The biggest hurdle perovskite solar cells face before they can reach market is that standard devices made with three-dimensional (3D) methylammonium-based perovskites degrade when exposed to light, moisture, and oxygen. Two-dimensional (2D) organic metal-halide perovskites, on the other hand, are much more stable but suffer from low power-conversion efficiency.
An artificially colored scanning electron microscope image of the cross section of a photovoltaic device shows a perovskite layer (brown) composed of 2D platelets of butylammonium lead halide standing up between the larger, flat 3D cesium formamidinium perovskite FA0.83Cs0.17Pb(I0.6Br0.4)3. The mixed 2D–3D perovskites result in efficient, air-stable solar cells. Credit: Nature Energy.
Oxford University researchers have now made a hybrid of the two by incorporating small 2D perovskite platelets into 3D perovskite films. The 2D platelets enhance stability and prevent charges from recombining nonradiatively, which increases sunlight-to-electricity conversion efficiency, according to their study published in Nature Energy (doi:10.1038/nenergy.2017.135).
Henry Snaith and colleagues turned to 2D Ruddlesden–Popper phases ((RNH3)2(A)n–1BX3n+1), which are layered 2D perovskite films known for their moisture stability. And they used formamidinium-based perovskites, which are more thermally stable than their methylammonium counterparts.
The researchers started with a precursor for a cesium formamidinium perovskite FA0.83Cs0.17Pb(I0.6Br0.4)3, to which they added butylammonium (C4H9NH+ 3) iodide. The resulting fully crystalline perovskite films had small 2D plate-like crystals of butylammonium lead halide standing between larger, flat 3D perovskite grains. The films also had remarkably enhanced crystallinity.
Best-performing solar cells made with the film had an efficiency of 19.5%, retained 80% of this efficiency after 1000 hours in air under simulated sunlight, and 4000 hours when encapsulated.
Researchers at the Swiss Federal Institute of Technology in Lausanne have combined 2D and 3D perovskites to make ultra-stable solar cells that have worked for more than 10,000 hours, or over 400 days, with no loss in efficiency. The solar cells are also low cost and have an efficiency of 11.2%.
Conventional solar cells struggle to reach a goal of less than 10% drop in efficiency for at least 1000 hours in lab-based accelerated aging tests, the researchers write in Nature Communications (doi:10.1038/ncomms15684). This corresponds to the 20–25 year warranty, with less than a 10% drop in performance that a marketable product would require.
The team led by Mohammad Khaja Nazeeruddin covered a methyl ammonium lead iodide perovskite film with a 2D perovskite based on aminovaleric acid (HOOC(CH2)4NH3)2PbI4. They also replaced the hole-transport layer found in conventional perovskite solar cells with hydrophobic carbon electrodes because of vulnerability of the former to moisture and oxygen in ambient operating conditions.
The researchers used an industrial-scale process to print a 10 cm × 10 cm solar module, and tested it under continuous light in the presence of oxygen and moisture.
A team from Columbia University and the Italian Institute of Technology has provided the first direct view of an unusual phenomenon thought to be responsible for the excellent optoelectronic properties of perovskite materials.
Charge carriers in lead halide perovskites last for an unusually long time and travel long distances, resulting in high efficiency despite defects in the material. Columbia’s Xiaoyang Zhu and his colleagues had proposed previously that these unique carrier properties are due to polarons—quasiparticles that represent electrons and their self-induced polarization in the lattice—that screen charge carriers and keep them from colliding with defects. But no one had been able to directly observe how, or if, they are formed.
Zhu, Filippo De Angelis, and their colleagues used time-resolved optical Kerr effect spectroscopy to give a time domain view of polaron formation in CH3NH3PbBr3 and CsPbBr3 perovskites. The results, reported in Science Advances (doi:10.1126/sciadv.1701217), revealed that deformations of the soft PbBr3– lattice are mainly responsible for the polaron formation, and having an organic cation is not essential. Polarons form more than twice as quickly in the methylammonium perovskite than the cesium one because of its higher frequency of PbBr3– vibrations. The researchers also confirmed the formation of the polarons using density functional theory calculations.
A new method to heal defects and make them electronically less reactive in hybrid halide perovskite films could provide a path to further improve the efficiency and stability of perovskite solar cells.
Perovskite surfaces and grain boundaries have a high density of ionic defects, where charges can get trapped and recombine, reducing efficiency. Oxygen or moisture can also seep into perovskite films at defects, setting off degradation, which makes devices less stable. So far, researchers have passivated charged defects in perovskites by adding molecules that act as electron donors or acceptors. But most passivation molecules only passivate one type of defect, either positively or negatively charged.
University of Nebraska–Lincoln’s Jinsong Huang reported in Nature Energy (doi:10.1038/nenergy.2017.102) that quaternary ammonium halides with the structure NR+ 4X–, where R is an alkyl or aryl group and X is a halide, can efficiently passivate charged defects in mixed-cation halide perovskites with quaternary ammonium and halide ions.
With a passivation layer of quarternary ammonium halides—choline chloride or choline iodide—deposited on the perovskite film, the efficiency of CH3NH3PbI3 solar cells went up from 17% to a certified value of over 20%.
