Enhancement of Morphological and Opto-Electronic Properties of Perovskite (Ch3nh3pbi3) Thin Films for Solar Cell Applications.
Moracha, Daniel Juma
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The functionality of the photovoltaic devices greatly depends on the film morphology, which is determined by the deposition methods and annealing techniques. Organometal halide perovskite based solar cells represents an upcoming photovoltaic technology. Perovskites have been widely and extensively studied for some years now, but comprehending their properties has slowed down their advancement. Finding a link between their morphology and the resulting properties is important in dealing with some of the basic issues like high band gap and sheet resistivity that hinder further development of this solar cell. Herein the perovskite thin films were prepared by single and double step deposition methods. Concentrations of solutions, annealing temperatures and dipping time were used as parameters to form different morphologies during self-assembly processes. The optical band gaps for the films prepared by single step were observed to decrease from 2.10eV to 1.96eV when the annealing temperature was increased from 80oC to 160oC. An increase of annealing temperature beyond 160oC led to the formation of yellowish substance on the substrate, this indicated the presence of lead iodide. Further these results implied the decomposition of methyl ammonium which resulted to an increase of the optical band gap to 2.16eV. Films that were deposited by double step displayed similar trend where the optical band gap was observed to decrease significantly from 2.14eV to 1.95eV when the dipping time was varied from 2 hours to 8 hours. The optical band gap was observed to increase to 2.15eV when the dipping time was prolonged to 10 hours and beyond. This indicated that the nucleation and film growth of perovskite is a reversible process and 8 hours was found to be ideal for the two process to that is nucleation and film growth to perfectly take place. The transmittance data was simulated using multi-peak fitting SCOUT software from which other optical constants like refractive index was obtained as n = 2.3803, extinction coefficient k= 0.2215 and absorption coefficient α= 69855.23 for single step film samples. For the double step deposition method the refractive index of n=2.4011, extinction coefficient k= 0.2998 and absorption coefficient α= 71450.42 were revealed. The sheet resistivity of the films was evaluated using four-point probe. It was observed that an increase in annealing temperature from 80oC to 160oC led to decrease in sheet resistivity of films from 0.7982 Ω cm-1 to 0.4231Ω cm-1.The sheet resistivity of films deposited by double step decreased from 0.5675Ω cm-1 to 0.2290Ω cm-1. From the results herein it was deduced that the morphology of the perovskite thin films can be enhanced by not only the deposition methods but also annealing temperatures and dipping times.