Neda Pourdavoud

Impact of Film Stoichiometry on the Ionization Energy and Electronic Structure of CH3 NH3 PbI3 Perovskites.

The electronic structure of a large sample set of CH3 NH3 PbI3 -based perovskites is studied. Combined investigations by UV/X-ray photoelectron spectroscopy and X-ray diffraction reveal that interstitials present in the film lead to changes in the occupied density of states close to the valence band, which in turn influences the performance of solar cells. Changes in elemental composition tune the ionization energy of the perovskite film by almost 1 eV without introducing significant amounts of gap states.

Photonic Nanostructures Patterned by Thermal Nanoimprint Directly into Organo-Metal Halide Perovskites

Photonic nanostructures are created in organo-metal halide perovskites by thermal nanoimprint lithography at a temperature of 100 °C. The imprinted layers are significantly smoothened compared to the initially rough, polycrystalline layers and the impact of surface defects is substantially mitigated upon imprint. As a case study, 2D photonic crystals are shown to afford lasing with ultralow lasing thresholds at room temperature.

Indium-Free Perovskite Solar Cells Enabled by Impermeable Tin-Oxide Electron Extraction Layers

Corrosive precursors used for the preparation of organic-inorganic hybrid perovskite photoactive layers prevent the application of ultrathin metal layers as semitransparent bottom electrodes in perovskite solar cells (PVSCs). This study introduces tin-oxide (SnOx ) grown by atomic layer deposition (ALD), whose outstanding permeation barrier properties enable the design of an indium-tin-oxide (ITO)-free semitransparent bottom electrode (SnOx /Ag or Cu/SnOx ), in which the metal is efficiently protected against corrosion. Simultaneously, SnOx functions as an electron extraction layer. We unravel the spontaneous formation of a PbI2 interfacial layer between SnOx and the CH3 NH3 PbI3 perovskite. An interface dipole between SnOx and this PbI2 layer is found, which depends on the oxidant (water, ozone, or oxygen plasma) used for the ALD growth of SnOx . An electron extraction barrier between perovskite and PbI2 is identified, which is the lowest in devices based on SnOx grown with ozone. The resulting PVSCs are hysteresis-free with a stable power conversion efficiency (PCE) of 15.3% and a remarkably high open circuit voltage of 1.17 V. The ITO-free analogues still achieve a high PCE of 11%.

Thermal nanoimprint to improve the morphology of MAPbX3 (MA = methylammonium, X = I or Br)

Perovskites have high potential for future electronic devices, in particular, in the field of opto-electronics. However, the electronic and optic properties of these materials highly depend on the morphology and thus on the preparation; in particular, highly crystalline layers with large crystals and without pinholes are required. Here, nanoimprint is used to improve the morphology of such layers in a thermal imprint step. Two types of material are investigated, MAPbI3 and MAPbBr3, with MA being methylammonium, CH3NH3+. The perovskite layers are prepared from solution, and the crystal size of the domains is substantially increased by imprinting them at temperatures of 100–150 °C. Although imprint is performed under atmospheric conditions which, in general, enhances the degradation, the stamp that covers the layer under elevated temperature is able to protect the perovskite largely from decomposition. Comparing imprinting experiments with pure annealing at a similar temperature and time proves this. Furthe...

Impact of body resistance on RF linearity of SOI MOSFET circuits

Body resistances (RB) of body-contacted (BC) partially-depleted (PD) SOI MOSFETs are adjusted by a proper layout structure. The transition in the output conductance frequency response due to the finite body resistance was eliminated. Transition-free (TF) curves for 45 nm SOI MOSFET at various biasing points were extracted. The TF concept was used to design a 2.4 GHz cascode low noise amplifier. Mixed-mode transient device and circuit simulation was performed to obtain the output signal spectrum. Simulation results of TF SOI LNA showed 6 dB and 9 dB reduction in HD3 and THD at Vin=0.1 mV, respectively, comparing with the conventional BC SOI circuit. Third order intercept point (IP3) was improved by 3 dB at Vin=0.1 mV in TF SOI LNA. Simulation results confirmed the linearity advantage of RF SOI circuit based on the transition-free operating regime.

Photonic nanopatterns in organo-metal halide perovskites by thermal nanoimprint lithography (Conference Presentation)

The recently re-discovered class of organometal-halide perovskites hold great promise for solar cells, LEDs and lasers.[1] Today, their potential has not been fully unlocked partially because of the lack of suitable nano-patterning techniques, which are mandatory to create resonator structures, waveguides etc. with a maximum level of precision directly into perovskite layers. Their chemical and thermal instability prevents the use of established wet-chemical patterning techniques.[2] In contrast to conventional inorganic semiconductors, crystal binding in these perovskites includes significant contributions of van der Waals interactions among the halide atoms and Hydrogen bonding.[3] The formation enthalpy per unit cell is only about 0.1eV in MAPbI3.[4] Here, we take advantage of the “soft-matter properties” of organo-metal halide perovskites and demonstrate that photonic nano-structures can be prepared by direct thermal nano-imprint lithography in MAPbI3 and MAPbBr3 at relatively low temperatures (<150°C). The resulting periodic patterns provide distributed feedback resonators, which afford lasing in MAPbI3 with ultra-low threshold levels on the order of 1 μJ/cm2.[5] Moreover, our results also state the first DFB lasers based on MAPbBr3. We will discuss the applicability of thermal imprinting for perovskite solar cells and LEDs. [1] B. R. Sutherland et al. Nat Photon 2016, 10, 295. [2] D. Lyashenko et al. physica status solidi (a) 2017, 214, 10.1002/pssa.201600302. [3] D. A. Egger et al. Journal of Physical Chemistry Letters 2014, 5, 2728. [4] A. Buin et al. Nano Lett 2014, 14, 6281. [5] N. Pourdavoud et al. Adv Mater 2017, 10.1002/adma.201605003.

Pushing the lifetime of perovskite solar cell beyond 4500 h by the use of impermeable tin oxide electron extraction layers (Conference Presentation)

Perovskite solar cells (PSCs) suffer from decomposition of the active material in the presence of moisture or heat. In addition, the corrosion of metal electrodes due to halide species needs to be overcome.[1,2] Here, we introduce ALD-grown tin oxide (SnOx) as impermeable electron extraction layer (EEL), which affords air resilient and temperature stable MAPbI3 PSCs. Being conductive, SnOx is positioned between the metal electrode and the perovskite. Its outstanding permeation barrier properties protect the perovskite against ingress of moisture or migrating metal atoms, while simultaneously the metal electrode is protected against leaking halide compounds.[2] Therefore, SnOx is also excellently suited to sandwich and protect ultra-thin metal layers (Ag or Cu) as cost efficient Indium-free semitransparent electrodes (SnOx/metal/SnOx) in PSCs. Using photoelectron spectroscopy, we unravel the formation of a PbI2 interfacial layer between a SnOx EEL and the perovskite. The resulting interface dipole between SnOx and the PbI2 depends on the choice of oxidant for ALD (water, ozone, oxygen plasma). SnOx grown by using ozone affords hysteresis-free devices with a stable efficiency of 16.3% and a remarkably high open circuit voltage of 1.17 V.[3] Finally, we fabricated semitransparent PSCs with efficiency >11% (Tvis = 17%) and an astonishing stability > 4500h under ambient conditions (>50% RH) or elevated temperatures (60°C).[4] [1] Y. Kato et al., Adv. Mater. Interf. 2015, 2, 150019 [2] K. Brinkmann et al., Nat. Commun. 8, 13938 [3] T. Hu et al. Adv. Mat. (submitted) [4] J. Zhao et al. Adv. Energ. Mat. (in press)

Losses, gain, and lasing in organic and perovskite active materials(Conference Presentation)

Organic solid state lasers (OSLs) based on semiconducting polymers or small molecules have seen some significant progress over the past decade. Highly efficient organic gain materials combined with high-Q resonator geometries (distributed feedback (DFB), VCSEL, etc.) have enabled OSLs, optically pumped by simple inorganic laser diodes or even LEDs. However, some fundamental goals remain to be reached, like continuous wave (cw) operation and injection lasing. I will address various loss mechanisms related to accumulated triplet excitons or long-lived polarons that in combination with the particular photo-physics of organic gain media state the dominant road-blocks on the way to reach these goals. I will discuss the recent progress in fundamental understanding of these loss processes, which now provides a solid basis for modelling, e.g. of laser dynamics. Avenues to mitigate these fundamental loss mechanisms, e.g. by alternative materials will be presented. In this regard, a class of gain materials based on organo-lead halide perovskites re-entered the scene as light emitters, recently. Enjoying a tremendous lot of attention as active material for solution processed solar cells with a 20+% efficiency, they have recently unveiled their exciting photo-physics for lasing applications. Optically pumped lasing in these materials has been achieved. I will discuss some of the unique properties that render this class of materials a promising candidate to overcome some of the limitations of “classical” organic gain media.