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The properties of 6 mM urea added perovskite solar cells (PSCs) according to ZrO2 electron transport layer (ETL) thicknesses of 204, 221, 254, and 281 nm were examined. A solar simulator was used to characterize the photovoltaic properties of the cell. Optical microscopy and field emission scanning electron microscope were used for the microstructure analysis, and a 3D profiler was used to analyze surface roughness. UV-VIS-NIR was used to analyze transmittance. From the photovoltaic analysis result, an energy conversion efficiency (ECE) of 14.93% was exhibited by the cell with a 221 nm-ZrO2 layer and added urea. From the analysis result of microstructure and surface roughness, 384 nm grain size was obtained through appropriate surface roughness of base layer for perovskite growth and the grain size coarsening by the urea under the 221 nm-ZrO2 condition. For this reason, ECE increased as the resistance of the grain boundary decreased. When the thickness of the ETL was increased above 250 nm-ZrO2, the ECE decreased due to the reduction in light transmittance, and light reaching the perovskite layer. Therefore, the ECE of PCS could be enhanced by selecting a ZrO2 layer with the appropriate thickness and the addition of urea.
