Effects of the Wrinkle Structure and Flat Structure Formed During Static Low-Temperature Annealing of ZnO on the Performance of Inverted Polymer Solar Cells

Abstract

Low-temperature annealing of Zinc oxide (ZnO) films as electron transport layers for inverted polymer solar cells was investigated. A wrinkled morphology of the ZnO film has previously been mostly observed after dynamic annealing (DA). In this study, we investigated the effect of static annealing (SA) of ZnO layers deposited by the sol–gel method at 25 °C, 150 °C, and 200 °C for 10 min in air. We observed the formation of the wrinkle structures on the surface of the ZnO sample annealed at 150 °C, while flat structures were formed at 200 °C. Here, a variable ramping/heating rate provided by a static annealing process resulted in a variable solvent evaporation rate and transformation of the precursor. The tensile stresses induced by slower heating (∼40 °C/min in a 0.75 M ZnO solution) and residual solvent resulted in an amorphous layer with a wrinkled structure at a low temperature of 150 °C. A flat structure was obtained with slightly different dynamics at a faster heating rate (∼56 °C/min) at 200 °C. Consequently, the SA process enabled us to fabricate the desired wrinkled morphology at lower annealing temperatures (e.g., 150 °C) than ever reported for DA processes; the ramp rate in the 200 °C SA process was too high to form the wrinkled structure. The short circuit current of the device using a wrinkle structure was better than that of the device with flat structure due to optical effects of internal reflection, scattering and light-trapping by wrinkles, while the transmittance and fill factor of the device using a flat ZnO layer annealed at 200 °C was better than those of the device with a ZnO layer annealed at 150 °C. This was due to better film quality from the higher processing temperature, a low surface roughness, and less defects. However, the power conversion efficiency of devices with both films was similar, meaning that the low temperature annealing process producing the wrinkle structure can be used for fabricating devices with polymer substrates and gas barriers for flexible electronics. In the case of the wrinkled structure, we observed that the wrinkle height was highly dependent on the ramping rate, ZnO solution concentration, and annealing temperature, compared with previous works.