PbI3 Perovskite Research Articles

Theoretical Treatment of CH3 NH3 PbI3 Perovskite Solar Cells.

Hybrid halide perovskite solar cells (PSCs) giving over 22 % power conversion efficiencies (PCEs) have attracted considerable attention. Although perovskite plays a significant role in the operation of PSCs, the fundamental theories associated with perovskites have not been resolved in spite of the increase in research. In this Minireview, we assess the current understanding, based on the first-principles calculations, of structural and electronic properties, defects, ionic diffusion, and shift current for CH3 NH3 PbI3 perovskite, and the effect of ionic transport on the hysteresis of current-voltage curves in PSCs. The shift current connected to the possible presence of ferroelectricity is also discussed. The current state-of-the-art and some open questions regarding PSCs are also highlighted, and the benefits, challenges, and potentials of perovskite for use in PSCs are stressed.

Solution-Processed Lithium-Doped ZnO Electron Transport Layer for Efficient Triple Cation (Rb, MA, FA) Perovskite Solar Cells.

The current work reports the lithium (Li) doping of a low-temperature processed zinc oxide (ZnO) electron transport layer (ETL) for highly efficient, triple-cation-based MA0.57FA0.38Rb0.05PbI3 (MA: methylammonium, FA: formamidinium, Rb: rubidium) perovskite solar cells (PSCs). Lithium intercalation in the host ZnO lattice structure is dominated by interstitial doping phenomena, which passivates the intrinsic defects in ZnO film. In addition, interstitial Li doping also downshifts the Fermi energy position of Li-doped ETL by 30 meV, which contributes to the reduction of the electron injection barrier from the photoactive perovskite layer. Compared to the pristine ZnO, the power conversion efficiency (PCE) of the PSCs incorporating lithium-doped ZnO (Li-doped) is raised from 14.07 to 16.14%. The superior performance is attributed to the reduced current leakage, enhanced charge extraction characteristics, and mitigated trap-assisted recombination phenomena in Li-doped devices, thoroughly investigated by means of electrochemical impedance spectroscopy (EIS) analysis. Li-doped PSCs also exhibit lower photocurrent hysteresis than ZnO devices, which is investigated with regard to the electrode polarization phenomena of the fabricated devices.

TiO_2 nanotube/TiO_2 nanoparticle hybrid photoanode for hole-conductor-free perovskite solar cells based on carbon counter electrodes

In this work, TiO2 nanotubes were assembled in the hole-conductor-free, carbon counter electrode-based (CH3NH3)PbI3 perovskite solar cells based on the TiO2 nanotube/TiO2 nanoparticle hybrid photoanode under ambient conditions, on which TiO2 nanotubes were uniformly embedded in the hybrid photoanode, and their structure was not affected by the assembly process. Compared with cells based on TiO2 nanoparticles, the cells exhibited a power conversion efficiency of 9.16% that was improved by 40% under similar conditions. The optimized device structure provided effective transport pathways for carrier and carrier recombination was effectively suppressed. In addition, the TiO2 hybrid photoanode ensured efficient light trapping by the sensitizers, leading to an increase in the number of photogenerated carriers.

Top-Down Fabrication of Stable Methylammonium Lead Halide Perovskite Nanocrystals by Employing a Mixture of Ligands as Coordinating Solvents.

A top-down method is demonstrated for the fabrication of CH3 NH3 PbBr3 and CH3 NH3 PbI3 perovskite nanocrystals, employing a mixture of ligands oleic acid and oleylamine as coordinating solvents. This approach avoids the use of any polar solvents, skips multiple reaction steps by employing a simple ultrasonic treatment of the perovskite precursors, and yields rather monodisperse blue-, green-, and red-emitting methylammonium lead halide nanocrystals with a high photoluminescence quantum yield (up to 72 % for the green-emitting nanocrystals) and remarkably improved stability. After discussing all relevant reaction parameters, the green-emitting CH3 NH3 PbBr3 nanocrystals are employed as a component of down-conversion white-light-emitting devices.

Reduced Interface-Mediated Recombination for High Open-Circuit Voltages in CH3 NH3 PbI3 Solar Cells.

Perovskite solar cells with all-organic transport layers exhibit efficiencies rivaling their counterparts that employ inorganic transport layers, while avoiding high-temperature processing. Herein, it is investigated how the choice of the fullerene derivative employed in the electron-transporting layer of inverted perovskite cells affects the open-circuit voltage (VOC ). It is shown that nonradiative recombination mediated by the electron-transporting layer is the limiting factor for the VOC in the cells. By inserting an ultrathin layer of an insulating polymer between the active CH3 NH3 PbI3 perovskite and the fullerene, an external radiative efficiency of up to 0.3%, a VOC as high as 1.16 V, and a power conversion efficiency of 19.4% are realized. The results show that the reduction of nonradiative recombination due to charge-blocking at the perovskite/organic interface is more important than proper level alignment in the search for ideal selective contacts toward high VOC and efficiency.

Indium-Free Perovskite Solar Cells Enabled by Impermeable Tin-Oxide Electron Extraction Layers.

Corrosive precursors used for the preparation of organic-inorganic hybrid perovskite photoactive layers prevent the application of ultrathin metal layers as semitransparent bottom electrodes in perovskite solar cells (PVSCs). This study introduces tin-oxide (SnOx ) grown by atomic layer deposition (ALD), whose outstanding permeation barrier properties enable the design of an indium-tin-oxide (ITO)-free semitransparent bottom electrode (SnOx /Ag or Cu/SnOx ), in which the metal is efficiently protected against corrosion. Simultaneously, SnOx functions as an electron extraction layer. We unravel the spontaneous formation of a PbI2 interfacial layer between SnOx and the CH3 NH3 PbI3 perovskite. An interface dipole between SnOx and this PbI2 layer is found, which depends on the oxidant (water, ozone, or oxygen plasma) used for the ALD growth of SnOx . An electron extraction barrier between perovskite and PbI2 is identified, which is the lowest in devices based on SnOx grown with ozone. The resulting PVSCs are hysteresis-free with a stable power conversion efficiency (PCE) of 15.3% and a remarkably high open circuit voltage of 1.17 V. The ITO-free analogues still achieve a high PCE of 11%.

CsI Pre-Intercalation in the Inorganic Framework for Efficient and Stable FA1-x Csx PbI3 (Cl) Perovskite Solar Cells.

Engineering the chemical composition of organic and inorganic hybrid perovskite materials is one of the most feasible methods to boost the efficiency of perovskite solar cells with improved device stability. Among the diverse hybrid perovskite family of ABX3 , formamidinium (FA)-based mixed perovskite (e.g., FA1-x Csx PbI3 ) possesses optimum bandgaps, superior optoelectronic property, as well as thermal- and photostability, which is proven to be the most promising candidate for advanced solar cell. Here, FA0.9 Cs0.1 PbI3 (Cl) is implemented as the light-harvesting layer in planar devices, whereas a low temperature, two-step solution deposition method is employed for the first time in this materials system. This paper comprehensively exploits the role of Cs+ in the FA0.9 Cs0.1 PbI3 (Cl) perovskite that affects the precursor chemistry, film nucleation and grain growth, and defect property via pre-intercalation of CsI in the inorganic framework. In addition, the resultant FA0.9 Cs0.1 PbI3 (Cl) films are demonstrated to exhibit an improved optoelectronic property with an elevated device power conversion efficiency (PCE) of 18.6%, as well as a stable phase with substantial enhancement in humidity and thermal stability, as compared to that of FAPbI3 (Cl). The present method is able to be further extended to a more complicated (FA,MA,Cs)PbX3 material system by delivering a PCE of 19.8%.

Electrical Stress Influences the Efficiency of CH3NH3PbI3 Perovskite Light Emitting Devices

Organic-inorganic hybrid perovskite materials are emerging as semiconductors with potential application in optoelectronic devices. In particular, perovskites are very promising for light-emitting devices (LEDs) due to their high color purity, low nonradiative recombination rates, and tunable bandgap. Here, using pure CH3 NH3 PbI3 perovskite LEDs with an external quantum efficiency (EQE) of 5.9% as a platform, it is shown that electrical stress can influence device performance significantly, increasing the EQE from an initial 5.9% to as high as 7.4%. Consistent with the enhanced device performance, both the steady-state photoluminescence (PL) intensity and the time-resolved PL decay lifetime increase after electrical stress, indicating a reduction in nonradiative recombination in the perovskite film. By investigating the temperature-dependent characteristics of the perovskite LEDs and the cross-sectional elemental depth profile, it is proposed that trap reduction and resulting device-performance enhancement is due to local ionic motion of excess ions, likely excess mobile iodide, in the perovskite film that fills vacancies and reduces interstitial defects. On the other hand, it is found that overstressed LEDs show irreversibly degraded device performance, possibly because ions initially on the perovskite lattice are displaced during extended electrical stress and create defects such as vacancies.

Amino-Acid-Induced Preferential Orientation of Perovskite Crystals for Enhancing Interfacial Charge Transfer and Photovoltaic Performance.

Interfacial engineering of perovskite solar cells (PSCs) is attracting intensive attention owing to the charge transfer efficiency at an interface, which greatly influences the photovoltaic performance. This study demonstrates the modification of a TiO2 electron-transporting layer with various amino acids, which affects charge transfer efficiency at the TiO2 /CH3 NH3 PbI3 interface in PSC, among which the l-alanine-modified cell exhibits the best power conversion efficiency with 30% enhancement. This study also shows that the (110) plane of perovskite crystallites tends to align in the direction perpendicular to the amino-acid-modified TiO2 as observed in grazing-incidence wide-angle X-ray scattering of thin CH3 NH3 PbI3 perovskite film. Electrochemical impedance spectroscopy reveals less charge transfer resistance at the TiO2 /CH3 NH3 PbI3 interface after being modified with amino acids, which is also supported by the lower intensity of steady-state photoluminescence (PL) and the reduced PL lifetime of perovskite. In addition, based on the PL measurement with excitation from different side of the sample, amino-acid-modified samples show less surface trapping effect compared to the sample without modification, which may also facilitate charge transfer efficiency at the interface. The results suggest that appropriate orientation of perovskite crystallites at the interface and trap-passivation are the niche for better photovoltaic performance.

Controlled Ostwald ripening mediated grain growth for smooth perovskite morphology and enhanced device performance

Here we report, a novel two-step dipping technique via post-immersion polar solvent engineering for controlled secondary grain growth (Ostwald Ripening) to fabricate efficient mixed organic cation based MA0.6FA0.4PbI3 perovskite solar cell (PSC) in conjunction with low temperature (140°C) processed sol-gel ZnO ETL for full process compatibility with flexible substrates. The reported MTD-SE method (stands for Modified Two Step Dipping - Solvent Engineering) limits the grain coarsening effect during post-immersion stage of two-step dipping method and provides substantially smooth perovskite surface morphology for enhanced charge transport properties compared to conventional two-step techniques by means of controlled Ostwald Ripening process. The grain coarsening process and concomitant irregular grain size distribution are judiciously controlled by increasing the chemical potential or free energy change (ΔG) of the system at the post-immersion. The photovoltaic performance and photo-current hysteresis phenomena of the reported MTD-SE PSC have been compared with PSCs fabricated with conventional two-step techniques, incorporating 2-Propanol or ethyl alcohol as dipping solvents. The enhanced device performance of MTD-SE PSCs is correlated with the conducive role of the evenly distributed grain boundaries in them, which act as carrier dissociation interfaces and carrier transport pathways to charge selective contacts for superior charge separation and extraction properties. Adding to the merits, MTD-SE PSCs also demonstrate significantly suppressed photo-current hysteretic behaviour which has been elucidated in the context of faster ion migration kinetics with the increased grain boundaries, which exhibit higher ionic diffusivity. The favourable ion migration kinetics with MTD-SE PSC have also been comprehensively analysed from the frequency-dependent capacitive spectra.

SrCl2 Derived Perovskite Facilitating a High Efficiency of 16% in Hole-Conductor-Free Fully Printable Mesoscopic Perovskite Solar Cells.

Despite the breakthrough of over 22% power conversion efficiency demonstrated in organic-inorganic hybrid perovskite solar cells (PVSCs), critical concerns pertaining to the instability and toxicity still remain that may potentially hinder their commercialization. In this study, a new chemical approach using environmentally friendly strontium chloride (SrCl2 ) as a precursor for perovskite preparation is demonstrated to result in enhanced device performance and stability of the derived hole-conductor-free printable mesoscopic PVSCs. The CH3 NH3 PbI3 perovskite is chemically modified by introducing SrCl2 in the precursor solution. The results from structural, elemental, and morphological analyses show that the incorporation of SrCl2 affords the formation of CH3 NH3 PbI3 (SrCl2 )x perovskites endowed with lower defect concentration and better pore filling in the derived mesoscopic PVSCs. The optimized compositional CH3 NH3 PbI3 (SrCl2 )0.1 perovskite can substantially enhance the photovoltaic performance of the derived hole-conductor-free device to 15.9%, outperforming the value (13.0%) of the pristine CH3 NH3 PbI3 device. More importantly, the stability of the device in ambient air under illumination is also improved.

Controlling Nucleation and Growth of Metal Halide Perovskite Thin Films for High-Efficiency Perovskite Solar Cells.

Metal halide perovskite thin films can be crystallized via a broad range of solution-based routes. However, the quality of the final films is strongly dependent upon small changes in solution composition and processing parameters. Here, this study demonstrates that a fractional substitution of PbCl2 with PbI2 in the 3CH3 NH3 I:PbCl2 mixed-halide starting solution has a profound influence upon the ensuing thin-film crystallization. The presence of PbI2 in the precursor induces a uniform distribution of regular quadrilateral-shaped CH3 NH3 PbI3 perovskite crystals in as-cast films, which subsequently grow to form pinhole-free perovskite films with highly crystalline domains. With this new formulation of 3CH3 NH3 I:0.98PbCl2 :0.02PbI2 , this study achieves a 19.1% current-voltage measured power conversion efficiency and a 17.2% stabilized power output in regular planar heterojunction solar cells.

A high performance and low-cost hole transporting layer for efficient and stable perovskite solar cells.

Here we report a small molecule oxidant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4TCNQ) doped, low cost 2',7'-bis(bis(4-methoxyphenyl)amino)spiro[cyclopenta[2,1-b:3,4-b']dithiophene-4,9'-fluorene] (FDT) hole transporting layer (HTL) for efficient mixed organic cation based MA0.6FA0.4PbI3 (MA = methyl ammonium, FA = formamidinium) perovskite solar cells (PSCs), fabricated via a highly reproducible controlled nucleation assisted restricted volume solvent annealing method, having full temperature compatibility with flexible substrates. The optimized (1 wt%) F4TCNQ doped FDT HTL based devices (F-FDT devices) demonstrate simultaneous enhancement of photovoltaic performance and device stability as well as significant reduction in photo-current hysteresis, as compared to conventional bis(trifluoromethylsulfonyl)amine lithium (Li-TFSI) additive based FDT HTL devices (L-FDT devices). Adding to the merits, F-FDT PSCs exhibit about 75% higher device stability compared to conventional L-FDT devices during the course of three weeks. Mott-Schottky analysis and in-depth charge transport characterization were carried out using electrochemical impedance spectroscopy (EIS) of the fabricated devices to understand the superior performance of the F-FDT devices. In addition, detailed polaronic intensity characterization of the doped HTL films was performed via ultraviolet-visible near-infrared (UV-vis-NIR) spectroscopy to investigate the underlying mechanism. Mitigated photocurrent hysteresis in the F-FDT devices has also been examined in terms of the inherent electrode polarization phenomenon. Furthermore, the superior device stability of the F-FDT PSCs has been probed in terms of variation in electronic properties, surface wettability, crystallinity, and microstrain dislocation density, and a detailed picture of the underlying mechanism behind stability enhancement is presented.

Enabling the sunlight driven response of thermally induced shape memory polymers by rewritable CH3NH3PbI3 perovskite coating

(CH3NH3)PbI3 perovskite coating not only makes normal widely used thermal responsive shape memory polymers (SMPs) responsive to direct sunlight due to the strong photo-thermal effect of the coating, but also allows the material to be able to flexibly switch between photo-sensitive and photo-inert states by erasing and re-applying the coating.

Bandgap tunable Csx(CH3NH3)1-xPbI3 perovskite nanowires by aqueous solution synthesis for optoelectronic devices.

To date, all the lead halide based full-inorganic or organic-inorganic hybrid perovskites have been synthesized from organic solvent, such as N,N-dimethylformamide (DMF) or dimethyl sulfoxide (DMSO), by a solution method. Herein, water has been utilized as a 'green' solvent to develop an efficient synthetic route to grow various kinds of lead halide perovskite nanowires (NWs). By controlling the proportion of the hybrid cations, Csx(CH3NH3)1-xPbI3 perovskite NWs were successfully synthesized. Every Csx(CH3NH3)1-xPbI3 perovskite NW demonstrated single crystal characteristics with uniform stoichiometric element distribution. Because of the controllable cation composition, the NW bandgaps could be finely tuned from 1.5 to 1.7 eV. Transient photoluminescence spectra showed superior NW quality when compared with those of the conventional DMF-based NWs. Based on the abovementioned high quality single Cs0.5(CH3NH3)0.5PbI3 perovskite NW, a reliable single-NW photodetector was fabricated to investigate the optoelectronic application. It demonstrated a responsivity of 23 A/W, exceeding most of the reported values in the perovskite nanowire photoconductive detectors, and the shot-noise normalized detectivity was 2.5 × 1011 Jones comparable to the parameters of the commercial silicon-based nanowires. The green and robust synthesis method, finely tunable NW bandgaps, and superior optoelectronic properties are expected to open a new door for the development of perovskite optoelectronic devices.

Room-Temperature Atomic Layer Deposition of Al2 O3 : Impact on Efficiency, Stability and Surface Properties in Perovskite Solar Cells.

In this work, solar cells with a freshly made CH3 NH3 PbI3 perovskite film showed a power conversion efficiency (PCE) of 15.4 % whereas the one with 50 days aged perovskite film only 6.1 %. However, when the aged perovskite was covered with a layer of Al2 O3 deposited by atomic layer deposition (ALD) at room temperature (RT), the PCE value was clearly enhanced. X-ray photoelectron spectroscopy study showed that the ALD precursors are chemically active only at the perovskite surface and passivate it. Moreover, the RT-ALD-Al2 O3 -covered perovskite films showed enhanced ambient air stability.

Single Vs Mixed Organic Cation for Low Temperature Processed Perovskite Solar Cells

The present work reports a comparative study between single and mixed organic cation based MAPbI3 and MA0.6FA0.4PbI3 perovskite devices fabricated in conjunction with low temperature processed (<150°C) ZnO electron transport layers. MA0.6FA0.4PbI3 perovskite devices demonstrate 37% higher power conversion efficiency compared to MAPbI3 perovskite devices developed on the ZnO ETL. In addition, MA0.6FA0.4PbI3 devices exhibit very low photocurrent hysteresis and they are three-fold more stable than conventional MAPbI3 PSCs (perovskite solar cells). An in-depth analysis on the charge transport properties in both fresh and aged devices has been carried out using electrochemical impedance spectroscopy analysis to comprehend the enhanced device stability of the mixed perovskite devices developed on the ZnO ETL. The study also investigates into the interfacial charge transfer characteristics associated with the ZnO/mixed organic cation perovskite interface and concomitant influence on the inherent electronic properties.

Thin-Film Transformation of NH4 PbI3 to CH3 NH3 PbI3 Perovskite: A Methylamine-Induced Conversion-Healing Process.

Methylamine-induced thin-film transformation at room-temperature is discovered, where a porous, rough, polycrystalline NH4 PbI3 non-perovskite thin film converts stepwise into a dense, ultrasmooth, textured CH3 NH3 PbI3 perovskite thin film. Owing to the beneficial phase/structural development of the thin film, its photovoltaic properties undergo dramatic enhancement during this NH4 PbI3 -to-CH3 NH3 PbI3 transformation process. The chemical origins of this transformation are studied at various length scales.

Ion-Exchange-Induced 2D-3D Conversion of HMA1-x FAx PbI3 Cl Perovskite into a High-Quality MA1-x FAx PbI3 Perovskite.

High-quality phase-pure MA1-x FAx PbI3 planar films (MA=methylammonium, FA=formamidinium) with extended absorption and enhanced thermal stability are difficult to deposit by regular simple solution chemistry approaches owing to crystallization competition between the easy-to-crystallize but unwanted δ-FAPbI3 /MAPbI3 and FAx MA1-x PbI3 requiring rigid crystallization conditions. Here A 2D-3D conversion to transform compact 2D mixed composition HMA1-x FAx PbI3 Cl perovskite precursor films into 3D MA1-x FAx PbI3 (x=0.1-0.9) perovskites is presented. The designed Cl/I and H/FA(MA) ion exchange reaction induced fast transformation of compact 2D perovskite film, helping to form the phase-pure and high quality MA1-x FAx PbI3 without δ-FAPbI3 and MAPbI3 impurity. In all, we successfully developed a facile one-step method to fabricate high quality phase-pure MA1-x FAx PbI3 (x=0.1-0.9) perovskite films by 2D-3D conversion of HMA1-x FAx PbI3 Cl perovskite. This 2D-3D conversion is a promising strategy for lead halide perovskite fabrication.

Strontium Insertion in Methylammonium Lead Iodide: Long Charge Carrier Lifetime and High Fill-Factor Solar Cells.

The addition of Sr2+ in CH3 NH3 PbI3 perovskite films enhances the charge carrier collection efficiency of solar cells leading to very high fill factors, up to 85%. The charge carrier lifetime of Sr2+ -containing perovskites is in excess of 40 μs, longer than those reported for perovskite single crystals.