A new publication from Opto-Electronic Science; DOI 10.29026/oes.2022.210005 overviews the crucial charge carrier transfer processes and dynamics within perovskite active layers by means of time-resolved ultrafast laser spectroscopy.
This article provides an overview of how the charge carrier dynamics vary with respect to the crystalline phase of the organic-inorganic perovskite. Despite that organic-inorganic lead halide perovskites have attracted enormous scientific attention for energy conversion applications over the recent years, the influence of temperature and the type of the employed hole transport layer (HTL) on the charge carrier dynamics and recombination processes in perovskite photovoltaic devices is still largely unexplored. In particular, significant knowledge is missing on how these crucial parameters for radiative and non-radiative recombinations, as well as for efficient charge extraction vary among different perovskite crystalline phases that are induced by temperature variation.
The present article presents micro photoluminescence (μPL) and ultrafast time resolved transient absorption spectroscopy (TAS) results in a reference Glass/Perovskite architecture and two different Glass/ITO/HTL/Perovskite configurations at temperatures below room temperature. The objective of this work is to probe and shed light on the charge carrier dynamics of different perovskite crystalline phases, while considering also the effect of the employed hole transport layer (HTL) polymer. Namely, CH3NH3PbI3 films were deposited on Glass, PEDOT:PSS and PTAA polymers, and the developed Glass/CH3NH3PbI3 and Glass/ITO/HTL/CH3NH3PbI3 architectures were studied from 85 up to 215 K in order to explore the charge extraction dynamics of the CH3NH3PbI3 orthorhombic and tetragonal crystalline phases. Interestingly enough, the article reports evidence that the charge carrier dynamics at low temperatures, are not only affected by the employed hole transport layer, but in addition are strongly correlated to the different perovskite crystal phases.
In particular, μPL spectroscopy reveals an unusual blueshift of the bandgap with temperature, which is discord of the Varshni behavior of the typical semiconductor, below 120 K for the orthorhombic phase of the perovskite and the dual emission at temperature below of 100 K. Moreover, in the three studied temperatures by means of TAS, at 85 K (orthorhombic phase), at 120 K (coexistence of the orthorhombic and the tetragonal phase) for each peak, and at 180 K (tetragonal phase) the Glass/ITO/PTAA/CH3NH3PbI3architecture exhibits faster hole injection from the perovksite layer to the HTL and slower recombination rates (k2) when compared with the Glass/ITO/PEDOT:PSS/CH3NH3PbI3 configuration. It is discussed within the article that this is due to the better crystalline quality of the perovskite film when it is grown on the PTAA polymer. Furthermore, as the temperature increases for each perovskite crystal phase (orthorhombic and tetragonal), the τ2 time components and k2 bimolecular recombination rate decrease, for both configurations. Thus, it was found that the charge carrier dynamics at low temperatures, are not only affected by the employed hole transport layer, as we have already shown for the room temperature measurements but are strongly correlated to the different perovskite crystal phases.
Based on the above, it is believed that the new insights of this article pave the way towards the design of more efficient and stable PSCs, particularly for low temperature applications, such as the use of PSCs in satellites or in space stations. More importantly, this articles demonstrates the validity of TAS as a figure of merit technique on shedding light on the physical origins and mechanisms within and from the perovskite active layers, that are the key component of PSC devices.
The Ultrafast Laser Micro- and Nano-Processing (ULMNP) laboratory of Prof. E. Stratakis at the Institute of Electronic Structure and Laser (IESL) of the Foundation for Research and Technology - Hellas (FORTH) focuses, among other topics, on the application of lasers in photovoltaic and thermoelectric materials and devices. We have recently start working on the development of novel PSCs upon following laser assisted crystallization techniques, as well as, by employing different types of hole transport layer polymers, aiming to enhance photovoltaic performance while optimizing device stability. Notably, the TAS setup in ULMNP Laboratory allows the performance of measurements under dry ambient, as well as in-situ probing at variable temperatures. Our continuous mission and vision are to provide research facilities for state-of-the-art research in the multidisciplinary field of nanotechnology and advanced electronics for next-generation energy scavenging materials and devices. More information and related publications on these topics may be found on the webpages of ULMNP laboratory, under the title “Applications of lasers in photovoltaic and thermoelectric devices”.
Article reference Serpetzoglou E, Konidakis I, Kourmoulakis G, Demeridou I, Chatzimanolis K et al. Charge carrier dynamics in different crystal phases of CH3NH3PbI3 perovskite. Opto-Electron Sci 1, 210005 (2022). doi: 10.29026/oes.2022.210005
Keywords: transient absorption spectroscopy / μ-photoluminescence / variable temperature / perovskite crystalline phases / hole transport layer / charge carrier dynamics
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In the Ultrafast Laser Micro- and Nano- processing group (ULMNP) of IESL research is focused on the development of novel ultrafast pulsed laser processing schemes for controlled biomimetic structuring at micro- and nano- scales of a variety of materials, including biopolymers. By applying ultrafast laser pulses novel surface structures with sub-micron sized features are produced while the physical properties of semiconductor, dielectric and metallic surfaces are significantly modified. The biomimetic surfaces developed exhibit controlled dual-scale morphology, that mimics the hierarchical structuring of natural surfaces with exciting properties (i.e. the Lotus leaf, the Shark Skin, the Butterfly wings). As a result, the biomimetic morphology attained gives rise to notable multifunctional properties including water repellence, self-cleaning, antibacterial, anti-sticking, anti-fogging, anti-reflection and combination of those (b) smart, i.e show the ability to change their functionality in response to different external stimuli. The ability to tailor the morphology and chemistry is an important advantage for the use of the biomimetic structures as models to study the dependence of growth, division and differentiation of cells on the surface energy of the culture substrate, as well as 3D scaffolds for tissue regeneration. At the same time, novel ultrafast non-linear imaging tools are employed to characterize the biological processes taking place during the development of tissue into 3D scaffolds. At the same time, ULMNP focuses on the ultrafast laser-based development of various types of nanomaterials, nanolayers and processes applied in photovoltaic, gas sensing and energy storage applications. The exploitation of ultrashort pulses for the doping, functionalization, spectroscopic diagnosis and quality control of graphene and other 2D materials is additionally explored, placing emphasis on the understanding of the fundamental physical properties of such materials.
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