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Microgroove lens with 500-800 µm in depth is proposed on the glass substrate of thin-film solar cell. The objective is to improve photovoltaic characteristics under weak-light illumination. First, the micro-optical characteristics were theoretically studied in connection with micro-lens structure; then a diamond micro-grinding was employed to fabricate microgroove lens; finally, indoor conversion efficiency and outdoor electrical generation were measured. It is shown that the 800-µm-depth microgroove lens is able to absorb and scatter the inclined light to solar cell for improving weak-light conversion efficiency. It enhances the electricity generation by 118% and 185% in cloudy and overcast days, respectively.
© 2015 Optical Society of America
1. Introduction
Due to the poor absorption of crystalline silicon (c-Si) solar cells [1], alternative materials with stronger absorption such as amorphous silicon (α-Si) [2] and organic materials [3] are used for thin film solar cells. A challenge in thin film solar cells is that they have not achieved conversion efficiency as high as c-Si. Besides, the solar cell technology has its limit of photovoltaic performance, such as the ideal thermodynamic limit [4]. Hence, a new design is required to improve conversion efficiency without sacrificing other photovoltaic characteristics.
Unfortunately, the finite optical absorption dominates the minimum thickness of the absorber, while the radiative recombination sets the maximum electrical thickness limit of the solar cell [5]. The traditional strategies that address these issues are about relying on decoupling optical and electrical thicknesses of solar cells. Such as the pattering of pyramid structure on the front side of c-Si cell [6] and the grating of microcrystalline thin-film silicon solar cell [7]. These strategies are based on the increase of the surface area for recombination which may degrade the charge collection and ultimately negate any gain from light trapping [8]. Besides, a separate dielectric/metallic scattered layer was designed to enhance the absorption of the incident light path or to scatter the light going into the absorber. For example, a hole-array layer was inserted into a tandem solar cell as an intermediate electrode [9]. However, it would lead to complex designs for practical application.
The influence of glass substrate solar harvest has been focused on recently. The overall reflectivity on the glass sheet is as high as 5% [10, 11]. Thus, additional micro-structures patterned on the air/glass substrate interface are required to minimize reflection losses and enhance the light in-coupling. The microgrooved glass surface with a 150 nm period and a 150 nm deep decreased the reflectivity from 5% to 1% [11]. A polymer hemi-spherical microlens array was moulded on the surface, resulting in a 15–60% increase in overall cell efficiency [12]. Compared with anti-reflective layer, the main advantage in patterning micro-structures was the spectral independence of the anti-reflect effect as long as the texture features were much larger than the interest wavelengths [13]. However, most of the microlenses composed of polymer/resin materials were directly applied to glass surface [12], which led to lower outdoor durability.
It has been reported that the light trapping configuration showed a probability of photon absorption [14–16]. The micro-lens array as trapping element was mainly machined by the imprinting [13, 14], the wet-etching [15], or the lithography [11, 12] along with a self-aligned array of micro apertures located in a highly reflecting mirror or mirror-like electrodes. However, this configuration was rather sensitive to light incident angle. Although the laser induced backside wet etching was proposed to process microgrooves on the sapphire substrate [17], the quality of the machining surface was poor. Besides, the lithography technique was expensive and time-consuming [18]. The wet-etching produced poor microstructure form accuracy [19]. The imprint fabrication [13, 14] depended on the mold and its nano/submicron structure limited industrial solar application.
The industrial product designers frequently encounter the problems regarding the harvesting prediction of solar energy [20]. As solar cells are likely to be operated close to field work, the light intensities are often below one sun intensity in most cases. In particular, when the PV solar cells are used indoors, the light intensities are even much lower. The issues about light incident angles related to photovoltaic characteristics were rarely focused as a function of the spectral and irradiance intensity. Until now, it has not yet been clear about the relationship between micro-optic behavior and photovoltaic behavior.
As a mechanical micro-machining, the micro-grinding along with #600 diamond wheel V-tip was developed to fabricate microstructure on hard and brittle materials, such as silicon [21] and quartz glass [22]. This Eco-fabrication is effective and easy. It may assure Si micro-structure accuracy by dressing diamond wheel V-tip [21]. It may also control the microgroove angle of quartz glass by precision form-truing of diamond wheel V-tip [22]. However, the mirror micro-grinding of microlens has not been performed.
In this paper, a diamond micro-grinding along with diamond wheel V-tip is proposed to directly fabricate accurate and mirror microgroove array on the glass substrate of α-Si TF solar cell. Thus, the light will not suffer from a second reflection at the interface between microlens and solar cell substrate. This microgroove glass lens may produce new micro-optic characteristics for absorbing scattered light. The objective is to improve the photovoltaic characteristics under weak-light illumination. First, the micro-optical characteristic was studied by using Monte-Carlo simulation and optical effects of microgroove lens structures were theoretically analyzed; Second, the fabrication of microgroove lens with 500-800 µm depth at the glass/air interface of thin-film solar cell was employed by a diamond grinding wheel; then, the optical and photovoltaic characteristics were studied concerning the light intensity and the incidence; finally, the microgroove lens was applied to a solar device to conduct the electricity charge-discharge experiments under weak and strong daylight illuminations to investigate the electricity generation capability.
2. Optical modeling of micro-optic lens array solar cell
In the study, the solar cell with a superstrate configuration is described by a cell consisting of a 1µm thick α-Si absorber layer with the dimension of 50 × 50 × 3 mm3 (l × w × h). To investigate the optical effects of microgroove lens, the optical phenomena of reflection and refraction are needed to be analyzed by using the theory of geometric optics.
2.1 Micro-optical performance
Figure 1 shows the incident light trajectories inside glass substrate for micro-hemisphere lens and microgroove lens on solar cell. The light trajectories were analyzed by Monte Carlo ray-tracing. It was likely to predict solar cell [23] and optical element performances [24–26]. It is shown that the light reflected several times after the incident light hit the micro-lens structured surface and refracted into large propagating angle. The microlens array may scatter light on solar cell surface compared with the plane. Besides, the microgroove lens harvests more light than the micro-hemisphere lens at cell entrance.
Fig. 1 Monte Carlo simulated light trajectories on the micro-hemisphere lens (left) and microgroove lens (right) solar cell.
Figure 2 shows the light flux γ and the light transmittance t of solar cell versus incident angle α. It is shown that the microgroove lens scatters more light to solar cell than the micro-hemisphere lens (see Fig. 2(a)). It contributes the effective utilization of light photons in solar cell. This is because it protects overly centralized light from solar cell.
Fig. 2 The light flux γ and the light transmittance t of solar cell versus incident angle α. (a) The light flux γ and (b) the light transmittance t.
It is also seen that the light transmittance t decreases with increasing incident angle α. This trend is similar to the result reported in Ref [18]. In the case of the incident angle of 0-50°, the microgroove lens averagely enhances the light transmittance t by 0.6-2.8% against the micro-hemisphere lens and by 1.4-3.2% against the traditional plane, respectively. In the case of the incident angle of 50-80°, the microgroove lens averagely enhances the light transmittance t by 1.0-7.2% and by 26.4-31.4%, respectively. As a result, the microgroove lens produces much stronger ability of transmittance for surrounding scattered light.
2.2 The structure design of microgroove lens
Figure 3 shows the light reflection and refraction inside microgroove structure. For the microgroove surface, the incident angle β is described as follows:
Fig. 3 The light reflection and refraction inside microgroove structure. (a) Double internal total reflection and (b) one internal total reflection.
The angle of refraction:
where nair is the air refractive index and mmat is the material refractive index. The critical angle of total reflection:In order to fulfill the two internal total reflection condition, first, the facet should not block the reflect light, that isSecond, the angle β2 should be larger than the critical angle βc, that isThirdly, the angle β3 should be also larger than the critical angle βc to satisfy the second total reflection:Combined with Eqs. (1) to (6), the incident angle β should be satisfied with following conditions:
In the case of normal illumination (α = 0°), the nair = 1.0 and nmat = 1.46 (sodalime glass). The first condition is β≤64°, the second condition is β≥32.4°, and the third is β≤65.9°. Therefore, slope angle φ lies between 32.4° and 64°. In other words, the V-angle θ should be in the range of 52°-115.2°, thus aspect ratio (p/d) is between 0.64 and 2.05. In this study, we choose the V-angle of 60°.
3. Experiment
3.1 Fabrication of micro-optic lens array on the glass
Before micro-grinding the microgroove lens, the diamond wheel needs to be trued to be V-shape. First, a grinding wheel was driven along the tool path to make a mutual-wear with a dresser. The dresser was composed of green silicon carbide abrasive and ceramic bond (GC). Then, the working surface profile of grinding wheel was gradually formed to be a V-shape no matter what the initial shape the grinding wheel used to be. The trued wheel V-tip angle is equal to the angle of V-shape tool path [21]. The detailed truing conditions are shown in Table 1.
Table 1. Form-truing conditions of SD600 wheel V-tip
Figure 4 shows the micro-grinding process of microgroove lens on the glass substrate. In micro-grinding, the on-machine V-tip truing of diamond grinding wheel was first performed on grinder. Then, the sharpened diamond wheel V-tip was used to perform a micro-grinding of microgroove lens on the glass with micron-scale depth of cut in CNC grinding system. In this micro-grinding, the microgroove lenses were gradually patterned on through a traverse grinding tool path. In order to assure the high accuracies of the surface and smoothness of the ground microgroove lens, the spark-out grinding was performed to polish these ground structures after the rough grinding and the fine grinding. The micrographics of microgroove lens before and after polishing are showed in Fig. 5.
Fig. 4 Fabrication of microgroove lens on the glass substrate. (a) Machining scheme and (b) machining scene.
The microgroove lens surface was measured by surface profiler Talysurf CLI2000. Before polishing, the surface roughness Ra was 1.84-2.27μm; after polishing, the Ra was 0.06-0.20 μm. It was quite smooth and integrated without edge-damaged compared with the micro-hemisphere fabricated by wet etching [19]. Detailed micro-grinding conditions are shown in Table 2.
Table 2. Micro-grinding conditions
Figure 6 shows the topography of microgroove lens structured solar cells. It is seen that each microgroove lens had a flat roof. From the bottom reflection of the glass, the particular band pattern of dark and bright was observed (Fig. 6(a)). The base of microgroove lens was much dimmer than the one of flat roof. It may suggest that the light was trapped in the microgroove lens. Hence, it is worth investigating how the microgroove lens affects the photovoltaic performance of solar cell.
Fig. 6 The topography of TF microgroove lens solar cell. (a) The photo of microgroove lens solar cell, (b) the measured 3D microgroove lens surface and (c) the microgroove lens profile.
3.2 Measurement
To qualify the photovoltaic characteristics of the solar cell, the IV-testing measurement was carried out using a solar simulator (Sun 3000 Class AAA, Abet Technologies) in a standard test condition (STC, 25°C, AM 1.5g) with different incident angles and light illumination intensities.
In measurement, an angle-adjusting gauge was placed on the testing platform of the solar simulator. The incident angle α is defined as the angle between the incidence light and the normal direction of solar cell. The desired incident angle was attained by controlling the gauge angle. Therefore, the I-V characteristics of solar cell could be measured at different incident angle α. Thus, if the incidence light slanted onto the cell surface with the light intensity of I0, the actual light intensity I1 should be I1 = I0cosα.
In order to evaluate the electricity generation ability of the microgroove lens structured solar cells, which were then applied to solar device, the testing setups of electricity charge-discharge were conducted by using No.51 microcontrollers. In this experiment, ten pieces of the original TF solar cells were chosen as the samples. They were divided into two groups. One group was fabricated with same dimensions of microgroove lens and the other group was prepared with traditional surface. During the experiment, each solar device was responsible for charging two storage batteries under the same outdoor condition. (Fig. 7(a)). The electricity discharge time of storage battery was introduced to evaluate the electricity generation ability of the solar device (Fig. 7(b)).
Fig. 7 The experimental setups of testing the electricity generation ability: (a) outdoor charging and (b) discharging.
4. Results and discussion
Figure 8 shows the simulation of light efficiency versus visible light wavelength of microgroove lens in depth of 300-1000 µm. The light efficiency ƞ is defined as the ratio of received flux to the emitted flux. It is shown that an increase in light wavelength led to a slight increase in light efficiency at the normal incidence (Fig. 8(a)). Besides, the microgroove lens increased the light efficiency by more than 3% compared with traditional surface for all visible light wavelengths. It may be explained by that the reflective light was trapped and once again absorbed. Although the microgroove lens with a large depth increased light efficiency, the largest efficiency was obtained for the microgroove depth of 800 µm. Hence, the microgroove lens with the depth of 800 µm may be the optimal one to confine the light scattering. It is further verified by the practical test results of photovoltaic characters as shown in Fig. 11.
Fig. 8 The simulation light efficiency of the glass substrate in different microgroove depth d. (a) Light efficiency ƞ versus visible light wavelength λ and (b) light efficiency ƞ versus incident angle α.
Figure 8(b) shows the light efficiency versus incident angle for microgroove lens in depth of 600-800 µm for white light illumination. It is shown that the light efficiency decreased with increasing incident angle. The light efficiency of microgroove lens surface was also averagely 3% larger than the one of traditional surface. And the increase was greater for incident angle α≥50°. Besides, the microgroove lens in depth of 800 μm showed the highest light efficiency at α<50°, but the one with depth of 600 μm showed the highest at α>50°. It may be because the microgroove lens surface enlarged actual lighting area for oblique illumination in contrast to traditional surface.
Figure 9(a) shows the photovoltaic effects of the solar cell before and after patterned with the microgroove lens at different incident angle. The light irradiance intensity is 1000 W/m2. In the case of solar cells with microgroove lens, the I-V characteristic was slightly poorer than the traditional one for normal incidence (α = 0°), but it was much better for incident angle α = 45° and α = 80°. Two possible reasons may explain: first, the photovoltaic efficiency of the solar cell has reached its photocurrent generation limit for α = 0° at one solar illumination. Thus, enhancing light intensity may be of no avail. Second, for an incident angle α>0°, the microgroove lens decreased the light reflection loss, thus leading to an improvement of I-V characteristics. In fact, the short currents have gained 13.5% and 80% for incident angle of α = 45° and α = 80°, respectively. Even though the microgroove lens was not polished, its I-V characteristic was better than the traditional surface in the case of α = 45° and α = 80°, just slightly poorer than the polished microgroove lens.
Fig. 9 Effects of the solar cell before and after patterned with the microgroove lens. (a) I-V characteristics at different incident angle α and (b) LED glowing effects (See Media 1 and Media 2).
Figure 9(b) shows the LED glowing effect of the microgroove lens solar cell in contrast to traditional one. In experiment, the solar cells were linked with the same LEDs and were exposed to the only lighting source: the 30 lumen incandescent light. It is seen that the LED lighted by the microgroove lens solar cell was brighter than the one lighted by the traditional one. It is implied that the solar cell with microgroove lens may generate more electricity than the traditional solar cell at dim illumination.
Figure 10 shows the experimental conversion efficiency Ef as a function of incident angle α for microgroove lens solar cell at the light irradiance intensities of 1000W/m2, 750W/m2 and 500W/m2, respectively. It is shown that the conversion efficiency Ef decreased with the increasing incident angle α. It is because, on one hand, the oblique incidence light was partly reflected, on the other hand, the actual light intensity was decreased according to the function of I1 = I0cosα.
Fig. 10 Conversion efficiency Ef as a function of incident angle α for microgroove lens solar cell at the light intensities of (a) 1000W/m2, (b) 750W/m2 and (c) 500W/m2.
Under one solar illumination conditions, the conversion efficiency of the microgroove lens solar cell was slightly less than the traditional one for incident angle α<45°, but it was obviously improved as for incident angle α>50°, especially on the condition that the illumination is below one solar (Fig. 10(b) and Fig. 10(c)). In fact, for the all measured incident angles, the average gains of Ef were more than 1%, 6% and 17% at the light intensities of 1000W/m2, 750W/m2 and 500W/m2, respectively. It is because the microgroove lens solar cell produces an enhanced I-V photovoltaic characteristic (Fig. 9(a)). As a result, the microgroove lens may improve the conversion ability under low light intensity illumination.
Fig. 11 The photovoltaic characters of solar cell versus the incident angle and groove depths at the light illumination intensity of 500W/m2.
Figure 11 shows the photovoltaic characteristics of solar cell versus the incident angle and groove depth at the light illumination intensity of 500W/m2. It is shown that the short current ISC decreased with the increasing light incidence (Fig. 11(a)). It is because the actual light intensity decreases as the incident angle increases. Even though the ISC of microgroove lens with the groove depth of 700 μm was less than the ones with depths of 600 μm and 800 μm, it was still higher than the ISC of the traditional solar cell. Actually, the ISC has averagely gained 14.6%, 12% and 8% for groove depth of 800 μm, 700 μm, 600 μm, respectively, compared with the traditional one. It is because the microgroove lens may increase the percentage of photos reaching the absorber layer thereby increasing the photocurrent.
As for open voltage, it decreased with increasing incident angles. When the VOC of microgroove lens is contrasted with the one of traditional, the distinction is tiny at lower incident angle α<80° (Fig. 11(b)). It is because the patterning of microgroove lens may have little effects on the VOC. It is identical with the result reported in Ref [19].
On the contrary, the fill factor FF increased with the increasing incident angle (Fig. 11(c)). It showed quite different trend with VOC and ISC. It may be the reason of decreasing VOC and decreasing ISC resulting in the increase of FF. The fill factors of microgroove lens increased more than 25%, 16.7% and 8.3% for microgrooved depth of 800 μm, 600 μm and 700 μm, respectively. The FF even increased from 47% to 61% at the incident angle close to 90°. It suggests that the microgroove lens could optimize the photovoltaic properties of TF solar cell at low light illumination.
Figure 11(d) shows that the conversion efficiency Ef of microgroove lens was larger than the traditional surface at all measured incident angles. It could be explained as the gains of ISC and FF. The microgroove lens in depth of 800μm showed the highest efficiency at α<50°, but the one with depth of 600 μm showed the highest at α>50°. The simulations of light efficiency exhibit similar results as shown in Fig. 8(a). It indicated that the incident angle may influence the microgroove lens to perform its effects on photovoltaic conversion efficiency.
In conclusion, compared with the traditional solar cell, the short circuit current of the microgroove lens solar cell increased by about 16.2% without sacrificing open circuit voltage or fill factor at low light intensity. On the contrary, the fill factor and the conversion efficiency were gained by over 25% and 38%, respectively, at the groove depth of 800 μm. Thus, the patterning of microgroove lens with micrometric scale at the glass of TF solar cell is an efficient alternative to minimize reflection loss at the cell entrance.
5. Electricity generation capacity
Due to the geometric peculiarities of the microgroove lens, it is important to ensure that the optical gain achieved at weak-light intensity doesn’t vanish in practical application related to electricity generation capacity. For this purpose, the electric charge-discharge experiment was conducted under weak daylight and strong daylight illuminations. In experiments, the sunny, cloudy and overcast days were chosen, respectively. The details were shown in Table 3.
Table 3. Electricity charge-discharge results
From the table, it is seen that the electricity discharging lasted the longest in the sunny weather and the shortest in overcast. It means that the weather condition has great influence on the solar cell performance, especially for traditional solar cell whose discharge time was decreased by 60% in the overcast day. Nevertheless, the microgroove lens only decreased by about 25%. As for different weather conditions, the discharge time of microgroove lens has gained about 51%, 118% and 185% at sunny, cloudy and overcast days, respectively, compared with the traditional. Thus, it is further verified that the TF solar cell with microgroove lens proves better performance at weak light illumination when associates with the practical application.
6. Conclusion
The microgroove lens arrays with the depth of 500-800 µm were presented to improve micro-optic characteristics. They may be micro-machined by a grinding at the front of the glass substrate for the thin film solar cell to absorb inclined light and scatter light. The photovoltaic characteristics of micro-optic lens array structured solar cell may be greatly improved at low light intensity illumination. It enhances electricity generation much more greatly against traditional plane under weak daylight illumination than under strong daylight.
- The light efficiency of microgroove lens surface was averagely 3% larger than the one of traditional surface at all measured light incidence. And the increase was greater for incident angle larger than 50°. Besides, the microgroove lens in depth of 800 μm showed the highest light efficiency for incident angle less than 50°, but the one with depth of 600 μm showed the highest for incidence larger than 50°.
- At one solar illumination, the photovoltaic characteristics of the microgroove lens TF solar cell was slightly poorer than the traditional solar cell in the case of normal incidence, but it was much better than the traditional one in the case of oblique incidence.
- The conversion efficiency decreased with the increasing incident angle. At the all measured incident angles, the average gains of conversion efficiency of the microgroove lens TF solar cell were more than 1%, 6% and 17% at the light intensities of 1000W/m2, 750W/m2 and 500W/m2, respectively, compared with the traditional solar cell.
- In the case of light illumination intensity less than 500W/m2, microgroove lens with depth of 800 µm has improved to be the optimum structure, which improved the conversion efficiency of microgroove lens about 38%, and the short current and fill factor increased about 16% and 25%, respectively, without sacrificing open voltage. Thus, the microgroove lens surface may absorb the scattered light for electricity generation in low light intensity.
- When the microgroove lens surface is applied to a solar device, it may improve the electricity generation ability by 51%, 115% and 185% compared with the traditional surface during sunny, cloudy and overcast days, respectively. The microgroove lens structures produce stronger electricity generation ability than the traditional surface.
Acknowledgments
This project was supported by the National Natural Science Foundation of China (Grant No. 61475046).
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