Efficiency enhancement of the photovoltaic cells using a converter, methods for producing thereof, and a solar cell device

Abstract

The present invention relates to Method for producing an efficiency-enhancing converter layer for a photovoltaic cell, comprising the steps of: - Providing a sheet-like polymeric substrate; - Irradiating onto a main surface of the substrate with particles of a particle source to create fractal tree-like conduits extending from the main surface of the substrate into the bulk material of the substrate.

Description

[0001] Description

[0002] Efficiency enhancement of the photovoltaic cells using a converter, methods for producing thereof, and a solar cell device

[0003] Field of the invention

[0004] The present invention relates to the field of solar cell devices, in particular to photovoltaic cells with efficiency-enhancing micro-structured layers.

[0005] Background art

[0006] Solar energy harvesting using photovoltaic cells is not inexpensive and causes relatively high initial costs. Typical solar energy received in 1 m2is ~ 1 kW in day time and 250 W at night time due to the earth background emission. The current widely used photovoltaic cells made of silicon substrate can only capture a fraction of the available solar energy due to the intrinsic material properties of silicon. Much of the ultraviolet and near-infrared radiation is not efficiently harvested and generates heat and should therefore be blocked from the active area of the photovoltaic cell.

[0007] The solar harvesting of silicon-based photovoltaic cells is restricted to a narrow range of the visible spectrum (570- 620 nm) so that the highest achievable efficiency of a silicon photovoltaic cell cannot exceed 23%. Most photovoltaic devices do not capture infrared radiation, losing more than half of the whole solar radiation spectrum.

[0008] Besides the importance of advanced optical materials and their role in the performance of photovoltaic cells, designing the cell structure is essential. In fact, the photovoltaic cell structure has a significant effect on the conversion efficiency. Thus, in research, different aspects are targeted related to the configuration and design of photovoltaic cells such as the number of layers, materials, and circuits in order to increase the overall efficiency. Applying advanced optical structures including elaborated surface structures in a polymeric optical convertor is a key approach to provide optimal designs to increase the efficiency of the photovoltaic cell. The purpose of these designs is twofold: First, the outer surface of a top layer should reduce light reflections and secondly, a suitable photon path to the active area of the photovoltaic cell should be provided.

[0009] It is an object of the present invention to provide an efficiency-enhancing converter layer for photovoltaic cells, which allows to increase the efficiency of energy harvesting.

[0010] Summary of the invention

[0011] This object is achieved by a method for producing an efficiency-enhancing converter layer according to claim 1 , the efficiency-enhancing converter layer for a photovoltaic cell, and a solar cell device according to further independent claims.

[0012] Further embodiments are indicated in the dependent subclaims.

[0013] According to a first aspect, a method for producing an efficiency-enhancing converter layer for a photovoltaic cell is provided, comprising the steps of:

[0014] Providing a sheet-like polymeric substrate;

[0015] Irradiating onto a main surface of the substrate with a particle source to create tracks, particularly with fractal tree-like conduits, extending from the main surface of the substrate into the bulk material of the substrate.

[0016] According to a further aspect, an efficiency-enhancing converter layer for a photovoltaic cell device is provided, produced by the above method.

[0017] According to a further aspect, a solar cell device is provided comprising a photovoltaic cell and the above efficiency-enhancing converter layer which is attached on the active surface of the photovoltaic cell.

[0018] It may be provided that the particle source emits alpha particles, protons or heavy particles having an energy upon impact with the surface of the polymeric substrate of between 1 ,5 MeV and 5 MeV. Furthermore, the dosage of particle radiation onto the main surface is 103to 5*106particles per cm2.

[0019] As mentioned above, the efficiency of photovoltaic cells may generally be improved by covering the active area of photovoltaic cells with the converter layer serving different functions.

[0020] The converter layer according to the present invention is made of a polymer as a converter material, such as a polycarbonate, particularly a PCTD (Polycarbonate track detector) material which may be modified by particle radiation. The particle radiation is directed onto a main surface so that microstructural latent tracks arise from the particle energy transfer into the polymer chain, leading to chain scissions and the formation of active sites. Latent tracks are the trail of defects inside the substrate created in the path of the particles during the irradiation. Formation of these tracks depends on the amount of energy lost inside the material.

[0021] As a converter layer material, a polycarbonate track detector material (PCTD) or bisphenol-A-based polymers, such as MakrofolODE or LEXAN® may be used, which are polycarbonates that are susceptible to degradation or modification by particle radiation. The bisphenol-A-based polymers, have emerged as promising candidates with applications ranging from track-etched membranes to medical uses. These have found utility in dosimetry, optics, electronics, space applications, ion tracking, nanomaterial synthesis, and space exploration. When polymeric materials are exposed to ionizing radiation, such as the accelerated electrons and protons and other heavy particles, their initial structure undergoes chain scission, cross-linking, and free radical formation, leading to molecular fragmentation and the creation of saturated / unsaturated chemical groups. Furthermore, the interaction of protons or heavy / alpha particles with polymers causes chain scission, cross-linking, double bond formation, and molecular emission that give rise to notable changes in the structural, chemical, and optical properties of the polymer under various radiation conditions.

[0022] The tracks, particularly with fractal tree-like microstructural conduits, may be formed by particles as hollow nano- / microstructures on their way through the bulk material of the converter layer. The polymeric efficiency-enhancing converter layer can be optically closely coupled with a photovoltaic cell surface. Due to the radiation exposure modified converter material, it has been found that light reflections on the main surface of the converter layer are significantly reduced compared to the non-modified converter material.

[0023] Furthermore, the tracks / conduits extending from the main surface (surface of the converter layer which will be arranged opposite to the surface of the photovoltaic cell) into the bulk material of the converter layer define a suitable photon path across the converter layer.

[0024] MZ

[0025] The linear energy transfer (LET) of the irradiation is proportional to —2where M, Z and E represent particle mass, charge and energy, so LET is larger for low energy and heavier particles inducing larger latent tracks.

[0026] It may be provided that an electrochemical etching is applied to widen the conduits in a first region (from surface to a depth where the tracks / conduits starts branching) close to the main surface, particularly to a width of 1 to 20 pm at the surface of the substrate. The electrochemical etching provides different bulk and track etching rates to support widening of the tracks / conduits and the formation of the fractal tree-like microstructures.

[0027] Furthermore, the electrochemical etching may be performed with a high voltage of between 1000 and 5000 VAC to induce the electrical breakdown in the form of sparks to promote the formation of additional branches of fractal tree-like microstructures in the bulk material and to develop latent tracks on nano-to-micro scales depending on etching time. Thereby, tracks are formed having a high number of micro conduits of different diameters and lengths produced by electrical breakdown. In fact, the etching time can control the mean track size, particularly the track diameter at the surface of the substrate.

[0028] The electrical voltage applied for electrochemical etching depends on the substrate thickness. If the substrate thickness is about 500 pm, the electric field should be about 20-100 kV / cm.

[0029] Since the tracks or conduits provide hollow channels the etching electrolyte may enter the microstructures during the etching process thereby widening or extending the channels based on the difference between bulk and track etching rates, alongside with the effect of electrical break down.

[0030] The track selectively conducts the photons in a certain spectral region and traps the photons in several other regions according to the wavelengths to propagate across the polymer thickness or may trap / reflect accordingly.

[0031] Both particle irradiation and electrochemical etching are promoting the formation of fractal tree-shaped conduits. The special optical properties of these fractal structures arise from its distinctive fractal shape, which results in spectral selectivity, allowing a certain bandwidth of light pass through. They are capable of trapping photons in certain range of wavelengths, while transmitting photons in other spectral regions.

[0032] For better efficiency, the etching condition is configured to match the optimal spectra of the solar cells (570-620 nm). For example, the etching condition may include that the electrolyte is potassium hydroxide KOH (6 M) + Ethanol+ Water (PEW) (e.g. 20-25 mass % KOH, 60-70 mass % water and 5-15 mass % ethanol), an etching time of between 4 - 8 hours and a voltage applied over the thickness of the substrate of between 1 ,5 and 5kV, for example at 2kV, and a frequency of between 30 Hz to 1 kHz, for example at 50 Hz. The electrical parameters may be selected depending on the polymer thickness. However, with an appropriate etching time, it is possible to develop the mean diameter of tracks and achieve the appropriate size of fractals after optimal development by etching.

[0033] By or while irradiating or during the electrochemical etching the tracks are formed in two regions, a first region where each particle makes a single track and a second branching region where the tracks form numerous branches in a fractal-like manner. During the electrochemical etching process, the first part of the tracks is formed as a conical head (track diameter) whose size is developed in terms of etching time. The top layer is gradually removed by a bulk etching rate and the depths of track grows with a depth etching rate. The latter is several times greater than the bulk one. In the second region the microstructure includes many branched tracks which are induced by the electric field of the electrochemical etching process. Furthermore, the density of branched tracks exceeds several thousand per cm2, however the exact density strongly depends on the applied voltage and can be estimated by confocal scanning microscopy in cross position after proper dying process. The etching duration may be selected to obtain a width of track openings at the surface of about 1 to 20 pm, preferably at 10pm. A relation between track width an etching duration is e.g. described in Kandaiya, S. (1988), “Characterization of CR 39 nuclear track detector for use as a radon / thoron dosemeter (No. KFK--4348)”, Kernforschungszentrum Karlsruhe GmbH (Germany). However, numerous studies have established a linear relationship between etching time and track diameter in the etching process. For instance, a track diameter of 10 pm can be obtained by irradiating with a particle energy of 2MeV at the surface of the substrate and an etching duration of about 6 hours with above indicated electrolyte.

[0034] The density of branches arises from electrical break down depending on voltage across the substrate.

[0035] In addition, the photon guiding tracks or conduits can be filled with an up-conversion or down-conversion material to further improve the efficiency gain. So, according to a further embodiment, after removal of the etching electrolyte the conduits may be filled with optical up-conversion materials, such as lanthanides, or down-conversion materials such dye substances, such as Coumarin, Cholarzol, Rd6G and RdB, in order to shift the wavelength of incident photons passing through the filled conduits to a specified wavelength range in which the active area of the photovoltaic cell beneath the converter layer is efficient or sensitive. To fill the tracks / conduits with the up-conversion material it may be applied on the main surface of the converter layer / substrate from which it may enter the tracks / conduits.

[0036] The concentration and the type of the up-conversion materials may be significant to give out maximum efficiency. In fact, the optimal efficiency is achieved through the application of this polymeric converter, which can be applied to any commercially available or laboratory-tested solar cell, resulting in typically and an excess of ~ 2% increase in efficiency. Note that UV will be absorbed by polymer, and thermal IR radiation should be blocked via different techniques e.g. Fresnel lens, diffraction gratings etc.

[0037] The above efficiency-enhancing converter layer for a photovoltaic cell provides a low- cost polymer substrate that has an efficiency gain of up to 2%. The above converter combines the effect of a photon conductor through the distributed conduits of various sizes along a fractal tree towards the photovoltaic active area with the reduction of the reflection of the main surface of the converter layer.

[0038] The formation of the 3D fractal tree microstructures in the bulk material of the converter layer using particle irradiation and subsequent electrochemical etching results in the distribution of a high number of conduits or tracks with a density of 103to 5*106tracks / cm2. It has been found that the optimal track density is around 106particles / cm-2. They extend from the main surface with a diameter of 1 to 20 pm, e.g. 10pm, tapering with their extension into the depth of the bulk material, which can be controlled by the particle fluence exposure from the radiation source.

[0039] The formation of the fractal tree-like microstructures notably results in a further spectral broadening of the light bandwidth propagation through the converter layer caused by average unit cells formed between the tracks / conduits of the fractal tree-like branches. The average unit cell is related to or corresponds the average number of tracks per cm2. Larger fluences are correlated to the smaller average unit cells. In fact, average unit cell is equivalent to the mean distance of tracks.

[0040] The transmitted or reflected wavelengths depend on the unit cell. The unit cell is defined as the mean distance between two adjacent conduits when density is adequately high. The efficiency-enhancing converter layer acts like a lattice. Although photonic crystal formalism cannot be applied in this case due to the fractal structure, the corresponding lattice constant for the photonic crystal must be comparable to the wavelength of light, typically in the range of 0.4 to 1 pm up to 10pm. This range is three orders of magnitude larger than the lattice constants found in solids and hence photonic crystals need to be synthesized. Here, average unit cell may be close to 10 pm. The structures of interest here are not photonic crystals but random fractal-like microstructural conduits. For this reason, according to the simulation based on fractal structure phenomenon the transmission bandwidth becomes widened and allows more transmission over certain spectral regions. The transmission bandwidth range start from few nanometers to tens of nanometer. The transmission bandwidth range depends on scattered tracks, density of tracks, distribution of tracks and shape of fractals. More heterogeneity of track distribution led to wider spectral range. The mean dimensions of the unit cells define the transmission bandwidth. This bandwidth should match the optimum efficiency spectra of the solar cell. The optimal density of the random fractal-like conduits has to be determined experimentally. The amount of the broadening of the light bandwidth propagation depends on the mean distance between neighboring tracks or conduits formed by the alpha particle exposure. As a material for the layer, a polycarbonate track detector material (PCTD) such as MakrofolODE can be used.

[0041] Moreover, the fractal tree-like microstructures may be promoted using the electrochemical etching process applying high-frequency electric discharge between the ends of the conduits. The particle radiation initiates the formation of tracks, while the etching process may further develop the size of the tracks and may create, deepen and / or further develop fractal-like branched track structures. Therefore, both processes may contribute to achieve these microstructures. The transmitted light bandwidth is affected based on population of tracks, diameter, conical structure of the tracks / conduits and its fractal-treeing structure. It also depends on the density of tracks / conduits and the degree of non-uniformity of distribution of the conduits.

[0042] With increasing conduit / track density, the transmission bandwidth starts broadening because of the mean conduit distance approaches to the dimensions of a unit cell assuming a uniform track distribution.

[0043] It is preferred to adapt the optical bandwidth of the converter layer to overlap the effective spectral range of the photovoltaic cell (e.g. 570nm - 620nm for silicon photovoltaic cells) by adjusting the involved production parameters that address the depth, width and / or the density of the radiation-generated and etched tracks or conduits.

[0044] Basically, the etched tracks or conduits act as tapered waveguides. Thus, when photons enter the tracks / conduits from the main surface, the tracks / conduits not only guide the photons within the effective spectral range across the converter layer, but also spectrally select the photons outside of spectral range by trapping them within the conduits.

[0045] These conduits can therefore advantageously be filled with up- and / or downconversion materials to shift the wavelength of the trapped photons to the desired spectral range when travelling through the tapered conduits of the fractal tree-like microstructures. The above structure of a converter layer can eliminate unwanted light spectra and may shift the remaining spectra into the optimal region. This goal is partially achieved through the inherent properties of fractal conduits and further enhanced by filling these conduits with up-conversion and down-conversion materials. Unlike traditional methods that merely place up-conversion materials on a surface layer, our approach embeds these materials within the depths of the conduits, resulting in a more efficient conversion process. This approach utilizes numerous fractal conduits, which improves system efficiency by approximately 1-2% and enhances stability.

[0046] On the other hand, it may be essential to block photons of undesired wavelength of the solar light spectrum in UV and IR spectral ranges so as not to allow them reaching the active area of the photovoltaic cell. In fact, those undesired spectral ranges of sunlight induce heat, and may degrade the active area of the photovoltaic cell leading to a reduced efficiency and lifetime of the photovoltaic cell. While polymer totally absorbs UV and does not allow it to propagate into the solar panel, infrared light (IR) is thermal radiation and is not blocked by the polymer material. Infrared light (IR) should be blocked in different ways using an architectured Fresnel lens and diffraction gratings or according to given macrostructures to reflect IR radiation. IR and UV both should be removed to enhance the efficiency inherently. Note that one of the optical properties of fractals is the IR reflection.

[0047] Brief description of the drawings

[0048] Embodiments of the present invention are described in more detail in conjunction with the accompanying drawings in which:

[0049] Fig. 1 is a schematic view on the solar cell device having an efficiency-enhancing converter layer according to the present invention;

[0050] Fig. 2 is a cross-sectional view through the solar cell device of Fig. 1;

[0051] Fig. 3 is a flowchart illustrating the method step for producing an efficiencyenhancing converter layer for a photovoltaic cell; Fig. 4 illustrates the apparatus for performing an alpha particle radiation of a converter layer material;

[0052] Fig. 5 schematically illustrates the apparatus for performing the process of electrochemical etching of the radiated converter layer;

[0053] Fig. 6 shows a diagram of spectral absorbance in terms of bare and a-irritated converter layer material after electrochemical etching;

[0054] Fig. 7 shows a diagram of spectral transmission in terms of bare and a-irritated converter layer material after electrochemical etching;

[0055] Fig. 8 shows diagram of spectral reflectivity in terms of bare and a-irritated converter layer material after electrochemical etching; and

[0056] Figure 9 depicts a diagram showing the efficiency gain depending on the treatment of the polycarbonate material to produce a converter layer for a photovoltaic cell.

[0057] Detailed description of embodiments

[0058] Figs. 1 and 2 shows a perspective schematic view and a cross-sectional view on a solar cell device 1 having a photovoltaic cell layer 2 having an active area on its main surface 21. The photovoltaic cell layer 2 may comprise a silicon substrate or the like.

[0059] Furthermore, an efficiency-enhancing converter layer 3 is attached and optically coupled by means of a transparent matching layer 4 onto the main surface 21 of the photovoltaic cell layer 2.

[0060] The efficiency-enhancing converter layer 3 is formed with a substrate of PCTD material, such as polycarbonate, such as MakrofolODE. The efficiency-enhancing converter layer may have a thickness of about 300 to 1000 pm. In the substrate fractal tree-like microstructures may be formed by alpha particle radiation followed by an electrochemical etching as described below.

[0061] The fractal tree-like microstructures are formed by randomly distributed tracks of alpha particles traces and secondary particles which affect the bulk material. The mean depth where secondary particles are generated depends on the energy of the alpha particles but may be preferably between 30pm and 200pm. The secondary particles cause the conduits to spread into a tree-like microstructure 31 which may be formed with a depth range of 30 pm - 200pm to 400 pm. The linear energy transfer (LET) of the irradiation

[0062] MZ is proportional to —2where M, Z and E represent particle mass, charge and energy, so LET is larger for low energy and heavier particles inducing larger latent tracks.

[0063] The etching process forms a conical opening 32 of the latent tracks in a first region extending from the main surface 33 of the efficiency-enhancing converter layer 3 to the depth where secondary particles are generated. The conical opening is made by widening the track / conduit or channel formed by the alpha particle shortly after it enters into the bulk material of the substrate using an electrochemical etching process.

[0064] The conduits or tracks, formed by the alpha particle radiation and by the etching, may be filled with up- and / or down-conversion materials, such as Rd6G or RdB or lanthanides.

[0065] Fig. 3 shows a flowchart illustrating the process of producing the above efficiencyenhancing converter layer 3.

[0066] In a first step S1 , a converter layer material such as PCTD or bisphenol-A-based polymers, such as MakrofolODE or LEXAN® is provided, which is a polycarbonate that is susceptible to degradation or modification by alpha particle radiation as described above.

[0067] In step S2, the main surface 33 of the PCTD material is radiated by means of an alpha emitter source, such as a241Am source.

[0068] The alpha particle irradiation of the converter layer 3 can be carried out, for example, by an arrangement as shown in Fig. 4. An alpha emitter source 11 may be arranged within a collimator 12 at a distance of between 30 and 40 mm from the surface of the converter layer 3. Alpha particles having an energy of 5.29 MeV and a specific activity of 3.43 Ci / g may be used to irradiate the converter layer material having a thickness of about 300 to 500 pm using a collimated241Am alpha source, resulting in a fluence of 2 MeV energy exposure at the surface of the converter layer material due to energy loss in air.

[0069] The irradiation source may be securely positioned at the end of a short brass cylinder, which is inserted into another brass cylindrical collimator with an aperture of 8 mm diameter and 0.5 cm2area. This allows precise adjustment of the distance between the layer and the source, thereby facilitating control of the alpha particle energy applied to the converter layer and the corresponding dose by adjusting the air gap at room temperature. The alpha dose and source energy may be calibrated to a distance using a surface barrier detector coupled to a multi-channel analyzer. The dose in alpha particles / cm2can be adjusted between 103and 5*106alpha / cm2.

[0070] In a next step S3, the irradiated converter layer 3 is subjected to an electrochemical etching process in an etching device 20 shown in Fig. 5. The reagent or electrolyte 23 of the etching process can be a mixture of potassium hydroxide (KOH), ethanol and water. For example, an etching electrolyte of 20-25 mass % KOH, 60-70 mass % water and 5-15 mass % ethanol can be used.

[0071] The etching time can be selected between 4-8 hours and the high voltage of a voltage source 22 can be set to 50 Hz and 1-5 kVAC (at e.g. 50 Hz) which is applied via electrodes 24 to connect the reagent 23. The high voltage is applied through the electrolyte mixture (reagent) separated by the converter layer 3 so that the voltage is applied across the thickness of the converter layer 3 to support etching.

[0072] Basically, it is advantageous to select the etching reagent so that electrochemical etching and chemical etching occur simultaneously in a single process.

[0073] The chemical etching helps the formation of latent tracks within the bulk material along the alpha particle paths. Chemical etching of the tracks can further increase the track structure dimensions. The etching time determines how the nanometer latent tracks will develop to micrometer size. According to etching time, the mean diameter can be controlled. The electrochemical etching process enhances the process of chemical latent track etching and may cause frequent sparks across the tip of the tracks (due to the increased conductivity of the tracks compared to the bulk polymer material) to the opposite substrate surface through the bulk material of the converter layer 3 resulting in the formation or enhancement of random fractal tree-like conduits through the thickness of the bulk material of the converter layer 3. In fact, the electric sparks inside the converter material promote the formation of micro conduits of different diameters, most tapered at the end.

[0074] At the beginning of the etching process, the reagent rapidly diffuses into the preexisting fine tracks or channels formed by the passage of alpha particles, creating conductive paths that penetrate the electrically insulating bulk material. Due to the high voltage applied in the etching process, treeing (formation of conduits induced by high voltage (spark) discharge into a dielectric material) occurs at the tip (end of the tracks in the bulk material of the substrate) of the tracks, causing or promoting multiple branch formation in the bulk of the converter layer material

[0075] The reagent for the electrochemical etching process may be selected to have a higher track etch rate meaning a higher etching rate for radiated regions than the bulk etch rate, meaning e.g. the etching rate for non-radiated regions creates conical tracks through the thickness of the converter layer material. The high voltage applied across the converter material by the electrochemical etching creates frequent sparking across the dielectric converter layer to promote the formation of tree-shaped fractal conduits through the bulk material.

[0076] Alpha radiation and electrochemical etching create numerous adjacent traces / conduits through the bulk material allowing photons to be trapped and light reflections to be suppressed, and these photons to be selectively guided and directed to the active area of the photovoltaic layer. This allows an efficiency gain of about 1%.

[0077] The fractal tree-like microstructures result in absorption or trapping of photons of solar light irradiation and act as tracks or conduits to select the wavelength of light to be directed to the active area of the photovoltaic layer. In step S4, the etched converter layer 3 is, after removal of the etching reagent, immersed in the up- or down-conversion material so that the up- or down-conversion material enters the microstructures (tracks or conduits) formed by the step of alpha particle irradiation and etching.

[0078] Thereby, the filled microstructures may act as reactive sites for up- or downconversion. Down-conversion materials may include e.g. Rd6G or RdB, Coumarin, up- conversion materials may include lanthanides and their fluorides, Er, Yb, Gd and Tm, and the like, e.g. NaYF4: Era+, Yba+ of an optimized density such as e.g. 1mg / ml.

[0079] The main up-conversion mechanism in b- NaYF4: Er3+, Yb3+ is energy transfer up conversion (ETU). Excitation in the 2F7 / 2-2F5 / 2 transition of Yb3+ is followed by a two- step energy transfer process to neighboring Er3+ ions (see Fig. 4), which brings Er3+ to the 4F7 / 2 level. Fast relaxation to the 2H11 / 2 / 4S3 / 2 level is followed by green emission (2H 11 / 2-4115 / 2, 4S3 / 2-4I15 / 2) and further relaxation to the 4F9 / 2 level yields red emission (4F9 / 2-4115 / 2). The Yb3+ ion has only one excited state and is an ideal sensitizer for Er3+ because of the relatively high oscillator strength of the 2F7 / 2-2F5 / 2 transition and the fact that Er3+ has a state with similar energy (4111 / 2), which is populated by energy transfer from Yb3+.

[0080] After applying the up- or down-conversion material, the converter layer is attached onto the active surface of a photovoltaic cell layer 2.

[0081] Figure 6 shows a diagram illustrating the spectral absorbance for wavelength between 400 and 900 nm for bare (not irradiated / untreated) and a-radiated polycarbonate substrate materials (pc) after electrochemical etching (ECE) with an irradiation of 104, 105and 106alpha / cm2. It can be seen that the absorbance over 400-900 nm (nearly plateau) notably increases at 106alpha / cm2against bare polycarbonate and also smaller track densities.

[0082] Fig. 7 shows the spectral transmission of the same converter materials, which shows the inverse of spectral absorbance accordingly. Similarly, the transmission over 400- 900 nm (nearly plateau region) attests the pc films with larger a dose undergo smaller transmittance against the bare (not irradiated) polycarbonate material. Fig.8 shows a diagram illustrating the characteristics of diffusion reflection spectra (DRS) of the a-irradiated polycarbonate material and the bare polycarbonate material. Note that the DRS demonstrates a large value of 12 % for 105- 106alpha / cm2, against bare polycarbonate material (~ 11 %), emphasizing a 2% increase over the visible range of 400-700 nm. The UV region shows a light increase with respect to the bare one. In NIR region 105-6depict the same behavior.

[0083] Figure 9 depicts a diagram showing the efficiency gain depending on the treatment of the polycarbonate material to produce a converter layer for a photovoltaic cell.

Claims

Claims1. Method for producing an efficiency-enhancing converter layer (3) for a photovoltaic cell (1), comprising the steps of:Providing (S1) a sheet-like polymeric substrate;Irradiating (S2) onto a main surface of the substrate with particles of a particle source to create conduits, particularly fractal tree-like conduits, extending from the main surface of the substrate into the bulk material of the substrate.

2. Method according to claim 1, wherein the particle source emits alpha particles, protons or heavy particles having an energy upon impact with the surface of between 1 ,5 MeV and 5 MeV.

3. Method according to claim 1 or 2, wherein the dosage of particle radiation is 103to 106particles per cm2.

4. Method according to any of the claims 1 to 3, wherein a polycarbonate track detector material (PCTD) such as MakrofolODE is used as the polymeric substrate.

5. Method according to any of the claims 1 to 4, wherein an electrochemical etching is applied (S3) to widen the fractal tree-like conduits in a first region close to the main surface, particularly to a width of 1 to 20 pm, preferably to a width of 5 to 12 pm.

6. Method according to claim 5, wherein the electrochemical etching is performed with a high voltage of between 1000 and 5000 VAC (at a frequency of 30 Hz to 1Khz) or an electrical field of 20-100 kV / cm to induce spark breakthroughs between the tips of the fractal tree-like microstructures in the substrate and an opposing surface of the substrate.

7. Method according to any of the claims 1 to 6, wherein conduits are filled with an optical up-conversion material, such as lanthanides, and / or a down-conversionmaterial such dye substances, such as Coumarin, Cholarzol, Rd6G and RdB, in order to shift the wavelength of incident photons passing through the filled conduits to a predetermined wavelength range.

8. An efficiency-enhancing converter layer (3) for a photovoltaic cell (1), produced by the method according to one of the claims 1 to 7.

9. A solar cell device comprising a photovoltaic cell (1) and an efficiencyenhancing converter layer (3) of claim 8.

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