Researchers from the University of North Carolina at Chapel Hill have reported bifacial minimodules with front efficiency comparable to opaque monofacial counterparts, while gaining additional energy from albedo light. Their new design strategy could help to improve the efficiency and stability of bifacial perovskite solar cells.
The scientists added a hydrophobic additive to the hole transport layer to protect the perovskite films from moisture. They also integrated silica nanoparticles with proper size and spacing in perovskite films to recover the absorption loss induced by the absence of reflective metal electrodes. The small-area single-junction bifacial perovskite cells achieved a power-generation density of 26.4 mW cm−2 under 1 sun illumination and an albedo of 0.2. The bifacial minimodules showed front efficiency of over 20% and bifaciality of 74.3% and thus a power-generation density of over 23 mW cm−2 at an albedo of 0.2. The bifacial minimodule retained 97% of its initial efficiency after light soaking under 1 sun for over 6,000 hours at 60 ± 5 °C.
By simultaneously harvesting both direct sunlight and albedo light, bifacial solar cells could improve the energy yield of solar technologies. So far, however, researchers only developed a few bifacial perovskite cells and modules, and existing ones exhibited far lower efficiencies than their mono-facial counterparts. The researchers' goal was to demonstrate perovskite bifacial minimodule with high energy yield and long operational lifetime. They developed a novel module structure and rear electrode, adding hydrophobic additives for improved moisture stability, and enhancing long-wavelength light absorption with embedded dielectric nanoparticles.
The team first designed a bifacial structure in which individual sub-cells are connected by indium tin oxide (ITO), and with silver grids that are separated by an optimal space on the rear ITO electrode to reduce resistance loss (i.e., the loss of electrical energy that can occur when a current flows inside wires). This design is straightforward and scalable, which means that it could enable the large-scale fabrication and commercialization of perovskite bifacial mini-modules.
According to the team, they also found that adding tris(pentafluorophenyl)borane (TPFB) as an additive in the hole transport layer alleviated the damage of moisture to the perovskite film during the process of SnO2 deposition. Also, the addition of TPFB decreased resistivity of hole transport layer and enhanced the energy alignment. Finally, the researchers introduced nanoparticles (NPs) into the perovskite to scatter the incident sunlight, thereby increasing the optical path to overcome their absorption loss particularly in long wavelength range in bifacial modules.
Using their proposed design, the researchers developed small area bifacial perovskite solar cells that achieved results that are significantly better than the power generation densities exhibited by both previously developed perovskite single-junction solar cells and minimodules.
Remarkably, the prototype minimodule developed by the team was also found to achieve a remarkably operational stability, only losing 3% of its initial efficiency after working for more than 6,000 hours. Combined, the design strategies introduced by this team of researchers could thus pave the way towards the large-scale fabrication of highly efficient and stable solar energy solutions based on bifacial perovskite structures.
This work affirms the promising potential of bifacial structures, the team stated, saying they would now like to identify new strategies to continually improve the energy yield and stability of bifacial perovskite solar modules.