Quantifying and Reducing Ion Migration in Metal Halide Perovskites through Control of Mobile Ions.

The presence of intrinsic ion migration in metal halide perovskites (MHPs) is one of the main reasons that perovskite solar cells (PSCs) are not stable under operation. In this work, we quantify the ion migration of PSCs and MHP thin films in terms of mobile ion concentration (No) and ionic mobility (µ) and demonstrate that No has a larger impact on device stability. We study the effect of small alkali metal A-site cation additives (e.g., Na+, K+, and Rb+) on ion migration. We show that the influence of moisture and cation additive on No is less significant than the choice of top electrode in PSCs. We also show that No in PSCs remains constant with an increase in temperature but µ increases with temperature because the activation energy is lower than that of ion formation. This work gives design principles regarding the importance of passivation and the effects of operational conditions on ion migration.

Improved charge extraction in inverted perovskite solar cells with dual-site-binding ligands.

Inverted (pin) perovskite solar cells (PSCs) afford improved operating stability in comparison to their nip counterparts but have lagged in power conversion efficiency (PCE). The energetic losses responsible for this PCE deficit in pin PSCs occur primarily at the interfaces between the perovskite and the charge-transport layers. Additive and surface treatments that use passivating ligands usually bind to a single active binding site: This dense packing of electrically resistive passivants perpendicular to the surface may limit the fill factor in pin PSCs. We identified ligands that bind two neighboring lead(II) ion (Pb2+) defect sites in a planar ligand orientation on the perovskite. We fabricated pin PSCs and report a certified quasi-steady state PCE of 26.15 and 24.74% for 0.05- and 1.04-square centimeter illuminated areas, respectively. The devices retain 95% of their initial PCE after 1200 hours of continuous 1 sun maximum power point operation at 65°C.

Tuning Film Stresses for Open-Air Processing of Stable Metal Halide Perovskites.

Challenges to upscaling metal halide perovskites (MHPs) include mechanical film stresses that accelerate degradation, dominate at the module scale, and can lead to delamination or fracture. In this work, we demonstrate open-air blade coating of single-step coated perovskite as a scalable method to control residual film stress after processing and introduce beneficial compression in the thin film with the use of polymer additives such as gellan gum and corn starch. The optoelectronic properties of MHP films with compression are improved with higher photoluminescence yields. MHP film stability is significantly improved under compression, under humidity, heat, and thermal cycling. By measuring the evolution of film stresses, we demonstrate for the first time that stress relaxation occurs in MHP films with tensile stress that correlates with film degradation. This discovery of a new mechanism underpinning MHP degradation shows that film stress can be used as a parameter to screen MHP devices and modules for quality control before deployment as a design for reliability criterion.

Robust and Manufacturable Lithium Lanthanum Titanate-Based Solid-State Electrolyte Thin Films Deposited in Open Air.

State-of-the-art solid-state electrolytes (SSEs) are limited in their energy density and processability based on thick, brittle pellets, which are generally hot pressed in vacuum over the course of several hours. We report on a high-throughput, open-air process for printable thin-film ceramic SSEs in a remarkable one-minute time frame using a lithium lanthanum titanium oxide (LLTO)-based SSE that we refer to as robust LLTO (R-LLTO). Powder XRD analysis revealed that the main phase of R-LLTO is polycrystalline LLTO, accompanied by selectively retained crystalline precursor phases. R-LLTO is highly dense and closely matched to the stoichiometry of LLTO with some heterogeneity throughout the film. A minimal presence of lithium carbonate is identified despite processing fully in ambient conditions. The LLTO films exhibit remarkable mechanical properties, demonstrating both flexibility with a low modulus of ∼35 GPa and a high fracture toughness of >2.0 . We attribute this mechanical robustness to several factors, including grain boundary strengthening, the presence of precursor crystalline phases, and a decrease in crystallinity or ordering caused by ultrafast processing. The creation of R-LLTO-a ceramic material with elastic properties that are closer to polymers with higher fracture toughness-enables new possibilities for the design of robust solid-state batteries.

Robust, High-Performing Maize-Perovskite-Based Solar Cells with Improved Stability.

Herein, we focus on improving the long-term chemical and thermomechanical stability of perovskite solar cells (PSCs), two major challenges currently limiting their commercial deployment. Our strategy incorporates a long-chain starch polymer into the perovskite precursor. The starch polymer confers multiple beneficial effects by forming hydrogen bonds with the methylammonium iodide precursor, templating perovskite growth that results in a compact and homogeneous film deposited in a simple one-step coating (antisolvent-free). The inclusion of starch in the methylammonium lead iodide films strongly improves their thermomechanical and environmental stability while maintaining a high photovoltaic performance. The fracture energy (G c) of the film is increased to above 5 J/m2 by creating a nanocomposite that provides intrinsic reinforcement at grain boundaries. Additionally, improved optoelectronic properties achieved with the starch polymer enable good photostability of the active layer and enhanced resistance to thermal cycling.

Open-Air Plasma-Deposited Multilayer Thin-Film Moisture Barriers.

Emerging moisture sensitive devices require robust encapsulation strategies to inhibit water ingress and prevent premature failure. A scalable, open-air plasma process has been developed to deposit alternating layers of conformal organosilicate and dense SiO2 thin-film barriers to prevent moisture ingress. The in situ low-temperature process is suitable for direct deposition on thermally sensitive devices and is compatible with flexible polymeric substrates. Using optical calcium testing, low water vapor transmission rates on the order of 10-3 g/m2/day at an accelerated aging condition of 38 °C and 90% relative humidity (RH) are achieved. Using moisture-sensitive perovskite devices as a representative moisture-susceptible device, devices retain over 80% of their initial performance for over 660 h in a 50 °C 50% RH accelerated aging environment. The ability of the multilayer barrier to enable device resistance to humid environments is crucial toward realizing longer operating lifetimes.

Comment on "Light-induced lattice expansion leads to high-efficiency perovskite solar cells".

Tsai et al (Reports, 6 April 2018, p. 67) report a uniform light-induced lattice expansion of metal halide perovskite films under 1-sun illumination and claim to exclude heat-induced lattice expansion. We show that by controlling the temperature of the perovskite film under both dark and illuminated conditions, the mechanism for lattice expansion is in fact fully consistent with heat-induced thermal expansion during illumination.

Hole-Transport Layer Molecular Weight and Doping Effects on Perovskite Solar Cell Efficiency and Mechanical Behavior.

The effect of tuning molecular weight ( Mn) in poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) to increase both mechanical properties of the film and electrical properties of perovskite solar cells is reported. Perovskite solar cell devices are fabricated to investigate the effect of Mn on power conversion efficiency. Moisture stability for various Mn is also studied in PTAA films exposed to mechanical loads in humid environments. Furthermore, cohesion and tensile tests are employed to determine the mechanical properties of PTAA, where higher Mn leads to more robust films. To elucidate the effect of Mn on the debonding kinetics, a viscoelastic fracture kinetic model is proposed as a function of Mn, and the debonding mechanism is found to be dependent on Mn. Finally, the effect of small-molecule-based dopants on the mechanical stability of PTAA is investigated.

Unravelling Atomic Structure and Degradation Mechanisms of Organic-Inorganic Halide Perovskites by Cryo-EM.

Despite rapid progress of hybrid organic-inorganic halide perovskite solar cells, using transmission electron microscopy to study their atomic structures has not been possible because of their extreme sensitivity to electron beam irradiation and environmental exposure. Here, we develop cryogenic-electron microscopy (cryo-EM) protocols to preserve an extremely sensitive perovskite, methylammonium lead iodide (MAPbI3) under various operating conditions for atomic-resolution imaging. We discover the precipitation of lead iodide nanoparticles on MAPbI3 nanowire's surface after short UV illumination and surface roughening after only 10 s exposure to air, while these effects remain undetected in conventional x-ray diffraction. We establish a definition for critical electron dose, and find this value for MAPbI3 at cryogenic condition to be 12 e-/Å2 at 1.49 Å spatial resolution. Our results highlight the importance of cryo-EM since traditional techniques cannot capture important nanoscale changes in morphology and structure that have important implications for perovskite solar cell stability and performance.

Beyond Fullerenes: Indacenodithiophene-Based Organic Charge-Transport Layer toward Upscaling of Low-Cost Perovskite Solar Cells.

Phenyl-C61-butyric acid methyl ester (PCBM) is universally used as the electron-transport layer (ETL) in the low-cost inverted planar structure of perovskite solar cells (PeSCs). PCBM brings tremendous challenges in upscaling of PeSCs using industry-relevant methods due to its aggregation behavior, which undermines the power conversion efficiency and stability. Herein, we highlight these, seldom reported, challenges with PCBM. Furthermore, we investigate the potential of nonfullerene indacenodithiophene (IDT)-based molecules by employing a commercially available variant, 3,9-bis(2-methylene-(3-(1,1-dicyanomethylene)-indanone))-5,5,11,11-tetrakis(4-hexylphenyl)-dithieno[2,3- d:2',3'- d']- s-indaceno[1,2- b:5,6- b'] dithiophene (ITIC), as a PCBM replacement in ambient-processed PeSCs. Films fabrication by laboratory-based spin-coating and industry-relevant slot-die coating methods are compared. Although similar power-conversion efficiencies are achieved with both types of ETL in a simple device structure fabricated by spin-coating, the nanofibriller morphology of ITIC compared to the aggregated morphology of PCBM films enables improved mechanical integrity and stability of ITIC devices. Upon slot-die coating, the aggregation of PCBM is exacerbated, leading to significantly lower power-conversion efficiency of devices than spin-coated PCBM as well as slot-die-coated ITIC devices. Our results clearly indicate that IDT-based molecules have great potential as an ETL in PeSCs, offering superior properties and upscaling compatibility than PCBM. Thus, we present a short summary of recently emerged nonfullerene IDT-based molecules from the field of organic solar cells and discuss their scope in PeSCs as electron or hole-transport layer.

Cross-Linkable, Solvent-Resistant Fullerene Contacts for Robust and Efficient Perovskite Solar Cells with Increased JSC and VOC.

The active layers of perovskite solar cells are also structural layers and are central to ensuring that the structural integrity of the device is maintained over its operational lifetime. Our work evaluating the fracture energies of conventional and inverted solution-processed MAPbI3 perovskite solar cells has revealed that the MAPbI3 perovskite exhibits a fracture resistance of only ∼0.5 J/m2, while solar cells containing fullerene electron transport layers fracture at even lower values, below ∼0.25 J/m2. To address this weakness, a novel styrene-functionalized fullerene derivative, MPMIC60, has been developed as a replacement for the fragile PC61BM and C60 transport layers. MPMIC60 can be transformed into a solvent-resistant material through curing at 250 °C. As-deposited films of MPMIC60 exhibit a marked 10-fold enhancement in fracture resistance over PC61BM and a 14-fold enhancement over C60. Conventional-geometry perovskite solar cells utilizing cured films of MPMIC60 showed a significant, 205% improvement in fracture resistance while exhibiting only a 7% drop in PCE (13.8% vs 14.8% PCE) in comparison to the C60 control, enabling larger VOC and JSC values. Inverted cells fabricated with MPMIC60 exhibited a 438% improvement in fracture resistance with only a 6% reduction in PCE (12.3% vs 13.1%) in comparison to those utilizing PC61BM, again producing a higher JSC.