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Based on the off-Γ diffractions from a square photonic crystal (PhC), a flat-top far-field profile is achieved in a microsized thin-film light-emitting diode (mTFLED) for higher light-collection efficiency. The PhC mTFLED with a lattice constant of 370 nm and hole depth of 80 nm exhibits two diffractions at ±12°, resulting in a flat-top far-field profile. The off-Γ diffractions improve light-collection efficiency from 6.57% to 10.79% within a ±15° extraction cone. In addition, the light output power of the PhC mTFLED is 2.93 times than that of nonPhC mTFLED.