Charge Balance and Exciton Confinement in Phosphorescent Organic Light Emitting Diodes

Abstract

local density of excitons, into both the PHOLED charge transport layers and the EML. This is, to our knowledge, the fi rst time that sensing layers have been used to measure charge and exciton leakage into transport layers, and to directly determine the dependence of charge balance and the extent of the exciton formation zone on current. Our results suggest a modifi ed structure with improved blocking characteristics leading to higher effi ciency and potentially longer device lifetime.An understanding of charge transport and the exciton density distribution is necessary to determine the effi cacy of a particular choice of blocking layer material. Charges injected at the electrodes are transported into the EML where they eventually recombine to form excitons, or are transported into an adjacent layer and lost to nonradiative recombination or are collected at the electrodes. Driftdiffusion with thermionic emission over energy barriers between layers can describe the charge balance and carrier distribution in the PHOLED [ 5,8,9 ] yielding the following expressionswith the boundary conditions ofand ( ) 0 a b i . Here, x = 0 corresponds to the position at the anode side of the EML, and x = L at the cathode. Also, E(x,t) is the electric fi eld at x and time t , q is the charge, k is Boltzmann's constant, T is the temperature, and ε is the dielectric constant of the material. Also, J x t n ( , ) is the electron current density, x t n ( , ) μ is its mobility, n x t ( , ) is the electron density, p x t ( , ) is the hole density, Va is the applied voltage, Vbi is the built-in voltage, E 0 is a reference electric fi eld, φ is the frontier orbital energy difference across an interface, and t E t d ( ) ( ) φ Δ = is the potential difference across the interface of width d . The hole current density, J x t p ( , ), is found using an expression analogous to Equation ( 1) . The Einstein relation is used to relate mobility to the diffusion constant, and thermionic emission over barriers has a Poole-Frenkel type fi eld dependence common to organics. [ 10 ] The solution to these equations determine the charge balance, given by Charge Balance and Exciton Confi nement in Phosphorescent Organic Light Emitting DiodesCaleb Coburn , Jaesang Lee , and Stephen R. Forrest* DOI: 10.1002/adom.201600067We study the charge balance and exciton confi nement in blue phosphorescent organic light emitting devices (PHOLEDs) by strategically placing ultrathin, red phosphorescent layers whose emission intensity is proportional to the local density of excitons at several locations within the device. Based on measured exciton distributions, we derive a model for understanding those materials properties that determine charge and exciton transport and confi nement, as well as for interpreting the experimental exciton profi les. We fi nd that triplet exciton leakage results from the very large energy gaps characteristic of blue phosphors, and demonstrate increased lifetime and efficiency in devices with improved exciton confi nement within the diode emission...

Improved stability in organic light‐emitting devices by mixing ambipolar and wide energy gap hosts

“…In conventional mixed‐ and graded‐EML devices, electron‐ and hole‐transporting hosts are mixed to achieve high charge balance and a wide RZ . The absolute position of the RZ is also important, as proximity to a transport layer interface can exacerbate degradation, or influence exciton confinement and charge balance . In cases showing interfacial degradation, the RZ is typically pinned at the degrading interface.…”

Effects of Charge Balance and Exciton Confinement on the Operational Lifetime of Blue Phosphorescent Organic Light-Emitting Diodes

“…L/L 0 . The differences in leakage at J = 1 mA/cm 2 and 10 mA/cm 2 are due to shifts in the charge recombination zone from the ETL to the HTL side of the EML with current [7]. This leads to proportionately larger sensor emission at low currents at the ETL side, whereas the HTL side leakage does not change over the same range of J (see Fig.…”

“…Here, we employ red, δ-doped (i.e. ultrathin) 'sensing layers' in the charge transport regions to monitor changes in η CB and η EC of a blue PHOLED over its operational lifetime [5,7,8]. Charge and triplet exciton leakage are distinguished by comparing the emission intensities from the fluorescent vs. phosphorescent sensing layers.…”

“…Leakage into the HTL of analogous devices is attributed to exciton diffusion [7,21] that results from the low triplet energy of the HTL compared to the EML constituents. In addition, both NPD and CPD fluoresce in the wavelength range λ = 400-450 [21], which would be expected in the case of electron leakage into the HTL.…”

“…The exciton density is zero at the sensing layer, corresponding to exciton trapping on sensor molecules, and excitons are blocked at the HBL. We use D = 2.3×10 -7 cm 2 s -1 and τ = 100 µs for exciton diffusion and lifetime in the HTL and 7.6×10 -7 cm 2 s -1 for the EML [7]. Values for τ(t') in the EML are Table 1, Ref.…”