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International Journal of Photoenergy
Volume 2013 (2013), Article ID 513284, 5 pages
http://dx.doi.org/10.1155/2013/513284
Research Article

Advantages of N-Type Hydrogenated Microcrystalline Silicon Oxide Films for Micromorph Silicon Solar Cells

Solar Energy Technology Laboratory, National Electronics and Computer Technology Center, National Science and Technology Development Agency, 112 Thailand Science Park, Phahonyothin Road, Klong 1, Klong Luang, Pathum Thani 12120, Thailand

Received 9 May 2013; Revised 30 May 2013; Accepted 30 May 2013

Academic Editor: Leonardo Palmisano

Copyright © 2013 Amornrat Limmanee et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

We report on the development and application of n-type hydrogenated microcrystalline silicon oxide films (n µc-SiO:H) in hydrogenated amorphous silicon oxide/hydrogenated microcrystalline silicon (a-SiO:H/µc-Si:H) micromorph solar cells. The n µc-SiO:H films with high optical bandgap and low refractive index could be obtained when a ratio of carbon dioxide (CO2) to silane (SiH4) flow rate was raised; however, a trade-off against electrical property was observed. We applied the n µc-SiO:H films in the top a-SiO:H cell and investigated the changes in cell performance with respect to the electrical and optical properties of the films. It was found that all photovoltaic parameters of the micromorph silicon solar cells using the n top µc-SiO:H layer enhanced with increasing the CO2/SiH4 ratio up to 0.23, where the highest initial cell efficiency of 10.7% was achieved. The enhancement of the open circuit voltage () was likely to be due to a reduction of reverse bias at subcell connection—n top/p bottom interface—and a better tunnel recombination junction contributed to the improvement in the fill factor (FF). Furthermore, the quantum efficiency (QE) results also have demonstrated intermediate-reflector function of the n µc-SiO:H films.

1. Introduction

Wide-bandgap silicon oxide based materials have been widely studied for thin film silicon solar cell applications because of their attractive optical and electrical properties [14]. Characterizations of boron doped hydrogenated amorphous and microcrystalline silicon oxide films (p a-SiO:H and p µc-SiO:H) and their applications as window layer of solar cells have been reported by many research groups [5, 6]. N-type a-SiO:H and µc-SiO:H films also have been developed and applied in single junction and multijunction thin film silicon solar cells [711]. However, most works focused on an intermediate-reflector function of the n µc-SiO:H films in conventional a-Si:H/µc-Si:H micromorph solar cells and paid little attention to their effects on junction connection and band diagram continuity [710]. Moreover, application of the n µc-SiO:H films to a-SiO:H based solar cells has not yet been reported; thus there is still much room for further research.

Our group has been investigating the wide bandgap SiO:H based materials and previously reported performance of the a-SiO:H based solar cells with single junction and multijunction structures—a-SiO:H, a-SiO:H/a-Si:H, and a-SiO:H/µc-Si:H [1215]. In this work, we focus on properties of the n µc-SiO:H films and their appropriateness to the use as the n top layer in the a-SiO:H/µc-Si:H micromorph silicon solar cells. Properties of the n µc-SiO:H films are presented along with the performance of the micromorph silicon solar cells.

2. Experimental Details

2.1. Preparation of n µc-SiO:H Films

N µc-SiO:H films have been prepared by very high frequency plasma enhanced chemical vapor deposition (60?MHz VHF-PECVD) technique. The gas sources were silane (SiH4), hydrogen (H2), and carbon dioxide (CO2), and phosphine (PH3) was employed as a doping source. For film characterizations, the n µc-SiO:H films were deposited on Corning glass substrates at the deposition temperature of 180°C, a plasma power of 70?mW/cm2, deposition pressure of 0.5?Torr, H2/SiH4 ratio of 35, PH3/SiH4 ratio of 0.38, and CO2/SiH4 ratio in the range of 0~0.28. The thickness of the films was kept at about 350?nm, which was measured by step profilometer. The crystalline volume fraction () of the n µc-SiO:H films was estimated by Raman scattering experiment. The Raman scattering spectra of the n µc-SiO:H films in the 400–600?cm-1 region can be deconvoluted into three spectra. A peak distribution around 470–475?cm-1 is assigned to the transverse optical (TO) mode of amorphous silicon, whose corresponding integrated area is identified as . A sharp peak arising at around 519–522?cm-1 corresponds to the transverse optical vibrational mode of crystalline silicon, and the associated integrated area is identified as . And the intermediate component corresponding to a peak at around 506–510?cm-1 is identified as . The crystalline volume fraction is calculated by using the simplified empirical relation as follows [16]: We have measured the absorption data (a) of the films at visible range by UV/Visible spectrophotometer. Due to the varying structure of the films from microcrystalline to amorphous phase, we avoided Tauc’s plots, and to give a numerical presentation of the shift in the absorption spectra we determined , that is, the energy corresponding to a = 104?cm-1, as an indicator of relative optical bandgap (). Refractive index () spectra of the films were estimated by Spectroscopic Ellipsometry (SE) using Tauc-Lorentz model [17]. The dark conductivity () of the films was measured in a coplanar configuration with Al electrode at room temperature.

2.2. Fabrication of a-SiO:H/µc-Si:H Micromorph Silicon Solar Cells

We have applied the n µc-SiO:H films as the n top layer of the micromorph silicon solar cells with the structure of TCO glass/ZnO/p-µc-SiO:H/i-a-SiO:H/n-µc-SiO:H/p-µc-SiO:H/i-µc-Si:H/n-µc-Si:H/ZnO/Ag (cell active area was 0.75?cm2). Note that absorber layer of the top cell was wide-bandgap a-SiO:H film, and p µc-SiO:H films were used as p layer in both top and the bottom cells. There was no intermediate layer at the junction connection between the top and bottom cells. Thicknesses of the i top a-SiO:H and i bottom µc-Si:H layers were 400 and 1500?nm, respectively. The CO2/SiH4 ratio for the n top layer deposition was varied from 0 to 0.28, while other conditions in cell fabrication were kept as the same. The thickness of the n top layer was approximately 30?nm. The current-voltage (-) characteristics of the solar cells have been investigated under the standard testing conditions—AM1.5, 100?mW/cm2, and 25°C—in a Wacom solar simulator. Quantum efficiency (QE) of the solar cells also has been evaluated by spectral response measurements.

3. Results and Discussion

3.1. Properties of n µc-SiO:H Films

Figure 1 shows Raman spectra of the n µc-SiO:H films deposited with different CO2/SiH4 ratios. It is obviously shown that the peak corresponding to crystalline phase, peak (), gradually decreased with increasing the CO2/SiH4 ratio, and the amorphous silicon () became a dominant phase at the ratio above 0.23. With no CO2 addition the of the film was 69%, decreasing to 12% at the CO2/SiH4 ratio of 0.28.

513284.fig.001
Figure 1: Raman spectra of n µc-SiO:H films deposited with different CO2/SiH2 ratios.

The optical bandgap of the films tended to increase while the refractive index measured at the wavelength of 550?nm showed an opposite change when the CO2/SiH4 ratio became higher, as shown in Figure 2. Incorporation of oxygen into the Si:H network has a direct consequence for optical gap widening. A component of the increase in optical bandgap is associated with the Si–O bonds because of the stronger bond energy of Si–O compared to those of Si–Si and Si–H [18]. Addition of oxygen atoms to Si:H films can widen optical bandgap; however, the more the participation of the oxygen atoms, the lower the conductivity of the films, as indicated in Figure 3.

513284.fig.002
Figure 2: Optical bandgap () and refractive index () of n µc-SiO:H films as a function of CO2/SiH2 ratio.
513284.fig.003
Figure 3: Dark conductivity of n µc-SiO:H films as a function of CO2/SiH2 ratio.
3.2. Characteristics of a-SiO:H/µc-Si:H Micromorph Silicon Solar Cells

As shown in Figure 4, open circuit voltage (), short circuit current density (), and fill factor (FF) of the solar cells obviously improved when the n µc-SiO:H film was applied as the n top layer instead of the n µc-Si:H film (CO2/SiH4 = 0). The best cell with initial conversion efficiency of 10.7% with ?V, ?mA/m2, and FF = 0.67 has been achieved at the CO2/SiH4 ratio of 0.23, where the of the film was approximately 35%. At the higher ratio, the of the cell began to drop, resulting in a decrease in the cell efficiency. Since the n µc-SiO:H films possess wide optical bandgap of about 2.3?eV and higher defect density compared to the n µc-Si:H film, these are supposed to allow a better continuity of band diagram and also a better tunnel recombination junction at the connection between the top and the bottom cells. As mentioned previously, the i top and p bottom layers in these solar cells were wide bandgap SiO:H based materials. The of the p bottom µc-SiO:H layer was estimated to be about 2.25?eV; thus the n µc-SiO:H film with the of 2.3?eV was probably better suited to the n top layer application for this cell structure. The enhancement in the was supposed to be due to a reduction of reverse bias at subcell connection—n top/p bottom interface. The series resistance () slightly increased while the shunt resistance () significantly enhanced from 1500 to 3200?O when the CO2/SiH4 ratio increased from 0 to 0.28, as shown in Figure 5. The increase of the was supposed to be caused by the better tunnel recombination junction, contributing to the improvement in the FF.

513284.fig.004
Figure 4: Photovoltaic parameters of a-SiO:H/µc-Si:H micromorph silicon solar cells using n top µc-Si(O):H layer deposited with various CO2/SiH2 ratios.
513284.fig.005
Figure 5: and of a-SiO:H/µc-Si:H micromorph silicon solar cells using n top µc-Si(O):H layer deposited with various CO2/SiH2 ratios.

According to the QE results shown in Figure 6, the spectrum response corresponding to the top a-SiO:H cell slightly enhanced while those of the bottom µc-Si:H cell decreased with increasing the CO2/SiH2 ratio. This suggested that, besides allowing ohmic and low resistive electrical connection between the two adjacent cells in the a-SiO:H/µc-Si:H micromorph silicon solar cell, the n top µc-SiO:H film also worked as an intermediate reflector to enhance light scattering, as verified by the increase of the spectrum response corresponding to the top cell. The drop of the at the CO2/SiH2 ratio of 0.28 was thought to be due to current mismatch between the top and the bottom cells.

513284.fig.006
Figure 6: QE of a-SiO:H/µc-Si:H micromorph silicon solar cells using n top µc-Si(O):H layer deposited with various CO2/SiH2 ratios.

Experimental results have verified the excellent multifunction of the n µc-SiO:H films when they are applied in the a-SiO:H/µc-Si:H micromorph solar cells, in addition to the conventional a-Si:H/µc-Si:H structure. Interestingly, the of our a-SiO:H/µc-Si:H micromorph solar cells was found to be high, compared to the conventional micromorph solar cells [710], and was further improved when the n top µc-SiO:H layer was used. The of the conventional cells was about 1.38–1.42?V, while the a-SiO:H/µc-Si solar cells showed the as high as 1.47–1.49?V. The multijunction thin film silicon solar cells with high are considered to have advantage of low temperature coefficients (TC) [15]. Although, at present, the efficiency of the a-SiO:H/µc-Si micromorph solar cells is lower than that of the micromorph solar cells using conventional structure, their advantages are expected to become more obvious when the cells are operating in high-temperature environment.

4. Conclusion

We have developed the n-type µc-SiO:H films and applied them as the n top layer of the a-SiO:H/µc-Si:H micromorph silicon solar cells. The solar cells using the n top µc-SiO:H layer showed higher , , and FF than the cell with the n top µc-Si:H layer. Enhancements in the cell parameters were supposed to be due to the better tunnel recombination junction, the better continuity of band diagram at the subcell connection, and, equally importantly, the more efficient intermediate reflector, all of which were mainly owing to the n µc-SiO:H films in the top cell.

Acknowledgment

This work was supported by Cluster and Program Management Office (CPM) of NSTDA, Thailand (P-00-10470).

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