light-trapping nanostructures for amorphous silicon solar cells (1).docx

5/24/2018 light-trapping nanostructures for amorphous silicon solar cells (1).docx

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PHYS-E0582 Special Course in Advanced Energy Technologies 2 V

Light-trapping nanostructures for

amorphous silicon solar cells

Salazar Martinez Arturo (introduction, nanocones)

Ilmanen Paulus (nanodomes)

Omelyanovich Mikhail (novel light-trapping nanostructures, conclusion)


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AALTO 2014

Introduction

Crystalline and amorphous Silicon films have been extensively studied and utilized inPhotovoltaic (PV) energy generation. While conventional crystalline Si (c-Si) films, which are

typically manufactured to thicknesses between 100-300 m, account for nearly 80% of the share

of PV generation technologies (Han, Sang Eon et al., 2011), they still suffer from high

production and installation costs, on top of low light trapping capabilities due to the high

reflection of light that is inherent to their design (Rui, Yu et al, 2011; Han, Sang Eon et al., 2011;

Zhu, Yia et al., 2009; Jeo, Sangmoo et al., 2012). Recent developments in 3 dimensional nano-

structuring of amorphous hydrogenated Silicon (a-Si:H) have yielded important results in terms

of cost reduction in the production of thin silicon films. Normally ranging in the 1-2 m

thickness, structures such as nanowires, nanocones, nanodomes, nanopillars and tapered

nanowires amongst others; have achieved a reduction in the amount of high quality Si needed for

their production, which accounts to roughly 40% of the total module cost according to many

authors, while at the same time improving the total cost per watt resulting from the increased

efficiency in light absorbance (Rui, Yu et al, 2011; Han, Sang Eon et al., 2011; Zhu, Yia et al.,

2009; Jeo, Sangmoo et al., 2012; Xingze Wang, Ken et al., 2012; Wensheng, Yan et al., 2012;

Demsy and John, 2012).

In this paper, simple and cost-efficient structures such as nanocones and nanodomes will be

reviewed, along with a presentation of more advanced structures currently being developed in the

areas of nano-scaled structures for performance improvement in the area of thin Si films for PV

generation.

Nanocones

Nanocone structuring in a-Si:H has been developed in recent years. While simpler structures

such as nanowires exist, and are relatively easier to be produced, and other more complex

structures are under development, nanocones represent one of the most promising solutions due


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to the fabrication techniques utilized in its production; Reactive Ion Etching (RIE) and

Langmuir-Blodgett (LB) assembly for their etching, which allow for good control of its

dimensions and aspect ratios of the different conic structures that can be built into a-Si:H thin

films (Xingze Wang, Ken et al., 2012).

Nanocones can be produced in multiple sizes of diameter and height, as well as placed either on

top or bottom of the Si layer depending on the type of application and the optimization of the

design of the film. Typical diameter size for nanocones ranges from 100-1000 nm, while the

height can range from 100-1500 nm; both variables are relevant to the wavelength of light to be

captured by the film (Zhu, Jia et al., 2009; Sangmoo et al., 2012; Xingze Wang, Ken et al., 2012;

Demsy and John, 2012). Figure 1 presents a typical sequence for the production of thin films

with nanocone texture.

Figure 1. (a-d) Schematic illustration of 1 m thick a-Si:H on ITO-coated glass substrate, a monolayer of

silica nanoparticles on top of a-Si:H thin film, NW arrays, and NC arrays. The effective refractive index

profiles of the interfaces between air and (e) a-Si:H thin film, (f) 600 nm a-Si:H NW arrays, and (g) 600 nm a-

Si:H NC arrays. Zhu, Jia et al., 2009

From the image, the process can be explained to start from a layer of (In this example) Indium

tin oxide (Although aluminum or silver, among other metals can be utilized as well), in which a

thicker layer of a-Si:H is deposited using chemical vapor deposition of 1000nm thickness. By the


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utilization of the LB process, Silica (SiO2) nanoparticles are deposited on top by spin coating,

which provide a shielding effect while the a-Si:H is reacted with a chlorine based reagent in the

RIE process. Due to the difference in reactive rates between silica and silicon, the process yields

either nanowires (c), or nanocones if reacted for a longer period (d) (Zhu, Jia et al., 2009). Figure

2 presents a series of electronic microscopy images that display silica nanoparticles on top of the

silicon wafer, as well as the final nanocone and nanowire structures after RIE.

Figure 2.(a, c, e) SEM images in a large area of a monolayer of silica nanoparticles, a-Si:H NC arrays, and a-

Si:H NW arrays, respectively. (b, d, f) Zoom-in SEM images of silica nanoparticles, a-Si:H NCs, and a-Si:H

NWs, respectively. Zhu, Jia et al., 2009.

One of the most important results from the preparation of nano-scaled structures in PV cell

surfaces has been the reduction of light reflectance, which results in both increased absorbance

and cell efficiency. Figure 3 displays the difference between a flat silicon surface and one with

nanocones, at determined visible light wavelengths, the light absorption on nanocones is nearly

100%, which can be seen by the black appearance of the surface.


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Figure 3.Reduction in light reflection between flat Si thin film , Nanowires and Nanocones (Zhu, Jia et al.,

2009).

The blackening effect of the nanocone structures can be explained due to the gradual decrease in

reflectivity resulting from the gradual decrease in reflectance at the silicone-air interface. With

an usable solar light spectrum ranging between 300-1100 nm in wavelength, the aforementioned

properties of thin layers with nanocones have been reported to achieve nearly 90% absorbance

(Kwon Beomjin, et al.; 2013). On the other hand, Xingze Wang, Ken et al. (2012), devised a

double grating design in which the top layer reduces reflectance, while the bottom grating with

nanocones reduces losses of absorbed lights resulting in increased absorption and light

conservation. In general, nanocones are proven to have near to optimal efficiency or close to the

bandgap limit for the electrical current generation. Figure 4 illustrates the different combinations

of top and bottom gratings with nanocones explained by Xingze Wang, Ken et al. (2012).


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Figure 3. Top and bottom gratings for increased absorption and decreased reflectance in silicon surfaces.

Xingze Wang, Ken et al. (2012).

On the other hand, by utilizing different combinations of p-n junctions, other authors, such as

Jeong, Sangmoo et al (2013). Such combinations refer to the placement of the dopings in the

Silicon surface, with the back doping resulting in the highest external quantum efficiency

(EQE) for their models, which fully exploits the benefits from reduced reflectance and increased

light storage within the Si layer. Figure 4 shows the back doping design on a 10 m film on an

Aluminum base.

Figure 4. Front and back doping comparisons for silicon films with nanocones and their achieved EQE.(Jeong, Sangmoo et al, 2013)

Another interesting design, is the one developed by Sangmoo et al. (2012). In their work, a

hybrid material film with nanocone top texture was developed by combining Silicon, a

conductive polymer and gold particles. In their design, as with others, the Si layer is prepared

with silica particles via the Langmuir-Blodgett method and RIE reaction. Consequently, a

conductive polymer layer poly(3,4-ethylene dioxythiophene):poly-(styrenesulfonate)) is applied

at low temperatures via spin coating and replaces the p,n junction. In order to increase the

conducting properties of the polymer surface, Au particles were deposited on top of the polymer

in the form of fingers running periodically along the top area of the film. The achieved results

are presented in Table 1, while a representation of their design is displayed in Figure 5.


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Table 1. Photovoltaic properties of a hybrid Si-polymer solar cells (Cones and flat). Jeong, Sangmoo et al,

2012

As it can be seen, a nanocone hybrid with Au grid resulted in a well balanced current of 31

mA/cm2, while keeping an open circuit voltage (Voc) of 0.5 V, and resulting in the highest power

conversion efficiency of 9.62% (Some efficiencies were reported up to 11% in their article).

Figure 6 represents a summary of absorption, reflection and currents with different aspect ratios

for the cones and at different wavelengths of light.

Figure 5. Schematicof hybrid Si-polymer film with Au grid. Jeong, Sangmoo et al, 2012.


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Figure 6. Comparison of results between conventional flat surface and nanocone designs of pure silicon and

polymer and gold hybrids. Jeong, Sangmoo et al, 2012.

As it can be observed, the absorption results are heavily benefited by the inclusion of nanocones.

Another finding comes from the optimal aspect ratio, which, close to 1, yields currents close to

40 mA/cm2, slightly short from 42.7 mA/cm

2; which represents the record for monocrystalline

silicon cells.

Regarding the optimal aspect ratio for nanocone structures, Wang and Leu (2012) determined a

surface plot for comparison between nanocones and nanowires and the delivered cell efficiency.

The plot can be seen in figure 7.


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Figure 7. (a) Ultimate efficiency of Silicon nanowire arrays (b) Contour plot for ultimate efficiency of

nanocones and nanowires as a function of the aspect ratio (diameter:height). Wang and Leu, 2012

As observed in the above plot, the optimal area lies in nanocone areas of bottom diameters close

to 600 nm and top diameters around 200nm. Table 2 presents the absorbance of light at different

wavelengths of visible light between nanowires and nanocones.

Table 2. Absorption of visible light at different wavelengths for nanowires and nanocones close to their

optimal aspect ratio according to Wang and Leu, 2012

As this paper approaches the findings for nanodomes, it is important to remark that, as nanowires

can be either a final design or an intermediate stage towards nanocones, nanocones in their own

can be the foundation for nanodome structures. This are achieved by either applying different

etching methods with silica or by coating the nanocones with, e.g., Transparent conducting

oxides (TCO), according to Zhu, Jia et al (2010).


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NanodomesAlthough nanocones as it is seems like a very good alternative as light trapping nanostructure, it

can be further modified to produce nanodomes. That can be done by depositing multilayers on

nanocones. Formed nanodome holds the same pattern as the nanocones used as a substrate. Zhu,

Jia et al. (2010) present two important advantages on using nanodome solar cells with thickness

less than 300 nm. Carriers of a-Si:H have a carrier diffusion length of 300 nm and Stabler-

Wronski effect is lesser in cells thinner than 300 nm. In very thick films carriers will recombine

and solar cell performance will be deteriorated. Stabler-Wronski effect means 10-30 %

efficiency reduction under light soaking. For these reasons it is very advantageous to try to

enhance performance of nanodome solar cells with thin film thickness under 300 nm.

Nanodome consist of multiple layers. Upper layer of the dome can be made for example from

silicon nitride or indium tin oxide. Domes on the surface scatter light along the plane of the film,

which enhances absorption. Nanodome structure also includes silver as a back reflector at the

bottom. Light absorbing a-Si thin film is in between those two layers. Because of its

multilayered structure nanodomes have both antireflective and light trapping abilities which

gives it an advantage over normal thin film solar cells. Typical nanodome structure is presented

in figure 8. At the bottom is 100 nm back reflective Ag film followed by 80 nm transparent

conducting oxide(TCO) film. In the middle is 280 nm Si thin film and at the top is another 80 nm

TCO film. (Zhu, Jia et al. (2010), Wu, X. et al. (2013)).

Figure 8. Typical structure of nanodome made by depositing layers of different materials on nanocone

substrate. Zhu, Jia et al., (2010)


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Zhu, Jia et al. (2010) conducted measurements with nanodome and normal flat film solar cells to

show anti reflection and light trapping enhancement. Measurements are presented in figure 9.

Nanodome solar cell using specs previously described reached up to 94 % absorption with

wavelengths of 400-800 nm whereas flat film with TCO layer on top as well reached only 65 %

absorption. Both devices had better results with TCO layer than without it. To evaluate

performance in normal environment, it is important to make the measurements with different

lighting. Nanodome had less loss in efficiency than flat film when angle was changed, which

indicated that it would be better option for solar cells in normal outside situations.

Figure 9. Absorption measurements with normal incidence, 30 and 60 angle of incidence. Indium tin oxide(ITO) was used as a TCO. Zhu, Jia et al., (2010)

To further prove effect of antireflection and light trapping Zhu, Jia et al. (2010) also performed

test in solar simulator (1 sun AM 1.5G illumination). Power conversion efficiency was 25 %

higher for nanodome. It had power efficiency of 5.9% (

while flat film only had 4.7 % ( . Improvement

efficiency can be explained with nanodomes big short-circuit current ( ). Theoretical

maximum value is so nanodome devices current value was very good. Authors suggestthat efficiency could be even further enhanced by improving material depositing to achieve

better values for open circuit voltage and fill factor. Figure 10a shows clearly the difference in


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light absorption between nanodome and flat film solar cells. Red colour can be observed in

picture of flat film solar cell, which indicates worse absorption than in nanodome solar cell.

Results of the test are shown in figure 10b.

Figure 10. (a) Photo of nanodome solar cell (left) and flat film solar cell (right). (b) Nanodome solar cell (left)

and flat film solar cell (right) power conversion. Zhu, Jia et al. (2010)

There has also been research with aim to reduce thickness of the a-Si thin film even further. Wu,

X. et al. (2013) performed analysis on ultrathin nanodome solar cells. Structure of the nanodome

used in simulations is presented in figure 11. They managed to design a structure with 26 nm

effective thickness of active layer. Simulations showed its active layer absorption to be 71.3 %,

which is very good considering how thin the film is. Changing the symmetrical structure of


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nanodomes to asymmetrical one was also tested. It enhanced active layer absorption from 71.3

% to 74.3 %. The result is promising for the future of ultrathin nanodome devices.

Figure 11. Cross-sectional structure of the simulated nanodome. . Wu, X. et al. (2013)

On top of nanocones and nanodomes there are also many alternative structures to be used to

enhance solar cell performance. Lately studies have concentrated on more complicated structures

than simple domes, wires or cones.


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Novel light-trapping nanostructures

In recent years scientist start develop quite exotic methods for light-trapping in thin film

amorphous solar cells. One of good example is work of Constantin Simovski, et al, 2013. In that

work scientist use of the nanoantenna arrays (Figure 12). That array of nano antennas should be

geometrically rather sparse. In Fig. 8 whose top view is similar to a square mesh of split wires.

The distance between tapered nano strips is sufficiently small for the excitation of domino modes

and sufficiently large to avoid the critical reflection beyond the band of these modes. This

explains the advantage of the suggested general design solution.

Figure 12. A schematic of thin-film solar cell with a light-trapping structure (left) and a top

view of the nanoantenna arrays (right).Constantin Simovski, et al, 2013.

This design employs the advantages of collective oscillations excited in the visible or infrared

spectral range by an incident plane wave in a lattice of Ag nanobar-based nano antennas. They

demonstrate that such weakly-resonant light-trapping structure can enhance substantially the

photo voltaic absorption in very thin (100-150 nm) underlying semiconductor layers. This light-

trapping structure are in-plane optically isotropic being also independent on the light

polarization. They develop also two different type of light-trapping structures for two different


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wavelength regions (Figure 13). Which may allow producers to further increase the performance

of the sensor in the desired range. That structure looks really not typical and wrong for typical

users.

Figure 13.Left: A unit cell of the interband TFSC based on CIGS. Right: A unit cell of the TFSC based on Si.

Side view and top view of the unit cell are given in scale with the reference length unit. P-doped and n-doped

parts of the PV are shown by different colors. Constantin Simovski, et al, 2013

Electric field in that structure we can see in Figure 14, were we can note that we have maximum

field efficiency across the borders of nano strips.

Figure 14. Electric field amplitude for =810 nm illustrating the concept of the LTS: (a) central vertical

cross section; (b) horizontal plane P1. The insulating layer of silica (2 nm) is not detectable. The incident wave

has the amplitude of 1 V/m. Constantin Simovski, et al, 2013


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In Figs. 14(a,b), they show the local field distribution at =810 nm in the central vertical cross

section of the unit cell and in the horizontal plane P1. This plane corresponds to the boundary

between nano antennas and a thin insulating layer of amorphous silica electrically isolating nano

antennas from the semiconductor (see also in Fig. 12). In Fig. 14(a) they observe a rather strong

reflected field. At this wavelength the power reflectance is R 0.2. An optimal blooming layer

without nano antennas at this wavelength allows R 0.1. As a result, the light-trapping structure

turns out to be more advantageous than the ARC. In Fig. 15(a), we can see the spectral photo

absorption A for three cases: with their light trapping structures, with a blooming layer, and

without any structure on the top of the photovoltaic layer. The impact of the reflectance is

illustrated by Fig. 15(b).

Figure 15. Left: spectral density of PV absorption for the interband TFSC based on CIGS in three cases: our

LTS, blooming layer (ARC), and open surface. Right: Power reflectanceR from our LTS (thick red curve), and

solar irradianceIs in arbitrary units (thin blue curve). Strong reflection at long waves does not result in the low

efficiency due to weak solar irradiance in this domain. Constantin Simovski, et al, 2013

In another work of previous author we also can find light trapping structures but with fully

different theory. A schematic view of their photovoltaic structure is shown in Fig. 16. A layer of

closely packed micron or submicron dielectric (e.g., polystyrene, silica, etc.,) spheroidal particles

is placed on the surface of a photovoltaic layer of thickness d=1000 nm. The photovoltaic film is

deposited on the aluminum-doped zinc oxide (AZO) substrate. This system can be either

obtained using the atomic layer deposition (ALD), grown using the self organization of colloidal

nanoparticles or simply mechanically applied using the centrifuge (micron-sized silica and


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polystyrene particles are available on the market). It is necessary to mention that the self-

organization is much cheaper than ALD-methods. Polystyrene spheres were chosen like cheapest

and easiest for mass manufactures product.

Figure 12.A schematic view of thin film solar cell with nanosphere coating. Polystyrene particles are packed

in a square array on the surface of the doped crystalline silicon film of thickness d=300 nm. SubstrateAZO.

In this paper, they proposed an array of non-resonant submicron dielectric particles (e.g.,

polystyrene spheres) with negligible optical losses for a significant increase of the photovoltaic

absorption in thin-film solar cells. Instead of the utilization of a resonator regime of light-

trapping, we suggest a broadband regime, which is the deal between the reduction of the

transmittance through the thin photovoltaic layer and the reduction of the overall reflectance.

Each sphere (or spheroid) transforms the incident light into a collimated light beam propagating

in the lossy photovoltaic layer with a strong attenuation. Stronger collimation of the beam leads

to its greater attenuation. Thus, it is possible to prevent an increase of the light transmission into

the substrate even for the silicon layer as thin as d=300 nm. The decrease of the reflection is the

collective effect. It can be described as the effect of an effective layer formed by closely packed

spheres. For larger spheres, the collimation of the beam in the photovoltaic layer is higher but the

reflection also increases, so that there is an optimal radius of the spheres. The best balance has

been achieved in his numerical example. This example corresponds to the gain in the


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photovoltaic absorption in 1.44%. Typical structure but with different theory was shown to us of

Jonathan Grandidier et al. in 2011 year.

In another work of Ragip A. Pala et all was shoen structure with long silver strip(Figure 13). In

their first simulations, they considered an array of Ag strips of thickness t=60nm, width w=80

nm, lateral period P=310 nm, and a spacing s=10nm from a 50 nm thick Si film(Figure 17a). For

reference, Figure 17b shows the simulated fields in the absence of metal structures. Figure 17c

and d show the dramatic field enhancements that can result from the strips under different

illumination conditions.

Figure 17. a) A schematic of the proposed plasmon-enhanced cell structure. Normalized and time-averaged

field intensity plots for normal incidence, TM illumination of b) a bare Si/SiO2 structure and c) and d) the samestructure with a periodic array of metal strips spaced at p=312 nm, a spacer layer thickness of s=10 nm, and an

absorbing Si film thickness equal to a=50 nm. The incoming wavelengths (energies) of b) and c) l=650nm

(1.91eV) and d) l=505nm (2.46eV) were chosen to demonstrate the effects of strong near-field light

concentration or excitation of waveguide modes by the strips. Ragip A. Pala et all (2009)


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By theoretical simulations their structure with silver strips where thickness 50 nm up the

photocurrent to 50%.

Another intresting structure is Core-Shell structure by Yiling Yu et all(2012). In Figure 18 we

can see that structure and comparing with another type of structures.

Figure 18.Solar absorption of standard coreshell structures (CS1 NWs). (a) Two-dimensional plot of Jsolar of

coreshell NWs as functions of the diameter of the semiconductor core (horizontal axis) and the thickness of

the dielectric shell (vertical axis). (b) Two-dimensional plot of Jsolar of planar thin films as functions of the

thickness of the semiconductor layer (horizontal axis) and the thickness of the ARC (vertical axis). The dashed

black line indicates the optimized thickness in the dielectric layer for the solar absorption. For the convenience

of comparison, the calculated Jsolar in a and b are normalized to their maximum values, respectively. (c) Spectral

absorption efficiency Qabs of CS1 NWs with a 300 nm diameter core as a function of the shell thickness

(vertical axis). The dashed white line indicates the result replotted in d. (d, upper) Absorption spectra of a

CS1 NW with a 300 nm diameter core and a 70 nm thick dielectric shell (red curve) and of a pure 300 nm

diameter semiconductor NW (blue curve); (lower) absorption spectra of planar semiconductor thin films in

thickness of 300 nm with (red curve) and without (blue curve) a 70 nm thick ARC. (e) Normalized J solar of

CS1 NWs with respect to pure semiconductor NWs in the same size as the core (red curve) and normalized


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Jsolar of planar thin films enhanced by the traditional ARC with respect to bare planar thin films. (f, upper)

Absorption spectra of a CS1 NW with a 100 nm diameter core and a 70 nm thick shell (red curve) and a pure

100 nm diameter semiconductor NW (blue curve); the peak 600 nm is from TM11/TE01 as indicated. (f, lower)Absorption spectra of 100 nm thick planar semiconductor thin films with (red curve) and without (blue curve)

a 70 nm thick ARC. Yiling Yu et al. (2012).

Author of that work tell that in his example of thin film solar cell with thickness 70nm of

semiconductor layer absorption up for 90% of all solar photons above the bandgap of a-Si:H.

Conclusions

Unfortunately many of that structure was testing only from theoretic simulations, which doesnt

allow us to say that one or another one is better. Anyway it much better then we have ideas for

their implementationbut real live is hardest thats why we are stay to use a gas.

At now time researchers start to check all of that theoretical works but in using that theory we

will see only after few years.

Anyway sun energy from our opinion is much green energy from all another, and science in

work with solar battery will go up to long and long time.


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References

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