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
The hydrogenated amorphous silicon (a-Si:H) thin-film solar cell is a popular candidate for future low cost commercial solar cells with reasonable efficiency . However, some serious challenges stemming from the material characteristics of a-Si:H, such as light-induced degradation due to the Staebler-Wronski effect and low electron and hole mobilities, are still keeping this material from finding widespread use in the photovoltaic market [2,3]. So far, the best solution to overcome these challenges is to decrease a-Si:H layer thicknesses, but a thinner film also leads to a lower optical absorption . Nanotextured interfaces are widely used to improve optical absorption by having light scattered into directions where it can propagate longer distances inside absorbing layers before it has a chance to escape. Therefore, different concepts of nanopatterning have gained tremendous interest, including periodic  and random surface gratings , plasmonic nanostructures [7–9], guided optical mode features [10,11], and photonic crystals . However, it is still not clear, which concept shows the best performance of light-trapping in spite of many experimental and theoretical studies .
Compared to other non-conventional nanopatterning techniques, nanoimprint lithography (NIL) on polymer sheets like polymethylmethacrylate (PMMA) , polydimethylsiloxane (PDMS) , polyethylene-teraphtalate (PET)  and polyethylene-naphtalate (PEN)  is still the most promising technique for mass production and large-area nanoscale-patterning required for solar cell applications . Besides, polymer sheets also have many advantages such as low cost, light weight, non-fragility, flexibility, and possibility of using roll-to-roll processes [19,20]. 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 .
In the present work, PMMA substrates are first patterned with nanostructures by thermal NIL. Then, aluminum (Al) and a-Si:H are coated on the surfaces as backside reflectors (BSRs) and absorbing layers, respectively. While silver (Ag) is a more common BSR material, Al is chosen in this work as its reflectivity is comparable to that of Ag, and it is much cheaper from a production point of view. Usually in a complete solar cell a buffer layer such of ZnO:Al between a-Si:H and the metal backside reflectors and an anti-reflection (AR) coating of In2O3:Sn (ITO) are used to reduce plasmonic losses and enhance the light absorption, respectively. A buffer ZnO:Al layer is crucial in reducing plasmonic losses for Ag BSRs, as Ag has strong plasmons in the red part of the spectrum where a-Si:H absorbs weakly. In Al, the strong plasmonic resonance is in the UV part of the spectrum while in the visible the intensity is much weaker than the one of Ag [22,23]. Moreover, the plasmonic losses are automatically reduced by a natural oxide layer of 2-3 nm forming on all Al surfaces . As it was found in a previous study that for grating type solar cells with Al BSRs the optimal period was largely independent of both AR and ITO layer thicknesses, these layers are omitted here . Therefore, we focus on studying a-Si:H films directly on Al BSRs with different surface geometries instead of working on a complete device, as it is a strong indicator of the optical efficiency of a solar cell based on the same BSR. In general, the optimal a-Si:H thickness might change with a different thickness of ITO. However, for mitigating some drawbacks, such as the light-induced degradation, the a-Si:H thickness should be as thin as possible. In the present work, a relatively thin a-Si:H layer, 200 nm, is chosen for further experimental and theoretical studies due to the reasonable light absorption. Finally, different imprint schemes, which generate different front textures, are investigated to demonstrate that surface textures with large height variations lead to improved optical efficiency.
2. Sample fabrication
In previous works [26,27], Al anodization was applied to fabricate nanostructured backside reflectors. In the present work, a similar technique is used to produce NIL stamps as follows: (1) An Al sheet is anodized in acid electrolytes to fabricate a porous alumina film on the surface, and (2) this film is selectively etched away in an acid mixture (chromic and phosphoric acid) at around 353K while stirring. The remaining structures on the surface show hexagonal arrays of hemispherical dimples in the centers and six rounded cones at the corners . The period of the array primarily depends on the electrolyte pH and the applied potential during anodization . Three different periods, 300, 420 and 600 nm, were obtained by anodization in oxalic acid (OX), phosphoric acid (PH) and citric acid (CI), respectively. The details and parameters for the anodization processes are given in a previous paper . In order to avoid bending during de-molding processes, the NIL stamps were glued onto cylindrical pieces of Al with Permabond ES550 epoxy paste. The schematic process flows of sample fabrication are as shown in Fig. 1.
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. 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
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.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 = 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 () 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 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 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), 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 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.
In Figs. 5(d)–5(f), the level of diffuse reflectance increases with the period. This is in line with what would be expected from a perfectly periodic structure, where a grating with a large period would have more non-specular diffraction orders than one with a short period. With disorder, the diffraction orders become diffuse, and light scattering for wavelength above the structural period becomes possible. In addition, Fig. 2 shows that the front textures become more and more disordered as the period increases from 300 to 600 nm. In a previous study of the bare nanostructured Al BSRs , it was shown that more diffuse light scattering would be generated by more disordered surface structures. As most of the total reflected light in Fig. 5(c) is in fact diffuse, optimizing the ordering of nanostructures on front textures may be crucial for enhancing optical absorption in thin-film solar cell applications.
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 () in the wavelength range from = 300 nm to = 800 nm is obtained as: , and the results are shown in Fig. 6(a). 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 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 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 more pronounced than in the case of P(OX).
If further increasing the a-Si:H thickness from 200 to 300 nm, the oscillations in the reflectance spectra become stronger, causing more spectral narrowing and blue shift. Besides, all in the short-wavelength region also increase due to the flatter front textures, so the decrease of is expected and the decreasing levels are strongly related to the geometry of the front textures. For example, the P(OX) sample has the largest decrease, which comes from the largest specular reflectance due to the flattest surface textures. Among all the samples studied, the P(PH) sample with a 200 nm a-Si:H coating seems the best candidate for efficient light-trapping in a-Si thin-film solar cells because it can absorb around 80.5% of the incident light inside the sample. It is noticed that this is achieved by applying a simple absorbing layer without any anti-reflection coatings or other complex concepts. However, using for evaluating the optical efficiency of solar cells is only an initial estimate, as it does not contain any information of the solar spectrum, nor of whether the absorption happens in the active layer or some other part of the cell. In a ‘perfect’ cell, with no absorption in the BSR and no recombination losses, it would be comparable to the experimental short-circuit current density under a homogeneous light source. But in the present case it is not possible to measure the exactly fraction of absorption occurring in the a-Si:H film nor the recombination, contact and plasmonic losses as the sample is not a complete solar cell. Therefore, the optical efficiency of a-Si:H deposited on Al BSRs is calculated and characterized by using finite-difference time-domain (FDTD) simulations and integrated external quantum efficiency (IQE).
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) [30–32] 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 to as:33] 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.
In general, RQE is a complex quantity, depending on materials involved, thicknesses of p-, i- and n-layers, and structures at interfaces. However, a series of FDTD calculations have shown that for Al BSRs with nanostructures of 300 – 700 nm and a thin a-Si:H film, the RQE becomes a smooth curve in the shape of a Fermi function, that is shifted by a change of film thickness but mostly unchanged by the size, shape and period of the surface features . For smaller BSR nanostructures (< 200 nm) the feature-independence is no longer true, as in this case will show plasmonic oscillations in the 600 – 800 nm wavelength range that depend on the feature-shape and -size. Likewise, a generalized expression for cannot be made for structures with a silver BSR due to plasmonic oscillations. General expressions for can therefore be calculated from the FDTD model based on a given set of a-Si:H layer thicknesses, and the IQE can be calculated from Eq. (1) by integrating and the measured over the AM 1.5G spectrum. for 100, 200 and 300 nm a-Si:H films on nanostructured Al BSRs was calculated. A broad-band plane wave impulse of 300 – 900 nm light with a length of 1.99 fs was used at normal incidence to the samples, and the simulations were run until the total energy of the field was reduced to in time steps of 5.82 attoseconds with a Courant stability factor of 0.99. Experimental values of the a-Si:H refractive index were taken from a previous paper  and the Al refractive index was from Palik . Both data sets were fitted to 20th degree polynomials on the 300 – 900 nm range for the simulation. As no absorption in the a-Si:H at wavelengths above 800 nm is expected, and a test of the materials data, calculated from the Sopra S.A. parameters for a-Si:H , was found to have the same trend. According to Figs. 5 and 6(a), the P(PH) is interesting for IQE calculations not only for the highest but also for low diffused reflectance and the results are shown in Fig. 6(b). A sample obtained by a-Si:H coating directly onto the original Al NIL stamp with 420 nm period (named O(PH)) has also been studied for comparison with the IQE of the P(PH) sample due to the similar surface textures. As shown in Fig. 6(b), the P(PH) sample has a higher IQE than the O(PH) sample when the a-Si:H layer thickness is 100 nm. For a thickness of 200 nm, the obtained IQEs of the two samples are almost identical, in particular, bearing the uncertainty in film thickness in mind. Among the dome feature samples, the T(PH) sample has an obviously higher IQE than the N(PH) sample because of the larger height variation as seen in Fig. 3.
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).
In order to further support the observed relation between surface features and improved absorption, the optical properties of several samples are modelled by using FDTD. To this end, four different samples, P(PH), O(PH), T(PH), and N(PH) with the same 200 nm a-Si:H coatings are studied as shown in Fig. 8.In contrast to the experimental samples, the model structures are perfectly periodic with hexagonal unit cells based on symmetrized AFM cross-section profiles. The dashed and the solid lines represent the profiles of the front textures and BSRs, respectively. In the simulations, a plane wave light pulse is injected in the negative -direction, perpendicular to the surface, and the photon flux in the 300 – 800 nm spectral range under the AM 1.5G spectrum is calculated as:Fig. 8. The regions of strong intensity are seen as red colored, and the reflected photon density is shown in the small region above the cross sections. It can be seen that there are some strong local fields close to the sharp features. As expected from the measurements, the strongest reflection is seen for the N(PH) sample (Fig. 8(d)). The T(PH) sample (Fig. 8(c)) has a weaker reflection, but it is still stronger than the two positive NIL BSRs (Figs. 8(a) and 8(b)). The calculated values of for the four simulated samples are 86.2, 81.0, 77.0, and 54.8%, respectively. The value computed for P(PH) is seen to be slightly higher than the experimental value of 80.5% (Fig. 6(a)). However, as the nanostructures of the model are completely periodic, the calculated are expected to differ slightly from the experiment. Using the simulated we may next compute IQE values for the four samples. In this manner, the IQEs are found to be 39.9, 37.6, 36.0, and 26.8% for P(PH), O(PH), T(PH), and N(PH) samples, respectively. These values are seen to agree rather well with the IQEs based on the experimental reflectance in Fig. 6(b). The model values are somewhat higher than the experimental ones, but both the range and distribution of the efficiencies are in good agreement. For the given structures, a comparison of the IQEs from the experimental and modelled reflectance indicates that increasing the ordering and the height variation of the nanostructures on the front textures seems a promising idea for optical efficiency enhancements in a-Si:H films on nanostructured Al BSRs.
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.
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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