Abstract
AbstractA rational host–guest concept design for the attainment of high efficiency at strong luminance from light‐emitting electrochemical cells (LECs) by suppression of exciton‐polaron quenching [Tang et al., Nature Communications 2017, 8, 1190] has been reported. However, a practical drawback with the presented host–guest LEC devices was that the operational stability is insufficient for many applications. Here, a systematic study is performed, revealing that a major culprit for the limited operational stability is that the employed n‐type host, 1,3‐bis[2‐(4‐tert‐butylphenyl)‐1,3,4‐oxadiazo‐5‐yl]benzene (OXD‐7), has a strong propensity for crystallization and that this crystallization results in a detrimental phase separation of the constituents in the active material during device operation. The authors, therefore, identify an alternative class of concept‐functional n‐type hosts in the form of spirobifluorene‐phosphine‐oxide compounds, and report that the replacement of OXD‐7 with amorphous 2,7‐bis(diphenylphosphoryl)‐9,9′‐spirobifluorene results in a much improved operational lifetime of 700 h at >100 cd m−2 during constant‐bias driving at an essentially retained high current efficacy of 37.9 cd A−1 and a strong luminance of 2940 cd m−2.
Highlights
Injection, transport, and recombination tert-butylphenyl)-1,3,4-oxadiazo-5-yl]benzene (OXD-7), has a strong propensity of electrons and holes
We note that similar spirobifuorene-phosphine-oxide derivatives have been employed in organic light-emitting diode (OLED),[9] and that Pertegas and co-workers introduced such a compound in an light-emitting electrochemical cells (LECs) that delivered a respectable luminance of 420 cd m−2 at a high current efcacy of 5 cd A−1 during pulsed-bias operation, but with a short operational lifetime of 1 h above 100 cd m−2.[10]. Here, we show that an appropriately designed host-guest LEC device based on the amorphous n-type host 2,7-bis(diphenylphosphoryl)-9,9′-spirobifuorene (SPPO13) delivers a strong luminance of 2940 cd m−2 at a high current efcacy of 37.9 cd A−1 and a much improved operational lifetime of 700 h at >100 cd m−2 during constant-bias operation
The diferential scanning calorimetry (DSC) data were recorded on bulk neat materials, but we have investigated the four n-type hosts in thin blend flms that were used as the active material in the LEC devices
Summary
We frst call attention to that all four active-material flms feature a uniform and pinhole-free surface morphology, which is an obvious requirement in order to avoid catastrophic short circuits between the two electrodes in planar light-emitting devices, such as LECs and OLEDs. A closer inspection reveals that the most fat and uniform surface belongs to the flms based on the n-type hosts SPPO13 (root-mean-square, RMS = 0.63 nm) and SPPO21 (RMS = 0.50 nm), which is in line with that all of the constituents in these flms were measured to be amorphous in the DSC (see Figure 2a). The OXD-7 based active-material flm exhibits an intermediate roughness (RMS = 0.83 nm), despite that OXD-7 was found to be highly crystalline in neat form in the DSC This observation indicates that the propensity of the n-type host OXD-7 for crystallization is suppressed by its good chemical compatibility with the other constituents in the active material, primarily the p-type host PVK. We were not able to ft the high-voltage data with the space-charge-limitedcurrent equation and a constant value for the mobility, but had better success with the equation for the space-charge-limited current modifed by the Poole–Frenkel efect:
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