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Impact of porous SiC-doped PVA based LDS layer on electrical parameters of Si solar cells
Optical Materials 80 (2018) 225–232
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Impact of porous SiC-doped PVA based LDS layer on electrical parameters of Si solar cells
T
S. Kacia,∗, R. Rahmounea, F. Kezzoulab, Y. Boudiafb, A. Keffousa, A. Manseria, H. Menaria, H. Cheragaa, L. Guerbousc, Y. Belkacema, R. Chalala, I. Bozetinea, A. Boukezzataa, L. Talbia, K. Benfadela, M.-A. Ouadfela, Y. Ouadaha a
Research Center on Semiconductor Technology for Energetic, CMSI Division, CRTSE, 2 Bd Frantz Fanon, PB 140, 7M, Algeria Research Center on Semiconductor Technology for Energetic, DDCS Division, CRTSE, 2 Bd Frantz Fanon, PB 140, 7M, Algeria c Algiers Nuclear Research Center (CRNA), 2 Bd Frantz Fanon, BP 399, Algiers, Algeria b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silicon solar cells Photoluminescent downshifting Porous SiC micropowder PVA
Nowadays, the advanced photon management is regarded as an area of intensive research investment. Ever since the most widely used commercial photovoltaic cells are fabricated with single gap semiconductors like silicon, photon management has offered opportunities to make better use of the photons, both inside and outside the single junction window. In this study, the impact of new down shifting layer on the photoelectrical parameters of silicon based solar cell was studied. An effort to enhance the photovoltaic performance of textured silicon solar cells through the application of porous SiC particles-doped polyvinyl alcohol (PVA) layers using the spin-coating technique, is reported. Current-voltage curves under artificial illumination were used to confirm the contribution of LDS (SiC-PVA) thin layers. Experiment results revealed that LDS based on SiC particles which were etched in HF/K2S2O8 solution at T = 80 °C under UV light of 254 nm exhibited the best solar cell photoelectrical parameters due to its strong photoluminescence.
1. Introduction One of the most studied attempts to make better use of the photons is the introduction of luminescent down-shifting (LDS) materials on the top of the silicon solar cells [1–3]. This type of coating absorbs high energy photons and reemits lower energy photons which are more favorable to the solar cells. Although the number of photons after the down-shifting may decrease, it is still possible to increase the output current of the solar cells by LDS due to the better spectral response of those reemitted longer wavelength photons [4]. The downshifter consists of a luminescent layer composed of chromophors embedded in a transparent matrix that is optically coupled to the solar cell. Ideal luminescent down shifting material should exhibit; a wide absorption band in the range of enhancement, high absorption coefficient, narrow emission band in the peak conversion efficiency region of the photoactive material, good separation between absorption and emission bands, and low cost [5]. The most frequently used host materials for the luminescent species are inorganic crystalline materials such as SiO2 [6], Al2O3 [7] and CaF2 [8]. Among many materials, polymers have found application in the PV industry as matrix for luminescent species. Very different
∗
Corresponding author. E-mail address: [email protected] (S. Kaci).
https://doi.org/10.1016/j.optmat.2018.05.006 Received 31 March 2018; Received in revised form 29 April 2018; Accepted 1 May 2018
Available online 07 May 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
polymer properties are required for DC layers such as: a high barrier to water and oxygen, photostable, optical transparency in the UV and visible domain, easy to apply and environmentally friendly processing. Having said this, these properties have to be preserved in use conditions under sunlight heating exposure (85 °C). Studied polymeric materials such as poly methyl methacrylate PMMA and ethyl vinyl acetate (EVA) [9] exhibited high transparency in the UV-Visible region of the solar spectrum, an adequate resistance to heat and humidity variations, and a high mechanical resistance [10]. They also provide a very good host environment for inorganic and organic dye molecules [11]. Recently, poly (vinyl alcohol) (PVA) based soft gels with luminescent properties were investigated [12]. Poly vinyl alcohol (PVA) is a semi-crystalline polymer that is water soluble and completely biodegradable, and has attractive traits such as hydrophilicity, chemical resistance, emulsifying, adhesivity and excellent film forming capability [13]. As PMMA, PVA is extensively used in optical devices owing to its high transparency in the UV-visible spectral range, its oxygen barrier effect and its good dissolubility in many organic solvents and water [14] to form gels. Recently, highly transparent PVA gels, prepared from Dimethyl sulfoxide-water mixtures, were reported [15,16]. PVA gels, consisting of network structure of crystalline
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texturing of the substrate. Indeed, texturing is important to decrease the front reflectance of the cell and to improve the light trapping in order to increase the generated current. We followed a modified Marrero's Method [26], in which, Na2CO3/NaHCO3 was used as texturing solution to texture (100) Cz Si wafers, to fabricate solar cells. For purpose of comparison, we employed NaOH based solution besides Na2CO3 one to carry out the anisotropic etching of crystalline silicon wafers. The influence of the textured surface, especially, the surface resulting from the Na2CO3 texturation method on different solar cell processing steps was studied. The parameters of the reaction in this case were found optimum as following: 20 wt% Na2CO3 and deionized water at 95 °C, during 20 min to achieve minimum reflection.
and amorphous regions, are very interesting complex materials. The amorphous regions consist of long flexible chains that connect these junctions [17,18], while crystalline regions, the aggregation of ordered polymer sequences, act as junction points. In practice, the degree of polymerization of PVA (of formula [CH2CH(OH)]n with « n » being the number of monomer mers in a macromolecule) is related to the degree of hydrolysis (each monomer mer contain one OH groupment) and both affect its solubility. It is well known that when the degree of polymerization of PVA increases, its molecular weight increases. It has been shown that, at a given temperature, the solubility of PVA decreases with increasing molecular weight [19–21]. The main factor to control when preparing PVA/fillers based suspensions is their stability, ensuring that the suspensions have the ability to last longer as possible before deposition of the composites thin films. Mendizabal et al. have discussed in their study the stabilty of the PVA based suspensions as function of degree of hydrolysis and polymerization and deduced that: (a) PVAs with a high degree of hydrolysis (> 96%), regardless of their degree of plymerization, dot not form good suspensions. The suspensions coalesce in less than 2 min, (b) Partially hydrolyzed (88% OH) PVAs of small degree of polymerization (low molecular weight < 70 000) yield very stable suspensions, even after 24 h, the suspensions do not coalesce, (c) Partially hydrolyzed (88% OH) PVAs of high degree of polymerization (high molecular weight > 70000) yield less stable suspensions (12–44 min) than do PVAs of the same degree of hydrolysis but smaller molecular weights [22]. In consequence, the water solubility of PVA essentially depends upon degree of hydrolysis. PVA with 87–89% OH have high degree of solubility, even in cold water, but for the complete dissolution heating to 85 °C is required. The distribution of OH groups in either side to backbone depends also on the degree of polymerization of PVA. Three distinct PVA configurations can be generated when n ≥ 3: isotactic (i-PVA), syndiotactic (s-PVA) and atactic (a-PVA). All the three configurations co-exist, in general, with s-PVA as the prominent phase (as much as 62% content). It seems that, under favorable conditions, the sequential distribution of OH groups in either side to backbone in s-PVA can be explored to design a regular interchain bridging in a layer structure. The distribution of OH groups in opposites sides in alternate sites to backbone facilitates a regular H-bonding between the adjacent chains. A small s-PVA molecule in this particular conformer structure, offers many free OH groups in the backbone after the interchain bridging by H-bonding between adjacent chains. The OH groups in this specific structure are supposed to confer H bonding functionality to planarize the polymer backbone in a specific conformer which seems influence the PL property of PVA [23]. We will expose, through the present work and based on our previous investigations [24.25], the possible application of a new down shifting layer based on SiC-PVA composite thin films, trying by this, to enhance the light conversion efficiency of single crystalline silicon solar cells.
2.2. LDS layer elaboration The elaboration of the porous SiC micropowder and their incorporation in PVA matrix to perform the SiC/PVA composite thin films were detailed in our previous reports [24,25]. We note that the porous SiC micropowders, used in the present study, were chosen on the base of their photoluminescence properties, it means that, those which demonstrated the best PL intensity were selected to the LDS investigations. We will just specify the etching conditions followed in the present study to prepare the porous SiC micropowder and their corresponding thin films. Thus, PVA thin film were obtained by dissolving PVA powder in deionized water to form PVA gel which is spin coated on the substrate and annealed at 100 °C for 10–15 min [24]. PVA/SiC(I) were prepared, firstly, by etching SiC powder in HF/K2S2O8 under UV light with 254 nm during 40 min at room temperature, followed by its incorporation in PVA gel to form the composite thin film as PVA thin film. PVA/SiC(II) were prepared like as PVA/Si(II) but at T = 80 °C. All the prepared LDS layers were deposited by spin coating method. A typical spin process consists of a dispense step in which the composite fluid is deposited onto the substrate surface, a high-speed spin step to thin the fluid, and a drying step to eliminate excess solvents from the resulting film. Two common methods of dispense are Static dispense, and Dynamic dispense. Static dispense is simply depositing a small puddle of fluid on the center of the substrate. This can range from 1 to 10 ml depending on the viscosity of the fluid and the size of the substrate to be coated. Higher viscosity and or larger substrates typically require a larger puddle to ensure full coverage of the substrate during the high-speed spin step. Dynamic dispense is the process of dispensing while the substrate is turning at low speed. After the dispense step it is common to accelerate to a relatively high speed to thin the composite fluid to near its final desired thickness. Typical spin speeds for this step range from 1000 to 6000 RPM, again depending on the properties of the composite fluid as well as the substrate. This step can take from 10 s to several minutes. A separate drying step is usually added after the high speed spin step to further dry the film without substantially thinning it (Scheme 1). Final film thickness and other properties depend on the nature of the composite fluid (viscosity, percent fillers, surface tension, etc.) and the parameters chosen for the spin process. Factors such as final rotational speed, acceleration contribute to how the properties of coated films are defined. The combination of spin speed and time defines generally the final film thickness. In our work, we have chosen the static dispense step to deposit the composite fluid. Thin films of pure PVA, PVA mixed with 10% by weight of porous SiC microparticles (etched in different condistions) were prepared by spin coating. The deposition parameters were: Rotation speed ω = 1000 tr/min, Acceleration a = 500 tr. min/s, total rotation time t = 120 s. The prepared samples were dried in Oven at 363K and kept in air tight container.
2. Experimental part In our investigations, Czochralski grown B-doped mc-Si wafers (as cut) with low resistivity and (100) orientation were used to manufacture the solar cells. A 7 μm Silicon carbide micropowder was employed and subjected to etching process to produce luminescent porous SiC powder by mean of photo-assisted electroless etching as described in Refs. [24,25]. PVA, having an average molar weight of 17000 g/mol and 88% OH was used as received. The chemicals used for the pyramidal texturation were sodium carbonate anhydrous (Na2CO3) and sodium hydroxide (NaOH) with a purity ≥ 99% in pellets. The chemicals were solved in 18 MΩ cm deionised water. 2.1. Solar cell processing Within the wafer-based manufacturing technology, basic step is 226
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Scheme 1. Scheme of the spin coating deposition [27].
varied dramatically with the etching solution. Compared with the bare Si substrate, the sample textured in SOL B presented a reduction in reflectance across the entire range of wavelengths. The reflectance of the sample textured in SOL B was lower than that textured in SOL A. The use of the Na2CO3 solution for silicon texturation with the optimum concentration seems to be more advantageous than the one based on NaOH commonly used. The solar spectrum presents a high density of UV photons. Yet, these photons are strongly absorbed in the very doped silicon forming the barrier of potential and have, consequently, only little chance to exceed this zone N+. It is thus necessary to reduce the depth of junction; this one was optimized preferably in the range of 0,4–0,5 μm [28]. The phosphorous concentration profile of the fabricated emitter and its depth are given in Fig. 3. The later was measured using SIMS apparatus. It is usually controlled by varying the temperature and time of the diffusion process. In this work, the resulting emitter has a thickness of approximately 0.5 μm (as shown in Fig. 3) and a sheet resistance about40 Ω/sq. Fig. 4 illustrates the optical reflectance of the diffused textured silicon wafers after antireflection coating deposition. Herein, the impact of the SiNx:H deposition on the reflectivity of the wafers is clearly observed. Compared to the reflectivity of silicon wafers textured in NaOH based solution, that of the silicon wafers textured in Na2CO3 based solution manifests the lower value in a large range of wavelength allowing thus a maximum light recovering compared to the NaOH based one. We used the following relation to estimate the solar weighted spectral reflectance, Rw for each spectrum:
2.3. Technical characterizations A Millipore Alpha-Q water purification system (18.2 MΩ Millipore Corporation, USA) was used to obtain ultrapure water. A scanning electron microscope (SEM) (PHILIPS SEM505) was used to carry out the morphology analysis. Optical measurements were carried out using an UV–VIS-NIR spectrophotometer (Cary 500 Version 8, 01) in the wavelength range of 250–2500 nm. Secondary ion mass spectrometry “SIMS” (CAMECA 4FE7-CRTSE) was used to carry out the phosphorous diffusion profile. Photoluminescence spectra of samples excited under a wavelength of 325 nm were measured using a PERKIN-ELMER LS 50B luminescence spectrometer. The (current–voltage) characterizations were realized by using ITEC 6121 voltage source model coupled with a Keithley 6485 picammeter. 3. Results and discussion 3.1. Solar cell characterizations The silicon solar cells studied in this work were alkaline textured by means of two solutions: NaOH-based solution (SOL A) and Na2CO3based one (SOL B). Fig. 1 shows the SEM pictures of two silicon wafers textured in NaOH based solution (Fig. 1-a) and in carbonate based solution (Fig. 1-b). The texturation is well established in both cases. It was observed the formation of small pyramids in the first case besides de formation of larger ones in the second case. The shape of the pyramids formed with NaOH based solution seems to be softened, loosing, thus, their tetragonal shape with defined triangular faces. In the case of pyramids obtained by using Na2CO3 as texturing solution, they are more prominent and their shapes are well regular. We predict that the light will have most chance to be recovered by the surface of the silicon wafers textured in carbonate based solution compared to the one that being treated in conventional NaOH based solution. The optical reflectance of Silicon substrates with the following configurations are illustrated in Fig. 2: a bare Si substrate, a Si substrate textured in NaOH-based solution (SOL A), and a Si substrate textured in Na2CO3-based solution (SOL B). The reflectance of the Si substrate
1100
Rw =
1100
∫ R (λ) φ (λ) d (λ)/ ∫ φ (λ) d (λ) 400
400
Where, R is the reflectivity and φ the solar flux at each wavelength under AM1.5 standard conditions. The different reflectivities obtained on p-Si (100) wafers, calculated in the spectral range 400–1100 nm, are summarized in Table 1. The Rw values after antireflection coating addition confirm that the
Fig. 1. SEM images of a silicon wafers: (a) textured with NaOH based solution and (b) textured with the carbonate based solution. 227
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Fig. 2. Reflectance spectra of p-Cz wafers (a) without texturation; with texturation in (b) NaOH based solution (SOL A) (c) Na2CO3 based solution (SOL B).
surface state of the silicon wafer. This later can be easily enhanced by choosing the suitable of both texturing conditions of the silicon wafer and the antireflection coating. Indeed, front surface texturation of silicon surface reduces cell reflectivity and contributes to more photocurrent generation within active wavelengths range. Hence, we focused our research efforts, in our early investigations, on reduction of the optical losses of Si wafers via texturation. Thus, we have combined pyramidal structured silicon wafer with SiNx, deposited as an antireflection coating (ARC) and passivation layer, in order to minimize the optical losses of the silicon wafer (Fig. 5). 3.2. Influence of the SiC-PVA based LDS on the solar cells performances Silicon carbide (SiC) possesses interesting photoluminescence properties which were highlighted in numerous research studies [29,30]. SiC is a wide gap semiconductor with advanced characteristics, greatly superior to conventional semiconductors. Compared with a band gap of 1.1 eV of silicon, it has a much wider one of 2.3, 3.0, or 3.2 eV for the main polytypes, 3C, 6H, or 4H, respectively [29]. These properties make SiC a good candidate for light emitting sources [23,31]. Luminescent silicon carbide powder can be obtained by etching chemically their surfaces [24,25]. It seems that, under favorable conditions, the sequential distribution of –OH groups in either side to backbone in s-PVA can be explored to design a regular inter-chain bridging in a layer structure. A small s-PVA molecule offers many free –OH groups in the backbone after the inter-chain bridging by H bonding between adjacent chains, with n number of the monomers. The –OH groups in this specific structure are supposed to confer H bonding functionality to planarize the polymer backbone in a specific conformer. A promptly strong PL thus appears, in the near UV–Visible regions, in such examples of planar PVA polymer molecules. Prior the preparation of the luminescent downshifting layer (LDS) based on SiC/PVA, we advised useful to measure the photoluminescence intensity of the SiC micropowder before and after the etching process. Room temperature photoluminescence (RTPL) measurements were performed using Xenon lamp as excitation source. The obtained results are reported in Fig. 6. The etching process was conducted in HF/K2S2O8 solution under UV illumination of the SiC powder, consisting on an acidic attack (HF) assisted with an oxidant entity (S2O82−), in presence of metallic catalyst which was deposited on the top of SiC microparticles surfaces in prior. An enhancement of the PL intensity of the SiC powder after the etching process was clearly observed.
Fig. 3. SIMS profile of phosphorous diffused in textured silicon wafer. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
reflectance of the wafer textured with a carbonate based solution (SOL B) is lower compared with the one submitted to standard NaOH etching. As results, the textured silicon wafers in Na2CO3 based solution will be selected to perform the reference solar cell. Subsequently, metallic contacts were deposited on the textured silicon cells by means of the screen-printing technique on the front side and vacuum evaporation on the back side. In most cases, silicon solar cells absorbs within visible wavelengths range (300–700 nm). Of course, for enhancing the gain in short-circuit current by LDS approach, photons should be re-emitted at wavelengths where the solar cell converts them efficiently. As consequence, the spectral range of the emission should match the spectral range where the external quantum efficiency of the solar cell is high. Yet, the absorbance of silicon solar cell depends strongly on the 228
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Fig. 4. Reflectance spectra of the different textured silicon wafers before and after SiNx:H deposition. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
achieved in HF/K2S2O8 under UV light at room temperature (SiC(I)) and the second was prepared in HF/K2S2O8 under UV light at T = 80 °C (SiC(II)). We prepared three LDS layers: the first was based on a pure PVA without doping, the second was prepared as a composite based on PVA and porous SiC powder (type I) and the third one was prepared with PVA and porous SiC powder (type II). We characterized the three designed thin films prior their application on the silicon solar cells. UV/Vis/NIR spectroscopy was used to measure the transmittance characteristics of the LDS layers (deposited on glass substrates) investigated in this study. The results are illustrated in Fig. 7-a. All the layers exhibit a high transparency in the Vis-NIR range. The PL emission measurements were also performed to establish the comparison between the three prepared LDS layers. Herein, the PVA/SiC(II) composite thin film exhibit the best PL property as shown in Fig. 7-b. All the PVA-SiC based LDS exhibited a PL property within the 350–550 nm. The best emission intensity was assigned to that of PVA-
Table 1 Rw values of the different prepared silicon cells. Rw (%) Si textured in SOLA
Rw (%) Si textured in SOLB
With SiNx:H coating 31.21
With SiNx:H coating 15.77
Without SiNx:H coating 6.51
Without SiNx:H coating 3.25
Knowing that the photoluminescence spectra were excited with λ = 325 nm, it means that the SiC powder can absorb UV light at 325 nm and reemits it in the blue region. It is this noteworthy result which promoted the idea to build a luminescent down shifting layer by the incorporation of the porous SiC micropowder in a transparent matrix like PVA and tests its impact on the photovoltaic performances of a real solar cell. We used two solution processes to etch the SiC powder. The first was
Fig. 5. Comparison between the reflectivity of the Si plane and the surface modified Si. 229
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Fig. 8. Pictures of the as-made silicon solar cell before and after cutting. We measured short-circuit current (Isc), open-circuit voltage (Voc), and the fill factor (FF) for both the four solar cells parts.
Fig. 6. Photoluminescence spectra of the SiC powder before un and after etching process.
attributed to a π*→n electronic transition in free –OH groups. It was reported in previous reports that the PL property of PVA depended strongly on the spatial distribution of –OH groups within oriented PVA molecules [23]. In the case of our study, a prominent peak observed for the pure PVA thin film at 410 nm which can be attributed to transitions in syndiotactic (s) PVA. It seems that, the incorporation of porous SiC particles in PVA tends to maintain the polymer backbone in coplanar configuration through H-bonds interactions, hence, intensify the PL property of the resulting composites. It can clearly be seen that, structuring can significantly increase the amount of light absorbed by the solar cell. Therefore, the PL emission of the different LDS layers (light emission within 350–500 nm) can be easily absorbed by the fabricated solar cell. This phenomenon was attested in Fig. 5. We noticed that the reflectivity of silicon decreases at 400 nm (wavelength corresponding to the maximum of PL emission of our PVA-SiC based LDS). We have cut the solar cell in four parts. The quarters of solar cell with an identical area of 19.5 cm2, each one, were investigated for the electrical measurements. Fig. 8 shows the as-made silicon solar cell. Fig. 9 shows the photoelectrical Current-Voltage (I-V) curves of all quarters Silicon solar cell with the following configurations: a reference solar cell, a reference solar cell coated with a pure PVA layer, a reference solar cell coated with microporous SiC powder (type I and II) embedded in PVA matrix applied over the reference solar cell. The photovoltaic performances of all solar cell pieces are summarized in Table 2. The reference quarter solar cell was characterized under artificial light, using a lamp of 75 Watt, exhibited a short-circuit current (Isc) of about 51.2 mA, an open-circuit voltage (Voc) of about 0.527 V and a FF of 26.58%. the weakness of the FF is due to the nature of the back metallic contact deposited via simple evaporation. This will probably be avoided if screen printed method was employed. This also can be avoided if a buffer layer such as SiO2 was grown prior the metal evaporation deposition. This is not the aim of our study but it will be performed in our future study. The electrical parameters of the textured Silicon solar cells coated with PVA, PVA/SiC(I) and PVA/SiC(II) were compared to those of the reference cell. The obtained current–voltage (I–V) characteristics are shown in Fig. 9 and summarized in Table 2. A distinct increase for all the parameters were immediately achieved after coating PVA/SiC(I) and PVA/SiC(II) on the solar cell. The highest increase in Isc reaching 125 mA, corresponding to an FF of 37,50% compared to that of the reference cell, was achieved by incorporating PVA/SiC(II) based LDS on the cell surface. For the pure PVA based LDS, we measured an Isc of 51.7 mA, and an Isc of 125 mA was for that of PVA-SiC(II) based LDS. We noted that the pure PVA
Fig. 7. (a) Optical transmittance spectra and (b) PL spectra of the different elaborated LDS layers. PVA/SiC(I) thin film: SiC powder etched in HF/K2S2O8 under UV light at room Temperature. PVA/SiC(II) thin film: SiC powder etched in HF/K2S2O8 under UV light at T = 80 °C.
SiC(II) based LDS. The pure PVA based LDS, used basically as host matrix, showed also a PL property. The photoluminescence emission of PVA molecules in the visible region (within 400–500 nm) was 230
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Fig. 9. Current-Voltage (I–V) Characterizations for all quarters of the solar cell.
have a good spectral sensitivity), thus, before the interaction with the solar cells occurs. The short-circuit current, Isc, which is the current through the solar cell when the voltage across the solar cell is zero (when the solar cell is short circuited), is mostly due to the generation and collection of lightgenerated carriers. It was reported, in previous studies [33,34], that Isc is proportional to the light absorbance amount integrated in the wavelength between 300 and 1100 nm. Yet, changing the light intensity incident on a solar cell seems to change all solar cell parameters, including the short-circuit current and the open-circuit voltage. We have found that the cell performance is enhanced with increasing light intensity originating from PL. As result, the solar cell power changed consequently. Our investigations showed that the PL emission intensity increased from that corresponding to the pure PVA based LDS to that of PVA-SiC(II) based LDS. This means that the amount of light received by the studied solar cell increased following the PL variation, hence, the variation of all the electrical parameters, Isc, amongst others. As consequence, the improvement in the solar cell performance is observed to be dependent on the nature of the porous SiC powder incorporated in the LDS layer.
Table 2 Photoelectrical parameters of all the solar cell quarters.
quarter quarter quarter quarter
of of of of
Reference solar cell Solar cell with PVA Solar cell with PVA-SiC(I) Solar cell with PVA-SiC(II)
Isc (mA)
Voc (V)
FF (%)
51.2 51.7 75.2 125.0
0.527 0.563 0.572 0.564
26.58 26.28 31.42 37.50
based LDS not influence the parameters. It was observed that the incorporation of porous SiC powder, etched in special conditions, enhances significantly the PL emission, consequently, the amount of light emitted in increased. In reality, intermolecular forces could operate at the polymer/SiC interfaces owing to the existence of OH groups with different distribution in the backbone of the PVA. It was shown that the SiC low-dimensional structures are very sensitive to the intrinsic nonstoichiometry and that SiC crystals can be covered by a very thin layer of amorphous silicon oxide or possibly silicon oxycarbon [32], we suppose that their incorporation within the PVA matrix will be accompanied by a crosslinking network resulting from H bonding interactions between the oxycarbon atoms coating SiC particles and the –OH functional groups of PVA. In addition, there may occur many trapping levels on the border SiC/PVA, which also could influence the PL property of the resulting composite thin film. We note that, Isc is proportional to the light absorbance amount integrated in the wavelength between 400 and 1100 nm. Our investigations showed that the PL emission intensity increased from that corresponding to the pure PVA based LDS to that of PVA-SiC(II) based LDS. It means that the amount of light received by the solar cell was increased according the PL variation. Therefore, it leads to the enhancement of the Isc. We believe these findings result from an enhancement of light absorption and photo-response. Indeed, in this study, we have tried to apply a concept based on the exploitation of the solar spectrum known by photon converter or downshifting approach; in order to minimize, even if lightly, the lower solar cell response to high energy photons, which is one form of energy losses limiting the efficiency of solar cells. A thin layer of polymer, polyvinyl alcohol (PVA), doped with luminescent semiconductor, porous SiC particles, was employed as luminescent downshifting layer (LDS), used to convert some high energy photons to lower energy ones (within visible region, where solar cells
4. Conclusion In this study, the incorporation of spectral conversion layers (LDS) and their results on different types of solar cells were studied. The fabrication and characterization of textured Si solar cells were reported. SiC-doped PVA coating solar cells were used as LDS layer. The electrical and optical properties were shown to depend largely on the nature of the porous SiC particles which play the role of luminescent species in the LDS layer. The application porous SiC powder which was etched at 80 °C in HF/K2S2O8 solution under UV light led to significant improvements in light conversion proven by the increase of the shortcircuits current (Isc), the later being the factor that made the greatest contribution to efficiency. The photons downshifted by the PVA/SiC based LDS have wavelength which better match the photosensitivity spectral response of the silicon solar cell. This was confirmed through the application of PVA/SiC based LDS layer on a textured Si solar cell which led to an increase in the solar cell photoelectrical parameters. Finally, we conclude that, it will be interesting if these results will be investigated to fabricate slight and resistant encapsulating material, based on polymeric host (such as PMMA or PVA), strengthened by 231
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inorganic filler like porous SiC micropowder which exhibits at the same time a luminescent down shifting effect.
[16] M. Ohkura, T. Kanaya, K. Kaji, Gelation rates of poly(vinyl alcohol) solution, Polymer 33 (1992) 5044, http://dx.doi.org/10.1016/0032-3861(92) 90056–3. [17] C.M. Hassan, N.A. Peppas, Structure and Applications of Poly(Vinyl Alcohol) Hydrogels Produced by Conventional Crosslinking or by Freezing/Thawing Methods, Springer, Berlin, Heidelberg, 2000, p. 37, http://dx.doi.org/10.1007/3540-46414-X_2 Chapter. [18] C.M. Hassan, N.A. Peppas, Structure and morphology of freeze/thawed PVA hydrogels, Macromolecules 33 (2000) 2472, http://dx.doi.org/10.1021/ma9907587. [19] C.M. Hassan, N.A. Peppas, Structural and application of poly vinyl alcohol hydrogels produced by conventional crosslinking or by freezing/Thawing methods, Adv. Polym. Sci. 153 (2000) 37–65, http://dx.doi.org/10.1007/3-540-46414-X_2. [20] S. Mallakpour, A. Barati, Efficient preparation of hybrid nanocomposite coatings based on poly(vinyl alcohol) and silane coupling agent modified TiO2 nanoparticlesProg, Org. Coat. 71 (2011) 391–398, http://dx.doi.org/10.1016/j. porgcoat.2011.04.010. [21] C.A. Finch, Poly (Vinyl Alcohol): Properties and Applications, Wiley, NewYork, 1973, http://dx.doi.org/10.1002/pol.1974.130120212. [22] E. Mendizabal, J.R. Castellanos-Ortega, J.E. Puig, A method for selecting a polyvinyl alcohol as stabilizer in suspension polymerization, Colloid. Surface. 63 (1992) 209–217, http://dx.doi.org/10.1016/0166-6622(92)80242-T. [23] S. Ram, T.K. Mandal, Photoluminescence in small isotactic, atactic and syndiotactic PVA polymer molecules in water, Chem. Phys. 303 (2004) 121–128, http://dx.doi. org/10.1016/j.chemphys.2004.05.006. [24] S. Kaci, H. Mansouri, I. Bozetine, A. Keffous, L. Guerbous, Y. Siahmed, S. Aissiou, Elaboration and characterization of luminescent porous SiC microparticles/poly vinyl alcohol thin films, Opt. Mater. 64 (2017) 75–81, http://dx.doi.org/10.1016/j. optmat.2016.11.029. [25] S. Kaci, A. Keffous, A. Kheloufi, I. Bozetine, L. Guerbous, H. Mansouri, Y. Siahmed, S. Bouanik-Aissiou, PL investigations of spin coated thin films of porous SiC powder embedded in PVA, J. Alloy. Comp. 709 (2017) 668–676, http://dx.doi.org/10. 1016/j.jallcom.2017.03.128. [26] N. Marrero, B. González-Díaz, R. Guerrero-Lemus, D. Borchert, C. HernándezRodríguez, Optimization of sodium carbonate texturization on large area crystalline silicon solar cells, Sol. Energy Mater. Sol. Cells 91 (2007) 1943–1947, http://dx.doi. org/10.1016/j.solmat.2007.08.001. [27] L.A. Dobrzański, M. Szindler, Sol gel TiO2 antireflection coatings for silicon solar cells, J. Achiev. in Mat. Manufact. Eng. 52 (1) (2012) 7–14. [28] J. Lindmayer, J.F. Allison, The violet cell: an improved silicon solar cell, Sol. Cell. 29 (1990) 151, http://dx.doi.org/10.1016/0379-6787(90)90023-X. [29] A. Keffous, A. Chriet, Y. Belkacem, A. Mansri, N. Gabouze, M. Kechouane, A. Brighet, A. Boukezzata, S. Kaci, I. Menous, G. Nezzal, L. Guerbous, H. Menari, Investigation properties of a-Si1-xCx :H films elaborated by Co-sputtering of Si and 6H-SiC, Mod. Phys. Lett. B 24 (2010) 2101–2112, http://dx.doi.org/10.1142/ S0217984910024262. [30] A.O. Konstantinov, C.I. Harris, E. Janzén, Photoluminescence studies of porous silicon carbide, Appl. Phys. Lett. 65 (1994) 2699, http://dx.doi.org/10.1063/1. 113182. [31] J.Y. Fan, X.L. Wu, F. Kong, T. Qiu, G.S. Huang, Luminescent silicon carbide nanocrystallites in 3C-SiC/polystyrene films, Appl. Phys. Lett. 86 (2005) 171903, http:// dx.doi.org/10.1063/1.1914962. [32] I.V. Kityk, Specific features of band structure in large-sized Si2−xCx (1.04≤x < 1.10) nanocrystallites, Semicond. Sci. Technol. 18 (2003) 1001–1009 S0268-1242(03) 53124–53128. [33] A. El-Shaer, M.T.Y. Tadros, M.A. Khalifa, Effect of light intensity and temperature on crystalline silicon solar modules parameters, Intern. J. Emerg. Tech. Adv. Eng. 4 (2014) 311. [34] X. Cai, S. Zeng, X. Li, J. Zhang, S. Lin, A. Lin, B. Zhang, Effect of light intensity and temperature on the performance of GaN-based p-i-n solar cells, Elect. Control Eng., IEEE (2011) 1535, http://dx.doi.org/10.1109/ICECENG.2011.6058463.
Acknowledgments This work was completed thanks to the National Funds of Research, DGRSDT/MESRS (Algeria). References [1] E. Klampaftis, D. Ross, K.R. McIntosh, B.S. Richards, Enhancing the performance of solar cells via luminescent down-shifting of the incident spectrum: a review, Sol. Energy Mater. Sol. Cells 93 (2009) 1182–1194, http://dx.doi.org/10.1016/j.solmat. 2009.02.020. [2] E. Klampaftis, B.S. Richards, Improvement in multi-crystalline silicon solar cell efficiency via addition of luminescent material to EVA encapsulation layer, Prog. Photovoltaics Res. Appl. 19 (2011) 345–351, http://dx.doi.org/10.1002/pip.1019. [3] R. Rothemund, Optical modelling of the external quantum efficiency of solar cells with luminescent down-shifting layers, Sol. Energy Mater. Sol. Cells 120 (2014) 616–621, http://dx.doi.org/10.1016/j.solmat.2013.10.004. [4] G. Shao, C. Lou, J. Kang, H. Zhang, Luminescent down shifting effect of Ce-doped yttrium aluminum garnet thin films on solar cells, Appl. Phys. Lett. 107 (2015) 253904, http://dx.doi.org/10.1063/1.4938748. [5] B.C. Rowan, L.R. Wilson, B.S. Richards, Advanced material concepts for luminescent solar concentrators, IEEE J. Selected Topics in Quantum Electronics 14 (2008) 1312–1322, http://dx.doi.org/10.1109/JSTQE.2008.920282. [6] Z.J. Cheng, F.F. Su, L.K. Pan, M.L. Cao, Z. Sun, CdS quantum dot-embedded silica film as luminescent down-shifting layer for crystalline Si solar cells, J. Alloy. Comp. 494 (2010) L7–L10, http://dx.doi.org/10.1016/j.jallcom.2010.01.047. [7] H.J. Hovel, R.T. Hodgson, J.M. Woodall, The effect of fluorescent wavelength shifting on solar cell spectral response, Sol. Energy Mater. 2 (1979) 19–29, http:// dx.doi.org/10.1016/0165-1633(79)90027–3. [8] K. Kawano, N. Hashimoto, R. Nakata, Effects on solar cell efficiency of fluorescence of rare-earth ions, Mater. Sci. Forum 239 (1997) 311–314, http://dx.doi.org/10. 4028/www.scientific.net/MSF.239-241.311. [9] M. Abderrezek, M. Fathi, F. Djahli, M. Ayad, Study of the effect of luminescence down-shifting on GaAs solar cells with several optical windows layers, J. Sol. Energy Eng. 136 (2014) 011014, http://dx.doi.org/10.1115/1.4025593. [10] E. Klampaftis, M. Congiu, N. Robertson, B.S. Richards, Luminescent ethylene vinyl acetate encapsulation layers for enhancing the short wavelength spectral response and efficiency of silicon photovoltaic modules, IEEE J. Photovoltaics 1 (2011) 29–36, http://dx.doi.org/10.1109/JPHOTOV.2011.2162720. [11] B.S. Richards, A. Shalav, The role of polymers in the luminescence conversion of sunlight for enhanced solar cell performance, Synth. Met. 154 (2005) 61–64, http:// dx.doi.org/10.1016/j.synthmet.2005.07.021. [12] M.L. Di Lorenzo, M. Cocca, M. Avella, G. Gentile, D. Gutierrez, M.D. Pirriera, E. Torralba-Calleja, M. Kennedy, H. Ahmed, J. Doran, Down shifting in poly(vinyl alcohol) gels doped with terbium complex, J. Colloid Interface Sci. 477 (2016) 34–39, http://dx.doi.org/10.1016/j.jcis.2016.05.032. [13] J. Wang, X. Wang, C. Xu, M. Zhanga, X. Shang, Preparation of graphene/poly(vinyl alcohol) nanocomposites with enhanced mechanical properties and water resistance, Polym. Int. 60 (2011) 816, http://dx.doi.org/10.1002/pi.3025. [14] Yao Zhu, ZnO nanoparticles as a luminescent down-shifting layer for solar cells, http://theses.insa-lyon.fr/publication/2015ISAL0090/these.pdf. [15] M. Ohkura, T. Kanaya, K. Kaji, Gels of poly(vinyl alcohol) from dimethyl sulphoxide/water solutions, Polymer 33 (1992) 3686, http://dx.doi.org/10.1016/00323861(92)90656-H.
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Impact of porous SiC-doped PVA based LDS layer on electrical parameters of Si solar cells
T
S. Kacia,∗, R. Rahmounea, F. Kezzoulab, Y. Boudiafb, A. Keffousa, A. Manseria, H. Menaria, H. Cheragaa, L. Guerbousc, Y. Belkacema, R. Chalala, I. Bozetinea, A. Boukezzataa, L. Talbia, K. Benfadela, M.-A. Ouadfela, Y. Ouadaha a
Research Center on Semiconductor Technology for Energetic, CMSI Division, CRTSE, 2 Bd Frantz Fanon, PB 140, 7M, Algeria Research Center on Semiconductor Technology for Energetic, DDCS Division, CRTSE, 2 Bd Frantz Fanon, PB 140, 7M, Algeria c Algiers Nuclear Research Center (CRNA), 2 Bd Frantz Fanon, BP 399, Algiers, Algeria b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silicon solar cells Photoluminescent downshifting Porous SiC micropowder PVA
Nowadays, the advanced photon management is regarded as an area of intensive research investment. Ever since the most widely used commercial photovoltaic cells are fabricated with single gap semiconductors like silicon, photon management has offered opportunities to make better use of the photons, both inside and outside the single junction window. In this study, the impact of new down shifting layer on the photoelectrical parameters of silicon based solar cell was studied. An effort to enhance the photovoltaic performance of textured silicon solar cells through the application of porous SiC particles-doped polyvinyl alcohol (PVA) layers using the spin-coating technique, is reported. Current-voltage curves under artificial illumination were used to confirm the contribution of LDS (SiC-PVA) thin layers. Experiment results revealed that LDS based on SiC particles which were etched in HF/K2S2O8 solution at T = 80 °C under UV light of 254 nm exhibited the best solar cell photoelectrical parameters due to its strong photoluminescence.
1. Introduction One of the most studied attempts to make better use of the photons is the introduction of luminescent down-shifting (LDS) materials on the top of the silicon solar cells [1–3]. This type of coating absorbs high energy photons and reemits lower energy photons which are more favorable to the solar cells. Although the number of photons after the down-shifting may decrease, it is still possible to increase the output current of the solar cells by LDS due to the better spectral response of those reemitted longer wavelength photons [4]. The downshifter consists of a luminescent layer composed of chromophors embedded in a transparent matrix that is optically coupled to the solar cell. Ideal luminescent down shifting material should exhibit; a wide absorption band in the range of enhancement, high absorption coefficient, narrow emission band in the peak conversion efficiency region of the photoactive material, good separation between absorption and emission bands, and low cost [5]. The most frequently used host materials for the luminescent species are inorganic crystalline materials such as SiO2 [6], Al2O3 [7] and CaF2 [8]. Among many materials, polymers have found application in the PV industry as matrix for luminescent species. Very different
∗
Corresponding author. E-mail address: [email protected] (S. Kaci).
https://doi.org/10.1016/j.optmat.2018.05.006 Received 31 March 2018; Received in revised form 29 April 2018; Accepted 1 May 2018
Available online 07 May 2018 0925-3467/ © 2018 Elsevier B.V. All rights reserved.
polymer properties are required for DC layers such as: a high barrier to water and oxygen, photostable, optical transparency in the UV and visible domain, easy to apply and environmentally friendly processing. Having said this, these properties have to be preserved in use conditions under sunlight heating exposure (85 °C). Studied polymeric materials such as poly methyl methacrylate PMMA and ethyl vinyl acetate (EVA) [9] exhibited high transparency in the UV-Visible region of the solar spectrum, an adequate resistance to heat and humidity variations, and a high mechanical resistance [10]. They also provide a very good host environment for inorganic and organic dye molecules [11]. Recently, poly (vinyl alcohol) (PVA) based soft gels with luminescent properties were investigated [12]. Poly vinyl alcohol (PVA) is a semi-crystalline polymer that is water soluble and completely biodegradable, and has attractive traits such as hydrophilicity, chemical resistance, emulsifying, adhesivity and excellent film forming capability [13]. As PMMA, PVA is extensively used in optical devices owing to its high transparency in the UV-visible spectral range, its oxygen barrier effect and its good dissolubility in many organic solvents and water [14] to form gels. Recently, highly transparent PVA gels, prepared from Dimethyl sulfoxide-water mixtures, were reported [15,16]. PVA gels, consisting of network structure of crystalline
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texturing of the substrate. Indeed, texturing is important to decrease the front reflectance of the cell and to improve the light trapping in order to increase the generated current. We followed a modified Marrero's Method [26], in which, Na2CO3/NaHCO3 was used as texturing solution to texture (100) Cz Si wafers, to fabricate solar cells. For purpose of comparison, we employed NaOH based solution besides Na2CO3 one to carry out the anisotropic etching of crystalline silicon wafers. The influence of the textured surface, especially, the surface resulting from the Na2CO3 texturation method on different solar cell processing steps was studied. The parameters of the reaction in this case were found optimum as following: 20 wt% Na2CO3 and deionized water at 95 °C, during 20 min to achieve minimum reflection.
and amorphous regions, are very interesting complex materials. The amorphous regions consist of long flexible chains that connect these junctions [17,18], while crystalline regions, the aggregation of ordered polymer sequences, act as junction points. In practice, the degree of polymerization of PVA (of formula [CH2CH(OH)]n with « n » being the number of monomer mers in a macromolecule) is related to the degree of hydrolysis (each monomer mer contain one OH groupment) and both affect its solubility. It is well known that when the degree of polymerization of PVA increases, its molecular weight increases. It has been shown that, at a given temperature, the solubility of PVA decreases with increasing molecular weight [19–21]. The main factor to control when preparing PVA/fillers based suspensions is their stability, ensuring that the suspensions have the ability to last longer as possible before deposition of the composites thin films. Mendizabal et al. have discussed in their study the stabilty of the PVA based suspensions as function of degree of hydrolysis and polymerization and deduced that: (a) PVAs with a high degree of hydrolysis (> 96%), regardless of their degree of plymerization, dot not form good suspensions. The suspensions coalesce in less than 2 min, (b) Partially hydrolyzed (88% OH) PVAs of small degree of polymerization (low molecular weight < 70 000) yield very stable suspensions, even after 24 h, the suspensions do not coalesce, (c) Partially hydrolyzed (88% OH) PVAs of high degree of polymerization (high molecular weight > 70000) yield less stable suspensions (12–44 min) than do PVAs of the same degree of hydrolysis but smaller molecular weights [22]. In consequence, the water solubility of PVA essentially depends upon degree of hydrolysis. PVA with 87–89% OH have high degree of solubility, even in cold water, but for the complete dissolution heating to 85 °C is required. The distribution of OH groups in either side to backbone depends also on the degree of polymerization of PVA. Three distinct PVA configurations can be generated when n ≥ 3: isotactic (i-PVA), syndiotactic (s-PVA) and atactic (a-PVA). All the three configurations co-exist, in general, with s-PVA as the prominent phase (as much as 62% content). It seems that, under favorable conditions, the sequential distribution of OH groups in either side to backbone in s-PVA can be explored to design a regular interchain bridging in a layer structure. The distribution of OH groups in opposites sides in alternate sites to backbone facilitates a regular H-bonding between the adjacent chains. A small s-PVA molecule in this particular conformer structure, offers many free OH groups in the backbone after the interchain bridging by H-bonding between adjacent chains. The OH groups in this specific structure are supposed to confer H bonding functionality to planarize the polymer backbone in a specific conformer which seems influence the PL property of PVA [23]. We will expose, through the present work and based on our previous investigations [24.25], the possible application of a new down shifting layer based on SiC-PVA composite thin films, trying by this, to enhance the light conversion efficiency of single crystalline silicon solar cells.
2.2. LDS layer elaboration The elaboration of the porous SiC micropowder and their incorporation in PVA matrix to perform the SiC/PVA composite thin films were detailed in our previous reports [24,25]. We note that the porous SiC micropowders, used in the present study, were chosen on the base of their photoluminescence properties, it means that, those which demonstrated the best PL intensity were selected to the LDS investigations. We will just specify the etching conditions followed in the present study to prepare the porous SiC micropowder and their corresponding thin films. Thus, PVA thin film were obtained by dissolving PVA powder in deionized water to form PVA gel which is spin coated on the substrate and annealed at 100 °C for 10–15 min [24]. PVA/SiC(I) were prepared, firstly, by etching SiC powder in HF/K2S2O8 under UV light with 254 nm during 40 min at room temperature, followed by its incorporation in PVA gel to form the composite thin film as PVA thin film. PVA/SiC(II) were prepared like as PVA/Si(II) but at T = 80 °C. All the prepared LDS layers were deposited by spin coating method. A typical spin process consists of a dispense step in which the composite fluid is deposited onto the substrate surface, a high-speed spin step to thin the fluid, and a drying step to eliminate excess solvents from the resulting film. Two common methods of dispense are Static dispense, and Dynamic dispense. Static dispense is simply depositing a small puddle of fluid on the center of the substrate. This can range from 1 to 10 ml depending on the viscosity of the fluid and the size of the substrate to be coated. Higher viscosity and or larger substrates typically require a larger puddle to ensure full coverage of the substrate during the high-speed spin step. Dynamic dispense is the process of dispensing while the substrate is turning at low speed. After the dispense step it is common to accelerate to a relatively high speed to thin the composite fluid to near its final desired thickness. Typical spin speeds for this step range from 1000 to 6000 RPM, again depending on the properties of the composite fluid as well as the substrate. This step can take from 10 s to several minutes. A separate drying step is usually added after the high speed spin step to further dry the film without substantially thinning it (Scheme 1). Final film thickness and other properties depend on the nature of the composite fluid (viscosity, percent fillers, surface tension, etc.) and the parameters chosen for the spin process. Factors such as final rotational speed, acceleration contribute to how the properties of coated films are defined. The combination of spin speed and time defines generally the final film thickness. In our work, we have chosen the static dispense step to deposit the composite fluid. Thin films of pure PVA, PVA mixed with 10% by weight of porous SiC microparticles (etched in different condistions) were prepared by spin coating. The deposition parameters were: Rotation speed ω = 1000 tr/min, Acceleration a = 500 tr. min/s, total rotation time t = 120 s. The prepared samples were dried in Oven at 363K and kept in air tight container.
2. Experimental part In our investigations, Czochralski grown B-doped mc-Si wafers (as cut) with low resistivity and (100) orientation were used to manufacture the solar cells. A 7 μm Silicon carbide micropowder was employed and subjected to etching process to produce luminescent porous SiC powder by mean of photo-assisted electroless etching as described in Refs. [24,25]. PVA, having an average molar weight of 17000 g/mol and 88% OH was used as received. The chemicals used for the pyramidal texturation were sodium carbonate anhydrous (Na2CO3) and sodium hydroxide (NaOH) with a purity ≥ 99% in pellets. The chemicals were solved in 18 MΩ cm deionised water. 2.1. Solar cell processing Within the wafer-based manufacturing technology, basic step is 226
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Scheme 1. Scheme of the spin coating deposition [27].
varied dramatically with the etching solution. Compared with the bare Si substrate, the sample textured in SOL B presented a reduction in reflectance across the entire range of wavelengths. The reflectance of the sample textured in SOL B was lower than that textured in SOL A. The use of the Na2CO3 solution for silicon texturation with the optimum concentration seems to be more advantageous than the one based on NaOH commonly used. The solar spectrum presents a high density of UV photons. Yet, these photons are strongly absorbed in the very doped silicon forming the barrier of potential and have, consequently, only little chance to exceed this zone N+. It is thus necessary to reduce the depth of junction; this one was optimized preferably in the range of 0,4–0,5 μm [28]. The phosphorous concentration profile of the fabricated emitter and its depth are given in Fig. 3. The later was measured using SIMS apparatus. It is usually controlled by varying the temperature and time of the diffusion process. In this work, the resulting emitter has a thickness of approximately 0.5 μm (as shown in Fig. 3) and a sheet resistance about40 Ω/sq. Fig. 4 illustrates the optical reflectance of the diffused textured silicon wafers after antireflection coating deposition. Herein, the impact of the SiNx:H deposition on the reflectivity of the wafers is clearly observed. Compared to the reflectivity of silicon wafers textured in NaOH based solution, that of the silicon wafers textured in Na2CO3 based solution manifests the lower value in a large range of wavelength allowing thus a maximum light recovering compared to the NaOH based one. We used the following relation to estimate the solar weighted spectral reflectance, Rw for each spectrum:
2.3. Technical characterizations A Millipore Alpha-Q water purification system (18.2 MΩ Millipore Corporation, USA) was used to obtain ultrapure water. A scanning electron microscope (SEM) (PHILIPS SEM505) was used to carry out the morphology analysis. Optical measurements were carried out using an UV–VIS-NIR spectrophotometer (Cary 500 Version 8, 01) in the wavelength range of 250–2500 nm. Secondary ion mass spectrometry “SIMS” (CAMECA 4FE7-CRTSE) was used to carry out the phosphorous diffusion profile. Photoluminescence spectra of samples excited under a wavelength of 325 nm were measured using a PERKIN-ELMER LS 50B luminescence spectrometer. The (current–voltage) characterizations were realized by using ITEC 6121 voltage source model coupled with a Keithley 6485 picammeter. 3. Results and discussion 3.1. Solar cell characterizations The silicon solar cells studied in this work were alkaline textured by means of two solutions: NaOH-based solution (SOL A) and Na2CO3based one (SOL B). Fig. 1 shows the SEM pictures of two silicon wafers textured in NaOH based solution (Fig. 1-a) and in carbonate based solution (Fig. 1-b). The texturation is well established in both cases. It was observed the formation of small pyramids in the first case besides de formation of larger ones in the second case. The shape of the pyramids formed with NaOH based solution seems to be softened, loosing, thus, their tetragonal shape with defined triangular faces. In the case of pyramids obtained by using Na2CO3 as texturing solution, they are more prominent and their shapes are well regular. We predict that the light will have most chance to be recovered by the surface of the silicon wafers textured in carbonate based solution compared to the one that being treated in conventional NaOH based solution. The optical reflectance of Silicon substrates with the following configurations are illustrated in Fig. 2: a bare Si substrate, a Si substrate textured in NaOH-based solution (SOL A), and a Si substrate textured in Na2CO3-based solution (SOL B). The reflectance of the Si substrate
1100
Rw =
1100
∫ R (λ) φ (λ) d (λ)/ ∫ φ (λ) d (λ) 400
400
Where, R is the reflectivity and φ the solar flux at each wavelength under AM1.5 standard conditions. The different reflectivities obtained on p-Si (100) wafers, calculated in the spectral range 400–1100 nm, are summarized in Table 1. The Rw values after antireflection coating addition confirm that the
Fig. 1. SEM images of a silicon wafers: (a) textured with NaOH based solution and (b) textured with the carbonate based solution. 227
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Fig. 2. Reflectance spectra of p-Cz wafers (a) without texturation; with texturation in (b) NaOH based solution (SOL A) (c) Na2CO3 based solution (SOL B).
surface state of the silicon wafer. This later can be easily enhanced by choosing the suitable of both texturing conditions of the silicon wafer and the antireflection coating. Indeed, front surface texturation of silicon surface reduces cell reflectivity and contributes to more photocurrent generation within active wavelengths range. Hence, we focused our research efforts, in our early investigations, on reduction of the optical losses of Si wafers via texturation. Thus, we have combined pyramidal structured silicon wafer with SiNx, deposited as an antireflection coating (ARC) and passivation layer, in order to minimize the optical losses of the silicon wafer (Fig. 5). 3.2. Influence of the SiC-PVA based LDS on the solar cells performances Silicon carbide (SiC) possesses interesting photoluminescence properties which were highlighted in numerous research studies [29,30]. SiC is a wide gap semiconductor with advanced characteristics, greatly superior to conventional semiconductors. Compared with a band gap of 1.1 eV of silicon, it has a much wider one of 2.3, 3.0, or 3.2 eV for the main polytypes, 3C, 6H, or 4H, respectively [29]. These properties make SiC a good candidate for light emitting sources [23,31]. Luminescent silicon carbide powder can be obtained by etching chemically their surfaces [24,25]. It seems that, under favorable conditions, the sequential distribution of –OH groups in either side to backbone in s-PVA can be explored to design a regular inter-chain bridging in a layer structure. A small s-PVA molecule offers many free –OH groups in the backbone after the inter-chain bridging by H bonding between adjacent chains, with n number of the monomers. The –OH groups in this specific structure are supposed to confer H bonding functionality to planarize the polymer backbone in a specific conformer. A promptly strong PL thus appears, in the near UV–Visible regions, in such examples of planar PVA polymer molecules. Prior the preparation of the luminescent downshifting layer (LDS) based on SiC/PVA, we advised useful to measure the photoluminescence intensity of the SiC micropowder before and after the etching process. Room temperature photoluminescence (RTPL) measurements were performed using Xenon lamp as excitation source. The obtained results are reported in Fig. 6. The etching process was conducted in HF/K2S2O8 solution under UV illumination of the SiC powder, consisting on an acidic attack (HF) assisted with an oxidant entity (S2O82−), in presence of metallic catalyst which was deposited on the top of SiC microparticles surfaces in prior. An enhancement of the PL intensity of the SiC powder after the etching process was clearly observed.
Fig. 3. SIMS profile of phosphorous diffused in textured silicon wafer. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
reflectance of the wafer textured with a carbonate based solution (SOL B) is lower compared with the one submitted to standard NaOH etching. As results, the textured silicon wafers in Na2CO3 based solution will be selected to perform the reference solar cell. Subsequently, metallic contacts were deposited on the textured silicon cells by means of the screen-printing technique on the front side and vacuum evaporation on the back side. In most cases, silicon solar cells absorbs within visible wavelengths range (300–700 nm). Of course, for enhancing the gain in short-circuit current by LDS approach, photons should be re-emitted at wavelengths where the solar cell converts them efficiently. As consequence, the spectral range of the emission should match the spectral range where the external quantum efficiency of the solar cell is high. Yet, the absorbance of silicon solar cell depends strongly on the 228
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Fig. 4. Reflectance spectra of the different textured silicon wafers before and after SiNx:H deposition. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
achieved in HF/K2S2O8 under UV light at room temperature (SiC(I)) and the second was prepared in HF/K2S2O8 under UV light at T = 80 °C (SiC(II)). We prepared three LDS layers: the first was based on a pure PVA without doping, the second was prepared as a composite based on PVA and porous SiC powder (type I) and the third one was prepared with PVA and porous SiC powder (type II). We characterized the three designed thin films prior their application on the silicon solar cells. UV/Vis/NIR spectroscopy was used to measure the transmittance characteristics of the LDS layers (deposited on glass substrates) investigated in this study. The results are illustrated in Fig. 7-a. All the layers exhibit a high transparency in the Vis-NIR range. The PL emission measurements were also performed to establish the comparison between the three prepared LDS layers. Herein, the PVA/SiC(II) composite thin film exhibit the best PL property as shown in Fig. 7-b. All the PVA-SiC based LDS exhibited a PL property within the 350–550 nm. The best emission intensity was assigned to that of PVA-
Table 1 Rw values of the different prepared silicon cells. Rw (%) Si textured in SOLA
Rw (%) Si textured in SOLB
With SiNx:H coating 31.21
With SiNx:H coating 15.77
Without SiNx:H coating 6.51
Without SiNx:H coating 3.25
Knowing that the photoluminescence spectra were excited with λ = 325 nm, it means that the SiC powder can absorb UV light at 325 nm and reemits it in the blue region. It is this noteworthy result which promoted the idea to build a luminescent down shifting layer by the incorporation of the porous SiC micropowder in a transparent matrix like PVA and tests its impact on the photovoltaic performances of a real solar cell. We used two solution processes to etch the SiC powder. The first was
Fig. 5. Comparison between the reflectivity of the Si plane and the surface modified Si. 229
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Fig. 8. Pictures of the as-made silicon solar cell before and after cutting. We measured short-circuit current (Isc), open-circuit voltage (Voc), and the fill factor (FF) for both the four solar cells parts.
Fig. 6. Photoluminescence spectra of the SiC powder before un and after etching process.
attributed to a π*→n electronic transition in free –OH groups. It was reported in previous reports that the PL property of PVA depended strongly on the spatial distribution of –OH groups within oriented PVA molecules [23]. In the case of our study, a prominent peak observed for the pure PVA thin film at 410 nm which can be attributed to transitions in syndiotactic (s) PVA. It seems that, the incorporation of porous SiC particles in PVA tends to maintain the polymer backbone in coplanar configuration through H-bonds interactions, hence, intensify the PL property of the resulting composites. It can clearly be seen that, structuring can significantly increase the amount of light absorbed by the solar cell. Therefore, the PL emission of the different LDS layers (light emission within 350–500 nm) can be easily absorbed by the fabricated solar cell. This phenomenon was attested in Fig. 5. We noticed that the reflectivity of silicon decreases at 400 nm (wavelength corresponding to the maximum of PL emission of our PVA-SiC based LDS). We have cut the solar cell in four parts. The quarters of solar cell with an identical area of 19.5 cm2, each one, were investigated for the electrical measurements. Fig. 8 shows the as-made silicon solar cell. Fig. 9 shows the photoelectrical Current-Voltage (I-V) curves of all quarters Silicon solar cell with the following configurations: a reference solar cell, a reference solar cell coated with a pure PVA layer, a reference solar cell coated with microporous SiC powder (type I and II) embedded in PVA matrix applied over the reference solar cell. The photovoltaic performances of all solar cell pieces are summarized in Table 2. The reference quarter solar cell was characterized under artificial light, using a lamp of 75 Watt, exhibited a short-circuit current (Isc) of about 51.2 mA, an open-circuit voltage (Voc) of about 0.527 V and a FF of 26.58%. the weakness of the FF is due to the nature of the back metallic contact deposited via simple evaporation. This will probably be avoided if screen printed method was employed. This also can be avoided if a buffer layer such as SiO2 was grown prior the metal evaporation deposition. This is not the aim of our study but it will be performed in our future study. The electrical parameters of the textured Silicon solar cells coated with PVA, PVA/SiC(I) and PVA/SiC(II) were compared to those of the reference cell. The obtained current–voltage (I–V) characteristics are shown in Fig. 9 and summarized in Table 2. A distinct increase for all the parameters were immediately achieved after coating PVA/SiC(I) and PVA/SiC(II) on the solar cell. The highest increase in Isc reaching 125 mA, corresponding to an FF of 37,50% compared to that of the reference cell, was achieved by incorporating PVA/SiC(II) based LDS on the cell surface. For the pure PVA based LDS, we measured an Isc of 51.7 mA, and an Isc of 125 mA was for that of PVA-SiC(II) based LDS. We noted that the pure PVA
Fig. 7. (a) Optical transmittance spectra and (b) PL spectra of the different elaborated LDS layers. PVA/SiC(I) thin film: SiC powder etched in HF/K2S2O8 under UV light at room Temperature. PVA/SiC(II) thin film: SiC powder etched in HF/K2S2O8 under UV light at T = 80 °C.
SiC(II) based LDS. The pure PVA based LDS, used basically as host matrix, showed also a PL property. The photoluminescence emission of PVA molecules in the visible region (within 400–500 nm) was 230
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Fig. 9. Current-Voltage (I–V) Characterizations for all quarters of the solar cell.
have a good spectral sensitivity), thus, before the interaction with the solar cells occurs. The short-circuit current, Isc, which is the current through the solar cell when the voltage across the solar cell is zero (when the solar cell is short circuited), is mostly due to the generation and collection of lightgenerated carriers. It was reported, in previous studies [33,34], that Isc is proportional to the light absorbance amount integrated in the wavelength between 300 and 1100 nm. Yet, changing the light intensity incident on a solar cell seems to change all solar cell parameters, including the short-circuit current and the open-circuit voltage. We have found that the cell performance is enhanced with increasing light intensity originating from PL. As result, the solar cell power changed consequently. Our investigations showed that the PL emission intensity increased from that corresponding to the pure PVA based LDS to that of PVA-SiC(II) based LDS. This means that the amount of light received by the studied solar cell increased following the PL variation, hence, the variation of all the electrical parameters, Isc, amongst others. As consequence, the improvement in the solar cell performance is observed to be dependent on the nature of the porous SiC powder incorporated in the LDS layer.
Table 2 Photoelectrical parameters of all the solar cell quarters.
quarter quarter quarter quarter
of of of of
Reference solar cell Solar cell with PVA Solar cell with PVA-SiC(I) Solar cell with PVA-SiC(II)
Isc (mA)
Voc (V)
FF (%)
51.2 51.7 75.2 125.0
0.527 0.563 0.572 0.564
26.58 26.28 31.42 37.50
based LDS not influence the parameters. It was observed that the incorporation of porous SiC powder, etched in special conditions, enhances significantly the PL emission, consequently, the amount of light emitted in increased. In reality, intermolecular forces could operate at the polymer/SiC interfaces owing to the existence of OH groups with different distribution in the backbone of the PVA. It was shown that the SiC low-dimensional structures are very sensitive to the intrinsic nonstoichiometry and that SiC crystals can be covered by a very thin layer of amorphous silicon oxide or possibly silicon oxycarbon [32], we suppose that their incorporation within the PVA matrix will be accompanied by a crosslinking network resulting from H bonding interactions between the oxycarbon atoms coating SiC particles and the –OH functional groups of PVA. In addition, there may occur many trapping levels on the border SiC/PVA, which also could influence the PL property of the resulting composite thin film. We note that, Isc is proportional to the light absorbance amount integrated in the wavelength between 400 and 1100 nm. Our investigations showed that the PL emission intensity increased from that corresponding to the pure PVA based LDS to that of PVA-SiC(II) based LDS. It means that the amount of light received by the solar cell was increased according the PL variation. Therefore, it leads to the enhancement of the Isc. We believe these findings result from an enhancement of light absorption and photo-response. Indeed, in this study, we have tried to apply a concept based on the exploitation of the solar spectrum known by photon converter or downshifting approach; in order to minimize, even if lightly, the lower solar cell response to high energy photons, which is one form of energy losses limiting the efficiency of solar cells. A thin layer of polymer, polyvinyl alcohol (PVA), doped with luminescent semiconductor, porous SiC particles, was employed as luminescent downshifting layer (LDS), used to convert some high energy photons to lower energy ones (within visible region, where solar cells
4. Conclusion In this study, the incorporation of spectral conversion layers (LDS) and their results on different types of solar cells were studied. The fabrication and characterization of textured Si solar cells were reported. SiC-doped PVA coating solar cells were used as LDS layer. The electrical and optical properties were shown to depend largely on the nature of the porous SiC particles which play the role of luminescent species in the LDS layer. The application porous SiC powder which was etched at 80 °C in HF/K2S2O8 solution under UV light led to significant improvements in light conversion proven by the increase of the shortcircuits current (Isc), the later being the factor that made the greatest contribution to efficiency. The photons downshifted by the PVA/SiC based LDS have wavelength which better match the photosensitivity spectral response of the silicon solar cell. This was confirmed through the application of PVA/SiC based LDS layer on a textured Si solar cell which led to an increase in the solar cell photoelectrical parameters. Finally, we conclude that, it will be interesting if these results will be investigated to fabricate slight and resistant encapsulating material, based on polymeric host (such as PMMA or PVA), strengthened by 231
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inorganic filler like porous SiC micropowder which exhibits at the same time a luminescent down shifting effect.
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