Recent Submissions

  • Electropolymerization of robust conjugated microporous polymer membranes for rapid solvent transport and narrow molecular sieving

    Zhou, Zongyao; Li, Xiang; Guo, Dong; Shinde, Digambar; Lu, Dongwei; Chen, Long; Liu, Xiaowei; Cao, Li; Aboalsaud, Ammar M.; Hu, Yunxia; Lai, Zhiping (Nature Communications, Springer Science and Business Media LLC, 2020-10-21) [Article]
    Abstract Pore size uniformity is one of the most critical parameters in determining membrane separation performance. Recently, a novel type of conjugated microporous polymers (CMPs) has shown uniform pore size and high porosity. However, their brittle nature has prevented them from preparing robust membranes. Inspired by the skin-core architecture of spider silk that offers both high strength and high ductility, herein we report an electropolymerization process to prepare a CMP membrane from a rigid carbazole monomer, 2,2’,7,7’-tetra(carbazol-9-yl)-9,9’-spirobifluorene, inside a robust carbon nanotube scaffold. The obtained membranes showed superior mechanical strength and ductility, high surface area, and uniform pore size of approximately 1 nm. The superfast solvent transport and excellent molecular sieving well surpass the performance of most reported polymer membranes. Our method makes it possible to use rigid CMPs membranes in pressure-driven membrane processes, providing potential applications for this important category of polymer materials.
  • Spin transport in multilayer graphene away from the charge neutrality point

    He, Xin; Wen, Yan; Zhang, Chenhui; Li, Peng; Zheng, Dongxing; Chen, Aitian; Manchon, Aurelien; Zhang, Xixiang (Carbon, Elsevier BV, 2020-10-17) [Article]
    Graphene is considered as a promising material in spintronics due to its long spin relaxation time and long spin relaxation length. However, its spin transport properties have been studied at low carrier density only, beyond which much is still unknown. In this study, we explore the spin transport and spin precession properties in multilayer graphene at high carrier density using ionic liquid gating. We find that the spin relaxation time is directly proportional to the momentum relaxation time, indicating that the Elliott-Yafet mechanism still dominates the spin relaxation in multilayer graphene away from the charge neutrality point.
  • Metagrating-Based Terahertz Polarization Beam Splitter Designed by Simplified Modal Method

    Ma, Xinyu; Li, Yanfeng; Lu, Yongchang; Han, Jiaguang; Zhang, Xixiang; Zhang, Weili (Frontiers in Physics, Frontiers Media SA, 2020-10-15) [Article]
    Terahertz waves are finding important applications in diverse fields, and meanwhile the manipulation of terahertz waves calls for the development of various functional devices. Here, we have designed and fabricated a metagrating-based polarization beam splitter for terahertz waves using the simplified modal method. By only considering two propagation modes and treating the grating as a Mach-Zehnder interferometer, the method can greatly simplify the reverse grating design process. The parameters of the grating are first obtained under the guidance of the simplified modal method and then improved upon by the finite element method. The fabricated device is finally experimentally demonstrated with a terahertz time-domain spectroscopy system. The diffraction efficiencies of the polarization beam splitter at 0.9 THz are measured to be 69 and 63% for TE and TM waves relative to that of a silicon plate, respectively. The corresponding extinction ratios are 12 and 17 dB for TE and TM waves, respectively. The experiment results agree well with the simulations.
  • Suppressing Co-Crystallization of Halogenated Non-Fullerene Acceptors for Thermally Stable Ternary Solar Cells

    Hultmark, Sandra; Paleti, Sri Harish Kumar; Harillo, Albert; Marina, Sara; Nugroho, Ferry Anggoro Ardy; Liu, Yanfeng; Ericsson, Leif K. E.; Li, Ruipeng; Martín, Jaime; Bergqvist, Jonas; Langhammer, Christoph; Zhang, Fengling; Yu, Liyang; Campoy-Quiles, Mariano; Moons, Ellen; Baran, Derya; Müller, Christian (Advanced Functional Materials, Wiley, 2020-10-14) [Article]
    While photovoltaic blends based on non-fullerene acceptors are touted for their thermal stability, this type of acceptor tends to crystallize, which can result in a gradual decrease in photovoltaic performance and affects the reproducibility of the devices. Two halogenated indacenodithienothiophene-based acceptors that readily co-crystallize upon mixing are studied, which indicates that the use of an acceptor mixture alone does not guarantee the formation of a disordered mixture. The addition of the donor polymer to the acceptor mixture readily suppresses the crystallization, which results in a fine-grained ternary blend with nanometer-sized domains that do not coarsen due to a high Tg ≈ 200 °C. As a result, annealing at temperatures of up to 170 °C does not markedly affect the photovoltaic performance of ternary devices, in contrast to binary devices that suffer from acceptor crystallization in the active layer. The results indicate that the ternary approach enables the use of high-temperature processing protocols, which are needed for upscaling and high-throughput fabrication of organic solar cells. Further, ternary devices display a stable photovoltaic performance at 130 °C for at least 205 h, which indicates that the use of acceptor mixtures allows to fabricate devices with excellent thermal stability.
  • Beryllene: A Promising Anode Material for Na- and K-Ion Batteries with Ultrafast Charge/Discharge and High Specific Capacity

    Sun, Minglei; Yan, Yuan; Schwingenschlögl, Udo (The Journal of Physical Chemistry Letters, American Chemical Society (ACS), 2020-10-12) [Article]
    We predict two-dimensional Be materials, α- and β-beryllene. In α-beryllene each Be atom binds to six other Be atoms in a planar scheme, whereas β-beryllene consists of two stacked α-beryllene monolayers. Both α- and β-beryllene are found to be highly stable, as demonstrated by high cohesive energies close to that of bulk Be, an absence of imaginary phonon modes, and high melting points. Both materials are metallic, indicating potential applications in Na-ion and K-ion batteries, which are explored in detail. The diffusion barriers of Na (K) on α- and β-beryllene are found to be only 9 (3) and 4 (5) meV, respectively. In particular, the diffusion barrier of K on α-beryllene exhibits the lowest ever recorded value in two-dimensional materials, suggesting the possibility of ultrafast charge/discharge. As the theoretical specific capacities of Na/K on α- and β-beryllene are found to be 1487/1322 and 743/743 mA h g–1, respectively, the storage capacity is ultrahigh.
  • Metal Halide Perovskites for High-Energy Radiation Detection

    Kakavelakis, George; Gedda, Murali; Panagiotopoulos, Apostolis; Kymakis, Emmanuel; Anthopoulos, Thomas D.; Petridis, Konstantinos (Advanced Science, Wiley, 2020-10-12) [Article]
    Metal halide perovskites (MHPs) have emerged as a frontrunner semiconductor technology for application in third generation photovoltaics while simultaneously making significant strides in other areas of optoelectronics. Photodetectors are one of the latest additions in an expanding list of applications of this fascinating family of materials. The extensive range of possible inorganic and hybrid perovskites coupled with their processing versatility and ability to convert external stimuli into easily measurable optical/electrical signals makes them an auspicious sensing element even for the high-energy domain of the electromagnetic spectrum. Key to this is the ability of MHPs to accommodate heavy elements while being able to form large, high-quality crystals and polycrystalline layers, making them one of the most promising emerging X-ray and γ-ray detector technologies. Here, the fundamental principles of high-energy radiation detection are reviewed with emphasis on recent progress in the emerging and fascinating field of metal halide perovskite-based X-ray and γ-ray detectors. The review starts with a discussion of the basic principles of high-energy radiation detection with focus on key performance metrics followed by a comprehensive summary of the recent progress in the field of perovskite-based detectors. The article concludes with a discussion of the remaining challenges and future perspectives.
  • The elemental analysis and multi-nuclear NMR study of an alkali molten salt used to digest reference and commercial SWCNT powders

    Simoes, Filipa R. F.; Abou-Hamad, Edy; Kamenik, Jan; Kučera, Jan; Da Costa, Pedro M. F. J. (Journal of Analytical Atomic Spectrometry, Royal Society of Chemistry (RSC), 2020-10-09) [Article]
    For quite some time, alkaline oxidation (or fusion) has been used to solubilize refractory materials and mineral ores. Recently, its application scope was extended to facilitate batch-scale elemental analysis of nanomaterials such as carbon nanotubes. Here, a sodium tetraborate salt was used to digest four different types of single-walled carbon nanotubes. These samples were produced employing Co–Mo or Fe catalyst systems. Their graphitic matrix was exposed to different melt temperatures for a short period of time, following which the concentration of six transition metals was measured. Recoveries in excess of 80% were obtained, with the melt temperature affecting more the elemental extraction in Fe-catalyzed nanotubes. Together with previous results, the work described allows drawing pertinent conclusions on the advantages and limitations of alkaline oxidation as an alternative sample digestion approach for the routine chemical analysis of nanocarbons.
  • Cyclized polyacrylonitrile anode for alkali metal ion batteries

    Zhang, Wenli; Sun, Minglei; Yin, Jian; Abou-Hamad, Edy; Schwingenschlögl, Udo; Da Costa, Pedro M. F. J.; Alshareef, Husam N. (Angewandte Chemie International Edition, Wiley, 2020-10-05) [Article]
    Alkali metal (Li, Na, and K) ion batteries are vital in portable and large-scale stationary energy storage. Recently, organic anodes have attracted increasing attention for alkali metal ion batteries due to their chemical diversity and potential high capacity. In this work, we discovered that cyclized polyacrylonitrile (cPAN) can serve as a superior anode for alkali metal ion batteries. Remarkably, upon activation cycling, as an anode of lithium-ion battery, cPAN exhibits a reversible capacity as high as 1238 mAh g-1 under a current density of 50 mA g-1. Based on electrochemical experiments and first-principles calculations, it is demonstrated that the hexagonal carbon ring, piperidine ring, and pyridine nitrogen in ladder cPAN are the main active sites for lithium-ion storage. In addition, we show that cPAN displays a unique potential-dependent solid electrolyte interphase formation from 0.1 to 0.01 V vs. Li/Li+. Furthermore, cPAN displays decent performance as an anode in SIBs and PIBs. The discovery of cPAN anode could pave the way for the future development of organic anodes for alkali metal ion batteries.
  • Cyclized polyacrylonitrile anode for alkali metal ion batteries

    Zhang, Wenli; Sun, Minglei; Yin, Jian; Abou-Hamad, Edy; Schwingenschlögl, Udo; Da Costa, Pedro M. F. J.; Alshareef, Husam N. (Angewandte Chemie International Edition, Wiley, 2020-10-05) [Article]
    Alkali metal (Li, Na, and K) ion batteries are vital in portable and large-scale stationary energy storage. Recently, organic anodes have attracted increasing attention for alkali metal ion batteries due to their chemical diversity and potential high capacity. In this work, we discovered that cyclized polyacrylonitrile (cPAN) can serve as a superior anode for alkali metal ion batteries. Remarkably, upon activation cycling, as an anode of lithium-ion battery, cPAN exhibits a reversible capacity as high as 1238 mAh g-1 under a current density of 50 mA g-1. Based on electrochemical experiments and first-principles calculations, it is demonstrated that the hexagonal carbon ring, piperidine ring, and pyridine nitrogen in ladder cPAN are the main active sites for lithium-ion storage. In addition, we show that cPAN displays a unique potential-dependent solid electrolyte interphase formation from 0.1 to 0.01 V vs. Li/Li+. Furthermore, cPAN displays decent performance as an anode in SIBs and PIBs. The discovery of cPAN anode could pave the way for the future development of organic anodes for alkali metal ion batteries.
  • Quantum Confinement and Thickness-Dependent Electron Transport in Solution-Processed In 2 O 3 Transistors

    Isakov, Ivan; Faber, Hendrik; Mottram, Alexander D.; Das, Satyajit; Grell, Max; Regoutz, Anna; Kilmurray, Rebecca; McLachlan, Martyn A.; Payne, David J.; Anthopoulos, Thomas D. (Advanced Electronic Materials, Wiley, 2020-10-05) [Article]
    The dependence of charge carrier mobility on semiconductor channel thickness in field-effect transistors is a universal phenomenon that has been studied extensively for various families of materials. Surprisingly, analogous studies involving metal oxide semiconductors are relatively scarce. Here, spray-deposited In2O3 layers are employed as the model semiconductor system to study the impact of layer thickness on quantum confinement and electron transport along the transistor channel. The results reveal an exponential increase of the in-plane electron mobility (µe) with increasing In2O3 thickness up to ≈10 nm, beyond which it plateaus at a maximum value of ≈35 cm2 V−1 s−1. Optical spectroscopy measurements performed on In2O3 layers reveal the emergence of quantum confinement for thickness <10 nm, which coincides with the thickness that µe starts deteriorating. By combining two- and four-probe field-effect mobility measurements with high-resolution atomic force microscopy, it is shown that the reduction in µe is attributed primarily to surface scattering. The study provides important guidelines for the design of next generation metal oxide thin-film transistors.
  • Identifying Carrier Behavior in Ultrathin Indirect-Bandgap CsPbX3 Nanocrystal Films for Use in UV/Visible-Blind High-Energy Detectors

    Xin, Bin; Alaal, Naresh; Mitra, Somak; Subahi, Ahmad; Pak, Yusin; Almalawi, Dhaifallah; Alwadai, Norah M.; Lopatin, Sergei; Roqan, Iman S. (Small, Wiley, 2020-10-02) [Article]
    High-energy radiation detectors such as X-ray detectors with low light photoresponse characteristics are used for several applications including, space, medical, and military devices. Here, an indirect bandgap inorganic perovskite-based X-ray detector is reported. The indirect bandgap nature of perovskite materials is revealed through optical characterizations, time-resolved photoluminescence (TRPL), and theoretical simulations, demonstrating that the differences in temperature-dependent carrier lifetime related to CsPbX3 (X = Br, I) perovskite composition are due to the changes in the bandgap structure. TRPL, theoretical analyses, and X-ray radiation measurements reveal that the high response of the UV/visible-blind yellow-phase CsPbI3 under high-energy X-ray exposure is attributed to the nature of the indirect bandgap structure of CsPbX3. The yellow-phase CsPbI3-based X-ray detector achieves a relatively high sensitivity of 83.6 μCGyair−1 cm−2 (under 1.7 mGyair s−1 at an electron field of 0.17 V μm−1 used for medical diagnostics) although the active layer is based solely on an ultrathin (≈6.6 μm) CsPbI3 nanocrystal film, exceeding the values obtained for commercial X-ray detectors, and further confirming good material quality. This CsPbX3 X-ray detector is sufficient for cost-effective device miniaturization based on a simple design.
  • Bending strain tailored exchange bias in epitaxial NiMn/γ′-Fe4N bilayers

    Shi, Xiaohui; Mi, Wenbo; Zhang, Qiang; Zhang, Xixiang (Applied Physics Letters, AIP Publishing, 2020-09-28) [Article]
    The strain tunable exchange bias has attracted much attention due to its practical applications in flexible and wearable spintronic devices. Here, the flexible epitaxial NiMn/c0-Fe4N bilayers are deposited by facing-target reactive sputtering. The maximum strain-induced change ratios of exchange bias field HEB and coercivity HC (jDHEB/HEBj and jDHC/HCj) are 51% and 22%, respectively. A large strain-induced jDHEB/HEBj appears in a thicker ferromagnetic layer, but a large jDHC/HCj) appears in a thinner ferromagnetic layer. At a compressive strain, the antiferromagnetic anisotropy of the tetragonal NiMn layer increases, resulting in an increased HC of NiMn/c0-Fe4N bilayers. The bending-strain induced changes of anisotropy magnetoresistance and planar Hall resistance are also observed at low magnetic fields. The bending-strain tailored magnetic properties can be ascribed to the distributions of ferromagnetic and antiferromagnetic anisotropies.
  • Electrochemical multi-analyte point-of-care perspiration sensors using on-chip three-dimensional graphene electrodes

    Bauer, Meike; Wunderlich, Lukas; Weinzierl, Florian; Lei, Yongjiu; Duerkop, Axel; Alshareef, Husam N.; Baeumner, Antje J. (Analytical and Bioanalytical Chemistry, Springer Science and Business Media LLC, 2020-09-28) [Article]
    Multi-analyte sensing using exclusively laser-induced graphene (LIG)-based planar electrode systems was developed for sweat analysis. LIG provides 3D structures of graphene, can be manufactured easier than any other carbon electrode also on large scale, and in form of electrodes: hence, it is predestinated for affordable, wearable point-of-care sensors. Here, it is demonstrated that LIG facilitates all three electrochemical sensing strategies (voltammetry, potentiometry, impedance) in a multi-analyte system for sweat analysis. A potentiometric potassium-ion-selective electrode in combination with an electrodeposited Ag/AgCl reference electrode (RE) enabled the detection of potassium ions in the entire physiologically relevant range (1 to 500 mM) with a fast response time, unaffected by the presence of main interfering ions and sweat-collecting materials. A kidney-shaped interdigitated LIG electrode enabled the determination of the overall electrolyte concentration by electrochemical impedance spectroscopy at a fixed frequency. Enzyme-based strategies with amperometric detection share a common RE and were realized with Prussian blue as electron mediator and biocompatible chitosan for enzyme immobilization and protection of the electrode. Using glucose and lactate oxidases, lower limits of detection of 13.7 ± 0.5 μM for glucose and 28 ± 3 μM for lactate were obtained, respectively. The sensor showed a good performance at different pH, with sweat-collecting tissues, on a model skin system and furthermore in synthetic sweat as well as in artificial tear fluid. Response time for each analytical cycle totals 75 s, and hence allows a quasi-continuous and simultaneous monitoring of all analytes. This multi-analyte all-LIG system is therefore a practical, versatile, and most simple strategy for point-of-care applications and has the potential to outcompete standard screen-printed electrodes. [Figure not available: see fulltext.].
  • All-Perovskite Tandem Solar Cells: A Roadmap to Uniting High Efficiency with High Stability

    Zheng, Xiaopeng; Alsalloum, Abdullah Yousef; Hou, Yi; Sargent, E.; Bakr, Osman (Accounts of Materials Research, American Chemical Society (ACS), 2020-09-25) [Article]
    Organic−inorganic halide perovskite photovoltaics (PVs)only a decade-old fieldhave reached impressive power conversion efficiencies (PCEs) and passed industrial stability requirements (IEC 61215:2016 Damp Heat and Humidity Freeze tests), solidifying their status among candidates for next generation PVs. Among the various perovskite PV technologies, all-perovskite tandem solar cells (PTSCs) are frontrunners for commercialization. PTSCs unite a narrow-bandgap (NBG; Eg ≈1.2 eV) perovskite back cell with a wide-bandgap (WBG; Eg ≈1.7−1.9 eV) perovskite front cell. Despite their nascency, PTSCs have achieved certified PCEs of 24.8% and 24.2% for small-area (0.049 cm2) and large-area devices (1.041 cm2), respectively. With further advances in materials development, PTSCs are capable of moving beyond the PCE limits of single-junction cells due to reduced thermalization losses and improved utilization of the solar spectrum. By contrast, the PCE of single-junction perovskite devices is already approaching its saturation level, which is already very close to the device’s Shockley−Queisser limit for a bandgap of around 1.55 eV. The tandem architecture, thus, provides the most viable path forward to further exploiting the potential of perovskite solar cells. However, PTSC technology faces a set of challenges distinct from those in perovskite single-junction devices because (i) NBG perovskitestypically achieved by Pb−Sn alloyingare prone to oxidation (Sn2+ to Sn4+), which results in a high density of Sn vacancies that degrade the optoelectronic performance of NBG perovskite films, (ii) practically complete photon absorption and charge extraction require thick, NBG perovskite films having long carrier diffusion lengths, and (iii) WBG perovskites with high Br/(I + Br) ratio experience large voltage losses and inferior light stability due to surface trap states and phase segregation. In this Account, we discuss how to manage these considerations and maximize the power output in PTSCs via light management. We then review strategies, including composition- and additive-engineering, defect passivation, and matching charge transport layers, for enhancing the carrier diffusion length of NBG perovskite cells and mitigating voltage losses in WBG perovskite cells. We also summarize the advances made in the fabrication of PTSCs on the device level, especially the evolution of tunnel recombination junctions and tandem device architectures. Finally, we highlight further research efforts needed to overcome roadblocks to commercialization (e.g., improving the environmental, thermal, and operating stability of these devices) and offer our perspective on the future development of this rapidly advancing field.
  • GaN and InGaN nanowires prepared by metal-assisted electroless etching: Experimental and theoretical studies

    Devi, Assa Aravindh Sasikala; Xin, Bin; Mitra, Somak; Roqan, Iman S.; Najar, Adel (Results in Physics, Elsevier BV, 2020-09-25) [Article]
    We investigate the optical and structural properties of GaN and InGaN nanowires (NWs) fabricated by metal-assisted electroless etching in a hydrofluoric acid (HF) solution. The emission spectra of GaN and InGaN NWs exhibit a red shift compared to the as-grown samples resulting from an increase in the surface-to-volume ratio and stress relaxation in these nanostructures. The carrier lifetimes of GaN and InGaN NWs were measured. In addition, density functional theory (DFT) investigations were carried out on GaN and InGaN NWs using the generalized gradient approximation (GGA), including the Hubbard U parameter. The presence of compressive stress in the NWs was confirmed by the DFT calculations, which indicated that it induces a change in the lattice parameter along the c-direction. Formation energy calculations showed that In is a much more stable dopant in the GaN NWs compared to the native point defects, such as Ga and N vacancies. Moreover, electronic structure analysis revealed that the complex defects formed by the presence of In along with vacancy defects shifts the valence band maximum, thus changing the conducting properties of the NWs.
  • How Humidity and Light Exposure Change the Photophysics of Metal Halide Perovskite Solar Cells

    Ugur, Esma; Alarousu, Erkki; Khan, Jafar Iqbal; Vlk, Aleš; Aydin, Erkan; de Bastiani, Michele; Albalawi, Ahmed; Gonzalez Lopez, Sandra P.; Ledinský, Martin; De Wolf, Stefaan; Laquai, Frédéric (Solar RRL, Wiley, 2020-09-24) [Article]
    Metal halide perovskites exhibit outstanding optical and electronic properties, but are very sensitive to humidity and light-soaking. In this work, the photophysics of perovskites that have been exposed to such conditions are studied and, in this context, the impact of excess lead iodide (PbI2) is revealed. For exposed samples, the formation of subbandgap states and increased trap-assisted recombination is observed, using highly sensitive absorption and time-resolved photoluminescence (TRPL) measurements, respectively. It appears that such exposure primarily affects the perovskite surface. Consequently, on n–i–p device level, the spiro-OMeTAD/perovskite interface is more rapidly affected than its buried electron-collecting interface. Moreover, both stoichiometric and nonstoichiometric MAPbI3-based solar cells show reduced device performance mainly due to voltage losses. Overall, this study brings forward key points to consider in engineering perovskite solar cells with improved performance and material stability.
  • Unveiling the Origin of Multi-Domain Structures in Compositionally Modulated Cylindrical Magnetic Nanowires

    Bran, Cristina; Fernandez-Roldan, Jose Angel; P. Del Real, Rafael; Asenjo, Agustina; Chen, Yu-Shen; Zhang, Junli; Zhang, Xixiang; Fraile Rodríguez, Arantxa; Foerster, Michael; Aballe, Lucia; Chubykalo-Fesenko, Oksana; Vazquez, Manuel (ACS Nano, American Chemical Society (ACS), 2020-09-24) [Article]
    CoNi/Ni multisegmented cylindrical nanowires were synthesized via electrochemical route. The wires are 140 nm in diameter, with 1000 nm long Ni segments, and CoNi segments between 600 and 1400 nm in length. The magnetic configuration was imaged by XMCD-PEEM in the demagnetized state and at remanence after magnetizing axially and perpendicularly. Ni segments, with cubic crystal symmetry, show an axial magnetic configuration with a small curling component at the surface. In turn, CoNi segments, with hexagonal crystal symmetry and a strong magnetocrystalline anisotropy perpendicular to the nanowires, show a single vortex state in the shorter segments and multi-vortex or multi-transverse magnetic configurations in medium and long segments, respectively. A detailed study by micromagnetic simulations reveals that the magnetic configuration is determined mainly by the coupling between soft Ni and harder CoNi segments. For short CoNi segments, Ni segments are magnetostatically coupled and the chirality of the single vortex formed in CoNi remains the same as that of the curling in neighbouring Ni segments. For longer CoNi segments, the remanent state is either multi-vortex or multi-transverse state depending on whether the previously applied field was parallel or perpendicular to the magnetocrystalline axis. The results point out the relevance of the cylindrical geometry to promote the occurrence of complex magneto-chiral effects and provide key information for the design of cylindrical magnetic nanowires for multiple applications.
  • Temperature-dependent optical constants of monolayer [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]: spectroscopic ellipsometry and first-principles calculations.

    Liu, Hsiang-Lin; Yang, Teng; Chen, Jyun-Han; Chen, Hsiao-Wen; Guo, Huaihong; Saito, Riichiro; Li, Ming-yang; Li, Lain-Jong (Scientific reports, Springer Science and Business Media LLC, 2020-09-18) [Article]
    The temperature-dependent ([Formula: see text]) optical constants of monolayer [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] were investigated through spectroscopic ellipsometry over the spectral range of 0.73-6.42 eV. At room temperature, the spectra of refractive index exhibited several anomalous dispersion features below 800 nm and approached a constant value of 3.5-4.0 in the near-infrared frequency range. With a decrease in temperature, the refractive indices decreased monotonically in the near-infrared region due to the temperature-dependent optical band gap. The thermo-optic coefficients at room temperature had values from [Formula: see text] to [Formula: see text] for monolayer transition metal dichalcogenides at a wavelength of 1200 nm below the optical band gap. The optical band gap increased with a decrease in temperature due to the suppression of electron-phonon interactions. On the basis of first-principles calculations, the observed optical excitations at 4.5 K were appropriately assigned. These results provide basic information for the technological development of monolayer transition metal dichalcogenides-based photonic devices at various temperatures.
  • Nanoporous GaN/n-type GaN: a cathode structure for ITO-free perovskite solar cells

    Lee, Kwangjae; Min, Jung-Wook; Turedi, Bekir; Alsalloum, Abdullah Yousef; Min, Jung-Hong; Kim, Yeong Jae; Yoo, Young Jin; Oh, Semi; Cho, Namchul; Subedi, Ram Chandra; Mitra, Somak; Yoon, Sang Eun; Kim, Jong Hyun; Park, Kwangwook; Chung, Tae-Hoon; Jung, Sung Hoon; Baek, Jong-Hyeob; Song, Young Min; Roqan, Iman S.; Ng, Tien Khee; Ooi, Boon S.; Bakr, Osman (ACS Energy Letters, American Chemical Society (ACS), 2020-09-17) [Article]
    Introducing suitable electron/hole transport layers and transparent conductive layers (TCLs) into perovskite solar cells (PSCs) is key to enhancing the selective extraction of charge carriers and reducing surface recombination losses. Here, we introduce nanoporous gallium nitride (NP GaN)/n-type GaN (n-GaN) as a dual-function cathode structure for PSCs, acting as both the TCL and the electron transport layer (ETL). We demonstrate that the hierarchical NP GaN structure provides an expanded interfacial contact area with the perovskite absorber, while the n-GaN under the NP GaN displays high transmittance in the visible spectrum as well as higher lateral electric conductivity than that of a conventional ITO film. Prototype MAPbI3 PSCs based on this NP GaN/n-GaN cathode structure (without an extra ETL) show a power conversion efficiency of up to 18.79%. The NP GaN/n-GaN platform demonstrated herein paves the way for PSCs to take advantage of the widely available heterostructures of mature III-nitride-based technologies.
  • Photophysical Processes in Lead Halide Perovskite Solar Cells Revealed by Ultrafast Spectroscopy

    Ugur, Esma (2020-09-16) [Dissertation]
    Advisor: Laquai, Frédéric
    Committee members: De Wolf, Stefaan; Ooi, Boon S.; Albrecht, Steve
    Metal halide perovskites have emerged as photoactive materials in solution-processed devices thanks to their unique properties such as high absorption coefficient, sharp absorption edge, long carrier diffusion lengths, and tunable bandgap, together with ease of fabrication. The single-junction perovskite solar cells have reached power conversion efficiencies of more than 25%. Although the efficiency of perovskite devices has increased tremendously in a very short time, the efficiency is still limited by carrier recombination at defects and interfaces. Thus, understanding these losses and how to reduce them is the way forward towards the Shockley-Queisser limit. This thesis aims to apply ultrafast optical spectroscopy techniques to investigate the recombination pathways in halide perovskites, and understand the charge extraction from perovskite to transport layers and nonradiative losses at the interface. The first part focuses on perovskite solar cells with planar n–i–p device architecture which offers significant advantages in terms of large scale processing, the potential use of flexible substrates, and applicability to tandems. In addition to the optimization of MAPbI3 solar cell fabrication using a modified sequential interdiffusion protocol, the photophysics of perovskites exposed to humid air and illumination are discussed. The MAPbI3 film processed with the addition of glycol ethers to the methylammonium iodide solution results in the control of PbI2 to perovskite conversion dynamics, thus enhanced morphology and crystallinity. For samples exposed to humid air and illumination, the formation of sub-bandgap states and increased trap-assisted recombination are observed, using highly-sensitive absorption and time-resolved photoluminescence measurements, respectively. It appears that such exposure primarily affects the perovskite surface. The second part discusses the hole extraction from Cs0.07Rb0.03FA0.765MA0.135PbI2.55Br0.45 to the polymeric hole transport layer and interfacial recombination using ultrafast transient absorption spectroscopy technique. To illustrate this, PDPP-3T was used as HTL, since its ground state absorption is red-shifted compared to the perovskite’s photobleach, thereby allowing direct probing of the interfacial hole extraction and recombination. Moreover, carrier diffusion is investigated by varying the perovskite film thickness, and carrier mobility is found to be 39 cm2V-1s-1. Finally, hole extraction is found to be one order of magnitude faster than the recombination at the interface.

View more