Supplementary Material

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SUPPLEMENTARY INFORMATION for
Micro-Textured Conductive Polymer/Silicon Heterojunction Photovoltaic Devices with
High Efficiency
By Ting-Gang Chen, Bo-Yu Huang, En-Chen Chen, Peichen Yu, and Hsin-Fei Meng
Contents:
1. Simulation Parameters
2. Optical Modeling of Micro-textured Silicon with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS): Methodology and Results
3. Ultraviolet photoelectron spectroscopy (UPS) spectrum of PEDOT:PSS
* E-mail: [email protected], [email protected]
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SUPPLEMENTARY INFORMATION
1. Simulation Parameters
Table SI. Fitting parameters for micro-textured hybrid solar cells [S1-S3].
Parameters and units
Thickness (nm)
Electron affinity (eV)
Band gap (eV)
Effective conduction
band density (cm-3)
Effective valence
band density (cm-3)
Electron mobility
(cm2V-1s-1)
Hole mobility
(cm2V-1s-1)
Doping concentration
of acceptors (cm-3)
Doping concentration
of donors (cm-3)
Midgap density of
states (cm-3eV-1)
PEDOT:PSS
40
3.6
1.57
2x1021
2x1021
1.04x1019
1
1350
40
450
1x1019
0
0
3x1015
2.7x1019
2
Silicon
200000
4.05
1.12
2.8x1019
1x1012
2. Optical Modeling of Micro-textured Silicon with PEDOT:PSS
A rigorous couple wave analysis (RCWA) method was employed to investigate the
reflection properties of silicon micro-pyramids and planar surface with a thin PEDOT:PSS layer.
In this work, we use a commercial implementation of the three-dimensional RCWA
(DiffractMod, Rsoft Corp.) to examine the consistency between simulation and experiment. The
material properties of PEDOT:PSS are taken into account using the measured refractive index
and extinction coefficients by ellipsometer and shown in Fig. S1. In order to minimize the
calculation errors arising from boundary conditions, a unit-cell is defined with dimensions at
least an order of magnitude larger than the incident wavelength.
Fig. S1. (a) Measured refractive index (N) and extinction coefficient (K) of a PEDOT:PSS film
by ellipsometer. (b) The N and K of silicon from the software database.
The micro-texture and planar architectures were established for optical simulation that the
models consist of micro-scale silicon pyramids and planar surface with a 40-nm-thick
PEDOT:PSS layer conformally laid on the top of the it. The film thickness is measured by alpha
step with a PEDOT:PSS spin rate of 8000 nm. The side-walls of pyramids intersect at 54.7°
corresponding to the (111) plane with an average height of 4 µm. Fig. S2(a) shows the unit cell
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of a simulated 3D structural profile for micro-textured silicon structure. The respective unit cells
are 16 µm×21 µm with a spatial resolution of 10 nm in x, y, and z directions. The corresponding
refractive index profiles could be examined in a cross-sectional index contour map at 500 nm
wavelength as shown in Fig. S2(b).
(b)
Z (µm)
(a)
X (µm)
Fig. S2. (a) 3D structural profile for the micro-textured model. (b) The corresponding index
contours of (a) obtained at 500 nm wavelength. For viewing purpose, the thin PEDOT:PSS layer
is not shown in (a) but can be examined in (b).
The RCWA solves Maxwell’s equations in the Fourier domain. The diffraction efficiencies
are then calculated at the end of simulation and spatial field distributions are derived from the
Fourier harmonics. To ensure the convergence, the harmonic number of plane waves used to
expand the electromagnetic field in Fourier space is 9 for micro-textured models. The electric
field was set to 45-degree linearly polarized to account for the un-polarized solar radiation. Since
the scattering or diffusive diffraction dominates the reflection loss of sub-wavelength structures,
the reflectance is calculated by summing over all the diffraction orders to take into account to the
scattered optical loss components. Fig. S3 shows that the calculated reflectance agrees well with
the measured reflectance using an integrating sphere for both micro-textured and planar
structures for a 40-nm-thick PEDOT:PSS layer capped on top.
4
100
Reflectance (%)
80
Micro-texture (measured)
Micro-texture (calculated)
Planar (measured)
Planar (calculated)
60
40
20
0
400
500
600
700
800
900
1000
Wavelength (nm)
Fig. S3. Calculated and measured reflectance spectra using an integrating sphere for micropyramids and planar structures with a PEDOT:PSS spin rate of 8000 rpm.
In our calculation, the spatial field distribution can be derived by solving Maxwell`s
equations. Figures S4(a) and 4(b) show the steady-state electrical field distributions of microtextured silicon at incident wavelengths of 500 nm and 900 nm, respectively. The corresponding
absorption distributions are also calculated to depict the total power absorbed in the material, as
shown in Fig. S4(c) and 4(d) for 500 nm and 900 nm wavelengths, respectively. We can observe
that the absorbed power is close to the textured surface due to a high absorption coefficient of
silicon at a short incident wavelength. On the contrary, the absorption distribution is rather
uniform for the 900 nm wavelength. From the calculated electrical field, one could calculate the
spatial distribution of carrier generation rate by summing over the wavelength components and
weighted with the air mass 1.5 global solar spectrum [S4].
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(a)
(b)
1.0
Z (µm)
Z (µm)
1.0
-1.0
(c)
X (µm)
-1.0
(d)
X (µm)
1.0
Z (µm)
Z (µm)
1.0
0.0
0.0
X (µm)
X (µm)
Fig. S4. (a) and (b) Calculated steady-state electrical field distribution at incident wavelengths of
500 nm and 900 nm, respectively for micro-textured silicon. (c) and (d) The corresponding
absorption distribution for 500 nm and 900 nm.
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3. Ultraviolet photoelectron spectroscopy spectrum of PEDOT:PSS
The UPS has been applied to study the valence band electronic structure for the
PEDOT:PSS layer. First, the substrate coated with a clean gold coating is used for calibration
due to a well-known work function. A PEDOT:PSS dispersion (Clevios PH1000) with 5 wt%
dimethyl sulfoxide and 0.2 wt% hexaethylene glycol monododecyl ether is subsequently spincoated on top and send into the chamber. Our UPS system employs a helium discharge lamp
with energy of 21.2 eV as a photon source and the kinetic energy distribution of the emitted
photoelectrons as well as the UPS spectrum is shown in Fig. S5. By evaluating the offset with
and without the PEDOT:PSS coating, the positions of work function and ionization energy of the
Intensity (a.u.)
PEDOT:PSS layer are obtained for values of 4.75 eV and 5.17 eV, respectively.
18
16
14
12
10
8
6
4
2
0
Binding energy (eV)
Fig. S5. The UPS spectrum for the PEDOT:PSS layer.
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Reference
[S1] A. M. Nardes, On the conductivity of PEDOT:PSS thin films, (Technische Universiteit
Eindhoven, Eindhoven, 2007).
[S2] A. Elschner, S. Kirchmeyer, W. Lovenich, U. Merker, and K. Reuter, PEDOT: Principles
and Applications of an Intrinsically Conductive Polymer, (Taylor & Francis Group, Boca Raton,
2011 ).
[S3] J. Liu, Poly(ethylenedioxythiophene) based electronic devices for sensor applications,
(ProQuest, Ann Arbor, 2011).
[S4] F. Monestier, J.J. Simon, Ph. Torchio, L. Escoubas, F. Flory, S. Bailly, R. de Bettignies, S.
Guillerez, C. Defranoux, Sol. Energ. Mater. Sol. Cells 91, 405 (2007).
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