Rahul Dewan - Jacobs University

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4. µC-SI SOLAR CELLS WITH INTEGRATED LAMELLAR GRATINGS Incoming Light Glass Glass Glass Transparent Conductive Oxide ZnO: Al ZnO: Al Microcrystalline Silicon Diode µc-Si Profile Height µc-Si Metal Back Contact Metal Back Contact Metal Back Contact Period (a) (b) (c) Figure 4.1: Schematic sketch of a thin-film microcrystalline silicon solar cell (a) on a smooth substrate and (b) on a randomly textured substrate. The periodic unit cell investigated in this chapter is shown in (c). For each unit cell, the period and height of the grating were varied. the optical spectrum [47, 90]. For the blue and green parts of the optical spectrum, the short circuit current and the quantum efficiency remain almost constant, since the absorption length for blue and green light is significantly smaller than the thickness of the solar cell. Subsequently the blue and green light will be absorbed within the first few hundreds of nanometers of the solar cell. Whilst we observe the enhancements in quantum efficiency by introducing the texturing, in order to fully understand and optimize the nanotexturing process, it is imperative to use numerical models to analyze the optical losses in all layers of the thin-film silicon solar cells [91]. However, the analysis of the wave propagation within a randomly textured solar cell is complex. Therefore as a first learning block, a simple model system based on an integrated grating coupler was selected for investigation. The model system allows for studying the influence of the grating parameters on the solar cell parameters. A schematic cross section of a microcrystalline silicon solar cell deposited on a smooth substrate and on a randomly textured substrate is shown in Fig. 4.1(a) and 4.1(b), respectively. The microcrystalline solar cell structure, investigated in the study, consists of a 500 nm thick aluminum doped zinc oxide (ZnO:Al) front contact, followed by a hydrogenated microcrystalline silicon p-i-n diode (µc-Si:H) with a total 44
4.2. Solar Cells on Smooth Substrates thickness of 1000 nm and a back reflector consisting of an 80 nm thick ZnO:Al layer and a perfect metal reflector. The thickness of the p- layer was assumed to be 30 nm. The device structure is consistent with the standard microcrystalline silicon solar cell process used by several research groups like the Research Center Jülich [92] and <strong>University</strong> of Neuchâtel [93]. The complex refractive indices of the aluminum doped zinc oxide and microcrystalline silicon layers were determined by optical characterization of individual films on glass substrates by transmission/reflection, ellipsometric, and photothermal deflection spectroscopies [94]. The cross section of the investigated periodic structure in this chapter is shown in Fig. 4.1(c). The optical model used in this study is based on the periodic arrangement of grooves. It is assumed that the solar cell on a textured substrate can be described by a unit cell, which provides all information on the behavior of the entire solar cell. Key parameters of the unit cell are the period, the groove height, and the width of the groove. The groove height of the front and back gratings in the case of Fig. 4.1(c) is just the height of the one dimensional line grating. In the simulations, the period of the unit cell was varied from 500 to 3000 nm and the groove height of the structure was simulated for grooves from 0 to 500 nm. The width of the groove was always kept constant at 50% of the period. 4.2 Solar Cells on Smooth Substrates The investigation of solar cells on smooth substrates allows us to gain insights in the optical wave propagation within a microcrystalline silicon solar cell. Furthermore, these simulation results are then used as a reference to compare the performance of cells with integrated grating couplers. The structure of the solar cell on a smooth substrate used in this investigation is consistent with the standard microcrystalline silicon solar cell structure developed by the Research Center Jülich (Institute of Photovoltaics). A detailed description of the device fabrication is given in Ref. [95]. A schematic cross section of a microcrystalline silicon solar cell on a smooth substrate is shown in Fig. 4.2(a). In this simulation based study, the metal back contact of the solar cell was substituted with a perfect reflector. A silver back contact, like those on the fabricated cell, was not used because the optical properties of the zinc-oxide/silver contact is unknown and it also differs significantly if only properties of bulk-silver is used. The simulations of the optical wave propagation within the solar cell on a smooth substrate were carried out for wavelengths ranging from 300 nm to 1100 nm. The complex refractive indices for each layer were used to simulate the electric field distribution throughout the device structure for the entire wavelength range. Using the electric field distributions, the time average power loss, Q (x, z), within the individual regions of the solar cell was calculated using the equation 45
Page 1: OPTICS IN THIN-FILM SILICON SOLAR C
Page 4 and 5: CONTENTS 4.5 Summary . . . . . . .
Page 6 and 7: 1. INTRODUCTION thinner than the ab
Page 9 and 10: Chapter 2 Fundamental Concepts Unde
Page 11 and 12: 2.1. Characteristic Parameters of S
Page 19 and 20: 2.2. Thin-film silicon solar cells
Page 21 and 22: 2.2. Thin-film silicon solar cells
Page 23 and 24: 2.3. Solar Cell Material Properties
Page 25 and 26: 2.3. Solar Cell Material Properties
Page 27 and 28: 2.4. Light Confinement in Thin-film
Page 29 and 30: 2.4. Light Confinement in Thin-film
Page 31: 2.4. Light Confinement in Thin-film
Page 34 and 35: 3. COMPUTATIONAL MODELING OF OPTICA
Page 47: Chapter 4 µc-Si Solar Cells with I
Page 51 and 52: 4.3. Solar Cells with Integrated Gr
Page 59 and 60: 2 4.3. Solar Cells with Integrated
Page 61 and 62: 4.4. Toward an Optimal Grating Stru
Page 67 and 68: Chapter 5 µc-Si Solar Cells with T
Page 69 and 70: 5.2. Simulation Results sists of a
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Page 77: 5.4. Summary on a smooth substrate
Page 80 and 81: 6. SIMPLER AND FASTER METHOD FOR PE
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7. µC-SI SOLAR CELLS WITH 3-DIMENS
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Chapter 8 Summary and Outlook 8.1 S
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8.2. Further Areas to Investigate 8
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8.2. Further Areas to Investigate m
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8.2. Further Areas to Investigate Q
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BIBLIOGRAPHY [9] A. Lin and J. Phil
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BIBLIOGRAPHY Photovoltaic Solar Ene
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BIBLIOGRAPHY [57] J. I. Owen, J. H
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BIBLIOGRAPHY [79] K. Yee, “Numeri
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BIBLIOGRAPHY [102] C. Heine and R.
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BIBLIOGRAPHY [124] H. W. Deckman, C
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BIBLIOGRAPHY nanoparticles,” Jour
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LIST OF SYMBOLS AND ACRONYMS Ag Sil
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10. I. Vasilev, R. Dewan, M. Marink
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Acknowledgement • I would like to
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Rahul Dewan - Jacobs University

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