There are many theories that attempt to explain the particular advantages of perovskite solar cells. For example, polarons and a huge Rashba effect are believed to play a significant role. These hypotheses have been disproven experimentally by the BESSY II team. They have also narrowed down possible causes of the transport properties, allowing for better optimization strategies for this class of material.
For many years, research on organic and hybrid lead halide perovskites is booming. This class of materials is known for its unique properties. For example, perovskite semiconductors can convert the energy-rich blue wavelength of sunlight into electricity. Solar cells that are based on perovskites and silicon sub-cells achieve efficiency of 30%. Perovskite semiconductors can also be used as light-emitting devices, such as semiconductor lasers and radiation detectors. These materials are cheaper than conventional semiconductors and require less energy to produce thin films.
Despite years of intense research, it is still not clear how the microscopic processes that allow for superior charge transport in perovskite semiconductors are achieved. One thing is certain: the charge carriers released by sunlight in the material appear to have long lives and are less likely to be lost through defects or recombination.
Scientists have proposed hypotheses that explain this behavior. A team from BESSY II is now testing them experimentally. Prof. Oliver Rader led the team. Eva Unger, a perovskite expert at HZB provided advice and the HySPRINT laboratory facilities for sample preparation.
One theory is that polarons may form in lead-halide perovskites, and thus contribute to charge transport. These polarons can be described as oscillations of ions within the crystal lattice that react to the movement electrons due to their charge. The assumption that polarons play an important role in perovskites' formation of both negative (here lead), and positive (here cerium) ions was evident. Another group of measurements also supported this hypothesis.
This hypothesis can however be experimentally tested at BESSY II. It is possible to scan electronic band structures using angle-resolved photostamps (ARPES). With a higher effective mass, polarons would make up a large portion of the charge transport. ARPES measures electron kinetic energy, i.e. 1/2 mv2 for mass m and velocity. The kinetic energy of electrons is measured by ARPES.
Maryam Sajedi's measurements on crystal samples of CsPbBr3 showed no evidence of smaller curvatures. This disproves the hypothesis that large polarons exist. Maryam Sajedi says that the measured effective mass is no larger than what was theoretically predicted. Oliver Rader adds: "To ensure that we considered all possible effects, such as polarons and the repulsion electrons from each other, we collaborated with theoreticians at Forschungszentrum Julich. There is however no evidence of an increase in mass, so one does not need to assume polarons.
Rashba is not a huge effect
This second hypothesis assumes that there is a huge Rashba effect that limits losses due to recombination. The Rashba effect relies on strong spin-orbit coupling, which could be produced by heavy metal lead in lead-halide-perovskites. This effect was also suggested in earlier research as an explanation for the long lives of charge carriers. Maryam Sajedi analyzed measurement data from both inorganic CsPbBr3 as well as hybrid-organic MAPbBr3 with spun ARPES samples. She comments that the effect is probably at least 100 times smaller than expected.
Falsification helps progress
Rader states, "We were able to experimentally disprove 2 common hypotheses regarding the transport properties of perovskites. This is an important result." It is important to eliminate invalid hypotheses in order to optimize these materials further.
M. Sajedi et al, Absence of a giant Rashba effect in the valence band of lead halide perovskites, Physical Review B (2020).DOI: 10.1103/PhysRevB.102.081116