Understanding the Degradation of Methylenediammonium and Its Role in Phase-Stabilizing Formamidinium Lead Triiodide.

Formamidinium lead triiodide (FAPbI3) is the leading candidate for single-junction metal-halide perovskite photovoltaics, despite the metastability of this phase. To enhance its ambient-phase stability and produce world-record photovoltaic efficiencies, methylenediammonium dichloride (MDACl2) has been used as an additive in FAPbI3. MDA2+ has been reported as incorporated into the perovskite lattice alongside Cl-. However, the precise function and role of MDA2+ remain uncertain. Here, we grow FAPbI3 single crystals from a solution containing MDACl2 (FAPbI3-M). We demonstrate that FAPbI3-M crystals are stable against transformation to the photoinactive δ-phase for more than one year under ambient conditions. Critically, we reveal that MDA2+ is not the direct cause of the enhanced material stability. Instead, MDA2+ degrades rapidly to produce ammonium and methaniminium, which subsequently oligomerizes to yield hexamethylenetetramine (HMTA). FAPbI3 crystals grown from a solution containing HMTA (FAPbI3-H) replicate the enhanced α-phase stability of FAPbI3-M. However, we further determine that HMTA is unstable in the perovskite precursor solution, where reaction with FA+ is possible, leading instead to the formation of tetrahydrotriazinium (THTZ-H+). By a combination of liquid- and solid-state NMR techniques, we show that THTZ-H+ is selectively incorporated into the bulk of both FAPbI3-M and FAPbI3-H at ∼0.5 mol % and infer that this addition is responsible for the improved α-phase stability.

Kinetic-Controlled Crystallization of α-FAPbI3 Inducing Preferred Crystallographic Orientation Enhances Photovoltaic Performance.

Crystallization kinetic controls the crystallographic orientation, inducing anisotropic properties of the materials. As a result, preferential orientation with advanced optoelectronic properties can enhance the photovoltaic devices' performance. Although incorporation of additives is one of the most studied methods to stabilize the photoactive α-phase of formamidinium lead tri-iodide (α-FAPbI3 ), no studies focus on how the additives affect the crystallization kinetics. Along with the role of methylammonium chloride (MACl) as a "stabilizer" in the formation of α-FAPbI3 , herein, the additional role as a "controller" in the crystallization kinetics is pointed out. With microscopic observations, for example, electron backscatter diffraction and selected area electron diffraction, it is examined that higher concentration of MACl induces slower crystallization kinetics, resulting in larger grain size and [100] preferred orientation. Optoelectronic properties of [100] preferentially oriented grains with less non-radiative recombination, a longer lifetime of charge carriers, and lower photocurrent deviations in between each grain induce higher short-circuit current density (Jsc ) and fill factor. Resulting MACl40 mol% attains the highest power conversion efficiency (PCE) of 24.1%. The results provide observations of a direct correlation between the crystallographic orientation and device performance as it highlights the importance of crystallization kinetics resulting in desirable microstructures for device engineering.

Intermediate-phase engineering via dimethylammonium cation additive for stable perovskite solar cells.

Achieving the long-term stability of perovskite solar cells is arguably the most important challenge required to enable widespread commercialization. Understanding the perovskite crystallization process and its direct impact on device stability is critical to achieving this goal. The commonly employed dimethyl-formamide/dimethyl-sulfoxide solvent preparation method results in a poor crystal quality and microstructure of the polycrystalline perovskite films. In this work, we introduce a high-temperature dimethyl-sulfoxide-free processing method that utilizes dimethylammonium chloride as an additive to control the perovskite intermediate precursor phases. By controlling the crystallization sequence, we tune the grain size, texturing, orientation (corner-up versus face-up) and crystallinity of the formamidinium (FA)/caesium (FA)yCs1-yPb(IxBr1-x)3 perovskite system. A population of encapsulated devices showed improved operational stability, with a median T80 lifetime (the time over which the device power conversion efficiency decreases to 80% of its initial value) for the steady-state power conversion efficiency of 1,190 hours, and a champion device showed a T80 of 1,410 hours, under simulated sunlight at 65 °C in air, under open-circuit conditions. This work highlights the importance of material quality in achieving the long-term operational stability of perovskite optoelectronic devices.

Dynamic structural property of organic-inorganic metal halide perovskite.

Unique organic-inorganic hybrid semiconducting materials have made a remarkable breakthrough in new class of photovoltaics (PVs). Organic-inorganic metal (Pb and/or Sn) halides (-I, -Br, and -Cl) are the semiconducting absorber with the crystal structure of the famous "Perovskite". It is widely called "perovskite solar cells (PSCs)" in PV society. Now, the power conversion efficiency (PCE) of PSCs is recorded in 25.5%. Prototypical composition of the absorbers is (A = methylammonium [MA], formamidinium [FA], and Cs), (M = Pb and/or Sn), and (X = I, Br, and Cl) in the form of perovskite AMX3. Since the report on the stable all solid-state PSCs in 2012, the average annual growth rate of PCE is well over ∼10%. Such an outstanding PV performance attracts huge number of scientists in our research society. Their chemical as well as physical properties are dramatically different from monocrystalline Si, GaAs, other III-IV semiconductors, and many oxides with the crystal structure of perovskite. In this review, different fundamental aspects, in particular, the dynamic properties of A site cationic molecules and PbI6 octahedrons linked with their corners, from other semiconducting and dielectric materials are reviewed and summarized. Upon discussing unique properties, perspectives on the promising PV applications based on the comprehension in dynamic nature of the orientation in A site molecule and PbI6 octahedron tilting will be given.

Atomic layer deposition for efficient and stable perovskite solar cells.

Organic-inorganic hybrid metal halides are now the most attractive photovoltaic absorber materials, typically, methylammonium lead triiodides (MAPbI3). These unique semiconducting materials as absorbers demonstrate a remarkably improved power conversion efficiency of over 20% and now with a certified efficiency of 23.3%. Considering the Shockley-Queisser limit and their bandgaps, there is still much room to increase the efficiency. Stable devices with reproducibility and long-term use are essential for their commercialization. Atomic layer deposition (ALD) is a powerful technique to deposit high-quality thin films with excellent thickness accuracy and conformality, as well as with no pin-holes in a large area at low temperatures. ALD could be an ideal tool for efficient and stable perovskite solar cells. In particular, ALD will emerge for the production of tandem as well as flexible solar cells. This review contains the following recent research topics; underlying charge transport layers onto transparent conducting oxides (TCO), interfacial layers, overlying electron transport layers (ETLs), and encapsulation techniques utilized by ALD. Several extended understandings by recent studies and challenges toward further enhancing the efficiency and stability will be addressed.

Atomic layer deposition of a SnO2 electron-transporting layer for planar perovskite solar cells with a power conversion efficiency of 18.3.

High-efficiency planar type perovskite solar cells were fabricated by atomic layer deposition (ALD) of SnO2 and subsequent annealing at 180 °C. As-dep. SnO2 layers prepared by post-annealing at 180 and 300 °C, respectively, were used as electron transporting layers (ETLs). ALD-TiO2 layers were also prepared by post annealing at 400 °C, and the thicknesses of all ETLs were around 12 nm. PL quenching, optical band gap measurement, UPS, and conductive AFM results show that SnO2 can more appropriately be used as an ETL compared to TiO2.

Perovskite Solar Cells with Inorganic Electron- and Hole-Transport Layers Exhibiting Long-Term (≈500 h) Stability at 85 °C under Continuous 1 Sun Illumination in Ambient Air.

Despite the high power conversion efficiency (PCE) of perovskite solar cells (PSCs), poor long-term stability is one of the main obstacles preventing their commercialization. Several approaches to enhance the stability of PSCs have been proposed. However, an accelerating stability test of PSCs at high temperature under the operating conditions in ambient air remains still to be demonstrated. Herein, interface-engineered stable PSCs with inorganic charge-transport layers are shown. The highly conductive Al-doped ZnO films act as efficient electron-transporting layers as well as dense passivation layers. This layer prevents underneath perovskite from moisture contact, evaporation of components, and reaction with a metal electrode. Finally, inverted-type PSCs with inorganic charge-transport layers exhibit a PCE of 18.45% and retain 86.7% of the initial efficiency for 500 h under continuous 1 Sun illumination at 85 °C in ambient air with electrical biases (at maximum power point tracking).

p-Type CuCrO2 particulate films as the hole transporting layer for CH3NH3PbI3 perovskite solar cells.

CuCrO2 with a crystal structure of delafossite is a promising material as a transparent conducting oxide. It shows unique properties, for example, wide band gap, good chemical stability, and p-type carrier transporting character. The oxide layers with delafossite structure have been suggested as hole transporting materials for organic-inorganic CH3NH3PbI3 perovskite solar cells. In this study, we fabricated inverted (p-i-n) type planar perovskite solar cells with CuCrO2 nanoparticles synthesized by the hydrothermal method and their films were formed by spin-coating without any further heat treatment. The champion device gave a 13.1% of power conversion efficiency and CuCrO2 based devices show improved stability in ambient air compared with the standard PEDOT:PSS based perovskite solar cells.

Effect of Rubidium Incorporation on the Structural, Electrical, and Photovoltaic Properties of Methylammonium Lead Iodide-Based Perovskite Solar Cells.

We report the electrical properties of rubidium-incorporated methylammonium lead iodide ((RbxMA1-x)PbI3) films and the photovoltaic performance of (RbxMA1-x)PbI3 film-based p-i-n-type perovskite solar cells (PSCs). The incorporation of a small amount of Rb+ (x = 0.05) increases both the open circuit voltage (Voc) and the short circuit photocurrent density (Jsc) of the PSCs, leading to an improved power conversion efficiency (PCE). However, a high fraction of Rb+ incorporation (x = 0.1 and 0.2) decreases the Jsc and thus the PCE, which is attributed to the phase segregation of the single tetragonal perovskite phase to a MA-rich tetragonal perovskite phase and a RbPbI3 orthorhombic phase at high Rb fractions. Conductive atomic force microscopic and admittance spectroscopic analyses reveal that the single-phase (Rb0.05MA0.95)PbI3 film has a high electrical conductivity because of a reduced deep-level trap density. We also found that Rb substitution enhances the diode characteristics of the PSC, as evidenced by the reduced reverse saturation current (J0). The optimized (RbxMA1-x)PbI3 PSCs exhibited a PCE of 18.8% with negligible hysteresis in the photocurrent-voltage curve. The results from this work enhance the understanding of the effect of Rb incorporation into organic-inorganic hybrid halide perovskites and enable the exploration of Rb-incorporated mixed perovskites for various applications, such as solar cells, photodetectors, and light-emitting diodes.

In-Situ Formed Type I Nanocrystalline Perovskite Film for Highly Efficient Light-Emitting Diode.

Excellent color purity with a tunable band gap renders organic-inorganic halide perovskite highly capable of performing as light-emitting diodes (LEDs). Perovskite nanocrystals show a photoluminescence quantum yield exceeding 90%, which, however, decreases to lower than 20% upon formation of a thin film. The limited photoluminescence quantum yield of a perovskite thin film has been a formidable obstacle for development of highly efficient perovskite LEDs. Here, we report a method for highly luminescent MAPbBr3 (MA = CH3NH3) nanocrystals formed in situ in a thin film based on nonstoichiometric adduct and solvent-vacuum drying approaches. Excess MABr with respect to PbBr2 in precursor solution plays a critical role in inhibiting crystal growth of MAPbBr3, thereby forming nanocrystals and creating type I band alignment with core MAPbBr3 by embedding MAPbBr3 nanocrystals in the unreacted wider band gap MABr. A solvent-vacuum drying process was developed to preserve nanocrystals in the film, which realizes a fast photoluminescence lifetime of 3.9 ns along with negligible trapping processes. Based on a highly luminescent nanocrystalline MAPbBr3 thin film, a highly efficient green LED with a maximum external quantum efficiency of 8.21% and a current efficiency of 34.46 cd/A was demonstrated.

An ultra-thin, un-doped NiO hole transporting layer of highly efficient (16.4%) organic-inorganic hybrid perovskite solar cells.

NiO is a wide band gap p-type oxide semiconductor and has potential for applications in solar energy conversion as a hole-transporting layer (HTL). It also has good optical transparency and high chemical stability, and the capability of aligning the band edges to the perovskite (CH3NH3PbI3) layers. Ultra-thin and un-doped NiO films with much less absorption loss were prepared by atomic layer deposition (ALD) with highly precise control over thickness without any pinholes. Thin enough (5-7.5 nm in thickness) NiO films with the thickness of few time the Debye length (LD = 1-2 nm for NiO) show enough conductivities achieved by overlapping space charge regions. The inverted planar perovskite solar cells with NiO films as HTLs exhibited the highest energy conversion efficiency of 16.40% with high open circuit voltage (1.04 V) and fill factor (0.72) with negligible current-voltage hysteresis.

Nanotubular Heterostructure of Tin Dioxide/Titanium Dioxide as a Binder-Free Anode in Lithium-Ion Batteries.

Titanium dioxide (TiO2 ), tin dioxide (SnO2 ), and heterostructured TiO2 /SnO2 nanotube (NT) arrays have been fabricated by template-assisted atomic-layer deposition (ALD) for use as anodes in a lithium-ion battery (LIB). TiO2 NT arrays with 8 nm thick walls showed higher capacity (≈250 mA h g(-1) after the 50th cycle at a rate of C/10) than the typical theoretical capacity of bulk TiO2 and a radically improved capacity retention property upon cycling. SnO2 NT arrays with different wall thicknesses (8, 10, 13, and 20 nm) were also fabricated and their electrochemical performances were measured. All of the SnO2 NT arrays showed substantially higher initial irreversible capacity and higher reversible capacity than those of bulk TiO2 . Thinner walls of the SnO2 NTs result in better capacity retention. Heterotubular structures of TiO2 (5 nm)/SnO2 (10 nm)/TiO2 (5 nm) were successfully fabricated, and displayed a sufficiently high capacity (≈300 mA h g(-1) after 50 cycles) with exceptionally improved cycling performance up to the 50th cycle.