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Abstract
We report on digitally printed distributed feedback lasers on flexible polyethylene terephthalate substrates based on methylammonium lead iodide perovskite gain material. The perovskite lasers are printed with a digital drop-on-demand inkjet printer, providing full freedom in the shape and design of the gain layer. We show that adjusting the perovskite ink increases the potential processing window and decreases the surface roughness of the active layer to less than 7 nm, which is essential for low lasing thresholds. Prototype inkjet-printed perovskite lasers processed on top of nanopatterned rigid as well as flexible substrates are demonstrated. Optimized perovskite gain layers printed on PET substrates demonstrated lasing and showed a linewidth of 0.4 nm and a lasing threshold of 270 kW/cm2. In addition, printing of a distinct shape shows a high level of uniformity, demonstrated by a low spatial resolved full width half maximum variation over the whole printing area. These results reveal the possibilities of digital printed perovskite layers towards large-scale and low-cost laser applications of arbitrary shape.
© 2018 Optical Society of America
1. Introduction
Solution processed solid state lasers are of particular interest due to diverse potential advantages compared to their epitaxial counterparts, such as low processing costs, broad wavelength choice and ease of integration into a manifold of optoelectronic devices [1,2]. In the last two decades, the development of solution processed lasers was centered around gain materials made of conjugated polymers and colloidal quantum dots [1–4]. Continuous improvement of materials and devices led to the demonstration of the suitability of such solution processed devices for various applications ranging from spectroscopy to sensing and visible light communication [5–8]. However, with organic polymers and colloidal quantum dots continuous wave operation, direct electrical pumping as well as long term stability remains a major challenge [9–12]. In the past five years, metal halide perovskites have emerged as a new class of high performance solution processed semiconductors which gives hope that these challenges can be overcome [13,14]. In 2014 Xing et al. reported tunable amplified spontaneous emission (ASE) from the blue to the near infrared region using this material class [15]. Nearly simultaneously, Deschler et al. demonstrated lasing from a perovskite layer sandwiched in a vertical cavity [16]. Since then, several reports demonstrating ASE or random lasing from thin films or lasing from whispering gallery mode perovskite nanostructures occurred [17–19]. Recently, solution processed perovskite thin films were also deposited on more practical cavities, such as surface emitting distributed feedback resonators or photonic crystals cavities [20–23]. The thin films of these lasers were so far spin-coated. While this is a suitable method for lab demonstration, it does not allow a structured deposition and flexibility in design without post-processing which is very difficult for perovskites thin films [24].
In this work, we demonstrate for the first time inkjet-printed perovskite lasers. Inkjet printing is highly advantageous for the digital deposition of perovskite layers of arbitrary shape, which is desirable and needed in many applications such as in lab on chip system or integrated optics with perovskites as light emitter [25]. Furthermore, it minimizes the needed amount of solution and it is compatible to large area and even roll-to-roll processing. We have developed and optimized the inkjet printing process in order to achieve smooth perovskite films, which is necessary for the minimization of waveguide losses in the thin films [26]. We have deposited those perovskite films successfully on silica gratings as well as on nanoimprinted polymer gratings on flexible polyethylene terephthalate (PET) substrates.
2. Methods
2.1 Nanoimprint lithography (NIL)
Master gratings were fabricated by electron beam (e-beam) lithography in silica. These gratings were replicated onto the flexible substrates by nanoimprint lithography, which is a low cost, easily up scalable fabrication technique, able to replicate laser resonators with a large variety of materials and geometries [27–30]. The elastomer Polydimethylsiloxane (PDMS, Dow Corning, Sylgard 184 silicone elastomer kit) was used to fabricate a negative pattern from the silica grating master. The commercial UV resist OrmoComp® was drop casted onto polyethylene terephthalate (PET) substrates and imprinted with the PDMS stamps. The detailed process parameters are reported elsewhere [22].
2.2 Inkjet printing of perovskite layer
A stock solution of 1.4 M methylammonium lead iodide (CH3NH3PbI3) was prepared by dissolving lead iodide (Alfa Aesar Product No. 44314) and methylammonium iodide (Dyesol Product No. MS101000.) in dimethyl sulfoxide (DMSO) and gamma-butyrolactone (GBL) (3:7) (both Sigma Aldrich) and stirred for at least for 1h at 65°C. This stock solution was diluted to 0.7 M by adding either pure GBL or a mixture DMSO:GBL (3:7). The solution was then filtered with a 0.45 µm polytetrafluoroethylene (PTFE) filter and filled in the print head (DMC11610; nozzle diameter 21 µm) of a Fujifilm Dimatix DMP 2800 inkjet printing system. The ink was held at 35°C while printing and deposited at a plate substrate of 22 °C. The drops were generated by a single bias pulse with a peak voltage of 20 V and a pulse width of 15 µs. The jetting frequency was set to 8 kHz. The drop spacing was set to 45 µm to reach a perovskite thickness of 180 nm. The wet film was transferred to a vacuum chamber (~1 mbar) to outgas the excess solvent. The films were then annealed at 100°C for 10 min on a hotplate under a nitrogen flow on a hotplate. The nanoimprinted polyethylene terephthalate (PET) substrate was mildly pretreated for 20 s with an argon plasma. The rigid silicon dioxide (SiO2) was treated for 60 s with O2 plasma, followed by rinsing for 10 s with 2-propanol. All fabrication steps were performed under cleanroom conditions with an air humidity of 45 ( ± 5) %.
2.3 Optical characterization
For the photoluminescence (PL) characterization, we used the 532 nm output of a frequency-doubled solid state laser (FTSS355-Q2, CryLaS GmbH) emitting pulses of ~1 ns. The pump pulse energy was varied by a neutral density filter and continuously monitored using a calibrated GaAsP-photodiode connected to an oscilloscope (Tektronix, TDS2024C). The perovskite film emission was coupled to an optical fiber and detected by a spectrometer (Acton Standard Series SP-2358 Imaging, variable gratings) connected to an intensified CCD camera (Princeton Instruments PI-MAX4:1024f-HR). A spot size of 1.5 × 10−5 cm2 was measured with the moving knife-edge method [31]. All measurements were performed at room temperature in N2 atmosphere. In order to distinguish lasing from related effects, methods as formulated by Samuel et al. were applied [32]. White light interferometer images are obtained with a Sensofar Plu neox system with a Nikon objective 10x magnification. Atomic force microscopy images were obtained with a DME DS 95 Dualscope AFM in tapping mode using cantilevers from NanoWorld (Arrow NCR).
3. Results and discussion
In this work, we demonstrate inkjet printing of perovskite gain layers for distributed feedback (DFB) lasers. In order to obtain wave guiding in the active material and the grating, a smooth surface with an appropriate thickness to support mode propagation is essential. Figure 1(a) sketches the process flow of the fabrication of a flexible laser device by nanoimprinting and inkjet printing. Details on the fabrication process can be found in literature [22,30]. Figure 1(b) shows the colored diffraction of a nanoimprinted grating on a flexible PET substrate. A smooth and structured perovskite layer is subsequently digitally inkjet-printed on via a multipass inkjet printing approach [see Fig. 1(c)]. Suited film thicknesses and grating periods can be estimated by applying the transfer matrix method to determine the effective index neff of the guided modes. Lasing is then expected in the spectral range determined by the Bragg equation [33,34]:
Fig. 1 (a) Schematic of the replication and the inkjet printing process. The grating period was 380 nm and a grating height z of 80 nm was used. Drop diameter ~80 µm are indicated. (b) Photography of a Flexible PET foil with the nanoimprinted pattern. (c) Image of a perovskite layer on PET structured as the logo of our university.
A film thickness of around 180 nm was targeted, leading to an effective refractive index of 2.1 and a suiting grating period Λ between 370 to 380 nm to match the ASE spectrum of the perovskite film [22]. A parameter optimization of the perovskite ink was performed to match this requirement. As reported in previous work, the inkjet-printed perovskite layer thickness can be controlled by changing the drop distance between two printed drops [35]. In order to achieve good control about the surface crystallization of the inkjet-printed perovskite layer, an additional vacuum drying step was introduced. This way, inkjet-printed perovskite layers can be prepared with similar morphological and electrical properties as in a reference spin coating process. To reach the optimal thickness of 180 ( ± 10 nm), we used a 0.7 M ink of CH3NH3PbI3 in DMSO and GBL and a drop spacing of 45 µm.
A second key parameter to control the surface roughness is the ink composition and processing, i.e., the perovskite concentration and vacuum drying time. We obtained optimal printing behavior and lowest surface roughness for 0.7 M perovskite solution with a GBL to DMSO ratio of 85:15. A low surface roughness in order to minimize the waveguide scattering losses is beneficial to decrease the gain threshold [26]. As shown in Fig. 2(a), the determined surface roughness on the imprinted grating can be reduced to values below 7 nm. The surface roughness, given as a root mean square value (rms) is determined from white light interferometer images and confirmed by atomic force microscopy images [Fig. 2(b)]. Seok et al. described the mechanism of DMSO and an antisolvent dripping to form smooth perovskite layer [36]. We observe a similar effect with our vacuum drying step before the perovskite is formed via an annealing process [37]. Therefore, we notice for all three inks a decrease in surface roughness with increasing vacuum time with a minimal surface roughness of 7 nm achieved after 180 s. The effect is more pronounced with less concentrated inks, indicating a slower drying of the solvent rich wet film. Additionally, we observe that for long vacuum drying times there is a strong increase of surface roughness for inks with low GBL content. This indicates that lower amounts of the high boiling point solvent GBL promotes a faster crystallization, thus leading to rougher surfaces. For a higher GBL content the surface roughness remains at low values around 10 nm even after 480 s vacuum drying. The GBL-rich ink demonstrates that its high boiling point supports the crystallization of a flat perovskite surface, thereby increasing the processing window of the vacuum drying tremendously.
Fig. 2 (a) Surface roughness of printed CH3NH3PbI3 inks on rigid glass substrate in dependency of ink concentration, ink composition and vacuum drying time calculated from white light interferometer images (b) shows corresponding atomic force microscopy images of CH3NH3PbI3 underlining the improved process control for low surface roughness of polycrystalline perovskite films. Values on the bottom left are the extracted root mean square (rms) values.
To underline the versatility of our inkjet printing process, we demonstrate the lasing of inkjet-printed perovskite gain layers on imprint textured flexible PET foils. This route offers digital printed low cost DFB lasers of potential key relevance for photonic microsystems and lab-on-a-chip devices. The substrates utilized offer structured areas with different grating pitches and also planar, unstructured regions. When the sample is excited, we clearly can distinguish between photoluminescence (below threshold), amplified spontaneous emission (above threshold on unstructured, planar regions) and lasing (above threshold on structured regions with appropriate pitch) for different pumping levels and positions. Figure 3(a) shows the different spectra measured on the imprinted PET foil when excited with the 532 nm pump laser at a spot size of 1.5 × 10−5 cm2. At the peak emission wavelength, the emission intensity was measured as the function of the pump pulse intensity. A kink occurs at around 270 kW/cm2, which is comparable to the value of 235 kW/cm2 obtained on a rigid substrate [Fig. 3(b)]. The result is consistent with the similar film coverage and grain size observed in atomic force microscopy images of printed perovskite layers on rigid SiO2 and flexible PET (see Appendix Fig. 5). Our obtained threshold values are slightly higher compared to spin coated or directly nanoimprinted layers [22,38]; this can be attributed to the remaining roughness. The high resolution spectrum of the lasing peak in Fig. 3(c) shows a full width half maximum (FWHM) value of 0.4 nm, which is in the range of the spectrometer resolution. Furthermore, as shown in Fig. 3(d), the light emission of the lasing sample exhibits a highly polarized output, with high intensities between 90° to 120° and 270° to 300°. For polarizer angles from 0° to 30° and 180° to 210°, the emission is nearly completely blocked. According to the measurement guidelines on “how to recognize lasing” as formulated by Samuel et al [32], the combination of the small FWHM of the emission peak, the threshold behavior and the highly polarized emission are very strong indicators for true lasing in our inkjet-printed CH3NH3PbI3 layer on top of the nanoimprinted PET substrate. The stability of the printed laser devices, showed in Appendix Fig. 6, is lower compared to spin cast devices as demonstrated in an earlier publication [22]. A reason for the reduced stability could be a non-ideal conversion of precursors to the perovskites phase, leaving unreacted PbI2 in the film. Li et al recently showed that remnant PbI2 can be detrimental for the stability of perovskite solar cells [39]. Another recent study on optical gain in mixed-halide perovskites supported this assumption by demonstration that a deficiency of lead in the precursors solutions leads to a significant improved lasing stability of perovskite thin films [40].
Fig. 3 Inkjet-printed CH3NH3PbI3 laser on a flexible PET substrate with nanoimprinted grating and planar regions. (a) Photoluminescence, amplified spontaneous emission (on planar regions) and lasing (on structured regions) spectra. (b) Emission intensity at the peak wavelength as a function of the pump laser pulse fluence. The lasing threshold is 270 kW/cm2. The lasing threshold of an inkjet-printed gain layer on top of a rigid SiO2 layer shows a similar threshold of 235 kW/cm2. (c) High resolution spectrum of the laser peak on a grating with 380 nm period, showing a full width at half maximum of only 0.4 nm. (d) Signal strength detected for lasing emission as a function of the rotation angle of a linear polarizer placed between the sample and detector.
In order to highlight the freedom in design provided by the digital inkjet printing process, we printed the shape of the logo of our university on an unstructured glass substrate. Figure 4(a) shows the color coded spatial resolved FWHM obtained by a Gaussian fit through the spectra at each individual point of the scanned area, which has a relative intensity significantly above the noise level. The blue areas have a FWHM below 10 nm which correspond to areas where ASE occurred, indicating flat and smooth perovskite structures. The green areas with a FWHM of around 50 nm correspond to locations, which do not exhibit ASE but photoluminescence. This is mainly the case at the boarders of the printed structures. The inactive area in red correspond to not printed areas. A spatial resolved peak intensity map is depicted in Fig. 4(b). It can be seen that the peak intensity measurement shows the strongest ASE signal in the middle area and flattens out to the printing edges. Large area white light interferometer images suggest, that these inhomogeneous patterns origin from the differences in film thickness and surface roughness of the printed layer [Fig. 4(c)]. The weak or disappearing ASE signal at the printed edges can be explained by the strong coffee ring effect, which leads to an increase of the layer thickness. If the printed layers become too thick, the waveguide mode exhibits less gain due to the reduced average excited charge carrier density throughout the film. However, this proof of concept study emphasizes the potential of uniform inkjet-printed perovskite layers for structured DFB lasers in optoelectronic devices.
Fig. 4 (a) Spatially resolved full width at half maximum (FWHM) of an inkjet-printed KIT-Logo on a glass substrate. Photoluminescence is seen between 10 and 50 nm (blue to green). Amplified spontaneous emission below 10 nm (dark blue). (b) Normalized intensity distribution of the printed layer. Red indicates high intensities with strong ASE signal. (c) Large area white light interferometer image stitched together from 72 single images. Layer is inhomogeneous at the edges of the printed areas is attributed to the coffee-ring-effect.
4. Summary
In summary, we have demonstrated a versatile and flexible digital inkjet-printed process for CH3NH3PbI3 distributed feedback laser on rigid as well as on flexible PET substrates. The realized prototype perovskite laser shows low linewidth of around 0.4 nm and a threshold of 235 kW/cm2 and 270 kW/cm2 on rigid substrates and nanopatterned flexible PET substrates, respectively. Moreover, we used inkjet printing to process a laser in customized shape. Our distinct printing design shows a high level of uniformity, approved by a low FWHM variation over nearly the whole area. The combination of nanoimprinted grating structures onto a flexible PET substrate and the printed perovskite layer underlines the potential of this process technology. These results pave the way for digital printed perovskite DFB lasers for nanopatterned photonic microsystems and lab-on-a-chip devices in customized shapes.
Appendix
Fig. 5 Atomic force microscopy images of CH3NH3PbI3 on electron-beam structured SiO2 grating and on Nanoimprinted OrmoComp® substrate respectively. The calculated rms values are 27 nm on SiO2 and 32 nm on PET, using a 1.4M CH3NH3PbI3 ink and a 180s vacuum annealing step.
Fig. 6 Emission intensity stability of the inkjet-printed CH3NH3PbI3 laser on a flexible PET substrate with a nanoimprinted grating in N2 atmosphere. The intensity drops to half of its initial value after 0.7 x 107 pulses. The samples were excited with a repetition rate of 5 kHz and an excitation density of 480 kW/cm2.
Funding
German Federal Ministry of Education and Research (BMBF) (03SF0483); Karlsruhe School of Optics & Photonics (KSOP); Deutsche Forschungsgemeinschaft; Initiating and Networking funding of the Helmholtz Association (HYIG); Helmholtz Energy Materials Foundry.
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