Four-terminal tandem organic solar cell achieves 16.94% PCE

Researchers and team members of the SOREC2 European Project, Francisco Bernal-Texca and Professor Jordi Martorell, have fabricated a four-terminal tandem organic solar cell that achieved a 16.94% power conversion efficiency (PCE). Central to this development was the fabrication of an ultrathin transparent silver electrode, a component that played a pivotal role in optimising the performance of the tandem solar cell.

Two-terminal organic solar cells (OSCs) represent a promising approach to address the transmission and thermalisation losses in single-junction solar cells. These organic solar cells consist of front and rear subcells with varying bandgaps, enabling broader absorption and use of the solar spectrum. However, achieving optimal performance in such configurations requires a sufficient current balance between the two subcells. Moreover, fabricating this type of tandem OSC is challenging because they need a robust interconnection layer capable of facilitating efficient charge recombination while maintaining high transparency.

To fabricate the new device, the researchers first explored the organic materials destined for the photoactive layer of both cells. They examined the effectiveness of three blends for the front cell, which is designed to harvest the high-energy photons. The blend that performed the best, named PM6:L8-BO, was selected. For the back opaque cell, the researchers used the PTB7-Th:O6T-4F blend, with a narrow bandgap, which makes it better suited to absorb the infrared 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 conventional inverse problem-solving methodology to find the final configuration of the solar device.

The fabrication of an ultra-thin transparent silver electrode with a thickness of 7 nm was critical; this element was placed at the back of the front cell, ensuring a good light transmission to power the back cell. Conventional top Ag electrodes utilised for transparent solar cell applications typically range in thickness from 9 to 15 nm.

The electrode was then stacked with three dielectric layers alternating tungsten trioxide (WO₃) and lithium fluoride (LiF). This photonic multilayer structure was positioned between the two cells to facilitate efficient and uniform light distribution. According to researchers, this structure exhibits a high transmission in the 750–1000 nm range and a high reflectivity in the 500–700 nm range.

Francisco Bernal, first author of the study, said the development of a transparent silver intermediate electrode is crucial for the efficient operation of the solar cell. It must be transparent enough to allow light to reach the back cell while maintaining high electrical conductivity to ensure the optimal performance of the front cell. “Being able to fabricate an electrode of only 7 nm without observing losses in the front transparent cells is a significant advancement in the field of transparent cells,” Bernal said.

The researchers tested the device’s photovoltaic performance under 1 sun of illumination with a solar simulator and measured its quantum efficiency. This revealed that the device can achieve a 16.94% power conversion efficiency. The research findings could have potential applications in photoelectochemical cells (PEC); according to Martorell, the methodology for the design and implementation of the four-terminal tandem structure could also be applied to design new systems where an adequate distribution of light in the elements is crucial for the performance of a certain device.

By optimising the methodology and design strategies, the researchers aim to unlock the full potential of these devices in harnessing solar energy for diverse energy conversion processes, such as CO₂ conversion and valorisation.

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