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It is well established that high-Qmicrocavities are crucial for polariton condensation.202 The fabrication of the highly reflective DBRs necessary for these high-Qcavities relies on techniques like molecular beam epitaxy, sputtering or electron-beam evaporation. These are associated with high substrate temperatures, plasmas or charged particles.236,237 Organic materials, by contrast, often degrade easily and are very sensitive, whether this regards exposure to heat, light, water or oxygen.238,239Hence, elaborate approaches have been developed to avoid sput- tering on top of the organic film in the fabrication of high-Qmicrocavities.221 Yet, there have been reports of depositing DBRs on top of organic films,240,241 which could in more recent years even demonstrate good functionality of the organic film.60,61Establishing a protocol for incorporating organic films of C545T between sputtered, highly reflective DBRs without dam- aging them was thus expected to be possible, but challenging. This is why the effect of different parameters of the sputtering process on the organic film was studied with the help of PLQY measurements. The effect of storing the samples in air on the PLQY was also tested. It was nec- essary to determine the impact of storage in air because all setups for sample characterisation and excitation were established outside gloveboxes, i.e., in air.

substrate temperature relative to ambient temperature. Any amount of heating during the sputtering process (Tmin,test =50◦) turned out to have a negative effect on the homogeneity and stability of the organic film. Although this was initially unexpected from the reported glass transition temperature of C545T (Tg = 100 ◦C)226, the sensitivity at lower tempera- tures is likely to be caused by the high vacuum in the chamber before the start of the pro- cess. Once sample damage by heating of the substrate was avoided, the quality of the organic film might still be deteriorated by the conditions inherent to the sputtering process or by the sample storage. First, sputtering on top of organic films could be detrimental because of the high-energy particles or heat present in the plasma.236–238,242Second, the exposure to oxygen and water can quench luminescence, particularly in electroluminescent or phosphorescent de- vices.75,239,243In our system, degradation—though not as severe—is also expected, especially in combination with irradiation.244Both effects were investigated by PLQY measurements and the results are presented in this section.

In order to quantify the damage to the sample by sputtering on top of organic films, thin films of C545T:MADN were produced in the same evaporation run. These are expected to have the same C545T concentration and thus the same PLQY. On the surface of some of these samples, an oxide film was deposited by sputtering, after which the PLQY of these different films was compared. Figure 5.5a shows the results of two sample sets, one set of samples containing 17% C545T and the other 1% C545T in MADN. The first set distinguishes three scenarios: (1) only taking the sample into air, (2) sputtering SiO₂ on top and subsequently taking the sample into air and (3) taking the sample into air and only then sputtering SiO2on top. The second set compares the PLQY of one film taken directly out of the glove box and the other one taken out after having a Ta2O5film sputtered onto its surface. Both the effect of SiO₂ and of Ta₂O₅ is tested because the materials are sputtered with different gas mixtures. Ta2O5 required 17% of oxygen to be mixed into the argon gas in order to obtain films at the right stoichiometry. Figure 5.5a clearly shows that all sputtering processes degrade the film quality. Comparing the cases of direct sputtering and sputtering after exposure to air, it becomes apparent that the exposure to air in itself decreases the PLQY. Moreover, while direct sputtering of SiO2 decreases the PLQY by less than 20% (0.30 → 0.25), the deposition of Ta₂O₅reduces the PLQY by more than 50% (0.70_→0.34). However, due to the high refractive index of Ta₂O₅ (n(Ta₂O₅)_≈2.15), some of the apparent reduction in PLQY could be caused by wave-guiding. An estimation of this effect assumes isotropic emission and complete loss of

Figure 5.5: Degradation of C545T:MADN films with different concentrations (1% to 100% C545T) from measurements of their PLQY. The impact of (a) sputtering on top of the organic film and (b) the storage in air on the PLQY was measured. In the legend, C is an abbrevi- ation for C545T and the green and blue symbols represent different samples from different fabrication runs. The dashed lines are intended to act as guide to the eye.

the photons when wave-guided. This puts an upper limit to the reduction in PLQY caused by wave-guiding of 17%. Hence, it is probable that although the damage to the material might be less than 50%, it is still larger than when depositing SiO2. Overall, it should be noted that while the sputtered samples show a reduced PLQY, the material is not completely destroyed; but its luminescence efficiency is decreased by50%. This is in agreement with literature, where the damage of sputtering to organic films is reported to not quench a major fraction of the luminescence.242 However, the extent of the damage depends on processing parameters like pressure, sputtering power and gas mixture and the correlation is sometimes counter- intuitive.245,246

The development of the PLQY with time of storage in air is shown in Figure 5.5b, where different concentrations and film treatments are compared. All PLQY values are normalised to the PLQY on the first day of measurement. Thereafter, samples are stored in a dark, ambient environment and measured at intervals. Three main observations follow from this investiga- tion. First, although all untreated samples (no sputtering, filled symbols) deteriorate when ex- posed to air, the PLQY is never entirely suppressed but remains above 50% of its original value even after 200 days. Second, the higher the C545T concentration, the greater the degradation over time. In particular, the film with the lowest concentration barely shows any degradation over the investigated time span of 19 days. Third, the films which were sputtered upon (open symbols) behave very differently: their PLQY increases with time. Since three independent

samples (two shown) confirm this trend of an increasing PLQY with time after sputtering on the surface, this is understood as a real effect. A stabilisation of the PLQY with time could be explained by the sputtered layer protecting the surface of the organic material from e.g. oxygen. Some reports also have shown that the sputtering process need not necessarily de- teriorate the luminescence properties.246The continuing increase in PLQY, by contrast, is not currently understood.

To summarise the investigations regarding the damage of the material over time and with sputtering, a detrimental impact of both aspects is found. The damage over time by exposure to air can be reduced by measuring samples soon after fabrication and storing them in an oxygen-free environment (i.e., the glovebox). This was generally the followed protocol.