Stabilization of the Trigonal High-Temperature Phase of Formamidinium Lead Iodide

Formamidinium lead iodide (FAPbI₃) has the potential to achieve higher performance than established perovskite solar cells like methylammonium lead iodide (MAPbI₃), while maintaining a higher stability. The major drawback for the latter material is that it can crystallize at room temperature in a wide bandgap hexagonal symmetry (<i>P</i>6₃<i>mc</i>) instead of the desired trigonal (<i>P</i>3<i>m</i>1) black phase formed at a higher temperature (130 °C). Our results show that employing a mixture of MAI and FAI in films deposited via a two-step approach, where the MAI content is <20%, results in the exchange of FA molecules with MA without any significant lattice shrinkage. Additionally, we show with temperature-dependent X-ray diffraction that the trigonal phase exhibits no phase changes in the temperature range studied (25 to 250 °C). We attribute the stabilization of the structure to stronger interactions between the MA cation and the inorganic cage. Finally, we show that the inclusion of this small amount of MA also has a positive effect on the lifetime of the photoexcited species and results in more efficient devices

Charge Transport Limitations in Self-Assembled TiO₂ Photoanodes for Dye-Sensitized Solar Cells

Solid-state dye-sensitized solar cells offer the possibility of high-power conversion efficiencies due to theoretically lower fundamental losses in dye regeneration. Despite continuous progress, limitations in charge diffusion through the mesoporous photoanode still constrain the device thickness and hence result in reduced light absorption with the most common sensitizers. Here we examine block copolymer-assembled photoanodes with similar surface area and morphology but a large variation in crystal size. We observe that the crystal size has a profound effect on the electron transport, which is not explicable by considering solely the ratio between free and trapped electrons. Our results are consistent with the long-range mobility of conduction band electrons being strongly influenced by grain boundaries. Therefore, maximizing the crystal size while maintaining high enough surface area will be an important route forward

Guided in Situ Polymerization of MEH-PPV in Mesoporous Titania Photoanodes

Incorporation of conjugated polymers into porous metal oxide networks is a challenging task, which is being pursued via many different approaches. We have developed the guided in situ polymerization of poly­(2-methoxy-5-(2′-ethylhexyloxy)-<i>p</i>-phenylenevinylene) (MEH-PPV) in porous titania films by means of surface functionalization. The controlled polymerization via the Gilch route was induced by an alkoxide base and by increasing the temperature. The selected and specially designed surface-functionalizing linker molecules mimic the monomer or its activated form, respectively. In this way, we drastically enhanced the amount of MEH-PPV incorporated into the porous titania phase compared to nonfunctionalized samples by a factor of 6. Additionally, photovoltaic measurements were performed. The devices show shunting or series resistance limitations, depending on the surface functionalization prior to in situ polymerization of MEH-PPV. We suggest that the reason for this behavior can be found in the orientation of the grown polymer chains with respect to the titania surface. Therefore, the geometry of the anchoring via the linker molecules is relevant for exploiting the full electronic potential of the conjugated polymer in the resulting hybrid composite. This observation will help to design future synthesis methods for new hybrid materials from conjugated polymers and n-type semiconductors to take full advantage of favorable electronic interactions between the two phases

Contactless Visualization of Fast Charge Carrier Diffusion in Hybrid Halide Perovskite Thin Films

Organic–inorganic metal halide perovskite solar cells have recently attracted considerable attention with reported device efficiencies approaching those achieved in polycrystalline silicon. Key for an efficient extraction of photogenerated carriers is the combination of low nonradiative relaxation rates leading to long carrier lifetimes and rapid diffusive transport. The latter, however, is difficult to assess directly with reported values varying widely. Here, we present an experimental approach for a contactless visualization of the charge carrier diffusion length and velocity in thin films based on time-resolved confocal detection of photoluminescence at varying distances from the excitation position. Our measurements on chloride-treated methylammonium lead iodide thin films, the material for which the highest solar cell efficiencies have been reported, reveal a charge carrier diffusion length of 5.5–7.7 μm and a transport time of 100 ps for the first micrometer corresponding to a diffusion constant of about 5–10 cm² s^–1, similar to GaAs thin films

Recycling Perovskite Solar Cells To Avoid Lead Waste

Methylammonium lead iodide (MAPbI₃) perovskite based solar cells have recently emerged as a serious competitor for large scale and low-cost photovoltaic technologies. However, since these solar cells contain toxic lead, a sustainable procedure for handling the cells after their operational lifetime is required to prevent exposure of the environment to lead and to comply with international electronic waste disposal regulations. Herein, we report a procedure to remove every layer of the solar cells separately, which gives the possibility to selectively isolate the different materials. Besides isolating the toxic lead iodide in high yield, we show that the PbI₂ can be reused for the preparation of new solar cells with comparable performance and in this way avoid lead waste. Furthermore, we show that the most expensive part of the solar cell, the conductive glass (FTO), can be reused several times without any reduction in the performance of the devices. With our simple recycling procedure, we address both the risk of contamination and the waste disposal of perovskite based solar cells while further reducing the cost of the system. This brings perovskite solar cells one step closer to their introduction into commercial systems

Grain Boundaries Act as Solid Walls for Charge Carrier Diffusion in Large Crystal MAPI Thin Films

Micro- and nanocrystalline methylammonium lead iodide (MAPI)-based thin-film solar cells today reach power conversion efficiencies of over 20%. We investigate the impact of grain boundaries on charge carrier transport in large crystal MAPI thin films using time-resolved photoluminescence (PL) microscopy and numerical model calculations. Crystal sizes in the range of several tens of micrometers allow for the spatially and time resolved study of boundary effects. Whereas long-ranged diffusive charge carrier transport is observed within single crystals, no detectable diffusive transport occurs across grain boundaries. The observed PL transients are found to crucially depend on the microscopic geometry of the crystal and the point of observation. In particular, spatially restricted diffusion of charge carriers leads to slower PL decay near crystal edges as compared to the crystal center. In contrast to many reports in the literature, our experimental results show no quenching or additional loss channels due to grain boundaries for the studied material, which thus do not negatively affect the performance of the derived thin-film devices

Synthesis of Perfectly Oriented and Micrometer-Sized MAPbBr₃ Perovskite Crystals for Thin-Film Photovoltaic Applications

Wide band gap perovskites such as methylammonium lead bromide are interesting materials for photovoltaic applications because of their potentially high open-circuit voltage. However, the fabrication of high-quality planar films has not been investigated in detail for this material. We report a new synthesis approach for the fabrication of bromide-based perovskite planar films based on the control of the deposition environment. We achieve dense layers with large and perfectly oriented crystallites 5–10 μm in size. Our results show that large crystal sizes can be achieved only for smooth indium-doped tin oxide substrates, whereas lateral perovskite crystal growth is limited for the rougher fluorine-doped tin oxide substrates. We additionally correlate photocurrent and perovskite crystal properties in photovoltaic devices and find that this parameter is maximized for ordered systems, with internal quantum efficiencies approaching unity. Hence, our work not only gives a new pathway to tune morphology and crystal orientation but also demonstrates its importance for planar perovskite solar cells

Unraveling the Function of an MgO Interlayer in Both Electrolyte and Solid-State SnO₂ Based Dye-Sensitized Solar Cells

The coating of n-type mesoporous metal oxides with nanometer thick dielectric shells is a route that has proven to be successful at enhancing the efficiency of some families of dye-sensitized solar cells. The primary intention is to introduce a “surface passivation layer” to inhibit recombination between photoinduced electrons and holes across the dye-sensitized interface. However, the precise function of these dielectric interlayers is often ambiguous. Here, the role of a thin MgO interlayer conformally deposited over mesoporous SnO₂ in liquid electrolyte and solid-state dye-sensitized solar cells is investigated. For both families of devices the open-circuit voltage is increased by over 200 mV; however, the short-circuit photocurrent is increased for the solid-state cells, but reduced for the electrolyte based devices. Through electronic and spectroscopic characterization we deduce that there are four distinct influences of the MgO interlayer: It increases dye-loading, slows down recombination, slows down photoinduced electron transfer, and results in a greater than 200 mV shift in the conduction band edge, with respect to the electrolyte redox potential. The compilation of these four factors have differing effects and magnitudes in the solid-state and electrolyte DSCs but quantitatively account for the difference in device performances observed for both systems with and without the MgO shells. To the best of our knowledge, this is the most comprehensive account of the role of dielectric shells in dye-sensitized solar cells and will enable much better interfacial design of photoelectrodes for DSCs

Charge Transport Limitations in Perovskite Solar Cells: The Effect of Charge Extraction Layers

Understanding the charge transport characteristics and their limiting factors in organolead halide perovskites is of great importance for the development of competitive and economically advantageous photovoltaic systems derived from these materials. In the present work, we examine the charge carrier mobilities in CH₃NH₃PbI₃ (MAPI) thin films obtained from a one-step synthesis procedure and in planar n–i–p devices based on these films. By performing time-of-flight measurements, we find mobilities around 6 cm²/V s for electrons and holes in MAPI thin films, whereas in working solar cells, the respective effective mobility values are reduced by 3 orders of magnitude. From complementary experiments on devices with varying thicknesses of electron and hole transport layers, we identify the charge extraction layers and the associated interfaces rather than the perovskite material itself as the major limiting factors of the charge carrier transport time in working devices

Light-Emitting Electrochemical Cells Based on Hybrid Lead Halide Perovskite Nanoparticles

Methylammonium lead bromide (MAPbBr₃) perovskite nanoparticles (NPs) have been recently proposed as a new material for light-emitting diodes, as well as a new paradigm to elucidate the operational mechanism in perovskite solar cells. Here, we have expanded the synthesis concept to fabricate NPs based on formamidinium lead bromide (FAPbBr₃). Importantly, we have demonstrated that the photophysical features of this novel material can be easily tuned by exchanging the organic cation, achieving lower radiative bimolecular recombination rate for FAPbBr₃ NPs. Additionally, we report for the first time light-emitting electrochemical cells (LECs) based on perovskite NPs by an easily up-scalable spray-coating technique. Stable luminance of 1–2 cd/m² at low driving currents was achieved for both types of materials. Overall, this work opens a new avenue of research into the field of organic–inorganic metal halide nanoparticles bearing different alkyl ammonium groups and their application in the developing field of thin-film lighting devices