Probing Molecular and Crystalline Orientation in Solution‐Processed Perovskite Solar Cells

The microstructure of solution‐processed organometallic lead halide perovskite thin films prepared by the “gas‐assisted” method is investigated with synchrotron‐based techniques. Using a combination of GIWAXS and NEXAFS spectroscopy the orientational alignment of CH₃NH₃PbI₃ crystallites and CH₃NH₃⁺ cations are separately probed. The GIWAXS results reveal a lack of preferential orientation of CH₃NH₃PbI₃ crystallites in 200–250 nm thick films prepared on both planar TiO₂ and mesoporous TiO₂. Relatively high efficiencies are observed for device based on such films, with 14.3% achieved for planar devices and 12% for mesoporous devices suggesting that highly oriented crystallites are not crucial for good cell performance. Oriented crystallites however are observed in thinner films (≈60 nm) deposited on planar TiO₂ (but not on mesoporous TiO₂) indicating that the formation of oriented crystallites is sensitive to the kinetics of solvent evaporation and the underlying TiO₂ morphology. NEXAFS measurements on all samples found that CH₃NH₃⁺ cations exhibit a random molecular orientation with respect to the substrate. The lack of any NEXAFS dichroism for the thin CH₃NH₃PbI₃ layer deposited on planar TiO₂ in particular indicates the absence of any preferential orientation of CH₃NH₃⁺ cations within the CH₃NH₃PbI₃ unit cell for as‐prepared layers, that is, without any electrical poling.     

Thermal and Photo-Degradation Study of α-FAPbI3-Based Perovskite Using In Situ X-Ray Diffraction

Hybrid halide perovskite has established its credibility as high performance thin film photovoltaic technology. Perovskite based on formamidinium cation is at the core composition to top performances and stability. Herein, a depth study based on temperature-controlled in situ X-ray diffraction focusing on the photo-active formamidinium lead iodide (α-FAPbI₃) is reported. In particular, the thermal stability of the latter and the degradation pathways under different experimental conditions are clarified. Based on this in situ technique, the lattice thermal expansion coefficient is reported that provides relevant information on possible mechanical stress created upon temperature cycling or damp heat test. The results support that α-FAPbI₃ degradation is substantially accelerated when temperature is combined to illumination and when it is interfaced with the extraction layers. In addition, by contrast to in darkness for which α-FAPbI₃ degrades directly into PbI₂, the existence of a temperature gap under illumination involving an intermediate step with a non-crystalline phase resulting from the perovskite degradation and contributing to the formation of PbI₂ by-product is revealed.     

Solution‐based processing of Cu(In,Ga)Se2 absorber layers for 11% efficiency solar cells via a metallic intermediate

In this work, a low cost solution‐based method for the deposition of uniform Cu‐In‐Ga layers compatible with roll‐to‐roll processing is described. As ink system we use metal carboxylates dissolved in a mixture of a nitrogen containing base and an alcohol. This solution can be coated homogeneously under inert atmosphere using a doctor blade technique. With this method and appropriate precursor concentrations, crack‐free metal layers with dry‐film thicknesses of more than 700 nm can be deposited in one fast step. For the controlled film formation during the drying of the solvents a flow channel has been used to improve the evaporative mass transport and the convective gas flows of any unwanted organic species. Due to the absence of organic binders with high molecular weight, this step allows the formation of virtually pure metal layers. Elementary analyses of the dried thin films reveal less than 5 wt% of carbon residues at 200°C. In situ X‐ray diffraction data of the drying step show the formation of Cu‐In‐Ga alloys. The subsequent processing of Cu(In,Ga)Se₂ chalcopyrites with evaporated elemental selenium takes place in a separate tube oven under inert atmosphere. Photoelectric measurements of cells with CdS buffer and ZnO window layer reveal a short‐circuit current of 29 mA/cm², an open‐circuit voltage of 533 mV, and a fill factor of 0.69 under standard conditions. Thus efficiencies of up to 11% on 0.5 cm² area without antireflective coating have been achieved. Copyright © 2014 John Wiley & Sons, Ltd.     

Plasmonic‐Induced Photon Recycling in Metal Halide Perovskite Solar Cells

Organic–inorganic metal halide perovskite solar cells have emerged in the past few years to promise highly efficient photovoltaic devices at low costs. Here, temperature‐sensitive core–shell Ag@TiO₂ nanoparticles are successfully incorporated into perovskite solar cells through a low‐temperature processing route, boosting the measured device efficiencies up to 16.3%. Experimental evidence is shown and a theoretical model is developed which predicts that the presence of highly polarizable nanoparticles enhances the radiative decay of excitons and increases the reabsorption of emitted radiation, representing a novel photon recycling scheme. The work elucidates the complicated subtle interactions between light and matter in plasmonic photovoltaic composites. Photonic and plasmonic schemes such as this may help to move highly efficient perovskite solar cells closer to the theoretical limiting efficiencies.