As the world races to combat climate change, solar energy has emerged as one of the most promising solutions. Solar energy is one of the most promising solutions to the world’s growing energy demands. Among the many solar technologies being explored, perovskite solar cells have gained attention for their efficiency and low manufacturing costs. However, a major challenge stands in the way: they degrade too quickly, limiting their commercial use. Scientists are now working to make these solar cells more stable while maintaining high performance.
A recent study has focused on how stabilising additives influence perovskite solar cells. By monitoring how electrons move within these cells, researchers aim to improve their efficiency and durability. This research could bring perovskite solar panels one step closer to real-world application.
Perovskite solar cells face a critical problem: instability. They last less than a year before breaking down. To tackle this, scientists are adding stabilising molecules to the perovskite layer to cover defects and improve performance.
Hannu P. Pasanen
The challenge of stability in perovskite solar cells
Perovskite solar cells work by absorbing sunlight and converting it into electricity. At their core, they contain a thin layer of perovskite material sandwiched between two charge-selective layers. One layer extracts electrons to generate current, while the other replenishes the electrons back into the perovskite.
Despite their advantages, these solar cells face a critical problem instability. Most perovskite materials last less than a year before breaking down, making them unsuitable for widespread use. To tackle this, scientists are adding stabilising molecules to the perovskite layer to cover defects and improve performance. Think of the perovskite layer as a busy highway. Without proper maintenance (passivation), potholes (defects) can slow down traffic (electrons), causing delays (energy loss). But how do these additives affect the way electrons move inside the solar cell? Understanding this is key to designing more efficient solar panels.
The need for durable and high-performing solar cells is growing as renewable energy sources become more integrated into national energy grids. In many regions, solar farms depend on traditional silicon-based cells, which are reliable but costly. Perovskite technology presents a more affordable alternative, but only if its stability issues are resolved. Ongoing research in laboratories worldwide is focused on bridging this gap, ensuring that perovskite cells can withstand environmental conditions without compromising efficiency.
Tracking electrons at lightning speed
Electrons move incredibly fast within solar cells, making it difficult to track their journey. To study these movements, researchers use ultrafast time-resolved spectroscopy. This technique involves sending rapid laser pulses into the material and measuring how electrons react within a trillionth of a second.
By analysing how electrons travel between layers, scientists can identify weak spots that cause energy loss. If electrons lose energy before reaching the charge-collecting layer, they release heat instead of producing electricity. Understanding these processes helps in designing materials that minimise energy loss and boost efficiency.
New advancements in spectroscopy allow researchers to observe electron behaviour with greater accuracy than ever before. By refining these techniques, scientists can detect previously unnoticed inefficiencies and develop more precise methods for improving electron transport. These insights are crucial for creating perovskite solar cells that not only generate power efficiently but also sustain performance over extended periods.
The role of surface passivation
One strategy to improve perovskite solar cells is surface passivation. This involves applying a thin coating over the perovskite layer to reduce defects and improve electron flow. A common material used for collecting electrons in perovskite solar cells is C60, but its interface with perovskite is imperfect, leading to energy losses.
A promising approach is to use organic molecules that form an ultra-thin 2D perovskite layer. This layer sits on top of the bulk perovskite and creates a more stable pathway for electrons to travel. By reducing defects at the interface, surface passivation can significantly improve solar cell performance.
In addition to improving electron flow, surface passivation helps enhance the structural integrity of perovskite solar cells. The protective layers shield the perovskite from moisture and oxygen, both of which contribute to rapid degradation. This means that a well-designed passivation layer not only optimises electrical performance but also extends the overall lifespan of the solar cell.
Comparing different passivation methods
In the study, researchers tested five passivating molecules, of which ethylenediammonium iodide (EDAI) and octylammonium iodide (OAI) were found to be best and worst performing molecules, respectively.
Results showed that EDAI-treated solar cells achieved an average efficiency of 19.6 percent, while OAI-treated devices reached 15.5 percent. The unpassivated control achieved 16.5 percent. These differences in efficiency were linked to how electrons moved through each material.
Time-resolved spectroscopy revealed that while OAI improved the lifespan of excited electrons in the perovskite layer, it slowed down their transfer to C60. In contrast, EDAI allowed electrons to move faster, reducing charge transfer time to 65 nanoseconds compared to 95 nanoseconds in the unpassivated cell. OAI, on the other hand, extended this time to 133 nanoseconds, making it less effective for improving overall efficiency.
Why some passivates work better than others
The study also highlighted an issue with OAI: its molecules tended to clump together, creating uneven coverage. This inconsistency led to variations in performance across different areas of the solar cell. Even with better preparation techniques, OAI struggled to retain electrons efficiently.
One of the key findings was that charge retention, the ability to keep electrons from slipping back into the perovskite layer, was much higher with EDAI. The charge retention time for unpassivated cells was 400 nanoseconds, while OAI only slightly improved this to 500 nanoseconds. EDAI, however, doubled this time to 900 nanoseconds, making it the best performer in the study.
A surprising discovery: Passivation and electron extraction in one
Beyond improving stability, the research uncovered an unexpected finding. EDAI did not just act as a passivation layer, it also showed potential to replace C60 as the electron extraction layer. The 2D perovskite layer formed by EDAI was able to collect and retain electrons, even without the presence of C60.
This is significant because C60, while commonly used in perovskite solar cells, has its own stability issues, including the risk of delamination. If a single material like EDAI could serve both as a passivate and an electron extraction layer, it could simplify the design of solar cells and improve their long-term stability.
The future of perovskite solar cells
This research brings scientists closer to overcoming the instability issues that have held perovskite solar cells back from large-scale use. By fine-tuning surface passivation techniques, researchers can create solar cells that not only last longer but also operate with greater efficiency.
Further studies will explore whether other 2D perovskite materials can provide similar benefits. If materials like EDAI continue to show promise, they could lead to a new generation of perovskite solar cells that are both high-performing and commercially viable.
With solar energy playing a key role in the future of renewable power, breakthroughs like these could pave the way for cleaner and more accessible electricity. What do you think? Could perovskite solar cells soon become the leading technology in solar energy?
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Journal reference
Pasanen, H. P., Khan, R., Odutola, J. A., & Tkachenko, N. V. (2024). Transient absorption spectroscopy of films: impact of refractive index. The Journal of Physical Chemistry C, 128(15), 6167-6179. https://doi.org/10.1021/acs.jpcc.4c00981
