Detailed Characterization of Methylammonium and Formamidinium Bismuth Iodide Perovskite Thin Films, Focusing on Electrical Properties for Optoelectronic Applications

In the world of solar energy, scientists have long been searching for materials that can capture sunlight and turn it into electricity more efficiently than current silicon panels. For years, a family of materials known as perovskites has shown great promise, but many of the best performers contain lead, a toxic metal that poses environmental and health risks. This has driven researchers to look for safer, lead-free alternatives that still possess the same ability to conduct electrical charges. Among the most promising candidates are compounds made with bismuth, a much less toxic metal. However, simply swapping lead for bismuth changes how the material behaves. To make these new materials useful, scientists must understand exactly how electricity moves through them, how they absorb light, and how their internal crystal structures are arranged. The challenge lies in the fact that these materials are complex; their performance depends heavily on the specific atoms used to build them and how the thin layers of the material are manufactured.

A team of researchers from universities in Slovenia and North Macedonia set out to investigate two specific versions of these bismuth-based materials. They focused on compounds where the bismuth and iodine atoms are paired with either methylammonium or formamidinium, which are small organic molecules that act as structural spacers within the crystal. The researchers created these materials in the lab, first growing them as solid crystals and then turning them into very thin, uniform films on glass slides, a process essential for making them into solar cells. By carefully examining these films, the team aimed to determine their optical properties, such as how much energy is needed to free an electron, and their electrical behavior, specifically how easily charges can move through the material at different temperatures.

The team began by confirming that they had successfully created the correct chemical structures. Using X-ray diffraction, a technique that reveals the arrangement of atoms by bouncing X-rays off a sample, they verified that the crystals formed the expected patterns. They also looked at the films under powerful microscopes to check their surface quality. The images showed that both types of films were smooth and continuous, with no large gaps or cracks, which is crucial for a device to function well. One subtle difference emerged: the film made with the formamidinium molecules appeared slightly more regular and orderly on the surface than the one made with methylammonium, suggesting that the size and shape of the organic molecule influence how the crystal settles during manufacturing. Chemical analysis confirmed that the ratio of bismuth to iodine atoms was exactly as predicted, ensuring the materials were pure and free of unwanted impurities.

Next, the researchers measured how these films interact with light. When light hits a material, it can be absorbed if the light carries enough energy to push an electron into a state where it can move and carry a current. The team found that both materials absorb light very well, particularly in the blue and green parts of the visible spectrum, giving the films a distinct orange or dark red color. By analyzing the absorption data, they calculated the energy gap, which is the minimum amount of energy required to free an electron. They determined that the methylammonium version has an energy gap of 1.98 electron volts, while the formamidinium version has a slightly smaller gap of 1.74 electron volts. This difference is significant because a smaller gap allows the material to harvest a broader range of sunlight. The data indicated that both materials behave as indirect semiconductors, a specific type of behavior where the movement of electrons involves a change in momentum, making the process slightly more complex than in direct semiconductors.

To understand how electricity actually travels through these films, the team performed impedance spectroscopy, a method that measures how the material resists the flow of electrical current across a wide range of frequencies and temperatures. They observed that as the temperature increased, the resistance of the material decreased, a behavior typical of semiconductors where heat helps free up charge carriers. The measurements revealed that the movement of electricity is not smooth and continuous but rather happens through a "hopping" mechanism. In this process, charges jump from one localized spot to another, overcoming small energy barriers along the way. The researchers calculated that the energy required for these jumps is approximately 0.5 electron volts for both materials. This value is higher than what is typically seen in lead-based perovskites, suggesting that while bismuth materials are safer, they currently move charges more slowly.

The study also uncovered details about where the resistance comes from within the material. The electrical signals indicated that the boundaries between the tiny crystals, or grains, that make up the film are more resistive than the grains themselves. This means that the path of least resistance is often blocked by the interfaces where these crystals meet. Furthermore, the relaxation of electrical charges, or how quickly they respond to a changing field, did not follow a simple, uniform pattern. Instead, the charges exhibited a spread of response times, indicating a complex internal environment with many different types of obstacles. Despite these hurdles, the researchers confirmed that the materials possess the fundamental properties needed for optoelectronic devices. They concluded that while the bismuth-based films are not yet as efficient as their lead-based counterparts, their stability and low toxicity make them strong candidates for future development, especially if scientists can find ways to improve the connections between the crystals to allow electricity to flow more freely.