Interestingly, upon increasing the thickness of TCA to approximately 19 nm by spinning a higher concentration of 6 mM TCA, we observed a decline in EQE relative to the optimal 3 mM TCA. While the thicker TCA layer would prevent back-FRET energy transfer further, the triplets within the TCA now face non-radiative recombination pathways before reaching the TCA-rubrene/DBP interface (i.e., φBT decreases, φET2 also faces a drop with thicker TCA films). .