Advanced Light Management Approaches for Thin-Film Silicon Solar Cells

Energy Procedia 15 (2012) 189 – 199

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.

doi:10.1016/j.egypro.2012.02.022

Energy

Procedia

www.elsevier.com/locate/procedia

International Conference on Materials for Advanced Technologies 2011, Symposium O

Advanced Light Management Approaches

for Thin-Film Silicon Solar Cells

M. Zeman

_{, O. Isabella}

_{, K. Jäger}

_{, R. Santbergen}

_,

S. Solntsev

, M. Topic

and J. Krc

a_{Delft University of Technology, DIMES, P.O. Box 5053, 2600 GB Delft, The Netherlands} b_{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 three-dimensions. 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.

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of Solar Energy 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: m.zeman@tudelft.nl

© 2011 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of the organizing committee of International Conference on Materials for Advanced Technologies.

2 M. Zeman et al. / Energy Procedia 00 (2011) 000–000

i. efficient trapping and enhanced absorption of incident light in the desired parts of solar cell structures (inside absorber layers);

ii. minimisation of reflection losses at the front interfaces and absorption losses in the solar cell structure outside the absorber layers;

iii. 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, a-SiOx 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 n- doped 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 multi-junction devices [29]. Multi-multi-junction solar cells like tandem a-Si:H/ c-Si:H micromorph [30] and triple-junction, 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 all-silicon 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

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

4 M. Zeman et al. / Energy Procedia 00 (2011) 000–000

 





_

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

 

 

exp , .

ik  ikz x y

 _ _

Z (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.

 

T T tot tot K K K K k λ AID AID T λ T λ T λ H λ T λ T λ AID         





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_.

2.2. Examples of 3-D optical simulations

To determine optical situation inside thin-film solar cells by considering exact geometry of the struc-ture and the morphology of the texstruc-tured 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 trapezoid-like 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].

textured flat

(a) (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. Wavelength [nm] 350 400 450 500 550 600 650 700 750 800 Ab so rpta nc e an d op tica l lo ss es 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 R ZnO:Al p n ZnO Ag i - Text i - Flat

6 M. Zeman et al. / Energy Procedia 00 (2011) 000–000

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).

(a) (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 Y90 corresponds to the yield

(percentage of cells that reach at least 90 % of the efficiency level of the best cell).

Wavelength [nm] 300 400 500 600 700 800 Haze in tra nsm ission 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

flat glass / etched AZO (ref.) etched glass / etched AZO etched glass Wavelength [nm] 400 500 600 700 800 EQE 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0

flat glass / etched AZO (13.83 mA/cm2_{) (ref.)}

etched glass / etched AZO (14.98 mA/cm2₎

etched glass / not-etched AZO (12.18 mA/cm2₎

etched glass/etched AZO

VOC = 0.899 V

JSC= 14.98 mA/cm2

FF = 0.713  = 9.59 % Y90 = 70.0% over 30 dots

etched glass / etched AZO

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.

Wavelength [nm] 300 400 500 600 700 800 EQ E 0.0 0.2 0.4 0.6 0.8 1.0 without nanoparticles with nanoparticles (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 (nano-particles) 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

8 M. Zeman et al. / Energy Procedia 00 (2011) 000–000

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 surface-textured 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 demon-strated 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.

(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/cm}2 _{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

Wavelength (nm) 300 400 500 600 700 800 EQE 0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 AZO / 1-D PC AZO / Ag

metal via metal (Ag)

a part of PC (previous cell dot)

Wavelength (nm) 350 450 550 650 750 850 RTOT 0.0 0.2 0.4 0.6 0.8 1.0 air / 1-D PC air / Ag

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.

References

[1] Sato K, Gotoh Y, Wakayama Y, Hayashi Y, Adachi K, Nishimura H. Highly textured SnO2:F TCO films for a-Si solar

cells. Rep. Res. Lab. Asahi Glass Co. Ltd. 1992; 42:129-137.

[2] Dominé D, Buehlmann P, Bailat J, Billet A, Feltrin A, Ballif C. Optical management in high-efficiency thin-film silicon micromorph solar cells with a silicon oxide based intermediate reflector. Phys. Status Solidi RRL 2008; 2:163.

10 M. Zeman et al. / Energy Procedia 00 (2011) 000–000

[3] Berginski M, Hüpkes J, Schulte M, Schöpe G, Stiebig H, Rech B, Wuttig M. The effect of front ZnO:Al surface texture and optical transparency on efficient light trapping in silicon thin-film solar cells. J. Appl. Phys. 2007; 101:074903.

[4] Haug FJ, Soederstroem T, Python M, Terrazzoni-Daudrix V, Niquille X, Ballif C. Development of micromorph tandem solar cells on flexible low-cost plastic substrates. Sol. Energy Mat. Sol. Cells 2009; 93:884-997.

[5] Söderström T, Haug FJ, Terrazzoni-Daudrix V, Ballif C. Flexible micromorph tandem a-Si/c-Si solar cells. J. App. Phys. 2010; 107:014507.

[6] Heijna MCR, Goris MJAA, Soppe WJ, Schipper W, Wilde R. Roll-to-roll nanotexturisation of layers on steel foil sub-strates for nip silicon solar cells. Proc. 25th European Photovoltaic Solar Energy Conf., Valencia, Spain; 2010, p. 3090-3, 3AV.1.73.

[7] Isabella O, Campa A, Heijna MCR, Soppe WJ, van Erven R, Franken RH, Borg H, Zeman M. Diffraction gratings for light trapping in thin film silicon solar cells. Proc. 23rd _{European Photovoltaic Solar Energy Conf., Valencia, Spain; 2008, p.}

2320-4, 3AV.1.48.

[8] Campa A, Isabella O, van Erven R, Peeters P, Borg H, Krc J, Topic M, Zeman M. Optimal design of periodic surface texture for thin-film a-Si:H solar cells. Prog. Photovolt.: Res. Appl. 2010; 18:160-7.

[9] Atwater HA, Polman A. Plasmonics for improved photovoltaic devices. Nat. Mater. 2010; 9:205-13.

[10] Stuart HR, Hall DG. Island size effects in nanoparticle-enhanced photodetectors. Appl. Phys. Lett. 1998;73: 3815-7. [11] Santbergen R, Liang R, Zeman M. A-Si solar cells with embedded silver nanoparticles. Proc. 35th _{IEEE Photovoltaic}

Specialist Conf., Honolulu, Hawaii; 2010, p.748-53.

[12] Meier J, Kroll U, Spitznagel J, Büchel A. Progress in up-scaling of thin film silicon solar cells by large-area PECVD KAI systems. Proc. 31st IEEE Photovoltaic Specialists Conf., Orlando, Florida; 2005, p. 1464-7.

[13] Berger O, Inns D, Aberle AG. Commercial white paint as back surface reflector for thin-film solar cells. Sol. Energy Mat.

Sol. Cells; 2007; 91:1215-21.

[14] Lipovsek B, Krc J, Isabella O, Zeman M, Topic M. Analysis of thin-film silicon solar cells with white paint back reflectors.

phys. status solidi c 2010; 7(3-4):1041-4.

[15] Hüpkes J, Wätjen T, Van Aubel R, Schmitz R, Reetz W, Gordijn A. Material study on ZnO/Ag back reflectors for silicon thin film solar cells. Proc. 23th European Photovoltaic Solar Energy Conf., Valencia, Spain; 2008, p. 2419 - 21, 3AV.2.24. [16] Zeng L, Bermel P, Yi Y, Alamariu BA, Broderick KA, Liu J, Hong C, Duan X, Joannopoulos J, Kimerling LC. Demonstra-tion of enhanced absorpDemonstra-tion in thin film Si solar cells with textured photonic crystal back reflector. Appl. Phys. Lett. 2008;

93:221105.

[17] Curtin B, Biswas R, Dalal V. Mater. Res. Soc. Symp. Proc. 2010; 1245:1245, A07-21.

[18] Isabella O, Krč J, Zeman M. Application of photonic crystals as back reflectors in thin-film silicon solar cells. Proc. 24th

European Photovoltaic Solar Energy Conf., Hamburg, Germany; 2009, p. 2304-9.

[19] Krč J, Zeman M, Luxembourg S, Topic M. Modulated photonic-crystal structures as broadband back reflectors in thin-film silicon solar cells. Appl. Phys. Lett. 2009; 94:153501.

[20] Fischer D, Dubail S, Anna Selvan JA, Pellaton Vaucher N, Platz R, Hof Ch. et al. The ''micromorph'' solar cell: extending a-Si:H technology towards thin film crystalline silicon. Proc. 25th IEEE Photovoltaic Specialist Conf., Washington, DC; 1996, p. 1053.

[21] Yamamoto K, Yoshimi M, Tawada Y, Fukuda S, Sawada T, Meguro T, Takata H, Suezaki T, Koi Y, Hayashi K, Suzuki T, Ichikawa M, Nakajima A. Large area thin film Si module. Sol. Energy Mat. Sol. Cells 2002; 74:449.

[22] Buehlmann P, Bailat J, Dominé D, Billet A, Meillaud F, Feltrin A, Ballif C. In situ silicon oxide based intermediate reflector for thin-film silicon micromorph solar cells. Appl. Phys. Lett. 2007; 91:143505.

[23] Despeisse M, Bugnon G, Feltrin A, Stueckelberger M, Cuony P, Meillaud F, Billet A, Ballif C. Resistive interlayer for improved performance of thin film silicon solar cells on highly textured substrate. Appl. Phys. Lett. 2010; 96:073507. [24] Krč J, Smole F, Topic M. Optical simulation of the role of reflecting interlayers in tandem micromorph silicon solar cells.

Sol. Energy Mat. Sol. Cells 2005; 86:537-50.

[25] Fay S, Steinhauser J, Oliveira N, Vallat-Sauvain E, Ballif C. Opto-electronic properties of rough LP-CVD ZnO:B for use as TCO in thin-film silicon solar cells. Thin Solid Films 2007; 515:8558-61.

[26] Berginski M, Huepkes J, Reetz W, Rech B, Wuttig M. Recent development on surface-textured ZnO:Al films prepared by sputtering for thin-film solar cell application. Thin Solid Films 2008; 516:5836-41.

[27] Kambe M, Matsu Ti, Sai H, Taneda N, Masumo K, Takahashi A, Ikeda T, Oyama T, Kondo M, Sato K. Improved light-trapping effect in a-Si:H/uc-Si:H tandem solar cells by using high haze SnO2:F thin films. Proc. 34th IEEE Photovoltaic Specialists Conf., Philadelphia, PA; 2009, p. 1891-4.

[28] Das C, Lambertz A, Huepkes J, Reetz W, Finger F. A constructive combination of antireflection and intermediate-reflector layers for a-Si/mu c-Si thin film solar cells. Appl. Phys. Lett. 2008; 92:053509.

[29] Shockley W, Queisser HJ. Detailed balance limit of efficiency of P-N junction solar cells. J. Appl. Phys. 1961; 32:510-9. [30] Yamamoto K, Nakajima A, Yoshimi M, Sawada T, Fukuda S, Suezaki T et al. High efficiency thin film silicon hybrid cell

[31] Benagli S, Borrello D, Vallat-Sauvain E, Meier J, Kroll U, Hötzel Jet al. High-efficiency amorphous silicon devices on LPCVD-ZNO TCO prepared in industrial KAI-M R&D reactor. Proc. 24th European Photovoltaic Solar Energy Conf.,

Hamburg, Germany; 2009.

[32] Guha S et al. Proc. 15th _{International Photovoltaic Science and Engineering Conf., Shanghai, China; 2005, p. 35.}

[33] Barnett A, Kirkpatrick D, Honsberg C, Moore D, Wanlass M, Emery K et al. Very high efficiency solar cell modules.

Prog. Photovolt.: Res. Appl. 2009; 17:75-83.

[34] Ginley D, Green MA, Collins R. Solar energy conversion toward 1 terawatt. MRS Bulletin 2008; 33:355.

[35] Ivanova S, Pellé F, Esteban R, Laroche M, Greffet JJ, Collin S, Pelouard JL, Guillemoles JF. Thin film concepts for photon addition materials. Proc. 23rd _{European Photovoltaic Solar Energy Conf., Valencia, Spain; 2008, 734-6.}

[36] Loeper P, Goldschmidt JC, Fischer S, Peters M, Meijerink A, Biner D, Krämer K, Schultz-Wittmann O, Glunz SW, Luther J. Upconversion for silicon solar cells: material and system characterization. Proc. 23rd _{European Photovoltaic Solar}

Energy Conf., Valencia, Spain; 2008, p.173-80, 1CO.3.1.

[37] Zeman M, Isabella O, Tichelaar FD, Luxembourg SL. Amorphous silicon based multilayers for photovoltaic applications,

phys. status solidi c 2010; 7 (3-4): 1057-60.

[38] Conibeer G, Patterson R, Huang L, Guillemoles J, König D, Shrestha S, Green MA. Modelling of hot carrier solar cell absorbers. Sol. Energy Mat. Sol. Cells 2010; 94:1516-21.

[39] Luque A, Marti A, Nozik AJ. Solar cells based on quantum dots: Multiple exciton generation and intermediate bands. MRS

Bulletin 2007; 32:236-41.

[40] Zeman M, Willemen JA, Vosteen LLA, Tao G, Metselaar JW. Computer modeling of current matching in a-Si:H/a-Si:H tandem solar cells on textured substrates. Sol. Energy Mat. Sol. Cells 1997; 46:81.

[41] Krc J, Smole F, Topic M. Analysis of light scattering in amorphous Si : H solar cells by a one-dimensional semi-coherent optical model. Prog. Photovolt.: Res. Appl .2003; 11:15-26.

[42] Springer J, Poruba A, Vanecek M. Improved three-dimensional optical model for thin-film silicon solar cells. J. Appl. Phys. 2004; 96:5329-37.

[43] Lanz T, Reinke NA, Prucco B, Rezzonico D, Ruhstaller, B. Light scattering simulation for thin film silicon solar cells.

Optical nanostructures for PV, OSA technical digest (CD) (Optical Society of America 2010), paper PTuB3.

[44] Krc J, Zeman M, Smole F, Topic M. Optical modeling of a-Si:H solar cells deposited on textured glass/SnO2 substrates. J. Appl. Phys. 2002; 92:749-55.

[45] Jäger K, Zeman M. A scattering model for surface-textured thin films. Appl. Phys. Lett. 2009; 95:171108.

[46] Dominé D, Haug FJ, Battaglia C, Ballif C. Modeling of light scattering from micro- and nanotextured surfaces. J. Appl.

Phys. 2010; 107:044504.

[47] Beckmann P, Spizzichino A. The Scattering of Electromagnetic Waves from Rough Surfaces. Pergamon Press 1963. [48] Carniglia CK. Scalar scattering theory for multilayer optical coatings. Opt. Eng. 1979; 18:104-15.

[49] Born M, Wolf E. Principles of optics. 7th _{ed. Cambridge University Press, Cambridge 1999, chapter 13.}

[50] Born M, Wolf E. Principles of optics. 7th _{ed. Cambridge University Press, Cambridge 1999, chapter 8.}

[51] Haase C, Stiebig H. Thin-film silicon solar cells with efficient periodic light trapping texture. Appl. Phys. Lett. 2007;

91:061116.

[52] Campa A, Krc J, Topic M. Analysis and optimisation of microcrystalline silicon solar cells with periodic sinusoidal textured interfaces by two-dimensional optical simulations. J. Appl. Phys. 2009; 105:083107.

[53] Bittkau K, Beckers T, Fahr S, Rockstuhl C, Lederer F, Carius R. Nanoscale investigation of light-trapping in a-Si:H solar cell structures with randomly textured interfaces. phys. stat. sol. a 2008; 205:2766-76.

[54] Dewan R, Knipp D. Light trapping in thin-film silicon solar cells with integrated diffraction grating. J. Appl. Phys. 2009;

106:074901.

[55] Rockstuhl C, Lederer F, Bittkau K, Beckers T, Carius R. The impact of intermediate reflectors on light absorption in tandem solar cells with randomly textured surfaces. Appl. Phys. Lett. 2009; 94:211101.

[56] Fahr S, Rockstuhl C, Lederer F. Engineering the randomness for enhanced absorption in solar cells. Appl. Phys. Lett. 2008;

92:171114.

[57] Chen J, Wang O, Li H. Microstructured design for light trapping in thin-film silicon solar cells. Opt. Eng. 2010; 49:088001. [58] Ansys HFSS official website. http://www.ansoft.com/products/hf/hfss/, 2011.

[59] Isabella O, Krc J, Zeman M. Modulated surface textures for enhanced light trapping in thin-film silicon solar cells. Appl.