Nobuya Sakai

A mixed-cation lead mixed-halide perovskite absorber for tandem solar cells

Perovskites for tandem solar cells Improving the performance of conventional single-crystalline silicon solar cells will help increase their adoption. The absorption of bluer light by an inexpensive overlying solar cell in a tandem arrangement would provide a step in the right direction by improving overall efficiency. Inorganic-organic perovskite cells can be tuned to have an appropriate band gap, but these compositions are prone to decomposition. McMeekin et al. show that using cesium ions along with formamidinium cations in lead bromide–iodide cells improved thermal and photostability. These improvements lead to high efficiency in single and tandem cells. Science, this issue p. 151 Addition of cesium cations creates a robust ideal inorganic-organic perovskite absorber for tandem silicon solar cells. Metal halide perovskite photovoltaic cells could potentially boost the efficiency of commercial silicon photovoltaic modules from ∼20 toward 30% when used in tandem architectures. An optimum perovskite cell optical band gap of ~1.75 electron volts (eV) can be achieved by varying halide composition, but to date, such materials have had poor photostability and thermal stability. Here we present a highly crystalline and compositionally photostable material, [HC(NH2)2]0.83Cs0.17Pb(I0.6Br0.4)3, with an optical band gap of ~1.74 eV, and we fabricated perovskite cells that reached open-circuit voltages of 1.2 volts and power conversion efficiency of over 17% on small areas and 14.7% on 0.715 cm2 cells. By combining these perovskite cells with a 19%-efficient silicon cell, we demonstrated the feasibility of achieving >25%-efficient four-terminal tandem cells.

Emergence of Hysteresis and Transient Ferroelectric Response in Organo-Lead Halide Perovskite Solar Cells.

Although there has been rapid progress in the efficiency of perovskite-based solar cells, hysteresis in the current-voltage performance is not yet completely understood. Owing to its complex structure, it is not easy to attribute the hysteretic behavior to any one of different components, such as the bulk of the perovskite or different heterojunction interfaces. Among organo-lead halide perovskites, methylammonium lead iodide perovskite (CH3NH3PbI3) is known to have a ferroelectric property. The present investigation reveals a strong correlation between transient ferroelectric polarization of CH3NH3PbI3 induced by an external bias in the dark and hysteresis enhancement in photovoltaic characteristics. Our results demonstrate that the reverse bias poling (-0.3 to -1.1 V) of CH3NH3PbI3 photovoltaic layers prior to the photocurrent-voltage measurement generates stronger hysteresis whose extent changes significantly by the cell architecture. The phenomenon is interpreted as the effect of remanent polarization in the perovskite film on the photocurrent, which is most enhanced in planar perovskite structures without mesoporous scaffolds.

Lead-Free Halide Double Perovskites via Heterovalent Substitution of Noble Metals

Lead-based halide perovskites are emerging as the most promising class of materials for next-generation optoelectronics; however, despite the enormous success of lead-halide perovskite solar cells, the issues of stability and toxicity are yet to be resolved. Here we report on the computational design and the experimental synthesis of a new family of Pb-free inorganic halide double perovskites based on bismuth or antimony and noble metals. Using first-principles calculations we show that this hitherto unknown family of perovskites exhibits very promising optoelectronic properties, such as tunable band gaps in the visible range and low carrier effective masses. Furthermore, we successfully synthesize the double perovskite Cs2BiAgCl6, perform structural refinement using single-crystal X-ray diffraction, and characterize its optical properties via optical absorption and photoluminescence measurements. This new perovskite belongs to the Fm3̅m space group and consists of BiCl6 and AgCl6 octahedra alternating in a rock-salt face-centered cubic structure. From UV-vis and photoluminescence measurements we obtain an indirect gap of 2.2 eV.

Atmospheric Influence upon Crystallization and Electronic Disorder and Its Impact on the Photophysical Properties of Organic–Inorganic Perovskite Solar Cells

Recently, solution-processable organic-inorganic metal halide perovskites have come to the fore as a result of their high power-conversion efficiencies (PCE) in photovoltaics, exceeding 17%. To attain reproducibility in the performance, one of the critical factors is the processing conditions of the perovskite film, which directly influences the photophysical properties and hence the device performance. Here we study the effect of annealing parameters on the crystal structure of the perovskite films and correlate these changes with its photophysical properties. We find that the crystal formation is kinetically driven by the annealing atmosphere, time and temperature. Annealing in air produces an improved crystallinity and large grain domains as compared to nitrogen. Lower photoluminescence quantum efficiency (PLQE) and shorter photoluminescence (PL) lifetimes are observed for nitrogen annealed perovskite films as compared to the air-annealed counterparts. We note that the limiting nonradiative pathways (i.e., maximizing PLQE) is important for obtaining the highest device efficiency. This indicates a critical impact of the atmosphere upon crystallization and the ultimate device performance.

Atomistic origins of CH3NH3PbI3 degradation to PbI2 in vacuum

We study the mechanisms of CH3NH3PbI3 degradation and its transformation to PbI2 by means of X-ray diffraction and the density functional theory. The experimental analysis shows that the material can degrade in both air and vacuum conditions, with humidity and temperature-annealing strongly accelerating such process. Based on ab initio calculations, we argue that even in the absence of humidity, a decomposition of the perovskite structure can take place through the statistical formation of molecular defects with a non-ionic character, whose volatility at surfaces should break the thermodynamic defect equilibria. We finally discuss the strategies that can limit such phenomenon and subsequently prolong the lifetime of the material.

The mechanism of toluene-assisted crystallization of organic–inorganic perovskites for highly efficient solar cells

We investigate the influence of solvent drenching in hybrid organic–inorganic perovskite (CH3NH3PbX) crystallization process with a non-solvent, toluene, during film fabrication process. We use three different precursor compositions, CH3NH3I (MAI):PbI2, 3MAI:PbI2 and 3MAI:PbCl2 to unravel the crystallization mechanism with toluene drenching. The mixed halide precursor (3MAI:PbCl2) results in the highest quality films with the toluene treatment, including high surface coverage, large grains, long PL lifetimes and high photoluminescence quantum efficiency (PLQE). The neat halide-based precursors (MAI:PbI2 and 3MAI:PbI2) with the treatment have increased photo-physical properties (PL lifetime and PLQE) and a surface coverage, but slightly decrease grain size in the film, while the 3MAI:PbI2 precursor has still formed numerous pinholes in the film. Mechanistically, we visually observe that the toluene drenching accelerates the nucleation at early stage of crystallization in 3MAI:PbCl2 precursor. X-ray diffraction pattern in this stage confirms the formation of both MAPbI3 and MAPbCl3, nucleation. During the crystallization process MAPbCl3 is transformed into MAPbI3 phase by the anion exchange. Toluene treatment strongly affects the ratio of MAPbI3 and MAPbCl3, nucleation and hence plays a critical role in deciding the final film morphology, their optoelectronic properties and hence their device performances.

A Universal Deposition Protocol for Planar Heterojunction Solar Cells with High Efficiency Based on Hybrid Lead Halide Perovskite Families

A robust and expedient gas quenching method is developed for the solution deposition of hybrid perovskite thin films. The method offers a reliable standard practice for the fabrication of a non-exhaustive variety of perovskites exhibiting excellent film morphology and commensurate high performance in both regular and inverted structured solar cell architectures.

Similar Structural Dynamics for the Degradation of CH3 NH3 PbI3 in Air and in Vacuum.

We investigate the degradation path of MAPbI3 (MA=methylammonium) films over flat TiO2 substrates at room temperature by means of X-ray diffraction, spectroscopic ellipsometry, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The degradation dynamics is found to be similar in air and under vacuum conditions, which leads to the conclusion that the occurrence of intrinsic thermodynamic mechanisms is not necessarily linked to humidity. The process has an early stage, which drives the starting tetragonal lattice in the direction of a cubic atomic arrangement. This early stage is followed by a phase change towards PbI2 . We describe how this degradation product is structurally coupled with the original MAPbI3 lattice through the orientation of its constituent PbI6 octahedra. Our results suggest a slight octahedral rearrangement after volatilization of HI+CH3 NH2 or MAI, with a relatively low energy cost. Our experiments also clarify why reducing the interfaces and internal defects in the perovskite lattice enhances the stability of the material.

Interface-Dependent Ion Migration/Accumulation Controls Hysteresis in MAPbI3 Solar Cells

Hysteresis in the current-voltage characteristics of hybrid organic-inorganic perovskite-based solar cells is one of the fundamental aspects of these cells that we do not understand well. One possible cause, suggested for the hysteresis, is polarization of the perovskite layer under applied voltage and illumination bias, due to ion migration within the perovskite. To study this problem systemically current-voltage characteristics of both regular (light incident through the electron conducting contact) and so-called inverted (light incident through the hole conducting contact) perovskite cells were studied at different temperatures and scan rates. We explain our results by assuming that the effects of scan rate and temperature on hysteresis are strongly correlated to ion migration within the device, with the rate-determining step being ion migration at/across the interfaces of the perovskite layer with the contact materials. By correlating between the scan rate with the measurement temperature we show that the inverted and regular cells operate in different hysteresis regimes, with different activation energies of 0.28+-0.04 eV and 0.59+-0.09 eV, respectively. We suggest that the differences, observed between the two architectures are due to different rates of ion migration close to the interfaces, and conclude that the diffusion coefficient of migrating ions in the inverted cells is 3 orders of magnitude higher than in the regular cells, leading to different accumulation rates of ions near the interfaces. Analysis of VOC as a function of temperature shows that the main recombination mechanism is trap-assisted (Shockley-Read Hall, SRH) in the space charge region, similar to what is the case for other thin film inorganic solar cells.

Plastic based dye-sensitized solar cells using Co9S8 acicular nanotube arrays as the counter electrode

Novel one-dimensional (1D) Co9S8 acicular nanotube arrays (ANTAs) are fabricated on a conducting plastic substrate by a two-step approach. Layered cobalt carbonate hydroxide (Co(CO3)0.5(OH)x·11H2O, LCCH) acicular nanorod arrays (ANRAs) are fabricated on a conducting plastic substrate by chemical bath deposition (CBD), followed by a simple ionic-exchange process to convert the LCCH ANRAs into Co9S8 ANTAs. The compositions of the films are verified by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and energy dispersive spectroscopy (EDS) mapping; their morphologies are examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The films of Co9S8 ANTAs, obtained after various periods of the CBD, are analyzed by cyclic voltammetry (CV) measurements and Tafel polarization curves, and the Co9S8 ANTAs obtained from 150 min of CBD show higher electrocatalytic ability towards the I−/I3− reaction than sputtered Pt. In addition, the long-term stability of the Co9S8 ANTAs film in I−/I3− electrolyte was tested by CV. The plastic based Co9S8 ANTAs electrodes are used as the counter electrodes (CEs) of flexible dye-sensitized solar cells (DSSCs), and a high power conversion efficiency of 5.47% is achieved, which is comparable to that of the DSSC using sputtered Pt (5.62%). Therefore, the Co9S8 ANTAs are proposed to be a reliable material to replace Pt as plastic based CEs of DSSCs.