The Role of Chlorine in the Formation Process of “CH3NH3PbI3‐xClx” Perovskite

CH₃NH₃PbI_3‐xCl_x is a commonly used chemical formula to represent the methylammonium lead halide perovskite fabricated from mixed chlorine‐ and iodine‐containing salt precursors. Despite the rapid progress in improving its photovoltaic efficiency, fundamental questions remain regarding the atomic ratio of Cl in the perovskite as well as the reaction mechanism that leads to its formation and crystallization. In this work we investigated these questions through a combination of chemical, morphological, structural and thermal characterizations. The elemental analyses reveal unambiguously the negligible amount of Cl atoms in the CH₃NH₃PbI_3‐xCl_x perovskite. By studying the thermal characteristics of methylammonium halides as well as the annealing process in a polymer/perovskite/FTO glass structure, we show that the formation of the CH₃NH₃PbI_3‐xCl_x perovskite is likely driven by release of gaseous CH₃NH₃Cl (or other organic chlorides) through an intermediate organometal mixed halide phase. Furthermore, the comparative study on CH₃NH₃I/PbCl₂ and CH₃NH₃I/PbI₂ precursor combinations with different molar ratios suggest that the initial introduction of a CH₃NH₃⁺ rich environment is critical to slow down the perovskite formation process and thus improve the growth of the crystal domains during annealing; accordingly, the function of Cl^? is to facilitate the release of excess CH₃NH₃⁺ at a relatively low annealing temperatures.     

FAPbI3 Flexible Solar Cells with a Record Efficiency of 19.38% Fabricated in Air via Ligand and Additive Synergetic Process

Compared with silicon‐based solar cells, organic–inorganic hybrid perovskite solar cells (PSCs) possess a distinct advantage, i.e., its application in the flexible field. However, the efficiency of the flexible device is still lower than that of the rigid one. First, it is found that the dense formamidinium (FA)‐based perovskite film can be obtained with the help of N‐methyl‐2‐pyrrolidone (NMP) via low pressure‐assisted method. In addition, CH₃NH₃Cl (MACl) as the additive can preferentially form MAPbCl_3?xI_x perovskite seeds to induce perovskite phase transition and crystal growth. Finally, by using FAI·PbI₂·NMP+x%MACl as the precursor, i.e., ligand and additive synergetic process, a FA‐based perovskite film with a large grain size, high crystallinity, and low trap density is obtained on a flexible substrate under ambient conditions due to the synergetic effect, e.g., MACl can enhance the crystallization of the intermediate phase of FAI·PbI₂·NMP. As a result, a record efficiency of 19.38% in flexible planar PSCs is achieved, and it can retain about 89% of its initial power conversion efficiency (PCE) after 230 days without encapsulation under ambient conditions. The PCE retains 92% of the initial value after 500 bending cycles with a bending radii of 10 mm. The results show a robust way to fabricate highly efficient flexible PSCs.     

Pure-Iodide Wide-Bandgap Perovskites for High-Efficiency Solar Cells by Crystallization Control

Wide-bandgap perovskite is a vital part of perovskite-based tandem solar cells. Currently, wide-bandgap perovskites are typically based on mixed-halide (I/Br) materials, but suffer from photoinduced phase separation. The pure-iodide formamidine/cesium (FA/Cs) based FA_xCs_1−xPbI₃ perovskites with high Cs content are good candidates, whereas the control of crystallization is challenging due to the complex crystallization kinetics. Here, pure-iodide FA_0.5Cs_0.5PbI₃ wide-bandgap perovskite solar cells is reported. As an acidic diammonium salt, methylenediaminium dichloride (MDACl₂) is applied as an additive to control the whole crystallization process of perovskite films, including both nucleation and crystal growth. Starting from the solution chemistry, the MDACl₂ additive with acidity and strong solvation properties can effectively regulate the chemical composition of perovskite precursor, thus inhibiting the growth of undesired 1D intermediates during the nucleation process. Besides, the incorporation of larger-sized MDA²⁺ into the lattice compensates for the tolerance factor and accelerates the ion exchange reaction between FA⁺ and Cs⁺ in the crystal growth process. As a result, the crystallinity of the perovskite films is significantly improved, benefitting from the dual function of MDACl₂. Finally, the efficiency of hole transport layer-free carbon electrode-based wide-bandgap perovskite solar cells reaches 18.52%, which is the highest reported so far.     

Precursor Solution Annealing Forms Cubic‐Phase Perovskite and Improves Humidity Resistance of Solar Cells

Solar cells with light‐absorbing layers comprising organometal halide perovskites have recently exceeded 22% efficiency. Despite high power‐conversion efficiencies, the stability of these devices, particularly when exposed to humidity and oxygen, remains poor. In the current study, a pathway to increase the stability of methylammonium lead iodide (CH₃NH₃PbI₃) based solar cells towards humidity is demonstrated, while maintaining the simplicity and solution‐processability of the active layers. Thermal annealing of the precursor solution prior to deposition induces the formation of cubic‐phase perovskite films in the solid state at room temperature. The experiments demonstrate that this improved ambient stability is correlated with the presence of the cubic phase at device operating temperatures, with the cubic phase resisting the formation of perovskite monohydrate—a pathway of degradation in conventionally processed perovskite thin films—on exposure to humidity.     

Improved performance of perovskite solar cells with a TiO2/MoO3 core/shell nanoparticles doped PEDOT:PSS hole-transporter

We demonstrate improved performance of inverted planar heterojunction CH₃NH₃PbI_3-xCl_x perovskite solar cells with a TiO₂/MoO₃ core/shell nanoparticles (NPs) doped poly(3,4-ethylene dioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) hole-transporting layer (HTL). TiO₂/MoO₃ Core/shell NPs with size of approximately 40 nm are successfully prepared with a simple wet solution method and are interspersed into PEDOT: PSS layer to construct the HTL. The optimized device shows a high power conversion efficiency of 13.63%, which is dramatically improved compared with the reference device with a pristine PEDOT:PSS HTL. The improvement is mainly attributed to the increased crystalline of the CH₃NH₃PbI_3-xCl_x film with large-scale domains and a compact morphology. More interesting, the cells exhibit superior stability in ambient conditions, which is attributed to the inhibited penetration of moisture due to the compact morphology of the CH₃NH₃PbI_3-xCl_x film and the reduced hygroscopicity of the PEDOT:PSS film.     

In situ recycle of PbI2 as a step towards sustainable perovskite solar cells

The development of organometal halide perovskite solar cells has grown rapidly and the highest efficiency of the devices has recently surpassed 22%. Because these solar cells contain toxic lead, a sustainable strategy is required to prevent environmental pollution and avoid healthy hazard caused by possible lead outflow. Here, in situ recycling PbI₂ from thermal decomposition CH₃NH₃PbI₃ perovskite films for efficient perovskite solar cells was developed. The thermal behavior of CH₃NH₃PbI₃ perovskite and its individual components were examined by thermogravimetric analysis. By optimizing the process of thermal decomposition CH₃NH₃PbI₃ film, the complete conversion from CH₃NH₃PbI₃ to pure PbI₂ layer with a mesoporous scaffold was achieved. The mesoporous structure readily promotes the conversion efficiency of perovskite and consequently results in high‐performance device. A perovskite crystal growth mechanism on the mesoporous PbI₂ structure was proposed. These results suggest that in situ recycled PbI₂ scaffolds can be a new route in manipulating the morphology of the perovskite active layer, providing new possibilities for high performance. Meanwhile, the risk of lead outflow can be released, and the saving‐energy fabrication of efficient solar cells can be realized.     

Morphological Control for High Performance,Solution‐Processed Planar Heterojunction Perovskite Solar Cells

Organometal trihalide perovskite based solar cells have exhibited the highest efficiencies to‐date when incorporated into mesostructured composites. However, thin solid films of a perovskite absorber should be capable of operating at the highest efficiency in a simple planar heterojunction configuration. Here, it is shown that film morphology is a critical issue in planar heterojunction CH₃NH₃PbI_3‐xCl_x solar cells. The morphology is carefully controlled by varying processing conditions, and it is demonstrated that the highest photocurrents are attainable only with the highest perovskite surface coverages. With optimized solution based film formation, power conversion efficiencies of up to 11.4% are achieved, the first report of efficiencies above 10% in fully thin‐film solution processed perovskite solar cells with no mesoporous layer.     

High‐Performance Planar Solar Cells Based On CH3NH3PbI3‐xClx Perovskites with Determined Chlorine Mole Fraction

Solution‐processable hybrid perovskite solar cells are a new member of next generation photovoltaics. In the present work, a low‐temperature two‐step dipping method is proposed for the fabrication of CH₃NH₃PbI_3‐xCl_x perovskite films on the indium tin oxide glass/poly(3,4‐ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) substrate. The bandgaps of the CH₃NH₃PbI_3‐xCl_x perovskite films are tuned in the range between 1.54 and 1.59 eV by adjusting the PbCl₂ mole fraction (n_Cl/(n_Cl + n_I)) in the initial mixed precursor solution from 0.10 to 0.40. The maximum chlorine mole fraction measured by a unique potentiometric titration method in the produced CH₃NH₃PbI_3‐xCl_x films can be up to 0.220 ± 0.020 (x = 0.660 ± 0.060), which is much higher than that produced by a one‐step spin‐coating method (0.056 ± 0.015, x = 0.17 ± 0.04). The corresponding solar cell with the CH₃NH₃PbI_2.34±0.06Cl_0.66±0.06 perovskite film sandwiched between PEDOT:PSS and C₆₀ layers exhibits a power conversion efficiency as high as 14.5%. Meanwhile, the open‐circuit potential (V_oc) of the device reaches 1.11 V, which is the highest V_oc reported in the perovskite solar cells fabricated on PEDOT:PSS so far.     

High‐Quality Cuboid CH3NH3PbI3 Single Crystals for High Performance X‐Ray and Photon Detectors

Organic–inorganic halide hybrid perovskite materials are promising materials for X‐ray and photon detection due to their superior optoelectronic properties. Single‐crystal (SGC) perovskites have increasingly attracted attention due to their substantially low crystal defects, which contribute to improving the figures of merit of the devices. Cuboid CH₃NH₃PbI₃ SGC with the naturally favorable geometry for device fabrication is rarely reported in X‐ray and photon detection application. The concept of seed dissolution‐regrowth to improve crystal quality of cuboid CH₃NH₃PbI₃ SGC is proposed and a fundamental understanding of the nucleation and growth is provided thermodynamically. The X‐ray detector fabricated from cuboid CH₃NH₃PbI₃ SGC demonstrates the firstly reported high sensitivity of 968.9 µC^?1 Gy^?1 cm^?2 under ?1 V bias. The results also show that the favorable crystal orientation and high quality of cuboid CH₃NH₃PbI₃ leads to better responsivity and faster response speed than the more common dodecahedral CH₃NH₃PbI₃ in photodetection. Consequently, the work paves a way to synthesize high‐quality perovskite SGCs and benefits the application of MAPbI₃ SGCs with preferred crystal orientation and favorable crystal geometry for emerging device applications.     

Deciphering the NH4PbI3 Intermediate Phase for Simultaneous Improvement on Nucleation and Crystal Growth of Perovskite

The NH₄PbI₃‐based phase transformation is realized by simply adding NH₄I additive, in order to simultaneously control perovskite nucleation and crystal growth. Regarding the nucleation process, the NH₄⁺ with small ionic radius preferentially diffuses into the [PbI₆]^4? octahedral layer to form NH₄PbI₃, which compensates the lack of CH₃NH₃I (MAI) precipitation. The generation of NH₄PbI₃ intermediate phase results in extra heterogeneous nucleation sites and reduces the defects derived from the absence of MA⁺. Regarding the crystal growth process, the cation exchange process between MA⁺ and NH₄⁺, instead of the MAs directly entering, successfully retards the crystal growth. Such NH₄PbI₃ consumption process slows down the crystal growth, which effectively improves the perovskite quality with lowered defect density. The cooperation of these two effects eventually leads to the high‐quality perovskite with enlarged grain size, prolonged photoluminescence lifetime, lowered defect density, and increased carrier concentration, as well as the finally enhanced photovoltaic performance. Moreover, NH₃ as a byproduct further facilitates the proposed transformation process and no external residue remains even without any post‐treatment. Such methodology of introducing a novel phase transformation to simultaneously control nucleation and crystal growth processes is of universal significance for further devotion in the foreseeable perovskite solar cells (PSCs) evolution.     

Low‐Voltage,Optoelectronic CH3NH3PbI3−xClx Memory with Integrated Sensing and Logic Operations

Nonvolatile optoelectronic memories integrated with the functions of sensing, data storage, and data processing are promising for the potential Internet of things (IoT) applications. To meet the requirements of IoT devices, multifunctional memory devices with low power consumption and secure data storage are highly desirable. This study demonstrates an optoelectronic resistive switching memory integrated with sensing and logic operations by adopting organic–inorganic hybrid CH₃NH₃PbI_3?xCl_x perovskites, which possess unusual defect physics and excellent light absorption. The CH₃NH₃PbI_3?xCl_x cell exhibits low operation voltage of 0.1 V with the assistance of light illumination, long‐term retention property, and multiple resistance states. Its unique optoelectronic characteristics enable to perform logic operation for inputting one electrical pulse and one optical signal, and detect the coincidence of electrical and optical signal as well. This design provides possibilities for smart sensor in IoT application.     

N and p-type properties in organo-metal halide perovskites studied by Seebeck effects

Organo-metal halide perovskites can exhibit co-existed electrical polarizations and semiconducting properties respectively from organic and inorganic components. Here, we find that the Seebeck coefficient can be changed between positive and negative values when the concentration of chloride ions is varied between single-halide (CH₃NH₃PbI₃) and mixed-halide structures (CH₃NH₃PbI_xCl_3−x). This indicates that varying the concentration of chloride ions can tune the semiconducting properties between the n-type and p-type regimes in the organo-metal halide perovskites. Our temperature-dependent capacitance measurement shows that increasing temperature can cause a change on internal electrical polarization. As a result, we can propose that the internal polarization functions as the underlying mechanism responsible for large temperature-dependent Seebeck coefficients in organo-metal halide perovskites operating between n-type and p-type regimes.     

Calculation studies on point defects in perovskite solar cells

The close-to-optimal band gap, large absorption coefficient, low manufacturing cost and rapid increase in power conversion efficiency make the organic-inorganic hybrid halide (CH₃NH₃PbI₃) and related perovskite solar cells very promising for commercialization. The properties of point defects in the absorber layer semiconductors have important influence on the photovoltaic performance of solar cells, so the investigation on the defect properties in the perovskite semiconductors is necessary for the optimization of their photovoltaic performance. In this work, we give a brief review to the first-principles calculation studies on the defect properties in a series of perovskite semiconductors, including the organic-inorganic hybrid perovskites and inorganic halide perovskites. Experimental identification of these point defects and characterization of their properties are called for.     

Controlled Growth and Reliable Thickness‐Dependent Properties of Organic–Inorganic Perovskite Platelet Crystal

Organolead halide perovskites (e.g., CH₃NH₃PbI₃) have caught tremendous attention for their excellent optoelectronic properties and applications, especially as the active material for solar cells. Perovskite crystal quality and dimension is crucial for the fabrication of high‐performance optoelectronic and photovoltaic devices. Herein the controlled synthesis of organolead halide perovskite CH₃NH₃PbI₃ nanoplatelets on SiO₂/Si substrates is investigated via a convenient two‐step vapor transport deposition technique. The thickness and size of the perovskite can be well‐controlled from few‐layers to hundred nanometers by altering the synthesis time and temperature. Raman characterizations reveal that the evolutions of Raman peaks are sensitive to the thickness. Furthermore, from the time‐resolved photoluminescence measurements, the best optoelectronic performance of the perovskite platelet is attributed with thickness of ≈30 nm to its dominant longest lifetime (≈4.5 ns) of perovskite excitons, which means lower surface traps or defects. This work supplies an alternative to the synthesis of high‐quality organic perovskite and their possible optoelectronic applications with the most suitable materials.     

Combination of Hybrid CVD and Cation Exchange for Upscaling Cs‐Substituted Mixed Cation Perovskite Solar Cells with High Efficiency and Stability

Mixed cation hybrid perovskites such as Cs_xFA_1?xPbI₃ are promising materials for solar cell applications, due to their excellent photoelectronic properties and improved stability. Although power conversion efficiencies (PCEs) as high as 18.16% have been reported, devices are mostly processed by the anti‐solvent method, which is difficult for further scaling‐up. Here, a method to fabricate Cs_xFA_1?xPbI₃ by performing Cs cation exchange on hybrid chemical vapor deposition grown FAPbI₃ with the Cs⁺ ratio adjustable from 0 to 24% is reported. The champion perovskite module based on Cs_0.07FA_0.93PbI₃ with an active area of 12.0 cm² shows a module PCE of 14.6% and PCE loss/area of 0.17% cm^?2, demonstrating the significant advantage of this method toward scaling‐up. This in‐depth study shows that when the perovskite films prepared by this method contain 6.6% Cs⁺ in bulk and 15.0% at the surface, that is, Cs_0.07FA_0.93PbI₃, solar cell devices show not only significantly increased PCEs but also substantially improved stability, due to favorable energy level alignment with TiO₂ electron transport layer and spiro‐MeOTAD hole transport layer, increased grain size, and improved perovskite phase stability.     

A simple fabrication of CH3NH3PbI3 perovskite for solar cells using low-purity PbI2

The CH₃NH₃PbI₃ (MAPbI₃) perovskite was usually prepared by high-purity PbI₂ with high cost. The low cost and low-purity PbI₂ was seldom reported for fabrication of MAPbI₃ because it cannot even dissolve well in widely adopted solvent of DMF. We developed an easy method to adapt low-purity PbI₂ for fabrication of high quality MAPbI₃ just by the simple addition of some hydrochloric acid into the mixture of low-purity PbI₂, MAI and DMF. This straightforward method can not only help dissolve the low quality PbI₂ by reacting with some impurities in DMF, but also lead to a successful fabrication of high-quality perovskite solar cells with up to 14.80% efficiency comparable to the high quality PbI₂ precursors.     

Hill climbing hysteresis of perovskite‐based solar cells: a maximum power point tracking investigation

The surge of the power conversion efficiency of metal halide lead perovskite solar cells comes with concerns, such as the long‐term ecotoxicity of lead compounds, their sensitivity toward moisture and oxygen, or the scarcity of some of their components. Most perovskite solar cells still suffer from serious stability problems when measured under real working conditions (maximum power point tracking at 60°C). In the long run, stability will certainly decide on the fate of CH₃NH₃PbI₃ and related lead perovskites for their use in photovoltaic modules. Herein, we show an effective and inexpensive strategy to perform ageing of perovskite solar cells under maximum power point tracking. For the first time, we analyze the issue of power extraction from solar cells exhibiting hysteresis. We show that a standard tracking algorithm such as perturb and observe fails to converge to the maximum power point of the solar cell if it exhibits j (V ) hysteresis, and we present an effective strategy to stabilize the algorithm. We show that enforcing oscillations in forward bias can boost the mean power output of some perovskite solar cells by more than 10%, in contrast to a reference crystalline silicon solar cell. Copyright © 2017 John Wiley & Sons, Ltd.     

Precisely Controlling the Grain Sizes with an Ammonium Hypophosphite Additive for High‐Performance Perovskite Solar Cells

A facile approach to precisely control the perovskite grain sizes is proposed and demonstrated for high‐performance photovoltaic (PV) solar cells. With the introduction of various amounts of NH₄H₂PO₂ (AHP) additives into the PbI₂/CH₃NH₃I precursors, the grain scale of CH₃NH₃PbI₃ films can be finely turned from hundreds of nanometer to micrometer scale, allowing evaluating the effects of crystalline grain boundary on trap densities, charge recombination, and PV device performance. The X‐ray diffraction and X‐ray photoelectron spectroscopy measurements indicate that the formation of intermediates plays a key role in assisting the perovskite crystal growth. The optimized devices show much larger open‐circuit voltages (V_OC) up to 1.10 ± 0.02 V and significantly enhance power conversion efficiencies (PCEs) of 16.5 ± 0.7%, as compared to the control devices with PCE of 9.4 ± 1.0% and V_OC of 1.00 ± 0.03 V. Further investigations confirm that the boosted PV performance origins from the decreased defect densities due to enlarged grain sizes. It is also demonstrated that the approach is general and applicable to other perovskite systems, e.g., HC(NH₂)₂PbI₃. The results suggest the promising application of AHP in achieving high‐performance perovskite PV devices, and shed light on understanding the grain boundary effects on perovskite optoelectronics.     

Efficiencies of perovskite hybrid solar cells influenced by film thickness and morphology of CH3NH3PbI3−xClx layer

Perovskite hybrid solar cells (pero-HSCs) have been intensively investigated due to their promising photovoltaic performance. However, the correlations between the efficiencies of pero-HSCs and thin film thicknesses and morphologies of CH₃NH₃PbI_3−xCl_x perovskite layers are rarely addressed. In this study, we report the correlation between the efficiencies of “planar heterojunction” (PHJ) pero-HSCs and the thin film thicknesses and morphologies of solution-processed CH₃NH₃PbI_3−xCl_x perovskite layers. Investigation of absorption spectra, X-ray diffraction patterns, atomic force microscopy and scanning electron microscopy images of CH₃NH₃PbI_3−xCl_x layers indicate that the efficiencies of PHJ pero-HSCs are dependent on the film thickness, as the thickness of CH₃NH₃PbI_3−xCl_x is less than 400 nm; whereas the efficiencies are significantly dependent on the film morphologies of CH₃NH₃PbI_3−xCl_x layers as the thickness is larger than 400 nm. Our studies provide a promising pathway for fabricating high efficiency PHJ pero-HSCs.     

A Self‐Powered and Stable All‐Perovskite Photodetector–Solar Cell Nanosystem

Hybrid organic–inorganic perovskites have attracted intensive interest as light absorbing materials in solid‐state solar cells. Herein, we demonstrate a high‐performance CH₃NH₃PbI₃‐based perovskite photodetector constructed on the flexible indium tin oxide (ITO) coated substrate even after 200 bending cycles. The as‐fabricated devices show high responsivity, broad spectrum response from ultraviolet to whole visible light, long‐term stability, and high on‐off ratio. Particularly, atomic layer deposition technique was used to deposit the ultrathin Al₂O₃ film on devices, functioning as a protection layer to effectively enhance the stability and durability of perovskite photodetectors. The first all‐perovskite self‐powered nanosystem was successfully assembled by integrating a perovskite solar cell with a perovskite photodetector. Driven by the perovskite solar cell, the photodetector exhibits fast and stable response to illuminated light at a low working voltage less than 1.0 V. This stable integrated nanosystem has promising applications in which photodetectors can work in harsh environments without external power sources.