RESUMEN
Halide perovskites are a well-known class of materials with many interesting applications. Great attention has been devoted to investigating halide perovskites containing triple methylammonium (MA+), formamidinium (FA+), and guanidinium (GA+) cations. Despite presenting very good applied perspectives so far, the lack of fundamental information for this system, such as its structural, thermal, and optoelectronic characteristics, prompts a step back before any technological leap forward. In the present work, we investigate the physical properties of mechanochemically solvent-free synthesized GAxFAxMA1-2xPbI3 halide perovskite powders with compositions of 0.00 ≤ x ≤ 0.15. We demonstrate that the synthesis of the powders can be performed by a simple manual mechanical grinding of the precursors for about 40 minutes, leading to solid solutions with an only minor content of unreacted precursors. X-ray diffraction, differential scanning calorimetry, and infrared spectroscopy techniques were used to investigate the structure, tetragonal-to-cubic phase transition, and vibrational characteristics of the organic cations with increasing GA+ and FA+ contents, respectively. The band gap and Urbach energies, obtained from ultraviolet-visible spectroscopy analyses, ranged from 1.58 to 1.65 eV and 23 to 36 meV, respectively, depending on the composition. These parameters demonstrate a non-random variation with x composition, which offers the possibility of a rational composition design for a given set of desired properties, demonstrating potential for optoelectronic applications. Finally, the system appears to have adequately tolerated heating for 12 hours at 120 °C in an ambient atmosphere, indicating high thermal stability and low ionic conductivity, which are desirable characteristics for solar cell applications.
RESUMEN
Despite the proven enhancement of MAPbI3 optoelectronic properties and stability by guanidinium substitution, divergences persist regarding fundamental knowledge on this system. This work shows that GAxMA1-xPbI3 solid solutions have guanidinium content-dependent phase transitions, dielectric permittivities, ionic conductivities, activation energies, and relaxation times.
RESUMEN
Methylammonium lead iodide (MAPbI3) emerges as a promising halide perovskite material for the next generation of solar cells due to its high efficiency and flexibility in material growth. Despite intensive studies of their optical and electronic properties in the past ten years, there are no reports on dc bias electric field effects on conductivity in a wide temperature range. In this work, we report the combined effects of frequency, temperature, and dc bias electric field on the ac conductivity of MAPbI3. We found that the results of dc bias electric fields are very contrasting in the tetragonal and cubic phases. In the tetragonal phase, sufficiently high dc bias electric fields induce a conductivity peak appearance â¼290 K well evidenced at frequencies higher than 100 kHz. Excluding possible degradation and extrinsic factors, we propose that this peak suggests a ferroelectric-like transition. In the absence of a dc bias electric field, the ac conductivity in the tetragonal phase increases with temperature while decreases with temperature in the cubic phase. Also, ac activation energies for tetragonal and cubic phases were found to be inversely and directly proportional to the dc bias electric field, respectively. This behavior was attributed to the ionic conduction, possibly of MA+and I-ions, for the tetragonal phase. As for the cubic phase, the ac conduction dynamics appear to be metallic-like, which seems to change to a polaronic-controlled charge transport to increased dc bias electric fields.