Highly reproducible vapor deposition technique, device physics and structural instability of perovskite solar cells

We describe a liquid-free, sequential vapor deposition process for Pb iodide based n-i-p perovskite solar cells, using both methyl-ammonium iodide (MAI) and formamidine iodide (FAI) as the organic molecule. The process consists of sequential deposition of both PbI2 and then vaporization in a vapor of MAI or FAI. The process produces highly reproducible high efficiency devices with efficiencies of ~13.6% for MAI devices and 11.2% for FAI devices. We also study the mid-gap defect densities using capacitance-frequency-temperature techniques. We find two distinct defects, one at 0.24 eV below the conduction band, and one at 0.64 eV below the conduction band. The attempt to escape frequency is also calculated for the midgap defect and it is in the range of 1012 /s. We also study the diffusion length of carriers in the devices by using Quantum Efficiency (QE) vs. voltage techniques for different thicknesses of the perovksite, and we find that for thicker devices, there is a definite loss in QE with increasing forward bias, implying a loss in collection due to shrinking depletion width. From such measurements, we conclude that the field-free diffusion length IS <;0.3 micrometer, and that in thin devices (~0.3 micrometer thickness), the transport is mainly controlled by field assisted drift and not by diffusion. We also study the thermally induced phase change using x-ray diffraction, and decomposition of perovskite devices using thermal gravimetric analysis, and find that at very modest temperatures (~100°C), the MAI devices change their phase with the perovskite degenerating into PbI2 over a period of 24 hours. The devices produced using FAI are more stable, with the phase change not occurring until ~125°C.