- Email: [email protected]
The optical properties of solar cells before and after encapsulation
Available online at www.sciencedirect.com
ScienceDirect Solar Energy 122 (2015) 718–726 www.elsevier.com/locate/solener
The optical properties of solar cells before and after encapsulation Wusong Tao a, Ying Du b,⇑ b
a Changzhou Trina Solar Energy Co., Ltd., Changzhou, China College of Science, Zhejiang University of Technology, Hangzhou 310023, China
Received 28 January 2015; received in revised form 26 June 2015; accepted 6 October 2015
Communicated by: Associate Editor Antoine Bittar
Abstract In this work we investigate how the optical properties of monocrystalline silicon and polycrystalline silicon wafers are affected by texturing techniques and encapsulation. For monocrystalline wafers, the KOH etching is better than acid etching while reactive ion etching (RIE) is proven to be preferred compared to acid etching for polycrystalline wafers. The differences in reflectance (R) between two textures are apparent before encapsulation, but when the textured wafers are encapsulated with glass especially antireflectance coated (ARC) glass, the difference can be reduced from about five percentage points to below a percentage point. More important, the optical losses caused by reflectance (R) losses and parasitic absorption losses (A) for four types of commercial monocrystalline and polycrystalline silicon module are quantified and compared. Analyses are carried out for eight configurations using a stratified model consisting of solar cell wafers as well as cover glass and ethylene vinyl acetate (EVA). The model is to first obtain the external reflectance (R) and transmittance (T) in each layer, and the spectrally parasitic absorption loss associated with the cover glass and EVA is calculated with the aid of R and T measurements, which allows us to do a complete optical loss analysis for the solar module. The results show that the KOH texture with ARC glass encapsulation may be better choice for monocrystalline wafers, which gives a 94.04% of effective light collected by silicon. The RIE technique with ARC glass is suitable for polycrystalline substrates with excellent light trapping of 94.14%. After all, reflectance from glass surface and silicon surface accounts for over 70% of total loss, while the absorption of glass and EVA accounts for the rest and less loss. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Texturing; Encapsulation; Monocrystalline silicon; Polycrystalline silicon
1. Introduction Solar cells are textured with suitable surface texturing and then encapsulated, which is used to minimize the total reflectance losses from the glass/silicon interface and increase the short-circuit current. During the last decade, many works were reported on the different texturing techniques such as, mechanical grooving (Willeke et al., 1992a,b), wet chemical etch (Stocks et al., 1996), laser ⇑ Corresponding author.
E-mail address: [email protected] (Y. Du). http://dx.doi.org/10.1016/j.solener.2015.10.007 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
sculpturing, plasma etching and reactive ion etching (RIE) Winderbaum et al., 1997, which have been used to texture monocrystalline and polycrystalline silicon. Laser texturing is slow and is incompatible with thin substrates, and plasma etching is likely to be relative expensive. However, wet chemical isotropic etching is a low cost method (Stocks et al., 1994), while masked and maskless RIE texturing for polycrystalline wafers produces lower reflectance than the standard wet etching used today in industry (Macdonald et al., 2004). It has been proven that the efficiency of solar cells is affected not only by texturing but also by encapsulation (Wohlgemuth et al., 2005).
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
Therefore, the transition from a solar cell to an encapsulated solar cell, i.e. a solar module, has a huge impact on the optical performance of the device (Gjessing and Marstein, 2013), which is easily ignored when comparing efficiency of silicon solar cells with different textures. Simultaneously, analyzing the optical property of photovoltaic (PV) modules and quantifying their optical losses are important for PV module designers for optimizing the design of solar cells and PV modules. In this work optical analysis was carried out on industrial wafers with different texturing techniques before and after the encapsulation, and it was found that a suitable surface texturing was needed to prevent optical loss between glass and encapsulated material. KOH texturing, which would be the least costly to implement, produces a modest improvement in reflectance for monocrystalline wafers before encapsulation, whereas RIE texturing, generates a larger gain in light absorption for polycrystalline silicon solar cells in spite of its high cost. At the same time, it was found the difference in reflectance between the textures was reduced after encapsulation. This is attributed to both light-trapping and oblique coupling of incident light into the cell. ARC glass retained their antireflective properties by encapsulation and showed huge improvement on reflectance compared to bare glass, especially for the monocrystalline solar cells with KOH texture and the polycrystalline solar cells with RIE texture. There is considerable potential for use of ARC glass as the encapsulated glass. Moreover, we make a stratified model to calculate the optical loss of glass and ethylene vinyl acetate (EVA), and the effective light collected by silicon can be quantified for eight different configurations. It will be helpful to reduce the light loss by optimizing the process and encapsulation materials, which will enhance the photoelectric conversion efficiency of solar modules. 2. Experimental 2.1. Surface texturing An effective texturing process reduces reflectance from the top surface of the solar cell and aids in the retention of long wavelength, weakly absorbed light within the silicon (light trapping) Stocks et al., 1994. It also has the side benefit of increasing the optical path of light in the bulk of the material, because of the change in the angle of incidence after multiple reflectances occurred on the surface (Winderbaum et al., 1997). For monocrystalline solar cells, standard random pyramid (the KOH texture) and isotropic textures (the acidic texture) are made with wet etching in solutions based on KOH and HN03/H3P04/ HF, respectively. The random pyramids are formed by etching in a 1% (wt) KOH, 4% (wt) isopropanol solution at 78 °C for 40 min after a damage removal in 30% (wt) KOH at 75 °C for 3 min. The isotropic etched samples are etched in an acidic solution (10:5:2, HN03: H3P04: HF) at 20–25 °C for 70 s. In addition, because of the
719
random grain orientation, traditional anisotropic etchants does not work on polycrystalline silicon. Two methods for texturing polycrystalline solar cells that yields good performance are selected: one is to texture samples using acidic solution (5:1, HN03: HF), while the other is the maskless RIE method. The RIE equipment used in these experiments was a JuSung plasma-enhanced chemical vapor deposition (PECVD). The wafer was etched by RIE with SiF6 and Cl2 for appropriate time. In the experiment, the base pressure before each run was less than 5 106 Tort. Then, the samples were cleaned prior to the optical measurement and subsequent process by acetone and ethanol under sonication to remove contaminants introduced by the fabrication process. SiNx antireflective coatings are deposited on the solar cells by PECVD after the texturing process. The SiNx thickness is 75 ± 5 nm with a refractive index of 2.02 ± 0.03 at 600 nm, which was determined by ellipsometry SEMILAB SE400. 2.2. Encapsulation technique Encapsulation method is based on the heat laminating, a process which basically consists of supplying thermal energy on outside of package to soften/melt the sealants (Notte et al., 2014). To facilitate the measurement, the window glass used for the encapsulation is cut into pieces of about 20 20 cm2, and the edges of the glass are polished. A monocrystalline silicon or polycrystalline silicon wafer is encapsulated with window glass from Almaden and EVA from FIRST at 145 degrees for 16 min using a Boostsolar BSL2236OAC laminator. The physical thickness of the EVA was found from the cross-sectional microscope images to be about 0.50 ± 0.05 mm thick, while the glass is 3.2 ± 0.2 mm thick. The electrical contacts are extracted from the encapsulated device by means of metal ribbons (Notte et al., 2014). The reflectance in the external surface of the glass may also be reduced by using the antireflective coatings (SiO2) with the thickness of 120 ± 15 nm on the glass, then the optical properties of bare glass and the ARC glass are compared. The ARC glass is confirmed to be lower reflectance losses. 2.3. Measurement Reflectance and transmittance measurements of samples were made using a spectrophotometer (Perkin Elmer, Lambda 950) with an attached barium sulfate coated integrating sphere, which collects reflected and transmitted light through a sample from all directions. The samples were carefully placed below the measurement spot (16 mm 3.5 mm). Light reflected from the rear side will also contribute to the measured reflectance. The measurement accuracy of the spectrophotometer was ±0.4%. Scans were made from 300 nm to 1100 nm with a PbS detector in the wavelength range above 800 nm and a photomultiplier below 800 nm. The uncertainty of wavelength is ±1 nm.
720
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
Incident light was 3 degrees from normal incidence to ensure specularly reflected light did not escape the integrating sphere. The encapsulating layer consisted of several millimeters of EVA topped by 3.2 ± 0.2 mm thick Vycor, a low absorptivity glass. Optical constants of glass and EVA (the refractive index and thickness) were found using ellipsometry, while the absorption loss of glass and EVA were calculated from reflectance and transmittance measurements of a glass and glass-EVA conjunction. 3. Results and discussion 3.1. Reflectance measurement We measured reflectance as a function of wavelength (300–1100 nm) for monocrystalline and polycrystalline solar cells textured using different techniques with and without encapsulation. Fig. 1 shows the reflectance of the encapsulated monocrystalline solar cells with bare glass and ARC glass. KOH and acid etch are used to produce the KOH texture (Fig. 1(a)) and the acidic texture (Fig. 1 (b)), respectively. For comparison, the reflectance of monocrystalline solar cell before encapsulation was also shown. Comparing Fig. 1(a) with Fig. 1(b), reflectance measurements from samples show reflectance losses on the KOH texture are comparable with those from the acidic texture, but the KOH textured sample seems to produce a reflectance somewhat lower than the acid etch. Additionally, the reflectance from encapsulated samples with ARC glass after the etch is lower than those from unencapsulated samples over the whole solar spectrum, especially in the region below 450 nm, and the reflectance from bare glass follows a similar trend resembling that from ARC glass. Therefore, when encapsulated under glass, the samples with textured surfaces will have relatively improved reflectance due to light-trapping (Macdonald et al., 2004). However, the encapsulated sample with bare glass exhibits poor antireflective performance than that with ARC glass. It is worth noticing that the apparent
reduction in reflectance in the UV region after encapsulation, which is caused by absorption within the encapsulating material. At wavelength greater than 1000 nm, the reflectance increases for all samples, but they increase the least for the encapsulated sample with ARC glass. In fact, such a module achieves a higher generation current than an unencapsulated solar cell. In order to represent the antireflective property of those samples more accurately, averaged front side reflectance values for all two types of textures both with and without encapsulation are shown in Fig. 2. We also show reflectance values encapsulated with ARC glass. Before encapsulation, samples textured with acidic solution show little improvement in reflectance loss compared to the KOH textured samples, and the difference is very apparent (5%). The data presented in Fig. 2 shows that the monocrystalline solar cell with KOH texture has a much lower reflectance (9.61%) than that with acidic texture (15.00%) before encapsulation. For the encapsulated samples with bare glass, the reflectance of the two textures decreases to 7.04% and 8.20%, respectively. After encapsulated with ARC glass, there will be a reflectance of about 4.14% for the KOH texture and a reflectance of about 5.38% for the acidic texture at the module surface, and the difference between two textures becomes too small (1%), which represents the excellent light trapping properties of encapsulated textures silicon. Therefore, the reflectance of samples encapsulated with ARC glass is lower than that with bare glass. It is shown that for monocrystalline solar cells textured using KOH etch, the encapsulated samples with ARC glass is at advantage due to relatively low reflectance. The uncertainty on the average reflectance includes both the uncertainty on the representativeness of the sample used for analysis and the systematic measurement error. Our model illuminates the whole module area, whereas our spectrometer only illuminates a small spot. And we measure the reflectance on an area of the cell where there are fingers, but no busbars. The associated uncertainty
Fig. 1. Front side reflectance (300–1100 nm) of the encapsulated monocrystalline modules with bare glass and ARC glass for (a) the KOH texture and (b) the acidic texture. For comparison, the reflectance of monocrystalline wafer before encapsulation is also shown.
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
Fig. 2. Average reflectance (300–1100 nm) comparison of monocrystalline modules with KOH texture and acidic texture for the following samples: unencapsulated, encapsulated with bare glass, encapsulated with ARC glass. The relative error of average reflectance is estimated to be about ±1.5%.
for representativeness of the sample was ±0.5%, ±0.4% for measurement accuracy, ±0.5% for systematic error (assuming 100% of reflectance). Therefore, we estimated the average reflectance with a relative error of about ±1.5%. Both reflectance and transmittance measurements suffer this error which will affect the calculation of absorption. The total reflectance from encapsulated polycrystalline solar cells with bare glass and ARC glass as a function of wavelength are shown in Fig. 3. Acid solution and maskless RIE etch are used to produce surface texturing: the acidic texture (Fig. 3(a)) and the RIE texture (Fig. 3(b)). For comparison, the reflectance of polycrystalline solar cell before encapsulation is also shown. Comparing Fig. 3(a) with Fig. 3(b), the acidic textured sample is certainly not as good as the maskless RIE textured sample, since the RIE textured sample in particularly exhibits good light-trapping. Similarly, we observe that the encapsulated sample with bare glass exhibits poor antireflective performance than that with ARC glass. The reflectance for all samples before and after encapsulation from Fig. 3 follows
721
a similar trend resembling those from Fig. 1 even though their textures differ greatly with each other. It can be seen that the reflectance of solar cells are reduced effectively by encapsulation with ARC glass. The average reflectance results of polycrystalline solar cells are summarized in Fig. 4. The data presented in Fig. 2 shows that the polycrystalline solar cell with RIE texture has a much lower reflectance (9.18%) than that with acidic texture (13.11%) before encapsulation. The relative difference between the two textures is substantially reduced after encapsulation. After encapsulation with bare glass there will be a reflectance of about 7.42% for the RIE texture and a reflectance of about 8.14% for the acidic texture at the module surface. For the samples encapsulated with ARC glass, the reflectance of the two textures decreases to 4.28% and 5.11%, respectively. When the solar cells are encapsulated, the difference in reflectance between the two textures is reduced from about 4% to only 0.7%.
3.2. Optical model To quantify the optical loss from encapsulated modules, a stratified model is shown in Fig. 5, which consists of solar cells as well as encapsulation layers of glass and EVA. In the model, glass is the first layer, and the second layer is EVA while the third layer is solar cell. After encapsulation, the light impinging on the module is either reflected, parasitically absorbed in the encapsulation layers, or absorbed by the cell (assuming T (%) is the fraction of light arrived in the wafer surface which is totally absorbed by the solar cell). Firstly, the incident light is assumed to be I = 1 (100%). To achieve the effective light T (%) absorbed by solar cell, the optical loss (reflectance and absorption) from each layer of the encapsulated module should be considered. We define the optical loss as the fraction of the incident light, which is contributing to the externally reflectance R1 (%) from glass, internally reflectance R2 (%) at the glass/EVA interface, R3 (%) at the EVA/cell interface, and spectral absorption A1 (%) of glass and A2
Fig. 3. Front side reflectance (300–1100 nm) of the encapsulated polycrystalline modules with bare glass and ARC glass for (a) the acidic texture and (b) the RIE texture. For comparison, the reflectance of monocrystalline solar cell before encapsulation is also shown.
722
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
3.3. Optical analysis on glass (1st layer) Fig. 6 shows the reflectance R1 and absorption A1 as a function of wavelength for bare glass and ARC glass, respectively. Herein R1 is the one-side reflectance from the front side of glass by blackening the back side of glass before encapsulation. Assuming that the one-side reflectance and absorption of the glass is not changed by the encapsulation process, A1 can be obtained in terms of measurable reflectance and transmittance quantities of glass according to Eq. (2), in which T1 represents the transmitted light T (%) of glass. A1 ¼ 100 R1 T 1 Fig. 4. Average reflectance (300–1100 nm) comparison of polycrystalline modules with acidic texture and RIE texture for the following configurations: unencapsulated, encapsulated with bare glass, encapsulated with ARC glass. The relative error of average reflectance is estimated to be about ±1.5%.
Fig. 5. The schematic diagram of the encapsulated module for analysis on the optical losses.
(%) of EVA. n0 of 1 represents the refractive index of air. Both bare glass and ARC glass are analyzed in the optical model with a refractive index n1 of 1.47 ± 0.02. The refractive index n2 of EVA is set to be 1.50 ± 0.01. Solar cell with a SiNx coating on the silicon surface has a refractive index of 2.05 ± 0.05. n1, n2 and n3 were found using ellipsometry. The spectral optical losses are weighted in the wavelength range 350–1100 nm to quantify the optical losses as a percentage value. Here the reflectance R2 is negligible because of the little difference of refractive index between glass and EVA. Light that is neither reflected nor parasitically absorbed by the encapsulation layer is then absorbed by the cell, giving:
ð2Þ
It can be concluded that over the whole solar spectrum, on the one hand, a SiO2 layer (n = 1.3) of suitable thickness coated on the glass would reduce the average reflectance to 1.72%, in comparison to 4.55% for bare glass. On the other hand, ARC glass shows up to 1.63% little lower absorption loss than bare glass of 1.69%. The absorption of ARC glass with wavelength lower than 500 nm is less 0.06% than that of bare glass, this maybe due to a little more scattering from the surface of antireflective coating than that of the smooth glass, or it might be caused by the presence of a reflectance from the interface of coating and glass in the measured samples that is not considered in the model. The relative error of average reflectance and transmittance is estimated to be ±1.5%. R1 and T1 can be written as R1 Bareglass ¼ 4:55 0:07, T 1 Bareglass ¼ 93:76 1:41. The relative error of average absorption Ar can be calculated from the R (%) and T (%) using the following equation: DA Ar ¼ 100% ¼ A
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 R 1:5% þ T 1:5% A
100% ð3Þ
T ¼ 100 R1 R2 R3 ðA1 þ A2 Þ 100 R1 R3 ðA1 þ A2 Þ
ð1Þ
In which R1 was measured values, while A1 and A2 were calculated values according to the equation A = 100 R T, and R3 was calculated from R3 (R1 + R2 + R3) R1. We have following reflectance and absorption analysis of each layer to describe the incident light loss in the modules.
Fig. 6. Front side reflectance and absorption of bare glass and ARC glass as a function of wavelength.
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
Therefore, the relative errors of average absorption A1_bare glass and A1_ARC glass are calculated to be about ±83.2% and ±88.9%, i.e. A1 Bareglass ¼ 1:69 1:41, A1 ARCglass ¼ 1:63 1:45. 3.4. Optical analysis on glass and EVA (1st + 2nd layer) Any light which does not escape is absorbed parasitically by the solar cells or by encapsulation materials (glass and EVA). Consequently, light must be collected by the solar cells with a high absorption, while the light absorbed by glass and EVA is as little as possible. In order to evaluate the total absorption loss of encapsulation materials (A1 + A2) after encapsulation, a glass-EVA configuration is prepared by heat sealing, like being encapsulated, which is shown in Fig. 7(a), and we measured the total reflectance amount (R1 + R2 + R3) and transmittance (T2) of this configuration. Its reflectance (R1 + R2 + R3) and transmittance (T2) are shown in Fig. 7(b). As can be seen, incident light reflected from ‘‘bare glass + EVA” configuration is about 7.93%, which is higher than 6.68% for ‘‘ARC glass + EVA” configuration, while the transmittance of both configurations are 90.24% and 91.51%, respectively. It is also clear that glass and EVA will absorb a significant
723
share of the light, which may be achieved by Eq. (4) (Data is presented in Fig. 7(c)). The incident light absorbed from ‘‘bare glass + EVA” configuration is about 1.91%, which is higher than 1.82% from ‘‘ARC glass + EVA” configuration. A1 þ A2 ¼ 100 ðR1 þ R2 þ R3 Þ T 2
ð4Þ
The light absorbed by EVA at these wavelengths with the average value of about 0.2% from (ARC glass + EVA) and (Bare glass + EVA) configurations is calculated according to Eq. (5), as shown in Fig. 8. Especially light with wavelength greater than 400 nm is negligibly absorbed by EVA, which in agreement with the result in Fig. 5 of Ref. (Khoo et al., 2012). The good match of spectral absorption also verifies the absorption calculation in the model. However, the absorption of EVA at wavelengths below 400 nm, is no longer negligible and varies rapidly with wavelength. Because of the same absorption of EVA, the less absorption of ARC glass especially in wavelength less than 500 nm, dominates the less absorption loss of the configuration. A2 ¼ ðA1 þ A2 Þ A1
ð5Þ
The average absorption A2 is so small that the relative error can be up to ±1027.4% in theory according to Eq. (3).
Fig. 7. (a) The schematic diagram of glass-EVA configuration, (b) reflectance and transmittance measurements for two configurations and (c) calculated absorption A1 + A2 for two configurations.
724
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
it might be caused by the increase of carrier recombination nearby the cell surface due to the production of damages resulting from the bombardment of ions, which will actually enhance the reflectance at the silicon surface, especially in the blue response. Therefore more fractions of light reflected from the silicon surface are totally internally reflected at the coating/glass interface for ARC glass, while this fraction of light escapes into the air for bare glass. R3 ðR1 þ R2 þ R3 Þ R1
Fig. 8. Calculated absorption A2 from (ARC glass + EVA) and (Bare glass + EVA) configurations.
ð6Þ
The relative errors of calculated reflectance R3 for monocrystalline modules are ±2.2% with KOH texture and ±1.4% with Acidic texture, while for polycrystalline modules are ±3.1% with RIE texture and ±4.1% with Acidic texture, respectively.
3.5. Reflectance on glass/EVA/solar cell interface (1st + 2nd + 3rd layer)
3.6. Loss analysis of encapsulated sample
We calculate the reflectance on glass/EVA/solar cell interface for monocrystalline and polycrystalline modules according to Eq. (6), in which R2 is negligible. Fig. 9(a) shows calculated reflectance R3 (300–1100 nm) from four different samples of monocrystalline modules: the first case is the encapsulated monocrystalline module with bare glass for the KOH texture (abbreviated as KOH texture/bare glass), consequently, the other three samples are KOH texture/ARC glass, acidic texture/bare glass, and acidic texture/ARC glass, respectively. In theory, an identical R3 will be obtained on the same wafers with the same texture. The good match between KOH texture/bare glass and KOH texture/ARC glass, as well as between acidic texture/ bare glass and acidic texture/ARC glass, as shown in Fig. 9, illustrates that R3 in the model is valid. For polycrystalline samples, the results are equivalent to that of monocrystalline samples except for some deviation between RIE texture/bare glass and RIE texture/ARC glass for the wavelengths of less than 500 nm. The reason for the short wavelength deviation between ARC glass and bare glass is not completely understood. We believe
In order to compare the light loss from air to the surface of the solar cells due to the reflectance of glass, EVA and solar cell as well as the absorption of all materials, eight kinds of configurations were performed on Fig. 10. It is clear that the reflectance R1 and R3 account for the great proportion of light loss, which is over 70% of total loss, and has caused the light arriving in the silicon surface is diminished by at least 5%. However, the glass and EVA will absorb a secondary share of the light, which is 0.2%, 1.7% and 1.63% for EVA, bare glass and ARC glass, respectively. For monocrystalline modules (in Fig. 10(a)), it gives a comparison of the R1, R3, A1, A2 and T of four configurations, on average. Reductions in reflectance losses R1 of ARC glass in textured samples are almost exclusively due to the lower refractive index of SiO2 coating (1.45). Reduction in R3 of the KOH textured sample compared to the acidic textured sample is due to the better properties of standard random pyramid resulting from anisotropic etching. Therefore, the KOH texture encapsulated with ARC glass may be better choice for monocrystalline modules, which gives a 94.31% transmittance light collected by
Fig. 9. Calculated reflectance R3 (300–1100 nm) of (a) monocrystalline solar cells and (b) polycrystalline solar cells, encapsulated with bare glass and ARC glass for different textured technology.
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
725
Fig. 10. The average transmittance and reflectance together with absorption comparison of (a) monocrystalline modules and (b) polycrystalline modules. (The global relative error of average transmittance T is about ±2.5%).
silicon, while the acidic textures sample with ARC glass yields a gain of 92.8%. However, due to the greater reflectance from the bare glass, the transmittance of 91.05% in KOH textured sample is achieved in comparison to the 89.89% of acidic textured samples. Polycrystalline modules show similar excellent light trapping results when textured with RIE technology and encapsulated with ARC glass, as shown in Fig. 10(b). The greater R3 from the acidic textured polycrystalline sample may be contributed to the slight anisotropic nature of the texture causing variation in the tub shape on the different grains (Stocks et al., 1996), while RIE texturing has the excellent anisotropic etch property regardless of grain orientation. The polycrystalline samples with RIE texture/ARC glass shows up to 5% greater transmittance than polycrystalline samples with acidic texture/bare glass. By RIE texturing, the results show that the transmittance increased from 90.8% to 94.14% after encapsulated with bare glass and ARC glass, respectively. However, the acid etch is not as good as RIE etch, and the transmittances become 89.95% and 93.07%, respectively. Generally speaking, these results certainly show that RIE texturing provides significant gains in future chance of more current in solar cells. 4. Conclusions In this work the influences of texturing techniques and encapsulation on the optical performance of monocrystalline and polycrystalline modules were investigated. Firstly, it was found that the KOH texture for monocrystalline wafers (9.61%) and the RIE texture for polycrystalline wafers (9.18%) exhibited significantly lower reflectance than the acidic texture (15.00% and 13.11%, respectively) without encapsulation. After encapsulation, the difference between two different textures was significantly reduced, particularly for an ARC glass with a reflectance loss of 1.72% in comparison to 4.55% for bare glass. Moreover, we made a stratified model to quantify optical losses for the encapsulated modules using different
encapsulation glass and different texture. Monocrystalline module with KOH texture and ARC glass obtains a high effective absorption of 94.31%, while polycrystalline modules show similar excellent light trapping result of 94.14% when textured with RIE technique and encapsulated with ARC glass. Summing up, the reflectance R1 and R3 accounts for the great proportion of light loss, which is over 70% of total loss, while the absorption of glass and EVA accounts for the rest and less loss. Further reflectance improvements are possible with better texturing structures by adjusting experimental parameters or adding the antireflection coatings above and below the encapsulating layer without affecting the excellent light trapping properties. Moreover, the absorption of glass and EVA should be further reduced even though their contribution to light loss is much less. Acknowledgement The authors gratefully acknowledge Zhen Xiong for his measurements of reflectance and transmittance. References Gjessing, J., Marstein, E.S., 2013. Optical performance of solar modules. Energy Procedia 38, 348–354. Khoo, Yong Sheng, Walsh, Timothy M., Lu, Fei, Aberle, Armin G., 2012. Method for quantifying optical parasitic absorptance loss of glass and encapsulant materials of silicon wafer based photovoltaic modules. Sol. Energy Mater. Sol. Cells 102, 153–158. Macdonald, D.H., Cuevas, A., Kerr, M.J., et al., 2004. Texturing industrial multicrystalline silicon solar cells. Sol. Energy 76, 277–283. Notte, L.L., Polino, G., Verzola, P., et al., 2014. Influence of encapsulation materials on the optical properties and conversion efficiency of heat-sealed flexible polymer solar cells. Surf. Coat. Technol. 255, 69– 73. Stocks, M.J., Carr, A.J., Blakers, A.W., 1994. Texturing of polycrystalline silicon. In: First WCPEC, CH3365-4(94), pp. 1551–1554. Stocks, M.J., Carr, A.J., Blakers, A.W., 1996. Texturing of polycrystalline silicon. Sol. Energy Mater. Sol. Cells 40, 33–42. Willeke, G., Nussbaumer, H., Bender, H., et al., 1992. High efficiency silicon solar cells. In: Proc. 11th European PV Solar Energy Conference, pp. 41–44.
726
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
Willeke, G., Nussbaumer, H., Bender, H., et al., 1992b. A simple and effective light trapping technique for polycrystalline silicon solar cells. Sol. Energy Mater. Sol. Cells 26 (4), 345–356. Winderbaum, S., Reinhold, O., Yun, F., 1997. Reactive ion etching (RIE) as a method for texturing polycrystalline silicon solar cells. Sol. Energy Mater. Sol. Cells 46, 239–248.
Wohlgemuth, J., Cunningham, D., Shaner, J., et al., 2005. Crystalline silicon photovoltaic modules with antireflective coated glass. In: Proceedings of IEEE Photovoltaic Specialists Conference (PVSC), 2005, pp. 1015–1018.
ScienceDirect Solar Energy 122 (2015) 718–726 www.elsevier.com/locate/solener
The optical properties of solar cells before and after encapsulation Wusong Tao a, Ying Du b,⇑ b
a Changzhou Trina Solar Energy Co., Ltd., Changzhou, China College of Science, Zhejiang University of Technology, Hangzhou 310023, China
Received 28 January 2015; received in revised form 26 June 2015; accepted 6 October 2015
Communicated by: Associate Editor Antoine Bittar
Abstract In this work we investigate how the optical properties of monocrystalline silicon and polycrystalline silicon wafers are affected by texturing techniques and encapsulation. For monocrystalline wafers, the KOH etching is better than acid etching while reactive ion etching (RIE) is proven to be preferred compared to acid etching for polycrystalline wafers. The differences in reflectance (R) between two textures are apparent before encapsulation, but when the textured wafers are encapsulated with glass especially antireflectance coated (ARC) glass, the difference can be reduced from about five percentage points to below a percentage point. More important, the optical losses caused by reflectance (R) losses and parasitic absorption losses (A) for four types of commercial monocrystalline and polycrystalline silicon module are quantified and compared. Analyses are carried out for eight configurations using a stratified model consisting of solar cell wafers as well as cover glass and ethylene vinyl acetate (EVA). The model is to first obtain the external reflectance (R) and transmittance (T) in each layer, and the spectrally parasitic absorption loss associated with the cover glass and EVA is calculated with the aid of R and T measurements, which allows us to do a complete optical loss analysis for the solar module. The results show that the KOH texture with ARC glass encapsulation may be better choice for monocrystalline wafers, which gives a 94.04% of effective light collected by silicon. The RIE technique with ARC glass is suitable for polycrystalline substrates with excellent light trapping of 94.14%. After all, reflectance from glass surface and silicon surface accounts for over 70% of total loss, while the absorption of glass and EVA accounts for the rest and less loss. Ó 2015 Elsevier Ltd. All rights reserved.
Keywords: Texturing; Encapsulation; Monocrystalline silicon; Polycrystalline silicon
1. Introduction Solar cells are textured with suitable surface texturing and then encapsulated, which is used to minimize the total reflectance losses from the glass/silicon interface and increase the short-circuit current. During the last decade, many works were reported on the different texturing techniques such as, mechanical grooving (Willeke et al., 1992a,b), wet chemical etch (Stocks et al., 1996), laser ⇑ Corresponding author.
E-mail address: [email protected] (Y. Du). http://dx.doi.org/10.1016/j.solener.2015.10.007 0038-092X/Ó 2015 Elsevier Ltd. All rights reserved.
sculpturing, plasma etching and reactive ion etching (RIE) Winderbaum et al., 1997, which have been used to texture monocrystalline and polycrystalline silicon. Laser texturing is slow and is incompatible with thin substrates, and plasma etching is likely to be relative expensive. However, wet chemical isotropic etching is a low cost method (Stocks et al., 1994), while masked and maskless RIE texturing for polycrystalline wafers produces lower reflectance than the standard wet etching used today in industry (Macdonald et al., 2004). It has been proven that the efficiency of solar cells is affected not only by texturing but also by encapsulation (Wohlgemuth et al., 2005).
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
Therefore, the transition from a solar cell to an encapsulated solar cell, i.e. a solar module, has a huge impact on the optical performance of the device (Gjessing and Marstein, 2013), which is easily ignored when comparing efficiency of silicon solar cells with different textures. Simultaneously, analyzing the optical property of photovoltaic (PV) modules and quantifying their optical losses are important for PV module designers for optimizing the design of solar cells and PV modules. In this work optical analysis was carried out on industrial wafers with different texturing techniques before and after the encapsulation, and it was found that a suitable surface texturing was needed to prevent optical loss between glass and encapsulated material. KOH texturing, which would be the least costly to implement, produces a modest improvement in reflectance for monocrystalline wafers before encapsulation, whereas RIE texturing, generates a larger gain in light absorption for polycrystalline silicon solar cells in spite of its high cost. At the same time, it was found the difference in reflectance between the textures was reduced after encapsulation. This is attributed to both light-trapping and oblique coupling of incident light into the cell. ARC glass retained their antireflective properties by encapsulation and showed huge improvement on reflectance compared to bare glass, especially for the monocrystalline solar cells with KOH texture and the polycrystalline solar cells with RIE texture. There is considerable potential for use of ARC glass as the encapsulated glass. Moreover, we make a stratified model to calculate the optical loss of glass and ethylene vinyl acetate (EVA), and the effective light collected by silicon can be quantified for eight different configurations. It will be helpful to reduce the light loss by optimizing the process and encapsulation materials, which will enhance the photoelectric conversion efficiency of solar modules. 2. Experimental 2.1. Surface texturing An effective texturing process reduces reflectance from the top surface of the solar cell and aids in the retention of long wavelength, weakly absorbed light within the silicon (light trapping) Stocks et al., 1994. It also has the side benefit of increasing the optical path of light in the bulk of the material, because of the change in the angle of incidence after multiple reflectances occurred on the surface (Winderbaum et al., 1997). For monocrystalline solar cells, standard random pyramid (the KOH texture) and isotropic textures (the acidic texture) are made with wet etching in solutions based on KOH and HN03/H3P04/ HF, respectively. The random pyramids are formed by etching in a 1% (wt) KOH, 4% (wt) isopropanol solution at 78 °C for 40 min after a damage removal in 30% (wt) KOH at 75 °C for 3 min. The isotropic etched samples are etched in an acidic solution (10:5:2, HN03: H3P04: HF) at 20–25 °C for 70 s. In addition, because of the
719
random grain orientation, traditional anisotropic etchants does not work on polycrystalline silicon. Two methods for texturing polycrystalline solar cells that yields good performance are selected: one is to texture samples using acidic solution (5:1, HN03: HF), while the other is the maskless RIE method. The RIE equipment used in these experiments was a JuSung plasma-enhanced chemical vapor deposition (PECVD). The wafer was etched by RIE with SiF6 and Cl2 for appropriate time. In the experiment, the base pressure before each run was less than 5 106 Tort. Then, the samples were cleaned prior to the optical measurement and subsequent process by acetone and ethanol under sonication to remove contaminants introduced by the fabrication process. SiNx antireflective coatings are deposited on the solar cells by PECVD after the texturing process. The SiNx thickness is 75 ± 5 nm with a refractive index of 2.02 ± 0.03 at 600 nm, which was determined by ellipsometry SEMILAB SE400. 2.2. Encapsulation technique Encapsulation method is based on the heat laminating, a process which basically consists of supplying thermal energy on outside of package to soften/melt the sealants (Notte et al., 2014). To facilitate the measurement, the window glass used for the encapsulation is cut into pieces of about 20 20 cm2, and the edges of the glass are polished. A monocrystalline silicon or polycrystalline silicon wafer is encapsulated with window glass from Almaden and EVA from FIRST at 145 degrees for 16 min using a Boostsolar BSL2236OAC laminator. The physical thickness of the EVA was found from the cross-sectional microscope images to be about 0.50 ± 0.05 mm thick, while the glass is 3.2 ± 0.2 mm thick. The electrical contacts are extracted from the encapsulated device by means of metal ribbons (Notte et al., 2014). The reflectance in the external surface of the glass may also be reduced by using the antireflective coatings (SiO2) with the thickness of 120 ± 15 nm on the glass, then the optical properties of bare glass and the ARC glass are compared. The ARC glass is confirmed to be lower reflectance losses. 2.3. Measurement Reflectance and transmittance measurements of samples were made using a spectrophotometer (Perkin Elmer, Lambda 950) with an attached barium sulfate coated integrating sphere, which collects reflected and transmitted light through a sample from all directions. The samples were carefully placed below the measurement spot (16 mm 3.5 mm). Light reflected from the rear side will also contribute to the measured reflectance. The measurement accuracy of the spectrophotometer was ±0.4%. Scans were made from 300 nm to 1100 nm with a PbS detector in the wavelength range above 800 nm and a photomultiplier below 800 nm. The uncertainty of wavelength is ±1 nm.
720
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
Incident light was 3 degrees from normal incidence to ensure specularly reflected light did not escape the integrating sphere. The encapsulating layer consisted of several millimeters of EVA topped by 3.2 ± 0.2 mm thick Vycor, a low absorptivity glass. Optical constants of glass and EVA (the refractive index and thickness) were found using ellipsometry, while the absorption loss of glass and EVA were calculated from reflectance and transmittance measurements of a glass and glass-EVA conjunction. 3. Results and discussion 3.1. Reflectance measurement We measured reflectance as a function of wavelength (300–1100 nm) for monocrystalline and polycrystalline solar cells textured using different techniques with and without encapsulation. Fig. 1 shows the reflectance of the encapsulated monocrystalline solar cells with bare glass and ARC glass. KOH and acid etch are used to produce the KOH texture (Fig. 1(a)) and the acidic texture (Fig. 1 (b)), respectively. For comparison, the reflectance of monocrystalline solar cell before encapsulation was also shown. Comparing Fig. 1(a) with Fig. 1(b), reflectance measurements from samples show reflectance losses on the KOH texture are comparable with those from the acidic texture, but the KOH textured sample seems to produce a reflectance somewhat lower than the acid etch. Additionally, the reflectance from encapsulated samples with ARC glass after the etch is lower than those from unencapsulated samples over the whole solar spectrum, especially in the region below 450 nm, and the reflectance from bare glass follows a similar trend resembling that from ARC glass. Therefore, when encapsulated under glass, the samples with textured surfaces will have relatively improved reflectance due to light-trapping (Macdonald et al., 2004). However, the encapsulated sample with bare glass exhibits poor antireflective performance than that with ARC glass. It is worth noticing that the apparent
reduction in reflectance in the UV region after encapsulation, which is caused by absorption within the encapsulating material. At wavelength greater than 1000 nm, the reflectance increases for all samples, but they increase the least for the encapsulated sample with ARC glass. In fact, such a module achieves a higher generation current than an unencapsulated solar cell. In order to represent the antireflective property of those samples more accurately, averaged front side reflectance values for all two types of textures both with and without encapsulation are shown in Fig. 2. We also show reflectance values encapsulated with ARC glass. Before encapsulation, samples textured with acidic solution show little improvement in reflectance loss compared to the KOH textured samples, and the difference is very apparent (5%). The data presented in Fig. 2 shows that the monocrystalline solar cell with KOH texture has a much lower reflectance (9.61%) than that with acidic texture (15.00%) before encapsulation. For the encapsulated samples with bare glass, the reflectance of the two textures decreases to 7.04% and 8.20%, respectively. After encapsulated with ARC glass, there will be a reflectance of about 4.14% for the KOH texture and a reflectance of about 5.38% for the acidic texture at the module surface, and the difference between two textures becomes too small (1%), which represents the excellent light trapping properties of encapsulated textures silicon. Therefore, the reflectance of samples encapsulated with ARC glass is lower than that with bare glass. It is shown that for monocrystalline solar cells textured using KOH etch, the encapsulated samples with ARC glass is at advantage due to relatively low reflectance. The uncertainty on the average reflectance includes both the uncertainty on the representativeness of the sample used for analysis and the systematic measurement error. Our model illuminates the whole module area, whereas our spectrometer only illuminates a small spot. And we measure the reflectance on an area of the cell where there are fingers, but no busbars. The associated uncertainty
Fig. 1. Front side reflectance (300–1100 nm) of the encapsulated monocrystalline modules with bare glass and ARC glass for (a) the KOH texture and (b) the acidic texture. For comparison, the reflectance of monocrystalline wafer before encapsulation is also shown.
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
Fig. 2. Average reflectance (300–1100 nm) comparison of monocrystalline modules with KOH texture and acidic texture for the following samples: unencapsulated, encapsulated with bare glass, encapsulated with ARC glass. The relative error of average reflectance is estimated to be about ±1.5%.
for representativeness of the sample was ±0.5%, ±0.4% for measurement accuracy, ±0.5% for systematic error (assuming 100% of reflectance). Therefore, we estimated the average reflectance with a relative error of about ±1.5%. Both reflectance and transmittance measurements suffer this error which will affect the calculation of absorption. The total reflectance from encapsulated polycrystalline solar cells with bare glass and ARC glass as a function of wavelength are shown in Fig. 3. Acid solution and maskless RIE etch are used to produce surface texturing: the acidic texture (Fig. 3(a)) and the RIE texture (Fig. 3(b)). For comparison, the reflectance of polycrystalline solar cell before encapsulation is also shown. Comparing Fig. 3(a) with Fig. 3(b), the acidic textured sample is certainly not as good as the maskless RIE textured sample, since the RIE textured sample in particularly exhibits good light-trapping. Similarly, we observe that the encapsulated sample with bare glass exhibits poor antireflective performance than that with ARC glass. The reflectance for all samples before and after encapsulation from Fig. 3 follows
721
a similar trend resembling those from Fig. 1 even though their textures differ greatly with each other. It can be seen that the reflectance of solar cells are reduced effectively by encapsulation with ARC glass. The average reflectance results of polycrystalline solar cells are summarized in Fig. 4. The data presented in Fig. 2 shows that the polycrystalline solar cell with RIE texture has a much lower reflectance (9.18%) than that with acidic texture (13.11%) before encapsulation. The relative difference between the two textures is substantially reduced after encapsulation. After encapsulation with bare glass there will be a reflectance of about 7.42% for the RIE texture and a reflectance of about 8.14% for the acidic texture at the module surface. For the samples encapsulated with ARC glass, the reflectance of the two textures decreases to 4.28% and 5.11%, respectively. When the solar cells are encapsulated, the difference in reflectance between the two textures is reduced from about 4% to only 0.7%.
3.2. Optical model To quantify the optical loss from encapsulated modules, a stratified model is shown in Fig. 5, which consists of solar cells as well as encapsulation layers of glass and EVA. In the model, glass is the first layer, and the second layer is EVA while the third layer is solar cell. After encapsulation, the light impinging on the module is either reflected, parasitically absorbed in the encapsulation layers, or absorbed by the cell (assuming T (%) is the fraction of light arrived in the wafer surface which is totally absorbed by the solar cell). Firstly, the incident light is assumed to be I = 1 (100%). To achieve the effective light T (%) absorbed by solar cell, the optical loss (reflectance and absorption) from each layer of the encapsulated module should be considered. We define the optical loss as the fraction of the incident light, which is contributing to the externally reflectance R1 (%) from glass, internally reflectance R2 (%) at the glass/EVA interface, R3 (%) at the EVA/cell interface, and spectral absorption A1 (%) of glass and A2
Fig. 3. Front side reflectance (300–1100 nm) of the encapsulated polycrystalline modules with bare glass and ARC glass for (a) the acidic texture and (b) the RIE texture. For comparison, the reflectance of monocrystalline solar cell before encapsulation is also shown.
722
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
3.3. Optical analysis on glass (1st layer) Fig. 6 shows the reflectance R1 and absorption A1 as a function of wavelength for bare glass and ARC glass, respectively. Herein R1 is the one-side reflectance from the front side of glass by blackening the back side of glass before encapsulation. Assuming that the one-side reflectance and absorption of the glass is not changed by the encapsulation process, A1 can be obtained in terms of measurable reflectance and transmittance quantities of glass according to Eq. (2), in which T1 represents the transmitted light T (%) of glass. A1 ¼ 100 R1 T 1 Fig. 4. Average reflectance (300–1100 nm) comparison of polycrystalline modules with acidic texture and RIE texture for the following configurations: unencapsulated, encapsulated with bare glass, encapsulated with ARC glass. The relative error of average reflectance is estimated to be about ±1.5%.
Fig. 5. The schematic diagram of the encapsulated module for analysis on the optical losses.
(%) of EVA. n0 of 1 represents the refractive index of air. Both bare glass and ARC glass are analyzed in the optical model with a refractive index n1 of 1.47 ± 0.02. The refractive index n2 of EVA is set to be 1.50 ± 0.01. Solar cell with a SiNx coating on the silicon surface has a refractive index of 2.05 ± 0.05. n1, n2 and n3 were found using ellipsometry. The spectral optical losses are weighted in the wavelength range 350–1100 nm to quantify the optical losses as a percentage value. Here the reflectance R2 is negligible because of the little difference of refractive index between glass and EVA. Light that is neither reflected nor parasitically absorbed by the encapsulation layer is then absorbed by the cell, giving:
ð2Þ
It can be concluded that over the whole solar spectrum, on the one hand, a SiO2 layer (n = 1.3) of suitable thickness coated on the glass would reduce the average reflectance to 1.72%, in comparison to 4.55% for bare glass. On the other hand, ARC glass shows up to 1.63% little lower absorption loss than bare glass of 1.69%. The absorption of ARC glass with wavelength lower than 500 nm is less 0.06% than that of bare glass, this maybe due to a little more scattering from the surface of antireflective coating than that of the smooth glass, or it might be caused by the presence of a reflectance from the interface of coating and glass in the measured samples that is not considered in the model. The relative error of average reflectance and transmittance is estimated to be ±1.5%. R1 and T1 can be written as R1 Bareglass ¼ 4:55 0:07, T 1 Bareglass ¼ 93:76 1:41. The relative error of average absorption Ar can be calculated from the R (%) and T (%) using the following equation: DA Ar ¼ 100% ¼ A
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 2 R 1:5% þ T 1:5% A
100% ð3Þ
T ¼ 100 R1 R2 R3 ðA1 þ A2 Þ 100 R1 R3 ðA1 þ A2 Þ
ð1Þ
In which R1 was measured values, while A1 and A2 were calculated values according to the equation A = 100 R T, and R3 was calculated from R3 (R1 + R2 + R3) R1. We have following reflectance and absorption analysis of each layer to describe the incident light loss in the modules.
Fig. 6. Front side reflectance and absorption of bare glass and ARC glass as a function of wavelength.
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
Therefore, the relative errors of average absorption A1_bare glass and A1_ARC glass are calculated to be about ±83.2% and ±88.9%, i.e. A1 Bareglass ¼ 1:69 1:41, A1 ARCglass ¼ 1:63 1:45. 3.4. Optical analysis on glass and EVA (1st + 2nd layer) Any light which does not escape is absorbed parasitically by the solar cells or by encapsulation materials (glass and EVA). Consequently, light must be collected by the solar cells with a high absorption, while the light absorbed by glass and EVA is as little as possible. In order to evaluate the total absorption loss of encapsulation materials (A1 + A2) after encapsulation, a glass-EVA configuration is prepared by heat sealing, like being encapsulated, which is shown in Fig. 7(a), and we measured the total reflectance amount (R1 + R2 + R3) and transmittance (T2) of this configuration. Its reflectance (R1 + R2 + R3) and transmittance (T2) are shown in Fig. 7(b). As can be seen, incident light reflected from ‘‘bare glass + EVA” configuration is about 7.93%, which is higher than 6.68% for ‘‘ARC glass + EVA” configuration, while the transmittance of both configurations are 90.24% and 91.51%, respectively. It is also clear that glass and EVA will absorb a significant
723
share of the light, which may be achieved by Eq. (4) (Data is presented in Fig. 7(c)). The incident light absorbed from ‘‘bare glass + EVA” configuration is about 1.91%, which is higher than 1.82% from ‘‘ARC glass + EVA” configuration. A1 þ A2 ¼ 100 ðR1 þ R2 þ R3 Þ T 2
ð4Þ
The light absorbed by EVA at these wavelengths with the average value of about 0.2% from (ARC glass + EVA) and (Bare glass + EVA) configurations is calculated according to Eq. (5), as shown in Fig. 8. Especially light with wavelength greater than 400 nm is negligibly absorbed by EVA, which in agreement with the result in Fig. 5 of Ref. (Khoo et al., 2012). The good match of spectral absorption also verifies the absorption calculation in the model. However, the absorption of EVA at wavelengths below 400 nm, is no longer negligible and varies rapidly with wavelength. Because of the same absorption of EVA, the less absorption of ARC glass especially in wavelength less than 500 nm, dominates the less absorption loss of the configuration. A2 ¼ ðA1 þ A2 Þ A1
ð5Þ
The average absorption A2 is so small that the relative error can be up to ±1027.4% in theory according to Eq. (3).
Fig. 7. (a) The schematic diagram of glass-EVA configuration, (b) reflectance and transmittance measurements for two configurations and (c) calculated absorption A1 + A2 for two configurations.
724
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
it might be caused by the increase of carrier recombination nearby the cell surface due to the production of damages resulting from the bombardment of ions, which will actually enhance the reflectance at the silicon surface, especially in the blue response. Therefore more fractions of light reflected from the silicon surface are totally internally reflected at the coating/glass interface for ARC glass, while this fraction of light escapes into the air for bare glass. R3 ðR1 þ R2 þ R3 Þ R1
Fig. 8. Calculated absorption A2 from (ARC glass + EVA) and (Bare glass + EVA) configurations.
ð6Þ
The relative errors of calculated reflectance R3 for monocrystalline modules are ±2.2% with KOH texture and ±1.4% with Acidic texture, while for polycrystalline modules are ±3.1% with RIE texture and ±4.1% with Acidic texture, respectively.
3.5. Reflectance on glass/EVA/solar cell interface (1st + 2nd + 3rd layer)
3.6. Loss analysis of encapsulated sample
We calculate the reflectance on glass/EVA/solar cell interface for monocrystalline and polycrystalline modules according to Eq. (6), in which R2 is negligible. Fig. 9(a) shows calculated reflectance R3 (300–1100 nm) from four different samples of monocrystalline modules: the first case is the encapsulated monocrystalline module with bare glass for the KOH texture (abbreviated as KOH texture/bare glass), consequently, the other three samples are KOH texture/ARC glass, acidic texture/bare glass, and acidic texture/ARC glass, respectively. In theory, an identical R3 will be obtained on the same wafers with the same texture. The good match between KOH texture/bare glass and KOH texture/ARC glass, as well as between acidic texture/ bare glass and acidic texture/ARC glass, as shown in Fig. 9, illustrates that R3 in the model is valid. For polycrystalline samples, the results are equivalent to that of monocrystalline samples except for some deviation between RIE texture/bare glass and RIE texture/ARC glass for the wavelengths of less than 500 nm. The reason for the short wavelength deviation between ARC glass and bare glass is not completely understood. We believe
In order to compare the light loss from air to the surface of the solar cells due to the reflectance of glass, EVA and solar cell as well as the absorption of all materials, eight kinds of configurations were performed on Fig. 10. It is clear that the reflectance R1 and R3 account for the great proportion of light loss, which is over 70% of total loss, and has caused the light arriving in the silicon surface is diminished by at least 5%. However, the glass and EVA will absorb a secondary share of the light, which is 0.2%, 1.7% and 1.63% for EVA, bare glass and ARC glass, respectively. For monocrystalline modules (in Fig. 10(a)), it gives a comparison of the R1, R3, A1, A2 and T of four configurations, on average. Reductions in reflectance losses R1 of ARC glass in textured samples are almost exclusively due to the lower refractive index of SiO2 coating (1.45). Reduction in R3 of the KOH textured sample compared to the acidic textured sample is due to the better properties of standard random pyramid resulting from anisotropic etching. Therefore, the KOH texture encapsulated with ARC glass may be better choice for monocrystalline modules, which gives a 94.31% transmittance light collected by
Fig. 9. Calculated reflectance R3 (300–1100 nm) of (a) monocrystalline solar cells and (b) polycrystalline solar cells, encapsulated with bare glass and ARC glass for different textured technology.
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
725
Fig. 10. The average transmittance and reflectance together with absorption comparison of (a) monocrystalline modules and (b) polycrystalline modules. (The global relative error of average transmittance T is about ±2.5%).
silicon, while the acidic textures sample with ARC glass yields a gain of 92.8%. However, due to the greater reflectance from the bare glass, the transmittance of 91.05% in KOH textured sample is achieved in comparison to the 89.89% of acidic textured samples. Polycrystalline modules show similar excellent light trapping results when textured with RIE technology and encapsulated with ARC glass, as shown in Fig. 10(b). The greater R3 from the acidic textured polycrystalline sample may be contributed to the slight anisotropic nature of the texture causing variation in the tub shape on the different grains (Stocks et al., 1996), while RIE texturing has the excellent anisotropic etch property regardless of grain orientation. The polycrystalline samples with RIE texture/ARC glass shows up to 5% greater transmittance than polycrystalline samples with acidic texture/bare glass. By RIE texturing, the results show that the transmittance increased from 90.8% to 94.14% after encapsulated with bare glass and ARC glass, respectively. However, the acid etch is not as good as RIE etch, and the transmittances become 89.95% and 93.07%, respectively. Generally speaking, these results certainly show that RIE texturing provides significant gains in future chance of more current in solar cells. 4. Conclusions In this work the influences of texturing techniques and encapsulation on the optical performance of monocrystalline and polycrystalline modules were investigated. Firstly, it was found that the KOH texture for monocrystalline wafers (9.61%) and the RIE texture for polycrystalline wafers (9.18%) exhibited significantly lower reflectance than the acidic texture (15.00% and 13.11%, respectively) without encapsulation. After encapsulation, the difference between two different textures was significantly reduced, particularly for an ARC glass with a reflectance loss of 1.72% in comparison to 4.55% for bare glass. Moreover, we made a stratified model to quantify optical losses for the encapsulated modules using different
encapsulation glass and different texture. Monocrystalline module with KOH texture and ARC glass obtains a high effective absorption of 94.31%, while polycrystalline modules show similar excellent light trapping result of 94.14% when textured with RIE technique and encapsulated with ARC glass. Summing up, the reflectance R1 and R3 accounts for the great proportion of light loss, which is over 70% of total loss, while the absorption of glass and EVA accounts for the rest and less loss. Further reflectance improvements are possible with better texturing structures by adjusting experimental parameters or adding the antireflection coatings above and below the encapsulating layer without affecting the excellent light trapping properties. Moreover, the absorption of glass and EVA should be further reduced even though their contribution to light loss is much less. Acknowledgement The authors gratefully acknowledge Zhen Xiong for his measurements of reflectance and transmittance. References Gjessing, J., Marstein, E.S., 2013. Optical performance of solar modules. Energy Procedia 38, 348–354. Khoo, Yong Sheng, Walsh, Timothy M., Lu, Fei, Aberle, Armin G., 2012. Method for quantifying optical parasitic absorptance loss of glass and encapsulant materials of silicon wafer based photovoltaic modules. Sol. Energy Mater. Sol. Cells 102, 153–158. Macdonald, D.H., Cuevas, A., Kerr, M.J., et al., 2004. Texturing industrial multicrystalline silicon solar cells. Sol. Energy 76, 277–283. Notte, L.L., Polino, G., Verzola, P., et al., 2014. Influence of encapsulation materials on the optical properties and conversion efficiency of heat-sealed flexible polymer solar cells. Surf. Coat. Technol. 255, 69– 73. Stocks, M.J., Carr, A.J., Blakers, A.W., 1994. Texturing of polycrystalline silicon. In: First WCPEC, CH3365-4(94), pp. 1551–1554. Stocks, M.J., Carr, A.J., Blakers, A.W., 1996. Texturing of polycrystalline silicon. Sol. Energy Mater. Sol. Cells 40, 33–42. Willeke, G., Nussbaumer, H., Bender, H., et al., 1992. High efficiency silicon solar cells. In: Proc. 11th European PV Solar Energy Conference, pp. 41–44.
726
W. Tao, Y. Du / Solar Energy 122 (2015) 718–726
Willeke, G., Nussbaumer, H., Bender, H., et al., 1992b. A simple and effective light trapping technique for polycrystalline silicon solar cells. Sol. Energy Mater. Sol. Cells 26 (4), 345–356. Winderbaum, S., Reinhold, O., Yun, F., 1997. Reactive ion etching (RIE) as a method for texturing polycrystalline silicon solar cells. Sol. Energy Mater. Sol. Cells 46, 239–248.
Wohlgemuth, J., Cunningham, D., Shaner, J., et al., 2005. Crystalline silicon photovoltaic modules with antireflective coated glass. In: Proceedings of IEEE Photovoltaic Specialists Conference (PVSC), 2005, pp. 1015–1018.