Proceedings of nanoGe September Meeting 2015 (NFM15)
Publication date: 8th June 2015
In the last few years many papers devoted to the use of hybrid halide perovskites (HPVK) in solar cells has reported high efficiencies, up to more than 20%, due to their extremely high absorption coefficient (>104 cm-1 at the band absorption edge), but also attributed to a very low exciton binding energy. Moreover, HPVKs have been also successfully used as active layers in light emitting devices [1,2]. In Ref. 2, the Electroluminescence (EL) in field-effect transistor was observed below 200 K and the two observed recombination bands were associated to the tetragonal (dominant above 100 K) and ortorrombic (dominant below 160 K) ferroelectric phases present in the CH3NH3PbI3semiconductor. The observed behaviour of the EL was very similar to that observed in Photoluminescence (PL) experiments as a function of temperature [3], whose quenching above 160 K was attributed to the exciton dissociation, deducing an exciton binding energy as high as 62 meV, in contrast to a value as low as 6 meV reported in Ref. 4. Here, we present the main results from an optical study (light absorption, PL and time resolved PL as a function of temperature, from 4 to 300 K) carried out on HPVK films exhibiting different polycrystalline morphologies, in order to elucidate most of the aforementioned controversial results on binding energy and give light on the origin of the double bands observed in EL and PL spectra. Absorption spectra allow the deduction of an exciton Rydberg energy around 23 and 12 meV for the orthorhombic and tetragonal phases, respectively. At the lowest temperatures one can observe, depending on the film morphology (different grain size, mainly), the exciton related absorption and PL corresponding to the orthorhombic phase, but also a low energy band associated to defect grains whose crystal structure is assumed in between those of orthorhombic to tetragonal phases, because their emission band is extremely broad under low power excitation (even if its linewidth and importance decrease with power), practically unresolved from that of the tetragonal phase at the high energy side of the PL spectrum. The PL from the tetragonal phase disappears above 160 K and only that from the tetragonal phase is observed until room temperature.
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