Photoelectric efficiency enhancement of a polycrystalline silicon solar cell coated with an EVA film containing Eu3+ complex by addition of modified SiO2

Jin Dong and Baoping Lin*
School of Chemistry and Chemical Engineering, Southeast University, Nanjing 211189, PR China. E-mail: lbp@seu.edu.cn

Received 21st September 2016 , Accepted 27th October 2016

First published on 28th October 2016


Abstract

As an effective luminescent down-shifting (LDS) species, Eu3+ complexes have been doped in ethylene-vinyl acetate (EVA) films to improve the spectral response of silicon photovoltaic modules in the ultraviolet region. One key point about the improvement of photoelectric conversion efficiency by the LDS technique is increasing the fluorescence intensity of the EVA composite film as far as possible while staying within the visible light transmittance of the film. Around this point, we attempt to offer an idea for further studies. In this study, nano-SiO2 modified by a silane coupling agent was doped into EVA film with a fixed content of Eu3+ complex by the casting method. The experimental results show that when the modified SiO2 content is within 4%, the fluorescence of the EVA composite film gradually increases with increase in the modified SiO2 content and before formation of EVA composite film, the fluorescence intensities have no difference between the mixture with modified SiO2 and that without modified SiO2. SEM and AFM images of the EVA composite films show that the complex particles are probably enwrapped by aggregations of the modified SiO2 during the film-forming process, which strengthens the rigidity of the Eu3+ complex in the structure and results in the enhancement of fluorescence intensity. When the modified SiO2 content is within 2%, the visible light transmittance of the EVA film only slightly decreases. The photoelectric conversion efficiency of polycrystalline silicon solar cell coated with the composite film increases from 12.13% to 12.29%. Hence, the study provides a feasible approach to improve the LDS technique and has a potential application value.


1. Introduction

The spectral response of crystalline silicon solar cells to ultraviolet light is rapidly attenuated, and the ultraviolet light can also increase the temperature of the solar cells to reduce photoelectric efficiency and accelerate the aging of the cell modules.1,2 One way to improve the poor spectral response in the ultraviolet region is to use a luminescent down-shifting (LDS) layer, which is a composite of encapsulating film doped with a LDS species.1,3–8 Eu3+ complexes, which can absorb ultraviolet photons and emit visible photons, are excellent LDS species because of their large Stokes shift, narrow emission spectrum and high luminescent quantum efficiency (LQE).8–10 In many studies, Eu3+ complexes have been doped into polymer films, and then silicon solar cells have been encapsulated with the polymer films to improve the photoelectric efficiency.4,8,11,12

However, optical losses accompany the LDS layer due to the front surface reflection increase, concentration quenching and isotropic luminescence.1 To improve the photoelectric efficiency, it is important that the increase obtained by the LDS should be larger than the loss.8,12 In recent years, some studies have been carried out on this aspect. However, organic rare earth complexes with high LQE should be selected as LDS species to effectively improve the conversion of ultraviolet light. Liu et al. prepared a series of Eu3+ complexes as LDS species doped into polyvinyl acetate film separately and improved the mc-Si PV module efficiency from 16.05% to 16.37% by selecting the most effective Eu3+ complex.8 Wang et al. prepared Eu3+ complexes with high LQE up to 0.73, and photoelectric efficiency of mc-Si solar cells encapsulated by an Eu3+ complexes/EVA film was enhanced by 0.42%.12 On the other hand, optical losses caused by doping LDS species should be reduced through some ways, such as modifying the rare earth complex, changing the film-forming method and adding promoters. Chen et al. modified the first ligand and got a macromolecular complex.13 For introduction of the macromolecular ligand, the compatibility of the complex and the EVA film was strengthened and the visible light transmittance of the LDS layer was increased. For LDS films prepared by the soaking method, Wang et al. found that LDS species were well-distributed in the EVA, and the visible light transmittance of the EVA films did not decrease significantly.12

In some studies, surface modification of rare earth materials by appropriate amounts of SiO2 can improve the fluorescence intensity of the rare earth ions. Strek et al. offered a comparison of fluorescence intensity between Eu3+ complexes and silica glasses doped with the complexes, and the results showed that the latter increased slightly than the former.14 In addition, Jung15 and Han16 coated rare earth phosphors with SiO2 and this increased the fluorescence intensity. In particular, Yu17 and Tao18 coated nano-silica on the surface of Eu3+ complexes and found the emission intensity of SiO2/Eu3+ complexes were significantly stronger than that of the Eu3+ complexes alone. The cause of fluorescence enhancement is generally considered to be the structure of Eu3+ complex becoming more rigid after the surface modification by SiO2, which is beneficial to reducing the Eu3+ ion non-radiative transition probabilities.15–19 Furthermore, nano-SiO2 has been extensively applied in polymer matrixes, and the composites have better properties in terms of mechanics, optics, thermotics and electronics.20–22

In this work, nano-SiO2 was modified by a silane coupling agent, and the modified SiO2 and Eu3+ quaternary complexes were doped into EVA films. Dispersity and compatibility of modified SiO2 in the EVA matrix were discussed. UV-vis spectra and PL spectra were used to characterize the transmittance and fluorescence intensity of the EVA composite films. Finally, the EVA composite film was coated onto the surface of polycrystalline silicon solar cells and the photoelectric efficiency of the solar cell was tested.

2. Experimental section

2.1. Materials and equipment

Eu2O3 (>99.99%), 1,10-phenanthroline monohydrate (phen), 1-tridecanecarboxylic acid (TA) and methacryloxy propyl trimethoxy silane (KH570) were purchased from Sinopharm Chemical Reagent Co., Ltd. Dibenzoylmethane (DBM), EVA (33% VA) and nano-SiO2 were purchased from Aladdin Chemical Co., Ltd. All reagents were used as obtained.

FT-IR (Nicolet 5700) was used to characterize the modified SiO2 over the range of 200 to 4500 cm−1. The morphology and size of the modified SiO2 were observed by TEM (Tecnai G2 20). Transmittance of the EVA composite film was measured by an ultraviolet spectrophotometer (UV-2450). The surface morphology of the EVA composite films was observed by SEM (Inspect F50) and AFM (Dimension Edge). Fluorescence spectra of the EVA composite film were recorded on a fluorescence spectrometer (Fluoromax-4). IV performances of samples were tested under AM 1.5G (25 °C) by a solar simulator (Sol3A 94043A).

2.2. Synthesis of the europium quaternary complex

Eu2O3 (0.704 g) was dissolved in excess HCl solution with stirring. The solution was heated and concentrated to remove water and excess HCl, and then the remaining solid (EuCl3) was dissolved in ethanol. DBM (1.792 g), TA (0.913 g) and phen (0.793 g) were separately dissolved in ethanol. The solution of DBM was added into the solution of EuCl3, and the pH of the mixture was adjusted to 6–7 with ammonia–water. The coordination reaction was carried out at 50 °C with stirring. One hour later, the solution of TA was added into the reaction mixture dropwise and the pH of the mixture was adjusted to 6–7 with ammonia–water. One hour later, the solution of phen was also added dropwise and the pH was adjusted to 6–7 with ammonia–water. Two hours later, the mixture was allowed to stand at room temperature for 6 hours and the Eu3+ complex precipitated. This precipitate was collected on a filter, washed with deionized water and ethanol 3 times, and dried for 24 h. A pale yellow powder of Eu(DBM)2phen(TA) (shortened as EuDPT in this paper) was obtained.

2.3. Preparation of KH570-modified nano-SiO2

A mixture of silane coupling agent KH570 (2 g) and deionized water (0.75 g) was adjusted to pH 5–6 with acetic acid for the hydrolysis of KH570. After nano-SiO2 (2 g) was ultrasonically dispersed in toluene (80 mL) for 0.5 h, hydrolyzed KH570 solution (0.5 g) was added into this suspension. The suspension was ultrasonically dispersed for 0.5 h, and heated at 50 °C with stirring for 24 h. After centrifugation, washing with ethanol and vacuum drying for 24 h, the resultant product (KH570–SiO2) was obtained.

2.4. Preparation of the EVA films

EVA films were prepared by the film casting method. The KH570–SiO2 with different weight ratio (to EVA, the same below) was ultrasonically dispersed in 40 mL of toluene for 0.5 h. EVA (1 g) and EuDPT (0.1 g. Experimental results had showed that the appropriate amount of EuDPT was 1 wt%, which will be discussed in another paper) were dissolved in the mixture at 65 °C with stirring for 1 h. Then, the mixture was put in a fixed-size glass container. After drying at 75 °C for 3 h, EVA film with about 0.5 mm thickness was obtained.

2.5. Test of the IV characteristic of a polycrystalline silicon solar cell coated with the EVA composite film

The EVA composite film was covered on the surface of the polycrystalline silicon solar cell and lightly pressed. Because of adhesive ability of EVA, the film was coated on the surface of the solar cell. After the solar cell stood at 25 °C for 2 h, the IV characteristics of the sample was tested under AM 1.5G (optical density of 100 mW cm−2) by the solar simulator. After the test, the film was peeled off gently and the surface of the cell was lightly cleaned by a cotton swab moistened with alcohol.

3. Results and discussion

3.1. Characterization of KH570–SiO2

The infrared spectra of unmodified and modified SiO2 are shown in Fig. 1. In the spectrum of unmodified SiO2, the Si–O bending vibration band at 472 cm−1, Si–O–Si symmetrical and antisymmetric stretching vibration bands at 805 cm−1 and 1104 cm−1, O–H bending vibration band at 1636 cm−1 and –OH stretching vibration band at 3431 cm−1 are displayed. In the spectrum of modified SiO2, besides the characteristic absorption bands above, the band at 1703 cm−1, which is from the C[double bond, length as m-dash]O stretching vibration, and band at 2958 cm−1, which is from –CH3 stretching vibration, are observed. These additional bands indicate SiO2 has been chemically grafted with KH570.
image file: c6ra23497f-f1.tif
Fig. 1 FT-IR spectra of unmodified and modified SiO2.

Fig. 2a shows the TEM image of unmodified SiO2 particles in toluene. These particles with a clear surface edge and size of 20–30 nm are nearly spherical. In Fig. 2b, the shape of modified SiO2 particles with a size of 25–40 nm are irregular and their surface edges become indistinct. These changes demonstrate that KH570 has been successfully grafted onto the surface of nano-SiO2. As nano-SiO2 has a large specific surface area and is easy to unite, the modified SiO2 particles are aggregated into chain structures spanning hundreds of nanometers in toluene.


image file: c6ra23497f-f2.tif
Fig. 2 TEM images of (a) unmodified SiO2 and (b) SiO2 modified by KH570.

3.2. Transmittance of KH570–SiO2/EuDPT/EVA films

Fig. 3 gives the transmittance of EVA composite films doped with KH570–SiO2 in different contents. As seen in the figure, the absorption of the EVA composite film in the UV region is still very strong after the addition of KH570–SiO2; in the visible light region, transmittance of the EVA composite films decreases after the addition of KH570–SiO2, and the higher the content of KH570–SiO2, the lower the transmittance. When KH570–SiO2 was doped with contents of 2% or less, the visible light transmittance of the EVA films changes only slightly. The lipophilicity of SiO2 increases after surface modification by KH570, which improves the compatibility of SiO2 particles in EVA. So when the addition of KH570–SiO2 is relatively small, the KH570–SiO2 is uniformly dispersed in EVA and has little influence on the transmittance of the EVA films. However, when the additional amount of KH570–SiO2 is more than 2%, visible light transmittance of the EVA films decreases obviously. This is probably because some of the KH570–SiO2 particles have agglomerated in the EVA film and the dispersion becomes worse as the amount of KH570–SiO2 increases.
image file: c6ra23497f-f3.tif
Fig. 3 Transmittance of KH570–SiO2/EuDPT/EVA films with KH570–SiO2 in different mass contents. The content of EuDPT is 1%.

3.3. Fluorescence spectra of KH570–SiO2/EuDPT/EVA films

Fluorescence spectra of EVA composite films doped with KH570–SiO2 are shown in Fig. 4 and 5. As seen in the figures, with and without KH570–SiO2 added in EVA film, the shape of the excitation and emission spectra and the position of peaks remain unchanged (same amount of EuDPT added). This indicates that KH570–SiO2 is physically dispersed in EVA and does not lead to the variation of the chemical structure of the Eu3+ complex.
image file: c6ra23497f-f4.tif
Fig. 4 Excitation spectra of EuDPT/EVA and KH570–SiO2/EuDPT/EVA films. The content of EuDPT is 1%.

image file: c6ra23497f-f5.tif
Fig. 5 The normalized emission spectra of KH570–SiO2/EuDPT/EVA films with KH570–SiO2 in different mass contents. The content of EuDPT is 1%.

The excitation spectrum of the KH570–SiO2/EuDPT/EVA film displays broadband excitation and the maximum excitation wavelength is 366 nm, as shown in Fig. 4. The emission spectrum of KH570–SiO2/EuDPT/EVA film displays the characteristic emission of the Eu3+ ion. As shown in Fig. 5, the emission peaks at 579 nm, 591 nm, 611 nm, 651 nm and 691 nm, in this order, are assigned to the 5D07F0, 5D07F1, 5D07F2, 5D07F3 and 5D07F4 transitions of the Eu3+ ion, and the maximum emission wavelength is 611 nm.

In addition, the fluorescence intensity of the EVA composite film strengthens with the addition of KH570–SiO2, as can be seen from Fig. 5. Because the shape of the emission spectrum remains the same, the intensity of the maximum emission peak at 611 nm can be used to compare the emission intensity of the EVA composite film at different KH570–SiO2 content. And the relationship between relative intensity of the emission peak at 611 nm and KH570–SiO2 content is shown in Fig. 6. When the content of KH570–SiO2 is within 4%, the emission peak intensity increases and the ratio of intensity gain to KH570–SiO2 content decreases with increasing KH570–SiO2 content. When the content of KH570–SiO2 is more than 4%, the emission peak intensity is substantially unchanged and increases by about 10% compared with the EuDPT/EVA film without KH570–SiO2.


image file: c6ra23497f-f6.tif
Fig. 6 The 611 nm emission intensities of KH570–SiO2/EuDPT/EVA films with different contents of KH570–SiO2.

In order to study whether the fluorescence intensity was enhanced before formation of the EVA film when EuDPT and KH570–SiO2 were mixed in toluene, we prepared a toluene solution of EuDPT (2.5 g L−1) and a mixed toluene solution of EuDPT (2.5 g L−1) and KH570–SiO2 (10 g L−1) to compare the fluorescence intensities of both solutions. Fig. 7 gives the emission spectra of both, and the results show that fluorescence intensities of both solutions are almost the same, which indicates that the fluorescence enhancement occurs after formation of the film.


image file: c6ra23497f-f7.tif
Fig. 7 The normalized emission spectra of EuDPT in toluene solution with and without KH570–SiO2. The concentrations of EuDPT and KH570–SiO2 are 2.5 g L−1 and 10 g L−1, respectively.

3.4. SEM and AFM analyses of KH570–SiO2/EuDPT/EVA films

The surface microstructure of different EVA films was characterized by SEM. Fig. 8a shows the SEM image of the EuDPT/EVA film without KH570–SiO2. It is known that the surface of pure EVA film is generally uniform and flat. And as seen in Fig. 8a, the EuDPT/EVA film substrate interface shows scale-like morphology, which is the outcome of the influence of dispersion and aggregation degree of EuDPT in EVA. Fig. 8b shows the SEM image of KH570–SiO2/EVA film without EuDPT (the content of KH570–SiO2 is 2%). As can be seen, most of the KH570–SiO2 particles with uniform particle size are homogeneously dispersed in the EVA film and a few of the KH570–SiO2 particles unite into submicron grains with different sizes. Fig. 8c shows the SEM image of the KH570–SiO2/EuDPT/EVA film with 2% KH570–SiO2. As can be seen, the surface micro morphology is neither like the scaly shapes nor like the uniform dispersion of nano-size particles in the matrix, but a structure of many small protuberances on the film substrate interface. These protuberances that show irregular curved shapes are uniformly dispersed in the EVA matrix. As is well known, because of their large specific surface area and high surface energy, nano-SiO2 particles can absorb and enwrap neighboring particles. So the formation of the protuberances may be caused by aggregations of KH570–SiO2 enwrapping EuDPT particles. During the film-forming process, the content of KH570–SiO2 increases with solvent evaporation, which decreases the distance between EuDPT and KH570–SiO2 and improves the chances of contact. Then with the aggregation of KH570–SiO2, EuDPT particles are probably absorbed and enwrapped by the aggregations. Finally, with formation of the film, the protuberances appear. Although this kind of package is not a close core–shell structure, the rigidity of EuDPT in the structure is strengthened to a certain extent, which leads to the enhancement of fluorescence intensity. And because KH570–SiO2 has good compatibility with EVA, the protuberances are uniformly dispersed. At the same time, a small number of submicron grains, which are the agglomerations of KH570–SiO2 particles, can be seen in Fig. 8c. And the more the grains, the lower the visible light transmittance of the film. Fig. 8d shows the SEM image of KH570–SiO2/EuDPT/EVA film with 4% KH570–SiO2. As can be seen, the surface morphology is similar to that of the KH570–SiO2/EuDPT/EVA film with 2% KH570–SiO2. But in Fig. 8d, one difference is that the protuberances are a little more uniform in size. This is probably because EuDPT particles are more evenly enwrapped with increasing KH570–SiO2 content. Of course, when the content of KH570–SiO2 rises to be a certain degree, the quantity of KH570–SiO2 that enwraps EuDPT particles reaches saturation, which results in no further enhancement of fluorescence. In Fig. 8d, another difference is the submicron grains formed by KH570–SiO2 particles increase obviously with the increasing of KH570–SiO2 content, which results in a decrease of visible light transmittance of the film. Therefore, there exists an optimum amount of KH570–SiO2.
image file: c6ra23497f-f8.tif
Fig. 8 SEM images of (a) 1% EuDPT/EVA film, (b) 2% KH570–SiO2/EVA film, (c) 2% KH570–SiO2/EuDPT/EVA film and (d) 4% KH570–SiO2/EuDPT/EVA film.

To observe the changes of surface microstructure more clearly, three dimensional surface morphologies of different EVA films were characterized by AFM. Fig. 9 shows the AFM height images of the EuDPT/EVA and KH570–SiO2/EuDPT/EVA films. As seen in Fig. 9a, the scale-like morphology can be observed clearly. The distribution of these scale-like protrusions on the surface is dense and uniform in the 3D image, which indicates that the Eu3+ complex is uniformly dispersed in the EVA film. In Fig. 9b, the protrusions on the surface look like many hills. The distribution of these protrusions with different sizes and different shapes is relatively loose, which also indicates that the KH570–SiO2 particles have aggregated and some aggregations have enwrapped EuDPT particles to lead to the inhomogeneity of the film surface. In addition, the surface roughness (Rq) of the EuDPT/EVA film is 32.5 nm, and the Rq of the KH570–SiO2/EuDPT/EVA film is 34.7 nm. The latter is a little larger than the former.


image file: c6ra23497f-f9.tif
Fig. 9 AFM height images of (a) 1% EuDPT/EVA film and (b) 2% KH570–SiO2/EuDPT/EVA film. 2D image is on the left and 3D image is on the right.

3.5. Photoelectric efficiency of the polycrystalline silicon solar cell coated with KH570–SiO2/EuDPT/EVA film

Pure EVA film, EuDPT/EVA film and KH570–SiO2/EuDPT/EVA films with different KH570–SiO2 content were coated on the surface of the same polycrystalline silicon solar cell, individually. The IV characteristics of the samples based on the single device were measured in the solar simulator, and each sample was measured at least six times. The IV curves and the photovoltaic parameters of the corresponding samples are shown in Fig. 10 and Table 1.
image file: c6ra23497f-f10.tif
Fig. 10 IV curves of the mc-Si solar cell coated with EuDPT/EVA film and KH570–SiO2/EuDPT/EVA film.
Table 1 IV performances of mc-Si solar cell coated with different EVA composite film
Parameter Pure EVA EuDPT/EVA 1% KH570–SiO2/EuDPT/EVA 2% KH570–SiO2/EuDPT/EVA 3% KH570–SiO2/EuDPT/EVA
Voc (V) 0.5936 ± 0.0018 0.5932 ± 0.0016 0.5927 ± 0.0006 0.5932 ± 0.0013 0.5930 ± 0.0010
Isc (mA) 62.6 ± 0.3 62.8 ± 0.3 63.3 ± 0.3 63.4 ± 0.3 62.4 ± 0.3
Jsc (mA cm−2) 27.83 ± 0.10 27.93 ± 0.11 28.12 ± 0.13 28.18 ± 0.12 27.74 ± 0.10
Imax (mA) 58.1 ± 0.3 58.0 ± 0.3 58.6 ± 0.3 58.7 ± 0.3 57.5 ± 0.3
Vmax (V) 0.4706 ± 0.0027 0.4706 ± 0.0021 0.4705 ± 0.0012 0.4708 ± 0.0019 0.4705 ± 0.0024
Pmax (mW) 27.3 ± 0.3 27.3 ± 0.3 27.6 ± 0.3 27.7 ± 0.3 27.0 ± 0.3
Fill factor (%) 73.5 ± 0.3 73.2 ± 0.2 73.5 ± 0.2 73.5 ± 0.3 73.0 ± 0.4
Efficiency (%) 12.15 ± 0.13 12.13 ± 0.11 12.25 ± 0.09 12.29 ± 0.11 12.01 ± 0.12


As is known, the value of ΔJsc in theory can be expressed by the following equation:8

image file: c6ra23497f-t1.tif
where q is the photon's charge (1.6 × 10−19 C), EQE is the external quantum efficiency and Φ(λ) is the incident photon flux of a given spectra (AM 1.5G). So the interaction between the effect of LDS and the effect of the optical losses on the photoelectric efficiency can be evaluated by the value of ΔJsc; and if the former is greater than the latter, the value of ΔJsc is positive, otherwise the value is negative.8,12 As seen in Table 1 and Fig. 10, when the KH570–SiO2 content is 1% or 2%, the Jsc increases by 0.19 mA cm−2 or 0.25 mA cm−2, respectively, compared with the sample without KH570–SiO2, which means that the enhancement of fluorescence has a greater effect on the photoelectric efficiency than the slight decline in visible light transmittance of the film. Accordingly, the efficiency also increases. When the KH570–SiO2 content is 3% or 4%, the Jsc decreases compared the sample without KH570–SiO2, which means that the obvious decline in visible light transmittance of the film has a greater effect on the efficiency than the enhancement of fluorescence. Accordingly, the efficiency also decreases.

In Table 1, when the KH570–SiO2 content is 2%, the photoelectric efficiency is the highest. Thus, the optimum addition amount of KH570–SiO2 is 2%, and the photoelectric efficiency increases from 12.13% to 12.29%. Although the film is only coated on the surface of the solar cell, the results have referenced values. It can be believed that when the solar cell is encapsulated with the film, or another rare earth complex with a higher LQE is selected as the LDS species in the EVA film, the photoelectric conversion efficiency will be also increased with the suitable doping of KH570–SiO2.

4. Conclusion

In this work, nano-SiO2 that was modified by silane coupling agent KH570 has been doped into EVA films with an Eu3+ quaternary complex (1 wt% content). With KH570–SiO2 content within 2 wt%, the EVA composite film still has good visible light transmittance because KH570–SiO2 has good compatibility with EVA. And it was found that the fluorescence of the EVA composite film gradually increases when the KH570–SiO2 content is within 4 wt%. Based on the microstructure characterization of the different EVA composite films by SEM and AFM, it is conjectured that EuDPT particles may be enwrapped by the aggregations of KH570–SiO2 after film formation, which could lead to the enhancement of fluorescence of the EVA composite film. The test shows that the optimum addition amount of KH570–SiO2 is 2% and the photoelectric conversion efficiency of the polycrystalline silicon solar cell coated with the composite film increases by 0.16%. Therefore, modified SiO2 and a LDS species could be encapsulated into the EVA layer of mc-Si PV modules, which is worthy of further study to improve the photoelectric conversion efficiency.

Acknowledgements

This work is supported by the Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

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