Solving Instability in Halide Perovskite Solar Cells

Halide perovskite solar cells are a promising candidate for next-generation solar technology thanks to their exceptional characteristics, which include high power conversion efficiency, low-cost fabrication, and excellent optoelectronic properties.

 

Solar cells. Image used courtesy of Adobe Stock

 

The efficiency of halide perovskite solar cells has been increasing rapidly in recent years and now approaching the efficiency of traditional silicon-based solar cells, the dominant technology in the market. One of the key advantages of halide perovskite solar cells is their low-cost fabrication process. Unlike silicon-based solar cells, which require high-temperature and high-vacuum processes, halide perovskite solar cells are fabricated using low-temperature solution-based methods, such as spin- or spray-coating, reducing production cost and making it easier to scale up.

Another advantage of halide perovskite solar cells is their excellent optoelectronic properties. These materials have a high absorption coefficient, which means they can absorb more incident light. They also have a long carrier diffusion length, which allows carriers to travel long distances before recombining, resulting in high open-circuit voltages and high-power conversion efficiencies.

Despite these advantages, some challenges must be addressed before commercializing halide perovskite solar cells. One of the major challenges is their stability and durability. Halide perovskite materials are sensitive to moisture and heat, which can lead to degradation over time. Efforts are underway to improve the stability of these materials by developing new encapsulation techniques and modifying the chemical composition of the perovskite layer.

Contributing to these ongoing efforts, researcher Juan-Pablo Correa-Baena, assistant professor in the School of Materials Sciences and Engineering at Georgia Tech, reports halide perovskite solar cells are less stable than previously considered. The study highlights the thermal instability within the cell's interface layers and how it impacts the device's operation.

 

Impact of Bulky Organic Cations on Interfaces

Bulky organic cations are used to reduce recombinations at interfaces and improve the efficiency of metal halide perovskite photovoltaics. These cations interact with point defects at the perovskite surfaces and introduce a tunneling junction with the charge extraction layers, decreasing non-radiative recombinations. They are often added at the perovskite and charge-transport-layer interface by deposition on already-formed perovskite films to avoid transport losses.

While this approach is popular, it is still unclear if it is beneficial or hinders stability too much. Researchers at Georgia Tech report that the cations alter the material's structure at the interface where they are deposited, creating atomic-scale defects that limit the efficacy of current extraction from the solar cell. To demonstrate this theory, the researchers made a sample solar panel of perovskite films with eight independent solar cells, allowing them to investigate each cell's performance. They studied the performance of these cells, with and without the cation surface treatment, and analyzed the interfaces before and after prolonged thermal stress using synchrotron-based X-ray characterization techniques.

 

Demonstrating the Practicality of Passivation

The researchers treated the sample solar panels at 100 degrees Celsius for 40 minutes and measured the dynamics in chemical composition using X-ray photoelectron spectroscopy and another X-ray technology, allowing them to visualize the diffusion of cations into the lattice and changes in interface structure when exposed to heat. They then performed excitation correlation spectroscopy to understand how surface defects and structural changes due to cations impact solar cell performance.

 

Solar cell architecture used in the study and evolution of the composition with the addition of a bulky cation layer. Image used courtesy of Georgia Institute of Technology

 

The team found that cells treated with organic cations evolve in structure and composition under thermal stress and that these changes can cause a significant power loss. However, the speed of these changes depends on the type of cations used, which means suitable cations can lead to stable interfaces.

The researchers hope their studies will urge researchers to test these interfaces under thermal stress and solve instability to build more efficient and optimized solar technologies.