According to a report on The Elec website, Apple plans to use a special OLED technology, hybrid OLED technology, on future iPads. Hybrid OLED technology can combine rigid and flexible OLED panel materials.
Apple is reluctant to rely entirely on the flexible OLED technology it uses in the iPhone because of the panel’s tendency to “crinkle” and the effect becomes more pronounced as the display gets larger.
While hybrid OLED technology is not yet perfected and will take at least a year to become commercially viable, that means it will need to wait until around 2024 to be used in the iPad product line.
For now, it appears that hybrid OLED panels may be a better option because they are thinner than rigid panels and cheaper than flexible panels.