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International Conference on Materials for Advanced Technologies 2011, Symposium O
Advanced Light Management Approaches
for Thin-Film Silicon Solar Cells
M. Zemana,*, O. Isabellaa, K. Jägera, R. Santbergena,
S. Solntseva, M. Topicb and J. Krcb
b
a
Delft University of Technology, DIMES, P.O. Box 5053, 2600 GB Delft, The Netherlands
University of Ljubljana, Faculty of Electrical Engineering, Trzaska 25, SI-1000 Ljubljana, Slovenia
Abstract
Light management is important for improving the performance of thin-film solar cells. Advanced concepts of
efficient light scattering and trapping inside the cell structures need to be investigated. An important tool for design
and optimisation of the concepts present optical modelling and simulation. In the article a model of light scattering at
textured surfaces, which is based on first order Born approximation and the Fraunhofer diffraction, is presented.
Another approach presents rigorous solving of Maxwell’s equations for electromagnetic waves in two- or threedimensions. An example of a three-dimensional simulation, employing the finite element method, of an amorphous
silicon solar cell with periodically textured interfaces is shown. The second part of the article focuses on experimental
results related to three advanced light management approaches: i) modulated surface morphologies for enhanced
scattering and anti-reflection, ii) metal nano-particles introducing plasmonic scattering, and iii) one-dimensional
photonic crystals (Bragg stacks) for back reflectors. Improvements in output performance of amorphous silicon solar
cells are demonstrated and discussed.
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© 2011
2011Published
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Elsevier
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peer-review
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Energyof
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on
Materials
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Research Institute of Singapore (SERIS) – National University of Singapore (NUS).
Keywords: Thin-film silicon solar cells; light trapping; optical modelling; modulated surface texture; metal nano-particles;
plasmonics; photonic crystal
1. Introduction
The efficiency of thin-film silicon solar cells has to achieve a level of 20% on a laboratory scale in
order to stay competitive with bulk crystalline silicon solar cells and other thin-film solar cell
technologies. Light management is one of the key issues for improving the performance of thin-film
silicon solar cells and decreasing the production costs by shortening deposition times and using less
material. In particular light management is aiming for:
* Corresponding author. Tel.: +31 15 2782409; fax: +31 15 27829568
E-mail address: [email protected]
1876-6102 © 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee of International Conference
on Materials for Advanced Technologies. Open access under CC BY-NC-ND license.
doi:10.1016/j.egypro.2012.02.022
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efficient trapping and enhanced absorption of incident light in the desired parts of solar cell
structures (inside absorber layers);
minimisation of reflection losses at the front interfaces and absorption losses in the solar cell
structure outside the absorber layers;
effective use of the solar spectrum in a broad wavelength range.
Trapping of light inside the absorber layers (i) leads to prolongation of optical paths and consequently
to increased absorption of light in thin absorber layers. The following techniques of light trapping will be
addressed in this article:
 scattering at rough interfaces,
 scattering at nano-particles,
 reflection at the back side and at intermediate reflectors in case of tandem cells.
Rough (textured) interfaces are usually introduced in thin-film solar cells by using surface-textured
substrates. If a superstrate configuration of the cell is used the texture is introduced by surface-textured
transparent conductive oxides (TCOs), deposited on a glass carrier. Different TCO substrates such as
randomly textured fluorine doped tin oxide (FTO) of Asahi U type [1], LP-CVD boron doped zinc oxide
(BZO) [2] or magnetron sputtered aluminium doped zinc oxide (AZO) [3] have been researched and
optimised optically and electrically. Periodically textured substrates, where the texture is embossed in a
dedicated lacquer film on the substrate, are becoming of interest, especially in roll-to-roll production of
flexible PV modules [4-8]. Recently, layers with embedded metal [9-11] or dielectric nano-particles, such
as white paint [12-14], for efficient in-coupling and scattering of light into the absorber layer have
attracted a lot of attention. Metal-based reflectors such as ZnO/Ag [15], and alternative dielectric back
reflectors, such as white foils and photonic crystal [16-19], have been investigated in respect of high
reflection and efficient scattering at the back side of a solar cell. Intermediate reflectors, based on ZnO, aSiOx layers, are introduced in thin-film tandem devices for better manipulation of spectrum distribution
between the top and the bottom cell [20-24].
Minimisation of reflection and absorption losses outside the absorber layers (ii) relates to the
implementation of properly designed anti-reflecting layers and structures (sub-wavelength textures at the
front interfaces) and reduction of optical losses in the supporting layers, such as contacts and p- and ndoped layers in pin devices. Recently a lot of effort has been dedicated to the development of TCO
materials with low absorption in a wavelength region of interest (300 nm < λ < 1200 nm) [25-27]. To
decrease optical losses in doped layers a continuous attention is paid to the development of wide band gap
doped semiconductors based on a-Si:H and μc-Si:H such as hydrogenated amorphous/microcrystalline
silicon carbide (a-SiC:H/μc-SiC:H) and hydrogenated amorphous/microcrystalline silicon oxide
(a-SiO:H/μc-SiO:H) [2, 28]. Effective utilisation of the energy of the solar spectrum (iii) is related to
establishing a good matching between the energy of the incoming photons, from different parts of the
spectrum, and energy band gaps of the semiconductor absorbers used in the device. Improvements of the
conversion efficiency above the Shockley Queisser limit of a single-junction cell are possible with multijunction devices [29]. Multi-junction solar cells like tandem a-Si:H/ c-Si:H micromorph [30] and triplejunction, like a-Si:H/a-SiGe:H/c-Si:H solar cells [31, 32] are produced to meet efficient spectrum
utilisation. Recently novel absorber materials and cell concepts based on spectrum splitting on two or more
laterally dislocated cells [33], up- and down-converters [34-36], absorbers with multi-layer or quantum dot
superlattices [37, 38], intermediate-band cells [39] have been investigated for a generic approach of allsilicon multi-junction solar cells.
In this article the importance of optical modelling and simulation in designing the novel optical
concepts of thin-film solar cells is highlighted first. Main features and verification results of the developed
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one-dimensional (1-D) model of light scattering are presented. Simulation results of an a-Si:H solar cell
with periodically textured interfaces, obtained with a three-dimensional (3-D) electromagnetic model for
rigorous solving of Maxwell’s equations are shown. Next, three advanced approaches for light trapping
are presented: a) modulated surface textures for enhanced scattering, b) plasmonic scattering using metal
nano-particles, and c) one-dimensional (1-D) photonic crystal (PC) structures (Bragg stacks) for back
reflectors. The approach based on the use of modulated surface textures allows manipulation of scattering
in a broad wavelength range. The second approach takes advantage of the enhanced scattering due to
metal nano-particles embedded at the interface between two different materials, favouring light-in
coupling in the higher refractive index material. The third approach deals with the manipulation of
reflection and transmission at a particular interface inside a solar cell. Improvements related to the
presented approaches are demonstrated on example of single-junction a-Si:H solar cells.
2. Optical modelling and simulations
Optical modelling (developing models) and simulations (employing the models in analysis) enable to
investigate and optimise existing and develop new optical solutions for thin-film solar cells. Modelling
and simulations are indispensible tools in design of advanced optical concepts. Accurate and well
calibrated optical models and simulators are required. Different one-dimensional computer simulators like
the ASA program from Delft University of Technology [40] and the Sunshine program from Ljubljana
University [41] and others [42, 43] have been developed for research of thin-film solar cells. Still, reliable
models that can combine textured morphology of interfaces in solar cells and optical properties of
surrounding materials directly with light scattering properties are required. In the following we briefly
present a 1-D scattering model developed by the authors. Further on, simulation results of an a-Si:H solar
cell with periodically textured interfaces, obtained by 3-D finite element method simulator for rigorous
solving Maxwell’s equations will be presented.
2.1. Advanced one-dimensional optical model of light scattering
Determination of scattering properties of textured interfaces that are introduced in thin-film solar cells
is of prime importance for 1-D modelling and simulation of thin-film solar cells. Two descriptive
scattering parameters are used to evaluate scattering of light by a nano-textured interface: the angular
intensity distribution AID (in direct relation to angular distribution function, ADF, used in some other
works [44]) and the haze parameter, H for reflected and transmitted light. While the AID gives
information about the directionality (angles) of scattered light, the H describes how much of light is
scattered with respect to the total reflected or transmitted light at an interface. In order to determine the
scattering characteristics of a textured interface the relationship between the morphology of an interface
and the descriptive scattering parameters must be investigated.
Recently, two scattering models have been developed that calculate the AID and H in transmission [45,
46]. Both models are based on the scalar scattering theory [47, 48]. As input, these models do not use
statistical parameters like vertical root-mean-square roughness, but the surface height distribution z(x,y)
that can be obtained e.g. from atomic force microscopy (AFM) measurements on the surface. Models
reflect the insight that the AID of the scattered light is related to the Fourier transform of the textured
interface profile. Here, we briefly present the main features and upgrades of the model reported in [45],
developed by Jäger et al. The scattering model is based on the first order Born approximation [49] and on
Fraunhofer diffraction [50]. The normalised intensity AID(λ,θ) of transmitted light with wavelength ,
scattered at a rough surface under a scattering angle θ is given by
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AID  λ, θ  
Aopt
A
F
r
cos θ
A Z  z  x, y  e

i Kx x  K y y

2
dx.dy ,
(1)
where A is the area of the AFM scanned surface that serves as input for the model. F =
(k2/4π)[n2(r,λ) - 1] is the scattering potential that is dependent on the refractive index n and the
wavevector k = 2π/λ, r is the distance between sample and detector, and with Aopt the model is calibrated.
The function [z(x,y)] contains the height distribution of the surface. Good agreement between calculated
and measured AID values was obtained with

Z
 ik 
1
exp ikz  x, y  .
(2)
One sees that Eq. (1) contains the two dimensional Fourier transform of . The scattering angle θ is
connected to the Fourier space components Kx and Ky via θ=arcsin[( Kx2 +Ky2)0.5/k]. Figure 1(a) shows
measured and calculated AID for pyramid-like texture of FTO of Asahi U-Type [1], crater-like texture of
AZO that was etched after deposition in a solution of 0.5% hydrochloric acid for 20 s and 40 s,
respectively [3], and pyramid-like texture BZO [2] (the AFM of the last two textures are shown as inserts
in Fig. 1(b)). The measured and calculated intensities for the four samples at a selected wavelength of λ =
600 nm are shown in the figure. Calculations and measurements were performed for the case of TCO/air
interface. For all samples, the agreement between measured and calculated values is good, only for AZO
etched for 40 s, the deviation is larger in the central angle range. The haze in transmission is defined as
the diffuse transmittance Tdif divided by the total transmittance Ttot, where Ttot is the sum of Tdif and the
specular transmittance Tspec. Determining the haze can be realised as in Eq. (3), Ttot is then, up to a factor,
given by summing over all the AID components that lead to propagating modes. Since the AID is
determined via a discrete Fourier transform of the AFM date, this sum is discrete. Tdif is then (up to a
factor) given by subtracting the specular transmittance Tspec from Ttot.
Tdif ( λ) Ttot ( λ)  Tspec ( λ)
H


T  λ
Ttot ( λ)
Ttot ( λ)
K 2x

 K y2  k
 λ
T
AIDKT x , K y  AID0,0

K 2x  K y2  k  λ 
(3)
AIDKT x , K y
Figure 1(b) shows the calculated and measured haze values for the four different TCOs when the AID
is calculated according to Eq. (1). Application of the model to interfaces formed by other layers (such as
TCO/a-Si) is underway. Obtained scattering parameters for internal interfaces of solar cell structures can
be then imported in simulators for thin-film systems, such as ASA or SunShine.
(a)
(b)
Fig. 1. a) Angular intensity distributions for light scattered at four different surface-textured TCO materials at 600 nm; b) Haze as a
function of wavelength for the four TCO samples. The values in the brackets correspond to the vertical root mean square roughness
of the textured surface. AFM images of BZO (220 nm) and AZO (100 nm) are given as inserts. AFM image size is 55 m2.
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2.2. Examples of 3-D optical simulations
To determine optical situation inside thin-film solar cells by considering exact geometry of the structure and the morphology of the textured interfaces, rigorous methods of solving Maxwell’s equations can
be applied. There exist 2-D and 3-D simulation tools that are able to solve electromagnetic field situation
in the structures, using different approaches of solving the equations, such as Finite Integrating Technique
(FIT), Finite Element Method (FEM), Finite Difference Time Domain (FDTD) and Rigorous Coupled
Wave (RCWA) analysis [51-57]. The disadvantages of rigorous calculation with 2-D and especially 3-D
models are still long computation times and large memory requirements of the (super)computers used for
simulations. However, exact solution without assumptions and approximations used in the case of 1-D
modelling can be obtained. As an example of 3-D simulations we demonstrate simulation of an a-Si:H
single-junction solar cell with periodically textured interfaces. Periodical textures (diffractive gratings)
have a potential for efficient light scattering. In our case we show the simulation results for a trapezoidlike one-dimensional grating with lateral period P = 600 nm and vertical height h = 300 nm of the texture.
By considering proper boundary conditions, only one period of the structure can be included in
simulations. Simulations were carried out by a software package HFSS [58] which is based on FEM
approach. In Fig. 2(a) the results of the absolute value of the electric field distribution in the structure,
corresponding to  = 665 nm, are presented for the cell with textured and flat interfaces (reference). In
Fig. 2(b) simulation results of absorptances in individual layers of the textured cell structure are shown.
For the flat cell only the absorptance in the i-a-Si:H absorber layer, Ai, is indicated as a dashed line.
Assuming ideal extraction of charge carriers from the i-a-Si:H absorber layer and neglecting the
contributions from p- and n-doped layers. The Ai curve itself can be considered as the external quantum
efficiency, EQE of the device. A general increase in the Ai is observed at long-wavelength part of the
spectrum (550-750 nm) for the textured cell as a consequence of light scattering at the interfaces and
improved trapping in the structure. Experimental results of our a-Si:H solar cells deposited on periodic
gratings can be found in Ref. [7].
1.0
R
ZnO:Al
p
n
ZnO
Ag
i - Text
i - Flat
Absorptance and optical losses
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
350
400
450
500
550
600
650
700
750
800
Wavelength [nm]
textured
(a)
flat
(b)
Fig. 2. (Colour online) (a) Absolute value of the electric field strength of light inside the a-Si:H solar cell with
textured (left structure) and flat (right structure) interfaces as calculated by 3-D simulations The period and height of
the trapezoid-like texture are P = 600 nm and h = 300 nm. The inclination angle of the verticals in the texture is 80º.
The values of electric field corresponding to different colours shown on the left scale bar spans linearly from 0.00
V/m (blue colour – bottom of the scale) to 5.68×107 V/m (red colour – top of the scale). (b) Absorptances in the
individual layers of the structure for a cell with textured interfaces. For the cell with flat interfaces the absorptance for
the i-layer is given only to indicate lower level of light trapping inside the i-layer for wavelengths in the 550-700 nm
range.
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3. Advanced light management concepts
3.1. Modulated surface textures
Besides periodic textures that have been addressed in the previous section, a concept of modulated
surface textures is presented in further as an advanced approach for efficient light trapping in thin-film
silicon solar cells [59]. The term modulated texture we assign to a surface morphology that combines two
or more types of different textures (either random or periodic) which have different vertical and lateral
parameters of the surface morphology. The concept aims at enhanced light scattering at textured interfaces
in the cells in a broad wavelength range of the solar spectrum. In addition, anti-reflecting effects caused
by sub-wavelength features of the modulated texture can be employed at the front interfaces. In this article
one example of the modulated surface texture is presented; where we combine random large and random
small textures. The different textures were introduced at different interfaces of glass/AZO substrate,
namely large random features were introduced at the glass-AZO interface and smaller random features
were created at the AZO-air surface. The large texture (large crater shapes) was obtained by wet etching
of flat a Corning Eagle XG glass (covered with a sacrificial conductive layer) in a mix of HF and H2O2.
The AZO layers were rf-magnetron sputtered on the surface of glass. The smaller texture component
(smaller craters) was realised by wet etching of AZO surface in 0.5 % HCl solution. By applying etching
of glass, then sputtering and etching of AZO, the AZO top surface reassembles the modulated texture
consisting of the texture of glass (which is transferred through the ~1 m thick AZO film) and the texture
of etched AZO (see insert in Fig. 3(a)).
Haze parameter of transmitted light (illumination applied always from the glass side) is shown in Fig.
3(a) for the following samples: etched AZO on flat glass (ref.), etched glass, and etched AZO on etched
glass with the modulated texture. Results show the highest haze for the modulated texture. The large
texture of the glass significantly lifts up the entire haze level. We deposited a-Si:H cells on the analysed
substrates, selected results are shown in Fig. 3(b). The highest EQE, short-circuit current density JSC, and
the conversion efficiency , are obtained for the solar cell with the modulated texture. The texture still
needs to be further optimised in respect of achieving a broader angular distribution of scattered light (high
haze is already achieved as demonstrated).
1.0
1.0
0.9
0.9
0.8
flat glass / etched AZO (ref.)
etched glass / etched AZO
etched glass
0.7
0.6
0.5
0.7
0.6
EQE
Haze in transmission
0.8
etched glass / etched AZO
2
0.5
0.4
0.4
0.3
0.3
0.2
10 m
0.1
0.0
300
400
500
600
Wavelength [nm]
(a)
700
800
etched glass/etched AZO
VOC = 0.899 V
JSC= 14.98 mA/cm
FF = 0.713
 = 9.59 %
Y90 = 70.0% over 30 dots
0.2
flat glass / etched AZO (13.83 mA/cm2) (ref.)
0.1
etched glass / not-etched AZO (12.18 mA/cm2)
0.0
400
etched glass / etched AZO (14.98 mA/cm2)
500
600
700
800
Wavelength [nm]
(b)
Fig. 3. (Colour online) (a) Haze parameter of transmitted light, insert: SEM image of modulated surface texture;
(b) Solar cell characteristics deposited on substrates with different textures. The value of Y 90 corresponds to the yield
(percentage of cells that reach at least 90 % of the efficiency level of the best cell).
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3.2. Metal nano-particles
An alternative way to provide light trapping in solar cells is by means of scattering at metal nano-particles.
Light incident on the particles can induce a localised surface plasmon resonance. As a result, these particles can
be very efficient light scatterers in a tuneable wavelength range. The size and shape of the particles and their
position inside the solar cell are parameters that can be used to fine tune the scattering and reflectivity properties
[9-11]. To visualise the effects of selective behaviour in Fig. 4(a, top) three samples including nano-particles
surrounded by different materials are shown. Besides scattering of light, the particles can give rise to the
parasitic absorption of light. Larger nano-particles with a diameter in the order of 100 nm give rise to more
scattering and less absorption [9-11] and are therefore desirable for solar cell applications.
The effect of a plasmonic back reflector on the performance of a-Si:H solar cells was investigated
experimentally. AZO was deposited on glass. On this flat front TCO an a-Si:H p-i-n structure with a 150
nm thick intrinsic layer was deposited using PECVD. Finally, either a state-of-the-art 80 nm thick AZO
followed by an opaque Ag layer or a plasmonic back reflector was added to finish the solar cell. The plasmonic
back reflector has a similar structure, but has silver nano-particles embedded in the middle of the AZO layer.
The silver nano-particles were formed by depositing a 30 nm thin Ag layer followed by a 1-hr anneal at
180 °C. Due to surface tension the Ag film breaks up into islands. A scanning electron micrograph (SEM)
image of the islands is shown in Fig. 4(a). As can be seen, the Ag islands have a highly irregular shape
and a size of several hundreds of nanometers. The surface coverage is about 40%. Islands with a more
regular shape could be formed by annealing a thinner layer of Ag, however this would result in smaller
islands and give rise to more absorption and less scattering of light.
1.0
without nanoparticles
with nanoparticles
0.8
EQE
0.6
0.4
0.2
1 m
0.0
300
400
500
600
700
800
Wavelength [nm]
(a)
(b)
Fig. 4. (Colour online) (a) Top: Photographs of the samples with similar Ag nano-particles but in different dielectric
environments. Left: glass/particles, middle: glass/particles/ZnO, right: glass/ZnO/particles/ZnO. Bottom: A schematic
representation of the solar cell with integrated nano-particles at the rear side. An SEM image of Ag islands (nanoparticles) on AZO, formed by annealing a 30 nm thick Ag film is also shown; (b) EQE of a-Si:H cell device with and
without silver nano-particles embedded in AZO layer at the back.
The EQE of the devices with and without nano-particles were measured and the results are shown in
Fig. 4(b). It can be seen that the EQE of the device with nano-particles is somewhat higher in the
wavelength range 620-740 nm. This, and the fact that interference oscillations are less pronounced,
indicates that the particles scatter light into the absorber layer diffusely. As a result of the improved light
trapping total JSC has increased from 10.32 mA/cm2 to 10.87 mA/cm2 , which is 5 %. It is expected that even
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larger gains in JSC could be obtained if these large nano-particles could be fabricated with a more uniform size
and shape. Introduction of textured interfaces into the flat cell can lead to significantly larger increases in JSC,
however, in combination with metal nano-particles further optical improvements are expected.
3.3. 1-D photonic crystals
Optical losses can occur at the metallic back contact of thin-film silicon solar cells because the surfacetextured metal back reflectors suffer from undesired surface plasmon absorption [42]. This is a parasitic
effect which we want to avoid also when introducing metal nano-particles in the role of light scatterers
(see previous section). In case of back reflectors alternative solutions based on dielectric materials are
investigated [16-19]. High-reflectance characteristics can be achieved for example by one-dimensional
photonic crystal structures (1-D PC) which act as distributed Bragg reflector. 1-D PC is a multilayer
structure in which two layers with different optical properties (refractive indexes) are periodically
alternated. When light propagates through this structure, constructive and deconstructive interferences
arise, resulting in the wavelength-selective reflectance or transmittance behaviour. It has been demonstrated that different materials, such as a-Si:H, a-SiNx:H, a-SiOx, ZnO and others, can be employed to
form 1-D PC structures suitable for the back or intermediate reflectors in thin-film solar cells [18]. In this
paper we present the results related to a PC back reflector realised by 4 layers of a-Si:H (d = 35 nm) and 4
layers of a-SiNx:H (d = 85 nm) material. The measured reflectance of the PC stack deposited on glass
substrate is shown in the insert of Fig. 5(b). A-Si:H single junction solar cells were deposited with the PC
back reflector. As the front contact indium tin oxide (ITO) TCO layer was used (d = 400 nm). As the back
contact AZO layer (d = 500 nm) was sputtered on n-layer followed by the PC stack. Local contacts were
made through the PC stack using reactive ion etching and Ag evaporation. The SEM image of the
structure is given in Fig. 5(a). In Fig. 5(b) EQE results of the cells are presented. One can see that the
EQE curve of the a-Si:H cell with the PC back reflector approaches the one achieved with Ag back
reflector. Electrical parameters of the cells were not optimised.
1.0
metal via
metal (Ag)
0.9
AZO / 1-D PC
AZO / Ag
0.8
a part of PC
(previous cell dot)
0.7
EQE
0.6
0.5
1.0
0.8
RTOT
0.4
0.3
0.4
air / 1-D PC
air / Ag
0.2
0.2
0.0
350
0.1
0.0
300
0.6
450
550
650
750
850
Wavelength (nm)
400
500
600
700
800
Wavelength (nm)
(a)
(b)
Fig. 5. (a) SEM image showing the layout of the fabricated solar cell devices with PC back reflector; (b) (Colour
online) External quantum efficiencies of solar cells with PC and Ag back reflector. The corresponding JSC values
calculated from the EQE are 11.43 mA/cm2 for AZO/PC and 11.87 mA/cm2 for AZO/Ag back reflector. The inset
shows the reflectance of the stand alone PC stack and of a silver layer as measured in air.
Using a PC stack with higher number of layers or employing materials with higher contrast in
refractive indexes, the reflectivity of the stack can be raised up, close to 100 % in a broad wavelength
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region, leading to higher improvements in EQE of the cells with PC back reflectors. If a PC back reflector
is implemented in the cell where surface texture is present at the back side and the interfaces of the PC
stack appear to be textured as well, one should be aware of some losses in reflectance that may arise, if
coherency of light waves is destructed.
4. Conclusions
In this article the important issues of light management in thin-film silicon solar cells were highlighted.
The role of optical modelling and simulations in design and implementation of advanced optical concepts
in thin-film solar cells is discussed. A scattering model for the calculation of the angular intensity
distribution of the diffused light at surface-textured TCO layers was presented. Selected results of 3-D
simulations of an a-Si:H solar cell with periodically textured interfaces were presented. Three approaches of
advanced light trapping in thin-film solar cells were presented: i) modulated surface textures, ii) plasmonic
scattering using metal nano-particles, and iii) one-dimensional photonic-crystal-like structures for back reflectors.
The modulated surface textures were realised by etched glass / etched AZO substrates, showing high haze also
at longer wavelengths. A-Si:H solar cells deposited on the substrates with the modulated texture showed improved
EQE and JSC (8 % relative increase) in comparison to the cells deposited on flat glass / etched AZO substrates.
Modulated surface-textures have potential for enhanced scattering and improved light trapping in thin-film solar
cells. Metal nano-particles are efficient light scatterers and can be used for light trapping in solar cells. Silver
nano-particles were fabricated by annealing a film of silver. These particles were used to form the plasmonic
back reflector of a thin a-Si:H solar cell. Compared to a similar device without nano-particles the JSC increased
by 5 %.
1-D PC can be used to obtain high reflectance at the back contact in a broad and tuneable wavelength
region. 1-D PCs based on a-Si:H and a-SiNx:H layers were designed and implemented in a-Si:H solar
cells. Solar cell with implemented simple (8 layer) PC stack rendered similar output characteristics as the
reference cell with Ag back reflector.
Further optimisation of the presented concepts is required, resulting in even higher improvements in
the performance of thin-film solar cells.
Acknowledgements
This work was partially carried out with a subsidy of the Dutch Ministry of Economic Affairs under
EOS program (Projects EOSLT04029 and KTOT01028), Nuon Helianthos company and Slovenian
Research Agency (Project J2-0851-1538-08). The authors gratefully acknowledge financial support from
the NMP-Energy Joint Call FP7 SOLAMON Project (www.solamon.eu). The authors thank PV-LAB of
the École Polytechnique Fédérale de Lausanne, Switzerland, for BZO samples for scattering model
verification.
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