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Organic light‐emitting devices (OLEDs) are widely used for mobile displays, but the relatively short lifetime of blue OLEDs remains a challenge in many applications. Typically, instability is viewed as a material‐specific chemical degradation problem. It is known to be alleviated by reducing the operating current or otherwise decreasing the exciton density. It is shown here that this view is incomplete. For archetypical phosphorescent materials, it is observed that the dependence of photostability on the triplet exciton lifetime follows a cubic power law, steeper than its dependence on exciton density. It is demonstrated that the triplet exciton lifetime not only determines the energy stored within an OLED, it also determines the loss in luminescence by controlling the yield of quenching by defects. The dominant role of the triplet exciton lifetime suggests that the stability of the best OLED materials can be significantly improved …