Organometal Trihalide Perovskites Research Articles

The domination of ionic conductivity in tetragonal phase of the organometal halide perovskite CH3NH3PbI3-xClx

Organometal trihalide perovskites have recently gained extreme attention due to their high solar energy conversion in photovoltaic cells. Here, we investigate the contribution of iodide ions to a total conductivity of the mixed lead halide perovskite CH3NH3PbI3−xClx with a use of the modified DC Hebb–Wagner polarization method. It has been identified that an ionic conductivity dominates in tetragonal phase which is associated with room temperature. The obtained activation energy for this type of hopping mechanism is equal to (0.87 ± 0.02) eV, which is in a good agreement with previous literature reports. The high contribution of ionic conductivity at room temperature might be a reason of the observed hysteresis in halide perovskite solar cells.

Pressure dependence of excited-state charge-carrier dynamics in organolead tribromide perovskites

Excited-state charge-carrier dynamics governs the performance of organometal trihalide perovskites (OTPs) and is strongly influenced by the crystal structure. Characterizing the excited-state charge-carrier dynamics in OTPs under high pressure is imperative for providing crucial insights into structure-property relations. Here, we conduct in situ high-pressure femtosecond transient absorption spectroscopy experiments to study the excited-state carrier dynamics of CH3NH3PbBr3 (MAPbBr3) under hydrostatic pressure. The results indicate that compression is an effective approach to modulate the carrier dynamics of MAPbBr3. Across each pressure-induced phase, carrier relaxation, phonon scattering, and Auger recombination present different pressure-dependent properties under compression. Responsiveness is attributed to the pressure-induced variation in the lattice structure, which also changes the electronic band structure. Specifically, simultaneous prolongation of carrier relaxation and Auger recombination is achieved in the ambient phase, which is very valuable for excess energy harvesting. Our discussion provides clues for optimizing the photovoltaic performance of OTPs.

Ion Migration in Organometal Trihalide Perovskite Solar Cells

Recently, organometal trihalide perovskites (OTPs) are very prospective photovoltaic materials because of its low-cost, easy fabrication and high power conversion efficiency (PCE). However, the low stability of perovskite solar cell is one of the important problems to be solved for their commercialization. Among all those factors that affect the stability of OTPs, ion migration can be important. The presence of ion migration has received widespread attention since the report of photocurrent hysteresis of OTPs. According to experimental data, the ion migration must be an important factor for many unusual phenomena in OTP materials and device, such as current-voltage hysteresis. The direction of the diode polarity and photocurrents can be reversed repeatedly by applying a small electric field. Here we show that how ion migrates and what the effect of ion migration is.

Solvent-induced crystallization for hybrid perovskite thin-film photodetector with high-performance and low working voltage

Organometal trihalide perovskites have emerged as a class of solution-processed semiconductors exhibiting remarkable optoelectronic properties. Using a high-quality perovskite thin film prepared by solvent-induced crystallization method and adopting a novel device configuration based on photon recycling effect, a perovskite thin-film photodetector has been constructed with the highest external quantum efficiency of 4.1 × 104% and responsivity of 219 A W−1 at a low bias of 1 V so far. The device working mechanism was further disclosed based on energy band bending model. The high-performance and low working-voltage perovskite thin-film photodetector will find potential applications in photodetection and optoelectronic integrated circuits.

Improved planar perovskite solar cells using the modified hole transporting layer and solvent annealing

Organometal trihalide perovskites have been demonstrated as excellent light absorbers for high efficiency solar cell [1]. In this work, Glass/ITO/PEDOT:PSS/Perovskite/PCBM/Ag planar heterojunction structure was designed and investigated. For the reference cell, the efficiency of 12.5 % was achieved. An relative low Jsc was exhibited at the result, mainly owing to the low conductivity of the electric transport layer – PEDOT:PSS. By controlling the amount of DMSO doped in PEDOT:PSS, a superior device was obtained with the efficiency of 13.6% although the roughness of the PEDOT:PSS layer was also increasing with doping DMSO. Furthermore, another key factor for a perovskite solar cell is the high crystallization of the perovskite layer[2]. Solvent annealing was adopted to improve the crystallization in this work. With dropping IPA around the solar cell and annealing for 20 minute, a surprising characteristics of the device with the efficiency of 15%, VOC of 0.99 V, Fill Factor of 67.9%, JSC of 22.3mA/cm2 was obtained.

Revealing the stability and efficiency enhancement in mixed halide perovskites MAPb(I 1–x Cl x ) 3 with ab initio calculations

A little addition of Cl to MAPbI3 has been reported to improve the material stability as well as light harvesting and carrier conducting properties of organometal trihalide perovskites, the key component of perovskite solar cell (PSC). However, the mechanism of performance enhancement of PSC by Cl addition is still unclear. Here, we apply the efficient virtual crystal approximation method to revealing the effects of Cl addition on the structural, electronic, optical properties and material stability of MAPb(I1–xClx)3. Our ab initio calculations present that as the increase of Cl content cubic lattice constants and static dielectric constants decrease linearly, while band gaps and exciton binding energies increase quadratically. Moreover, we find the minimum of exciton binding energy at the Cl content of 7%, at which the chemical decomposition reaction changes coincidentally to be from exothermic to endothermic. Interactions among constituents of compound and electronic charge transferring during formation are carefully discussed. This reveals new prospects for understanding and designing of stable, high efficiency PSCs.

In Situ Growth of 120 cm2 CH3 NH3 PbBr3 Perovskite Crystal Film on FTO Glass for Narrowband-Photodetectors.

Organometal trihalide perovskites have been attracting intense attention due to their enthralling optoelectric characteristics. Thus far, most applications focus on polycrystalline perovskite, which however, is overshadowed by single crystal perovskite with superior properties such as low trap density, high mobility, and long carrier diffusion length. In spite of the inherent advantages and significant optoelectronic applications in solar cells and photodetectors, the fabrication of large-area laminar perovskite single crystals is challenging. In this report, an ingenious space-limited inverse temperature crystallization method is first demonstrated to the in situ synthesis of 120 cm2 large-area CH3 NH3 PbBr3 crystal film on fluorine-doped tin oxide (FTO) glass. Such CH3 NH3 PbBr3 perovskite crystal film is successfully applied to narrowband photodetectors, which enables a broad linear response range of 10-4 -102 mW cm-2 , 3 dB cutoff frequency (f 3 dB ) of ≈110 kHz, and high narrow response under low bias -1 V.

Structural Stabilities and Electronic Properties of High-Angle Grain Boundaries in Perovskite Cesium Lead Halides

Organometal trihalide perovskites are emerging as very promising photovoltaic materials, which is rivaling that of single crystal silicon solar cells despite their polycrystalline nature with relatively high density of grain boundaries (GBs). There is a lack of understanding of the effects of GBs on halide perovskites as their presence in silicon and other photovoltaic materials is generally detrimental to their photovoltaic properties. Using first-principles calculations, we systematically investigate the geometric structures of high-angle tilt GBs in halide perovskites CsPbX3 (X = Cl, Br, and I) starting from the coincidence site lattice model and refining using crystal shifts and lattice expansion. Electronic density of states calculations reveal that GBs in halides perovskites do not generate midgap states because of the large distance between the unsaturated atoms and the atomic reconstructions in the GB region. However, we show that the GBs can induce different very shallow states near the valence b...

In- and Ga-based inorganic double perovskites with direct bandgaps for photovoltaic applications.

Double perovskites in the form of A2B'B''X6 (A = Cs, B' = Ag, B'' = Bi) have been reported as potential alternatives to lead-containing organometal trihalide perovskites. However, all double perovskites synthesized to date exhibit indirect bandgaps >1.95 eV, which are undesirable for photovoltaic and optoelectronic applications. Herein, we report a comprehensive computer-aided screening of In- and Ga-based double perovskites for potential photovoltaic applications. To this end, several preconditions are implemented for the screening of optimal candidates, which include structural stability, electronic bandgaps, and optical absorption. Importantly, four In- and Ga-based double perovskites are identified to possess direct bandgaps within the desirable range of 0.9-1.6 eV for photovoltaic applications. Dominant optical absorption of the four double perovskites is found to be in the UV range. The structural and thermal stability of the four double perovskites are examined using both the empirical Goldschmidt ratio and convex-hull calculations. Only Cs2AgInBr6 is predicted to be thermodynamically stable.

Scaffold-reinforced perovskite compound solar cells

The relative insensitivity of the optoelectronic properties of organometal trihalide perovskites to crystallographic defects and impurities has enabled fabrication of highly-efficient perovskite solar cells by scalable solution-state deposition techniques well suited to low-cost manufacturing.

Ion Migration in Organometal Trihalide Perovskite and Its Impact on Photovoltaic Efficiency and Stability.

Organometal trihalide perovskites (OTPs) are emerging as very promising photovoltaic materials because the power conversion efficiency (PCE) of OTP solar cells quickly rises and now rivals with that of single crystal silicon solar cells after only five-years research. Their prospects to replace silicon photovoltaics to reduce the cost of renewable clean energy are boosted by the low-temperature solution processing as well as the very low-cost raw materials and relative insensitivity to defects. The flexibility, semitransparency, and vivid colors of perovskite solar cells are attractive for niche applications such as built-in photovoltaics and portable lightweight chargers. However, the low stability of current hybrid perovskite solar cells remains a serious issue to be solved before their broad application. Among all those factors that affect the stability of perovskite solar cells, ion migration in OTPs may be intrinsic and cannot be taken away by device encapsulation. The presence of ion migration has received broad attention after the report of photocurrent hysteresis in OTP based solar cells. As suggested by much direct and indirect experimental evidence, the ion migration is speculated to be the origin or an important contributing factor for many observed unusual phenomenon in OTP materials and devices, such as current-voltage hysteresis, switchable photovoltaic effect, giant dielectric constant, diminished transistor behavior at room temperature, photoinduced phase separation, photoinduced self-poling effect, and electrical-field driven reversible conversion between lead iodide (PbI2) and methylammonium lead triiodide (MAPbI3). Undoubtedly thorough insight into the ion-migration mechanism is highly desired for the development of OTP based devices to improve intrinsic stability in the dark and under illumination. In this Account, we critically review the recent progress in understanding the fundamental science on ion migration in OTP based solar cells. We look into both theoretical and experiment advances in answering these basic questions: Does ion migration occur and cause the photocurrent hysteresis in perovskite solar cells? What are the migrating ion species? How do ions migrate? How does ion migration impact the device efficiency and stability? How can ion migration be mitigated or eliminated? We also raise some questions that need to be understood and addressed in the future.

Enhanced Performance with the Incorporation of Organo-metal Trihalide Perovskite in Nanostructured ZnO Solar Cell

The tremendous developments in the field of organic photovoltaics (efficiency>12%) in recent years has opened up a new possibility for the commercialization of low cost, flexible, and third generation photovoltaics. However, the rapid degradation of the organic materials and their short exciton diffusion lengths are major hindrance for the complete development of organic solar cells for commercial use. The concept of organic/inorganic hybrid solar cell which combines the advantages of both organic and inorganic materials are readily applicable for low cost, low temperature, high yield, and solution processable techniques but suffers from low fundamental efficiency. Recently, a new class of orgnanic/inorganic hybrid material known as organometal trihalide perovskites have emerged which exhibit the properties of good absorbtion, free charge carrier generation and efficient transport of the generated carriers while maintaining the primary requirements of organic electronics viz. low temperature and solution processability.In our present work, we have demonstrated the enhancement in device efficiency by incorporating organometal trihalide perovskite (CH3NH3PbI3) into ZnO nanostructured/MDMO-PPV inverted solar cell. The devices with perovskite absorbers showed a photocoversion efficiency of 1.97% in contrast to mere 0.06% obtained for ZnO/MDMO-PPV devices.

Critical kinetic control of non-stoichiometric intermediate phase transformation for efficient perovskite solar cells.

Organometal trihalide perovskites (OTP) have attracted significant attention as a low-cost and high-efficiency solar cell material. Due to the strong coordination between lead iodide (PbI2) and dimethyl sulfoxide (DMSO) solvent, a non-stoichiometric intermediate phase of MA2Pb3I8(DMSO)2 (MA = CH3NH3(+)) usually forms in the one-step deposition method that plays a critical role in attaining high power conversion efficiency. However, the kinetic understanding of how the non-stoichiometric intermediate phase transforms during thermal annealing is currently absent. In this work, we investigated such a phase transformation and provided a clear picture of three phase transition pathways as a function of annealing conditions. The interdiffusion of MAI and DMSO varies strongly with the annealing temperature and time, thus determining the final film composition and morphology. A surprising finding reveals that the best performing cells contain ∼18% of the non-stoichiometric intermediate phase, instead of pure phase OTP. The presence of such an intermediate phase enables smooth surface morphology and enhances the charge carrier lifetime. Our results highlight the importance of the intermediate phase growth kinetics that could lead to large-scale production of efficient solution processed perovskite solar cells.

Correlation of energy disorder and open-circuit voltage in hybrid perovskite solar cells

Organometal trihalide perovskites have been demonstrated as excellent light absorbers for high-efficiency photovoltaic applications. Previous approaches to increasing the solar cell efficiency have focused on optimization of the grain morphology of perovskite thin films. Here, we show that the structural order of the electron transport layers also has a significant impact on solar cell performance. We demonstrate that the power conversion efficiency of CH3NH3PbI3 planar heterojunction photovoltaic cells increases from 17.1 to 19.4% when the energy disorder in the fullerene electron transport layer is reduced by a simple solvent annealing process. The increase in efficiency is the result of the enhancement in open-circuit voltage from 1.04 to 1.13 V without sacrificing the short-circuit current and fill factor. These results shed light on the origin of open-circuit voltage in perovskite solar cells, and provide a path to further increase their efficiency. Ongoing efforts are devoted to raising the efficiency of solar cells in converting energy from solar radiation. Now, improved structural order in the charge transport layers of perovskite solar cells is shown to increase the efficiency from 17.1% to 19.4%.

Recent Advances in Improving the Stability of Perovskite Solar Cells

Organometal trihalide perovskites have recently emerged as promising materials for low‐cost, high‐efficiency solar cells. In less than five years, the efficiency of perovskite solar cells (PSC) has been updated rapidly as a result of new strategies adopted in their fabrication process, including device structure, interfacial engineering, chemical compositional tuning, and crystallization kinetics control. To date, the best PSC efficiency has reached 20.1%, which is close to that of single crystal silicon solar cells. However, the stability of PSC devices is still unsatisfactory and is the main bottleneck impeding their commercialization. Here, we summarize recent studies on the degradation mechanisms of organometal trihalide perovskites in PSC devices, and the strategies for stability improvement.

Microstructures of Organometal Trihalide Perovskites for Solar Cells: Their Evolution from Solutions and Characterization.

The use of organometal trihalide perovskites (OTPs) in perovskite solar cells (PSCs) is revolutionizing the field of photovoltaics, which is being led by advances in solution processing of OTP thin films. First, we look at fundamental phenomena pertaining to nucleation/growth, coarsening, and microstructural evolution involved in the solution-processing of OTP thin films for PSCs from a materials-science perspective. Established scientific principles that govern some of these phenomena are invoked in the context of specific literature examples of solution-processed OTP thin films. Second, the nature and the unique characteristics of OTP thin-film microstructures themselves are discussed from a materials-science perspective. Finally, we discuss the challenges and opportunities in the characterization of OTP thin films for not only gaining a deep understanding of defects and microstructures but also elucidating classical and nonclassical phenomena pertaining to nucleation/growth, coarsening, and microstructural evolution in these films. The overall goal is to have deterministic control over the solution-processing of tailored OTP thin films with desired morphologies and microstructures.

Ultrafast photomodulation spectroscopy of π-conjugated polymers, nanotubes and organometal trihalide perovskites: A comparison

We compare the ultrafast dynamics of the primary photoexcitations in various π-conjugated organic semiconductors, semiconducting single-walled carbon nanotubes (S-NTs) and organometal trihalide perovskites including CH3NH3PbI3 (MAPbI3) and CH3NH3PbI1.1Br1.9 (MAPbI1.1Br1.9), using broadband pump–probe photomodulation spectroscopy in the spectral range of 0.2–2.7eV with 300fs time resolution. The primary photoexcitations in single polymer chains and isolated S-NTs have been found to be quasi-one-dimensional (q-1D) excitons, with characteristic photoinduced absorption (PA) band due to intra-band transitions. This conclusion is in agreement with the large exciton binding energy, Eb in polymers and S-NTs (where Eb>200meV), illustrating the universal optical characteristic features of q-1D excitons. In three dimensional (3D) semiconductors of organometal trihalide perovskites such as MAPbI3 we found that with above-gap excitation both photo-carriers and excitons are photogenerated; but only photocarriers are photogenerated with below-gap excitation. In contrast, mainly excitons are photogenerated in MAPbI1.1Br1.9. The contrast between MAPbI3 and MAPbI1.1Br1.9 is ascribed to the difference of Eb, which is ∼20meV and ∼110meV, respectively. Our work shows that Eb is one of the crucial parameters that determine the photophysics characteristic of semiconductors, that result in universal occurrence of an exciton PA band, regardless if the compound is q-1D carbon based π-conjugated semiconductors, NTs, or 3D crystalline perovskite semiconductors. At the same time, our work also shows that the broadband ultrafast photomodulation spectroscopy is a powerful tool in analyzing the photophysics of semiconductors, and emphasizes the need for a broad probe spectral range in order to decipher the primary photoexcitation species.

Molecular Engineering of Functional Materials for Energy and Opto-Electronic Applications.

This review presents an overview of the dedicated research directions of the Group for Molecular Engineering of Functional Materials (GMF). This includes molecular engineering aspects of sensitizers constructed from ruthenium complexes, organic molecules, porphyrins and phthalocyanines. Manipulation of organometal trihalide perovskites, and charge transporting materials for high performance perovskite solar cells and photo-detectors are also described. Controlling phosphorescence color, and quantum yields in iridium complexes by tailoring ligands for organic light emitting diodes are demonstrated. Efficient reduction of CO(2) to CO using molecular catalyst on a protected Cu(2)O photocathode, and cost-effective water-splitting cell using a high efficiency perovskite solar cell are presented.

Vacuum-free laminated top electrode with conductive tapes for scalable manufacturing of efficient perovskite solar cells

The efficiency of organometal trihalide perovskites (OTP) solar cells have reached that parity of single crystal silicon, and its nature abundant raw material and solution-process capability promise a bright future for commercialization. However, the vacuum based techniques for metal electrode deposition and additional encapsulation layer increase the cost of the perovskite solar cells dramatically and impede their commercialization process. Here, we report a vacuum-free low temperature lamination technique to fabricate the top electrode by commercial conductive tapes (C-tape). The simple fabrication method yields good quality contact and high efficiency device of 12.7%. The C-tapes also encapsulated the devices effectively, resulting in greatly improved device stability. The combination of lamination of electrodes and encapsulation layers into a single step significantly reduce the cost of device fabrication.

Molecular Engineering of Functional Materials for Energy and Opto-Electronic Applications.

This review presents an overview of the dedicated research directions of the Group for Molecular Engineering of Functional Materials (GMF). This includes molecular engineering aspects of sensitizers constructed from ruthenium complexes, organic molecules, porphyrins and phthalocyanines. Manipulation of organometal trihalide perovskites, and charge transporting materials for high performance perovskite solar cells and photo-detectors are also described. Controlling phosphorescence color, and quantum yields in iridium complexes by tailoring ligands for organic light emitting diodes are demonstrated. Efficient reduction of CO(2) to CO using molecular catalyst on a protected Cu(2)O photocathode, and cost-effective water-splitting cell using a high efficiency perovskite solar cell are presented.