Objective Stable luminescent efficiency in inorganic halogen perovskite quantum dots (QDs) is difficult to achieve because of their degradation when exposed to air. In addition to stability, increased luminescence efficiency of QD composite films is an important factor for their future applications. Due to a sharp difference in refractive index between the air and flat QD film interface, some light is limited in the film, which has difficulty in crossing the interface and this decreases the luminescence efficiency of the QD film. This light is absorbed by the film and converted into thermal energy, which shortens the device’s lifetime. Therefore, improvement of luminescence efficiency and stability of halogen perovskite QDs is essential. In this study, we report the fabrication of a flexible QD polydimethylsiloxane (PDMS) film with a microlens array (MLA) pattern using PDMS polymer mixed with lead cesium bromide (CsPbBr3) QDs to transfer the concave MLA master mold. The flexible MLA QD film effectively improves luminescence efficiency and stability of the CsPbBr3 QDs. Additionally, we find that introducing the MLA structure on the surface of the film significantly enhances surface hydrophobicity.
Methods The preparation procedure of CsPbBr3 QD film with a MLA pattern is divided into the following two steps. (1) CsPbBr3 QDs are synthesized by a conventional and simple chemical synthesis method and then dissolved into n-hexane (Fig. 1). Next, PDMS prepolymer and its crosslink agent are successively poured into the n-hexane QD solution in a weight ratio of 10∶1, and the solution is ultrasonically oscillated for uniformity. Finally, the CsPbBr3 QD PDMS system is obtained by removal of n-hexane using vacuum. During the preparation process, both PDMS and its crosslink agent, with a constant weight of 1.1 g, are added into the CsPbBr3 QD n-hexane solution in different volumes. The volume of CsPbBr3n-hexane solution is denoted by x (in mL) and the corresponding QD film is denoted as QD-x. (2) The second step of the preparation procedure is the transfer of the microlens array structure onto the surface of the CsPbBr3 QD film (Fig. 2). First, a micropillar array is prepared by the laser direct writing technique and then the array is heated to melt and cooled to form a convex MLA master mold. Next, the convex MLA master mold is transferred onto the PDMS surface to form a concave MLA pattern using a soft lithography technique. The PDMS MLA pattern surface is modified by a plasma surface treatment and then immersed into ethyl alcohol for 4 h. Third, the CsPbBr3 QD PDMS solution is poured onto the modified PDMS concave MLA surface, which is then followed by a degassing and curing process. Finally, a CsPbBr3 QD film with the MLA pattern is obtained by separation from the PDMS concave MLA surface.
Results and Discussions The luminescence efficiencies of the prepared CsPbBr3 QD films are measured using a fluorescence spectrometer (Fig. 3). With the increase of QD concentration, the number of QDs per unit volume of PDMS becomes large and the luminescence intensity of the QD film increases. However, the increase of QD concentration also increases the agglomeration of the QDs, which decreases the luminescence efficiency. In this experiment, QD-2.5 displays an optimum luminescence intensity and, thus, the QD PDMS system of QD-2.5 is selected as the prepared material for subsequent QD PDMS film fabrication. Moreover, this film displays stable luminescence intensity when repeatedly bent a thousand times or when immersed into water for ten days (Figs. 4 and 5). This demonstrates that the QD film is water-repellent and suitable for wearable flexible devices. The prepared MLA QD film shows about 10% increase in luminescence intensity compared to a flat QD film without any patterns (Fig. 6). This is mainly due to the antireflective property of the MLA pattern, which effectively avoids total reflection and decreases Fresnel reflection at the interface of air and the QD film (Fig. 7). When a 2 μL deionized water droplet is dropped on the surface of the prepared MLA QD film, the droplet shows a contact angle of 138.6°. By contrast, the contact angle is 96.7° when a 2 μL droplet is dropped on a flat QD film. The MLA pattern blocks the spreading of the water droplet and the three-phase contact line does not cross the obstacle, reaching a new thermodynamic equilibrium state. As a result, the droplet on the MLA pattern has a large contact angle. On a flat solid surface, the three-phase contact line will not encounter any retardation. The droplet reaches its corresponding thermodynamic equilibrium state and thus displays a small contact angle. The experimental results demonstrate that the MLA QD film is suitable for high-humidity environment because its hydrophobic surface is not conducive to water vapor accumulation.
Conclusions Inorganic CsPbBr3 QDs were synthesized by a conventional and simple chemical synthesis method. The synthesized CsPbBr QDs were uniformly added to PDMS by ultrasonic mixing, which was followed by removal of the n-hexane in the QD solution by vacuum. The PDMS QD film shows good luminescence stability under repeated bending or after immersion into water. After introducing the MLA pattern on the surface of the flexible QD film, the luminescence efficiency and surface hydrophobicity are significantly improved.
