Carbon Nanoparticles in High‐Performance Perovskite Solar Cells

In the past few years, organic–inorganic metal halide ABX₃ perovskites (A = Rb, Cs, methylammonium, formamidinium (FA); B = Pb, Sn; X = Cl, Br, I) have rapidly emerged as promising materials for photovoltaic applications. Tuning the film morphology by various deposition techniques and additives is crucial to achieve solar cells with high performance and long‐term stability. In this work, carbon nanoparticles (CNPs) containing functional groups are added to the perovskite precursor solution for fabrication of fluorine‐doped tin oxide/TiO₂/perovskite/spiro‐OMeTAD/gold devices. With the addition of CNPs, the perovskite films are thermally more stable, contain larger grains, and become more hydrophobic. NMR experiments provide strong evidence that the functional groups of the CNPs interact with FA cations already in the precursor solution. The fabricated solar cells show a power‐conversion efficiency of 18% and negligible hysteresis.     

Computational characterisation of the charge-transfer and T-shaped molecular complexes of N-methyl imidazoline-2-thione and N-methyl imidazolidine-2-thione with the dihalogens Br2 and I2

The computational characterisation of the molecular complexes of N-methyl imidazoline-2-thione (methimazole) and the related saturated analogue N-methyl imidazolidine-2-thione with Br₂ and I₂ is carried out using quantum mechanical electronic structure methods. Two kinds of molecular connectivity have been examined. The first displays a collinear S–X–X geometry (X = Br, I) and leads to charge-transfer (CT) type adducts, possible in two stereoisomeric conformations depending on the direction of the X₂-axis, either planar or perpendicular to the NCS plane. The second kind corresponds to T-shaped hypervalent complexes in which sulphur is connected to both the X atoms forming the linear X–S–X arrangement. The structural changes, the spectroscopic findings, the natural bond orbital analysis and the examination of the molecular orbital second-order perturbation energies give interesting information about the nature of the halogen bonding interaction between the electron-donor organic species and the electron-acceptor dihalogen molecule. Similar trends are followed by the energy and relative stability results including basis set superposition error corrections, which show the larger stabilisation of the planar CT conformers of both dihalogens vs. the perpendicular configurations. They also indicate the higher stability of the T-shaped bromine complexes relative to the CT species, opposite to the energy order of the corresponding diiodine adducts. A critical comparison is carried out with literature results on similar systems.     

Fabrication of Efficient and Stable CsPbI3 Perovskite Solar Cells through Cation Exchange Process

Inorganic lead halide perovskites have attracted attention due to their tolerance to higher processing temperature and higher bandgap suitable for tandem solar cell application. Not only do they improve cell stability and efficiency, they also reveal many interesting and un‐anticipated material qualities. This work reports a simple cation exchange growth (CEG) method for fabricating inorganic high‐quality cesium lead iodide (CsPbI₃) by adding methylammonium iodide (MAI) additive in the precursor. X‐ray diffraction results reveal a multi‐stage film formation process whereby i) MAPbI₃ perovskite first formed that acts as a perovskite template for ii) subsequent ion exchange whereby the MA⁺ ions in the MAPbI₃ are replaced by Cs⁺ (as temperature ramps up) and iii) form g‐phase perovskite CsPbI₃. Optical microscopy, photoluminescence, and electrical characterizations reveal that the CEG process produces high‐quality film with better absorption, uniform and dense film with better interface, lower defects, and better stability. Using the CEG approach, the power conversion efficiency of the best CsPbI₃ solar cell is significantly increased up to 14.1% for the device fabricated using 1.0 m MAI additive. The outcome is beneficial for further improvement of inorganic perovskite solar cells and their application in perovskite‐silicon tandem devices.     

Stabilization of Precursor Solution and Perovskite Layer by Addition of Sulfur

Efficient perovskite solar cells (PSCs) are mainly fabricated by a solution coating processes. However, the efficiency of such devices varies significantly with the aging time of the precursor solution used to fabricate them, which includes a mixture of perovskite components, especially methylammonium (MA), and formamidinium (FA) cations. Herein, how the inorganic–organic hybrid perovskite precursor solution of (FAPbI₃)_0.95(MAPbBr₃)_0.05 degrades over time and how such degradation can be effectively inhibited is reported on, and the associated mechanism of degradation is discussed. Such degradation of the precursor solution is closely related to the loss of MA cations dissolved in the FA solution through the deprotonation of MA to volatile methylamine (CH₃NH₂). Addition of elemental sulfur (S₈) drastically stabilizes the precursor solution owing to amine–sulfur coordination, without compromising the power conversion efficiency (PCE) of the derived PSCs. Furthermore, sulfur introduced to stabilize the precursor solution results in improved PSC stability.     

Understanding the Role of Cesium and Rubidium Additives in Perovskite Solar Cells: Trap States,Charge Transport,and Recombination

Adding cesium (Cs) and rubidium (Rb) cations to FA_0.83MA_0.17Pb(I_0.83Br_0.17)₃ hybrid lead halide perovskites results in a remarkable improvement in solar cell performance, but the origin of the enhancement has not been fully understood yet. In this work, time‐of‐flight, time‐resolved microwave conductivity, and thermally stimulated current measurements are performed to elucidate the impact of the inorganic cation additives on the trap landscape and charge transport properties within perovskite solar cells. These complementary techniques allow for the assessment of both local features within the perovskite crystals and macroscopic properties of films and full devices. Strikingly, Cs‐incorporation is shown to reduce the trap density and charge recombination rates in the perovskite layer. This is consistent with the significant improvements in the open‐circuit voltage and fill factor of Cs‐containing devices. By comparison, Rb‐addition results in an increased charge carrier mobility, which is accompanied by a minor increase in device efficiency and reduced current–voltage hysteresis. By mixing Cs and Rb in quadruple cation (Cs‐Rb‐FA‐MA) perovskites, the advantages of both inorganic cations can be combined. This study provides valuable insights into the role of these additives in multiple‐cation perovskite solar cells, which are essential for the design of high‐performance devices.     

Understanding Film Formation Morphology and Orientation in High Member 2D Ruddlesden–Popper Perovskites for High‐Efficiency Solar Cells

2D Ruddlesden–Popper (RP) perovskites have recently emerged as promising candidates for hybrid perovskite photovoltaic cells, realizing power‐conversion efficiencies (PCEs) of over 10% with technologically relevant stability. To achieve solar cell performance comparable to the state‐of‐the‐art 3D perovskite cells, it is highly desirable to increase the conductivity and lower the optical bandgap for enhanced near‐IR region absorption by increasing the perovskite slab thickness. Here, the use of the 2D higher member (n = 5) RP perovskite (n‐butyl‐NH₃)₂(MeNH₃)₄Pb₅I₁₆ in depositing highly oriented thin films from dimethylformamide/dimethylsulfoxide mixtures using the hot‐casting method is reported. In addition, they exhibit superior environmental stability over thin films of their 3D counterpart. These films are assembled into high‐efficiency solar cells with an open‐circuit voltage of ≈1 V and PCE of up to 10%. This is achieved by fine‐tuning the solvent ratio, crystal growth orientation, and grain size in the thin films. The enhanced performance of the optimized devices is ascribed to the growth of micrometer‐sized grains as opposed to more typically obtained nanometer grain size and highly crystalline, densely packed microstructures with the majority of the inorganic slabs preferentially aligned out of plane to the substrate, as confirmed by X‐ray diffraction and grazing‐incidence wide‐angle X‐ray scattering mapping.     

Exploring the Stability of Novel Wide Bandgap Perovskites by a Robot Based High Throughput Approach

Currently, lead‐based perovskites with mixed multiple cations and hybrid halides are attracting intense research interests due to their promising stability and high efficiency. A tremendous amount of 3D and 2D perovskite compositions and configurations are causing a strong demand for high throughput synthesis and characterization. Furthermore, wide bandgap (≈1.75 eV) perovskites as promising top‐cell materials for perovskite–silicon tandem configurations require the screening of different compositions to overcome photoinduced halide segregation and still yielding a high open‐circuit voltage (V_oc). Herein, a home‐made high throughput robot setup is introduced performing automatic perovskite synthesis and characterization. Subsequently, four kinds of compositions (i.e., cation mixtures of Cs–methylammonium (MA), Cs– formamidinium (FA), MA–FA, and FA–MA) with an optical bandgap of ≈1.75 eV are identified as promising device candidates. For Cs–MA and Cs–FA films it is found that the Br–I phase segregation indeed can be overcome. Moreover, Cs–MA, MA–FA, and Cs–FA based devices exhibit an average V_oc of 1.17, 1.17, 1.12 V, and their maximum values approached 1.18, 1.19, 1.14 V, respectively, which are among the highest V_oc (≈1.2 V) values for ≈40% Br perovskite. These findings highlight that the high throughput approach can effectively and efficiently accelerate the invention of novel perovskites for advanced applications.     

Targeting Ideal Dual‐Absorber Tandem Water Splitting Using Perovskite Photovoltaics and CuInxGa1‐xSe2 Photocathodes

Efficient sunlight‐driven water splitting devices can be achieved by pairing two absorbers of different optimized bandgaps in an optical tandem design. With tunable absorption ranges and cell voltages, organic–inorganic metal halide perovskite solar cells provide new opportunities for tailoring top absorbers for such devices. In this work, semitransparent perovskite solar cells are developed for use as the top cell in tandem with a smaller bandgap photocathode to enable panchromatic harvesting of the solar spectrum. A new CuIn_xGa_1‐xSe₂ multilayer photocathode is designed, exhibiting excellent performance for photoelectrochemical water reduction and representing a near‐ideal bottom absorber. When pairing it below a semitransparent CH₃NH₃PbBr₃‐based solar cell, a solar‐to‐hydrogen efficiency exceeding 6% is achieved, the highest value yet reported for a photovoltaic–photoelectrochemical device utilizing a single‐junction solar cell as the bias source under one sun illumination. The analysis shows that the efficiency can reach more than 20% through further optimization of the perovskite top absorber.     

Tris(triazolyl)borate ligands of intermediate steric bulk for the synthesis of biomimetic structures with hydrogen bonding and solubility in hydrophilic solvents

Tris(triazolyl)borate ligands (Ttz) of intermediate steric bulk were synthesized to investigate their potential for hydrogen bonding and improved solubility in hydrophilic solvents as applied to biomimetic chemistry. The crystal structure of 3-phenyl-5-methyl-1,2,4-triazole (Htz^Ph,Me) revealed hydrogen bonding and π stacking interactions. The new ligand salt, potassium tris(3-phenyl-5-methyl-1,2,4-triazolyl)borate (KTtz^Ph,Me) was synthesized as the first example of a Ttz ligand of intermediate steric bulk. Metathesis between KTtz^Ph,Me and NaCl followed by recrystallization produced [NaTtz^Ph,Me] · 6CH₃OH in which the geometry around the sodium is octahedral with an unusual N₃O₃ donor set; this structure also shows that a hydrogen bonding network is formed by methanol molecules and triazole nitrogens. (Ttz^Ph,Me)ZnCl was synthesized and characterized crystallographically as [(Ttz^Ph,Me)ZnCl] · 0.5CH₃OH in which the zinc is tetrahedral and the triazole rings are within hydrogen bonding distance of CH₃OH. All of these new compounds are methanol soluble to varying degrees and Htz^Ph,Me and KTtz^Ph,Me are soluble in methanol/water mixtures.     

Rational Core–Shell Design of Open Air Low Temperature In Situ Processable CsPbI3 Quasi‐Nanocrystals for Stabilized p‐i‐n Solar Cells

As a promising alternative, inorganic perovskite nanocrystals allow reinforced stability of photovoltaic device. Unfortunately, directly assembling these nanocrystals into film is uncontrollable. Instead, in situ assembling technology under low temperature in open air is attractive but limited due to the tendency of nonperovskite transition. The adverse shell ligands and unstable core lattices are known as the fundamental problems. In order to address this issue, here proposed is a rational core–shell design: 1) with respect to ligands, a new one, 4‐fluorophenethylammonium iodide, is used to enhance bonding force and charge coupling between ligands and nanocrystals; 2) with respect to lattices, a novel compound H₂PbI₄ is employed to assist divalent ion (Mn²⁺) doping into perovskite lattices. By low temperature in situ processing CsPbI₃ quasi‐nanocrystal film, the highest power conversion efficiency of 13.4% for p‐i‐n solar cells is achieved, which retains 92% after 500 h in ambient air. The current study underlines the significance of rational hierarchical design of inorganic perovskite nanocrystals, especially for low temperature in situ processable electronic devices.     

Roles of Organic Molecules in Inorganic CsPbX3 Perovskite Solar Cells

Over 25% efficiencies have been achieved by organic–inorganic hybrid perovskite solar cells (PSCs). However, their practical applications are limited by the instability of the hybrid perovskite materials. Replacing hybrid perovskites with inorganic CsPbX₃ perovskites shows great promise to address the above issue and much progress has been made. To achieve high efficiency and stable inorganic CsPbX₃ PSCs, organic molecular engineering has been playing a vital role. Herein, the progress of the organic molecular engineering in inorganic CsPbX₃ PSCs is systematically reviewed. First, structure evolution induced by organic molecular engineering for inorganic CsPbX₃ perovskites is demonstrated. Then, organic molecular engineering in CsPbX₃ PSCs is categorized and reviewed (alloying in perovskite structures, as sacrificial agents, forming 2D structures, and modifying surfaces and interfaces). Finally, future research directions are suggested to further improve the performance of inorganic PSCs.     

Halogen Engineering for Operationally Stable Perovskite Solar Cells via Sequential Deposition

The performance of perovskite solar cells (PSCs) relies on the synthesis method and chemical composition of the perovskite materials. So far, PSCs that have adopted two‐step sequential deposited perovskite with the state‐of‐art composition (FAPbI₃)_1?x(MAPbBr₃)_x (x < 0.05) have achieved record power conversion efficiency (PCE), while their one‐step antisolvent dripping counterparts with typical composition Cs_0.05FA_0.81MA_0.14Pb(I_0.85Br_0.15)₃ with more bromine have exhibited much better long‐term operational stability. Thus, halogen engineering that aims to elevate bromine content in sequential deposited perovskite film would push operational stability of PSCs toward that of antisolvent dripping deposited perovskite materials. Here, a Br‐rich seeding growth method is devised and perovskite seed solution with high bromine content is introduced into a PbI₂ precursor, leading to bromine incorporation in the resulting perovskite film. Photovoltaic devices fabricated by Br‐rich seeding growth method exhibit a PCE of 21.5%, similar to 21.6% for PSCs having lower bromine content. Whereas, the operational stability of PSCs with higher bromine content is significantly enhanced, with over 80% of initial PCE retained after 500 h tracking at maximum power point under 1‐sun illumination. This work highlights the vital importance of halogen composition for the operational stability of PSCs, and introduces an effective way to incorporate bromine into mixed‐cation‐halide perovskite film via sequential deposition method.     

Soft Template‐Controlled Growth of High‐Quality CsPbI3 Films for Efficient and Stable Solar Cells

The unfavorable morphology and inefficient utilization of phase transition reversibility have limited the high‐temperature‐processed inorganic perovskite films in both efficiency and stability. Here, a simple soft template‐controlled growth (STCG) method is reported by introducing (adamantan‐1‐yl)methanammonium to control the nucleation and growth rate of CsPbI₃ crystals, which gives rise to pinhole‐free CsPbI₃ film with a grain size on a micrometer scale. The STCG‐based CsPbI₃ perovskite solar cell exhibits a power conversion efficiency of 16.04% with significantly reduced defect densities and charge recombination. More importantly, an all‐inorganic solar cell with the architecture fluorine‐doped tin oxide (FTO)/NiO_x/STCG‐CsPbI₃/ZnO/indium‐doped tin oxide (ITO) is successfully fabricated to demonstrate its real advantage in thermal stability. By suppressing the inductive effect of defects during the phase transition and utilizing the unique reversibility of the phase transition for the high‐temperature‐processed CsPbI₃ film, the all‐inorganic solar cell retains 90% of its initial efficiency after 3000 h of continuous light soaking and heating.     

Charge Accumulation and Hysteresis in Perovskite‐Based Solar Cells: An Electro‐Optical Analysis

Organic–inorganic hybrid perovskite solar cells based on CH₃NH₃PbI₃ have achieved great success with efficiencies exceeding 20%. However, there are increasing concerns over some reported efficiencies as the cells are susceptible to current–voltage (I–V) hysteresis effects. It is therefore essential that the origins and mechanisms of the I–V hysteresis can clearly be understood to minimize or eradicate these hysteresis effects completely for reliable quantification. Here, a detailed electro‐optical study is presented that indicates the hysteresis originates from lingering processes persisting from sub‐second to tens of seconds. Photocurrent transients, photoluminescence, electroluminescence, quasi‐steady state photoinduced absorption processes, and X‐ray diffraction in the perovskite solar cell configuration have been monitored. The slow processes originate from the structural response of the CH₃NH₃PbI₃ upon E‐field application and/or charge accumulation, possibly involving methylammonium ions rotation/displacement and lattice distortion. The charge accumulation can arise from inefficient charge transfer at the perovskite interfaces, where it plays a pivotal role in the hysteresis. These findings underpin the significance of efficient charge transfer in reducing the hysteresis effects. Further improvements of CH₃NH₃PbI₃‐based perovskite solar cells are possible through careful surface engineering of existing TiO₂ or through a judicious choice of alternative interfacial layers.     

The structural chemistry of triorganotin(IV) derivatives of 3-amino-5-mercapto-1,2,4-triazole: Supramolecular structures involving intermolecular N-H?N and N-H?S hydrogen bonding

Four new triorganotin complexes of 3-amino-5-mercapto-1,2,4-triazole with the type of R₃Sn(SC₂N₃HNH₂-3) (R = Me, 1; n-Bu, 2; Ph, 3; PhCH₂, 4) have been synthesized. All the complexes have been characterized by elemental analysis, IR, ¹H NMR and ¹³C NMR spectra. Complexes 1, 3 and 4 have been characterized by X-ray crystallography analyses too. The geometry about Sn of complex 1 is distorted trigonal bipyramidal and the supramolecular structures of complex 1 has been found consist of channels built up by intermolecular N-H?N hydrogen bonding. The geometry of tin atoms in complexes 3 and 4 are distorted tetrahedron and 1D polymers connected by intermolecular N-H?N hydrogen bonding or N-H?N and N-H?S hydrogen bonding. Additionally, 1D polymer of complex 3 aggregated in 2D layer by intermolecular N-H?S hydrogen bonding.     

Quantum chemical topological analysis of hydrogen bonding in HX…HX and CH3X…HX dimers (X = Br,Cl, F)

We present a systematic investigation of the nature and strength of the hydrogen bonding in HX···HX and CH₃X…HX (X = Br, Cl and F) dimers using ab initio MP2/aug-cc-pVTZ calculations in the framework of the quantum theory of atoms in molecules (QTAIM) and electron localisation functions (ELFs) methods. The electron density of the complexes has been characterised, and the hydrogen bonding energy, as well as the QTAIM and ELF parameters, is consistent, providing deep insight into the origin of the hydrogen bonding in these complexes. It was found that in both linear and angular HX…HX and CH₃X…HX dimers, F atoms form stronger HB than Br and Cl, but they need short (～2 Å) X…HX contacts.     

Inorganic CsPb1−xSnxIBr2 for Efficient Wide‐Bandgap Perovskite Solar Cells

Recently, the stability of organic–inorganic perovskite thin films under thermal, photo, and moisture stresses has become a major concern for further commercialization due to the high volatility of the organic cations in the prototype perovskite composition (CH₃NH₃PbI₃). All inorganic cesium (Cs) based perovskite is an alternative to avoid the release or decomposition of organic cations. Moreover, substituting Pb with Sn in the organic–inorganic lead halide perovskites has been demonstrated to narrow the bandgap to 1.2–1.4 eV for high‐performance perovskite solar cells. In this work, a series of CsPb_1?xSn_xIBr₂ perovskite alloys via one‐step antisolvent method is demonstrated. These perovskite films present tunable bandgaps from 2.04 to 1.64 eV. Consequently, the CsPb_0.75Sn_0.25IBr₂ with homogeneous and densely crystallized morphology shows a remarkable power conversion efficiency of 11.53% and a high V_oc of 1.21 V with a much improved phase stability and illumination stability. This work provides a possibility for designing and synthesizing novel inorganic halide perovskites as the next generation of photovoltaic materials.     

Precursor Engineering for Ambient‐Compatible Antisolvent‐Free Fabrication of High‐Efficiency CsPbI2Br Perovskite Solar Cells

High temperature stable inorganic CsPbX₃ (X: I, Br, or mixed halides) perovskites with their bandgap tailored by tuning the halide composition offer promising opportunities in the design of ideal top cells for high‐efficiency tandem solar cells. Unfortunately, the current high‐efficiency CsPbX₃ perovskite solar cells (PSCs) are prepared in vacuum, a moisture‐free glovebox or other low‐humidity conditions due to their poor moisture stability. Herein, a new precursor system (HCOOCs, HPbI₃, and HPbBr₃) is developed to replace the traditional precursors (CsI, PbI₂, and PbBr₂) commonly used for solar cells of this type. Both the experiments and calculations reveal that a new complex (HCOOH?Cs⁺) is generated in this precursor system. The new complex is not only stable against aging in humid air ambient at 91% relative humidity, but also effectively slows the perovskite crystallization, making it possible to eliminate the popular antisolvent used in the perovskite CsPbI₂Br film deposition. The CsPbI₂Br PSCs based on the new precursor system achieve a champion efficiency of 16.14%, the highest for inorganic PSCs prepared in ambient air conditions. Meanwhile, high air stability is demonstrated for an unencapsulated CsPbI₂Br PSC with 92% of the original efficiency remaining after more than 800 h aging in ambient air.     

MACl‐Assisted Ge Doping of Pb‐Hybrid Perovskite: A Universal Route to Stabilize High Performance Perovskite Solar Cells

Interfacial engineering, grain boundary, and surface passivation in organic–inorganic hybrid perovskite solar cells (HyPSCs) are effective in achieving high performance and enhanced durability. Organic additives and inorganic doping are generally used to chemically modify the surface contacting charge transport layers, and/or grain boundaries so as to reduce the defect density. Here, a simple but tricky one‐step method to dope organic–inorganic hybrid perovskite with Ge for the first time is reported. Unlike Ge doping to all‐inorganic perovskites, application of GeI₂ in organic–inorganic perovskite precursors is challenging due to the extremely poor solubility of GeI₂ in hybrid perovskite ink, leading to failure in the formation of uniform films. However, it is found that addition of methylammonium chloride (MACl) into the precursor remarkably increases the solubility of GeI₂. This MACl‐assisted Ge doping of hybrid perovskites produces high‐quality crystalline film with its surface passivated with nonvolatile GeI₂ (GeO₂) and the volatile MACl additive also improves the uniformity of GeO₂ distribution in the perovskite films. The resulting Ge‐doped mixed cation and mixed halide perovskite films with composition FA_0.83MA_0.17Ge_0.03Pb_0.97(I_0.9Br_0.1)₃ show superior photoluminescence lifetime, power conversion efficiency above 22%, and greater stability toward illumination and humidity, outperforming photovoltaic properties of HyPSCs prepared without the Ge doping.     

Effect of solvent environment on the CO band position in the infrared spectrum of trans-[Fe(CN)4(CO)2]

Density functional theory (DFT) is used to understand the effect of hydrogen bonding solvents on the CO band position in the infrared (IR) spectrum of a mono-iron complex, trans-[Fe^II(CN)₄(CO)₂]²⁻. This mono-iron complex has received much attention recently due its potential relation to the biosynthesis of Fe-only hydrogenase enzymes. Our calculations show that the polar solvent molecules preferentially hydrogen bond to the cyano ligands in this complex. The effect of such hydrogen bonding on the electron density distribution is analyzed in terms of the population in natural bond orbitals (NBO). Our results show that the presence of hydrogen bonding to the cyano ligands decreases the extent of back bonding from the metal to the carbonyl ligand. This results in decreased electron density in the π^∗ orbitals of the carbonyl bond leading to a strengthening of the CO bond and a consequent blue shift in the IR band position of the carbonyl group. We also show that the extent of blue shift correlates with the number of nearest neighbor solvent molecules.