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New Insights into Methylammonium Lead Iodide Solar Cells

Source: Christopher Eames et al. - CC BY 4.0

For many years scientists have been looking for a new generation of highly efficient, low-cost and easy-to-process solar cells. To improve the materials for solar cells researchers need to get a better understanding of how these materials function at the atomic and electronic scales. Perovskite solar cells (PSCs) based on methylammonium lead iodide (CH3NH3PbI3) have shown outstanding performance in the recent years. Owing to their defect tolerance and long carrier lifetimes and diffusion lengths, they are exceptionally suitable semiconductors for solar cells and light-emitting devices. Nevertheless, perovskite solar cells (PSCs) based on methylammonium lead iodide suffer from poor moisture stability, thermal decomposition and device hysteresis, as the interaction between polar CH3NH3 + (MA+) and inorganic PbI3 −sublattices, CH3NH3PbI3 is very weak.

In a recent study (2019), researchers at the U.S. Department of Energy’s (DOE) Argonne National Laboratory have carried out a novel type of experiment that revealed important new insights into how energy moved in methylammonium lead iodide. At the atomic level, methylammonium lead iodide has an atypical nature. It is composed of two distinct substructures that interpenetrate each other at the nanometer level. The first, the organic methylammonium ion structure, has only light atoms and can vibrate at high energies. The second, lead iodide, is much heavier and vibrates only when exposed to low energies. In the experiment, an infrared laser pulse was used to selectively excite only the methylammonium ions in the crystal lattice.Then the scientists attempted to observe how those atoms transferred their energy to the inorganic lead iodide. Although the organic and inorganic molecules are very close to each other, the researchers noticed that the methylammonium molecules appeared to remain ​excited for quite some time before they transferred their energy directly to the lead iodide sublattice. The vibrational behaviour of the methylammonium molecules suggested that they may not be as important in the overall performance of the material as previously assumed. They seemed to have the sole function of a charge-balancing spacer material.

Research on the use of methylammonium lead iodide in solar cells has been conducted for more than 10 years. In 2013, scientists proposed to grow a thin layer of acceptor material on the methylammonium lead iodide layer which acted as ‘electron donor’ material in donor-acceptor polymer/organic planar solar cells and bulk-heterojunction solar cells. Thus, a donor-acceptor contact interface for charge separation was created in which the hybrid CH3NH3PbI3 perovskite/acceptor PHJ yielded the photovoltaic effect under irradiation. The acceptor layer consisted of fullerene (C60) or a C60 derivative on top of the donor material. A thin bathocuproine (BCP) film was combined with an aluminum (Al) layer and acted as the negative electrode.

In 2014, scientists demonstrated that with a slightly oxidized electron blocking layer the fill factor for the solar cells with a perovskite layer thickness of 900 nm increased to the same values as for the devices with 300 nm perovskite layers. As a result, the power conversion efficiencies for the cells with 300 and 900 nm were very similar, 12.7% and 12%, respectively. The scientists improved the performance of inverted devices, where the holes were extracted via the transparent conductor poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) situated on top of the transparent bottom electrodeby means of putting an evaporated CH3NH3PbI3 perovskite in between organic electron and hole blocking layers. This configuration led to stable and reproducible devices that did not suffer from strong hysteresis effects and when optimized led to efficiencies close to 15%.

In 2017, researchers showed that iodide ions in the methylammonium lead iodide seemed to migrate via interstitial sites at temperatures above 280K. This coincided with temperature dependent static distortions resulting in pseudocubic local symmetry. Based on bond distance analysis, the migrating and distorted iodines appeared to be consistent with the formation of I2 molecules, forming a 2I-à I2 + 2e-redox couple. The scientists found that a crucial feature of the tetragonal structure was that the methylammonium ions did not sit centrally in the A-site cavity, but disordered around two off-centre orientations that facilitated the interstitial ion migration via a gate opening mechanism.

Perovskite solar cells have a lot of advantages over common solar cells: the perovskite material has a direct optical band gap of around 1.5eV, long diffusion length and long minority carrier lifetimes. They have broad absorption range from the visible to the near infrared spectrum (800 nm) and a high absorption coefficient (105 cm-1). They show efficiencies of more than 22 percent. Perovskite materials, such as methylammonium lead halides, are fairly inexpensive and simple to produce. They have a fast charge separation process, a long transport distance of electrons and holes and a long carrier separation lifetime. This low-cost material is ideal for generating energy on windows of buildings, car tops and walls. It is cheaper than silicon, as perovskites use less material in order to absorb the same amount of light compared to silicon.

Despite all these advantages, scientists still have to overcome several problems concerning the large-scale production of methylammonium lead iodide Solar cells. The results of this study show that the delayed coupling between the organic and inorganic sublattices may indicate a diminished importance of the organic molecules. However, other possibilities of how the organic ions in methylammonium lead iodide might influence the optoelectronic properties of the entire material need thorough research until perovskite solar cells can eventually compete with existing silicon-based solar cells on the market.