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Abstract
In this paper the concept of new selectively transparent polymer layer, dedicated for improvement of innovative solar cell structure efficiency is presented. Various polymer materials differing in optical, mechanical and thermal properties are being considered. The texturisation of the front surface is proposed in order to assure difference in the transmission of radiation coming from both sides towards higher solar conversion efficiency. The theoretical approach is verified by the measurements. The fabrication process of different texturisation patterns on polymer foil using novel laser ablation method is described and the preliminary results are presented.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The concept of selectively transparent layer – a layer which transmission and reflection strongly depend on the side of incoming radiation, is very promising for the solar cell applications. The main motivation for undertaking this research has been the authors’ work on the down-shifting (DS) and down-converting (DC) layers containing ZnO nanoparticles or rare earth (RE) elements [1–5]. These structures, excited by the UV light, emit radiation in the highest sensitivity range of the considered thin-film PV cell, which is typically 400÷700 nm. Such approach has been reported to highly increase the solar cell efficiency, as has been proven in [6] and [7]. However, an important issue is that the luminescent particles re-emit photons in all directions equally, thus high percentage of the emitted energy is being lost, and therefore only a fraction of them reaches the solar cell. Hitherto applied solution to this problem have been proposed in a form of a Luminescent Solar Concentrator (LSC) hereof [8]. That device was assembled as a thin concentrator with the light trapping and guided light direction. Unfortunately, the practical realization of this idea brought several serious problems, which eliminated its advantageous application in photovoltaic cells. Motivated by this approach, authors designed modified converter structure. The idea is to apply semi-transparent “Venice mirror”-like layer, characterized by different transmission characteristics depending on the side of observation.
The selectively transparent layer placed on top of the down-converting layer with nanoparticles is meant to augment the efficiency of the device by directing the highest possible number of photons towards the solar cell. Moreover, a specific stack of optical layers for efficient light trapping is proposed, combined with the new approach of application of the selective reflective layer, which enables multiplication of the active device area. Figure 1 shows the schematic concept of the proposed solution.
Fig. 1. Scheme of the thin-film solar cell with the down-converting layer and selectively-transparent layer. Authors, based on [9].
There have been numerous approaches to the practical realization of the selectively transparent layer for the application on top of the down-shifting layer. One of such takes advantage of significant Stokes shift, while the absorption and emission spectra do not overlap each other. In this way, the selectively transparent layer should transmit the absorbed photons, but reflect the emitted ones. It can be completed using a photonic structure which mechanism works as a spectral photonic band stop filter for certain wavelengths [10]. Different approach of trapping the emitted photons in the converting layer is application of the metallic nanoparticles. Silver or gold particles embedded in a dielectric material are characterized by strong scattering properties above certain wavelength, due to their specific plasma oscillation frequency [7]. The oscillation frequency can be tuned by changing the shape and size of the particles, as well as the refractive index of the propagation medium [11]. These type of layers could also act as semi-transparent contacts.
For the proposed application, the selectively transparent layer applied on the top of flexible thin-film solar cell, is expected to be mechanically resistant on bending, which is not necessarily the case of the two approaches mentioned before. That is the reason of manufacturing the investigated layer, considered for this particular application, using a polymer foil, with one-dimensional pattern on the front side. This type of front surface texturisation minimizes the reflections and it has been proven to be very beneficial for the enhancement of the solar cells performance [12]. Effective light trapping through textured surfaces have been achieved previously by other authors using Al-doped ZnO (AZO) [13,14] or V-shaped light trapping configuration [15].
For the examination of proposed concept of the selectively transparent layer, experimental and reference samples have been prepared and investigated.
2. Materials and methods
2.1 Theoretical approach to the front surface texturisation
Transmission and reflection coefficient depend not only on the interface type, but also on the angle of light incidence. They are described by the Fresnel equations for two light polarizations $S$ and $P$:
Fig. 2. Fresnel transmittance and reflectance for a) air to polymer interface and b) polymer to air interface. Polymer of two different indices of refraction have been considered – 1.58 is the refraction index for PET foil and 1.76 is the refraction index for Teonex foil.
Dependencies shown in Fig. 2 bring to the conclusion that the transmission of air to polymer interface is fairly good and stable if the angle of incidence does not exceed 60°. On the other hand, the transmission of polymer to air interface descends to 0 when a certain angle is exceeded, which as a consequence causes total internal reflection. The value of this angle depends on the polymer index of refraction.
A concept which would allow high transmission of light from air to polymer would be assuring that the angles of incidence higher than 60° are eliminated. What follows, in order to trap the light inside the polymer, the light should strike the interface from the polymer side at angles larger than the angle of total internal reflection for the particular polymer kind. This goal can be achieved for instance by the texturisation of the foil. Two possible ways of texturisation patterns, triangular and sinusoidal grooves, are depicted in the Fig. 3.
Fig. 3. Schematic representation of two different texturisation patterns a) triangular grooves inclined at an angle α and b) sinusoidal grooves.
2.2 Triangular grooves
The advantage of texturisation in the form of triangular grooves (Fig. 3(a)), is that all effective angles of incidence at the air to polymer interface, are inferior or equal to α. If the rays would strike the surface of the foil perpendicularly, all effective angles would be equal to α. The optimal value of α for perpendicularly striking radiation, could be assessed using the performance factor (PF) which represents the product of the transmission for the air-to-polymer interface, and reflectance for the polymer-to-air interface.
It depends also on the index of refraction and is depicted in the Fig. 4.Fig. 4. Performance factor versus the inclination of triangular grooves allows to find the optimal angle of α.
The inclination angle which maximizes the performance factor is lower for higher indices of refraction, but at the same time with lower index of refraction higher performance factor is possible.
2.3 Sinusoidal grooves
The sinusoidal texturisation can be mathematically described analogously to a mechanical wave:
where A is the amplitude and T is the period of the sinusoid. The parameters A and T will influence the effective angles of incidence of the illuminating radiation which have an impact for the final transmission and reflection percentages. As in the section 2.2 we will define the performance factor of the texturisation as the product of the transmission for the air-to-polymer interface and reflectance for the polymer-to-air interface. Its value depends on A and T as well as on the index of refraction of the polymer foil (Fig. 5).According to Fig. 5, the amplitude to period ratio which maximises the performance is equal to 0.5. The index of refraction of the foil influences the performance for an amplitude to period ratio close to 1 or close to 0.1.
2.4 Comparison of the triangular and sinusoidal texturisation of similar dimensions
Comparing Fig. 4 and Fig. 5 it can be remarked that triangular texturisation is prone to achieve higher values of performance factor than for sinusoidal texturisation of the most optimal amplitude to period ratio.
Table 1 compares the performance of two types of considered texturisation patterns exposed to radiation of 589 nm striking the foil surface perpendicularly. The considered triangular grooves are inclined at an angle α equal to 48° and they are based on a triangle of a height 26 µm and base of 48 µm. An analogous pattern based on the sinusoid of the peak-to-peak amplitude equal to 26 µm and the period of 48 µm is considered. The total transmission and reflectance coefficients are calculated basing on the effective angles of impinging radiation.
Table 1. Calculations of total transmittance and reflectance for two texturisation patterns for λ=589 nm.
The calculations from Table 1 show that the sinusoidal texturisation is, in this case, equally effective when transmitting the light from air to polymer, however, it is less efficient when trapped inside the structure, as it allows a lot of effective angles of incidence lower than the angle of total internal reflection.
2.5 Verification of the concept
In order to verify the proposed concept of texturisation, a 3M Cool Mirror 330 foil textured in the form of triangular grooves, has been examined. Using the Keyence VK-X 3D Laser Scanning Confocal Microscope the dimensions could be verified (Fig. 6).
Fig. 6. a) 3D view of the polymer foil, b) photograph of the cross-section, c) scheme of the foil cross-section with dimensions.
The foil exhibits selective transparency depending on the side of illumination visible with the naked eye (Fig. 7(b) and Fig. 7(c)). In order to estimate the correct values, it has been placed on top of the silicon detector in SK300 Optel spectrophotometer with M250 monochromator, equipped with tungsten light source. A difference of transmission of 70% depending on the way that the foil is placed on the detector, has been recorded (Fig. 7(a)).
2.6 Considered materials
The selectively transparent layer for the considered application should not only possess possibly high transmittance in a wide spectral range for the radiation coming from outside of the cell and possibly low transmittance for the radiation generated inside, but also the selectively transparent layer for the considered application should be persistent to mechanical and environmental threats (humidity, high temperature), as well as possess flexibility and low weight. In order to meet these requirements, different types of polymers were considered: PET foil, optical PET, Kapton, Teonex. The considered foils are made of 4 different kinds of polymers: Poly(ethylene terephtalate) – PET, optical PET|, Polyimide - PI and, Polyethylene Napthalene – PEN. PET is a widespread material which results in its low cost, but its mechanical properties and temperature resistance are much lower than for the other two groups. Optical PET is a PET foil with an enhanced response in UV range, with the other parameters staying the same. Kapton HN, by DuPont, made from Polyimide, can be subjected to high temperatures (more than 400°C), but its transmission for wavelengths lower than 400 nm is very poor. What is more, it is quite expensive. Finally, Teonex, made from Polyethylene Napthalene, is also able to withstand high temperatures and harsh chemicals, but is cheaper than Kapton and has a transmission characteristics more suitable for the considered application (Fig. 8). The features of the polymer foils are gathered in Table 2.
Table 2. Some of the properties of the considered foils (taken from the datasheets of the manufacturers)
Taking into account mainly the characteristics of optical transmittance, optical PET foil has been chosen for further processing.
2.7 Fabrication of the selectively-transparent layer made of optical PET
In order to influence the parameters of the texturisation pattern, its fabrication was performed using a subtractive method by the 343 nm UV laser Tru Micro 5235c. The advantage of laser ablation method is that high precision – of the order of single µm – can be achieved. There are a lot of parameters which influence the process: the laser power, laser frequency, velocity of the sample holder, number of the laser beam passes.
3. Results
Within the research 17 samples have been fabricated using different parameters of laser ablation (velocity of the sample holder, distance between the lines, number of laser beam passes). They are gathered in Table 3 together with the transmission signal ratio - the most important parameter which allows to asses if the foil is selectively transparent or not. Figure 9 shows the whole transmission signal ratio in the whole measurement range.
Fig. 9. Transmission signal ratio versus wavelength for samples exhibiting the effect of selective transparency
Table 3. Samples fabrication parameters and the transmission signal ratio for 600 nm. The samples with the highest ratio have been marked in green, the samples where no effect was observed have been marked in red.
Among the compared samples there are samples P5 and P7 which exhibit the highest transmission signal ratio – up to around 30% difference. At the same time, the absolute value of transmittance in the “transmissive” direction should be as high as possible. The transmittance of the best samples is compared in the Fig. 10(a) and here sample P7 turns out to be much better than sample P5. Concluding the data in Fig. 10(a), it can be noticed that there is a severe degradation of transmittance in the UV-blue light in comparison to unmodified PET, which can be attributed to too high laser power which deteriorated the optical properties of the material.
Samples P5 and P7 have been subjected to further analysis using a 3D microscope. Their profile is shown in the Fig. 11. It can be seen that the inclination angle of triangular grooves is around 30° for P7 and 33° for P5 which justifies higher difference in signal depending on the side, but on the other hand, as shown in Fig. 4, the performance is prone to be much better if the inclination angle would exceed 40°.
At the same time, the shape of the grooves is not really triangular – there are double peaks present, which is especially visible in the case of P7. These peaks are identified as the burned material, which has been removed by the laser in order to form the grooves, however, it has precipitated on the border of the grooves. It has very low transparency and is responsible for the brownish colour of the foil (Fig. 10(b)). The transmittance of P7 is slightly better than P5 as there is some part of the foil which remains uncovered by the burned PET.
4. Conclusions
The paper describes the testing stage of the proposed concept based on selectively transparent flexible layer manufactured on foil substrate, which proved to indicate the prospective properties for application in photovoltaic structures with down-converting layers.
Engineering of selectively transparent layers is a compromise between the best optical, mechanical and thermal properties, as well as the costs of adding it to the final structure. It has been proved that the texturisation of the front surface enables to achieve even up to 70% difference in transmission, depending on the side from which the radiation arrives. The transmission in the visible wavelength range has fallen from 80% when illuminated perpendicularly to the textured side, to around 10% when illuminated perpendicularly to the planar side. Moreover, the method is scalable, as well as less expensive and technologically demanding than alternative selectively transparent layers, investigated by the authors.
Triangular and sinusoidal texturisation have been analyzed and compared. In theory, the triangular grooves are prone to produce higher performance factor, however, their manufacturing is slightly more complex. The first attempts of their production using laser ablation method on the optical PET have been conducted. The effect of selective-transparency has been observed. The preliminary results are promising, although further optimization of the manufacturing process parameters can result in more efficient layers. In the frame of the future research, the angle of inclination of the grooves should be increased. As the deterioration of the optical properties of the material has been observed, therefore the process should be either performed with lower power or the foil should be changed to the one which is more resistant to heat.
In further perspective, the authors intend to apply the foil on top of flexible solar cells equipped with elaborated down-shifting layer with luminescent nanoparticles suspended in the polymer medium. Optimal texturization technologies for this process are now under investigation of the authors. The most promising solution is the texturization of the actual polymer down-shifting layer in order to minimize the losses of reflection on the interface in between the polymers, together with the complexity and costs of the final photovoltaic device.
Disclosures
The authors declare no conflicts of interest.
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