Researchers at ICFO have fabricated a brand new four-terminal organic solar cell with a tandem configuration with a 16.94% power conversion efficiency (PCE). The brand new device consists by a highly transparent front cell that comes with a transparent ultrathin silver (Ag) electrode of only 7nm, which ensures its efficient operation.
Two-terminal tandem organic solar cells (OSCs) represent some of the promising approaches to handle the transmission and thermalization losses in single-junction solar cells. These organic solar cells consist of front and rear subcells with various bandgaps, enabling broader absorption and use of the solar spectrum. Nevertheless, achieving optimal performance in such configurations demands a sufficient current balance between the 2 subcells. Furthermore, fabricating tandem organic solar cells of those types are difficult because they need a strong interconnection layer able to facilitating efficient charge recombination while maintaining high transparency.
The four-terminal tandem configuration has emerged as a highly efficient alternative strategy in solar cell design. Unlike the two-terminal approach, this configuration features separate electrical connections for the transparent front cell and the opaque back cell. Consequently, the difficulty of electrical current matching is not any longer a limiting factor. This setup enables greater flexibility in choosing the bandgaps of every cell of the tandem, thereby optimizing photon absorption and enhancing the general efficiency of solar energy production.
Now, in a brand new study published within the Journal Solar RRL, ICFO researchers Francisco Bernal-Texca, and Prof. Jordi Martorell describe the fabrication of a four-terminal tandem organic solar cell that has achieved a 16.94% power conversion efficiency (PCE). Central to this achievement is the fabrication of an ultrathin transparent silver electrode, a critical component that played a pivotal role in optimizing the performance of the tandem solar cell.
To fabricate the brand new device, the researchers first explored the organic materials destined for the photoactive layer of each cells. They examined the effectiveness of three distinct blends for the front cell, which is designed to reap the high-energy photons. The mix that performed the perfect, named PM6:L8-BO, was finally chosen. For the back opaque cell, the researchers decided to make use of the PTB7-Th:O6T-4F mix, with a narrow bandgap, which makes it higher suited to soak up the infrared a part of the spectrum (low-energy photons).
After selecting the blends, the researchers used a numerical approach to design the four-tandem OSC’s final structure. They used the matrix formalism combined with the standard inverse problem-solving methodology to seek out the optimal performance and the ultimate configuration of the solar device.
The fabrication of an ultra-thin transparent silver electrode with a thickness of only 7nm was the important thing ingredient in the present research. This element was placed in the back of the front cell, ensuring a superb light transmission to power the back cell. Conventional top Ag electrodes utilized for transparent solar cell applications typically range in thickness from 9 to fifteen nm.
Its production demanded meticulous control of laboratory conditions to make sure precision and consistency. The electrode was then stacked with three dielectric layers alternating tungsten trioxide (WO3) and lithium fluoride (LiF). This photonic multilayer structure has an important role, since it is positioned between the 2 cells to facilitate efficient and uniform light distribution. “This structure exhibits a high transmission within the 750-1000 nm range and a high reflectivity within the 500-700 nm range,” researchers wrote.
“The event of a transparent silver intermediate electrode is crucial for the efficient operation of the solar cell. It must present a fragile balance, being transparent enough to permit light to succeed in the back cell while maintaining high electrical conductivity to make sure the optimal performance of the front cell,” said Francisco Bernal, ICFO researcher and first creator of the study. “Having the ability to fabricate an electrode of only 7nm without observing losses within the front transparent cells is a big advancement in the sector of transparent cells.”
The researchers tested the photovoltaic performance of the device under 1 sun of illumination with a solar simulator and measured its quantum efficiency. The device achieved a 16,94% of power conversion efficiency which, so far, can be the best reached for a four-terminal tandem organic cell. The authors of the study remark that the present official record in efficiency for organic tandem devices is 14,2% and that the last reported PCE for 4-terminal organic tandems is 6.5% .
“Our research holds potential applications in photoelectochemical cells (PEC), addressing crucial electrical requirements comparable to providing the mandatory voltage to surpass established for driving water splitting or CO2 reduction reactions like in SOREC2 project,” explains Prof. Jordi Martorell, researcher at ICFO and SOREC2 project coordinator. “The methodology for the design and implementation of the four-terminal tandem structure might be applied to design news systems where an adequate distribution of sunshine in the weather is crucial for the performance of a certain device.”
The researchers are currently directing their focus towards refining, tuning and enhancing the methodology and structural design tailored for applications comparable to solar fuels, where tandem devices hold widespread applicability. By optimizing the methodology and design strategies, researchers aim to unlock the total potential of those devices in harnessing solar energy for diverse and sustainable energy conversion processes, comparable to CO2 conversion and valorization.
