Study Reveals 4-Terminal Tandem Solar Cells Can Surpass Efficiency Limits

By Muhammad OsamaReviewed by Laura ThomsonMay 31 2024

In a recent article published in the journal Scientific Reports, researchers investigated the performance and optimization of 4-terminal organic/silicon tandem solar cells using numerical simulations and experimental measurements. They explored the potential of tandem solar cells to surpass the Shockley-Queisser efficiency limit of single-junction solar cells.

Background

Tandem solar cells have gained significant attention in recent years due to their potential to achieve high power conversion efficiency. These cells consist of multiple sub-cells with different bandgaps, allowing them to harness a broader range of the solar spectrum.

Despite their advantages, such as ease of processing, affordability, flexibility, and lightweight nature, organic solar cells typically exhibit lower efficiency than their silicon counterparts. Strategies to improve light absorption in organic solar cells include designing multi-layer tandem structures and incorporating mixtures of organic and inorganic materials (ternary cells) to capture a broader range of the solar spectrum.

About the Research

The authors aimed to optimize 4-terminal tandem solar cells by employing numerical simulations to assess the impact of varied physical parameters on efficiency. They primarily focused on three types of tandem solar cells: organic/organic, perovskite/silicon, and perovskite/organic.

These solar cells were selected due to their complementary absorption spectra, which offer the potential for enhanced light harvesting across the entire solar spectrum. The solar cell capacitance simulator one-dimensional (SCAPS-1D) software, a robust tool for numerical simulations, was utilized to analyze the performance of these 4-terminal tandem solar cells.

The study examined two different top-cell configurations: one with a poly 3-hexylthiophene (P3HT) organic active layer and the other with a cesium (Cs), silver (Ag), bismuth (Bi), antimony (Sb), and bromine (Br) atoms (Cs₂AgBi₀.₇₅Sb₀.₂₅Br₆) perovskite active layer. For the P3HT top cell, the impact of the active layer's thickness on the performance was thoroughly investigated.

Simulations were conducted with three different thicknesses: 50 nm, 100 nm, and 150 nm. To confirm the accuracy of the results, the simulation data for the 200 nm thick P3HT active layer was compared with experimental data, which showed a strong agreement.

Next, the researchers analyzed the transmission spectrum of the top cells, which was subsequently used as the input spectrum for the bottom cells. They simulated the performance of four distinct bottom-cell materials: polycrystalline silicon (P-Si), PBDB-T, PCPDTBT, and cesium tin iodide (CsSnI₃). Their performance under the filtered spectrum from the top cell was evaluated using the parameters and absorption data for each bottom-cell material.

The performance of the perovskite top cell was simulated with a 400 nm thick Cs₂AgBi₀.₇₅Sb₀.₂₅Br₆ active layer, using parameters and data obtained from relevant literature. The transmission spectrum of the perovskite top cell was then assessed and used as the input for the bottom cell simulations.

Research Findings

The outcomes demonstrated that the 4-terminal tandem solar cells exhibited significant efficiency improvements compared to their single-junction counterparts. The organic/organic tandem solar cell achieved an efficiency of 11.2%, which was 37% higher than the best single-junction organic solar cell.

The perovskite/silicon tandem solar cell reached an efficiency of 25.6%, 28% higher than the best single-junction silicon cell. The perovskite/organic tandem solar cell attained an efficiency of 18.4%, which was 46% higher than the best single-junction perovskite cell. Furthermore, the 4-terminal tandem structure with a silicon bottom cell displayed the highest efficiency, with the P3HT top cell reaching approximately 25.86% and the Cs₂AgBi₀.₇₅Sb₀.₂₅Br₆ perovskite top cell achieving 35.43%.

The top cell active layer's thickness increased, and the bottom cells' current density decreased due to the reduced light transmission. However, the thicker active layer improved the top cell's current density and efficiency. The bottom cells' open-circuit voltage and fill factor remained constant, while the top cell's fill factor decreased due to increased recombination. The peak efficiencies of the methylammonium lead iodide (MAPbI₃) and CsSnI₃ perovskite bottom cells were 10.68% and 9.68%, respectively, contributing to the overall efficiency of the 4-terminal tandem structure.

This research has significant implications for developing high-performance and commercially practical photovoltaic devices. The optimized tandem structures, with their ability to capture a broader range of the solar spectrum, offer a promising pathway to overcome the limitations of single-junction solar cells.

Conclusion

The paper thoroughly analyzed 4-terminal organic/silicon tandem solar cells, showcasing their potential to achieve high power conversion efficiencies beyond the Shockley-Queisser limit of single-junction solar cells. Its findings offered insights into the complex physics of tandem solar cells, contributing to the advancement of more efficient and cost-effective solar energy technologies.

The researchers suggested that future work optimize tandem structures by exploring new materials with enhanced properties, improving sub-cell interfaces, and developing advanced fabrication techniques. They also suggested prioritizing the experimental validation of simulation results to confirm the accuracy and reliability of the research outcomes. They emphasized the importance of assessing tandem solar cells' stability and long-term performance to identify and overcome any potential obstacles related to durability and commercialization.

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