Development and outdoor characterization of a hybrid bifacial HCPV module

Conversion of direct, diffuse, and albedo irradiance into electricity is demonstrated with a new kind of hybrid bifacial high‐concentration photovoltaic module named bifacial EyeCon. It consists of Fresnel lenses that concentrate the direct sunlight 321x onto III‐V triple‐junction solar cells that are mounted on the front surface of p‐PERC bifacial c‐Si cells. Thus, the Si absorbs the front and rear diffuse irradiance. Because III‐V and Si cells are electrically isolated (hence a 4‐terminal device) but thermally coupled by a dielectric adhesive, Si also acts as a heat distributing substrate. To accommodate the concentrator cells, we adapted the metallization layout and also optimized it for low intensity, ie, 200 W/m2 on the front and 100 W/m2 on the rear, using finite element network simulation. Additionally, when the concentrator cells are mounted on a bifacial Si cell instead of a metal heat distributor, their operating temperature is 16 K higher. However, we demonstrate with outdoor measurements that the power output of the bifacial EyeCon module reaches up to 326 W/m2 when the direct to global irradiance ratio is 92%. At a lower fraction of 70% the bifacial Si cells augment the power output of the III‐V string by 19%rel

For instance, Lee et al 6 reported a hybrid micro-CPV module with triple-junction (3J) and interdigitated-back-contact (IBC) c-Si solar cells mounted on a dual-axis tracker (A=2640 cm 2 , C geo =1000x). The module converted 30.5% of the global normal irradiance (GNI) because the Si cells boosted CPV power output by 3.5% rel when the DNI/GNI was 92%. Askins et al 7 presented another hybrid CPV module (A=1000 cm 2 , C geo =180x) that reached an efficiency of 24.5% when the DNI/GNI was 83%. In this case, the c-Si cells increased the 3J CPV string power generation by 6.3% rel .
At the submodule level, Yamada et al 8 applied 3J cells, PMMA primary optics, reflective secondary elements, and a c-Si solar cell in a hybrid CPV prototype (A=16 cm 2 , C geo =100x) that reached an efficiency of 29.4% when DNI/GNI was 81%. After optimization to improve diffuse light transmission, the efficiency was raised to 30.7%. Additionally, the improved hybrid submodule generates 10% more power if the c-Si cell is operated bifacially instead of only mono-facially 9 . In the same size category, we built a hybrid CPV submodule (A=144 cm 2 , C geo =226x), which we named EyeCon 10 , using silicone-on-glass (SoG) Fresnel lenses and 4J solar cells mechanically stacked on the surface of an IBC c-Si cell that also acts as the heat distributor. We showed that at a high DNI/GNI of 90%, efficiencies as high as 36.8% are possible, and that at an average DNI/GNI of 57%, the Si cell increases the CPV output by up to 30.6% rel .
These examples demonstrate that it is possible to harvest over 30% of GNI under real outdoor operating conditions using commercially available solar cells. On top of this, the hybrid monofacial energy yield could be improved between 10% and 30%, depending on the diffuse component, when bifacial Si cells are applied 9 .
In the following, we present the design, development, and characterization steps followed to upscale and transform our hybrid monofacial CPV submodule into an enhanced hybrid bifacial HCPV module (A=1088 cm 2 , C geo =321x). It is important to note that we followed the manufacturing processes of the FLATCON ® module 11 , a proven CPV technology capable of converting up to 36.7% of DNI 12 .   Table 1. Typical values achieved in our screen-printing process were used for the height and width of front and rear fingers. Another advantage of this design is that the cell can be measured with the same chuck used for commercially available 4BB cells. Further optimization was performed by fixing the front pitch at 2 mm (previous optimum) and varying the rear pitch (blue circles). A significant improvement from 18.2 to 18.8% is observed when the rear pitch increases from 2 to 3 mm. Therefore, we increased the rear pitch to 3 mm and varied the front one to confirm that 2 mm is indeed the optimum (red circles). Finally, a variation of the BB width shows that 0.7 mm was already the optimum; however, its impact is nearly insignificant (green circles).

| Metallization layout optimization of bifacial Si cell
Once the metallization layout was defined, we manufactured 87 cells in the 2BB configuration with a width of 0.7 mm, a 2 mm front pitch, and a 3 mm rear pitch. Batch processing and characterization were done at the Fraunhofer ISE PV-TEC. The cells were measured monofacially on front and rear at 200 W/m 2 . Table 2 provides a summary of their I-V characteristics and bifaciality factors, where the latter are calculated as the ratio of the rear over the front value.
Assuming superposition, the weighted average efficiency for 200 W/m 2 on the front and 100 W/m 2 on the rear is 17.7%.
Notably, this value is considerably lower than the best bifacial one-  In the present bifacial EyeCon design, the concentration ratio  Finally, to test the validity of our simulations, we assembled the cell stack shown at the bottom of Figure 3. The assembly consists of a glass base and one bifacial Si cell with six chip resistors (1.5 Ω) glued to its surface using the same dielectric adhesive as used in the hybrid CPV module. An electric power of 1.6 W/resistor was applied in order to emulate concentrated irradiance (321 × 900 W/m 2 ). In the experiment, we measured by contact probe a steady-state temperature on the resistor surface of 81.9 C (magenta circle). This measurement supports our modelling results since it agrees within 1 K with its equivalent simulation (Sim2) at a lens aperture of 22.7 cm 2 .

| Electrical interconnection of bifacial Si circuit
The

| Mechanical stacking of 3J on bifacial Si solar cells
Mounting

| Electrical interconnection of CPV 3J circuit
Interconnection of the mounted 3J solar cells was done with ultrasonic heavy-wire bonding (500-μm Al wire). Along the short dimension of the baseplate, four cells within a row were strung in parallel and then these 12 strings were interconnected in series. As described in Steiner et al 17 , the interconnection scheme was chosen to minimize power losses due to inhomogeneous current generation and temperature distribution within the module. Figure 4 shows a picture of the heavy-wire bonding process on the nearly finished EyeCon baseplate.

| Module assembly
Once the 3J and Si interconnections were electrically tested, the

| Acceptance angle of the hybrid HCPV module
It is well known that the tolerance to tracking misalignment of a CPV module mainly depends on the design of the primary optics and the size of the receiver. Typically, the half acceptance angle (α 1/2 ) of a CPV module is defined as the misalignment angle where the power output drops to 90% of its maximum capacity.
In order to obtain α 1/2 of our hybrid module, we deliberately misaligned the tracker and measured the maximum power output of the CPV and bifacial Si cells. The plots in Figure 5 show

| Concentrated sunlight effect on bifacial Si cells
The misalignment of the concentrator lens to the sun results in  where the annual DNI/GNI ratio falls below 80%.

| SUMMARY AND CONCLUSION
A hybrid bifacial HCPV module using metamorphic 3J solar cells Open access funding enabled and organized by Projekt DEAL.