Optical constants manipulation of formamidinium lead iodide perovskites: ellipsometric and spectroscopic twigging.

Unraveling the knowledge of the complex refractive index and photophysical properties of the perovskite layer is paramount to uncovering the physical process that occurs in a perovskite solar cell under illumination. Herein, we probed the optical and photophysical properties of FAPbI3 (FAPI) and Cs0.1FA0.9PbI3 (CsFAPI) thin films deposited from pre-synthesized powder, by the spectroscopic ellipsometer and time-resolved fluorescence spectra. We determined the complex refractive index of perovskite films by fitting the measured spectroscopic ellipsometer data with the three-oscillator Tauc-Lorentz (T-L) model. We deduced that the CsFAPI thin film had a slightly lower absorption coefficient than the FAPI, but a higher refractive index and dielectric constant than the FAPI. The peak photoluminescence (PL) emission of FAPI and CsFAPI thin film on glass substrates was observed around 803 nm and 799 nm, respectively, while on ITO substrates, both FAPI and CsFAPI thin film was quenched and red-shifted to 816 nm. The methylammonium free pure CsFAPI-based perovskite solar cell fabricated in p-i-n configuration, measured a competitive efficiency of 16.14%, characterized by a J SC of 23.995 mA cm-2, V OC of 912 mV, and FF of 73.74%.

Molecular Tailoring of Pyridine Core-Based Hole Selective Layer for Lead Free Double Perovskite Solar Cells Fabrication.

To solve the toxicity issues related to lead-based halide perovskite solar cells, the lead-free double halide perovskite Cs2AgBiBr6 is proposed. However, reduced rate of charge transfer in double perovskites affects optoelectronic performance. We designed a series of pyridine-based small molecules with four different arms attached to the pyridine core as hole-selective materials by using interface engineering. We quantified how arm modulation affects the structure-property-device performance relationship. Electrical, structural, and spectroscopic investigations show that the N3,N3,N6,N6-tetrakis(4-methoxyphenyl)-9H-carbazole-3,6-diamine arm's robust association with the pyridine core results in an efficient hole extraction for PyDAnCBZ due to higher spin density close to the pyridine core. The solar cells fabricated using Cs2AgBiBr6 as a light harvester and PyDAnCBZ as the hole selective layer measured an unprecedented 2.9% power conversion efficiency. Our computed road map suggests achieving ∼5% efficiency through fine-tuning of Cs2AgBiBr6. Our findings reveal the principles for designing small molecules for electro-optical applications as well as a synergistic route to develop inorganic lead-free perovskite materials for solar applications.

Lead-sulfur interaction induced damp and water stability in pure formamidinium lead triiodide.

Research efforts in various multitudes have been demonstrated to stabilize methylammonium (MA)- and bromide (Br)-free formamidinium lead triiodide (FAPI) perovskite thin films. Despite these commendable efforts, pure FAPI perovskite thin film is prone to critical phase-transition issues due to its thermodynamically stable non-perovskite phase (2H). Here, in this work, we propose a rational additivization strategy to overcome this challenge. Our multifunctional ammonium salt containing a sulfur heteroatom shifts the thermodynamic stability from the 2H phase to an intermediate phase closer to the cubic phase. Along with the high crystallinity, micron-sized grains with preferred (00h) facet orientation stem the Pb…S interaction to offer exceptional stability against high relative humidity, direct water incursion, and shelf-life aging. Our findings through experimental and theoretical studies substantiate the role of Pb…S interaction in stabilizing the perovskite cubic phase and the stoichiometric distribution of elemental components.

Overcoming Limitations in Water-Ethanol Sprayed Superstrate Solar Cells by Compositional Engineering of Cu2CdSn(S,Se)4.

The increasing demand for solar energy requires materials from earth-abundant elements to ensure cost-effective production. One such light harvester Cu2CdSn(S,Se)4 fulfills this property. We report the development of functional solar cells based on Cu2CdSn(S,Se)4, which has been previously unreported. Furthermore, we deposited the thin films of Cu2CdSn(S,Se)4 by spray pyrolysis using environmentally benign solvents, in a superstrate architecture, reducing the potential cost of upscaling, the environmental hazards, and enabling its use in semitransparent or tandem solar cells. We analyze the Cu2CdSn(S,Se)4 and its optoelectronic characteristics with different sulfur and selenium ratios in the composition. We noted that Se is homogeneously distributed in the absorber and electron transport layer, forming a Cd(S,Se) phase that impacts the optoelectronic properties. The introduction of Se, up to 30%, is found to have a positive impact on the solar cell performance, largely improving the fill factor and absorption in the infrared region, while the voltage deficit is reduced. The device with a Cu2CdSn(S2.8Se1.2) composition had a 3.5% solar-to-electric conversion efficiency, which is on par with the reported values for chalcogenides and the first report using Cu2CdSn(S,Se)4. We identified the critical factors that limit the efficiency, revealing pathways to further reduce the losses and improve the performance. This work provides the first proof of concept of a novel material, paving the way for developing cost-efficient solar cells based on earth-abundant materials.

Uncovering the Role of Electronic Doping in Lead-free Perovskite (CH3 NH3 )2 CuCl4-x Brx and Solar Cells Fabrication.

Lead halide perovskites are attractive pigments to fabricate solar cells in the laboratory, owing to their high power conversion efficiency. However, given the presence of Pb, such materials also have a high level of toxicity and are carcinogenic for humans and aquatic life. Arguably, this hampers their acceptability for immediate commercialization. This study entails the synthesis, optoelectronic properties, and photovoltaic parameters of two-dimensional copper-based perovskites as an environmentally benign alternative to lead-based perovskites. The perovskites - (CH3 NH3 )2 CuCl4-x Brx with x=0.3 and 0.66 - are derivatives of the stable (CH3 NH3 )2 CuCl4 . The single crystals and powders diffractograms suggest compositions with variations in Cl/Br ratio and dissimilar bromine localization in the inorganic framework. The copper mixed halide perovskite exhibits a narrow absorption with a bandgap of 2.54-2.63 eV related to the halide ratio disparity (crystal color variation). These findings demonstrate the impact of halides to optimize the stability of methylammonium copper perovskites and provide an effective pathway to design eco-friendly perovskites for optoelectronic applications.

Probing proton diffusion as a guide to environmental stability in powder-engineered FAPbI3 and CsFAPbI3 perovskites.

Formamidinium lead iodide-based solar cells show promising device reliability. The grain imperfection can be further suppressed by developing powder methodology. The water uptake capability is critical for the stability of α-formamidinium lead triiodide (FAPbI3) thin films, and elucidating the migration of hydrogen species is challenging using routine techniques such as imaging or mass spectroscopy. Here, we decipher the proton diffusion to quantify indirect monitoring of H migration by following the N-D vibration using transmission infrared spectroscopy. The technique allows a direct assessment of the perovskite degradation associated with moisture. The inclusion of Cs in FAPbI3, reveals significant differences in proton diffusion rates, attesting to its impact. CsFAPbI3's ability to block the active layer access by water molecules is five times higher than α-FAPbI3, which is significantly higher than methylammonium lead triiodide (MAPbI3). Our protocol directly probes the local environment of the material to identify its intrinsic degradation mechanisms and stability, a key requirement for optoelectronic applications.

Microstrain and Urbach Energy Relaxation in FAPbI3-Based Solar Cells through Powder Engineering and Perfluoroalkyl Phosphate Ionic Liquid Additives.

Structural and electronic imperfections are the origin of defects and lead to nonradiative recombination that is detrimental to fabricating efficient perovskite solar cells. Here, we propose a powder engineering methodology for α-FAPbI3 as a precursor material. Our developed methodology of α-FAPbI3 synthesis mitigates the notorious structural and electronic imperfections evidenced by a significant decline in the microstrain and Urbach energy as compared to reported δ-FAPbI3 powder and conventional precursor routes. In addition to the performance enhancement in photovoltaics, our engineered powder showed remarkable thermal and moisture stability along with cost-effectiveness through the employment of low-grade PbI2. Further, through additive engineering, with the use of ultrahydrophobic perfluoroalkyl phosphate anion-based ionic liquids, the microstrain and Urbach energy achieved the lowest values of 1.67 × 10-4 and 12.47 meV, respectively, as a result of defect passivation and a semi-ionic F-Pb interaction that stabilizes the surface.

Reducing the Trap Density in MAPbI3 Based Perovskite Solar Cells via Bromide Substitution.

The past decade has witnessed tremendous advancement in the field of halide perovskite (PSK) as a choice of material for high-performing solar cells fabrication. Here, we investigate the impact of the halide exchange through N-bromosuccinimide (NBS) treatment in MAPbI3 based solar cells. We observed the partial halide exchange (I- to Br- ) or the filling of halide (X- ) vacancy upon treatment of different NBS concentrations experimentally by spectroscopic and diffractogram studies. We noted that halide exchange impacts the crystallization and is beneficial in improving the photovoltaic performance. The optimized 0.5 % NBS treated PSC exhibited a power conversion efficiency of 17.87 % due to an increment in open-circuit voltage (Voc ) and short circuit current (Jsc ). We noted improved perovskite crystal growth upon Br- substitution; eventually, it helps to lower the trap density, reducing non-radiative recombination and renders the enhancement of long-term stability of PSC.

Leverage of Pyridine Isomer on Phenothiazine Core: Organic Semiconductors as Selective Layers in Perovskite Solar Cells.

To drive the development of perovskite solar cells (PSCs), hole-transporting materials are imperative. In this context, pyridine derivatives are being probed as small molecules-based hole-transporting materials due to their Lewis base and electron-deficient unit. Herein, we focused our investigation on pyridine isomer molecules 4,4'-(10-(pyridin-x-yl)-10H-phenothiazine-3,7-diyl)bis(N,N-bis(4-methoxyphenyl)aniline) (x = 2, 3, or 4), in which the pyridine nitrogen heteroatom is located at the 2, 3, and 4 positions, named as 2PyPTPDAn, 3PyPTPDAn, and 4PyPTPDAn, respectively. We decipher the structure-properties-device performance relationship impacted by the different N-atom positions in pyridine. In the case of 3PyPTPDAn, the partial orbital overlap between highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) favors the generation of neutral excitons and hole transport, as well as improves the film-formation ability, and this induces efficient hole extraction as compared to their 2,4 analogues. The solar cells fabricated with 3PyPTPDAn gave on-par photovoltaic performance as that of typical Spiro-OMeTAD, and higher performance than those of 2PyPTPDAn and 4PyPTPDAn. The hydrophobicity and homogeneous film properties of 3PyPTPDAn add merits to the stability. This work emphasizes the guidelines to develop small molecules for organic solar cells, organic light-emitting diodes, and thermally activated delayed fluorescence.

Deciphering the Orientation of the Aromatic Spacer Cation in Bilayer Perovskite Solar Cells through Spectroscopic Techniques.

Slowing the degradation of perovskite-based solar cells (PSCs) is of substantial interest. We engineered the surface by introducing a hydrophobic overlayer on a three-dimensional (3D) perovskite using fluorinated or nonfluorinated aryl ammonium cation spacers. The placement of a fluoroarene cation allows the formation of a bilayer structure, that is, layered/3D perovskites. By doing so, the surface hydrophobic character increases notably by the virtue of the perfluorinated benzene moiety. The fabricated devices thereof gave higher performance and longevity than control devices in addition to boosting reliability. The fluoro-phenethylammonium iodide (FPEAI)-based devices showed lower nonradiative carrier recombination. To decipher the orientation of the spacer cation in this bilayer structure, we probed the surface by polarization-modulated infrared reflection-absorption spectroscopy and noted substantial differences in the orientation due to the presence of fluorine substitution. We hypothesize that the stronger van der Waals interactions due to the higher electronegativity in FPEAI govern the orientation and performance enhancement and act as a barrier to moisture decomposition.

Tailoring of a Phenothiazine Core for Electrical Conductivity and Thermal Stability: Hole-Selective Layers in Perovskite Solar Cells.

Hole-selective layers are an indispensable component for the fabrication of effective perovskite solar cells. We designed and developed two phenothiazine-based hole transport materials: PTADAnCBZ with an electron-donating sulfur atom and PTODAnCBZ with an electron-withdrawing sulfone group in the core. PTODAnCBZ in contrast to PTADAnCBZ possesses a unique molecular orbital distribution and lower dihedral angles, which endowed it with excellent optoelectrical properties, improved charge transportation, and thermal stability. The solar cells fabricated with PTODAnCBZ yielded a higher photovoltaic (PV) performance as compared to PTADAnCBZ and were on par in terms of performance with those fabricated with Spiro-OMeTAD. Notably, the phenothiazine-based PV devices showed improved stability under multi-stress conditions including moisture, moisture and light, and moisture and heat. Phenothiazine-based molecules showed unparalleled thermal stability as compared to the doped Spiro-OMeTAD. Our findings pinpoint the advantages of cost-effective phenothiazine with dioxide as hole-selective layers and suggest its application in a variety of optoelectrical devices such as PVs and organic LED.

Protocol for deciphering the electrical parameters of perovskite solar cells using immittance spectroscopy.

Here, we present a protocol for the fabrication of inverted (p-i-n)-type perovskite solar cells, unraveling its electrical merits via immittance spectroscopy. The immittance spectroscopy is a prevailing technique for both qualitative and quantitative analyses of charge carrier dynamics in working devices. This technique integrates the temperature-dependent capacitance-frequency (C-f) spectra, impedance spectra, and Mott-Schottky analyses. This protocol is also applicable for typical (n-i-p) perovskite solar cells and other multilayer semiconductor devices. For complete details on the use and execution of this protocol, please refer to Khan et al. (2019, 2021).

A Machine Learning Approach for Metal Oxide Based Polymer Composites as Charge Selective Layers in Perovskite Solar Cells.

A library of metal oxide-conjugated polymer composites was prepared, encompassing WO3 -polyaniline (PANI), WO3 -poly(N-methylaniline) (PMANI), WO3 -poly(2-fluoroaniline) (PFANI), WO3 -polythiophene (PTh), WO3 -polyfuran (PFu) and WO3 -poly(3,4-ethylenedioxythiophene) (PEDOT) which were used as hole selective layers for perovskite solar cells (PSCs) fabrication. We adopted machine learning approaches to predict and compare PSCs performances with the developed WO3 and its composites. For the evaluation of PSCs performance, a decision tree model that returns 0.9656 R2 score is ideal for the WO3 -PEDOT composite, while a random forest model was found to be suitable for WO3 -PMANI, WO3 -PFANI, and WO3 -PFu with R2 scores of 0.9976, 0.9968, and 0.9772 respectively. In the case of WO3 , WO3 -PANI, and WO3 -PTh, a K-Nearest Neighbors model was found suitable with R2 scores of 0.9975, 0.9916, and 0.9969 respectively. Machine learning can be a pioneering prediction model for the PSCs performance and its validation.

Substance and shadow of formamidinium lead triiodide based solar cells.

The current decade has witnessed a surge of progress in the investigation of methyl ammonium lead iodide (MAPbI3) perovskites for solar cell fabrication due to their intriguing electro-optical properties, despite the intrinsic degradation of the material that has restricted its commercialisation. As a promising alternative, solar cells based on its formamidinium analogue, FAPbI3, are currently being actively pursued for having demonstrated a certified efficiency of 24.4%, while the room-temperature conversion to a non-perovskite δ-phase impedes its further commercialisation, and strategies have been adopted to overcome this phase instability. An in-depth and real-time understanding of microstructural relationships with optoelectronic properties and their underlying mechanisms using operando in situ spectroscopic techniques is paramount. Thus, the design and development of a new process, data driven methodology, characterization and evaluation protocols for perovskite absorber layers and the fabricated devices is a judicious research direction. Here, in this perspective, we shed light on the compositional, surface engineering and crystallization kinetics manipulations for FAPbI3, followed by a proposition for unified testing protocols, for scalling of devices from the lab to the market.

Mechanistic origin and unlocking of negative capacitance in perovskites solar cells.

We have unlocked the mechanistic behavior of negative capacitance in perovskite solar cells (PSCs) by analyzing impedance spectra at variable photovoltage and applied bias, temperature-dependent capacitance versus frequency (C-f) spectra, and current-voltage (J-V) characteristics. We noted that p-i-n type PSCs having PEDOT:PSS or PTAA as hole transport layer display negative capacitance feature at low and intermediate frequencies. The activation energies (E a ) for the observance of negative capacitance were found to be in a similar order of magnitude required for the ionic migration. Moreover, the kinetic relaxation time (τ kin ) estimated to be in the same order of magnitude required to activate the halide ion migration. Our investigation suggests that the primary reason for the appearance of negative capacitance in PSCs with a p-i-n configuration is associated with the migration of halide ions and vacancies in the perovskite layers.

Pyridine Bridging Diphenylamine-Carbazole with Linking Topology as Rational Hole Transporter for Perovskite Solar Cells Fabrication.

Developing cost-effective and rational hole transporting materials is critical for fabricating high-performance perovskite solar cells (PSCs) and to promote their commercial endeavor. We have designed and developed pyridine (core) bridging diphenylamine-substituted carbazole (arm) small molecules, named as 2,6PyDANCBZ and 3,5PyDANCBZ. The linking topology of core and arm on their photophysical, thermal, semiconducting, and photovoltaic properties were probed systematically. We found that the 2,6PyDANCBZ shows higher mobility and conductivity along with uniform film-forming ability as compared to 3,5PyDANCBZ. The PSCs fabricated with 2,6PyDANCBZ supersede the performance delivered by Spiro-OMeTAD and importantly also gave improved long-term stability. Our findings put forward small molecules based on core-arm linking topology for cost-effective hole selective layers designing.

Elucidating the Doping Mechanism in Fluorene-Dithiophene-Based Hole Selective Layer Employing Ultrahydrophobic Ionic Liquid Dopant.

Perovskite solar cells have set a new milestone in terms of efficiencies in the thin film photovoltaics category. Long-term stability of perovskite solar cells is of paramount importance but remains a challenging task. The lack of perovskite solar cells stability in real-time operating conditions erodes and impedes commercialization. Further improvements are essential with a view to delivering longer-lasting photovoltaic (PV) performances. An ideal path in this direction will be to identify novel dopants for boosting the conductivity and hole mobility of hole transport materials (HTMs), and by so doing, the usage of hygroscopic and deliquescent additive materials can be avoided. The present work demonstrates the employment of ionic liquids into a dissymmetric fluorene-dithiophene, FDT (2',7'-bis(bis(4-methoxyphenyl)amino) spiro[cyclopenta[2,1-b:3,4-b']dithiophene-4,9'-fluorene]) based HTM to understand the doping mechanisms. N-Heterocyclic hydrophobic ionic liquid, 1-butyl-3-methylpyidinium bis(trifluoromethylsulfonyl)imide (BMPyTFSI) as p-type dopant for FDT was found to increase the conductivity of FDT, to higher geometrical capacitance, to facilitate homogeneous film formation, and to enhance device stability. Our findings open up a broad range of hole-transport materials to control the degradation of the underlying water-sensitive active layer by substituting a hygroscopic element.

Electrical Methods to Elucidate Charge Transport in Hybrid Perovskites Thin Films and Devices.

The panchromatic light absorption and excellent charge carrier transport properties in organo lead halide perovskites allowed to achieve an unprecedented power conversion efficiency in excess of 25 % for thin film photovoltaics fabrication. To understand the underlying phenomena, various comprehensive set of optical and electrical techniques have been employed to investigate the charge carrier dynamics in such devices. In this perspective, we aim to summarize the electrical transport properties of perovskite thin films by using (i) impedance spectroscopy (IS), (ii) space charge limited current (SCLC), (iii) field-effect transistors (FETs) and (iv) time-of-flight (TOF) methods. We have deliberated various equivalent circuit used to model the perovskite solar cells by means of IS. The SCLC technique provide vital electrical parameters such as mobility, activation energy, traps density and distribution, carrier concentration, density of states, etc. The TOF technique measures mobility as a primary parameter while the FETs configuration provide a valuable insight into the in-plane charge transport in perovskites thin films. We believe that these notable understanding will provide insights into charge carrier dynamics in perovskite materials and devices.

Lead-Free Perovskites: Metals Substitution towards Environmentally Benign Solar Cell Fabrication.

Perovskite solar cells have attracted significant attention during the current decade owing to their efficacy and photovoltaics performance, which has reached a new milestone in the thin-film category. Perovskite solar cells have witnessed a remarkable 25.2 % light-to-electricity conversion efficiency; however, the toxicity of the commonly employed Pb counterpart towards humans as well as the environment, in addition to material instability, are current bottlenecks towards commercial application. The scientific community has explored other metal ions as substitutions for Pb, while preserving the unique properties of the material, to produce environment-friendly perovskites. In this Review, we highlight the recent developments and challenges of Pb-free halide perovskite-based light harvesters for solar cell applications. This summary is intended to aid in the further development of a materials library for this sustainable technology.

Appraisement of Crystal Expansion in CH3 NH3 PbI3 on Doping: Improved Photovoltaic Properties.

Three-dimensional hybrid perovskite materials (CH3 NH3 PbI3 ) suffer from intrinsic instability owing to organic cation evaporation and ion migration. The inclusion of a large organic cation such as guanidinium has been probed to stabilize the structure. This work proposes the inclusion of imidazolium iodide (C3 N2 H5 I) as an organic cation inside the CH3 NH3 PbI3 matrix, as a reservoir to control the spontaneous loss of iodide. The introduction of imidazolium iodide in amounts below 20 % has an impact on the crystallization process but not on the optical properties. It also positively controls non-radiative recombination and improves the open-circuit voltage of the solar cells. The present study paves way for a deeper insight into the limit of multi-dimensional perovskite to further push the performance.