Metal Ions in Halide Perovskite Materials and Devices

The influence of metal ions on perovskite properties and resulting device performance has been undeniable in progressing the field of inorganic and organic–inorganic hybrid perovskites. Here we provide a review on the capacity of metal ions to impart a broad range of effects from controlling crystallization to the alloying, doping, and passivation of perovskite materials. Although some metal ions have already proved effective in modulating bandgaps through alloying, their ability to control crystallization, carrier concentration, and emissive effects still require significant improvements in fundamental understanding. This presents enormous opportunities in research that may afford novel properties and applications through unparalleled control of perovskite materials

Imperfections and their passivation in halide perovskite solar cells

All highly-efficient organic–inorganic halide perovskite (OIHP) solar cells to date are made of polycrystalline perovskite films which contain a high density of defects, including point and extended imperfections. The imperfections in OIHP materials play an important role in the process of charge recombination and ion migration in perovskite solar cells (PSC), which heavily influences the resulting device energy conversion efficiency and stability. Here we review the recent advances in passivation of imperfections and suppressing ion migration to achieve improved efficiency and highly stable perovskite solar cells. Due to the ionic nature of OIHP materials, the defects in the photoactive films are inevitably electrically charged. The deep level traps induced by particular charged defects in OIHP films are major non-radiative recombination centers; passivation by coordinate bonding, ionic bonding, or chemical conversion have proven effective in mitigating the negative impacts of these deep traps. Shallow level charge traps themselves may contribute little to non-radiative recombination, but the migration of charged shallow level traps in OIHP films results in unfavorable band bending, interfacial reactions, and phase segregation, influencing the carrier extraction efficiency. Finally, the impact of defects and ion migration on the stability of perovskite solar cells is described

Synergistic Effect of Elevated Device Temperature and Excess Charge Carriers on the Rapid Light-Induced Degradation of Perovskite Solar Cells

With power conversion efficiencies now reaching 24.2%, the major factor limiting efficient electricity generation using perovskite solar cells (PSCs) is their long‐term stability. In particular, PSCs have demonstrated rapid degradation under illumination, the driving mechanism of which is yet to be understood. It is shown that elevated device temperature coupled with excess charge carriers due to constant illumination is the dominant force in the rapid degradation of encapsulated perovskite solar cells under illumination. Cooling the device to 20 °C and operating at the maximum power point improves the stability of CH3NH3PbI3 solar cells over 100× compared to operation under open circuit conditions at 60 °C. Light‐induced strain originating from photothermal‐induced expansion is also observed in CH3NH3PbI3, which excludes other light‐induced‐strain mechanisms. However, strain and electric field do not appear to play any role in the initial rapid degradation of CH3NH3PbI3 solar cells under illumination. It is revealed that the formation of additional recombination centers in PSCs facilitated by elevated temperature and excess charge carriers ultimately results in rapid light‐induced degradation. Guidance on the best methods for measuring the stability of PSCs is also given

Lattice-switch Monte Carlo

We present a Monte Carlo method for the direct evaluation of the difference between the free energies of two crystal structures. The method is built on a lattice-switch transformation that maps a configuration of one structure onto a candidate configuration of the other by `switching' one set of lattice vectors for the other, while keeping the displacements with respect to the lattice sites constant. The sampling of the displacement configurations is biased, multicanonically, to favor paths leading to `gateway' arrangements for which the Monte Carlo switch to the candidate configuration will be accepted. The configurations of both structures can then be efficiently sampled in a single process, and the difference between their free energies evaluated from their measured probabilities. We explore and exploit the method in the context of extensive studies of systems of hard spheres. We show that the efficiency of the method is controlled by the extent to which the switch conserves correlated microstructure. We also show how, microscopically, the procedure works: the system finds gateway arrangements which fulfill the sampling bias intelligently. We establish, with high precision, the differences between the free energies of the two close packed structures (fcc and hcp) in both the constant density and the constant pressure ensembles.Comment: 34 pages, 9 figures, RevTeX. To appear in Phys. Rev.

Low defects density CsPbBr3single crystals grown by an additive assisted method for gamma-ray detection

Metal halide perovskites have arisen as a new family of semiconductors for radiation detectors due to their high stopping power, large and balanced electron-hole mobility-lifetime (μτ) product, and tunable bandgap. Here, we report a simple and low-cost solution processing approach using additive-assisted inverse temperature crystallization (ITC) to grow cesium lead bromide (CsPbBr3) single crystals with low-defect density. Crystals grown from precursor solutions without additives tend to grow fastest along the [002] direction, resulting in crystals shaped as small elongated bars. The addition of choline bromide (CB) proves to mediate the crystallization process to produce large single crystals with a cuboid shape, allowing for more practical fabrication of gamma-ray detectors. This new additive-assisted growth method also improves the resulting crystal quality to yield a reduction in the density of trap states by over one order of magnitude, relative to a crystal grown without CB. The detector fabricated from a CB-assisted solution-grown perovskite CsPbBr3 single crystal is able to acquire an energy spectrum from a cesium-137 (137Cs) source with a resolution of 5.5% at 662 keV

Ultrafast Exciton Transport with a Long Diffusion Length in Layered Perovskites with Organic Cation Functionalization

Layered perovskites have been employed for various optoelectronic devices including solar cells and light-emitting diodes for improved stability, which need exciton transport along both the in-plane and the out-of-plane directions. However, it is not clear yet what determines the exciton transport along the in-plane direction, which is important to understand its impact toward electronic devices. Here, by employing both steady-state and transient photoluminescence mapping, it is found that in-plane exciton diffusivities in layered perovskites are sensitive to both the number of layers and organic cations. Apart from exciton–phonon coupling, the octahedral distortion is revealed to significantly affect the exciton diffusion process, determined by temperature-dependent photoluminescence, light-intensity-dependent time-resolved photoluminescence, and density function theory calculations. A simple fluorine substitution to phenethylammonium for the organic cations to tune the structural rigidity and octahedral distortion yields a record exciton diffusivity of 1.91 cm2 s−1 and a diffusion length of 405 nm along the in-plane direction. This study provides guidance to manipulate exciton diffusion by modifying organic cations in layered perovskites

Layer number dependent ferroelasticity in 2D Ruddlesden–Popper organic-inorganic hybrid perovskites

Ferroelasticity represents material domains possessing spontaneous strain that can be switched by external stress. Three-dimensional perovskites like methylammonium lead iodide are determined to be ferroelastic. Layered perovskites have been applied in optoelectronic devices with outstanding performance. However, the understanding of lattice strain and ferroelasticity in layered perovskites is still lacking. Here, using the in-situ observation of switching domains in layered perovskite single crystals under external strain, we discover the evidence of ferroelasticity in layered perovskites with layer number more than one, while the perovskites with single octahedra layer do not show ferroelasticity. Density functional theory calculation shows that ferroelasticity in layered perovskites originates from the distortion of inorganic octahedra resulting from the rotation of aspherical methylammonium cations. The absence of methylammonium cations in single layer perovskite accounts for the lack of ferroelasticity. These ferroelastic domains do not induce non-radiative recombination or reduce the photoluminescence quantum yield

Scalable Fabrication of Efficient Perovskite Solar Modules on Flexible Glass Substrates

Perovskite materials are good candidates for flexible photovoltaic applications due to their strong absorption and low-temperature processing, but efficient flexible perovskite modules have not yet been realized. Here, a record efficiency flexible perovskite solar module is demonstrated by blade coating high-quality perovskite films on flexible Corning Willow Glass using additive engineering. Ammonium chloride (NH4Cl) is added into the perovskite precursor solution to retard the nucleation which prevents voids formation at the interface of perovskite and glass. The addition of NH4Cl also suppresses the formation of PbI2 and reduces the trap density in the perovskite films. The implementation of NH4Cl enables the fabrication of single junction flexible perovskite solar devices with an efficiency of 19.72% on small-area cells and a record aperture efficiency of 15.86% on modules with an area of 42.9 cm2. This work provides a simple way to scale up high-efficiency flexible perovskite modules for various applications

Grain Engineering for Perovskite/Silicon Monolithic Tandem Solar Cells with Efficiency of 25.4%

Organic-inorganic halide perovskites are promising semiconductors to mate with silicon in tandem photovoltaic cells due to their solution processability and tunable complementary bandgaps. Herein, we show that a combination of two additives, MACl and MAH 2 PO 2 , in the perovskite precursor can significantly improve the grain morphology of wide-bandgap (1.64–1.70 eV) perovskite films, resulting in solar cells with increased photocurrent while reducing the open-circuit voltage deficit to 0.49–0.51 V. The addition of MACl enlarges the grain size, while MAH 2 PO 2 reduces non-radiative recombination through passivation of the perovskite grain boundaries, with good synergy of functions from MACl and MAH 2 PO 2 . Matching the photocurrent between the two sub-cells in a perovskite/silicon monolithic tandem solar cell by using a bandgap of 1.64 eV for the top cell results in a high tandem V oc of 1.80 V and improved power conversion efficiency of 25.4%. © 2018 Elsevier Inc.; Grain engineering through combined MACl and MAH 2 PO 2 additives in perovskite precursors improves the photovoltaic performance of perovskite/silicon tandem cells. MACl increases the grain size of wide-bandgap perovskite films and also produces smooth films. MAH 2 PO 2 suppresses non-radiative recombination sites at grain boundaries. The synergetic effects of MACl and MAH 2 PO 2 further promote grain growth and prolong the carrier recombination lifetime. This enables a power conversion efficiency of 25.4% for a perovskite/silicon tandem device. © 2018 Elsevier Inc.; The efficiency of organic-inorganic halide perovskite solar cells skyrocketed in the past 6 years, reaching 23.3%. Their pairing with silicon in tandem solar cells offers a promising path for further reducing the levelized cost of electricity of photovoltaics. Strategies such as compositional engineering and charge-transport-layer optimization have been reported to improve the tandem efficiency. However, the large open-circuit voltage deficit of wide-bandgap perovskite cells still limits the tandem performance. Here, we utilize combined additives to smooth the perovskite film, increase its grain size, and lower its defect density. The synergistic effect of the additives leads to increased photocurrent and reduced open-circuit voltage deficit for wide-bandgap perovskite solar cells. When additives are used to form a top cell with a bandgap of 1.64 eV, the perovskite and silicon sub-cells are current matched and yield a perovskite/silicon tandem device with an efficiency of 25.4%

Interfacial Molecular Doping of Metal Halide Perovskites for Highly Efficient Solar Cells

Tailoring the doping of semiconductors in heterojunction solar cells shows tremendous success in enhancing the performance of many types of inorganic solar cells, while it is found challenging in perovskite solar cells because of the difficulty in doping perovskites in a controllable way. Here, a small molecule of 4,4′,4″,4″′-(pyrazine-2,3,5,6-tetrayl) tetrakis (N,N-bis(4-methoxyphenyl) aniline) (PT-TPA) which can effectively p-dope the surface of FAxMA1−xPbI3 (FA: HC(NH2)2; MA: CH3NH3) perovskite films is reported. The intermolecular charge transfer property of PT-TPA forms a stabilized resonance structure to accept electrons from perovskites. The doping effect increases perovskite dark conductivity and carrier concentration by up to 4737 times. Computation shows that electrons in the first two layers of octahedral cages in perovskites are transferred to PT-TPA. After applying PT-TPA into perovskite solar cells, the doping-induced band bending in perovskite effectively facilitates hole extraction to hole transport layer and expels electrons toward cathode side, which reduces the charge recombination there. The optimized devices demonstrate an increased photovoltage from 1.12 to 1.17 V and an efficiency of 23.4% from photocurrent scanning with a stabilized efficiency of 22.9%. The findings demonstrate that molecular doping is an effective route to control the interfacial charge recombination in perovskite solar cells which is in complimentary to broadly applied defect passivation techniques