Chemistry Of Perovskites Research Articles

Virtual Special Issue on the Physical Chemistry of Perovskites

Virtual Special Issue on the Physical Chemistry of Perovskites

Facet orientation control of tin-lead perovskite for efficient all-perovskite tandem solar cells

Developing high-performance narrow-bandgap (NBG) perovskite solar cells based on tin-lead (Sn-Pb) perovskite is critical for the advancement of all-perovskite tandem solar cells. However, the limitations of the device current density and efficiency are magnified by the issues concerning poor carrier transport caused by a substantial number of defects in thick NBG films. This problem is further exacerbated by the quality of film crystallization, which is associated with the rapid and uncontrolled crystallization of Sn-rich perovskite chemistry using the antisolvent approach. We regulate the crystallization of Sn-contained perovskite with a mild gas-quench approach to fabricate a highly crystal-oriented and well-arranged NBG perovskite absorber. This strategy effectively boosts electron transport and light absorption of the NBG perovskite. Consequently, the average power conversion efficiency (PCE) of the NBG perovskite solar cells increases from 19.50 % to 21.18 %, with the best device achieving an excellent PCE of 21.84 %. Furthermore, when combined with a wide-bandgap perovskite subcell to form an all-perovskite tandem solar cell, a PCE of 25.23 % is achieved. After being stored in the glovebox for 1000 h, the unencapsulated device maintains over 90 % of its initial PCE, demonstrating long-term stability and durability. This work presents a promising approach for developing high-efficiency NBG perovskite solar cells.

Enhancing the Performance of the Mesoporous TiO2 Film in Printed Perovskite Photovoltaics through High‐Speed Imaging and Ink Rheology Techniques

AbstractMesoscopic carbon‐based perovskite solar cells (C‐PSCs) have the potential to be manufactured at an industrial scale by utilizing screen‐printing, a simple, affordable, and commercially mature process. As such, many recent publications have focused on enhancing performance through modifying cell architecture and perovskite chemistries. This work examines how ink rheology can be tuned to optimize cell performance through reducing the occurrence of common print defects to create higher quality m‐TiO2 films. Inks with different solvent dilutions and rheological profiles are assessed using high‐speed imaging through the screen‐printing visualization (SPV) technique, to investigate the impact of the viscosity and elasticity on ink separation mechanisms. The resultant film quality and its influence on device performances are then assessed. Ink separation lengths are minimized, and the formation of filaments ceases during printing, leading to improved TiO2 film topography and homogenous infiltration of the perovskite precursor. Consequently, PCE is improved by over 10% of the original efficiency in cells and 224 cm2 active area modules due to enhanced Voc and FF. These results not only provide key insights into tailoring ink rheology, to achieve improved print homogeneity and higher performing cells, but also aid further work on enhancing the performance of other screen‐printed functional films.

Supramolecular interactions using β-cyclodextrin in controlling perovskite solar cell performance

By incorporating β-cyclodextrin, mitigation of residual PbI2 crystallization, control of perovskite chemistry, and uniform crystal growth, leading to improved solar cell performance and stability were demonstrated.

Tertiary amine oxides as perovskite nanocrystal ligands and components of polymeric nanocomposites

AbstractAs a class of semiconductor nanocrystals that exhibit high photoluminescence quantum yield (PLQY) at tunable wavelengths, perovskite nanocrystals (PNCs) are attractive candidates for optoelectronic and light‐emitting devices. However, attempts to optimize PNC integration into such applications suffer from PNC instability and loss of PL over time. Here, we describe the impact of organic and polymeric N‐oxides when used in conjunction with PNCs, whereby a significant increase in PNC quantum yield is observed in solution, and stable PL emission is obtained in polymeric nanocomposites. Specifically, when using aliphatic N‐oxides in ligand exchange with CsPbBr3 PNCs in solution, a substantial boost in PNC brightness is observed (~40% or more PLQY increase), followed by an alteration of the perovskite chemistry. When N‐oxide substituents are positioned pendent to a poly(n‐butyl methacrylate) backbone, the optically clear flexible nanocomposite films obtained have bright PL emission and maintain optical clarity for months. X‐ray diffraction is useful for characterizing the PNC crystalline structure following exposure to aliphatic N‐oxides, while electron microscopy (EM) and small‐angle X‐ray scattering (SAXS) measurements of the PNC‐polymer nanocomposites show this polymeric N‐oxide platform to cleanly disperse PNCs in flexible polymer films.

Compositional texture engineering for highly stable wide-bandgap perovskite solar cells

The development of highly stable and efficient wide-bandgap (WBG) perovskite solar cells (PSCs) based on bromine-iodine (Br-I) mixed-halide perovskite (with Br greater than 20%) is critical to create tandem solar cells. However, issues with Br-I phase segregation under solar cell operational conditions (such as light and heat) limit the device voltage and operational stability. This challenge is often exacerbated by the ready defect formation associated with the rapid crystallization of Br-rich perovskite chemistry with antisolvent processes. We combined the rapid Br crystallization with a gentle gas-quench method to prepare highly textured columnar 1.75-electron volt Br-I mixed WBG perovskite films with reduced defect density. With this approach, we obtained 1.75-electron volt WBG PSCs with greater than 20% power conversion efficiency, approximately 1.33-volt open-circuit voltage (Voc), and excellent operational stability (less than 5% degradation over 1100 hours of operation under 1.2 sun at 65°C). When further integrated with 1.25-electron volt narrow-bandgap PSC, we obtained a 27.1% efficient, all-perovskite, two-terminal tandem device with a high Voc of 2.2 volts.

Structural Dynamics of Metal Halide Perovskites during Photoinduced Halide Segregation.

Despite substantial research effort, photoinduced halide segregation in mixed halide perovskites continues to limit the available perovskite chemistries for use in optoelectronic applications. In this study, we present new insights into halide-segregation process through in situ X-ray diffraction measurements that reveal substantial structural changes in mixed-halide perovskites under excitation. We observe that photoinduced halide segregation leads to the formation of one iodide-rich and one bromide-rich perovskite composition whose Br:I ratios are the same (at 20 and 93% bromine, respectively), for a range of compositions of the pristine initial perovskite phase. This segregation reverses in the dark to re-form a mixed halide perovskite with the same lattice spacing as the pristine perovskite. From these results, we determine a kinetic rate for the formation and dissolution of these new crystalline phases and observe that the crystalline orientation is preserved through the light-segregation and dark-relaxation processes. Our results are consistent with a model of halide segregation where excitation causes changes in the free energy of mixing and ultimately the formation of a miscibility gap in the MAPb(IxBr1-x)3 phase diagram and should inform future works to model and manipulate the halide-segregation process in mixed-halide perovskites.

Methods of Hysteresis Reduction in Perovskite Solar Cells

Maniell workmanElectrochemical SocietyPerovskite Solar cells have shown much promise in a relatively short time. Power conversion efficiencies have increased from 3.8% to 24.2 % in a span of ten years. Perovskite solar cells have attracted much attention in research because of the relatively low cost in manufacturing and production. Current silicon photovoltaic devices are more expensive than conventional fossil fuel. Perovskites are typically analyzed by assessing their current voltage relationship. Under test conditions emulating standard environmental conditions the voltage is incrementally increased in one scan in decreased in another. hysteresis, the variance between the scans, is a common issue amongst perovskites. large hysteresis creates instability in operation. There are currently a number of efforts in order to reduce hysteresis in Perovskite solar cells. The capacitive element innate to perovskites makes addressing this issue complicated. Hysteresis has many sources such as the interfacial boundary, morphology, perovskite chemistry, and defects. In this presentation some of the current methods of hysteresis reduction for perovskite solar cells are examines for their effectiveness. The electrical and quantum mechanical characteristics play a critical role in hysteresis and device performance.

Barriers to carriers: faults and recombination in non-stoichiometric perovskite scintillators

Tuning the efficiency and speed of charge carrier recombination in inorganic scintillators can potentially improve their performance in diverse applications. Recent work suggests that this maybe be achieved via a two-phase scintillator AB that naturally phase separates into A-rich and B-rich domains. In addition, a favorable electronic structure and band-edge alignment such that the charge carriers are confined or are thermodynamically driven to preferentially accumulate in one of the two domains, might lead to an improved radiative recombination rate. Here, we use density functional theory computations and ab initio molecular dynamics (AIMD), including non-adiabatic molecular dynamics (NAMD) simulations, to examine an alternative phase structure and its potential impact on recombination. Using a model perovskite SrTiO_3 system with one-, two- and three-dimensional Ruddlesden–Popper (RP) phases, we demonstrate that RP faults induce band structure changes in the material that can act as barriers to carrier transport. Our AIMD/NAMD simulations indicate competing effects of a lower mean free path (potentially enhancing the desired radiative recombination and overall scintillating efficiency) and faster non-radiative recombination (undesired) due to enhanced electron–phonon coupling in the faulted system. Full exploitation of such a rational design approach would require tuning of the effective scintillation efficiency by varying the perovskite chemistry using appropriate arrangements of RP faults in the bulk material. Finally, other effects, such as the tendency of point defects to segregate at the interface, that might affect the overall performance, are briefly discussed. We expect the basic results found here to apply to other nanostructured scintillators.Graphical

Bifacial Perovskite Solar Cells via a Rapid Lamination Process

Hybrid perovskite solar cells are considered a promising choice for next-generation thin-film photovoltaic technology. To meet commercialization requirements, more research efforts have now been focused on developing potentially high throughput fabrication methods compatible with perovskite chemistry. Here we show that bifacial perovskite solar cells can be made by a rapid lamination process in ambient air within 60 s. Upon optimization, these devices show an average single-side efficiency above 15% and bifaciality of 0.93. The bifacial feature enables light collection from both sides and can boost the overall performance by 10% or more. These results and further studies can provide unique opportunities to design new device architectures and fabrication methods for highly functional perovskites photovoltaics.

An inherent instability study using ab initio computational methods and experimental validation of Pb(SCN)2 based perovskites for solar cell applications

Perovskite materials with ABX3 chemistries are promising candidates for photovoltaic applications, owing to their suitable optoelectronic properties. However, they are highly hydrophilic and unstable in nature, limiting the commercialization of perovskite photovoltaics. Mixed halide ion-doped perovskites are reported to be more stable compared to simple ABX3 chemistries. This paper describes ab initio modeling, synthesis, and characterization of thiocyanate doped lead iodide CH3NH3PbI(3−x)(SCN)x perovskites. Several perovskite chemistries with an increasing concentration of (SCN)− at x = 0, 0.25, 0.49, 1.0, 1.45 were evaluated. Subsequently, ‘n-i-p’ and ‘p-i-n’ perovskite solar device architectures, corresponding to x = 0, 0.25, 0.49, 1.0 thiocyanate doped lead halide perovskite chemistry were fabricated. The study shows that among all the devices fabricated for different compositions of perovskites, p-i-n perovskite solar cell fabricated using CH3NH3PbI(3−x)(SCN)x perovskite at x = 1.0 exhibited the highest stability and device efficiency was retained until 450 h. Finally, a solar panel was fabricated and its stability was monitored.

A Perovskite Electronic Structure Descriptor for Electrochemical CO2 Reduction and the Competing H2 Evolution Reaction

The electrochemical reduction of CO2 (CO2RR) to high-value products is an attractive means to simultaneously address the negative effects of increasing CO2 emissions and the ever-present societal demand for chemicals and fuels. However, the rational development of novel catalyst chemistries for this reaction is needed to tune the activity and selectivity of the CO2RR, especially for increasingly complex chemistries, such as oxides, sulfides, and phosphides. In this work, we explore a diverse chemical range of transition-metal perovskite oxides (ABO3) by determining their surface binding energies toward COads and Hads using density functional theory (DFT) and by evaluating their CO2RR activity and selectivity. We find that tuning perovskite chemistry results in the ability to electrochemically reduce CO2 to CH4. We propose the O 2p-band center as a rational design parameter that faithfully captures the energetics of Hads and COads binding that influence the hydrogen evolution reaction (HER) and CO2RR activity. A higher O 2p-band center results in higher HER activity, while an intermediate O 2p-band center improves favorability for CH4 evolution. In the process, we identify a group of perovskites (i.e., LaCoO3, GdCoO3, NdCoO3) as materials that have not, to date, been reported previously for CH4 evolution activity at relatively low overpotentials (−0.6 to −0.8 VRHE).

Triamine‐Based Aromatic Cation as a Novel Stabilizer for Efficient Perovskite Solar Cells

AbstractOperational stability of perovskite solar cells has been a challenge from the beginning of perovskite research. In general, humidity and heat are the most well‐known degradation sources for perovskites, requiring ideal design of perovskite chemistry to withstand them. Although triple‐cation perovskite (Cs0.05(FA0.85MA0.15)0.95Pb(I0.85Br0.15)3) has been already introduced as the stable perovskite material, the high reactivity of methylammonium and formamidinium in the cation sites demands further modification. Herein, 1,2,4‐triazole is suggested as an effective cation solute to improve the performance and stability of perovskite solar cells. 1,2,4‐Triazole is an aromatic cation with low dipole moment that is stable under humidity and heat. It also possesses three nitrogen atoms, forming additional hydrogen bonds in the lattice, stabilizing the material. In this study, the solar cell utilizing 1,2,4‐triazole alloying achieves a power conversion efficiency of 20.9% with superior stability under extreme condition (85 °C/85% of relative humidity (RH), encapsulated) for 700 h. The 1,2,4‐triazole‐alloyed perovskite exhibits reduced trap density and film roughness and enhanced carrier lifetime with electrical conductivity, suggesting an ideal perovskite structure for efficient and stable optoelectronic applications.

Photovoltaic Effect in Indium(I) Iodide Thin Films

Halide semiconductors are undergoing a period of intense interest, buoyed by the outstanding optoelectronic properties of hybrid perovskites. It is worthwhile to consider whether related materials sharing unique aspects of the perovskite chemistry, such as the incorporation of metals that provide lone pair s electrons when integrated within the halide lattice, might yield semiconductors with comparable properties. One such metal that has not been widely studied in the context of photovoltaics is monovalent indium. In this work, we investigate a method of depositing films of indium(I) iodide, coupled with optoelectronic characterization and incorporation of these films into thin-film solar cells. We find that, although indium(I) iodide exhibits a photovoltaic effect, it is likely to be compromised by difficult-to-remove defects, e.g., iodine vacancies, which are expected to introduce recombination centers deep within the band gap.

Impact of Surfaces on Photoinduced Halide Segregation in Mixed-Halide Perovskites

Photoinduced halide segregation currently limits the perovskite chemistries available for use in high-bandgap semiconductors needed for tandem solar cells. Here, we study the impact of post-deposition surface modifications on photoinduced halide segregation in methylammonium lead mixed-halide perovskites. By coating a perovskite surface with the electron-donating ligand trioctylphosphine oxide (TOPO), we are able to both reduce nonradiative recombination and dramatically slow the onset of halide segregation in CH3NH3PbI2Br films. This result, coupled with the observation that the rate of halide segregation can be tuned by varying the selective contact, provides a direct link between surface modifications and photoinduced trap formation. On the basis of these observations, we discuss possible mechanisms for photoinduced halide segregation supported by drift-diffusion simulations. This work suggests that improved understanding and control of perovskite surfaces provides a pathway toward stable and high-perf...

Barrier Design to Prevent Metal-Induced Degradation and Improve Thermal Stability in Perovskite Solar Cells

Metal-contact-induced degradation and escape of volatile species from perovskite solar cells necessitate excellent diffusion barrier layers. We show that metal-induced degradation limits thermal stability in several perovskite chemistries with Au, Cu, and Ag gridlines even when the metal is separated from the perovskite by a layer of indium tin oxide (ITO). Channels in a sputtered ITO layer that align with perovskite grain boundaries are pathways for metal and halide diffusion into or out of the perovskite. Planarizing the perovskite morphology with a spin-cast organic charge-transport layer results in a subsequently deposited ITO layer that is uniform and impermeable. We show that it is critical to seal the edges of the active layers to prevent escape of volatile species. We demonstrate 1000 h thermal stability at 85 °C in CH3NH3PbI3 solar cells with complete-coverage silver contacts. Our barrier layer design enables long-term thermal stability of perovskite solar cells, a critical step to commercialization.

Complex octahedral tilt phases in the ferroelectric perovskite system LixNa1−xNbO3

High-temperature phase behavior in the system LixNa1-xNbO3 has been studied by using high-resolution powder neutron diffraction. Each of the three compositions studied in the Na-rich part of the phase diagram (x = 0.03, 0.08 and 0.12) shows evidence for distinct and complex structural modulations based on different tilting schemes of NbO6 octahedral units. Whilst octahedral tilting is prevalent in the structural chemistry of perovskites the nature of complexity of the phases observed here is unprecedented. Neither of the long-range tilt phases observed in NaNbO3 itself occurs here; instead a novel phase with a well-defined 4-fold superlattice is observed for the composition Li0.12Na0.88NbO3, and yet more complex phases with modulations based on 20-fold and 30-fold repeats are observed for Li0.03Na0.97NbO3 and Li0.08Na0.92NbO3, respectively. This peculiar structural behavior makes the system LixNa1-xNbO3 the most structurally complex 'simple' perovskite known.

Hybrid Dion–Jacobson 2D Lead Iodide Perovskites

The three-dimensional hybrid organic-inorganic perovskites have shown huge potential for use in solar cells and other optoelectronic devices. Although these materials are under intense investigation, derivative materials with lower dimensionality are emerging, offering higher tunability of physical properties and new capabilities. Here, we present two new series of hybrid two-dimensional (2D) perovskites that adopt the Dion-Jacobson (DJ) structure type, which are the first complete homologous series reported in halide perovskite chemistry. Lead iodide DJ perovskites adopt a general formula A'A n-1Pb nI3 n+1 (A' = 3-(aminomethyl)piperidinium (3AMP) or 4-(aminomethyl)piperidinium (4AMP), A = methylammonium (MA)). These materials have layered structures where the stacking of inorganic layers is unique as they lay exactly on top of another. With a slightly different position of the functional group in the templating cation 3AMP and 4AMP, the as-formed DJ perovskites show different optical properties, with the 3AMP series having smaller band gaps than the 4AMP series. Analysis on the crystal structures and density functional theory (DFT) calculations suggest that the origin of the systematic band gap shift is the strong but indirect influence of the organic cation on the inorganic framework. Fabrication of photovoltaic devices utilizing these materials as light absorbers reveals that (3AMP)(MA)3Pb4I13 has the best power conversion efficiency (PCE) of 7.32%, which is much higher than that of the corresponding (4AMP)(MA)3Pb4I13.

Charge-transfer-energy-dependent oxygen evolution reaction mechanisms for perovskite oxides

This work experimentally identifies the charge-transfer energy as a key factor governing the catalytic oxygen evolution reaction (OER) activity and mechanism across a wide range of perovskite chemistries.

Crystallographic modeling and impedance studies of gadolinium modified La1−xGdxMnO3 (x = 0.1–0.3) ceramics

The crystal chemistry of perovskite structured ceramic phases: La0.9Gd0.1MnO3 (LMGd-1), La0.8Gd0.2MnO3 (LMGd-2) and La0.7Gd0.3MnO3 (LMGd-3) have been investigated using General Structure Analysis System (GSAS) programming. The LMGd phases crystallize in the space group R-3c and Z=6. Powder diffraction data have been subjected to Rietveld refinement to arrive at a satisfactory structural convergence of Rietveld parameters: Rp, Rwp and goodness of fit. Lattice parameters, bond lengths and bond angles have been calculated from GSAS software. Their electrical behaviour investigated by impedance spectroscopy in the frequency range 42Hz–5MHz at temperatures between 50° and 300°C. Nyquist plots show the dominance of grain contribution and Debye – like relaxation in the materials. Conductivity data as a function of temperature and frequency suggest semiconductor behavior of the materials.