Improving lifetimes and efficiencies of blue organic light-emitting diodes is clearly a scientific challenge. Towards solving this challenge, we propose a unicolored phosphor-sensitized fluorescence approach, with phosphorescent and fluorescent emitters tailored to preserve the initial color of phosphorescence. Using this approach, we design an efficient sky-blue light-emitting diode with radiative decay times in the submicrosecond regime. By changing the concentration of fluorescent emitter, we show that the lifetime is proportional to the reduction of the radiative decay time and tune the operational stability to lifetimes of up to 320 h (80% decay, initial luminance of 1000 cd/m2). Unicolored phosphor-sensitized fluorescence provides a clear path towards efficient and stable blue light-emitting diodes, helping to overcome the limitations of thermally activated delayed fluorescence.

Organic light emitting diodes (OLEDs), utilize small organic molecules or polymers, in order to achieve an emissive electroluminescent layer. Ultra-thin, lightweight, and flexible characteristics offer a highly enticing substitute, in comparison to their inorganic counterpart. It is therefore understandable why there has been a large amount of research, focused on enhancing the efficiency and stability of OLEDs. At present, efficient and long-lasting, red [1] and green [2] OLEDs are achievable, with the weakest link being the blue OLED. Achieving a blue OLED, which is both efficient and stable, has proven to be problematic. The challenge originating with the limitations of the blue emitter: if operational lifetime is prioritized, stable fluorescent emitters can be used. Their efficiency, however, is limited by unfavorable spin statistics. To improve efficiency, phosphorescent emitters can be used. Their large coupling between the exciton spin and the orbital angular momentum allows for radiative decay from the triplet state to the ground state. Additional to this, the spin-orbit coupling allows for intersystem crossing to occur, such that the singlet excited state can also populate the triplet state, helping to achieve almost 100% internal quantum efficiency. The drawback of this highly efficient system is the long lifetime of the triplet state, typically in the order of several microseconds, much longer than the fluorescence lifetime, leading to degradation of the organic material. [3] Since stability is most important to achieve a long-lived consumer product, fluorescent emitters are the chosen source of blue OLEDs. But, with battery life on portable devices being the cost of this inefficiency, it is vital that the blue OLEDs become more efficient.Thermally activated delayed fluorescence (TADF) [4][5][6][7][8][9] is one of the existing approaches targeting the OLED efficiency, where a reverse intersystem crossing, from triplet to singlet, is achieved. Combination of TADF and conventional fluorescence emitters, in a sensitizing approach is also a possibility. [10,11] However, the decay times of TADF systems are similar to that of a phosphorescent only system, [12,13] meaning that a short-lived OLED is inevitable.A phosphor-sensitized fluorescence approach, [14][15][16][17][18] offers an alternative to TADF OLEDs, by utilizing a donor-acceptor concept with a phosphorescent donor and a fluorescent acceptor. In Unicolored phosphor-sensitized fluorescence (UPSF) is a dual emitting concept proposed for improving efficiencies and operational lifetimes of blue organic light emitting diodes (OLEDs). To overcome the limitations of the individual emitters, it uses a phosphorescent donor to sensitize a fluorescent acceptor. To quantify the potential of the concept, a multiscale model of a UPSF OLED is developed. It starts from atomistic morphologies, the rates of all processes on the available experimental data are parameterized, and the respective master equation is solved with the help of the kinetic Monte Carlo algorith...

In state-of-the-art blue phosphorescent organic light-emitting diode (PHOLED) device architectures, electrons and holes are injected into the emissive layer, where they are carried by the emitting and hole transporting units, respectively. Using transient electroluminescence measurements, we disentangle the contribution of the electrons and holes on the transport and efficiency of both pristine and degraded PHOLEDs. By varying the concentration of hole transporting units, we show that for pristine PHOLEDs, the transport is electron dominated. Furthermore, degradation of the PHOLEDs upon electrical aging is not related to the hole transport but is governed by a decrease in the electron transport due to the formation of electron traps.