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. 2016 Feb 25;9(3):128.
doi: 10.3390/ma9030128.

Spectroscopic Ellipsometry Studies of n- i- p Hydrogenated Amorphous Silicon Based Photovoltaic Devices

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Free PMC article

Spectroscopic Ellipsometry Studies of n- i- p Hydrogenated Amorphous Silicon Based Photovoltaic Devices

Laxmi Karki Gautam et al. Materials (Basel). .
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Abstract

Optimization of thin film photovoltaics (PV) relies on characterizing the optoelectronic and structural properties of each layer and correlating these properties with device performance. Growth evolution diagrams have been used to guide production of materials with good optoelectronic properties in the full hydrogenated amorphous silicon (a-Si:H) PV device configuration. The nucleation and evolution of crystallites forming from the amorphous phase were studied using in situ near-infrared to ultraviolet spectroscopic ellipsometry during growth of films prepared as a function of hydrogen to reactive gas flow ratio R = [H₂]/[SiH₄]. In conjunction with higher photon energy measurements, the presence and relative absorption strength of silicon-hydrogen infrared modes were measured by infrared extended ellipsometry measurements to gain insight into chemical bonding. Structural and optical models have been developed for the back reflector (BR) structure consisting of sputtered undoped zinc oxide (ZnO) on top of silver (Ag) coated glass substrates. Characterization of the free-carrier absorption properties in Ag and the ZnO + Ag interface as well as phonon modes in ZnO were also studied by spectroscopic ellipsometry. Measurements ranging from 0.04 to 5 eV were used to extract layer thicknesses, composition, and optical response in the form of complex dielectric function spectra (ε = ε₁ + iε₂) for Ag, ZnO, the ZnO + Ag interface, and undoped a-Si:H layer in a substrate n-i-p a-Si:H based PV device structure.

Keywords: hydrogenated silicon; infrared spectra; photovoltaic devices; spectroscopic ellipsometry.

Conflict of interest statement

The authors have no conflict of interest. The funding sponsors had no role in the design of the study, in the collection, analyses, or interpretation of data; in the writing of the manuscript; and in the decision to publish the results.

Figures

Figure 1
Figure 1
Complex dielectric function spectra, ε = ε1 + iε2, (arrow pointing left for ε1 axis, arrow pointing right for ε2 axis) from 0.734 to 5.88 eV for a semi-infinite Ag film parameterized with a combination of a Drude oscillator and two oscillators assuming critical point parabolic bands (CPPB) with parameters listed in Table 2.
Figure 2
Figure 2
Spectra in ε (arrow pointing left for ε1 axis, arrow pointing right for ε2 axis) from 0.734 to 5.0 eV for the 108 ± 1 Å thick ZnO + Ag interface layer parameterized with a Lorentz and a Drude oscillator with parameters listed in Table 3.
Figure 3
Figure 3
Spectra in ε (top panel, real part ε1; bottom panel, imaginary part ε2) from 0.04 to 5.0 eV for a 3010 ± 2 Å thick ZnO film on Ag, with ε for ZnO parameterized using a combination of two CPPB and three Lorentz oscillators with parameters listed in Table 4. The inset shows high-energy electronic transitions in ε2.
Figure 4
Figure 4
Mean square error (MSE), void fraction (fvoid), nanocrystalline volume fraction (fnc), and surface roughness thickness (ds) in the top ~10 Å of the bulk layer, plotted versus the accumulated bulk layer thickness for an intrinsic hydrogen diluted R = [H2]/[SiH4] = 50 Si:H film deposited on a 200 Å R = 50 n-type a-Si:H over-deposited onto a ZnO/Ag back reflector (BR), as determined by virtual interface analysis (VIA) applied to real time spectroscopic ellipsometry (RTSE) data. Spectrally averaged mean error for fvoid, fnc, and ds are 0.3%, 2.4%, and 0.8 Å respectively.
Figure 5
Figure 5
Spectra in ε (top panel, real part ε1; bottom panel, imaginary part ε2) of a-Si:H and nc-Si:H reference material used in VIA applied over a spectral range from 2.75 to 5.0 eV. Spectra in ε for a-Si:H and nc-Si:H were obtained from analysis of RTSE data and by numerical inversion at a bulk layer thickness of 200 and 1150 Å, respectively.
Figure 6
Figure 6
Growth evolution diagrams obtained from analysis of RTSE data for (a) p-type; (b) intrinsic; and (c) n-type Si:H as a function of variable hydrogen dilution R = [H2]/[SiH4] in the n-i-p solar cell device structure. The data values and connecting lines depict the a→(a+nc) and (a+nc)→nc structural transitions of doped and undoped Si:H prepared at conditions described in Table 1. Arrows pointing upward indicate the respective transition occurs beyond the maximum thickness measured.
Figure 7
Figure 7
Deposition rates of (a) n-, (b) i-, and (c) p-layers on ZnO/Ag, n-layer/ZnO/Ag, and i-layer/glass, respectively, as functions of R.
Figure 7
Figure 7
Deposition rates of (a) n-, (b) i-, and (c) p-layers on ZnO/Ag, n-layer/ZnO/Ag, and i-layer/glass, respectively, as functions of R.
Figure 8
Figure 8
Schematic of a single junction a-Si:H based solar cell prepared in the n-i-p configuration. Each amorphous or protocrystalline Si:H layer is optimized to a value of R with an intended thickness.
Figure 9
Figure 9
Comparison of lower energy features in ε2 as a function of photon energy for ZnO with (solid line) and without (dotted line) over-deposition of a-Si:H. Parameters describing the sample without and with over-deposition of are listed in Table 4 and Table 5, respectively.
Figure 10
Figure 10
Spectra in ε (top panel, real part ε1; bottom panel, imaginary part ε2) extracted over a spectral range from 0.04 to 5 eV for 3621 ± 2 Å R = 10 a-Si:H films on BR over-coated with a R = 50 n-layer. The inset shows lower energy features in ε2 as a function of photon energy representing Si-Hn vibrational modes as modeled by Gaussian oscillators.

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References

    1. Robertson J. Deposition mechanism of hydrogenated amorphous silicon. J. Appl. Phys. 2000;87:2608–2617. doi: 10.1063/1.372226. - DOI
    1. Collins R., Ferlauto A., Ferreira G., Chen C., Koh J., Koval R., Lee Y., Pearce J., Wronski C. Evolution of microstructure and phase in amorphous, protocrystalline, and microcrystalline silicon studied by real time spectroscopic ellipsometry. Sol. Energy Mater. Sol. Cells. 2003;78:143–180. doi: 10.1016/S0927-0248(02)00436-1. - DOI
    1. Robertson J. Growth mechanism of hydrogenated amorphous silicon. J. Non-Cryst. Solids. 2000;266:79–83. doi: 10.1016/S0022-3093(00)00012-0. - DOI
    1. Fujiwara H., Kondo M., Matsuda A. Real-time spectroscopic ellipsometry studies of the nucleation and grain growth processes in microcrystalline silicon thin films. Phys. Rev. B. 2001;63 doi: 10.1103/PhysRevB.63.115306. - DOI
    1. Schiff E.A., Deng X. Amorphous silicon-based solar cells. In: Luque S.H.A., editor. Handbook of Photovoltaic Science and Engineering. Wiley; New York, NY, USA: 2003. pp. 487–545.
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