Synthesis and Growth Mechanisms of One-Dimensional Strontium Hydroxyapatite Nanostructures

Strontium hydroxyapatite (SrHAp) nanowires with an aspect ratio of several hundreds were synthesized by controlling the growth conditions during a hydrothermal process. In the strontium phosphate system, it was found that the phase evolution changed with pH and that the aspect ratio of SrHAp was affected by the phases present before heating. Since the conditions for SrHAp nucleation prohibits one-dimensional growth, it was impossible to grow large-scale SrHAp nanowires using routine hydrothermal methods. Through thermodynamic considerations, the mechanisms of nanowire formation appear to involve the rapid release of the stored chemical potential in a metastable phase, which promotes the anisotropic growth of the most stable SrHAp nanostructures. Thereby, the conditions for both the nucleation of the SrHAp phase and the anisotropic growth were determined simultaneously, and considerable quantities of SrHAp nanowires were synthesized

Origins of Efficient Perovskite Solar Cells with Low-Temperature Processed SnO₂ Electron Transport Layer

Recently, SnO2 has been noticed as a promising material for electron-transport layer of planar perovskite solar cells. SnO2 layer presents advantages of low-temperature processability and high power-conversion efficiency, and understanding the correlations between the SnO2 properties and device performance will provide a key to realize more efficient perovskite solar cells. Herein, uniform electron-transport layer using SnO2 nanoparticles is fabricated, and the effect of annealing on the solar-cell performance is discussed. Solar cells with low-temperature processed SnO2-nanoparticle layer (below 120 °C or even at room temperature) exhibit desirable short-circuit current, open-circuit voltage, and fill factor with the highest efficiency of 19.0%. Using atomic force microscopy and ultraviolet photoelectron spectroscopy, both great surface uniformity and favorable band alignment of low-temperature processed SnO2 layer have been observed, which are responsible for the device performance. Furthermore, deep electronic-trap states at the SnO2/perovskite interface are investigated via impedance analysis. Compared to the cells processed over 160 °C, low-temperature processed cells exhibit trap states shifting toward the bandedge and reduced trap density, verifying that controlling the interfacial trap states holds a dominance on the open-circuit voltage and is a critical requisite to enable efficient perovskite solar cells. These less-defective solar cells fabricated below 120 °C show high thermal stability, suggesting further commercial applications

Understanding the Trap Characteristics of Perovskite Solar Cells via Drive-Level Capacitance Profiling

Perovskite solar cells (PSCs) are gaining significant interest as the future of photovoltaics owing to their superior performance and cost-effectiveness. Nevertheless, traps in PSCs have emerged as issues that adversely affect the efficiency and stability of the devices. In this study, the methylammonium chloride (MACl) additive and phenyltrimethylammonium iodide (PTMAI) posttreatment were applied to passivate bulk and surface defects. Furthermore, variations of the traps’ quantitative spatial arrangement have been monitored by using the drive-level capacitance profiling (DLCP) analysis. A similar magnitude of trap reduction was observed for the bulk perovskite layer and two interfaces (electron transport layer (ETL)/perovskite and hole transport layer (HTL)/perovskite) with an optimal concentration of the MACl additive. However, the effect of perovskite posttreatment in reducing the trap density was much more noticeable at the HTL/perovskite interface compared to the bulk and ETL/perovskite regions. This observation was reinforced by the outcomes of the 500 h thermal stability tests at 60 °C from seven independent batches, which demonstrated a substantial suppression of trap accumulation, particularly at the HTL/perovskite interface, by an order of magnitude

Interfacial Modification and Defect Passivation by the Cross-Linking Interlayer for Efficient and Stable CuSCN-Based Perovskite Solar Cells

The study of the inorganic hole-transport layer (HTL) in perovskite solar cells (PSCs) is gathering attention because of the drawback of the conventional PSC design, where the organic HTL with salt dopants majorly participates in the degradation mechanisms. On the other hand, inorganic HTL secures better stability, while it offers difficulties in the deposition and interfacial control to realize high-performing devices. In this study, we demonstrate polydimethylsiloxane (PDMS) as an ideal polymeric interlayer which prevents interfacial degradation and improves both photovoltaic performance and stability of CuSCN-based PSC by its cross-linking behavior. Surprisingly, the PDMS polymers are identified to form chemical bonds with perovskite and CuSCN, as shown by Raman spectroscopy. This novel cross-linking interlayer of PDMS enhances the hole-transporting property at the interface and passivates the interfacial defects, realizing the PSC with high power-conversion efficiency over 19%. Furthermore, the utilization of the PDMS interlayer greatly improves the stability of solar cells against both humidity and heat by mitigating the interfacial defects and interdiffusion. The PDMS-interlayered PSCs retained over 90% of the initial efficiencies, both after 1000 h under ambient conditions (unencapsulated) and after 500 h under 85 °C/85% relative humidity (encapsulated)

Electrochemical Promotion of Oxygen Reduction on Gold with Aluminum Phosphate Overlayer

The activities of Au electrodes with an AlPO4 overlayer were examined for oxygen-reduction reactions in alkaline media. Oxygen molecules on gold catalysts are mainly reduced by a two-electron path, forming hydrogen peroxide with half efficiency. On the AlPO4 overlayer deposited Au, larger current densities corresponding to a nearly four-electron path were recorded within the potential range of approximately 0.7−1.0 V, which were correlated with the decomposition (disproportionation) of hydrogen peroxide. This enhancement was attributed to the electronic interactions and changed activities of the intermediate state, as confirmed by X-ray photoelectron spectroscopy and the voltammetric profiles of hydrogen peroxide, respectively

Evolution of the Electronic Traps in Perovskite Photovoltaics during 1000 h at 85 °C

With growing demands on the stability of perovskite photovoltaics against various degradation factors, understanding and controlling the defect characteristics of devices have become the most essential issues to be resolved. In this work, the organometal halide perovskite is modified with a lithium–fluoride ionic passivator that enables highly stable and efficient solar cells with a power-conversion efficiency of over 21%, retaining up to ∼90% after 1000 h at 85 °C. The thermal degradation regressions of the films and devices have been temporally investigated, and the trap density of states has been scrutinized as a function of time. Surprisingly, the electronic traps of the solar cells exhibit exponential relaxations in both the trap densities and energy levels as thermally stressed, and the incorporation of LiF has greatly enhanced this relaxation with the mitigation of the following degradation. It is suggested that LiF not only passivates the initial formation of the traps but also controls their roles and behaviors under the thermal degradation of devices

Modification of Gold Catalysis with Aluminum Phosphate for Oxygen-Reduction Reaction

The activities of Au/AlPO4 nanocomposites with the variation of metal phosphates were examined for oxygen-reduction reactions, both in an alkaline and acidic environment. In an alkaline media, the activities of the Au/AlPO4 nanocomposites on the oxygen-reduction were enhanced. The steeper reduction slope as well as larger reduction current density were observed in the potential range of approximately 0.8−1.0 V (vs reversible hydrogen electrode) with the newly appeared peak at ∼0.85 V. In an acidic media, the oxygen reduction on the Au/AlPO4 nanocomposites presented both higher onset potential and steeper reduction slope than that on the Au catalyst. Such enhancements were attributed to the electronic interactions between Au and AlPO4, as confirmed by X-ray photoelectron spectroscopy

Micro/Nanostructural Analyses of Efficient and Stable Perovskite Solar Cells via KF Doping

The migration of ionic defects in grain boundaries plays a critical role in the stability and efficiency of organic–inorganic perovskite solar cells. Furthermore, the ionic defects of perovskites also contribute to the generation of hysteresis. Herein, we alloy KF with perovskites to passivate ionic defects, leading to the improved stability and performance. We obtain a power conversion efficiency of 21.2% by alloying KF with triple cation perovskites [Cs0.05(FA0.83MA0.17)0.95Pb­(I0.83Br0.17)3]. With KF, the current–voltage hysteresis of solar cells becomes negligible, and the trap density of the device is reduced. Photostability and thermal stability of the devices were also improved. Approximately 90% of the initial efficiency was maintained after 1200 h under 1 sun illumination, and ∼80% for 2000 h at 85 °C/85% relative humidity. This study confirmed that K+ ions bond to halide ions in the alloyed perovskite and suppress the ionic defects, thereby reducing the trap density and metallic lead impurities resulting in the improved stabilities

Additional file 1: Figure S1. of Tailoring the Mesoscopic TiO2 Layer: Concomitant Parameters for Enabling High-Performance Perovskite Solar Cells

SEM images showing the TiO2 nanostructures with the PbI2 pre-coating and MAPbI3(Cl) infiltration into the PS-templated TiO2. Figure S2. The effect of PS ratio and the concentration of precursor solution on the X-ray diffraction of MAPbI3(Cl) perovskite. Figure S3. Cross-sectional back scattered electron images exhibiting the MAPbI3(Cl) perovskite infiltration in the porous TiO2 layer. Figure S4. Cross-sectional elemental distributions from energy dispersive X-ray spectroscopy (SEM-EDS) showing the Sn, Ti, O, Pb, and I distributions for different porous TiO2 scaffolds. Figure S5. Microstructures of MAPbI3(Cl) on the TiO2 blocking layer. Figure S6. Photovoltaic parameters with the average and the standard deviation in each condition. Figure S7. Ideal one-diode model for the perovskite solar cell. Figure S8. Current density vs. bias under dark and the corresponding fitting results. Figure S9. The effect of TiO2 blocking layer by sputter deposition on the performance of the perovskite solar cell. Figure S10. Morphology comparison by the spin-coating and sputter deposition