Exceptionally omnidirectional broadband light harvesting scheme for multi-junction concentrator solar cells achieved via ZnO nanoneedles

Polarization-insensitive broadband omni-directional anti-reflection in ZnO nanoneedle array for efficient solar energy harvesting

Broadband omni-directional anti-reflection characteristics have been an important issue because they can maximize the optical absorption in photovoltaic devices. Here, we investigate the optical properties of ZnO nanoneedle arrays to demonstrate broadband anti-reflection, omni-directionality, and polarization insensitivity using optical simulations and experimental approaches. The results of this work clarify that the ZnO nanoneedle array plays an important role as a broadband anti-reflection layer due to its spatially graded refractive index, omni-directionality and polarization insensitivity. To take advantage of these structures, we prepared a ZnO nanoneedle array on the surface of conventional SiN _x /planar Si solar cells to prove the broadband omni-directional anti-reflection for solar energy harvesting. Current density-voltage results show that SiN _x /planar Si solar cells with ZnO nanoneedle arrays lead to a nearly 20% increase in power conversion efficiency compared to SiN _x /planar Si solar cells, and a 9.3% enhancement in external quantum efficiency is obtained under identical conditions. Moreover, the photocurrent results of SiN _x /planar Si solar cells with ZnO nanoneedle arrays clearly demonstrate the incident angle- and polarization-insensitive characteristics compared to those of typical SiN _x /planar Si solar cells. Our results demonstrate the optical multi-functionality of ZnO nanoneedle arrays and pave the way for high-performance optoelectronic devices that require broadband omni-directional anti-reflection and polarization insensitivity.

Efficiency Enhancement of GaAs Single-Junction Solar Cell by Nanotextured Window Layer

In order to improve efficiency of flexible III-V semiconductor multi-junction solar cells, it is important to enhance the current density for efficiency improvement and to attain an even efficiency of solar cells on a curved surface. In this study, the nanotextured InAlP window layer of a GaAs single-junction solar cell was employed to suppress reflectance in broad range. The nanotextured surface affects the reflectance suppression with the broad spectrum of wavelength, which causes it to increase the current density and efficiency of the GaAs single-junction solar cell and alleviate the efficiency drop at the high incident angle of the light source. Those results show the potential of the effectively suppressed reflectance of multi-junction solar cells and even performance of solar cells attached on a curved surface.

Performance comparison of III-V//Si and III-V//InGaAs multi-junction solar cells fabricated by the combination of mechanical stacking and wire bonding

The integration of III–V and Si multi-junction solar cells as photovoltaic devices has been studied in order to achieve high photovoltaic conversion efficiency. However, large differences in the coefficients of thermal expansion and the lattice parameters of GaAs, Si, and InGaAs have made it difficult to obtain high-efficiency solar cells grown as epilayers on Si and InP substrates. In this paper, two types of devices, including GaInP/GaAs stacked on Si (GaInP/GaAs//Si) and GaInP/GaAs stacked on InGaAs (GaInP/GaAs//InGaAs), are fabricated via mechanical stacking and wire bonding technologies. Mechanically stacked GaInP/GaAs//Si and GaInP/GaAs//InGaAs triple-junction solar cells are prepared via glue bonding. Current-voltage measurements of the two samples are made at room temperature. The short-circuit current densities of the GaInP/GaAs//Si and GaInP/GaAs//InGaAs solar cells are 13.37 and 13.66 mA/cm², while the open-circuit voltages of these two samples are measured to be 2.71 and 2.52 V, respectively. After bonding the GaInP/GaAs dual-junction with the Si and InGaAs solar cells, the conversion efficiency is relatively improved by 32.6% and 30.9%, respectively, compared to the efficiency of the GaInP/GaAs dual-junction solar cell alone. This study demonstrates the high potential of combining mechanical stacked with wire bonding and ITO films to achieve high conversion efficiency in solar cells with three or more junctions.