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Energy Express

  • Editor: Christian Seassal
  • Vol. 22, Iss. S3 — May. 5, 2014
  • pp: A651–A662
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Nanoimprinted backside reflectors for a-Si:H thin-film solar cells: Critical role of absorber front textures

Yao-Chung Tsao, Christian Fisker, and Thomas Garm Pedersen  »View Author Affiliations


Optics Express, Vol. 22, Issue S3, pp. A651-A662 (2014)
http://dx.doi.org/10.1364/OE.22.00A651


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Abstract

The development of optimal backside reflectors (BSRs) is crucial for future low cost and high efficiency silicon (Si) thin-film solar cells. In this work, nanostructured polymer substrates with aluminum coatings intended as BSRs were produced by positive and negative nanoimprint lithography (NIL) techniques, and hydrogenated amorphous silicon (a-Si:H) was deposited hereon as absorbing layers. The relationship between optical properties and geometry of front textures was studied by combining experimental reflectance spectra and theoretical simulations. It was found that a significant height variation on front textures plays a critical role for light-trapping enhancement in solar cell applications. As a part of sample preparation, a transfer NIL process was developed to overcome the problem of low heat deflection temperature of polymer substrates during solar cell fabrication.

© 2014 Optical Society of America

1. Introduction

Compared to other non-conventional nanopatterning techniques, nanoimprint lithography (NIL) on polymer sheets like polymethylmethacrylate (PMMA) [14

14. T. Khaleque, H. G. Svavarsson, and R. Magnusson, “Fabrication of resonant patterns using thermal nano-imprint lithography for thin-film photovoltaic applications,” Opt. Express 21(S4), A631–A641 (2013). [CrossRef] [PubMed]

], polydimethylsiloxane (PDMS) [15

15. V. E. Ferry, M. A. Verschuuren, H. B. T. Li, E. Verhagen, R. J. Walters, R. E. I. Schropp, H. A. Atwater, and A. Polman, “Light trapping in ultrathin plasmonic solar cells,” Opt. Express 18(S2), A237–A245 (2010). [CrossRef] [PubMed]

], polyethylene-teraphtalate (PET) [16

16. J. J. Lee, S. Y. Park, K. B. Choi, and G. H. Kim, “Nano-scale patterning using the roll typed UV-nanoimprint lithography tool,” Microelectron. Eng. 85(5–6), 861–865 (2008). [CrossRef]

] and polyethylene-naphtalate (PEN) [17

17. M. A. González Lazo, R. Teuscher, Y. Leterrier, J. A. E. Månson, C. Calderone, A. Hessler-Wyser, P. Couty, Y. Ziegler, and D. Fischer, “UV-nanoimprint lithography and large area roll-to-roll texturization with hyperbranched polymer nanocomposites for light-trapping applications,” Sol. Energy Mater. Sol. Cells 103, 147–156 (2012). [CrossRef]

] is still the most promising technique for mass production and large-area nanoscale-patterning required for solar cell applications [18

18. C. Battaglia, J. Escarré, K. Söderström, M. Charrière, F.-J. Haug, M. Despeisse, F.-J. Haug, and C. Ballif, “Nanomoulding of transparent zinc oxide electrodes for efficient light trapping in solar cells,” Nat. Photonics 5(9), 535–538 (2011). [CrossRef]

]. Besides, polymer sheets also have many advantages such as low cost, light weight, non-fragility, flexibility, and possibility of using roll-to-roll processes [19

19. S. H. Ahn and L. J. Guo, “Large-area roll-to-roll and roll-to-plate nanoimprint lithography: a step toward high-throughput application of continuous nanoimprinting,” ACS Nano 3(8), 2304–2310 (2009). [CrossRef] [PubMed]

,20

20. M. D. Fagan, B. H. Kim, and D. G. Yao, “A novel process for continuous thermal embossing of large-area nanopatterns onto polymer films,” Adv. Polym. Technol. 28(4), 246–256 (2009). [CrossRef]

]. However, the heat deflection temperature (HDT) of polymer sheets is normally less than 200 °C, so a low temperature plasma-enhanced chemical vapor deposition (PECVD) process must be used for a-Si:H deposition [21

21. T. Söderström, F.-J. Haug, V. Terrazzoni-Daudrix, and C. Ballif, “Optimization of amorphous silicon thin film solar cells for flexible photovoltaics,” J. Appl. Phys. 103(11), 114509 (2008). [CrossRef]

].

2. Sample fabrication

Following stamp fabrication, nanostructured PMMA substrates were produced by thermal NIL in a vacuum chamber using a stamping pressure of 1.25 bar. The processes were carried out in an EVG520HE semi-automated hot embossing system. As shown in Fig. 1, three different imprint techniques were applied resulting in negative (N), positive (P) and transfer (T) samples. Negative PMMA imprints were fabricated at 120 °C substrate temperature, which is above the HDT of PMMA (100 °C). After cooling and de-molding, negative imprints with nano-dome features were formed on the PMMA surface. The samples were named N(OX), N(PH) and N(CI) for the small, middle and large periods, respectively. Positive PMMA imprints were made by the positive NIL process as follows: First, a negative imprint was fabricated at 180 °C in TOPAS 5013L-10 sheets with HDT > 120 °C. Then, a 40 nm Al film was coated onto the surface by sputtering as an anti-sticking layer. The Al-coated TOPAS negative stamp was applied to a PMMA substrate similarly to the negative imprint described above. The geometry of the nanostructured PMMA surface is then practically identical to the original NIL stamp. The samples were named P(OX), P(PH) and P(CI) for the small, middle and large periods, respectively. The surface morphologies of these three negative and three positive nanostructured PMMA substrates were studied by scanning electron microscope (SEM) with a tilt angle of 45°, and the images are shown in Fig. 2.
Fig. 2 SEM images with a tilt angle of 45þ of all negative and positive imprints on PMMA surfaces.
For further processing, 80 nm Al films were coated onto the surfaces as reflectors by sputtering. Subsequently, 100 – 300 nm a-Si:H films were deposited as absorbing layers by PECVD at a substrate temperature of 50 °C, an aux-showerhead temperature of 170 °C, a monosilane gas (SiH4) flow of 50 sccm, a pressure of 100 mTorr, and an RF power of 50 W. After a-Si:H deposition, optical transmission spectra within the range of 300 – 1100 nm were recorded with an integrating sphere to confirm that the Al layer is sufficiently thick to completely block specular and diffuse light transmission.

In order to control the structural geometries on the front textures of the a-Si:H films, a transfer NIL process was developed as follows: (1) A NIL stamp with 420 nm period was anodized in 0.5M citric acid at a low applied voltage of 100V and a short time period of 5 minutes for creating a sacrificial Al2O3 layer of 200 ~300 nm. (2) A 200 nm a-Si:H film and a 80 nm Al film were deposited directly hereon by PECVD and sputtering as an absorbing layer and a reflector, respectively. (3) The whole stamp with the a-Si:H and Al films was imprinted into a PMMA substrate heated to 140 °C. (4) After cooling, the de-molding process was done by selective etching of Al2O3 using an acid mixture of phosphoric and chromic acid. This transferred the a-Si:H and Al films onto the PMMA substrate with an inverted sequence. Below, we refer to this sample as T(PH). It is interesting that this transfer NIL process provides a promising method of transferring films with negative imprints from arbitrary NIL stamps onto any polymer substrate with a geometry that can be easily designed and controlled. This enables making a-Si:H thin-film solar cells on a cheaper plastic substrate without restrictions from the PECVD working temperature because a complete cell with an inverted sequence can be first fabricated on a metal substrate and then transferred onto plastic substrates afterwards. In this way, the low HDT of polymer substrates like PMMA is not an issue during solar cell fabrication.

3. Structural and optical characterization

Fig. 3 2D AFM topography of a 200 nm a-Si:H film on T(PH) (dashed lines) and N(PH) (solid lines) showing (a) short- and (b) long-axis cross-section profiles.
All samples were carefully characterized using a combination of scanning electron microscopy (SEM), atomic force microscopy (AFM), and UV-Vis reflectance spectroscopy. The SEM topographic images were analyzed by a Zeiss 1540 XB, the AFM 2D topographic images, AFM cross-section profiles and surface roughness were characterized by a NT-MDT NTEGRA, and the UV-Vis reflectance spectra were measured by a PerkinElmer Lambda1050. In Fig. 3, an AFM scan along the short-axis and long-axis of the 2D hexagonal topography shows that the negative imprints of the NIL stamp were directly transferred onto the front textures of the a-Si:H film. The dashed and solid lines represent the AFM cross-section profiles of a 200 nm a-Si:H film in the T(PH) and N(PH) configurations, respectively. It is clear that the front textures of the N(PH) and T(PH) samples are both dome features, but the T(PH) sample retains a significantly larger height variation than the N(PH) sample because the front textures are fixed during the a-Si:H deposition.

In order to study the relation between the optical reflectance and the geometry of the front textures, different thicknesses of a-Si:H were coated onto the all negative and positive NIL BSRs. The surface morphologies of the P(PH) and N(PH) samples with 100 – 300 nm a-Si:H coatings were recorded by SEM with tilt angles of 0þ and 45þ as seen in Fig. 4.
Fig. 4 SEM images with tilt angles of 0þ and 45þ of (a-h) the N(PH) and (i-p) the P(PH) BSRs with 0 – 300 nm a-Si:H films.
Fig. 5 Total (a-c) and diffuse (d-f) reflectance spectra of all the positive and negative NIL BSRs with 100 – 300nm a-Si:H films.
The UV/Vis reflectance spectra of all the negative and positive NIL BSRs with 100 – 300 nm a-Si:H coatings were recorded as well. The incident angle was set at θ1 = 8þ, the range of wavelengths was from 300 to 1100 nm, and the total and diffuse reflectances (non-specularly reflected light) were recorded as presented in Figs. 5(a)5(c) and Figs. 5(d)5(f), respectively. Independent of a-Si:H film thickness, the nanostructure periods of the front textures remain identical to the ones of the BSRs. According to AFM measurements, the root-mean-square (rms) of the surface roughness of the P(PH) samples (Figs. 4(i)4(p)) is between 90 to 109 nm, but the roughness of N(PH) samples (Figs. 4(a)4(h)) is only between 39 to 46 nm. Therefore, it is clear that the positive NIL BSRs retain more structures on the front textures than the negative NIL BSRs after Al and a-Si:H coating. By comparison to the reflectance spectra, it follows that surfaces with a larger height variation have a lower total reflectance, trapping more light inside the samples. As the diffuse reflections for the smaller period samples are comparable in Figs. 5(d)5(e) and independent of height variations, Figs. 5(a)5(b) show that all positive NIL BSRs have a lower specular reflectance (R(λ)) than the negative ones. For the large period samples, almost all reflectance is diffuse (Figs. 5(c) and 5(f)). Also, the negative NIL structures are both flatter and more uniform than the positive ones, so larger oscillations in the reflectance spectra of the negative NIL BSRs are observed due to interference.

In general, the reflected optical field has contributions directly reflected from the front surface as well as components reflected from the BSR with multiple passes through the a-Si:H layer. However, a-Si:H absorbs strongly in the short-wavelength region (< 500 nm) eliminating the BSR contribution, so R(λ) is determined by front textures only in that range. On the other hand, optical absorption of a-Si:H is weaker in the long-wavelength region (> 500 nm), so many factors like structural geometry of both front textures and BSRs and a-Si:H layer thicknesses all have to be carefully considered when analyzing R(λ) For the case of negative NIL BSRs, the AFM cross-section profiles (not shown) of the front textures show that the nano-dome depths decrease from 75 to 50 nm as the a-Si:H layer thickness increases from 100 to 300 nm, which means that the front textures become flattened with film thickness. As seen in Fig. 5(b), R(λ) of the N(PH) sample increases with the a-Si:H layer thickness in the short-wavelength region as a consequence of flatter surfaces on the thicker films. Therefore, less light is coupled into the 300 nm sample by the surface textures.

During PECVD deposition of a-Si:H, more material was deposited on the top area than the bottom area. As shown in Figs. 4(i)4(p), the height variation and the width of the dimples become larger and narrower, respectively, when initially increasing the a-Si:H thickness. However, when the thickness reaches the same scale as the height of the dimples, the top areas start merging together. The six higher rounded peaks at the corners of the hexagonal cell gradually turn into flatter triangular cones. If further increasing the a-Si:H layer thickness, the sharp dimples would disappear, leaving only shallow triangular cones on the surface. For the small period P(OX) samples, the dimples are too shallow to retain the structure after a-Si:H coating much like the case of the negative NIL BSRs. Therefore, when the a-Si:H films increase from 100 to 300 nm, the surface roughness, rms, decreases from 63 to 52 nm. This causes the thicker a-Si:H film to show higher R(λ) in the short-wavelength region as seen in Fig. 5(a). In the meantime, a thicker uniform film will have more internal resonances, also resulting in a reflectance spectrum showing spectral narrowing and blue shift in the long-wavelength region.

So far, it seems that the positive NIL BSRs are promising for light-trapping enhancements. Thus, in order to comprehensively compare light absorptions, the average of the measured absorption (Aave) in the wavelength range from λ1 = 300 nm to λ2 = 800 nm is obtained as: Aave=λ1λ2(1R(λ))dλ/(λ1λ2), and the results are shown in Fig. 6(a).
Fig. 6 (a) Average absorption between 300 – 800 nm of all replicated BSRs as a function of the a-Si:H layer thickness (b) Integrated quantum efficiency (IQE) of O(PH), P(PH), N(PH) and T(PH) samples under AM1.5G as a function of a-Si:H layer thickness. The red and blue dashed arrows illustrate the increase and the decrease of the IQE, respectively.
It again shows that a thicker film is not a guarantee for stronger light absorption. As the a-Si:H thickness initially increases from 100 to 200 nm the Aave values increase as well. The reflectance plots in Figs. 5(a)5(c) show that this increase is a consequence of one of the reflectance minima moving from the infrared region to 700 ~750 nm, i.e. an interference effect based on the film thickness. However, this is still not an efficient approach if the period of the front texture is too small as in the case of P(OX). Some of the gain from the interferences effect in the long-wavelength region would be cancelled by the loss in the short-wavelength region as seen in Fig. 5(a), leading to Aave values slightly increasing. However, if the period is large enough as in the cases of P(PH) and P(CI), in addition to the interferences effect, the narrower dimples also make the increase in the Aave more pronounced than in the case of P(OX).

4. Results and discussions

Experimentally, solar cells are typically characterized by their external and internal quantum efficiencies, which are the number of conduction electrons measured from the cell per incident photon and absorbed photon, respectively. These quantities therefore contain recombination, contact and plasmonic losses. In a system with no losses, all generated electron-hole pairs contributed to the current, so that both quantities can be expressed as optical efficiencies. Therefore, an integrated external quantum efficiency (IQE) [30

30. A. S. Lin, Y.-K. Zhong, S.-M. Fu, C.-W. Tseng, S.-Y. Lai, and W.-M. Lai, “Lithographically fabricable, optimized three-dimensional solar cell random structure,” J. Opt. 15(10), 105007 (2013). [CrossRef]

32

32. L. Yuan, F. Chen, C. Zheng, J. Liu, and N. Alemu, “Parasitic absorption effect of metal nanoparticles in the dye-sensitized solar cells,” Phys. Status Solidi A 209(7), 1376–1379 (2012). [CrossRef]

] with no system losses is calculated to estimate the optical efficiency of the a-Si:H films on different kinds of Al BSRs. It is defined over a spectral range from λ1 to λ2 as:
IQE= λ1λ2λ Isun(λ)Atot(λ)RQE(λ)dλλ1λ2λ Isun(λ)dλ,
(1)
where Isun(λ) is the standard solar spectrum (AM 1.5G), Atot(λ) is the total optical absorption of the entire sample (including both Al and a-Si) and the relative quantum efficiency RQE(λ)=AaSi(λ)/Atot(λ) is a ratio of total optical absorption of a-Si:H layers to Atot(λ). This way, RQE(λ) corresponds to the internal quantum efficiency in a loss free system. Finite-difference time-domain (FDTD) calculations were made with the Lumerical FDTD software package [33

33. Lumerical FDTD Solutions (http://www.lumerical.com/tcad-products/fdtd/).

] and used to estimate the optical efficiencies of the samples. In all simulations, models with periodic structures based on the experimental AFM cross-section profiles were applied.

Fig. 7 AFM cross-sections of center dimples on the front surface of (a) O(PH) and (b) P(PH) with 100 – 300 nm a-Si:H coating. (c) Ratio of height to FWHM of O(PH) and P(PH) as a function of the a-Si:H layer thickness.
In order to explain why the P(PH) sample has a higher IQE than the O(PH) sample for thinner a-Si:H films, the AFM cross-section profiles of the specific dimple on both the O(PH) and P(PH) samples were measured and shown in Figs. 7(a) and 7(b). The height variation of the O(PH) sample increases slightly from 220 to 250 nm when the thickness of the a-Si:H layer increases from 100 to 200 nm, but it drops back to 220 nm when the thickness further increases to 300 nm. For the Al-coated P(PH) sample, the initial height variation on the Al surface is 300 nm, which is not identical with the variation on the O(PH) sample. This is due to the 80 nm Al coating, as more Al is deposited on the top areas. During the a-Si:H deposition, the height variation increases slightly to 310 nm when coating the 100 nm a-Si:H film, but it decreases to 280 and 150 nm as further increasing the a-Si:H film to 200 and 300 nm, respectively.

The ratio of height to full width at half maximum (FWHM) of the dimples was calculated and plotted as a function of the a-Si:H layer thickness in Fig. 7(c). This is a measure of feature sharpness and a higher number means that the dimple is sharper. It is interesting that the dimple on the P(PH) sample is sharper than the one on the O(PH) sample when the a-Si:H layer thicknesses are below 200 nm. However, if further increasing the thickness to 300 nm, the dimple keeps going sharper on the O(PH) sample, but it becomes smoother on the P(PH) sample. Therefore, a total thickness of around 300 nm, which now includes both an a-Si:H film and an Al film, yields the largest height variation and the sharpest front textures on the P(PH) or O(PH) samples. It is also clear that a higher IQE is strongly correlated to a larger height variation and a sharper geometry on front textures as shown in Fig. 6(b).

5. Conclusions

In this study, nanostructured Al BSRs were produced by positive and negative NIL, and the relationship between the optical properties and the geometry of surface textures was studied. A promising solar cell fabrication method called the transfer NIL process was developed for controlling geometries on the front textures of a-Si:H films and overcoming the low HDT problem of polymer substrates. In order to maximize optical absorption in a-Si:H layers, front textures with a larger height variation, sharper geometry and higher order are important. Imprinted BSRs with different periods and structural features were investigated and it was found out that the positive NIL BSR with 420 nm period is the best candidate for efficient light-trapping especially when the a-Si:H layer thickness is around 200 nm or less. The reflectance measurements demonstrate that as much as 80.5% of the incident light between 300 and 800 nm is absorbed in the positive NIL sample coated by 200 nm a-Si:H without any anti-reflection coating.

Acknowledgments

The authors gratefully acknowledge Peter K. Kristensen and Deyong Wang from the Dept. of Physics and Nanotechnology, Aalborg University, Denmark, for technical support on the experimental work. This project is supported by the Danish Strategic Research Council under the project “Thin-film solar cell based on nanocrystalline silicon and structured backside reflectors THINC”.

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E. D. Palik, Handbook of Optical Constants of Solids (Academic, 1998).

35.

S. A. Sopra, Materials data (http://www.sspectra.com/sopra.html).

OCIS Codes
(350.6050) Other areas of optics : Solar energy
(240.3695) Optics at surfaces : Linear and nonlinear light scattering from surfaces
(050.6624) Diffraction and gratings : Subwavelength structures
(050.6875) Diffraction and gratings : Three-dimensional fabrication

ToC Category:
Light Trapping for Photovoltaics

History
Original Manuscript: January 13, 2014
Revised Manuscript: February 20, 2014
Manuscript Accepted: March 7, 2014
Published: March 19, 2014

Citation
Yao-Chung Tsao, Christian Fisker, and Thomas Garm Pedersen, "Nanoimprinted backside reflectors for a-Si:H thin-film solar cells: Critical role of absorber front textures," Opt. Express 22, A651-A662 (2014)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-22-S3-A651


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