1. Weight problem of solar cells in HAPS

(Click here for an explanation of HAPS)
One of SoftBank's advanced initiatives is the development of High Altitude Platform Station (HAPS), a "flying base station" that flies in the stratosphere at an altitude of 20 km. Since there are almost no environmental changes in the stratosphere, the energy (demand) required to fly there is always constant. On the other hand, the energy (supply) from solar cells depends on solar radiation, which changes with the season and location.

(Click here for an explanation of the HAPS energy balance)
Since HAPS, which flies without landing on the ground, can only work with the energy generated itself, solar cells are placed in all parts of the aircraft. For large aircraft, the number of solar cell modules can be several thousands, so even a slight increase in the weight of a single solar cell module can have a significant impact on flight performance. For large aircraft, an increase in weight of 100 g/㎡ reduces the flightable latitude range by nearly 2 degrees of latitude, so reducing the weight of solar cell modules has become an extremely important issue.

2. Comparison of general solar cells and solar cells for HAPS

Typical solar cells installed on the ground or roofs are protected by glass or metal frames and weigh 11 to 17 kg/㎡. Lightweight modules with thinner glass weigh 6.5 to 8.0 kg/㎡, and ultra-lightweight flexible modules with glass replaced by resin and no frame weigh around 3.0 to 4.7 kg/㎡. On the other hand, the weight required for HAPS is 0.3 to 0.6 kg/㎡. The weight must be further reduced by one order of magnitude from the ultra-lightweight module for ground use. (Table 1)

Table 1. Weight comparison of solar cell modules

For this reason, SoftBank's solar cell modules development project for HAPS is developing with weight reduction in particular as one of its goals. The initial module weight target for the project was set at less than 700 g/㎡. This 700 g/㎡ value is based on the fact that ultra-lightweight crystalline silicon modules used in solar car racing and other events weigh around 800 to 1000 g/㎡, so the first target was to reduce the weight below that level.

3. Design and development points to achieve goals

Table 2 shows the results of module weight simulation. As a result of trial calculations using various materials and thicknesses, the weight was estimated to reach 700 g/㎡ when the cell thickness is 80 µm, the encapsulant material thickness is 150 µm, and the protective material thickness is 50 µm.

Table 2. Weight estimation of lightweight module (g/㎡)

Selection of solar cell

The lightest and most efficient solar cell currently available is the compound solar cell for space applications, but it is expensive and takes a long time to manufacture. For this reason, we selected the high-efficiency crystalline silicon cells, which are available at about 1/1000 the price. With the cooperation of the solar cell manufacturer, we aimed to develop the thinnest, lightest, and most efficient cell among the practical size cells currently available.

Selection of components

Ultra-thin, weather-resistant resin was used as the protective material on the front and back surfaces, and wiring components as thin as possible and ultra-thin encapsulants were used for the cell-to-cell direct connections and the laminations. Since the operating environment, weight requirements, and cost requirements are very different from those of solar cells for terrestrial application, we considered a wide range of selections, including high-performance components that are not normally used.

4. High technology to realize ultra-light weight and high efficiency

The solar cells were developed by LONGi (Head office: Xi'an, China), the world's largest solar cell manufacturer, and the modularization technology was developed in cooperation with Fujipream Corporation(Head office: Himeji City, Hyogo Prefecture), a manufacturer of lightweight solar cell modules.

Figure 1. Appearance of ultra-thin SHJ (Silicon Hetero Junction) cell

Ultra-thin SHJ (Silicon Hetero Junction) cell

The cell used in this project is an ultra-lightweight SHJ (Silicon Hetero Junction) cell of 182mm x 91mm with a cell thickness of 80µm (about half the thickness of a cell installed for the ground generally) developed by LONGi (Figure 1). LONGi achieved the world's highest cell efficiency of 26.81% for silicon solar cells in November 2022. The 80 µm-thick cell used in this project employs the same technology as the world's highest efficiency cell, and despite its thinness and size, it achieves an efficiency of more than 25%. Generally, as silicon solar cells become thinner, the amount of light absorbed by the solar cell decreases and the efficiency decreases, but this trade-off has been overcome with advanced technology, making this one of the best products available today.

Development of ultra-lightweight modularization technology

Fujipream Corporation. has been developing and producing complex large-screen flat displays and lightweight modules for terrestrial applications using the precise lamination technology that is the company's forte. This time, we took advantage of this technology and knowledge to develop the technology for ultra-lightweight modules for HAPS.
Thin copper wire with a diameter of 250 µm was selected for the cell-to-cell direct connections, which were connected using low-melting-point solder (Figure 2). Since the manufacturing equipment was not compatible with the cells and wires used in this project, the process was performed manually by skilled technicians.

Figure 2. Connection of ultrafine wires with low-melting-point solder

For the thickness of components other than cells, 25 µm was used for the surface protective material, 150 µm for the encapsulant, which is about 1/4 the thickness of general ground products, and 50 µm for the back surface protective material (Figure 3). The total thickness of 175 µm for the surface protective material and encapsulant is thinner than the 250 µm diameter of wiring wires, so in order to laminate without gaps, it is necessary to perfectly follow the unevenness created by the wires and other materials. In particular, even the slightest deviation in the conditions of the 25 µm surface protection material will cause wrinkling and peeling as shown in Figure 4, which is fatal damage to the ultra-thin 80 µm cells. We focused on finding conditions that would allow lamination without gaps under these difficult conditions.

Figure 3. Protective material and encapsulant lamination

Figure 4. Wrinkled module

5. Weight and efficiency

Figure 5. Appearance of ultra-lightweight solar module

Figure 5 is a photograph of the completed ultra-light solar module. The module measures 563 mm * 584mm and weighs 218.5 g. From this, a unit weight of 665 g/㎡ can be obtained, achieving the project target of less than 700 g/㎡. This lightness can be evaluated as the world's top class in silicon solar cell modules.

In addition, by optimizing the layout, temperature, etc. during modularization, other factors have achieved a uniform, high-quality finish with no wrinkles or peeling.

Furthermore, EL (Electro Luminescence) measurement was performed to check for invisible damage, and as shown in Figure 6, the result confirmed that there were no cracks in the cells or peeling of the wiring.

As for module performance, the maximum output power (Pm) of 71.1 W was obtained in I-V measurements under AM 1.5 (Figure 7), as a result, the effective module efficiency of 22.2% was obtained for the effective area (563 * 568mm²: cell spacing 2 mm, edge distance 6 mm). As an ultra-lightweight silicon solar cell module, in addition to achieving the weight target of less than 700g/㎡, the module achieved the world's top-class efficiency.

Figure 6. EL measurement results

Figure 7. I-V measurement test results (AM1.5)

6. Future prospects

In this research and development, thanks to the cooperation and contribution making full use of the world's highest SHJ (Silicon Hetero Junction) cell technology by LONGi and the advanced lamination technology by Fujipream Corporation, we were able to achieve a module weight of 665 g/㎡, which far exceeds the target module weight of 700 g/㎡ set for this development project. However, the ultimate goal of this project is less than 500 g/㎡. In order to achieve this goal, we will continue to develop modules with the aim of making each component of the module even thinner and higher-performance, as well as improving wiring and laminating technology.

Writer：Kohei Okada