# Outdoor Characterization of Solar Cells With Microstructured Antireflective Coating in a Concentrator Photovoltaic Monomodule

Arnaud Joel Kinfack Leoga, Arnaud Ritou, Mathieu Blanchard, Lysandre Dirand, Yanis Prunier, Philippe St-Pierre, David Chuet, Philippe-Olivier Provost, Maïté Volatier, Vincent Aimez, et al.

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## ► To cite this version:

Arnaud Joel Kinfack Leoga, Arnaud Ritou, Mathieu Blanchard, Lysandre Dirand, Yanis Prunier, et al. Outdoor Characterization of Solar Cells With Microstructured Antireflective Coating in a Concentrator Photovoltaic Monomodule. IEEE Journal of Photovoltaics, 2023, 10.1109/JPHO-TOV.2023.3295498. hal-04179111

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Outdoor Characterization of Solar Cells
with Micro-structured Anti-Reflective
Coating in a Concentrator Photovoltaic
Monomodule

Arnaud J. K. Leoga<fnref n="1" />,<fnref n="2" />, Arnaud Ritou<fnref n="1" />,<fnref n="2" />,<fnref n="3" />, Mathieu Blanchard<fnref n="1" />,<fnref n="2" />, Lysandre Dirand<fnref n="1" />,<fnref n="2" />, Yanis Prunier<fnref n="1" />,<fnref n="2" />, Philippe St-Pierre<fnref n="1" />,<fnref n="2" />, David Chuet<fnref n="1" />,<fnref n="2" />, Philippe-Olivier Provost<fnref n="1" />,<fnref n="2" />, Maïté Volatier<fnref n="1" />,<fnref n="2" />, Vincent Aimez<fnref n="1" />,<fnref n="2" />, Gwenaëlle Hamon<fnref n="1" />,<fnref n="2" />, Abdelatif Jaouad<fnref n="1" />,<fnref n="2" />, Christian Dubuc<fnref n="3" />, Maxime Darnon<fnref n="1" />,<fnref n="2" />

**Abstract**— Micro-structured anti reflective coatings (ARC) have been identified as a promising solution to reduce optical losses in Concentrator Photovoltaics modules (CPV). We fabricated and tested in field a CPV modules made of 4 monomodules with a concentration factor of 250x, that embed either solar cells with micro-structured encapsulating ARC or solar cells with multilayer ARC as a reference. The micro-structured encapsulating ARC was made of semi-buried silica beads in polydimethylsiloxane (PDMS). The module was in operation for 1 year in the severe climatic conditions of Sherbrooke, Quebec, Canada, before extracting the monomodules performance. Despite a sub-optimal module design, we report a monomodule efficiency of 29.7% at 900 W/m<fnref n="2" /> for a cell with micro-structured encapsulating ARC. This proves the potential of micro-structured encapsulating ARC to enable high performance CPV systems.

**Index Terms**— anti-reflective coating, concentrator photovoltaics, CPV, outdoor characterization,

I. Introduction

Concentrator photovoltaics (CPV) is the photovoltaic
technology with the highest conversion efficiency at the
system level. It relies on multi-junction solar cells that were
originally developed for space application. To mitigate the
high cost of multi-junction solar cells, concentrator
photovoltaics uses optical elements (mirrors or lenses) to
collect the sunlight and concentrate it onto a smaller area solar
cell [1]. Current commercial CPV technologies reach more
than 30% conversion efficiency at the system level, thanks to

few mm<fnref n="2" /> solar cells and Fresnel lenses with an optical concentration of typically 500×[2]. Because of the presence of multiple optical interfaces, optical efficiency losses due to reflection cannot be neglected. To reduce reflective losses at the surface of the triple junction solar cells, interferometric multi-layer anti-reflective coatings (ARC) such as TiO₂/AlOₓ [3] or SiN/SiO₂ [4] are deposited on the surface of the solar cells, with controlled thickness and optical index. These layers can eventually play the role of encapsulating layers to prevent interaction of the cell with moisture and improve the system reliability. However, such interferometric approach is designed for normal incidence, and is less effective when sunlight reaches the cell surface with an angular distribution, especially when angles are larger than 45° [5], as it is the case in some CPV systems. An alternative approach consists in micro/nano structuring the antireflective coating to provide a graded optical index on the cell [6,7]. Using indoor characterizations, we have shown that the short circuit current produced by an InGaP/InGaAs/Ge solar cell can be improved by 2.6 ±1%, by using 1 µm-diameter silica beads embedded into a Poly Dimethyl Siloxane (PDMS) layer—so-called micro-beads ARC [8], in comparison with a AlOₓ/TiOₓ/SiO₂ encapsulating antireflective coating.

We propose here to evaluate micro-beads ARC in real operating conditions. We built a dedicated CPV module with four 250× spherical lenses [9]. Two solar cells with micro-beads ARC fabricated using the process described in [8] and two reference solar cells with AlOₓ/TiOₓ/SiO₂ encapsulating ARC are integrated into the module. Each cell is independently characterized. The module has been installed for one year on an experimental sun tracker and exposed to harsh operating conditions in the solar park of Université de Sherbrooke, Quebec, Canada (including -30°C nights during winter and +30°C during summer).

II. EXPERIMENTAL SECTIONS

A. Solar cells

The solar cells are commercial triple junction solar cells
from AZUR SPACE with an efficiency rated at 37.8% at
1000× (AM1.5D – 1000W/m<fnref n="2" />). The top electrode design was

<footnote n="1">¹Institut Interdisciplinaire d'Innovation Technologique (3IT), Université de Sherbrooke, 3000 Boulevard Université, Sherbrooke, J1K 0A5 Québec, Canada</footnote>

<footnote n="2">²Laboratoire Nanotechnologies Nanosystèmes (LN2) - CNRS IRL-3463, Institut Interdisciplinaire d'Innovation Technologique (3IT), Université de Sherbrooke, 3000 Boulevard Université, Sherbrooke, J1K 0A5 Québec, Canada</footnote>

<footnote n="3">³Saint-Augustin Canada Electric Inc., Innovation and development of solar product, 75 rue d'Anvers, Saint-Augustin, Quebec Canada, G3A 1S5</footnote>

<footnote>Corresponding author: [maxime.darnon@usherbrooke.ca](mailto:maxime.darnon@usherbrooke.ca)</footnote>
<figure><img src="image_3.png" /><figcaption>Figure 1: Pictures of the module installed on the EKO tracker.</figcaption></figure>

optimized for operation under 500× concentration. The solar cells are hexagonal with an active area (excluding busbars) of 6.55 mm². The cells are coated with a bilayer AlOₓ/TiOₓ ARC optimized for minimizing reflection when used with an SiO₂ encapsulating layer. Two kinds of encapsulating layers are considered. As a reference, a 100 ±30 nm-thick layer of SiO₂ is deposited by atmospheric plasma enhanced chemical vapor deposition. The reference cells are bonded with a conductive epoxy onto an aluminum plate, and the front electrical contact is connected by gold wirebonding. This reference corresponds to encapsulation and assembly from commercial products. The second kind of encapsulating layer (micro-beads) is made of 1 µm-diameter silica beads semi-buried into a 6 ±0.2 µm-thick PDMS layer (Sylgard 184). Details on the fabrication methods and indoor characteristics of these cells are described in [8]. These cells with micro-beads are mounted on an Aluminum core printed circuit board (PCB) and the front contact is connected by gold wirebonding.

## B. Test module and tracker

The test module is made of two thick aluminum plates—one to support the lenses and one to support the cells—separated by aluminum posts. The lenses are plano-convex BK7 lenses from Thorlabs (LA1145) [10] glued using PDMS. They have a focal distance of 74.8 mm, a diameter of 2 inches, and a clear aperture of 16.42 cm². The lens is not coated by an ARC. Considering the active area of the solar cells, the optical concentration of the module is 250×. The cells on receiver are fixed onto micrometric plates for finetuning the cell-to-lens alignment. Each cell is individually connected to the electrical characterization setup. The module is made of 4 lens-cell couples, referred to as monomodules in the following. Two monomodules (A and B) embed solar cells with micro-beads while two other monomodules (C and D) embed reference cells. Once the test module is mounted onto the tracker, iterative corrections of the cell's positions combined with short circuit current measurements are performed to maximize the short circuit current of each monomodule, and therefore optimize the cell-to-lens alignment. Afterward, the module is closed and kept water-tight during operation. Figure 1 shows pictures of the 250× module.

The test module is installed onto a STR-22G tracker from

EKO [11]. This tracker has a tracking accuracy of ±0.01°. For the first year of operation, the tracker was operating in astronomical mode (open-loop tracking). However, this tracking mode induces tracking errors that depend on the hour and day of operation. To minimize the tracking errors, we implemented a new tracking algorithm called MCT (Maximum Current Tracking). It consists in measuring the short circuit current in 16 positions (4 x 4 resolution) of 2.25° range in azimuth and elevation centered on the astronomical position of the sun. From these measurements, we interpolate up to 100 positions (10 x 10 resolution)—this short circuit current map yielding the maximum short circuit current and the associated best position. While the sky is cloudless for a long enough period of time, the tracker then moves to this exact position before data acquisition. This tracking procedure is repeated for each monomodule before each measurement. Since the tracking procedure takes around 10s per monomodule, we consider the offset between the astronomical sun position and its real position to be constant over the 10s alignment and measurement time.

Based on the cell alignment procedure and the MCT tracking algorithm, we can assume a perfect alignment of the cell, the lens, and the sun.

## C. Instrumentation

The electrical characterization of the monomodules is performed using a 2601B SMU and a 3706A multiplexer from Keithley. Between each measurement, the solar cells are maintained in open circuit since the experimental setup does not enable continuous polarization of each individual cell. The SMU, the multiplexer and the tracker are driven using a Raspberry Pi with a dedicated controlling software written in Python. A Razon+ weather station from Kipp'n Zonen installed ~200 m away from the test site is used to provide meteorological data (i.e. DNI and temperature).

## D. Methodology

For all characterizations, we adhered to the recommendations from Muller et al. [12] for data filtering for characterization under standard conditions: ambient temperature between 10°C and 30°C, DNI larger than 750 W.m⁻², diffuse irradiance below 140 W/m², DNI deviation lower than 2% within 5 minutes, wind speed lower than 5 m/s, air mass below 2. However, due to the limitations of our experimental setup, specifically the unavailability of spectral content, we were unable to apply filters for precipitable water vapor or correct for cell or lens temperature. These recommendations aim to bring us as close as possible to Concentrator Standard Operating Conditions (CSOC) considering the constraints of our experimental setup.

To determine the I-V characteristics of each monomodule. The open-circuit voltage (Voc) is measured first. Then, we sweep the cell voltage between 0V and Voc. Eight points are linearly measured between 0V and 70% of Voc, then 16 points are measured between 70% and 95% of Voc, and 8 points between 95% of Voc and Voc. Since it takes around 10 seconds per cell for tracker positioning and cell characterization, each cell is characterized every 3 minutes 16 seconds. From the I-V characteristics, we extract the short circuit current, the fill factor, and the maximum power point.
<table><caption>TABLE 1
CHARACTERISTICS OF CPV MONOMODULES OBTAINED FROM THE CHARACTERIZATIONS CARRIED FROM MID-AUGUST TO MID-SEPTEMBER 2022.</caption><thead><tr><th>Monomodule</th><th>Type</th><th>J<sub>SC</sub> (mA/cm<sup>2</sup>)<br>from EQE<br>@Cell level @AM1.5D</th><th>I<sub>SC</sub> (mA)<br>@Module level<br>@900W.m<sup>-2</sup></th><th>Effective<br>concentration</th><th>Short circuit<br>current CTM</th><th>Pmax (mW)<br>@Module level<br>@900W.m<sup>-2</sup></th><th>Efficiency<br>(%)</th></tr></thead><tbody><tr><td>A</td><td>Micro-beads</td><td>13.24</td><td>196</td><td>226</td><td>90</td><td>439</td><td>29.7</td></tr><tr><td>B</td><td>Micro-beads</td><td>13.20</td><td>192</td><td>222</td><td>89</td><td>425</td><td>28.8</td></tr><tr><td>C</td><td>Reference</td><td>12.87</td><td>172</td><td>204</td><td>82</td><td>369</td><td>25</td></tr><tr><td>D</td><td>Reference</td><td>12.75</td><td>144</td><td>172</td><td>69</td><td>217</td><td>14.7</td></tr></tbody></table>

### III. RESULTS AND DISCUSSIONS

The test module has been in operation in open loop tracking mode in the solar park of Université de Sherbrooke from Summer 2021 to Fall 2022. The MCT mode has been implemented in mid-august 2022. Due to the data filtering, only data acquired until mid-September 2022 can be used for determining the test module performance, and the module has been dismounted on September 14th, 2022.

Figure 2.a. presents the evolution of the short circuit current (Isc) of the four monomodules as a function of the DNI. As expected, we observe a linear variation of the short circuit current with the DNI for the four monomodules. Based on the measured Isc and the current density (Jsc) derived from indoor EQE measurement before integrating the cells into the module, we can estimate the cell-to-module coefficient (CTM) on the short circuit current. For this, we calculate the ratio between the measured current at 900 W.m⁻² and the short circuit current calculated from EQE measurement, knowing the cell active area (0.0655 cm²). This ratio gives an effective sun concentration which should be as close as possible to the lens optical concentration ratio of 250x. The short-circuit current CTM ratio is then defined by the ratio of the effective concentration to 250x. Table 1 presents the current density derived from EQE, the current measured at the monomodule level for an irradiance of 900 W.m⁻², the effective concentration and the short circuit current CTM ratio for each monomodule. For all devices but device D, the CTM is between 82 and 90% which is close to record monomodules presented in the literature [13]. Device D has a CTM of 69%, which will be discussed later. Due to this low performance, we use only device C as a reference in the following. The CTM losses are attributed to optical losses at the lens (transmission losses through the BK7 glass, reflection losses, comatic aberrations, etc...), optical losses by reflection of off-normal light on the cell, and to the larger cell temperature. The very high value we measure for the short circuit current CTM confirms that the cell and tracker alignment procedure enable a quasi-perfect cell/lens/sun alignment.

Monomodules A and B with micro-beads encapsulating ARC present a short circuit current of 196 and 192 mA at 900 W.m⁻², 12 to 14% larger than the Isc measured on reference monomodule C (169 mA). This relates also in a larger CTM (89 to 90% compared to 82% for the reference). This improvement exceeds the improvement of 2.6% measured in laboratory condition [8]. This can be explained by a wider angular distribution of the sunlight on the cell into a module compared to indoor conditions. Indeed, the gain of the micro-beads ARC compared to interferometric ARC increases

when the angle of incidence deviates from the normal of the cell. In addition, solar spectrum received by the cell into a module may be different from AM1.5D, due to absorptions in the lens and due to atmospheric variations, which could enhance the sensitivity to the current of the top or middle-junction of the cells. The cell temperature in the module is also larger than in laboratory conditions which also contributes to increase the short circuit current when the top cell is limiting the current. Finally, we cannot rule out a faster

<figure><img src="image_4.png" /><figcaption>Figure 2: (a) Short-circuit current, (b) Maximum power point and (c) fill factor as a function of DNI of 250x CPV monomodules after data filtering according to ASTM E2527 method described by the IEC 62670-3 standard (mid-August to mid-September 2022).</figcaption></figure>
degradation of the reference cells compared to the micro-beads ARC, which would lead to a higher short circuit current after one year of operation for the cells with micro-beads encapsulating ARC. However, preliminary data measured when the module was installed and the tracking mode was in open-loop showed also an improvement of the monomodule with micro-beads compared to the reference [10].

Figure 2.b. presents the maximum power produced by the four monomodules as a function of the DNI. As for the short circuit current, a linear variation of maximum power point (Pmax) as a function of DNI is observed. The maximum power point at 900 W/m² is 439 and 425 mW corresponding to a monomodule efficiency of 29.7 and 28.8% for monomodules A and B with micro-beads, respectively. This power is 15 to 19% larger than the maximum power measured on reference monomodule C (369 mW, efficiency of 25%). The power and efficiency are reported in Table 1.

The improvement for monomodule A and B performance compared to monomodule C is mostly due to the increase of the short circuit current, as explained in the previous part. An additional improvement is attributed to a larger fill factor (3.7% and 2.4% fill factor increase for monomodules A and B, respectively) since the V<sub>OC</sub> is almost identical for all three monomodules (2.85 V ±0.01V). The slightly lower V<sub>OC</sub> and fill factor of monomodule C can be explained by a different assembly method between reference cells and cells with micro-beads encapsulating ARC, that may lead to different electrical and thermal resistances. It is therefore not attributed to the difference in anti-reflective coating.

The evolution of the fill factor with the DNI is presented
Figure 2.c. We can see that the evolution of the fill factor with
DNI is similar for monomodules A, B and C, with a slope of -
1.7.10⁻⁴ %/W.m² ±0.1%. On the contrary, monomodule D has
a dramatically low fill factor (<55% at 900W/m²) with a slope
of -5.4 %/W.m⁻². This is indicative of a defective device
associated with a strong increase of the series resistance. A
visual inspection of cell D (not shown here) when the module
was dismounted showed surface residues and degradation of
the wire bonding pad that confirm the device defectivity.

IV. CONCLUSIONS

We have integrated solar cells with a graded anti reflective
coating (micro-beads semi buried into a PDMS layer) into a
test module and rated the performance of the monomodules
after one year in the field. Their performance was compared
with monomodules made with reference cells with
AlOx/TiOx/SiO2 encapsulating ARC. Individual alignment of
the solar cells and tracker positioning using maximum current
tracking enable a quasi-perfect cell / lens / sun alignment.
Electrical characterization of the monomodules showed that
the solar cells with the micro-beads ARC have 12 to 14%
larger short circuit current than the reference, exceeding the
expectations of 2.6% determined by indoor EQE measurement
of the cells. We believe this improvement comes from the
broad angular distribution of light inside the module,
compared to normal light incidence during EQE
measurements, and from an actual spectrum received by the
cell that differs from AM1.5D. A monomodule efficiency of
29.7 and 28.8% is reported for the monomodules with micro-

beads ARC. This is 3.8 to 4.7% absolute better than the
reference, mostly explained by a better light collection and to
a lower extent by a different cell assembly method.

This paper demonstrates therefore that integrating a graded ARC, for instance using semi-buried silica micro-beads in PDMS, enables a significant efficiency gain for CPV modules compared to conventional multi-layer ARC.

ACKNOWLEDGMENT

We acknowledge the support from STACE, MITACS and Quebec Ministère de l'Économie et de l'Innovation for their financial support. LN2 is a joint International Research Laboratory (IRL 3463) funded and co-operated in Canada by Université de Sherbrooke (UdeS) and in France by CNRS as well as ECL, INSA Lyon, and Université Grenoble Alpes (UGA). It is also supported by the Fonds de Recherche du Québec Nature et Technologie (FRQNT).

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