Sigalit Aharon

Temperature- and Component-Dependent Degradation of Perovskite Photovoltaic Materials under Concentrated Sunlight

We report on accelerated degradation testing of MAPbX3 films (X = I or Br) by exposure to concentrated sunlight of 100 suns and show that the evolution of light absorption and the corresponding structural modifications are dependent on the type of halide ion and the exposure temperature. One hour of such exposure provides a photon dose equivalent to that of one sun exposure for 100 hours. The degradation in absorption of MAPbI3 films after exposure to 100 suns for 60 min at elevated sample temperature (∼45-55 °C), due to decomposition of the hybrid perovskite material, is documented. No degradation was observed after exposure to the same sunlight concentration but at a lower sample temperature (∼25 °C). No photobleaching or decomposition of MAPbBr3 films was observed after exposure to similar stress conditions (light intensity, dose, and temperatures). Our results indicate that the degradation is highly dependent on the hybrid perovskite composition and can be light- and thermally enhanced.

Temperature dependence of hole conductor free formamidinium lead iodide perovskite based solar cells

Organometal halide perovskite is a promising material in photovoltaic (PV) cells. Within a short time, its performance has increased dramatically to become a real competitor to silicon solar cells. Here we report on the temperature dependence (annealing temperature and the dependence of the photovoltaic parameters on temperature) of formamidinium (FA) lead iodide (FAPbI3), methylammonium (MA) lead iodide (MAPbI3) and their mixture (MAPbI3 : FAPbI3) in hole conductor free perovskite solar cells. These three types of perovskites function both as light harvesters and as hole conductors. Surface photovoltage and optical characterization reveal the p-type behavior and the band gap of the different perovskites. We observed that the ratio between the MA and FA cations might change during the annealing process, affecting the band gap and the stability of the layers. The PV parameters at different temperatures show better stability for the pure MAPbI3 and FAPbI3 solar cells compared to their mixture. Using intensity modulated photovoltage/photocurrent spectroscopy, we found that the diffusion length is weakly dependent on the light intensity, while the charge collection efficiency drops with light intensity for the FAPbI3-based cells. However, for MAPbI3 and the mixture, the charge collection efficiency remains constant for a wide range of light intensities.

Effect of Halide Composition on the Photochemical Stability of Perovskite Photovoltaic Materials.

The photochemical stability of encapsulated films of mixed halide perovskites with a range of MAPb(I1-x Brx )3 (MA=methylammonium) compositions (solid solutions) was investigated under accelerated stressing using concentrated sunlight. The relevance of accelerated testing to standard operational conditions of solar cells was confirmed by comparison to degradation experiments under outdoor sunlight exposure. We found that MAPbBr3 films exhibited no degradation, while MAPbI3 and mixed halide MAPb(I1-x Brx )3 films decomposed yielding crystallization of inorganic PbI2 accompanied by degradation of the perovskite solar light absorption, with faster absorption degradation in mixed halide films. The crystal coherence length was found to correlate with the stability of the films. We postulate that the introduction of Br into the mixed halide solid solution stressed its structure and induced more structural defects and/or grain boundaries compared to pure halide perovskites, which might be responsible for the accelerated degradation. Hence, the cause for accelerated degradation may be the increased defect density rather than the chemical composition of the perovskite materials.

New insights into exciton binding and relaxation from high time resolution ultrafast spectroscopy of CH3NH3PbI3 and CH3NH3PbBr3 films

High time resolution broadband pump-probe experiments on CH3NH3PbI3 and CH3NH3PbBr3 films are described. The improved time resolution delineates instantaneous and delayed relaxation related effects on sample absorption and assists in clarifying controversial assignment of the underlying mechanisms. Analysis of the data in terms of finite difference spectra and spectral band integrals reveals that photoexcitation is high in the inter-band continuum leading to partial bleaching and red-shifts of the exciton band just below the absorption-edge instantaneously. Increased pump intensity saturates the exciton bleach and progressively reduces inter-band absorption in a broad range extending from the band edge to higher photon energies. Both effects are attributed to reduced Coulomb enhancement due to hot carrier screening. The spectral extent of the inter-band absorption attenuation provides estimated binding energies in the range of 20–30 meV in both materials. Sub-picosecond carrier cooling reverses the initial exciton transition red-shift and induces transmission near the band edge due to state filling and stimulated emission. Finally, 1–100 ps signals are dominated by reverse state filling due to non-geminate recombination. These results demonstrate that both inter-band and exciton absorptions are essential for unraveling photo-induced dynamics in these materials, and that insights obtained from many-body theoretical analysis of dynamic screening are essential for correctly assigning the recorded spectral evolution.

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.

Self‐Assembly of Perovskite for Fabrication of Semitransparent Perovskite Solar Cells

DOI: 10.1002/admi.201500118 layer was deposited by evaporation technique. However, evaporation-based processes are very costly, require high capital investments, and are very complicated for upscaling, which is required for industrial applications. Semitransparent top electrode made of silver nanowires was introduced into perovskite-based solar cells. [ 24 ] The transparency in this case was controlled by the top electrode transparency and not by the perovskite fi lm. In addition, the silver nanowires can be used as an alternative top electrode made by solution-processed technique for semitransparent solar cells. Here, we report on a unique, simple wet deposition method for the fabrication of semitransparent perovskite-based solar cells. This deposition method is fundamentally different from previously reported deposition methods of CH 3 NH 3 PbI 3 (MAPbI 3 ) perovskite. The fi lm formation in this method is enabled by the mesh-assisted assembly of the perovskite solution through wetting along the wall of a conventional screen printing mesh, as described earlier. [ 25–27 ] Meaning, here the perovskite is deposited along a controlled pattern through solution-process and in ambient conditions. Semitransparent perovskite solar cells were fabricated; the perovskite grid was deposited upon a mesoporous TiO 2 layer, followed by 2,2′,7,7′-tetrakis-( N , N -di-4methoxyphenylamino)-9,9′-spirobifl uorene (spiro-OMeTAD) deposition and evaporation of gold back contact. In addition, semitransparent hole conductor free perovskite solar cells (without spiro-OMeTAD as the hole transporting material (HTM)) were prepared for comparison. Nontransparent HTM-free perovskite solar cells were already demonstrated to achieve 10.85% effi ciency. [ 26–29 ] Control of transparency is achieved by changing solution concentrations (wt%) and mesh openings of the printing screen. Using this method, semitransparent cells with 20–70% transparency were fabricated. Ultrahigh resolution scanning electron microscopy (UHR-SEM), optical microscope, and a profi lometer were used to characterize the perovskite grids. Intensity-modulated photovoltage spectroscopy (IMVS) was performed for the analysis of the recombination processes occurring in this unique structure of semitransparent perovskite solar cells.

A mesoporous–planar hybrid architecture of methylammonium lead iodide perovskite based solar cells

We report a hybrid mesoporous–planar architecture of methylammonium lead iodide perovskite based solar cells, to combine the benefits of both the mesoporous and planar architectures in a single device. A mesoporous-TiO2 grid was fabricated on a compact TiO2 layer, through a self-assembly process based on directional wetting, providing regions with and without mesoporous-TiO2, followed by perovskite deposition and back contact evaporation (hybrid cells). The hybrid cells showed up to 10.7% power conversion efficiency (PCE) as compared to 13.5% and 6.3% for their mesoporous and planar counterparts, respectively. Interestingly, the hybrid cells are found to show a short circuit current density (Jsc) as high as the Jsc of the mesoporous TiO2 based cells and proved to conserve the current density even in the absence of mesoporous-TiO2 from planar parts of the hybrid cells. The cells showed the best fill factor as compared to their mesoporous and planar counterparts. The areal variation in the meso to planar ratio has also been realized by changing the grid size to demonstrate the effect of the architecture on the cell performance. Charge extraction measurements have been used to obtain insight into the recombination inside different solar cells architectures. The hybrid cell structure emerged as a novel promising design for perovskite solar cells.

Reflectivity Effects on Pump–Probe Spectra of Lead Halide Perovskites: Comparing Thin Films versus 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.

Numerical simulation of HTM-free and WO x based perovskite cells: Effects of interface conditions

Hole transport material (HTM) free and WOx based perovskite solar cells are theoretically investigated by using driftdiffusion and small signal models. The influence of interface states and leakage current is studied, and the current-voltage (J-V) and capacitance-voltage (C-V) characteristics are reproduced in reasonable agreement with experimental data, including build in potential (Vbi) variation, open circuit voltage (VOC) loss and hysteresis effects.

Understanding the distribution of chlorine in perovskite solar cells via x-ray fluorescence microscopyl

Spatial heterogeneities in the chemical makeup of thin film photovoltaic devices are pivotal in determining device efficiency. In this study, the presence of chlorine is identified in perovskite films synthesized with Cl-containing precursors by means of nanoprobe X-ray fluorescence (Nano-XRF). Additionally, a spatial variation in the Cl incorporation is observed within a given film, and the standard deviation of Cl: I ratio across the film is large. Using Nano-XRF, the Cl incorporation in methylammonium lead iodide perovskite films can be manipulated by precursor stoichiometry ratio.