Hybrid Lead Halide Iodide and Lead Halide Bromide in Efficient Hole Conductor Free Perovskite Solar Cell

In this work we used CH₃NH₃PbI_<i>n</i>Br_3–<i>n</i> (where 0 ≤ <i>n</i> ≤ 3) as hole conductor and light harvester in the solar cell. Various concentrations of methylammonium iodide and methylammonium bromide were studied which reveal that any composition of the hybrid CH₃NH₃PbI_<i>n</i>Br_3–<i>n</i> can conduct holes. The hybrid perovskite was deposited in two steps, separating it to two precursors to allow better control of the perovskite composition and efficient tuning of its band gap. The X-ray diffraction reveals the change in the lattice parameter due to the introducing of the Br^– ions. The hybrid iodide/bromide perovskite hole conductor free solar cells show very good stability, their power conversion efficiency achieved 8.54% under 1 sun illumination with current density of 16.2 mA/cm². The results of this work open the possibility for graded structure of perovskite solar cells without the need for hole conductor

Free Carrier Emergence and Onset of Electron–Phonon Coupling in Methylammonium Lead Halide Perovskite Films

Sub-10 fs resolution pump–probe experiments on methylammonium lead halide perovskite films are described. Initial response to photoexcitation is assigned to localized hot excitons which dissociate to free carriers. This is attested to by band integrals of the pump–probe spectra where photoinduced bleaching rises abruptly 20 fs after photoexcitation. Later stages of spectral evolution are consistent with hot carrier cooling, during which state filling induced bleaching of interband and exciton transitions curiously more than doubles. Electron coupling to optical phonons is observed as periodic spectral modulations in the pump–probe data of both films. Fourier analysis identifies active phonons at ∼100 and 300 wavenumbers pertaining to the lead-halide framework and organic cation motions, respectively. Coupling strengths estimated from the depth of these modulations are in the weak coupling limit, in agreement with values extracted from temperature dependent emission line shape analysis. These findings support free carriers in these materials existing as large polarons. Accordingly, these modes are probably not dictating the moderate carrier mobility in this material

Impact of Antisolvent Treatment on Carrier Density in Efficient Hole-Conductor-Free Perovskite-Based Solar Cells

This work demonstrates antisolvent treatment of organo-metal halide perovskite film in hole-conductor-free perovskite-based solar cell, achieving impressive power conversion efficiency of 11.2% for hole-conductor-free cells with gold contact. We found that antisolvent (toluene) surface treatment affects the morphology of the perovskite layer, and importantly, it also affects the electronic properties of the perovskite. Conductive atomic force microscopy (cAFM) and surface photovoltage show that the perovskite film becomes more conductive after antisolvent treatment. Moreover, the antisolvent treatment suppresses the hysteresis commonly obtained for perovskite-based solar cells. When the perovskite alone is characterized, a <i>I</i>–<i>V</i> plot of a single perovskite grain measured by cAFM shows that hysteresis vanishes after toluene treatment. During toluene treatment, excess halide and methylammonium ions are removed from the perovskite surface, leading to a net positive charge on the Pb atoms, resulting in a more conductive perovskite surface, which is beneficial for the hole-conductor-free solar cell structure. The reliability of the surface treatment was proved by calculating the statistical parameters <i>Z</i> score and <i>p</i> value, which were 2.5 and 0.012, respectively. According to these values, it can be concluded with 95% confidence that the average efficiency of cells fabricated via surface treatment is greater than the average efficiency of cells without surface treatment. The statistical data support the impact of surface treatment on the photovoltaic performance of perovskite solar cells

Micrometer Sized Perovskite Crystals in Planar Hole Conductor Free Solar Cells

In this work we demonstrate the planar configuration on hole conductor (HTM) free perovskite based solar cells. The CH₃NH₃PbI₃ perovskite was deposited using the spray technique to achieve micrometer size perovskite crystals. The number of spray passes changes the CH₃NH₃PbI₃ film thickness; for example, 10 spray passes achieved a film thickness of 3.4 μm of perovskite. Surprisingly, power conversion efficiency of 6.9% was demonstrated for this novel, simple solar cell structure with thick perovskite film that has no HTM. Capacitance–voltage measurements reveal charge accumulation at the CH₃NH₃PbI₃/Au interface while the compact TiO₂/CH₃NH₃PbI₃ junction showed a space charge region, which inhibits the recombination. Studying these interfaces is key to understanding the operation mechanism of this unique solar cell structure. This simple planar HTM free perovskite solar cell demonstrates the potential to make large-scale solar cells while maintaining a simple, low-cost architecture

Tunable Length and Optical Properties of CsPbX₃ (X = Cl, Br, I) Nanowires with a Few Unit Cells

Perovskite nanostructures, both hybrid organo–metal and fully inorganic perovskites, have gained a lot of interest in the past few years for their intriguing optical properties in the visible region. We report on inorganic cesium lead bromide (CsPbBr₃) nanowires (NWs) having quantum confined dimensions corresponding to 5 unit cells. The addition of various hydrohalic acids (HX, X = Cl, Br, I) was found to highly affect the NW length, composition, and optical properties. Hydrochloric (HCl) and hydroiodic (HI) acids mixed in the reaction solution influence the crystal structure and optical properties and shorten the NWs, while the hydrobromic acid (HBr) addition results solely in shorter NWs, without any structural change. The addition of HX increases the acidity of the reaction solution, resulting in protonation of the oleylamine ligands from oleylamine into oleyl-ammonium cations that behave similarly to Cs⁺ during crystallization. Therefore, the positions of the Cs⁺ at the growing surface of the NWs are taken by the oleyl-ammonium cations, thus blocking further growth in the favored direction. The emission of the NWs is tunable between ∼423–505 nm and possesses a potential in the optoelectronic field. Moreover, electrical conductivity measurements of the NWs are discussed to give a new point of view regarding the conductivity of perovskite nanostructures

Reflectivity Effects on Pump–Probe Spectra of Lead Halide Perovskites: Comparing Thin Films <i>versus</i> Nanocrystals

Due to the sizable refractive index of lead halide perovskites, reflectivity off their interface with air exceeds 15%. This has prompted a number of investigations into the prominence of photoreflective contributions to pump–probe data in these materials, with conflicting results. Here we report experiments aimed at assessing this by comparing transient transmission from lead halide perovskite films and weakly quantum confined nanocrystals of cesium lead iodide (CsPbI₃) perovskite. By analyzing how complex refractive index changes impact the two experiments, results demonstrate that changes in absorption and not reflection dominate transient transmission measurements in thin films of these materials. None of the characteristic spectral signatures reported in such experiments are exclusively due to or even strongly affected by changes in sample reflectivity. This finding is upheld by another experiment where a methyl ammonium lead iodide (MAPbI₃) perovskite film was formed on high-index flint glass and probed after pump irradiation from either face of the sample. We conclude that interpretations of ultrafast pump–probe experiments on thin perovskite films in terms of photoinduced changes in absorption alone are qualitatively sound, requiring relatively minor adjustments to factor in photoreflective effects

Light Energy Conversion by Mesoscopic PbS Quantum Dots/TiO₂ Heterojunction Solar Cells

Solid state PbS quantum dots (QDs)/TiO₂ heterojunction solar cells were produced by depositing PbS QDs on a 500 nm thick mesoscopic TiO₂ films using layer-by-layer deposition. Importantly, the PbS QDs act here as photosensitizers and at the same time as hole conductors. The PbS QDs/TiO₂ device produces a short circuit photocurrent (<i>J</i>_sc) of 13.04 mA/cm², an open circuit photovoltage (<i>V</i>_oc) of 0.55 V and a fill factor (FF) of 0.49, corresponding to a light to electric power conversion efficiency (η) of 3.5% under AM1.5 illumination. The electronic processes occurring in this device were investigated by transient photocurrent and photovoltage measurements as well as impedance spectroscopy in the dark and under illumination. The investigations showed a high resistivity for the QD/metal back contact, which reduces drastically under illumination. EIS also indicated a shift of the depletion layer capacitance under illumination related to the change of the dipole at this interface

The Impact of Piezoelectricity in Low Dimensional Metal Halide Perovskite

Hybrid perovskites show piezoelectric properties due to polarization and centro-symmetry breaking of PbX6 pyramids (X = I-, Br-, Cl-). This study examines the piezoelectric response of quasi-2D perovskites using various barrier molecules: benzyl amine (BzA), phenylethyl amine (PEA), and butyl diamine (BuDA). Utilizing piezoelectric force microscopy measurements, we determine the piezoelectric coefficient (d33) where BuDA exhibits a substantial response with values of 147 pm V–1 for n = 5, better than the other quasi-2D and 3D perovskite counterparts. Density functional theory calculations reveal distorted bond angles in the PbBr6 pyramids for quasi-2D perovskites, enhancing symmetry breaking. Additionally, polarizabilities and dielectric constants, derived from ab initio many-body perturbation theory, are highest for BuDA, followed by PEA and BzA, aligning with experimental results. We demonstrate pressure sensor performance, emphasizing the quicker capacitance decay time of the quasi-2D perovskite based on BuDA. This research underscores the impact of perovskite dimensionality on piezoelectricity, paving the way for the development of sensitive and wide-ranging pressure sensors