Session A69: Doping and Additives in Metal Halide Perovskites

Stabilizing Hybrid Perovskite Materials through Supramolecular Templating in Photovoltaics

Hybrid organic-inorganic perovskites have emerged as one of the most promising semiconductor materials in photovoltaics, yet their instability under operating conditions interferes with their applications. To address this challenge, we rely on supramolecular tools by purposefully tailoring noncovalent interactions to template hybrid perovskite architectures through halogen bonding, π-based interactions, and host-guest complexation, which has been assessed by solid-state NMR spectroscopy. As a result, we have achieved perovskite solar cells with superior operational stabilities without compromising their photovoltaic performances, which provides a versatile strategy for advancing hybrid perovskite photovoltaics.   

Enhanced Optical Performance of Lead- Free Cs2AgInCl6 Perovskites via Potassium Substitution for White LEDs

As an alternative to toxic lead halide perovskites, inorganic halide double perovskites (DPs) of the form Cs₂AgInCl₆ have emerged as a robust luminescent material for greener lighting technologies. The direct bandgap nature and superior long-term stability compared to their lead counterparts further attract their commercial implications. However, their light emission efficiency is still lower than lead-based perovskites. Herein, the addition of K⁺ at the Ag⁺ site of Cs₂AgInCl₆ crystals imparts enhancement in photoluminescence (PL) properties that agree with the theoretical calculations. As prepared samples exhibit a broadband PL emission profile (405–820 nm at λ_max ~ 629 nm) with a wide bandgap (~ 3.3 eV) and a large Stokes shift.  Notably,  the photoluminescence quantum yield of Cs₂AgInCl₆ increases by almost eight folds on increasing the content of K+ to 20 mol% and retains its luminesce intensity by ~ 85% over 180 days, confirming its excellent moisture stability, owing to the better passivation of the non-radiative trap states. The combination of blue-emitting CsPbBr₃ and broadband DPs as color conversion layers results in a characteristic warm white with a correlated color temperature of 3878 K, color rendering index of 85, and CIE coordinates of (0.387, 0.385).   

Incorporation of Germanium Nanoparticles in Organometal Perovskite Solar Cells

Metal-organic perovskite solar cells have gained much attention over the past decade. Known for its tunable bandgap via stoichiometric halide ratios, methylammonium lead halide (MAPbX₃) can be tuned from E_g = 1.6 eV (X = I) to E_g = 2.9 eV (X = Br). However, the Shockley-Queisser limit shows that an ideal solar cell under typical sunlight conditions should have a bandgap energy of 1.34 eV. Accessing the broader infrared range of the solar spectrum would allow for an increase in quantum efficiency. One well-known small bandgap semiconductor is germanium. However, unlike organometal perovskites, germanium is not cheaply or easily incorporated into the thin-film device structure. To combat this deficiency, we propose incorporating colloidal germanium quantum dots (Ge QDs), which may be solution processed, into our solar cell heterostructure to broaden our active layer’s absorption spectrum. QDs have the added benefits of having tunable sizes and ligands, allowing for optimizing device performance. In this work, we share the optical and optoelectronic results of incorporating monodispersed Ge QDs into our MAPbI₃ neat films and device architecture, respectively. 

    

A Study of the Addition of Oxides and Polymers to Perovskite Quantum Dot Solution and the Effects on Film Formation and Device Performance.

Perovskite quantum dots have shown promise in creating light-emitting diodes (LED) due to their high luminescence and tunable bandgaps. Challenges are still present in creating consistent films for devices. This study is focused on adding oxides and polymers to the quantum dot solution to investigate if the surface morphology and uniformity can be improved. Methylammonium lead bromide perovskite quantum dots will be created using a ligand-assisted reprecipitation method with 3,3-Diphenylpropylamine (DPPA) and trans-cinnamic acid (TCA) ligands. A comparison between different concentrations of oxides and polymers will be discussed. Photoluminescence (PL), photothermal deflection spectroscopy (PDS), atomic force microscopy (AFM), and Fourier-transform infrared spectroscopy (FTIR) will be used to characterize the films. Devices, such as LEDs and single-carrier devices, will be fabricated and compared to see the effects of each additive.   

Investigation of strontium doping in formamidinium lead iodide: electronic and optical properties

Due to toxicity concerns of Pb-containing materials in solar cells, Pb-free and stable organic-inorganic perovskites have been a highly desirable option for space and commercial solar cell applications; however, Pb-containing perovskites are among the higher-performing materials for solar cells to date. One possible method for retaining the preferable optoelectronic properties of purely Pb-containing materials while reducing their toxicity is the partial substitution of Pb⁺² by the homovalent cation, Sr⁺². Given the almost identical bonding pattern and ionic radii of Pb and Sr, interchanging the two has been achieved. Upon substitution by the non-toxic alkaline earth metal, there is increased thermal/environmental stability and an inherent reduction in overall toxicity of the host material. Here we utilize first-principles DFT methodologies to investigate the effects of Sr-doping on the electronic and optical properties of FAPbI₃, such as the electronic structure, bandgap, dielectric function, absorption spectra, reflectivity, and conductivity of the doped/undoped materials. These results can provide insight into the role that partial substitution of the Pb-site in perovskite materials has in the development of non-toxic Pb-free materials for solar cell applications.   

Electronic stopping power of Sr-doped FAPbI3

FAPbI₃ perovskites are among the higher-performing materials for solar cells to date. It has been found that upon substitution of Pb by the non-toxic alkaline earth metal Sr, there is increased thermal & environmental stability. For space applications, the perovskite's radiation resistance, in the scope of electronic stopping power, is an important parameter to consider.  The electronic stopping power describes the energy transfer rate to electrons in the material during ion irradiation. Through the utilization of the software, Stopping and Range of Ions in Matter (SRIM), and first-principles time-dependent density functional theory (TD-DFT) methodologies, we calculate the electronic stopping power in pristine and Sr-doped formamidinium lead iodide perovskites (FAPbI₃) with the inclusion of several intrinsic defects. From simulations, we estimate the effects of Sr-doping on the stopping process of ions in these defective systems and their implications in the development of photovoltaic devices for space missions.   

Shape Controlled Emissive Properties of Mn-doped CsPbBr3 Nanocrystals

II-VI semiconductor nanocrystals have been extensively studied by the soft chemistry method. In addition to a control on the size, one has found modulation of the shape under various experimental conditions. This increases the surface area for similar-sized objects and facilitates use in various applications such as catalysis where the increased surface area would imply more active sites. While experiments on hybrid perovskites have found only cubic facets, recently it was shown for CsPbX3 (X= Br,Cl) under certain experimental conditions [1] one found a faceted polyhedron which could be transformed into a hexapod. In this talk, I will present our recent work [2] in which we have examined Mn doping into the low energy facets. Doping into different facets is found to lead to emissive facets in some instances, and non-emissive facets in other instances. These studies help us explain how the same dopant atom in different shapes of the same material could have different emissive properties.

[1] Arm growth and facet modulation in perovskite nanocrystals. Lucheng Peng, Sumit Kumar Dutta, Debayan Mondal, Biswajit Hudait,Sanjib Shyamal, Renguo Xie, Priya Mahadevan, and Narayan Pradhan, J. Am. Chem. Soc. , 2019 , 141, 40, 16160-16168.

[2] Shape Control of Emissive Properties of Mn-Doped CsPbBr3 Nanocrystals. Debayan Mondal and Priya Mahadevan J. Phys. Chem. C 2021, 125, 21, 11462–11467.   

Doping and Distortions as Tools to Tune the Optoelectronic Properties of Halide Double Perovskites

In recent times, halide double perovskites (HDP) have attracted considerable attention as promising optoelectronic materials. In a combined theoretical and experimental study, we propose a design strategy through doping and distortion which can lead to white light emission and long excited state lifetime in HDP. Taking the example of an inorganic compound Cs₂Ag(M, M′)Cl₆, where M and M′ are heavy sp-elements,  we discuss the results from first principles electronic structure calculations, empirical molecular-orbital picture, parity driven symmetry analysis to predict the tuning of optoelectronic phenomena in these materials. The theoretical results are corroborated through experimental evidences arising from far-infrared absorption, steady-state and time-resolved photoluminescence, bias-dependent photoluminescence measurements. The present study establishes the role of structural distortion modes in influencing the optoelectronic behavior in HDP.   

Failure to dope halide perovskites: insufficient optimization or a physically-mandated bottleneck?

Intentional chemical doping of halide perovskites (HP) produce very low carriers concentration, not exceeding 10¹⁴cm^-3. One wonders if this represents a temporary setback due to insufficient optimization of the synthesis process, or an intrinsic, physically-mandated bottleneck. To elucidate this we use 3 Design Principles (DP), needed to be satisfied for ideal doping that are also supported by density functional theory calculations. These are DP(i): The impurities or structural defect should have shallow thermodynamic transition levels, lying close to the conduction band (CB) for n-type, or valence band (VB) for p-type; DP(ii): The Fermi level (E_F) pinning energy representing the maximum and minimum position of E_F, should be closed to VBM for p-type or CBM for n-type; and DP(iii): The shift in the doping-induced equilibrium Fermi energy should be sufficiently large to have, after doping, E_F close to VBM for p-type or CBM for n-type. For inorganic HP, we find that there are numerous shallow level dopants that satisfy DP(i). In contrast DP(ii) is satisfied only for holes; and DP(iii) fail for both holes and electrons, being the ultimate bottleneck for the n-type and p-type doping in systems based on I, Br and Cl.   

Ultrafast Carrier Dynamics in Mn doped CsPbBr3 Nanocrystals

Doping the semiconductor nanocrystal with transition metal ions increases the stability of the host and offers new optoelectronic functionalities. Herein, we demonstrate the influence of Mn ion doping on the carrier dynamics in CsPbBr₃ nanocrystals using ultrafast transient absorption spectroscopy. It is observed that Mn doping greatly alters and slows down the decay dynamics. Because the optical bandgap of CsPbBr₃ nanocrystals is only slightly larger than the Mn emission energy, the nonradiative energy transfer from CsPbBr₃ nanocrystals to Mn takes place. The excited carriers in Mn are known to have longer lifetime (on the order of milli seconds) and therefore act as carrier storage. However, the carrier population here decays in nanosecond timescale due to the thermal energy activation than leads to back energy transfer from Mn to CsPbBr₃ host. Nevertheless, the decay in Mn doped CsPbBr₃ is relatively retarded in comparison to undoped CsPbBr₃ nanocrystals. These rich functionalities displayed by Mn-doped CsPbBr₃ nanocrystals highlights the attractiveness of this material for future fundamental and applied research.   

Stabilization and Metallization of Sodium Halide Perovskites

Halide perovskites with chemical formula ABX3 have been pointed out as viable candidates for next-generation photovoltaic materials due to their squander conversion efficiency that can reach 25.5% under controlled conditions. To avoid stability issues, devices are usually made of alloys at the A-site, combining atoms such as Cs and Rb, and molecules such as MA and FA. Sodium should also be a good candidate for this task; however, it is believed that it would not form a stable compound owing to its small ionic radius. Here we show that application of hydrostatic pressures at the 6.0-7.1 GPa range can stabilize NaPbX3, X = Cl, Br, I orthorhombic perovskites (γ) with respect to the ilmenite phase, that is the most stable phase for NaPbX3 in the uncompressed case. We will also discuss the general stability of these compounds in comparison to a wide variety of different crystal phases, and analyze the stability of Na impurities in pure inorganic compounds such as CsPbX3, showing that Na prefers the Pb substitutional position. We also find that application of pressures ≥ 50GPa is capable of producing semiconductor-to-metal transitions in γ-NaPbX3 perovskites.   

Organic-Inorganic Perovskite-Metal Sulfide Composite Nanorods for Solar Cell Application

One-dimensional (1D) Organic-inorganic perovskites (OIPs) have become widely used materials in numerous optoelectronic (OE) researches due to better optical performance and economical fabrication process. This work demonstrates the nanorod synthesis of CH₃NH₃PbI₃ (MAPbI₃) perovskite with metal sulfide (MS) composite nanorods vai one-step spin coating for solar cell (SC) application. The crystal structure of MAPbI₃-MS, determined using X-ray diffraction, revealed a decrease of cell volume and a phase transition with increasing MS concentration. Fourier transform infrared spectroscopy was performed and all preferred bonds were observed. Nanorod grains were observed via Scanning electron microscopy with a decreasing diameter from 200 nm to 100 nm via MS incorporation. The thin films showed a fine absorbance in the visible and near-infrared wavelength region with a high absorption coefficient up to 10⁴ cm^-1. The overall absorption coefficient increased after MS addition. All the thin films showed very less reflectance (<7.5%) hence very less optical loss. The bandgap of the thin films was calculated using Tauc plot ranges from 1.5 eV - 1.6 eV with increasing MS stoichiometry which is preferable for SC and other OE applications.