  To improve the light extraction efficiency of organic light emitting diodes
(OLEDs), we developed a novel substrate, i.e., a metamaterial based flat Moire
micro-lens array formed using double nanoimprint, termed Mlens-array,
consisting of a hexagonal moir\'e pattern pillar array. By choosing a low
refractive index dielectric material for the pillar array and a high refractive
index dielectric material for the layer capped on top of the pillar array, we
fabricated the Mlens-array behaving as a conventional convex optical micro-lens
array. The Mlens-array was fabricated on a 4-inch wafer-size glass substrate by
double-cycle compositional nanoimprint lithography (NIL) which is easy for
achieving high throughput fabrication in large-scale. Applying the Mlens-array
substrate in a typical green-emitting OLED, the light extraction efficiency was
enhanced by over 100% (2.08-fold) compared to a control device fabricated on
the conventional planar glass substrate.
