Proceedings of Perovskite Thin Film Photovoltaics (ABXPV16)
Publication date: 14th December 2015
Efficiencies of solar cells based on organic-inorganic halide perovskites are rapidly increasing, however, a solid understanding of the photo-physical properties is still lacking. In order to accurately determine the optoelectronic behavior of perovskites, high control over the film thickness, stoichiometry and composition is required. In this work we present a method to obtain ultra-smooth films of CH3NH3PbI3 with tunable thickness, prepared by sequential physical vapour deposition (PVD) of the precursors CH3NH3I and PbI2. SEM and AFM microscopy show that this preparation method yields pinhole-free, smooth films with a thickness variation less than 2 nm. The X-ray diffraction pattern shows only the (110) and (220) directions of the tetragonal phase, indicating that all crystalline domains are oriented in the same direction. We show that the I- can be exchanged to Br- and Cl-, while the film thickness and morphology are largely preserved. Hence, these vapour-deposited perovskites enable us to study material properties such as (transient) reflection and absorption, and time-resolved photo conductance (terahertz and microwave) as function of composition and thickness. We find mobilities > 50 cm2/Vs, which are related to large crystalline domains within the film. In addition, for low light intensities (<1012 photons/cm2) we find no relaxation of the charge carrier mobility on time scales up to 1 ns. We apply a kinetic model to describe the photoconductance traces recorded using laser intensities varying over 3 orders of magnitude with one set of kinetic parameters, yielding a trap density of 2 x 1015 cm-3 and a dark carrier concentration of 1016 cm-3. Additionally, we find that any residual PbI2 does not affect the charge carrier dynamics, whereas an excess of CH3NH3I results in lower mobilities. Altogether, we show how perfectly flat perovskite films with substantial control over thickness, stoichiometry and composition can be obtained using physical vapour deposition and how this relates to the optoelectronic properties.
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