Figure 1

(Color online) Spectrally resolved CBP/BCzVBi OSL step responses for a 200 ns ${\text{Ar}}^{+}$ laser pump pulse with intensity below, at, and above threshold. Only CBP/BCzVBi spontaneous emission is evident below threshold in (a). At threshold, in (b), spectral narrowing and an increase in intensity at the laser line ${\lambda }_L=474\text{nm}$ are observed. Lasing is established above threshold in (c), although it is quenched at $t>80\text{ns}$. (d) Fits (gray lines) to the intensity transient data of (a)–(c) at the lasing wavelength (black lines) using the model presented in the text.Reuse & Permissions

Figure 2

Double pulse lasing recovery of ${\text{Alq}}_3/\text{DCM}$ (circles), CBP/BCzVBi (squares), and CBP/DCM (triangles) OSLs. The lasing intensity due to the trailing ${\text{N}}_2$ laser pulse is shown vs delay, $\tau$, from the initial 200 ns ${\text{Ar}}^{+}$ laser pulse. Gray lines show monoexponential fits, where the ${\text{Alq}}_3/\text{DCM}$ OSL recovers with time constant ${\tau }_{\text{rec}}=\left(40\pm 3\right)\mu \text{s}$. The OSLs with a common CBP host recover with ${\tau }_{\text{rec}}\ge 700\mu \text{s}$. Inset: Photoluminescence step response of ${\text{Alq}}_3/\text{DCM}$ and CBP/BCzVBi films to an 800 ns ${\text{Ar}}^{+}$ laser pump pulse. Quenching in each transient is due to single-triplet annihilation (STA), as inferred from fits to the model in the text (gray lines). A strongly varying intensity profile through the CBP/BCzVBi film thickness due to its high absorption coefficient results in reduced apparent quenching of the initial transient and is accounted for in the fit.Reuse & Permissions

Figure 3

(a) Singlet $\left(N_S\right)$ and triplet $\left(N_T\right)$ densities extracted from the CBP/BCzVBi model for the cases of optical and electrical pumping. When the triplet formation fraction ${\delta }_T={\varphi }_{\text{ISC}}=0.03$, the rate of triplet generation is low and the singlet density exceeds that of triplets for $t<50\text{ns}$, allowing threshold to be reached (gray lines). When ${\delta }_T={\chi }_T=0.75$, triplets always outnumber singlets and singlet-triplet annihilation ( STA) prevents lasing (black lines). (b) Laser threshold criteria calculated from Eq. (4) in the text. The contours indicate the maximum cavity loss, ${\alpha }_{\text{cav}}/\Gamma$, capable of lasing for different combinations of rise time, $t_R$, and current density, $J$. In the presence of STA, with $K_{\text{ST}}=1.9\times {10}^{-10}$ (dashed lines), the curves asymptote vertically, making lasing impossible for any $J$ in devices with rise times that exceed $t_R$. Eliminating STA relaxes the rise-time requirement (solid lines) and leads to lower threshold current densities.Reuse & Permissions