This document analyzes the absorption losses of silver back reflectors in ultra-thin crystalline silicon solar cells using frequency domain finite difference simulations. It finds that for flat cells, losses are from intrinsic absorption, guided mode resonance, and plasma oscillations. For textured cells, losses are from guided mode resonance and plasma oscillations. Simulation results show multiple absorption peaks for both TE and TM modes in textured cells, with losses generally higher for TM modes. The study aims to improve light absorption in the thin active layers.

1. Absorption losses of Ultra-thin
Crystalline silicon solar cells
Presented by
Eng/ Ahmed Ayman Fahmy
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2. Outline:
• Abstract
• Introduction
• Light trapping
• Simulation method & cell structure
• Results & Discussion
• conclusion
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3. Abstract
• Absorption losses of Ag back reflector in two
typical solar cell has been analyzed by frequency
domain finite difference method
• Ag BR absorption losses of flat crystalline silicon
composed of intrinsic absorption, guided mode
resonance absorption and plasma oscillation
absorption.
• Ag BR absorption losses of textured crystalline
silicon composed of guided mode resonance
absorption and plasma oscillation absorption.
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4. Introduction
• Ultra-thin crystalline silicon solar cells have the
Advantages of low cost, high efficiency and good stability
so they have been regarded as one of the main directions
for future development of silicon cells.
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5. Introduction (cont.)
• Researchers have prepared that Ultra-thin
crystalline silicon cells with chip thickness
(Active layer) in range of several microns to
several ten of microns.
• Efficiency of some cells has achieved 20%.
• The active layer thickness is thin and the light
absorption ability is weak.
• To improve the efficiency we use light trapping.
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6. Introduction (cont.)
• An optimum solar cell structure will typically have "light
trapping" in which the optical path length is several times the
actual device thickness, where the optical path length of a
device refers to the distance that an unabsorbed photon may
travel within the device before it escapes out of the device.
• This will improve the photocurrent density and enhance the
absorbing light of the active layer.
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Light trapping

7. • At the same time of enhancing the absorbing light of
active layer, metal back reflector will produce some
absorption losses to the incident light field.
• The absorption losses is very small but the active
layer is very thin so it must be fully considered.
• The absorption of metal back reflector includes the
intrinsic absorption of metal, the oscillation
absorption of guided mode and surface
plasma oscillation.
• The main method to evaluate the absorption losses
of metal back reflector (FDTD) Finite Difference
Time Domain
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Introduction (cont.)

8. • FDFD is a method to deal with complex electromagnetic
field problem.
• It uses Yee grid to transform Maxwell equation into the
difference form in frequency domain.
• It is a numerical analysis technique finding approximate
solutions to the associated system of differential
equations.
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Simulation method & cell structure

9. Simulation method & cell structure
• The absorption probability of the medium can be
obtained from the formula
𝑨 = 𝟏 − 𝑻 − 𝑹
A: Absorption
T: transmittance
R: Reflectivity
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Src: www.epsilonengineer.com/radiation.html

10. • Cell structure
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Simulation method & cell structure
(a) Flat crystalline silicon cell (b) textured crystalline silicon cell

11. • Incident angel of light field and
absorption losses.
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Simulation method & cell structure

12. • The two cell structures can be considered as a
flat wave guide structure.
• For the two cell structure the light satisfies the
standing wave condition in the Y direction and
the wave vector component isn’t zero in the X
direction.
• Max 𝜃𝑖𝑛 = 16°.
• According to the absorption property of active
layer and Ag BR the guided mode is mainly
concentrated in the long wave length, this is the
weak absorption region in crystalline silicon.
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Simulation method & cell structure

13. • The thickness of Y coordination active layer is
much greater than the wave length of the light so
the cell structure is multi-mode wave guide
• At the wave length of the guided mode the light
will contact with Ag BR for many times resulting
strong absorption losses.
• The losses of Ag BR is
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Simulation method & cell structure

14. Results & Discussion
• When the light is vertically incident Ag BR curve
is generated by interaction between Active layer
guided mode oscillation and plasma oscillation.
• When the light field is tilted the absorption
caused by guided mode oscillation.
• Increasing 𝜃 the absorption beak of TM mode is
greater than TE mode
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Absorption losses of Ag BR in flat cell

15. 16
Results & Discussion
Absorption of flat crystalline silicon cell

16. Results & Discussion
• On the surface near of Ag BR TE mode showed
significant attenuation so the light field is
evanescent wave in Ag BR.
• TM mode show strong oscillation characteristics
resulting surface plasma wave was formed on
the surface of Ag BR.
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Absorption losses of Ag BR in textured cell

17. 18
Results & Discussion
Distribution of incident light inside textured cell a) TE mode b) TM mode

18. conclusion
• Ag BR absorption losses of flat crystalline silicon
composed of intrinsic absorption, guided mode
resonance absorption and plasma oscillation
absorption and because of the max 𝜃𝑖𝑛 = 16° TM-
mode cause excite weak plasma oscillation effect.
• Ag BR absorption losses of textured crystalline
silicon composed of guided mode resonance
absorption and plasma oscillation absorption TE
and TM mode show multiple absorption beaks and
TM is higher than TE mode
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