(3), 364–369 (2006). [CrossRef]
  36. M. G. Moharam, “Coupled-wave analysis of two-dimensional gratings,” Proc. SPIE 883, 8–11 (1988).
  37. X. Xiao, G. Dong, J. Shao, H. He, and Z. Fan, “Optical and electrical properties of SnO2:Sb thin films deposited by oblique angle deposition,” Appl. Surf. Sci. 256(6), 1636–1640 (2010). [CrossRef]
  38. J. W. Leem, Y. T. Lee, and J. S. Yu, “Optimum design of InGaP/GaAs dual-junction solar cells with different tunnel diodes,” Opt. Quantum Electron. 41(8), 605–612 (2009). [CrossRef]
  39. B. Sang, K. Dairiki, A. Yamada, and M. Konagai, “High-efficiency amorphous silicon solar cells with ZnO as front contact,” Jpn. J. Appl. Phys. 38(Part 1, No. 9A), 4983–4988 (1999). [CrossRef]
  40. Y. Huang, S. Dai, S. Chen, C. Zhang, Y. Sui, S. Xiao, and L. Hu, “Theoretical modeling of the series resistance effect on dye-sensitized solar cell performance,” Appl. Phys. Lett. 95(24), 243503 (2009). [CrossRef]

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