Photophysical origin of blue organic light-emitting diode degradation

Rapid degradation of blue phosphorescent organic light-emitting diodes (PHOLEDs) is arguably the biggest challenge facing the OLED display industry today. While various mechanisms are known to cause intrinsic PHOLED degradation, the reason that blue PHOLEDs are uniquely unstable compared to green and red devices remains unclear. Here, we show that the cation of many OLED host and transport layer materials possesses a strong absorption band in the blue (wavelengths < 450 nm) that leads to rapid molecular fragmentation when it is excited. Focusing on common hosts such as NPB, TAPC, mCBP, mCP, and CBP, we find that the action spectrum for degradation matches the high energy P2 transition of the cation, whereas the low energy P1 transition (at red and near-infrared wavelengths) is stable.

Analytical characterization with MALDI-TOF mass spectrometry, FTIR, and NMR confirm that the degradation products following P2 excitation are consistent with those observed in blue PHOLEDs [1-3]. DFT/TDDFT modeling indicates that certain bond dissociation energies of these molecular cations decrease relative to the neutral molecule to roughly the same energy as the optically accessible P2 transition, thereby providing an efficient path to deposit the energy required to break these bonds. The unique instability of blue OLEDs thus originates from the larger overlap between their emission spectra and the P2 absorption of cationic (and likely also anionic) transport layer, host, and emitter dopant molecules, which in turn allows for efficient Förster-based exciton-polaron annihilation as well as simple far-field absorption of blue photons trapped in the device. This result holds implications for designing molecules with less P2-blue spectral overlap, and also for blue device architectures, where minimizing the accumulation of electrons and holes (i.e. anions and cations) as well as their spatial overlap with the internal optical field becomes an important design consideration.