Influência das fases intermediárias nas propriedades ópticas da perovskita híbrida orgânica-inorgânica CH3NH3Pbl3

Abstract

The use of the organic-inorganic hybrid perovskites with the general formula ABX3 (A=CH3NH3+, HC(NH2)2+; B=Pb2+, Sn2+; X=I-, Br-, Cl-) as light absorbers has greatly intensified in recent years due to its optical and electronic properties. The high efficiency of converting solar radiation into electrical energy, the low production costs and the possibility of processing by low temperature solution, encourage the use of these materials in different electro-optical applications, including photovoltaic devices. However, this system still has some limitations, such as low long-term structural stability. Issues related to the formation and degradation mechanism of this class of material, as well its electronic conduction processes remain under investigation. In this work, CH3NH3PbI3 perovskite films were produced by two-step spin coating method, and the effects of heat treatment and precursor stoichiometry on the optical and structural properties were analyzed. The absorption spectra of the PbI2 film exhibits different coordination degree of PbI2 with the solvent dimethylformamide (DMF). After the deposition of the organic component CH3NH3I (MAI), diluted in isopropanol (IPA), over PbI2 film, the spectra continues to exhibit structures associated to PbI2/DMF complexes in the region of 360 to 520 nm, in addition to the characteristic transition of perovskite at 760 nm. In the spectral region between the PbI2 complexes and perovskite absorptions, is evidenced absorptions of phases not fully understood in the literature. By adding MAI/IPA, new complexes can be generated, for example (MA)2(DMF)2Pb3I8 phase, recently reported in the literature. This intermediate phase was observed in X-ray diffractograms, demonstrating the different complexations degrees between PbI2, MAI and solvents. X-ray analysis shows that the intensity of PbI2 and perovskite peaks increase with the annealing time, at the expense of the dissociation of this intermediate phase. It is shown that the intermediate phase favors crystallization of perovskite in nanowire structure. It is supposed that the absence of heat treatment of the PbI2 film is determinant for the formation of intermediate phases. It was also observed that different stoichiometric ratios lead to different phases compositions, morphologies and optical properties. It is possible that increased MAI concentration favors the formation of intermediate phases, while the continuous heat treatment leads to the dissociation of these intermediate phases into PbI2 complexes and perovskite.