Alkali ternary lead halides have been studied intensively in the past few years, with great interest focussed on perovskite materials. In this paper we report on novel rubidium lead chloride nanocrystals (NCs) with the formula Rb6Pb5Cl16, which adopt a tetragonal symmetry. The NCs were characterized and found to be active in theUVregion, with a band-gap of∼4.05 eV. The roles of the ligands, oleic acid and oleylamine, were investigated and found to strongly affect the morphology and composition of the NCs, through the stabilization of the facilitated crystallization of the ionic precursors. The effective masses were observed by density functional theory (DFT) calculations, using the dielectric function, and the the Bohr exciton radius and exciton binding energy of the NCs were estimated. Moreover, the results were supported by the DFT calculations of the electronic properties and atomic structure.
Organic–inorganic halide perovskite has excellent properties to function as light harvesters in solar cells due to the possibility to tune its optical properties and to use it as thin film absorber, at a few hundrednanometer thicknesses. Herein, we demonstrate the fabrication of perovskite solar cells with controlled transparency, by the mesh assisted deposition process. Sequential fabrication of perovskite was performed in air, wherein a PbI2 grid was formed in the first step, and in the second step, the grid reacted selectively with methylammoniumiodide, resulting in a perovskite grid pattern. The best cells were obtained with a photoanode composed of mesoporous TiO2 with Al2O3 nanoparticles. The resulting semi-transparent perovskite solar cells, including a semi-transparent contact composed of MoO3/Au/MoO3 yielded a power conversion efficiency of 5.5% with an average transparency of 26% and efficiency of 8% for cells fabricated with a gold contact.
Three-dimensional (3D) perovskite has attracted a lot of attention owing to its success in photovoltaic (PV) solar cells. However, one of its major crucial issues lies in its stability, which has limited its commercialization. An important property of organic–inorganic perovskite is the possibility of forming a layered material by using long organic cations that do not fit into the octahedral cage. These long organic cations act as a “barrier” that “caps” 3D perovskite to form the layered material. Controlling the number of perovskite layers could provide a confined structure with chemical and physical properties that are different from those of 3D perovskite. This opens up a whole new batch of interesting materials with huge potential for optoelectronic applications. This Minireview presents the synthesis, properties, and structural orientation of low-dimensional perovskite. It also discusses the progress of low-dimensional perovskite in PV solar cells, which, to date, have performance comparable to that of 3D perovskite but with enhanced stability. Finally, the use of low-dimensional perovskite in light-emitting diodes (LEDs) and photodetectors is discussed. The low-dimensional perovskites are promising candidates for LED devices, mainly because of their high radiative recombination as a result of the confined low-dimensional quantum well.
This work reports on the high power conversion efficiency (PCE) and high open circuit voltage (Voc) of bromide-based quasi 2D perovskite solar cells. A Voc of more than 1.4 V and, at the same time, a PCE of 9.5% for cells with hole transport material (HTM) were displayed, whereas a Voc value of 1.37 V and a PCE of 7.9% were achieved for HTM-free cells. Bromide quasi 2D perovskites were synthesized using various long organic barriers (e.g., benzyl ammonium, BA; phenylethyl ammonium, PEA; and propyl phenyl ammonium, PPA). The influence of different barrier molecules on the quasi 2D perovskite's properties was studied using absorbance, X-ray diffraction, and scanning electron microscopy. No change was observed in the exciton binding energy as a result of changing the barrier molecule. Density functional theory (DFT) with spin–orbit coupling calculations showed that in the case of BA, holes are delocalized over the whole molecule, whereas for PEA and PPA, they are delocalized more at the phenyl ring. This factor influences the electrical conductivity of holes, which is highest for BA in comparison with the other barriers. In the case of electrons, the energy onset for the nonzero conductivity is lowest for BA. Both calculations support the better PV performance observed for the quasi 2D perovskite based on BA as the barrier. Finally, using contact angle measurements, it was shown that the quasi 2D perovskite is more hydrophobic than the 3D perovskite. Stability measurements showed that cells based on the quasi 2D perovskite are more stable than cells based on the 3D perovskite.
Abstract The influence of the Schottky contact is studied for hole transport material (HTM) free CH3NH3PbI3 perovskite solar cells (PSCs), by using drift-diffusion and small signal models. The basic current-voltage and capacitance-voltage characteristics are simulated in reasonable agreement with experimental data. The build in potential of the finite CH3NH3PbI3 layer is extracted from a Mott-Schottky capacitance analysis. Furthermore, hole collector conductors with work-functions of more than 5.5 eV are proposed as solutions for high efficiency HTM-free CH3NH3PbI3 PSCs.
Methylammonium-mediated phase-evolution behavior of FA1@xMAxPbI3 mixed-organic-cation perovskite (MOCP) is studied. It is found that by simply enriching the MOCP precursor solutions with excess methylammonium cations, the MOCPs form via a dynamic composition-tuning process that is key to obtaining MOCP thin films with superior properties. This simple chemical approach addresses several key challenges, such as control over phase purity, uniformity, grain size, composition, etc., associated with the solutiongrowth of MOCP thin films with targeted compositions.
We report here the discovery of a fancy interaction between cesium iodide (CsI) and methylamine (CH3NH2) due to the presence of the hydrogen bond. The formed CsI$xCH3NH2 is a liquid phase, which facilitates the large scale fabrication of highly uniform cesium-containing perovskite films with strong (110) preferred orientation by the CH3NH2 gas healing process. With this method, at most 10% nonpolar Cs cations could fully dope into the crystal lattice and extremely enhance the interaction of the inorganic framework with a more symmetrical PbI6 octahedron, resulting in obvious improvement in moisture stability under continuous illumination.
An important property of hybrid layered perovskite is the possibility to reduce its dimensionality to provide wider band gap and better stability. In this work, 2D perovskite of the structure (PEA)2(MA)n–1PbnBr3n+1 has been sensitized, where PEA is phenyl ethyl-ammonium, MA is methyl-ammonium, and using only bromide as the halide. The number of the perovskite layers has been varied (n) from n = 1 through n = ∞. Optical and physical characterization verify the layered structure and the increase in the band gap. The photovoltaic performance shows higher open circuit voltage (Voc) for the quasi 2D perovskite (i.e., n = 40, 50, 60) compared to the 3D perovskite. Voc of 1.3 V without hole transport material (HTM) and Voc of 1.46 V using HTM have been demonstrated, with corresponding efficiency of 6.3% and 8.5%, among the highest reported. The lower mobility and transport in the quasi 2D perovskites have been proved effective to gain high Voc with high efficiency, further supported by ab initio calculations and charge extraction measurements. Bromide is the only halide used in these quasi 2D perovskites, as mixing halides have recently revealed instability of the perovskite structure. These quasi 2D materials are promising candidates for use in optoelectronic applications that simultaneously require high voltage and high efficiency.
Perovskite nanostructures, both hybrid organo−metal and fully inorganic perovskites, have gained a lot of interest in the past few years for their intriguing optical properties in the visible region. We report on inorganic cesium lead bromide (CsPbBr3) nanowires (NWs) having quantum confined dimensions corresponding to 5 unit cells. The addition of various hydrohalic acids (HX, X = Cl, Br, I) was found to highly affect the NW length, composition, and optical properties. Hydrochloric (HCl) and hydroiodic (HI) acids mixed in the reaction solution influence the crystal structure and optical properties and shorten the NWs, while the hydrobromic acid (HBr) addition results solely in shorter NWs, without any structural change. The addition of HX increases the acidity of the reaction solution, resulting in protonation of the oleylamine ligands from oleylamine into oleyl-ammonium cations that behave similarly to Cs+ during crystallization. Therefore, the positions of the Cs+ at the growing surface of the NWs are taken by the oleyl-ammonium cations, thus blocking further growth in the favored direction. The emission of the NWs is tunable between ∼423−505 nm and possesses a potential in the optoelectronic field. Moreover, electrical conductivity measurements
We describe a general synthesis of conductive gold thin films doped with entrapped organic molecules, and demonstrate, for the first time, the immobilization of a redox couple within an electrode in a single step. The resulting film is of dual properties: conductivity arising from the gold, and redox behavior originating from the entrapped molecule. Faster electron-transfer rates are found for the entrapped case, compared to adsorption. The conductivity of the film affects the organic molecule–metal interactions, as seen in resistivity measurements, in Raman spectroscopy of the metal-entrapped molecules and from a remarkable red shift of 30 nm in emission spectroscopy. Doping is found to affect the work function of gold. Thin conductive doped metal films are of relevance to a variety of applications such as electrochemical detectors, electrode materials for electrochemical impedance spectroscopy, micro and nano electronics interconnects for packaging and for printed circuit boards. The ability to fine-tune the work function opens the possibility to design the desired energy level gaps for optoelectronic applications such as light emitting diodes (LEDs), solar cells and transistors.
The photochemical stability of encapsulated films of mixed halide perovskites with a range of MAPb(I1xBrx)3 (MA=methylammonium) compositions (solid solutions) was investigated under accelerated stressing using concentrated sunlight. The relevance of accelerated testing to standard operational conditions of solar cells was confirmed by comparison to degradation experiments under outdoor sunlight exposure. We found that MAPbBr3 films exhibited no degradation, while MAPbI3 and mixed halide MAPb(I1xBrx)3 films decomposed yielding crystallization of inorganic PbI2 accompanied by degradation of the perovskite solar light absorption, with faster absorption degradation in mixed halide films. The crystal coherence length was found to correlate with the stability of the films. We postulate that the introduction of Br into the mixed halide solid solution stressed its structure and induced more structural defects and/or grain boundaries compared to pure halide perovskites, which might be responsible for the accelerated degradation. Hence, the cause for accelerated degradation may be the increased defect density rather than the chemical composition of the perovskite materials.
This work demonstrates antisolvent treatment of organo-metal halide perovskite film in hole conductor-free perovskite-based solar cell, achieving impressive power conversion efficiency of 11.2% for hole-conductor-free cells with gold contact. We found that antisolvent (toluene) surface treatment affects the morphology of the perovskite layer, and importantly, it also affects the electronic properties of the perovskite. Conductive atomic force microscopy (cAFM) and surface photovoltage show that the perovskite film becomes more conductive after antisolvent treatment. Moreover, the antisolvent treatment suppresses the hysteresis commonly obtained for perovskite-based solar cells. When the perovskite alone is characterized, a I−V plot of a single perovskite grain measured by cAFM shows that hysteresis vanishes after toluene treatment. During toluene treatment, excess halide and methylammonium ions are removed from the perovskite surface, leading to a net positive charge on the Pb atoms, resulting in a more conductive perovskite surface, which is beneficial for the hole-conductor-free solar cell structure. The reliability of the surface treatment was proved by calculating the statistical parameters Z score and p value, which were 2.5 and 0.012, respectively. According to these values, it can be concluded with 95% confidence that the average efficiency of cells fabricated via surface treatment is greater than the average efficiency of cells without surface treatment. The statistical data support the impact of surface treatment on the photovoltaic performance of perovskite solar cells.
Hybrid perovskite and all-inorganic perovskite have attracted much attention in recent years owing to their successful use in the photovoltaic field. Usually the perovskite is used in its bulk form, although recently, perovskites’ nanocrystalline form has received increased attention. Recent developments in the evolving research field of nanomaterial-based perovskite are reviewed. Both hybrid organic-inorganic and all-inorganic perovskite nanostructures are discussed, as well as approaches to tune the optical properties by controlling the size and shape of perovskite nanostructures. In addition, chemical modifications can change the perovskite nanostructures’ band-gap, similar to their bulk counterpart. Several applications, including light-emitting diodes, lasers, and detectors, demonstrate the latent potential of perovskite nanostructures.
In this work we study the kinetics of cesium lead halide perovskite nanoparticle (NP) growth; the focusing and de-focusing of the NP size distribution. Cesium lead halide perovskite NPs are considered to be attractive materials for optoelectronic applications. Understanding the kinetics of the formation of these all-inorganic perovskite NPs is critical for reproducibly and reliably generating large amounts of uniformly sized NPs. Here we investigate different growth durations for CsPbI3 and CsPbBr3 NPs, tracking their growth by high-resolution transmission electron microscopy and size distribution analysis. As a result, we are able to provide a detailed model for the kinetics of their growth. It was observed that the CsPbI3 NPs exhibit focusing of the size distribution in the first 20 seconds of growth, followed by de-focusing over longer growth durations, while the CsPbBr3 NPs show de-focusing of the size distribution starting from the beginning of the growth. The monomer concentration is depleted faster in the case of CsPbBr3 than in the case of CsPbI3, due to faster diffusion of the monomers, which increases the critical radius and results in de-focusing of the population. Accordingly, focusing is not observed within 40 seconds of growth in the case of CsPbBr3. This study provides important knowledge on how to achieve a narrow size distribution of cesium lead halide perovskite NPs when generating large amounts of these promising, highly luminescent NPs.
High time resolution broadband pump-probe experiments on CH3NH3PbI3 and CH3NH3PbBr3 films are described. The improved time resolution delineates instantaneous and delayed relaxation related effects on sample absorption and assists in clarifying controversial assignment of the underlying mechanisms. Analysis of the data in terms of finite difference spectra and spectral band integrals reveals that photoexcitation is high in the inter-band continuum leading to partial bleaching and red-shifts of the exciton band just below the absorption-edge instantaneously. Increased pump intensity saturates the exciton bleach and progressively reduces inter-band absorption in a broad range extending from the band edge to higher photon energies. Both effects are attributed to reduced Coulomb enhancement due to hot carrier screening. The spectral extent of the inter-band absorption attenuation provides estimated binding energies in the range of 20–30 meV in both materials. Sub-picosecond carrier cooling reverses the initial exciton transition red-shift and induces transmission near the band edge due to state filling and stimulated emission. Finally, 1–100 ps signals are dominated by reverse state filling due to non-geminate recombination. These results demonstrate that both inter-band and exciton absorptions are essential for unraveling photo-induced dynamics in these materials, and that insights obtained from many-body theoretical analysis of dynamic screening are essential for correctly assigning the recorded spectral evolution.
This review discusses various parameters that influence and control the organo-metal halide perovskite crystallization process. The effect of the perovskite morphology on the photovoltaic performance is a critical factor. Moreover, it has a dramatic effect on the stability of the perovskite, which has significant importance for later use of the organo-metal perovskite in assorted applications. In this review, we brought together several research investigations that describe the main parameters that significantly influence perovskite crystallization, for example, the annealing process, the precursor solvent, anti-solvent treatment, and additives to the iteite solutions.mechanisms. Control over morphology is a key parameter to understand this attractive material; morphology control will be an additional step throughout its commercialization process. The review is divided to four parts, presenting various parameters influencing perovskite crystallization and morphology. Section 2 discusses the effect of the annealing process; Section 3 describes the precursor solvent, presenting the possible solvents being used in the deposition process; Section 4 presents the anti-solvent treatment and its effect on the cell properties; and Section 5 discusses additives that can be added to the perovskite solution before deposition.
Spatial heterogeneities in the chemical makeup of thin film photovoltaic devices are pivotal in determining device efficiency. We report the in-plane spatial distribution and degree of chlorine incorporation in organic−inorganic lead halide perovskite absorbers by means of nondestructive synchrotronbased nanoprobe X-ray fluorescence. The presence of chlorine is positively identified in CH3NH3PbI3 films synthesized with Clcontaining precursors and as an impurity in some films synthesized with nominally Cl-free precursors. The impurity may be introduced from precursors or as contaminants during film synthesis. The films formed from Cl-containing precursors contain roughly an order of magnitude higher amount of chlorine, with Cl:I values greater than 0.02 found whether Cl is present in either the organic or the inorganic precursor for both one- and two-step fabrication processes. A spatial variation in the Cl incorporation is observed within single particles and as well as between particles within a given film, and the standard deviation of the Cl:I ratio across the films is up to 30% of the average value. Understanding and controlling the heterogeneous distribution of chlorine in hybrid perovskite layers may offer a path to improve their photovoltaic performance.
Organo-metal halide perovskite is an efficient light harvester in photovoltaic solar cells. Organometal halide perovskite is used mainly in its “bulk” form in the solar cell. Confined perovskite nanostructures could be a promising candidate for efficient optoelectronic devices, taking advantage of the superior bulk properties of organo-metal halideperovskite, as well as the nanoscale properties. In this paper, we present facile low-temperature synthesis of two-dimensional (2D) lead halide perovskite nanorods (NRs). These NRs show a shift to higher energies in the absorbance and in the photoluminescence compared to the bulk material, which supports their 2D structure. X-ray diffraction (XRD) analysis of the NRs demonstrates their 2D nature combined with the tetragonal 3D perovskite structure. In addition, by alternating the halide composition, we were able to tune the optical properties of the NRs. Fast Fourier transform, and electron diffraction show the tetragonal structure of these NRs. By varying the ligands ratio (e.g., octylammonium to oleic acid) in the synthesis, we were able to provide the formation mechanism of these novel 2D perovskite NRs. The 2D perovskite NRs are promising candidates for a variety of optoelectronic applications, such as light-emitting diodes, lasing, solar cells, and sensors.
We report a hybrid mesoporous–planar architecture of methylammonium lead iodide perovskite based solar cells, to combine the benefits of both the mesoporous and planar architectures in a single device. A mesoporous-TiO2 grid was fabricated on a compact TiO2 layer, through a self-assembly process based on directional wetting, providing regions with and without mesoporous-TiO2, followed by perovskite deposition and back contact evaporation (hybrid cells). The hybrid cells showed up to 10.7% power conversion efficiency (PCE) as compared to 13.5% and 6.3% for their mesoporous and planar counterparts, respectively. Interestingly, the hybrid cells are found to show a short circuit current density (Jsc) as high as the Jsc of the mesoporous TiO2 based cells and proved to conserve the current density even in the absence of mesoporous-TiO2 from planar parts of the hybrid cells. The cells showed the best fill factor as compared to their mesoporous and planar counterparts. The areal variation in the meso to planar ratio has also been realized by changing the grid size to demonstrate the effect of the architecture on the cell performance. Charge extraction measurements have been used to obtain insight into the recombination inside different solar cells architectures. The hybrid cell structure emerged as a novel promising design for perovskite solar cells.