Improving Roll-to-Roll Slot-die Coated Perovskite Solar Cell Efficiency with PEDOT:PSS-PTAA Coatings: Insights from XPS Mapping
A Smarter Way to Build Flexible Solar Cells
Perovskite solar cells (PSCs) are quickly becoming a standout technology in the push for more efficient and affordable solar power. These devices are lightweight, flexible, and capable of delivering high power conversion efficiencies. Their compatibility with low-cost manufacturing techniques makes them ideal candidates for large-scale renewable energy deployment. Despite their advantages, a major obstacle remains. PSCs manufactured using roll-to-roll (R2R) techniques often perform worse than those made in lab settings.
A recent study titled “Roll-to-roll slot-die coating of PTAA with PEDOT:PSS buffer layer for perovskite solar cells: coating analysis by XPS mapping” aims to solve this challenge. Led by a team of researchers from Swansea University, the study explores a new way to improve flexible PSCs by rethinking how their hole transport layers (HTLs) are constructed.
The HTL plays a key role in moving electrical charge through the solar cell. A material called PEDOT:PSS is often used because it is easy to apply using scalable processes. However, PEDOT:PSS has downsides, such as instability and the potential to reduce efficiency over time. Another material, PTAA, offers better performance and stability but is difficult to coat evenly on rough flexible surfaces.
Rather than choosing one over the other, the researchers decided to combine the two. By using PEDOT:PSS as a buffer layer beneath PTAA, they created a bilayer structure that addresses the shortcomings of each material. This approach not only improved efficiency, but it also proved compatible with scalable R2R manufacturing. The result is a solar cell design that bridges the gap between lab performance and real-world production.
Key Highlights
New bilayer HTL strategy improves efficiency of flexible perovskite solar cells using roll-to-roll coating
Power conversion efficiency increases from 12.6% to 15.2% when a PEDOT:PSS buffer layer is added
XPS mapping and AFM confirm improved surface coverage and film morphology
Reduced non-radiative recombination thanks to better interface management
Compatible with scalable slot-die coating methods, enabling industrial relevance
Minimal impact on crystal morphology, preserving perovskite film quality
Strong potential for commercial-scale production of flexible solar technologies
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Background: The Materials and the Problem
Flexible solar cells are often built on substrates like PET-ITO, which offer flexibility and conductivity. However, these substrates have rough surfaces that make it difficult to apply ultra-thin functional layers like PTAA. Incomplete or uneven coatings can lead to exposed areas of the conductive surface, which increases energy loss through recombination and ultimately reduces efficiency.
PEDOT:PSS, while far from ideal as a standalone HTL, forms smooth films that can cover the rough texture of PET-ITO. It acts as a good base layer, allowing subsequent materials like PTAA to be applied more effectively. PTAA itself is an efficient and stable HTL with excellent compatibility with perovskite materials, but its low conductivity and poor coating behavior on uneven surfaces limit its use in flexible formats.
The idea behind the study was simple but effective. Use PEDOT:PSS to smooth and protect the substrate surface, then deposit PTAA on top to perform the role of hole transport. This would combine the practical strengths of both materials in a single, scalable solution.
Methodology: How the Bilayer Was Made and Analyzed
The research team created three types of devices for comparison. One used PTAA alone as the HTL. Another used PEDOT:PSS alone. The third used a bilayer structure, with PEDOT:PSS applied first and PTAA coated on top. All layers were deposited using slot-die coating, a method that mimics roll-to-roll processing and is suited for large-area manufacturing.
To evaluate the effectiveness of each configuration, the researchers turned to advanced characterization methods. X-ray photoelectron spectroscopy (XPS) mapping was used to determine how well each material covered the PET-ITO surface. Atomic force microscopy (AFM) provided detailed images of the surface roughness and film morphology. Additional techniques such as photoluminescence (PL), electroluminescence (EL), scanning electron microscopy (SEM), and external quantum efficiency (EQE) were used to assess how well the devices transported charge and how efficiently they converted light into electricity.
How to test solar cells using a source measure unit and solar simulator.
Results: Improved Performance with the PEDOT:PSS-PTAA Bilayer
The data showed a clear advantage for the bilayer structure. Devices with PTAA alone struggled to cover the rough PET-ITO surface completely. XPS mapping revealed detectable signals from indium, a component of ITO, indicating exposed areas of the conductive substrate. This led to increased recombination and reduced power conversion efficiency.
When a PEDOT:PSS buffer layer was added before PTAA, the coverage improved dramatically. Indium signals were nearly eliminated, suggesting that the ITO surface was well protected. AFM images confirmed that PTAA applied on top of PEDOT:PSS resulted in a smoother and more uniform surface. This provided a better foundation for the perovskite layer and helped reduce defects.
Optical and electrical testing supported these findings. PL measurements showed better charge extraction in the bilayer devices, while EL testing indicated lower levels of non-radiative recombination. Voc measurements under varying light intensities revealed a lower ideality factor for the bilayer devices, indicating fewer losses due to defect states.
In terms of raw performance, the bilayer devices reached a power conversion efficiency of 15.2 percent. This was a significant improvement over the 12.6 percent achieved by devices using only PTAA. SEM imaging showed that the crystal structure of the perovskite layer was not negatively affected by the addition of the buffer layer. This confirmed that the bilayer approach does not interfere with the formation of high-quality perovskite films.
Discussion: Manufacturing Meets Materials Science
This study offers a practical solution to a real-world challenge in solar technology. The combination of PEDOT:PSS and PTAA into a bilayer HTL structure allows for more effective charge transport, better coverage, and improved efficiency. Just as important, the method works with roll-to-roll slot-die coating techniques, which are already used in commercial-scale production lines.
Rather than focusing only on optimizing materials in isolation, the research shows the value of engineering interfaces between layers. By smoothing the surface and protecting the substrate, PEDOT:PSS enables PTAA to perform as an efficient HTL, even in the challenging context of flexible PSCs.
This kind of layer-by-layer thinking will be essential as solar technologies move from the lab into everyday use. It is no longer enough to chase the highest possible efficiency in a controlled setting. Real impact will come from designs that work reliably in scalable, cost-effective manufacturing environments.
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Conclusion: A Step Toward Commercial Solar Solutions
The work presented in this study is more than a technical upgrade. It represents a step forward in bringing flexible perovskite solar cells closer to commercial reality. By combining the processability of PEDOT:PSS with the high performance of PTAA, the researchers have created a design that solves both material and manufacturing challenges.
This approach delivers higher efficiency, better stability, and compatibility with industrial roll-to-roll coating processes. The results also open the door to further innovations in interface design, multilayer architecture, and long-term durability studies. As flexible solar technology continues to evolve, solutions like the one demonstrated here will be essential for turning promise into widespread adoption.
Authors
Dr. Rahul Patidar
Dr. James McGettrick
Dr. Rodrigo Garcia-Rodriguez
Dr. Chris Griffiths
Kathryn Lacey
Dr. Ershad Parvazian
Dr. David Beynon
Prof. Matthew Davies
Prof. Trystan Watson
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