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C2.01 - Defect Densities, Mobility and Device Physics of Perovksite Solar Cells 
April 7, 2015   1:30pm - 1:45pm

Pb halide based perovskite materials are important for photovoltaic devices. In this paper, we systematically discuss the influence of different processing conditions of perovskite devices for both p-i-n and n-i-p configurations. In a p-i-n device, light enters from the p side whereas in the n-i-p device, light enters from the n side. We discuss properties of materials and devices produced using three processing conditions : complete solution growth, partial solution growth, and complete vapor growth where no liquids are used. The properties measured include grain size, structure using x-ray spectrum, deep defects using capacitance-frequency-temperature spectroscopy, drift mobility of holes, type of doping ( p or n), which carrier controls transport and minority carrier diffusion length measured directly using electronic measurements. We show that the complete vapor grown device has the lowest deep defect density, the largest grains size, and a high photovoltaic efficiency of ~15%. We also discuss the differences in properties of materials devices fabricated using either methyl-ammonium iodide (MAI) or formamidinium iodide (FAI). We show that the materials and devices prepared using the vapor process for FAI are far more stable physically and allow a much larger processing range, including processing in vacuum at higher temperatures. In contrast, the devices produced with MAI decompose in vacuum at elevated temperatures. The differences in physical properties between these two classes of materials lead to much better stability for the devices prepared using FAI compared to devices prepared using MAI. We also investigate systematically the influence of Chlorine (eg use of PbCl2 instead of PbI2)during the vapor deposition process for devices. We do not find that Cl has any influence on the electronic properties of the material when the grain size is significant. We find that the material is n type independently of the substrate on which it is deposited, and that the transport is unambiguously controlled by holes and not electrons. Therefore it is inaccurate to say that the device is controlled by ambipolar transport. We also find that the drift mobility of holes, measured using time of flight techniques, is ~10-1 cm2/V-s. The Urbach energy of valence band tails is~16 meV.

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