Organohalide Lead Perovskites for Photovoltaic Applications

Deep Insights into the Coupled Optoelectronic and Photovoltaic Analysis of Lead-Free CsSnI3 Perovskite-Based Solar Cell Using DFT Calculations and SCAPS-1D Simulations

CsSnI₃ is considered to be a viable alternative to lead (Pb)-based perovskite solar cells (PSCs) due to its suitable optoelectronic properties. The photovoltaic (PV) potential of CsSnI₃ has not yet been fully explored due to its inherent difficulties in realizing defect-free device construction owing to the nonoptimized alignment of the electron transport layer (ETL), hole transport layer (HTL), efficient device architecture, and stability issues. In this work, initially, the structural, optical, and electronic properties of the CsSnI₃ perovskite absorber layer were evaluated using the CASTEP program within the framework of the density functional theory (DFT) approach. The band structure analysis revealed that CsSnI₃ is a direct band gap semiconductor with a band gap of 0.95 eV, whose band edges are dominated by Sn 5s/5p electrons After performing the DFT analysis, we investigated the PV performance of a variety of CsSnI₃-based solar cell configurations utilizing a one-dimensional solar cell capacitance simulator (SCAPS-1D) with different competent ETLs such as IGZO, WS₂, CeO₂, TiO₂, ZnO, PCBM, and C₆₀. Simulation results revealed that the device architecture comprising ITO/ETL/CsSnI₃/CuI/Au exhibited better photoconversion efficiency among more than 70 different configurations. The effect of the variation in the absorber, ETL, and HTL thickness on PV performance was analyzed for the above-mentioned configuration thoroughly. Additionally, the impact of series and shunt resistance, operating temperature, capacitance, Mott-Schottky, generation, and recombination rate on the six superior configurations were evaluated. The J-V characteristics and the quantum efficiency plots for these devices are systematically investigated for in-depth analysis. Consequently, this extensive simulation with validation results established the true potential of CsSnI₃ absorber with suitable ETLs including ZnO, IGZO, WS₂, PCBM, CeO₂, and C₆₀ ETLs and CuI as HTL, paving a constructive research path for the photovoltaic industry to fabricate cost-effective, high-efficiency, and nontoxic CsSnI₃ PSCs.

Synthesis of Perovskite-Type BiScO3 Ceramics and their Dielectric and Infrared Characterization

BiScO₃ compound was obtained in the form of dense ceramic with a perovskite-type structure, and its complex characterization was determined for the first time. The corresponding synthesis procedure is described in detail. It is demonstrated that the temperature region of the phase stability at atmospheric pressure lies at T < 700 °C (973 K). It is shown that the crystal structure of the BiScO₃ ceramic is centrosymmetric. Dielectric measurements of the synthesized sample performed at frequencies 25 Hz to 1 MHz and at temperatures 10-340 K show no changes typical for phase transition. Room-temperature infrared (30-15600 cm^-1) and Raman (90-2000 cm^-1) spectra of the prepared BiScO₃ ceramic are measured, and information on the parameters of phonon resonances is obtained. The number of infrared modes exceeds that predicted by the factor group analysis of the noncentrosymmetric space group C2. The reason for selection rules violation can be associated with the disorder of the crystal structure and local distortions induced by the lone pair of electrons of Bi³⁺.

Electrode Spacing as a Determinant of the Output Performance of Planar-Type Photodetectors Based on Methylammonium Lead Bromide Perovskite Single Crystals

Methylammonium lead bromide is a very perspective hybrid semiconductor material, suitable for high-sensitive, filter-free photodetection of electromagnetic radiation. Herein, we studied the effect of electrode spacing on the output performance and stability of planar-type photodetectors based on high-quality MAPbBr₃ single crystals. Such crystals, as large as 4.5×4.5×1.2 mm³ were synthesized via the inverse temperature crystallization method and were further used for the fabrication of planar Au/MAPbBr₃/Au photodetectors with variable electrode spacing (in the range between 125 and 25 μm). We report that the electrode spacing has a profound impact on photocurrent densities and key detector parameters (responsivity R, external quantum efficiency EQE, and specific detectivity D*). In the studied fivefold electrode spacing, the photocurrent density increased over 4 times, with decreasing active area of the devices. This effect is attributed to intrinsic photocurrent amplification. Based on the transient photocurrent measurements and calculated key parameters, we determined the device sample with the best output performance. The champion sample with an electrode spacing of 50 μm exhibited great detection ability, especially for a low light intensity of 200 nWcm^-2, for which we calculated the R of 19.55 A W^-1, EQE of 4253%, and D* of 3.42 × 10¹² Jones (cm Hz^1/2 W^-1). Moreover, the functional stability of this device showed a minimal reduction of photodetection ability after 2000 cycles, which makes it very promising for the next generation of optoelectronic devices.

Thermodynamic and dynamic stability in a new potential Cs2AgAsCl6 perovskite: insight from DFT study

A schematic diagram of the possible energy band level for photocatalytic activity: (a) favorable energy band level, (b) unfavorable VBM, and (c) unfavorable CBM position.

Lewis Base-Mediated Perovskite Crystallization as Revealed by In Situ, Real-Time Optical Absorption Spectroscopy

The strategy of Lewis base modification has been shown to be rather effective in fabricating high-quality perovskite crystals; however, the underlying mechanisms remain controversial owing to the lack of any systematic characterization of the crystallization process. Herein, we report a novel non-invasive optical technique, termed vertical reflection-type in situ, real-time absorption spectroscopy, to investigate the mechanisms of Lewis base-mediated optimization of perovskite crystallinity by visualizing the entire energetic landscape of crystal growth. We show that by virtue of the urea additive, a prototypical Lewis base, the growth kinetics is accelerated prominently by decreasing the activation energy from 73.7 to 41.7 kJ/mol. In addition, the self-passivation of structural disorder during thermal annealing is identified, which is shown to be further strengthened by urea modification toward a shallower distribution of trap states.

Polymeric Dopant-Free Hole Transporting Materials for Perovskite Solar Cells: Structures and Concepts towards Better Performances

Perovskite solar cells are a hot topic of photovoltaic research, reaching, in few years, an impressive efficiency (25.5%), but their long-term stability still needs to be addressed for industrial production. One of the most sizeable reasons for instability is the doping of the Hole Transporting Material (HTM), being the salt commonly employed as a vector bringing moisture in contact with perovskite film and destroying it. With this respect, the research focused on new and stable "dopant-free" HTMs, which are inherently conductive, being able to effectively work without any addition of dopants. Notwithstanding, they show impressive efficiency and stability results. The dopant-free polymers, often made of alternated donor and acceptor cores, have properties, namely the filming ability, the molecular weight tunability, the stacking and packing peculiarities, and high hole mobility in absence of any dopant, that make them very attractive and a real innovation in the field. In this review, we tried our best to collect all the dopant-free polymeric HTMs known so far in the perovskite solar cells field, providing a brief historical introduction, followed by the classification and analysis of the polymeric structures, based on their building blocks, trying to find structure-activity relationships whenever possible. The research is still increasing and a very simple polymer (PFDT-2F-COOH) approaches PCE = 22% while some more complex ones overcome 22%, up to 22.41% (PPY2).

Unraveling the Charge Transport Mechanism in Mechanochemically Processed Hybrid Perovskite Solar Cell

The long-term operation of organic-inorganic hybrid perovskite solar cells is hampered by the microscopic strain introduced by the multiple thermal cycles during the synthesis of the material via a solution process route. This setback can be eliminated by a room temperature synthesis scheme. In this work, a mechanochemical synthesis technique at room temperature is employed to process CH₃NH₃PbI₂Br films for fabricating perovskite solar cell devices. The solar cell device has produced a 957 mV V_oc, a 16.92 mA/cm² short circuit current density, and a 10.5% efficiency. These values are higher than the published values on mechanochemically synthesized CH₃NH₃PbI₃. The charge transport properties of the devices are studied using DC conductivity and AC impedance spectroscopy, which show a multichannel transport mechanism having both ionic and electronic contributions. A much smaller defect density in the mechanochemically synthesized hybrid perovskite material is confirmed. A polarization assisted recombination mechanism is observed to have a dominant effect on the overall charge transport mechanism. However, no obvious grain boundary and intralayer lattice defect related responses are found in the perovskite layer. Interfacial charge transport and recombination are found to show major effects on both the temperature dependent and illumination dependent impedance spectra.

A Review on Emerging Efficient and Stable Perovskite Solar Cells Based on g-C3N4 Nanostructures

Perovskite solar cells (PSCs) have attracted great research interest in the scientific community due to their extraordinary optoelectronic properties and the fact that their power conversion efficiency (PCE) has increased rapidly in recent years, surpassing other 3rd generation photovoltaic (PV) technologies. Graphitic carbon nitride (g-C3N4) presents exceptional optical and electronic properties and its use was recently expanded in the field of PSCs. The addition of g-C3N4 in the perovskite absorber and/or the electron transport layer (ETL) resulted in PCEs exceeding 22%, mainly due to defects passivation, improved conductivity and crystallinity as well as low charge carriers' recombination rate within the device. Significant performance increase, including stability enhancement, was also achieved when g-C3N4 was applied at the PSC interfaces and the observed improvement was attributed to its wetting (hydrophobic/hydrophilic) nature and the fine tuning of the corresponding interface energetics. The current review summarizes the main innovations for the incorporation of graphitic carbon nitride in PSCs and highlights the significance and perspectives of the g-C3N4 approach for emerging highly efficient and robust PV devices.

Molecular-Level Insight into Correlation between Surface Defects and Stability of Methylammonium Lead Halide Perovskite Under Controlled Humidity

Perovskite-based photovoltaics (PVs) have garnered tremendous interest, enabling power conversion efficiencies exceeding 25%. Although much of this success is credited to the exploration of new compositions, defects passivation and process optimization, environmental stability remains an important bottleneck to be solved. The underlying mechanisms of thermal and humidity-induced degradation are still far from a clear understanding, which poses a severe limitation to overcome the stability issues. Herein, in situ X-ray diffraction (XRD), in operando liquid-cell transmission electron microscopy (TEM) and ex situ solid-state (ss)NMR spectroscopy are combined with time-resolved spectroscopies to reveal new insights about the degradation mechanisms of methylammonium lead halide (MAPbI₃ ) under 85% relative humidity (RH) at different length scales. Liquid-cell TEM enables the live visualizations from meso-to-nanoscale transformation between the perovskite particles and water molecules, which are corroborated by the changes in local structures at sub-nanometer distances by ssNMR and longer range by XRD. This work clarifies the role of surface defects and the significance of their passivation to prevent hydration and decomposition reactions.

A-site cation with high vibrational motion in ABX3 perovskite effectively induces dielectric phase transition

A hybrid perovskite material with dielectric phase transition obtained by the introduction of a moving group.

A Review on Lead-Free Hybrid Halide Perovskites as Light Absorbers for Photovoltaic Applications Based on Their Structural, Optical, and Morphological Properties

Despite the advancement made by the scientific community in the evolving photovoltaic technologies, including the achievement of a 29.1% power conversion efficiency of perovskite solar cells over the past two decades, there are still numerous challenges facing the advancement of lead-based halide perovskite absorbers for perovskite photovoltaic applications. Among the numerous challenges, the major concern is centered around the toxicity of the emerging lead-based halide perovskite absorbers, thereby leading to drawbacks for their pragmatic application and commercialization. Hence, the replacement of lead in the perovskite material with non-hazardous metal has become the central focus for the actualization of hybrid perovskite technology. This review focuses on lead-free hybrid halide perovskites as light absorbers with emphasis on how their chemical compositions influence optical properties, morphological properties, and to a certain extent, the stability of these perovskite materials.

Non-Fullerene Small Molecule Electron-Transporting Materials for Efficient p-i-n Perovskite Solar Cells

PC₆₁BM is commonly used in perovskite solar cells (PSC) as the electron transport material (ETM). However, PC₆₁BM film has various disadvantages, such as its low coverage or the many pinholes that appear due to its aggregation behavior. These faults may lead to undesirable direct contact between the metal cathode and perovskite film, which could result in charge recombination at the perovskite/metal interface. In order to overcome this problem, three alternative non-fullerene electron materials were applied to inverted PSCs; they were evaluated on suitability as electron transport layers. The roles and effects of these non-fullerene ETMs on device performance were studied using photoluminescence (PL) measurements, field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), internal resistance in PSC measurements, and conductive atomic force microscopy (C-AFM). It was found that one of the tested materials, IT-4f, showed excellent electron extraction ability and was associated with reduced recombination. The PSC with IT-4f as the ETM produced better cell-performance; it had an average PCE of 11.21%, which makes it better than the ITIC and COi8DFIC-based devices. Finally, IT-4f was compared with PC₆₁BM; it was found that the two materials have quite comparable efficiency and stability levels.

Carbazole-Terminated Isomeric Hole-Transporting Materials for Perovskite Solar Cells

A set of novel hole-transporting materials (HTMs) based on π-extension through carbazole units was designed and synthesized via a facile synthetic procedure. The impact of isomeric structural linking on their optical, thermal, electrophysical, and photovoltaic properties was thoroughly investigated by combining the experimental and simulation methods. Ionization energies of HTMs were measured and found to be suitable for a triple-cation perovskite active layer ensuring efficient hole injection. New materials were successfully applied in perovskite solar cells, which yielded a promising efficiency of up to almost 18% under standard 100 mW cm^-2 global AM1.5G illumination and showed a better stability tendency outperforming that of 2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)-9,9'-spirobifluorene. This work provides guidance for the molecular design strategy of effective hole-conducting materials for perovskite photovoltaics and similar electronic devices.

Dielectric phase transition of an A2BX4-type perovskite with a pentahedral to octahedral transformation

Organic-inorganic hybrid compounds that undergo reversible dielectric phase transitions are a very attractive class of smart materials due to their wide applications in data storage, data communication and signal sensing. Here, a piperidine ring, C₅H₁₁N, was introduced into the inorganic lead halide perovskite scaffold to obtain three hybrid perovskite compounds, [C₅H₁₂N]₂PbCl₄ (1), [C₅H₁₂N]₂PbBr₄ (2), and [C₅H₁₂N]PbI₃ (3). When compound 2 and compound 3 feature static two-dimensional (2D) and one-dimensional (1D) perovskite structures, respectively, it is striking that compound 1 shows a reversible pentahedral to octahedral transformation. It undergoes an above-room-temperature dielectric phase transition at T_c≅ 352 K, wherein the high dielectric constant is more than twice the low dielectric constant. Structural analysis shows that 1 undergoes a phase transition from the space group Pnma at the low temperature phase (LTP) to C2/c at the high temperature phase (HTP). The phase transition originates from the order-disorder conversion of piperidinium cations. It is interesting to note that, the Pb²⁺ cations in the inorganic moieties change from five-coordinate at the LTP to six-coordinate at the HTP. The discovery of dielectric phase transition hybrid organic-inorganic lead halide perovskite materials further enhances the potential applications of high temperature responsive dielectric switchable materials.

Mutual Insight on Ferroelectrics and Hybrid Halide Perovskites: A Platform for Future Multifunctional Energy Conversion

An insight into the analogies, state-of-the-art technologies, concepts, and prospects under the umbrella of perovskite materials (both inorganic-organic hybrid halide perovskites and ferroelectric perovskites) for future multifunctional energy conversion and storage devices is provided. Often, these are considered entirely different branches of research; however, considering them simultaneously and holistically can provide several new opportunities. Recent advancements have highlighted the potential of hybrid perovskites for high-efficiency solar cells. The intrinsic polar properties of these materials, including the potential for ferroelectricity, provide additional possibilities for simultaneously exploiting several energy conversion mechanisms such as the piezoelectric, pyroelectric, and thermoelectric effect and electrical energy storage. The presence of these phenomena can support the performance of perovskite solar cells. The energy conversion using these effects (piezo-, pyro-, and thermoelectric effect) can also be enhanced by a change in the light intensity. Thus, there lies a range of possibilities for tuning the structural, electronic, optical, and magnetic properties of perovskites to simultaneously harvest energy using more than one mechanism to realize an improved efficiency. This requires a basic understanding of concepts, mechanisms, corresponding material properties, and the underlying physics involved with these effects.

High-Performance Planar-Type Ultraviolet Photodetector Based on High-Quality CH3NH3PbCl3 Perovskite Single Crystals

A hybrid perovskite MAPbCl₃ (MA = CH₃NH₃⁺) single crystal is considered to be one of the most viable candidates for the development of photodetectors because of its outstanding optoelectronic properties. However, the relatively lower crystalline quality of the reported MAPbCl₃ single crystals fabricated by the traditional one-step inverse temperature crystallization results in momentous degradation in the performance of their photodetectors. Here, we present a novel two-step temperature process to fabricate high-quality MAPbCl₃ single crystals, namely, lower temperature nucleation and higher temperature crystallization. These MAPbCl₃ single crystals present low defect density (∼7.9 × 10⁹ cm^-3) commensurate with the best-quality crystals of hybrid organic-inorganic lead halide perovskites reported so far. Moreover, a high-performance ultraviolet photodetector was demonstrated on MAPbCl₃ single crystals. At 30 V, the peak responsivity at 415 nm of the photodetector is as high as 3.73 A W^-1 (light intensity = 1 mW cm^-2), ∼2-3 orders of magnitude higher than that of the previously reported MAPbCl₃ photodetectors. Meanwhile, the device has an ultrafast response speed with a rise time of 130 ns, which is one of the shortest values of MAPbX₃-based photodetectors. Our findings open a new way to obtain high-quality perovskite single crystals and their high-performance photodetectors.

Mixed Cs and FA Cations Slow Electron-Hole Recombination in FAPbI3 Perovskites by Time-Domain Ab Initio Study: Lattice Contraction versus Octahedral Tilting

Using time domain density functional theory combined with nonadiabatic (NA) molecular dynamics, we show that electron-hole recombination takes subnanoseconds in FAPbI₃, showing excellent agreement with experiment. Cs doping retards charge recombination by factors of 1.1 and 3.1 due to lattice contraction and octahedral tilting, respectively. Lattice contraction decreases the NA coupling and increases the coherence time arising from the suppressed atomic fluctuations, slightly slowing recombination because the two factors have an opposite influence on quantum transition. In contrast, octahedral tilting simultaneously decreases the NA coupling, thanks to the reduced overlap between Pb and I orbitals, and the coherence time, extending the excited-state lifetime over 1 ns. Our simulations provide a mechanistic understanding for delayed charge losses in the mixed Cs and FA system, suggesting a rational strategy to improve perovskite solar cell performance.

Rapid Oxidation of the Hole Transport Layer in Perovskite Solar Cells by A Low-Temperature Plasma

Herein we report a strategy of rapid oxidation of the hole transport layer (HTL) in perovskite solar cells by using oxygen/argon mixture plasma. This strategy offers a promising approach for simple manufacturing, mass production, and industrial applications. Compared to the conventional process of overnight oxidation, only ~10 s of oxygen/argon mixture plasma treatment is enough for the solar cell devices with FTO/ETL/perovskite/HTL/Au structure demonstrating a high power conversion efficiency. It is found that the high concentration of atomic oxygen generated in plasma oxidizing the HTL improves the conductivity and mobility, and therefore the process time is considerably shortened. This novel approach is compatible with continuous mass production, and it is suitable for the fabrication of large-area perovskite solar cells in the future.

CsBr-Induced Stable CsPbI3- xBr x ( x < 1) Perovskite Films at Low Temperature for Highly Efficient Planar Heterojunction Solar Cells

All-inorganic cesium lead perovskites have emerged as alternative absorbing layers in solar cells owing to their superb thermal stability compared with the organic-inorganic hybrid perovskites. However, the desired cubic CsPbI₃ phase forms at a high temperature and suffers from a phase transition to the orthorhombic yellow phase at room temperature. A developed nonstoichiometric method is applied to fabricate CsPbI_{3- x}Br _x ( x < 1) films by adding excess CsBr into the precursor solution. The excess CsBr in the precursor solution helps to produce a microstrain in the lattice to stabilize the cubic CsPbI₃ phase at low temperature and incorporate a small part of Br^- into the CsPbI₃ lattice. At the optimal CsBr concentration (0.5 M), the corresponding solar cell achieves a power conversion efficiency of 10.92%. This work provides an effective way to stabilize the cubic CsPbI_{3- x}Br _x ( x < 1) phase at low temperature to further improve the performance of all-inorganic perovskite solar cells.

Lead-free hybrid perovskites for photovoltaics

This review covers the state-of-the-art in organo-inorganic lead-free hybrid perovskites (HPs) and applications of these exciting materials as light harvesters in photovoltaic systems. Special emphasis is placed on the influence of the spatial organization of HP materials both on the micro- and nanometer scale on the performance and stability of perovskite-based solar light converters. This review also discusses HP materials produced by isovalent lead(II) substitution with Sn2+ and other metal(II) ions, perovskite materials formed on the basis of M3+ cations (Sb3+, Bi3+) as well as on combinations of M+/M3+ ions aliovalent to 2Pb2+ (Ag+/Bi3+, Ag+/Sb3+, etc.). The survey is concluded with an outlook highlighting the most promising strategies for future progress of photovoltaic systems based on lead-free perovskite compounds.

The 3 D Structure of Twisted Benzo[ghi]perylene-Triimide Dimer as a Non-Fullerene Acceptor for Inverted Perovskite Solar Cells

Here, we introduced benzo[ghi]perylenetriimide (BPTI) derivatives including monomer and twisted dimer (t-BPTI) as an alternative electron-transport layer (ETL) material to replace the commonly used PC₆₁ BM in inverted planar heterojunction perovskite solar cells (PSCs). Moreover, the double ETL was applied in our PSCs with structure of glass/ITO/PEDOT:PSS/perovskite/BPTI/C₆₀ or PDI-C4/BCP/Al. The use of a double ETL structure can effectively eliminate the leakage current. The devices with the t-BPTI/C₆₀ double ETL yield an average power conversion efficiency of 10.73 % and a maximum efficiency of 11.63 %. The device based on the complete non-fullerene electron acceptors of t-BPTI/PDI-C4 as double ETL achieved maximum efficiency of 10.0 %. Moreover, it was found that the utilization of alloy t-BPTI+BPTI as ETL can effectively reduce the hysteresis effect of PSCs. The results suggest that BPTI-based electron-transport materials are potential alternatives for widely used fullerene acceptors in PSCs.

Exploring the potential energy surface of small lead clusters using the gradient embedded genetic algorithm and an adequate treatment of relativistic effects

Theoretical inclusion of relativistic effects (scalar and spin–orbit) play a crucial role to assure an adequate structural assignment on lead clusters.

First-principles study of intrinsic defects in formamidinium lead triiodide perovskite solar cell absorbers

Based on first-principles calculations, the intrinsic defects in FAPbI₃ are investigated systematically. It is found that antisites FA_I and I_FA create deep levels in the band gap which can act as recombination centers.

Effect of Low Temperature on Charge Transport in Operational Planar and Mesoporous Perovskite Solar Cells

Low-temperature optoelectrical studies of perovskite solar cells using MAPbI₃ and mixed-perovskite absorbers implemented into planar and mesoporous architectures reveal fundamental charge transporting properties in fully assembled devices operating under light bias. Both types of devices exhibit inverse correlation of charge carrier lifetime as a function of temperature, extending carrier lifetimes upon temperature reduction, especially after exposure to high optical biases. Contribution of bimolecular channels to the overall recombination process should not be overlooked because the density of generated charge surpasses trap-filling concentration requirements. Bimolecular charge recombination coefficient in both device types is smaller than Langevin theory prediction, and its mean value is independent of the applied illumination intensity. In planar devices, charge extraction declines upon MAPbI₃ transition from a tetragonal to an orthorhombic phase, indicating a connection between the trapping/detrapping mechanism and temperature. Studies on charge extraction by linearly increasing voltage further support this assertion, as charge carrier mobility dependence on temperature follows multiple-trapping predictions for both device structures. The monotonously increasing trend following the rise in temperature opposes the behavior observed in neat perovskite films and indicates the importance of transporting layers and the effect they have on charge transport in fully assembled solar cells. Low-temperature phase transition shows no pattern of influence on thermally activated electron/hole transport.

Spatial Inhomogeneity of Methylammonium Lead-Mixed Halide Perovskite Examined by Space- and Time-Resolved Microwave Conductivity

Reducing the spatial inhomogeneity of solution-processed, multicrystalline methylammonium lead iodide (MAPbI₃) perovskite is of great importance for improving its power conversion efficiency, suppressing point-to-point deviations, and delaying degradation during operation. Various techniques, such as conducting-mode atomic force microscopy and photoluminescence mapping, have been applied for this intriguing class of materials, revealing nonuniform electronic properties on the nanometer-to-micrometer scale. Here, we designed a new space- and time-resolved microwave conductivity system that enables mapping of the transient photoconductivity with resolution greater than ∼45 μm. We examined the effects of the precursor concentration of MAPbI₃ and the mixing of halides (I^- and Br^-) on the charge carrier dynamics, crystal size, and inhomogeneity of the films. The optoelectronic inhomogeneity of MAPbI₃ and MAPb(I_1-x Br _x )₃ on the sub-millimeter and millimeter scales shows a general correlation with their crystallite sizes, whereas the precursor concentration and halide mixing affect the inhomogeneity in a different way, providing a basis for uniform processing of a multicrystalline perovskite film.

Enhanced optical absorption via cation doping hybrid lead iodine perovskites

The suitable band structure is vital for perovskite solar cells, which greatly affect the high photoelectric conversion efficiency. Cation substitution is an effective approach to tune the electric structure, carrier concentration, and optical absorption of hybrid lead iodine perovskites. In this work, the electronic structures and optical properties of cation (Bi, Sn, and TI) doped tetragonal formamidinium lead iodine CH(NH₂)₂PbI₃ (FAPbI₃) are studied by first-principles calculations. For comparison, the cation-doped tetragonal methylammonium lead iodine CH₃NH₃PbI₃ (MAPbI₃) are also considered. The calculated formation energies reveal that the Sn atom is easier to dope in the tetragonal MAPbI₃/FAPbI₃ structure due to the small formation energy of about 0.3 eV. Besides, the band gap of Sn-doped MAPbI₃/FAPbI₃ is 1.30/1.40 eV, which is considerably smaller than the un-doped tetragonal MAPbI₃/FAPbI₃. More importantly, compare with the un-doped tetragonal MAPbI₃/FAPbI₃, the Sn-doped MAPbI₃ and FAPbI₃ have the larger optical absorption coefficient and theoretical maximum efficiency, especially for Sn-doped FAPbI₃. The lower formation energy, suitable band gap and outstanding optical absorption of the Sn-doped FAPbI₃ make it promising candidates for high-efficient perovskite cells.

Fully-Inorganic Trihalide Perovskite Nanocrystals: A New Research Frontier of Optoelectronic Materials

All-inorganic trihalide perovskite nanocrystals (NCs) are emerging as a new class of superstar semiconductors with excellent optoelectronic properties and great potential for a broad range of applications in lighting, lasing, photon detection, and photovoltaics. This article provides an up-to-date review on the developments of fully-inorganic trihalide perovskite NCs by emphasizing their controllable solution fabrication strategies, structural phase transformation, tunable optoelectronic properties, stability, as well as their photovoltaic and optoelectronic applications. Among the properties to be surveyed, particular focus is on the size-, shape-, and composition-dependent photoluminescence properties. Finally, by identifying new challenges, suggestions are provided for further research and potential development of this area.

Solution-phase synthesis of rubidium lead iodide orthorhombic perovskite nanowires

Recently, metal halide perovskite nanocrystals have demonstrated outstanding properties in various optoelectronic applications. Cesium lead halides (CsPbX₃) are the most studied perovskites in nanoscale dimensions. However, halide perovskite nanocrystals with other cations have rarely been reported. It is important to develop new perovskite compositions to further expand their application in various fields. In this paper, we first report the synthesis of colloidal rubidium lead iodide (RbPbI₃) nanowires (NWs). RbPbI₃ NWs have an orthorhombic crystal structure and are single-crystalline in nature. The diameter of the NWs is around 32 nm with lengths up to several tens of micrometers. RbPbI₃ NWs absorb strongly below 450 nm. RbPbI₃ devices exhibited good photoresponsive behavior, suggesting a potential use in optoelectronics.

Origin of Open-Circuit Voltage Loss in Polymer Solar Cells and Perovskite Solar Cells

Herein, the open-circuit voltage (V_OC) loss in both polymer solar cells and perovskite solar cells is quantitatively analyzed by measuring the temperature dependence of V_OC to discuss the difference in the primary loss mechanism of V_OC between them. As a result, the photon energy loss for polymer solar cells is in the range of about 0.7-1.4 eV, which is ascribed to temperature-independent and -dependent loss mechanisms, while that for perovskite solar cells is as small as about 0.5 eV, which is ascribed to a temperature-dependent loss mechanism. This difference is attributed to the different charge generation and recombination mechanisms between the two devices. The potential strategies for the improvement of V_OC in both solar cells are further discussed on the basis of the experimental data.

Inverted perovskite solar cells based on lithium-functionalized graphene oxide as an electron-transporting layer

A perovskite solar cell with an inverted p–i–n architecture employing GO and GO-Li as functional charge transport components.

Solvent-free, mechanochemical syntheses of bulk trihalide perovskites and their nanoparticles

For the first time, we have synthesized APbBr₃ (A = Cs⁺/MA⁺/FA⁺, where MA⁺ = CH₃NH₃⁺ and FA⁺ = CH(NH₂)₂⁺) bulk as well as nanoparticles (NPs) by solid-state reactions at room temperature.

Enhanced performance of perovskite solar cells by strengthening a self-embedded solvent annealing effect in perovskite precursor films

Smooth perovskite films with large grains are fabricated by strengthening the self-embedded solvent annealing effect in the perovskite precursor film via pre-depositing a protective layer.

A Low-Temperature, Solution-Processable Organic Electron-Transporting Layer Based on Planar Coronene for High-performance Conventional Perovskite Solar Cells

A low-temperature, solution-processable organic electron-transporting material (ETM) is successfully developed for efficient conventional n-i-p perovskite solar cells (PVSCs). This ETM can show a high efficiency over 17% on rigid device and 14.2% on flexible PVSC. To the best of our knowledge, this efficiency is among the highest values reported for flexible n-i-p PVSCs with negligible hysteresis thus far.

Enhanced performance of perovskite solar cells by modulating the Lewis acid-base reaction

The Lewis acid-base reaction between PbI₂ and solvent molecules is popular in fabricating PbI₂ films by a two-step method for making perovskite solar cells. Here, we control the microstructure of PbI₂ films through modulating the Lewis acid-base reaction by adding a small amount of N-methyl pyrrolidone into PbI₂/DMF solution. PbI₂ films with excellent crystallinity and full coverage are fabricated by spin-coating the mixed solution on the substrate, which leads to high quality perovskite layers with low recombination rate and high efficiency for carrier transfer. As a result, the power conversion efficiency of the best perovskite solar cells increases from 13.3% to 17.5%.

Graphene-Perovskite Solar Cells Exceed 18 % Efficiency: A Stability Study

Interface engineering is performed by the addition of graphene and related 2 D materials (GRMs) into perovskite solar cells (PSCs), leading to improvements in the power conversion efficiency (PCE). By doping the mesoporous TiO₂ layer with graphene flakes (mTiO₂ +G), produced by liquid-phase exfoliation of pristine graphite, and by inserting graphene oxide (GO) as an interlayer between the perovskite and hole-transport layers, using a two-step deposition procedure in air, we achieved a PCE of 18.2 %. The obtained PCE value mainly results from improved charge-carrier injection/collection with respect to conventional PSCs. Although the addition of GRMs does not influence the shelf life, it is beneficial for the stability of PSCs under several aging conditions. In particular, mTiO₂ +G PSCs retain more than 88 % of the initial PCE after 16 h of prolonged 1 sun illumination at the maximum power point. Moreover, when subjected to prolonged heating at 60 °C, the GO-based structures show enhanced stability with respect to mTiO₂ +G PSCs, as a result of thermally induced modification at the mTiO₂ +G/perovskite interface. The exploitation of GRMs in the form of dispersions and inks opens the way for scalable large-area production, advancing the possible commercialization of PSCs.

Photoluminescence Blinking of Single-Crystal Methylammonium Lead Iodide Perovskite Nanorods Induced by Surface Traps

Photoluminescence (PL) of organometal halide perovskite materials reflects the charge dynamics inside of the material and thus contains important information for understanding the electro-optical properties of the material. Interpretation of PL blinking of methylammonium lead iodide (MAPbI₃) nanostructures observed on polycrystalline samples remains puzzling owing to their intrinsic disordered nature. Here, we report a novel method for the synthesis of high-quality single-crystal MAPbI₃ nanorods and demonstrate a single-crystal study on MAPbI₃ PL blinking. At low excitation power densities, two-state blinking was found on individual nanorods with dimensions of several hundred nanometers. A super-resolution localization study on the blinking of individual nanorods showed that single crystals of several hundred nanometers emit and blink as a whole, without showing changes in the localization center over the crystal. Moreover, both the blinking ON and OFF times showed power-law distributions, indicating trapping-detrapping processes. This is further supported by the PL decay times of the individual nanorods, which were found to correlate with the ON/OFF states. Furthermore, a strong environmental dependence of the nanorod PL blinking was revealed by comparing the measurements in vacuum, nitrogen, and air, implying that traps locate close to crystal surfaces. We explain our observations by proposing surface charge traps that are likely related to under-coordinated lead ions and methylammonium vacancies to result in the PL blinking observed here.

Hexaazatrinaphthylene Derivatives: Efficient Electron-Transporting Materials with Tunable Energy Levels for Inverted Perovskite Solar Cells

Hexaazatrinaphthylene (HATNA) derivatives have been successfully shown to function as efficient electron-transporting materials (ETMs) for perovskite solar cells (PVSCs). The cells demonstrate a superior power conversion efficiency (PCE) of 17.6 % with negligible hysteresis. This study provides one of the first nonfullerene small-molecule-based ETMs for high-performance p-i-n PVSCs.

High quality perovskite films fabricated from Lewis acid–base adduct through molecular exchange

High quality CH₃NH₃PbI₃ perovskite films without residual PbI₂ are fabricated from the Lewis adduct of PbI₂·xDMF through molecular exchange. The photovoltaic performances of the perovskite solar cells are thus improved significantly.