CH3NH3PbI3 xClx Research Articles

Replacement of Biphenyl by Bipyridine Enabling Powerful Hole Transport Materials for Efficient Perovskite Solar Cells

Here, 2,2'- and 3,3'-bipyridine are introduced for the first time as the core structure to get two new hole transport materials (HTMs), namely F22 and F33. The electron-withdrawing nature of bipyridine lowers the HOMO level of the new compounds and enhances the open-circuit voltage of perovskite solar cells. Especially for F33, the better planarity leads to better conjugation in the whole molecule and the molecular interaction is enhanced. Hole-mobility tests, steady-state photoluminescence (PL) spectra as well as time-resolved PL decay results demonstrate that the new HTMs exhibit good hole extraction and hole-transporting property. Impressive power conversion efficiencies of 17.71 and 18.48 % are achieved in conventional planar perovskite (CH3 NH3 PbI3-x Clx ) solar cells containing F22 and F33 as HTMs, respectively. As far as we know, this is the first report on bypiridine-based HTMs with leading efficiencies, and the design motif in this work opens a new way for devising HTMs in the future.

Synergetic Effect of Chloride Doping and CH3NH3PbCl3 on CH3NH3PbI3−xClx Perovskite‐Based Solar Cells

The chloride-doped CH3 NH3 PbI3-x Clx perovskite has attracted great attention owing to clear performance enhancement by using a Cl additive and by the controversial arguments on Cl function and the mechanism behind it. Herein, a series of CH3 NH3 PbI3-x Clx perovskites with various Cl content was prepared through a gas/solid reaction between CH3 NH2 gas and HPbI3-x Clx (x=0-1). The small amount of Cl doping in CH3 NH3 PbI3-x Clx (x=0.05) could lead to band gap broadening and significantly increase the perovskite grain size, and the phase-pure CH3 NH3 PbI2.95 Cl0.05 perovskites exhibited up to 17.44 % efficiency. For Cl contents higher than 0.1 (x>0.1), CH3 NH3 PbCl3 formed and coexisted with CH3 NH3 PbI3-x Clx , and CH3 NH3 PbCl3 could help to improve the thermal stability of CH3 NH3 PbI3-x Clx . However, the excessive co-existing wide-band-gap CH3 NH3 PbCl3 perovskites would inhibit the electron transfer and lead to a deterioration of photovoltaic performance.

Increased Efficiency for Perovskite Photovoltaics Based on Aluminum-Doped Zinc Oxide Transparent Electrodes via Surface Modification

A majority of perovskite solar cells use indium–tin oxide (ITO) or fluorine-doped tin oxide (FTO) as the transparent electrodes. As a promising transparent electrode material, aluminum-doped zinc oxide (AZO) possesses comparable transmittance and conductivity and is environment-friendly and low-cost. Herein, we report the fabrication of perovskite solar cells using AZO as the transparent electrodes. A fullerene derivative layer was used to adjust the energy level alignment and to promote electron transport between the CH3NH3PbI3–xClx absorption layer and the AZO electrode. Power conversion efficiency of 13.07% and open-circuit voltage of 1.02 V have been achieved. Our work demonstrates the potential of fabricating cost-effective solar cells on AZO with mild processing temperatures.

Atomistic Simulations of Methylammonium Lead Halide Layers on PbTiO3 (001) Surfaces

The substantial increase in the power conversion efficiency of hybrid perovskite solar cells, to date reaching more than 20% in the laboratory, has strongly motivated research on this class of organic–inorganic materials and related devices, particularly based on CH3NH3PbI3–xXx/TiO2 heterostructures (X = Cl,Br). Taking under consideration that a ferroelectric substrate may act as an efficient electron transporter, positively influencing charge collection across the interface and allowing the tuning of the halide perovskite (HP) - ferroelectric junction, we performed extensive density functional theory calculations on CH3NH3PbI3–xClx layers deposited on tetragonal PbTiO3 (PTO) (001) surfaces, to study their structural and electronic properties. The main findings of this study are as follows. (i) A ferroelectric polarization pointing from the PTO/HP interface to the PTO is favorable for the photogenerated electrons transfer across the interface and their transport to the collecting electrode. (ii) The PTO i...

Direct Experimental Evidence of Halide Ionic Migration under Bias in CH3NH3PbI3–xClx-Based Perovskite Solar Cells Using GD-OES Analysis

In recent decades, the development of organic–inorganic hybrid perovskite solar cells (PSCs) has been increasing very quickly due to their high initial efficiency and low-cost process. However, key points such as crystal growth mechanisms, current–voltage hysteresis, and instability remain still unexplained or misunderstood. Among several possibilities, ionic migration in PSCs has been suggested to explain the hysteresis effect. However, direct experimental evidence of ionic migration under operation or measurement conditions of PSCs is still missing. This work shows directly the ionic migration of halogen components (I– and Cl–) of a CH3NH3PbI3–xClx perovskite film under an applied bias using glow discharge optical emission spectrometry (GD-OES). Furthermore, no migration of lead and nitrogen ions is observed on a polarization time scale less than 2 min. The ratio of fixed to mobile iodide ions is deduced from the evolution of the GD-OES profile lines as a function of the applied bias. The average length...

Direct Evidence of Chlorine-Induced Preferential Crystalline Orientation in Methylammonium Lead Iodide Perovskites Grown on TiO2

Mixed halide hybrid perovskites CH3NH3PbI3–xClx (MAPICl) show higher charge carrier diffusion lengths, improved solar cell characteristics, and enhanced stability as compared to their single halide counterparts CH3NH3PbI3 (MAPI). This is surprising in view of the fact that the actually observed Cl content is very low (x < 0.1) as the incorporation of large amounts of Cl in the MAPI lattice is crystallographically not possible. Understanding the role of chlorine in these systems is therefore of the utmost importance and has been the subject of several experimental and theoretical studies. The main conclusions are that Cl addition leads to larger grain sizes and preferential crystalline orientation; however, so far a more quantitative description of these aspects is lacking. Synchrotron X-ray diffraction microscopy is a very promising technique in this respect, as it allows for the direct imaging of crystalline domains oriented in a selected direction with a resolution of some hundreds of nanometers. When imaging perovskite thin films deposited on an FTO-covered glass substrate containing a dense polycrystalline TiO2 layer at the perovskite (220) reflection, diffraction micrographs show an increase in surface coverage of oriented grains from 2.5% (MAPI) to 5.5% (MAPICl; x = 0.07). This is a clear evidence of the increased preferential orientation in the mixed perovskite. At the same time the integrated intensity increases by a factor of 4, which can be related to a larger perovskite grain size. When a single crystalline TiO2(001) substrate is used for the perovskite film deposition, both effects are dramatically enhanced in the case of MAPICl, leading to a surface coverage of 80%, a 55-fold increase in integrated diffracted intensity with respect to MAPI, and a mean crystallite size of 2.5 μm. In contrast, MAPI does not show any differences when grown on TiO2(001), which demonstrates that chlorine locates preferentially at the TiO2–perovskite interface and induces the growth of larger crystallites showing preferential orientation along the (110) direction.

Efficient light harvesting from flexible perovskite solar cells under indoor white light-emitting diode illumination

This is the first report of an investigation on flexible perovskite solar cells for artificial light harvesting by using a white light-emitting diode (LED) lamp as a light source at 200 and 400 lx, values typically found in indoor environments. Flexible cells were developed using either low-temperature sol–gel or atomic-layer-deposited compact layers over conducting polyethylene terephthalate (PET) substrates, together with ultraviolet (UV)-irradiated nanoparticle TiO2 scaffolds, a CH3NH3PbI3–xClx perovskite semiconductor, and a spiro-MeOTAD hole transport layer. By guaranteeing high-quality carrier blocking (via the 10–40 nm-thick compact layer) and injection (via the nanocrystalline scaffold and perovskite layers) behavior, maximum power conversion efficiencies (PCE) and power densities of 10.8% and 7.2 μW·cm–2, respectively, at 200 lx, and 12.1% and 16.0 μW·cm–2, respectively, at 400 lx were achieved. These values are the state-of-the-art, comparable to and even exceeding those of flexible dye-sensitized solar cells under LED lighting, and significantly greater than those for flexible amorphous silicon, which are currently the main flexible photovoltaic technologies commercially considered for indoor applications. Furthermore, there are significant margins of improvement for reaching the best levels of efficiency for rigid glass-based counterparts, which we found was a high of PCE ~24% at 400 lx. With respect to rigid devices, flexibility brings the advantages of being low cost, lightweight, very thin, and conformal, which is especially important for seamless integration in indoor environments.

Aquointermediate Assisted Highly Orientated Perovskite Thin Films toward Thermally Stable and Efficient Solar Cells

The stability of single‐crystalline/monocrystalline‐like perovskite film is expected to be better than its microcrystalline counterparts. In the present work, highly orientated perovskite thin films (CH3NH3PbI3–xClx) are prepared by means of aquointermediates assisted solution process. It displays super‐duper preferred‐orientation along &lt;110&gt; direction that is close to the single crystal, and its diffraction intensity ratio of (110)/(310) is nearly two orders of magnitude higher in contrast to the films that prepared by traditional way. Owing to its superior performances, e.g., highly crystallized quality, stress‐free inside films, longer electron lifetime, faster temporal response time, etc., the highly orientated perovskite‐based solar cells accordingly allow realizing high efficiency while improving its thermal stability.

Dopant-Free Zinc Chlorophyll Aggregates as an Efficient Biocompatible Hole Transporter for Perovskite Solar Cells.

Chlorophylls (Chls) are abundant, naturally occurring pigments that play key roles in light-harvesting and electron/energy transfer in natural photosynthetic apparatus. To demonstrate the idea that Chls are suitable hole transporters, we employed two Chl derivatives, Chl-1 and Chl-2, which self-assembled readily into π-stacking aggregates through a simple spincasting process, in perovskite solar cells (PSCs). The Chl aggregate films exhibit an ultra-smooth film surface, high hole mobility, appropriate energy levels, and efficient hole injection efficiencies that are all key characteristics for efficient hole transporters in PSCs. CH3 NH3 PbI3-x Clx -based PSCs with these Chls as hole transporters were fabricated and compared with P3HT as a standard hole transporter. PSCs based on Chl-1 and Chl-2 without the use of typical additives, such as 4-tert-butylpyridine and lithium bis(trifluoromethanesulfinyl)imide, gave power conversion efficiencies of 11.44 and 8.06 %, respectively. This research provides a unique way to incorporate low-cost and environmentally friendly natural photosynthetic materials in the development of highly efficient photovoltaic devices.

Effect of Chlorine Substitution on Lattice Distortion and Ferroelectricity of CH3NH3PbI3

As a prototype of organic–inorganic hybrid perovskites, CH3NH3PbI3 is attracting extensive attention because of its applications in high-power-conversion-efficiency solar cells. However, the mixed-halide perovskite CH3NH3PbI3–xClx exhibited superior carrier diffusion properties in recent experiments. Using first-principles calculations and the Berry phase method, we calculated the crystal structures and ferroelectric properties of pure and Cl-doped CH3NH3PbI3. The off-center displacement of Pb within the PbI6 octahedron was found to introduce major intrinsic polarization, rather than the off-center displacement of CH3NH3+ within the inorganic lattice or charge polarization within the organic cation, as expected. With chlorine substitution for iodine, the larger electronegativity difference between the halogen and Pb increases the lattice distortion and, hence, the electric polarization increases by as much as ∼50%, which provides a possible mechanism to further assist carrier separation and diffusion in s...

Thermal Behaviors of Methylammonium Lead Trihalide Perovskites with or without Chlorine Doping

Methylammonium lead trihalide perovskites have become the harbinger in solar cells lately. However, many fundamental mechanisms, especially those related to the used materials, still need a deeper understanding, in order to further improve the cells’ stability and efficiency. This work discussed the influence of chlorine doping on the materials’ thermal stability and the energy loss by either releasing heat or emitting light in CH3NH3PbI3. The results indicated that CH3NH3PbI3–xClx had no phase transition in the range 25–100 °C, in contrast to CH3NH3PbI3 with a phase transition at ∼50 °C. It was discovered that the heat generated in absorbing photon processes in CH3NH3PbI3–xClx could be reduced, about 20% lower than that in CH3NH3PbI3. More heat released in CH3NH3PbI3 may cause the occurrence of the phase transition and then further decline the cell stability. Photoluminescence results also indicated that adding chlorine into CH3NH3PbI3 could further suppress the energy loss in the means emitting light.

Molecular Origins of Defects in Organohalide Perovskites and Their Influence on Charge Carrier Dynamics

The chemical origins of charge recombination centers in lead-based organohalide perovskites were investigated using a combination of quantitative solution chemistry, X-ray diffraction, and time-resolved photoluminescence spectroscopy. We explored the complex, concentration-dependent solution equilibria among iodoplumbate coordination complexes that have been implicated as potential midgap states in organohalide perovskites. High concentrations of PbI2, PbI3–, and PbI42– were found in precursor solutions that match those used to deposit perovskite films for solar cell applications. We found that the concentration of tetraiodoplumbate PbI42– is uniquely correlated with the density of charge recombination centers found in the final perovskite films regardless of the lead precursor used to cast the films. However, mixed-halide perovskites commonly referred to as CH3NH3PbI3–xClx suppressed the formation of PbI42– in comparison to perovskites that included only iodide, which is consistent with the longer charge...

Slow Organic‐to‐Inorganic Sub‐Lattice Thermalization in Methylammonium Lead Halide Perovskites Observed by Ultrafast Photoluminescence

Carrier dynamics in methylammonium lead halide (CH3NH3PbI3–xClx) perovskite thin films, of differing crystal morphology, are examined as functions of temperature and excitation wavelength. At room temperature, long‐lived (&gt;nanosecond) transient absorption signals indicate negligible carrier trapping. However, in measurements of ultrafast photoluminescence excited at 400 nm, a heretofore unexplained, large amplitude (50%–60%), 45 ps decay process is observed. This feature persists for temperatures down to the orthorhombic phase transition. Varying pump photon energy reveals that the fast, band‐edge photoluminescence (PL) decay only appears for excitation ≥2.38 eV (520 nm), with larger amplitudes for higher pump energies. Lower photon‐energy excitation yields slow dynamics consistent with negligible carrier trapping. Further, sub‐bandgap two‐photon pumping yields identical PL dynamics as direct absorption, signifying sensitivity to the total deposited energy and insensitivity to interfacial effects. Together with first principles electronic structure and ab initio molecular dynamics calculations, the results suggest the fast PL decay stems from excitation of high energy phonon modes associated with the organic sub‐lattice that temporarily enhance wavefunction overlap within the inorganic component owing to atomic displacement, thereby transiently changing the PL radiative rate during thermalization. Hence, the fast PL decay relates a characteristic organic‐to‐inorganic sub‐lattice equilibration timescale at optoelectronic‐relevant excitation energies.

Monitoring the Formation of a CH3NH3PbI3–xClx Perovskite during Thermal Annealing Using X‐Ray Scattering

Grazing incidence wide and small angle X‐ray scattering (GIWAXS and GISAXS) measurements have been used to study the crystallization kinetics of the organolead halide perovskite CH3NH3PbI3–xClx during thermal annealing. In situ GIWAXS measurements recorded during annealing are used to characterize and quantify the transition from a crystalline precursor to the perovskite structure. In situ GISAXS measurements indicate an evolution of crystallite sizes during annealing, with the number of crystallites having sizes between 30 and 400 nm increasing through the annealing process. Using ex situ scanning electron microscopy, this evolution in length scales is confirmed and a concurrent increase in film surface coverage is observed, a parameter crucial for efficient solar cell performance. A series of photovoltaic devices are then fabricated in which perovskite films have been annealed for different times, and variations in device performance are explained on the basis of X‐ray scattering measurements.

Mixed-Halide CH3 NH3 PbI3-x Xx (X=Cl, Br, I) Perovskites: Vapor-Assisted Solution Deposition and Application as Solar Cell Absorbers.

There have been recent reports on the formation of single-halide perovskites, CH3 NH3 PbX3 (X=Cl, Br, I), by means of vapor-assisted solution processing. Herein, the successful formation of mixed-halide perovskites (CH3 NH3 PbI3-x Xx ) by means of a vapor-assisted solution method at ambient atmosphere is reported. The perovskite films are synthesized by exposing PbI2 film to CH3 NH3 X (X=I, Br, or Cl) vapor. The prepared perovskite films have uniform surfaces with good coverage, as confirmed by SEM images. The inclusion of chlorine and bromine into the structure leads to a lower temperature and shorter reaction time for optimum perovskite film formation. In the case of CH3 NH3 PbI3-x Clx , the optimum reaction temperature is reduced to 100 °C, and the resulting phases are CH3 NH3 PbI3 (with trace Cl) and CH3 NH3 PbCl3 with a ratio of about 2:1. In the case of CH3 NH3 PbI3-x Brx , single-phase CH3 NH3 PbI2 Br is formed in a considerably shorter reaction time than that of CH3 NH3 PbI3 . The mesostructured perovskite solar cells based on CH3 NH3 PbI3 films show the best optimal power conversion efficiency of 13.5 %, whereas for CH3 NH3 PbI3-x Clx and CH3 NH3 PbI3-x Brx the best recorded efficiencies are 11.6 and 10.5 %, respectively.

Understanding the Role of the Mesoporous Layer in the Thermal Crystallization of a Meso-Superstructured Perovskite Solar Cell

Perovskite solar cells (PSCs) have been extensively studied in recent years due to their unexpected properties and low-temperature processing. In terms of morphology, the annealing conditions are crucial and highly determinant on the performance of the devices. Here, it is important to know the heat transfer in order to prevent detrimental effects in the photovoltaic performance principally produced by low crystallization and localized excessive thermal stress in the synthesized perovskite. In this work, differential scanning calorimetry (DSC) was used to reveal the thermal transitions occurring during crystallization of a mesoporous alumina-based PSC. We found that when the mixed-halide perovskite (CH3NH3PbI3–xClx) is crystallized in the presence of a mesoporous layer, the heat transfer flux is affected and therefore the perovskite formation shifts to higher temperatures, causing the infiltrated perovskite to crystallize differently than the capping layer. DSC analysis also indicated that when low annealing temperatures were used, the perovskite did not present good crystallinity, while at high temperatures the thermal stress generated on the infiltrated perovskite promoted low efficiencies. Finally, the optimal crystallization conditions were found to be 100 °C for 90 min, with a short postannealing at 130 °C for 10 min, which promoted a photoconversion efficiency up to 10.89%.

Iodine and Chlorine Element Evolution in CH3NH3PbI3–xClx Thin Films for Highly Efficient Planar Heterojunction Perovskite Solar Cells

Highly efficient planar heterojunction perovskite solar cells (PHJ–PSCs) with a structure of ITO/PEDOT:PSS/CH3NH3PbI3–xClx/PCBM/C60/Ag was fabricated, in which the compact and pinhole-free CH3NH3PbI3–xClx perovskite thin film was obtained using a mixture of precursors containing lead iodide (PbI2), lead chloride (PbCl2), and methylammonium iodide (CH3NH3I) at an optimized ratio of 1:1:4. The morphology and formation process of CH3NH3PbI3–xClx thin film was closely related to the annealing temperature and time, which would result in the controllable performance for the PHJ–PSC devices. The morphology, crystallization process, and element analysis suggested that the chlorine gradually diffused and sublimated from the film surface while the iodine moved to the surface, together with the removal of the pinholes in the film. The PHJ–PSCs with the as-prepared CH3NH3PbI3–xClx thin film showed good performance and excellent repeatability. The power conversion efficiency (PCE) up to 14.03% was achieved without obv...

Nonlinear Optical Response of Organic–Inorganic Halide Perovskites

Metal halide perovskites have exhibited excellent properties as absorbers in solar cells, but this may simply be the first of many applications for this intriguing class of materials. Here, we report the nonlinear optical response of triiodide (CH3NH3PbI3) and mixed halide (CH3NH3PbI3–xClx) perovskite absorbers. The results show that they have large nonlinear refractive index (NRI), 3 orders of magnitude larger than that of silicon. Particularly, the NRI of CH3NH3PbI3–xClx is more than two times larger compared to that of CH3NH3PbI3. Meanwhile, both of them have been proven to possess saturable absorption effects with small nonlinear absorption coefficients which indicate that they can maintain excellent absorption under high-intensity irradiation and are favorable to modulators toward large-energy pulsed laser. Taking into consideration the saturable absorption effect, we demonstrated a pulsed laser with the perovskite as a pulse modulator. These results above indicate the potential for perovskites to be...

Unraveling the Role of Monovalent Halides in Mixed-Halide Organic-Inorganic Perovskites.

The performance of perovskite solar cells is strongly influenced by the composition and microstructure of the perovskite. A recent approach to improve the power conversion efficiencies utilized mixed-halide perovskites, but the halide ions and their roles were not directly studied. Unraveling their precise location in the perovskite layer is of paramount importance. Here, we investigated four different perovskites by using X-ray photoelectron spectroscopy, and found that among the three studied mixed-halide perovskites, CH3 NH3 Pb(I0.74 Br0.26 )3 and CH3 NH3 PbBr3-x Clx show peaks that unambiguously demonstrate the presence of iodide and bromide in the former, and bromide and chloride in the latter. The CH3 NH3 PbI3-x Clx perovskite shows anomalous behavior, the iodide content far outweighs that of the chloride; a small proportion of chloride, in all likelihood, resides deep within the TiO2 /absorber layer. Our study reveals that there are many distinguishable structural differences between these perovskites, and that these directly impact the photovoltaic performances.

Polymer Stabilization of Lead(II) Perovskite Cubic Nanocrystals for Semitransparent Solar Cells

Lead (II) perovskite (PV, e.g., CH3NH3PbI3–xClx) with 3 wt% polyvinylpyrrolidone (PVP) produces an ≈90 nm film containing a PVP–PV composite consisting of ≈15-nm-sized PV nanocrystals. Solar cells using 3 wt% PVP–PV have several advantages over a device using nondoped PV, which include an increase in transparency, stability of the film, efficiency, and reproducibility.