Light-emitting materials

The chemical structures of the singly dendronised dendrimer (D1) and poly(dendrimer) (P1) investigated in this work are shown in Fig. 1a, with the synthesis of P1 described in the Supporting information. D1 is a first-generation dendrimer with a homoleptic fac-(2-phenylpyridyl)iridium(III) core and fluorenyl-carbazole-containing dendrons,17 and provides an important comparative material for the poly(dendrimer). Such carbazolyl-containing dendrons have previously been shown to provide improved hole transport for light-emitting dendrimers,18 and so were an integral part of the poly(dendrimer) design. In the case of P1, the iridium(III) complex core is heteroleptic, with two 2-phenylpyridyl ligands and a 1-methyl-5-phenyl-3-n-propyl-1H-[1,2,4]triazolyl co-ligand. As in the case of D1, the 2-phenylpyridyl ligands have the first-generation carbazole-containing dendrons attached and the phenyltriazolyl co-ligand provides the means of attachment to the polymer backbone. The norbornenyl-derived 1,3-divinylcyclopentane-polymer backbone was chosen as it could be formed by ring opening metathesis polymerisation to give materials with bulky dendritic side groups of high-molecular weight and relatively low polydispersity.19,20 Gel permeation chromatography against poly(styrene) standards showed P1 had an \({\bar{\mathrm{M}}\mathrm{w}}\) and dispersity of 1.6 × 105 Da and 1.3, respectively, and it is interesting to note that based on the number average molecular weight of the polymer and monomer, the degree of polymerisation can be calculated as ≈55 units, which would correspond to a polymer length (assuming an extended polymer chain) of ≈50 nm. Furthermore, it should be noted that P1 is a homopolymer, that is, every ‘monomer’ unit along the polymer backbone has a dendrimer attached.

Fig. 1 Chemical structures. Singly dendronised dendrimer D1 and poly(dendrimer) P1. Both materials have carbazole-containing first-generation dendrons attached to 2-phenylpyridyl ligands, which give rise to green emission from the iridium(III) complex. P1 is a homopolymer with a dendrimer attached to each ‘monomer’ unit of the polymer backbone, with two units shown Full size image

Photophysical properties

The photophysical properties of D1 and P1 were first compared to determine whether there was any effect on the luminescence properties caused by linking the dendrimers via the polymer backbone. Figure 2 shows the normalised PL spectra for D1 and P1 in solution, neat films, and when blended at 20 wt% with 4,4′-bis(N-carbazolyl)biphenyl (CBP)—the host used in the subsequent device studies. It can be seen in Fig. 2a that the solution photoluminescence spectra of D1 and P1 are identical in spite of D1 having a homoleptic core complex and P1 containing a heteroleptic complex. In heteroleptic complexes such as those found in P1, emission effectively occurs from the ligand with the smallest optical gap.21 The homoleptic complexes fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy) 3 ] and fac-tris(1-methyl-5-phenyl-3-n-propyl-1H-[1,2,4]triazolyl)iridium(III) emit green22 and blue23 light, respectively, and hence the emission from both D1 and P1 is green (emission peak at 520 nm and a shoulder at 548 nm) with Commision Internationale de L’Eclairage (CIE) 1931 chromaticity coordinates of (0.37, 0.54). There is little change in the PL spectra in moving from solution to the solid state (Fig. 2a), indicating that the dendrons are sufficient to inhibit strong interchromophore interactions and potential aggregate or excimer formation. It should be noted that the blend film PL spectra had a small component at 385 nm, which was due to the host material (CBP), and indicates that incomplete energy transfer has occurred in the photoluminescence experiment. The solution photoluminescence quantum yield (PLQY) of D1 and P1 were similar at 82 ± 8% and 72 ± 7%, respectively. Time-resolved PL measurements in solution (see Supplementary Fig. S1) show that both D1 and P1 had single exponential decays with an excited state lifetime of 1.7 and 1.5 μs, respectively, indicating that only one type of emissive species was present. This is strong evidence that, in spite of the relative closeness of the chromophores attached to the polymer backbone, interchromophore interactions do not lead to significant quenching of the luminescence. In the solid state the PLQY values of the neat films were 20 ± 3% for D1 and 30 ± 4% for P1, indicating that additional non-radiative decay pathways have emerged from intermolecular interchromophore interactions. This hypothesis is supported by time-resolved PL measurements, which showed that the PL decay for both D1 and P1 was faster than in solution and could be described in terms of two decay lifetimes of 0.2 μs and 0.7 μs (see Supplementary Table S1). When D1 and P1 were blended with CBP at 20 wt% there was an increase in the film PLQY, with the values (68 ± 8% for D1:CBP and 71 ± 8% for P1:CBP) returning close to those measured in solution. The PL decays of the CBP blends were closer to those measured in solution with the main decay component having a lifetime of ~1.5 μs (see Supplementary Table S1). This confirms that intermolecular interactions are the main source of non-radiative decay for D1 and P1 in the neat film, but that these interactions can be effectively suppressed by blending with a host such as CBP and suggests that both D1 and P1 are reasonably dispersed within the host.

Fig. 2 Photoluminescence and electroluminescence spectra. a Normalised PL spectra of D1 and poly(dendrimer) P1 for optically dilute solutions and neat films. b The PL and electroluminescence spectra of 20 wt% blends of D1 and P1 in CBP. Inset is a photo of two P1:CBP devices. The spectra are offset vertically for clarity. The samples were excited at 372 nm for the PL measurements Full size image

To gain insight into the structure of the CBP:P1 light-emitting film we measured the surface morphology using atomic force microscopy (AFM) and the distribution of P1 in the host using neutron reflectometry. AFM showed that the surface of the film deposited onto indium–tin oxide (ITO) was smooth with a root mean square roughness of around 0.4 nm (Fig. S2). The neutron reflectivity profiles and scattering length density versus distance from substrate plots are shown in Fig. S3. Deuterated CBP (d-CBP) was used to provide contrast with the protonated P1 (deuterated materials have a higher SLD) and it was found that there was a shallow SLD gradient across the film. The SLD at the interface with the substrate was (3.98 ± 0.01) × 10–6 Å–2 with that at the upper air interface being (4.28 ± 0.01) × 10–6 Å–2. That is, the results suggest the light-emitting layer in the device is slightly enriched with P1 near the substrate and CBP at the interface with the ETL but overall there was no significant vertical phase separation with respect to the substrate. The relatively uniform mixing observed in the neutron reflectometry experiment is consistent with the photophysical measurements.

Device performance

We next prepared bilayer devices with the structure ITO/PEDOT:PSS/emissive layer/ETL/LiF/Al [ITO = indium–tin oxide, PEDOT:PSS = poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate), ETL = 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) or 1,3,5-tris(m-pyrid-3-yl-phenyl)benzene (TmPyPB), LiF = lithium fluoride, Al = aluminium] where the emissive layer was deposited from solution either as a neat film of the emissive material or as a 20 wt% blend with CBP. The device performance characteristics are summarised in Table 1 with the electroluminescence spectra shown in Fig. 2b. The devices with the neat emissive layer and TPBi as the ETL had modest performance with turn on voltages of around 4–5 V (defined as the voltage at which 5 cd/m2 was measured). The maximum EQE for D1 was 4% at 90 cd/m2 and the device reached a maximum luminance of near 10,000 cd/m2 at around 8 V (Fig. S4). The OLEDs with a neat emissive layer of P1 reached a similar maximum luminance to the model dendrimer, but had a slightly higher EQE (7%) at similar luminance (Fig. S4).

Table 1 Summary of device performance and out-coupling of the OLEDs containing a neat emissive layer and when D1 or P1 were blended at a concentration of 20 wt% in CBP Full size table

The devices with a blended emissive layer and TPBi ETL showed a different level of performance (Fig. 3). Although the turn on voltages were slightly higher for the D1:CBP blend and lower for the P1:CBP blend, the key result was that in both cases the EQE was significantly higher than for the OLEDs containing the neat films (Table 1). The best D1:CBP device (Fig. S5) had a maximum EQE of 16% at a luminance of over 50 cd/m2, but more interestingly the EQE of the best P1:CBP blend device was 24% at 33 cd/m2 and it had a power efficiency of almost 54 lm/W (Fig. 3b). To confirm that the high EQE observed for the P1:CBP-based OLED was not limited to a single device architecture we changed the ETL to TmPyPB while keeping the rest of the structure the same. A slightly higher efficiency (30%) but at around the same luminance (30 cd/m2) was observed for the TmPyPB device (Fig. 3d). It should be noted that in all cases the electroluminescence spectra were similar to that of the photoluminescence (although there was no host emission) indicating that emission originates from the same chromophore (Fig. 2b) and suggestive that there were no strong optical cavity effects. In addition, the emission was green with CIE coordinates of (0.37, 0.54) at 1 mA/cm2.

Fig. 3 Device characteristics for the best OLEDs. a J–V–L curves of a device comprising a 20 wt% P1:CBP light-emitting layer with a TPBi ETL. b EQE and power efficiency as a function of the luminance of a device comprising a 20 wt% P1:CBP light-emitting layer with a TPBi ETL. c J–V–L curves of a device comprising a 20 wt% P1:CBP light-emitting layer with a TmPyPB ETL. d EQE and power efficiency as a function of the luminance of a device comprising a 20 wt% P1:CBP light-emitting layer with a TmPyPB ETL Full size image

Finally, we measured the angular dependence of the OLED emission from neat and blend films of D1 and P1 (Fig. 4).