Optimizing Flexible Organic/Silicon Tandem Solar Cells for High Efficiency

By Muhammad OsamaReviewed by Laura ThomsonJul 2 2024

In a recent article published in the journal Crystals, researchers explored the potential of flexible organic/silicon (organic/Si) tandem solar cells (TSCs) as a sustainable and efficient alternative to conventional solar energy technologies. They used technology computer-aided design (TCAD) simulations to investigate the design and optimization of these novel TSCs. Their goal was to overcome the limitations of single-junction solar devices and achieve higher power conversion efficiencies (PCEs).

Background

Solar energy is a crucial renewable resource capable of meeting global energy needs sustainably. However, single-junction solar devices face a theoretical efficiency barrier known as the Shockley-Queisser limit. To overcome this, TSCs have emerged as a promising solution. TSCs improve solar energy conversion efficiency by stacking multiple photoactive materials with complementary bandgaps, enabling broader spectrum absorption compared to single-junction cells.

Due to their beneficial properties, organic materials and silicon (Si) are excellent for tandem cells. Organic materials offer flexibility and adjustable bandgaps, while Si provides high-efficiency and well-established manufacturing processes. Flexible organic solar cells (OSCs) and thin-film Si solar cells are vital for developing efficient and flexible photovoltaic devices. These are ideal for applications requiring lightweight and adaptable power sources, such as wearable electronics.

About the Research

In this paper, the authors investigated a flexible two-terminal (2-T) organic/Si TSC, where an OSC is the front cell and a Si cell is the rear cell. These materials were chosen because of their non-toxicity, complementary bandgap properties, and cost-effectiveness. The OSC, with a bandgap of 1.78 eV, absorbs higher-energy photons, while the Si cell, with a bandgap of 1.12 eV, absorbs lower-energy photons efficiently. The study aimed to evaluate the performance of this tandem configuration and identify optimization strategies to enhance its efficiency.

The researchers conducted the simulations using the Atlas device simulator from Silvaco TCAD. Atlas models the behavior of solar cells by solving fundamental semiconductor transport equations, including carrier continuity and Poisson's equations. The simulations integrated various physical models and material parameters for organic and Si cells, including recombination models such as Shockley-Read-Hall (SRH) and Auger models.

Research Findings

Simulation results showed that the front OSC and rear Si cell achieved a PCE of 11.11% and 22.69%, respectively. These outcomes established a baseline for evaluating the integrated TSC's performance.

The authors further assessed the performance of 2-T organic/Si TSC, initially achieving a PCE of 20.03%. To enhance efficiency, they explored several optimization strategies. First, they removed the organic hole transport layer (HTL) and adjusted the front contact work function, significantly increasing the PCE to 23.27%. They also reduced the defect density in the organic absorber layer, further boosting the PCE to 23.27%.

Finally, the thicknesses of both the top and rear absorber layers were optimized. It showed that a combination of a 250 nm thick front organic layer and a 70 μm thick n-type Si layer achieved the highest PCE of 27.60%. This optimized tandem cell configuration resulted in an improved PCE of 27.60%, an open-circuit voltage (VOC) of 1.81 V, and a short-circuit current density (JSC) of 19.28 mA/cm², marking a significant enhancement over the initial 20.03% PCE of the unoptimized tandem cell.

Applications

The simulated results highlight the significant potential of the organic/Si TSC design for various implications, particularly in wearable electronics and flexible solar panels. The technology's flexibility, environmental friendliness, and high efficiency make it especially appealing for emerging applications.

The TSC's flexible nature allows it to seamlessly integrate into wearable electronic devices that demand adaptable and lightweight power sources. Its high efficiency and light weight make it an ideal candidate for portable solar power systems. Using non-toxic and abundant organic/Si TSC materials supports the development of sustainable and eco-friendly photovoltaic panels.

Conclusion

In summary, the simulations demonstrated the feasibility of flexible organic/Si TSCs as a promising alternative to conventional solar energy technologies. Through effective optimization, the tandem cell design achieved remarkable PCE improvements. This achievement offered valuable insights for future organic/Si tandem cell development, paving the way for practical applications in several fields.

The study underscored the potential impact of merging Si technology with low-cost printed organic photovoltaic technologies, advancing industrial and societal applications within the renewable energy sector.

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