Nanopatterning of hybrid perovskites by thermal nanoimprint for perovskite lasers – (NIPLAS)
Synthesis and Properties of Functional Materials
Final Report Abstract
In NIPLAS thermal nanoimprint was used to directly pattern (hybrid) halide perovskite semiconductors. As insights about the imprint mechanism and comparative studies of nanoimprint in MAPbI3 and other representatives of this family of materials were lacking, the project NIPLAS aimed to unravel the fundamental mechanisms underlying thermal nanoimprint in hybrid halide perovskites. Furthermore, we intended to exploit the exciting opportunities of patterning photonic structures by thermal nanoimprint for perovskite based optoelectronic devices. Based on promising preliminary work on MAPbI3, the focus in NIPLAS was directed on Br-based perovskites. Their optical bandgap in the mid-visible regime allows to cover the spectral region between 530-610 nm, which is difficult to address at room temperature with established inorganic semiconductor gain media, e.g. GaInN or AlGaInP etc. Initially, the research was geared towards developing a novel and more reproducible approach to prepare the starting layers. Thermal imprint with a flat stamp (planar hot pressing, PHP) was established to provide continuous polycrystalline layers with a flat surface (roughness < 1nm (rms)) and large grains; – in the case of MAPbBr3 grains with a mean diameter of up to ≈ 4 µm were achieved. We confirmed that, due to reduced scattering losses, smooth layers significantly decrease the pumping threshold for amplified spontaneous emission, which is a prerequisite for lasing. Large grains were enabled by processing at a temperature as high as 150°C; degradation of the perovskite was avoided as the flat stamp used shields the layer and prevents evaporation of potential gaseous decomposition products. The growth process during this ‘flat pressing’ was analysed in detail. It follows an exponential growth law (≈ t1/n), with a growth exponent n ≈ 3 as well as an activation energy for grain growth of Q ≈ 0.35 eV. Purity of the layer has been identified as an issue as impurities segregate to the grain boundaries and may limit grain growth by reducing grain boundary mobility. Aside from MAPbBr3, we also studied the PHP and thermal imprint of the all-inorganic variant CsPbBr3. The as-deposited layers were flattened by a thermal imprint process at 150°C and a pressure of 100 bar, essentially comparable to MAPbBr3. The grain growth occurring during flattening improved the PL quantum yield and increased the PL lifetime in CsPbBr3 films. While pristine layers did not show ASE under optical excitation, the layers show low threshold ASE after PHP. The impact of the concomitantly occurring 2D polymorph, CsPb2Br5, has been studied and the lowest ASE threshold has been found in the samples that contained only CsPbBr3. Aside from flattening, surface patterning of the perovskite layers was also performed by thermal imprint. For polycrystalline layers a higher pressure is required to replicate sub-micron sized structures compared to that with a single crystal (100 bar versus 25 bar). This (at first unexpected) result could be understood on the basis of gliding. Pressure-induced shear forces initiate gliding of dislocations on specific crystallographic glide planes in well-defined directions; with the perovskites investigated (the layers feature a preferential (100)-orientation in vertical direction) it is the (110)<110> glide system. As the glide planes are ‘disrupted’ by grain boundaries, a polycrystal requires an increased shear force to initiate gliding in differing directions – the lateral orientation differs from grain to grain. Moreover, the two effects of thermal nanoimprint, (i) patterning and (ii) grain growth could be coined into a specific, flexible processing procedure. When the pressure applied during heat-up is low (≈ 10 bar), grain growth occurs first, followed by patterning when the full pressure (100 bar) is applied. However, with the full pressure acting during the whole process, both occur simultaneously, and highly regular grain shapes can be obtained when the stamp structures are in the range of the grain size (‘geometry effects’). The imprint of DFB resonators afforded lasing in MAPbBr3 and CsPbBr3 layers with low thresholds (< 10 µJ/cm2) at room temperature for the first time. Even lower threshold levels were achieved in CsPbBr3 vertical cavity surface emitting lasers (VCSELs), where the active layer is sandwiched between two identical mirrors; the upper one was used as a stamp for the thermal imprint/flattening process (150°C, 100 bar). Upon pulsed excitation, the VCSEL shows lasing with a threshold of 2.2 µJ/cm2. Despite the substantial cuts in funding, the project as a whole can be considered as outstandingly successful.
Publications
- Distributed feedbeck lasers based on MAPbBr3, Adv. Mater. Technol. 3, 1700253 (2018)
N. Pourdavoud, A. Mayer, M. Buchmüller, K. Brinkmann, T. Haeger, T. Hu, R. Heiderhoff, I. Shutsko, P. Görrn, H.-C. Scheer, and T. Riedl
(See online at https://doi.org/10.1002/admt.201700253) - Ultra-smooth perovskite thin films for lasers, SPIE Optics and Photonics, San Diego (USA), 107240C (2018)
N. Pourdavoud, A. Mayer, T. Haeger, R. Heiderhoff, I. Shutsko, H.-C. Scheer, P. Görrn, and T. Riedl
(See online at https://doi.org/10.1117/12.2319364) - Imprint-induced grain growth in perovskite layers, MNE Conf. (Micro and Nano Engineering Conference), 2019, Rhodes, Greece
A. Mayer, N. Pourdavoud, T. Haeger, R. Heiderhoff, M. Leifels, J. Rond, J. Staabs, P. Görrn, T. Riedl, and H.-C. Scheer
- Recrystallized All-Inorganic Lead Halide Perovskite Thin-Films Show Low-Threshold Stimulated Emission and Lasing at Room Temperature, MRS Fall Meeting, Boston (USA) EN10.20.01 (2019)
N. Pourdavoud, T. Haeger, A. Mayer, M. Runkel, P. J. Cegielski, I. Shutsko, A. L. Giesecke, O. Charfi, R. Heiderhoff, S. Zaefferer, M. Lemme, D. Becker-Koch, Y. Vaynzof, H.-C. Scheer, W. Kowalsky, P. Görrn, and T. Riedl
- Room Temperature Stimulated Emission and Lasing in Recrystallized Cesium Lead Bromide Perovskite Thin Films, Adv. Mater. 31, 1903717 (2019)
N. Pourdavoud, T. Haeger, A. Mayer, P. J. Cegielski, A. L. Giesecke, R. Heiderhoff, S. Olthof, S. Zaefferer, I. Shutsko, A. Henkel, D. Becker-Koch, M. Stein, M. Cehovski, O. Charfi, H.-H. Johannes, D. Rogalla, M. Lemme, M. Koch, Y. Vaynzof, K. Meerholz, W. Kowalsky, H.-C. Scheer, P. Görrn, and T. Riedl
(See online at https://doi.org/10.1002/adma.201903717) - Direct patterning of methylammonium lead bromide perovskites by thermal imprint, MNE Conf. (Micro and Nano Engineering Conference), Turin, Italy (2021)
A. Mayer, T. Haeger, M. Runkel, J. Staabs, J. Rond, P. Görrn, T. Riedl, and H.-C. Scheer
- Relevance of processing parameters for grain growth of metal halide perovskites with nanoimprint, Appl. Phys. A 127, 717 (2021)
A. Mayer, T. Haeger, M. Runkel, J. Rond, J. Staabs, F. van gen Hassend, A. Röttger, P. Görrn, T. Riedl, and H.-C. Scheer
(See online at https://doi.org/10.1007/s00339-021-04830-0) - Upgrading of methylammonium lead halide perovskite layers by thermal imprint, Appl. Phys. A 127, 237 (2021)
A. Mayer, N. Pourdavoud, Z. Doukkali, K. Brinkmann, J. Rond, J. Staabs, A.-C. Swertz, F. van gen Hassend, P. Görrn, T. Riedl, and H.-C. Scheer
(See online at https://doi.org/10.1007/s00339-021-04366-3) - Direct patterning of methylammonium lead bromide perovskites by thermal imprint, Appl. Physics A 128, 399 (2022)
A. Mayer, T. Haeger, M. Runkel, J. Rond, J. Staabs, F. van gen Hassend, P. Görrn, T. Riedl, and H.-C. Scheer
(See online at https://doi.org/10.1007/s00339-022-05521-0)
