Comparative and comprehensive study on structural and optical properties of Methylammonium Lead Iodide and Methylammonium Bismuth Iodide

The search for cleaner energy has long focused on materials that can capture sunlight and turn it into electricity with remarkable efficiency. Among the most promising candidates are a family of crystals known as perovskites, which have recently revolutionized the field of solar power. These materials are built from a specific arrangement of atoms that allows them to absorb light intensely and move electrical charges quickly. For years, the most effective version of these crystals has relied on lead, an element that makes them perform exceptionally well but also introduces a serious problem: lead is toxic. It poses significant risks to human health and the environment, affecting the nervous system and organs, and it can accumulate in soil and water. This creates a difficult dilemma for scientists: how to keep the high performance of these solar materials while removing the dangerous element that makes them hazardous.

In a study published in August 2026, researchers Hasan Abbas and Zishan H. Khan from Jamia Millia Islamia in India tackled this challenge by comparing the standard lead-based crystal with a new, lead-free alternative. They set out to create two specific materials in the laboratory: the well-known methylammonium lead iodide, which contains lead, and a new compound called methylammonium bismuth iodide, which replaces the lead with bismuth, a much safer element. The team did not just mix chemicals; they carefully grew thin films of these materials on glass plates and then subjected them to a battery of tests to see how they were built and how they interacted with light. Their goal was to understand if the safer bismuth version could match the structural quality and light-absorbing abilities of the lead version, or if it would fall short in ways that would prevent it from being used in real-world solar panels.

The researchers began by synthesizing both materials using a liquid solution method, a process that involves dissolving the raw ingredients in a solvent and then drying them to form a solid film. Once the films were created, they examined them under powerful microscopes to see what they looked like on a tiny scale. The images revealed that the lead-free bismuth film was exceptionally smooth and dense, lacking the tiny holes or gaps that sometimes appear in the lead-based version. The bismuth crystals also formed slightly larger structures than their lead counterparts. When the team looked at the internal arrangement of the atoms using X-ray diffraction, a technique that reveals the crystal structure, they found that the bismuth material was actually more ordered and uniform than the lead material. This higher degree of order suggested that the lead-free film was of very high quality, potentially even better in terms of its physical structure than the traditional lead version.

Next, the team turned their attention to how these materials handled light, which is the most critical factor for a solar cell. They shone light across a wide range of colors, from the ultraviolet to the infrared, to see how much was absorbed and how much passed through. The results showed that both materials were excellent at capturing light, but they did so in slightly different ways. The lead-based material absorbed light most strongly at a wavelength of 760 nanometers, while the bismuth material peaked at 530 nanometers. The lead version was able to absorb a broader range of the visible spectrum, which is a key advantage for generating electricity. The researchers also calculated how light traveled through the materials and found that the lead-free film was more uniform, with fewer imperfections that could scatter light. This uniformity, combined with the smooth surface seen in the microscope images, indicated that the bismuth material had a very consistent internal structure.

Perhaps the most significant finding concerned the energy gap, or the specific amount of energy required to free an electron so it can carry a current. The lead-based material had an energy gap of 1.65 electron volts, a value that is very close to the theoretical ideal for a single-layer solar cell. This near-perfect match explains why lead-based perovskites have been so successful at converting sunlight into power. In contrast, the lead-free bismuth material had a wider energy gap of 2.23 electron volts. While this value is still useful, it means the material absorbs a different portion of the solar spectrum and is less efficient at capturing the full range of sunlight compared to the lead version. The study also looked at how the materials emitted light after absorbing it, a process called photoluminescence. The lead material glowed at a wavelength of 760 nanometers, while the bismuth material glowed at 590 nanometers, confirming the differences in their energy gaps.

Ultimately, the study paints a picture of two materials with distinct strengths. The lead-free bismuth perovskite proved to be structurally superior in many ways, forming a smoother, more uniform film with fewer defects than the lead version. It is also free from the toxicity that makes lead so dangerous. However, the lead-based material retains a crucial advantage: its energy gap is closer to the ideal for solar power, allowing it to harvest more energy from sunlight. The researchers concluded that while the lead-free option shows great promise and offers better morphological properties, it still needs further optimization to match the efficiency of its lead-based rival. The work suggests that the path forward involves refining these safer materials, perhaps by tweaking their composition, to bring their performance closer to the high standards set by the toxic but highly efficient lead crystals.