Berlin 2015 – scientific programme

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CPP: Fachverband Chemische Physik und Polymerphysik

CPP 68: Focus Session Hybrid Photovoltaics and Perovskites (joint session CPP, HL)

CPP 68.2: Invited Talk

Thursday, March 19, 2015, 15:30–16:00, C 130

Exciton stabilization in hybrid lead-halide perovskites: photophysical versus structural properties — •Annamaria Petrozza — Istituto Italiano di Tecnologya, Milan, Italy

Hybrid perovskites represent a new, disruptive technology in the field of optoelectronics. Hybrid halide perovskites, e.g. CH3NH3PbX3 [X = Cl, Br, or I], are usually deposited as polycrystalline thin-films with variable mesoscale morphology depending on the growth conditions. The obtained grain size ranges from tens to thousands of nm. Over the last two years the impressive improvement of photovoltaic performance has been driven by radical empirical evolution of the device architecture and processing methodologies. However, there is a considerable lack of understanding of material properties, both as pristine films and their embodiment in a device. Here we demonstrate, through a combination of femto-second transient absorption spectroscopy, structural analysis and multi-scale modeling as a function of crystal size and temperature, that the electron-hole interaction is sensitive to the microstructure of the material. We find that by control of the material processing during fabrication both free carrier and Wannier excitonic regimes are accessible. Thus, a definitive classification excitonic or free carrier semiconductor is not possible. The long-range order of the organic cation dipole field is disrupted by polycrystalline disorder introducing domain walls where dipole twinning breaks down. The variations in electrostatic potential found for smaller crystallites suppress exciton formation, while larger crystals of the same composition demonstrate an unambiguous excitonic state.