The colour purity of the emitted light from the device can be improved using both intrinsic and extrinsic solutions. These include modifying the structure of the emitter to produce a narrowing of the emission spectrum, 1 incorporating colour filters 2 or modifying the device optics with the use of microcavities. Colour filters reduce the light output and therefore efficiency of the display, while microcavities are optimised for a single emission colour and are difficult to implement in large area displays. The most desirable solution would be to develop narrowband emitters (Figure ) with much smaller full width at half maxima (FWHM), thereby obviating the need for extrinsic colour filters. Industry standards for 8k displays and beyond demand even wider colour gamuts and this will require even more saturated colour from each of the red, green and blue emitters. For example, the green colour point in the Rec.2020 colour space for UHDTVs has CIE1931 coordinates of [0.170, 0.797], 5 whereas the current HDTV standard (DCI-P3) has CIE coordinates of [0.265, 0.690] (Figure ). Current OLED displays use phosphorescent iridium complexes as the emitters for the red and green pixels. The band shape of the emission spectra from emissive iridium complexes is strongly tied to the nature of the emissive excited state. Complexes with mixed metal-to-ligand/ligand-to-ligand charge transfer MLCT/ 3 LLCT T1 states show broad, featureless emission, the result of large geometric reorganization in the excited triplet state, while complexes with locally excited 3 LC T1 states have narrower and vibronically structured emission profiles. Unfortunately, emission from LC states generally results in longer radiative lifetimes and lower photoluminescence quantum yields (ΦPL) than emission from MLCT/ 3 LLCT due to the reduced spin orbit coupling (SOC) as the iridium is not implicated in the former. To target narrowband phosphorescence while retaining fast, efficient emission from iridium complexes, the goal is to design complexes with mixed 3 MLCT/ 3 LLCT/ 3 LC excited states. This is very challenging as both states are sensitive to subtle changes in the electronics of the ligands about the metal centre. The brute-force way to access emitters with mixed 3 MLCT/ 3 LLCT/ 3 LC states is to screen a range of closely related molecules to assess how minor changes in the structure of the ligands impact the nature of the excited state. One of the key requirements for narrowband emission is reducing the vibrational coupling within the molecule that manifests in the presence of additional, lower energy emission bands to the main one. There are several design strategies to reduce this vibronic coupling. One is to closely match the geometry of both the ground and excited states, which can be achieved by employing a rigid molecular design. An alternative strategy is to reduce the amplitude of the vibrations that are strongly coupled to the electronic transitions, which can be achieved by increasing the steric bulk of the ligands on the complex as this dampens highfrequency C-H vibrational modes. There are a large array of different functional groups that can be substituted onto an emissive iridium(III) complex to suppress vibrational coupling without impacting significantly the emission energy. Examples of groups that can suppress vibronic coupling in iridium complexes can be as small as a methyl group, or longer alkyl chains, aryl groups or heteroatom-containing functional groups such as silanes. Previously reported iridium complexes containing trimethylsilyl groups on a ppy ligand are shown in Figure and their photophysical properties are reported in Table . One available yet underutilised tool to study vibrational coupling in emissive complexes is to calculate the vibrationally coupled emission spectrum. Several studies have demonstrated good correlation between experimental and simulated emission spectra of existing iridium-and platinum-containing emitters, but there are very few reports containing vibrational studies that are used as a tool for the design of new complexes. With a view to obtaining improved colour green emitters, here we modelled the vibrationally-coupled emission spectra, of a family of trimethylsilane-substituted greenemissive iridium complexes and then correlated these findings to their photophysical properties.