We investigate quenching processes which contribute to the roll-off in quantum efficiency of phosphorescent organic light-emitting diodes (OLED’s) at high brightness: triplet-triplet annihilation, energy transfer to charged molecules (polarons), and dissociation of excitons into free charge carriers. The investigated OLED’s comprise a host-guest system as emission layer within a state-of-the-art OLED structure—i.e., a five-layer device including doped transport and thin charge carrier and exciton blocking layers. In a red phosphorescent device, $N,N^{\prime }$-di(naphthalen-2-yl)- $N,N^{\prime }$-diphenyl-benzidine is used as matrix and tris(1-phenylisoquinoline) iridium $\left[\mathrm{Ir}\right(\mathrm{piq}{)}_3]$ as emitter molecule. This structure is compared to a green phosphorescent OLED with a host-guest system comprising the matrix 4,$4^{\prime }$,$4^{″}$-tris ($N$-carbazolyl)-triphenylamine and the well-known triplet emitter fac-tris(2-phenylpyridine) iridium $\left[\mathrm{Ir}\right(\mathrm{ppy}{)}_3]$. The triplet-triplet annihilation is characterized by the rate constant $k_{TT}$ which is determined by time-resolved photoluminescence experiments. To investigate triplet-polaron quenching, unipolar devices were prepared. A certain exciton density, created by continuous-wave illumination, is analyzed as a function of current density flowing through the device. This delivers the corresponding rate constant $k_P$. Field-induced quenching is not observed under typical OLED operation conditions. The experimental data are implemented in an analytical model taking in account both triplet-triplet annihilation and triplet-polaron quenching. It shows that both processes strongly influence the OLED performance. Compared to the red $\mathrm{Ir}(\mathrm{piq}{)}_3$ OLED, the green $\mathrm{Ir}(\mathrm{ppy}{)}_3$ device shows a stronger efficiency roll-off which is mainly due to a longer phosphorescent lifetime $\tau$ and a thinner exciton formation zone $w$.

Organic solar cells have the potential to become the cheapest form of electricity, beating even silicon photovoltaics. This article summarizes the state of the art in the field, highlighting research challenges, mainly the need for an efficiency increase as well as an improvement in long‐term stability. It discusses possible current and future applications, such as building integrated photovoltaics or portable electronics. Finally, the environmental footprint of this renewable energy technology is evaluated, highlighting the potential to be the energy generation technology with the lowest carbon footprint of all.

We examine spontaneous emission in organic electroluminescent devices and investigate the influence of the local photonic mode density on the emissive properties of molecular emitters. Spontaneous emission in organic electroluminescent devices is modeled by means of an approximate closed-form solution for the exciton rate equation, which yields the efficiency of conversion of electrical charges into molecular excited states. The exciton decay rate and the efficiency of conversion of molecular excitation into far-field radiated photons are described using a state-of-the-art classical electromagnetic formalism suitable to model multilayered organic light-emitting diodes (OLEDs). We present an in-depth analysis of the influence of optical microcavities and the corresponding resonant modes on the luminescent properties of organic molecules. Near-field coupling and coupling to metallic reflectors are demonstrated as the main effects responsible for environment-induced modifications of the rate and efficiency of spontaneous emission. The extent to which the excitonic decay rate is modified by the optical microcavity (Purcell effect) is shown to be strictly dependent on the intrinsic luminescence quantum yield of the molecular emitter. The modeling formalism is successfully validated against experimental results obtained on three series of small-molecule p-i-n OLED samples, featuring phosphorescent or fluorescent molecular emitters, with a widely varying thickness of the optical microcavity. We demonstrate that, within its limits of validity, the theoretical treatment in this work provides a rigorous quantitative description of spontaneous emission in organic luminescent devices and allows for the identification of the factors determining the OLED internal and external quantum efficiencies.