Thermal Transport in Engineered Hybrid Organic–Inorganic Perovskite Metasurfaces

Halide perovskites have recently gained widespread attention for their exceptional optoelectronic properties which have been illuminated by extensive spectroscopic investigations. In this article, nanophotonic surface-engineering using soft-lithography has been used to reproduce nanostructures with enhanced functionalities. A non-invasive optical technique based on Raman and photoluminescence (PL) spectroscopy is employed to investigate the interactive effect of the thermal and optical behaviour in surface-patterned hybrid organic-inorganic halide perovskite thin films. The thermophysical properties of the engineered perovskite films are extracted from the softening of the representative peak positions in the Raman and PL spectra of the samples which act as temperature markers. The investigation suggests a comparatively higher rise in the local temperature for the patterned thin films resulting from their enhanced absorption. Therefore, a cross-talk between the opto-thermal transport phenomena in imprinted perovskite thin films pertaining to both enhancing device properties along with maintaining device stability is established. The hybrid organic-inorganic halide perovskite CH3NH3PbI3 (MAPbI) with an ABX3 architecture has recently been demonstrated as successful optical and optoelectronic material due to its direct band gap, long carrier lifetimes, high electronic and hole mobilities and diffusion lengths [7, 8]. The low cost and facile fabrication techniques of MAPbI thin films in solution form are advantageous in terms of their large scale industrial generation [9-11]. Nano-patterning of the perovskite thin films have been shown to improve the performance in solar cells in terms of their efficiency through reduced surface reflectance, tailored light trapping, and higher external radiative efficiency and directionality [12-14]. In light emission applications, nanotexturing increases the out-coupling efficiency and imparts new features such as directionality and spectral tenability [15]. From the earliest techniques of fabrication by etching of the substrate and subsequent conformal coating to direct patterning of the perovskite film, researchers have demonstrated different approaches to create patterned perovskite metasurfaces. [16-19] Focused ion beam milling has been used to construct textures over small areas [20] but a scalable technique that takes advantage of the solution-based processing of the perovskite is more desirable. Toward this goal, nanoimprint lithography has been applied to pattern perovskite films through heated and pressurized contact with inflexible stamps. [21-23] Direct patterning of solution-deposited hybrid perovskites using flexible polydimethylsiloxane (PDMS) stamps was also demonstrated. [24-26] Unlike a silicon stamp, the PDMS can imprint its textures conformally on non-uniform surfaces and over defects. [27] The process is also recyclable enabling the usage of a single master to reproduce the same pattern on different 2 PDMS stamps ensuring the method to be cost effective. To ensure optimal optoelectronic functionalities in the visible region, the feature size on the stamp must be pushed to the nano-regime, which however requires attention to the integrity of the stamp. Features as small as a few nanometers have been replicated in molded polymers such as polyurethane and in silica sol–gel films. [27-29] Imprinting nanoscale features into crystalline materials, such as crystalline polymers, however, is far more challenging because interfaces with the stamp and substrate dramatically influence the crystallization process. [30] These nanophotonic textures can potentially be used to enhance the efficiency of perovskite solar cells, direct the emission of light emitting diodes, or generate perovskite distributed feedback lasers (DFB) lasers. Such imprinting techniques are particularly appropriate for halide perovskites because their sensitivity to water and plasma processing makes them incompatible with conventional lithography and etching. In order to develop high quality nanoscale imprinting processes, it is essential to control the crystallization dynamics of the perovskite materials. Lead halide perovskites have been known to be particularly sensitive to exposures to air, humidity and heat. Almost at the same time, many researchers found the instability of perovskites despite their excellent electronic properties. [31, 32] As far as we are concerned, perovskites are found to be sensitive to moisture, oxygen, UV light, light soaking, heat, electric field, and other potential factors. [32-35] The thermal stability and light soaking are the most challenging concerns affecting PSCs stability, since it is hard to avoid temperature increase and light illumination for solar cells during operation. One of the contributing factors for the heat sensitivity of the perovskites is the ultralow thermal conductivity of the material. The low thermal conductivity (0.2-0.5 WKm) is a result of the dynamics of the organic (MAI) component of the perovskite arising from the rotations of the MA cations as well as the overall tetragonalto-cubic phase transformations [36-41]. The thermal conductivity in MAPbI originates from phonon transport resulting from the anharmonically coupled interaction between the organic cation and inorganic cage. The Raman spectra of MAPbI perovskites have been studied in details by many researchers [42-48]. MAPbI perovskite structure feature a significant dynamical disorder which is aptly visualised through lattice vibrations (phonons) and molecular rotations [49-51]. Whereas electron-phonon interactions have a substantial influence on the solar cell operations, the phononphonon scattering process is directly related to the thermal properties of the perovskite [52-54]. The low thermal conductivity in perovskites are directly related to the phonon lifetimes. The phonon lifetime is essential as it provides critical insights into the different electron-phonon and phononphonon couplings occurring in the material [55]. Ab-initio molecular dynamics (AIMD) simulations along with third order perturbation theory have predicted very short phonon lifetimes ~ 20 ps and ~ 1 ps at the Brillouin zone center and Brillouin zone boundaries respectively, consistent with the findings from experimental inelastic scattering processes with X-rays and neutrons [56-59]. Such a short phonon lifetime means that the phonons do not efficiently dissipate heat and can therefore affect the carrier relaxation and phononphonon scattering processes. The inefficient dissipation of heat can result in localised increase in temperature accelerating the destabilisation process for the halide perovskites. As the lattice dynamics of MAPbI perovskites can provide an idea about the coordination environment as well as thermal properties of the material [56, 60-61] one of the easiest and non-destructive ways to probe its optical as well as thermal properties simultaneously is by using the Raman and PL spectroscopic techniques [62-64]. The laser beam effectively serves as: (i) a source of photons which are scattered inelastically by the material, and (ii) a heat source which increases the local temperature of the material at the point of incidence through absorption. The increase in the local temperature depends on the thermal conductivity of the material, therefore temperature and excitation power dependent optical measurements enable us to map the thermal properties of the materials to the lattice vibrations. In this paper, we use Raman spectroscopy to demonstrate the cross-talk