STUDENT TALKS

MONDAY 25

1. Catarina Ferreira, ICFO

Half Cylinder Photonic Plate with Broadband Polarized Light Emission and Light Recycling for Energy Efficient Liquid Crystal Displays

Energetic efficiency in many illumination devices is far from optimal: in liquid crystal displays a large fraction of the light emitted is lost since absorbing polarizers are used for the image formation. In this work we propose a light guiding structure capable of emitting diffuse polarized light, combined with light harvesting and conversion back into electricity of the light with the unwanted polarization. Key in achieving such double goal is the combination into a hybrid structure of a half-cylinder photonic plate for an ergodic light guided propagation with a non-absorbing reflective multilayer polarizer and perovskite solar cells. In a study interlacing ray optics tracing with a full wave vector inverse integration we were able to obtain a structure with a broadband polarization extinction ratio of 0.1 for the light diffused from the top surface, while close to 90% of the (unwanted) s-polarized light remains trapped and can be recycled back to electricity by the perovskite solar cells placed on the front and back ends of the guide.

2. Lingling Fan, Stanford

Controlling Optical Force with Coupled-mode Analysis and Wavefront Engineering

The frequency dependence of the optical force, i.e., the force spectrum, is important for controlling and manipulating nanoscale objects. Spectral resonances of light scatterers strongly influence the optical force spectrum. In this talk, we develop a theoretical formalism based on temporal coupled-mode theory. This formalism studies the lineshapes of force spectra on resonant scatterers, for different incident wavefronts. We discuss the theoretical conditions for achieving symmetric as well as asymmetric lineshapes, pertaining, respectively, to a Lorentzian and a Fano resonance. As an example, we discuss a scatterer size sorting mechanism that plays a role in biomedical diagnostics.Our work allows understanding and tailoring the optical force in the presence of the resonances.

3. Bin Chen, University of Toronto

Light management in perovskite tandem solar cells

Perovskite-based tandem solar cells hold the promise of better utilization of sun light at a lower cost. Light management is critical to match the current generated in the two series connected sub cells in a tandem for maximized power output. I will discuss our work on light management in perovskites tandem solar cells.

4. Shanti Maria Liga, ICFO

Colloidal synthesis of lead-free Cs2TiBr6−xIx perovskite nanocrystals

Over the last decade, metal halide perovskite materials have gained huge interest in photovoltaics research because of their excellent optoelectronic properties combined with the ease of solution processability. Despite perovskite solar cells reached 25.5% efficiency in 2021, the presence of lead in the structure of the best performing perovskites creates regulatory and environmental concerns due to its toxicity. For this reason, over the last few years, many efforts have been devoted to synthesizing lead-free perovskites, but despite the copious amount of novel materials, only few of them show suitable bandgaps for solar cell applications. Vacancy-ordered halide perovskites, with general formula A2B+4X6, are one of the alternative structures that have been designed to replace Pb2+ with non-toxic tetravalent cations. Among these, cesium titanium(IV) bromide perovskite is an environmentally friendly material that has attracted attention for its application in solar cells with power conversion efficiencies reaching 3.3% [1]. However, a low cost, scalable solution method suitable for the preparation of thin films has not yet been developed. We present here a new approach for the synthesis of Cs2TiBr6 and mixed bromide/iodide Cs2TiBr6−xIx, which consists of performing a colloidal synthesis using the hot-injection method. With this method, we were able to synthesize stable solutions of mixed bromide/iodide Cs2TiBr6−xIx nanocrystals, with bandgap tunable from 2.3 eV to 1.2 eV [2]. [1] Chen, M., Ju, M. G., Carl, A. D., Zong, Y., Grimm, R. L., Gu, J., … & Padture, N. P., Joule, 2018, 2(3), 558-570. [2] Liga, S. M., & Konstantatos, G., Journal of Materials Chemistry C, 2021, 9(34), 11098-11103.

5. Cora M. Went, Caltech

Transition Metal Dichalcogenides for Ultrathin Photovoltaics

Two-dimensional transition metal dichalcogenides (TMDCs) are promising candidates for ultrathin photovoltaics due to their high absorption coefficients and intrinsically passivated surfaces.¹ However, one-sun power conversion efficiencies greater than 5% have not been achieved in ultrathin (total cell thickness less than 150 nm) TMDC photovoltaics. We present advances towards efficient TMDC solar cells in two different cell geometries: Schottky-junction and carrier-selective contact devices. For Schottky-junction TMDC solar cells, we develop a new, simple procedure for transferring van der Waals metal contacts to limit Fermi level pinning at the semiconductor-metal interface. We measure more than an order of magnitude higher open-circuit voltage in cells with transferred contacts compared to cells with evaporated contacts, and a one-sun power conversion efficiency of 0.5%. For carrier-selective contact solar cells, we draw on a wealth of research on organic and polymer charge transporting layers in the perovskite solar cell community, and fabricate TMDC cells modeled on inverted perovskite device architectures. These devices achieve open-circuit voltages greater than 500 mV even with nonideal optical design. Using transfer matrix calculations, we demonstrate that short-circuit current greater than 20 mA/cm² is possible in a similar electronic architecture illuminated from the bottom through an ITO-coated glass substrate, with a 100-nm silver top contact as a back-reflector. We conclude by outlining a pathway to greater than 5% efficiency in TMDC photovoltaics.

References:

1. Jariwala, A. R. Davoyan, J. Wong, H. A. Atwater, Van der Waals Materials for Atomically-Thin Photovoltaics: Promise and Outlook. ACS Photonics 4, 2962–2970 (2017).

6. Xiaoyue Li, University of Toronto

Emission Tuning and Electroluminescence study of High Efficiency Dinuclear Cuprous Iodide Complexes

Dinuclear cuprous iodide [Cu2I2] complexes exhibit great potential as subtitude emitters for precious metals compounds applied in organic light emitting diodes (OLEDs) due to their adjustable energy levels, high efficiency, and low cost. This presentation is mainly exploring photoluminescence and electroluminescence of a series of dinuclear cuprous iodide complexes, which are either synthesized in organic solution or formed by in situ codeposition method. Moreover, construction of high quality cuprous complex based WOLEDs with dual emissive layers could be achieved by deposition strategy.

TUESDAY 26

1. Carles Ros, ICFO

Hydrogen generation, storage and recovery in graphene: reactions monitoring by Raman and FTIR spectroscopy

Hydrogen, as alternative energy vector to fossil fuels, still faces significant drawbacks in storage, transport and safety. Direct water splitting and fixation of hydrogen to graphene oxide (GO) by electrochemical means allows the production of an energy dense liquid capable to overcome the energy requiring gas pressurization and manipulation if these protons can be recovered back. In this work we in-situ study the reductive electrochemical reactions involved in direct water splitting and C-H sp3 bond formation in GO electrodes, the influence of various parameters, and the possibility to recover hydrogen on-demand.

Hydrogen incorporation at defective sites or epoxide groups and oxygen functional groups elimination were found to be competitive reactions and were followed by in-situ Raman spectroscopy. The influence of potential variations during electrochemical hydrogenation was found to increase up to a 160% the C-H bond formation. FTIR symmetric and asymmetric C-H stretching points at CH2 and CH3 groups forming at defective sites and edges. Under temperature treatments at different atmospheres hydrogen can be released from graphene, while the C-C sp2 bond structure is recovered. By DRIFT, TPD and TGA, two distinct desorption energies can be observed, pointing at different hydrogen bonding energies corresponding to different sites in the graphene structure.

2. Arun Nagpal, Caltech

Weyl semimetals as a platform for giant plasmonic non-reciprocity

A fundamental constraint on the efficiency of photothermovoltaic systems is a time-reversal symmetric cell response. To circumvent this, magneto-optical devices utilizing external magnetic fields have been well-characterized as platforms for time-asymmetric photonics and plasmonics, operating through the inducement of electronic cyclotron orbits. Recently, magnetic materials supporting massless fermions, magnetic Weyl semimetals, have been predicted to break time-reversal symmetry in the absence of an external field, through the appearance of nontrivial topological Berry curvature. These materials produce time-reversal-symmetry-breaking fields orders of magnitude stronger than observed in typical magnetic systems at similar energies. Here we discuss the Weyl semimetal Co₃Sn₂S₂ and its utilization in a Voigt configuration non-reciprocal plasmon measurement. We describe the crystalline and optical characteristics of single-crystal Co₃Sn₂S₂, and discuss its implementation in non-reciprocal energy-harvesting systems in the mid-infrared.

3. Sam Teale, University of Toronto

2D passivation in inverted perovskite photovoltaics

Recently 2D/3D heterostructure perovskite solar cells (PSCs) have produced > 25% power conversion efficiency (PCE). Unfortunately, due to energy misalignment this method does not translate to inverted PSCs. In this presentation I describe how we were able to increase the layer width of confined perovskites in 2D/3D heterostructures and demonstrate that this removes the barrier to electron extraction at the 2D/3D interface. Cells fabricated using this method demonstrate the best combination of efficiency and stability recorded in PSC literature.

4. Zhuoran Wang, ICFO

Superstrate Kesterite Solar Cells

Kesterite, or Cu2ZnSn(S,Se)4 (CZTS) is promising in developing sustainable PV technology due to its earth-abundant, non-toxic composition. However, issues including instability of interface, high density of defects that fails to allow the short charge-collection length to meet its light absorption needs, existence of Cd in its standard device configuration that fails to comply with the restriction of hazardous substances (RoSH), are promoting the development of alternative, eco-friendly device structure. Here, we present an important progress on this subject by adopting the superstrate configuration to kesterite, thus to realize advantageous light management and high defect tolerance in an ultrathin device.

5. Yubin Part, Stanford University

Violating Kirchhoff’s Law of Thermal Radiation in Semitransparent Structures

Kirchhoff’s law of thermal radiation characterizes the balance between emission and absorption of light. According to this law, whenever an object absorbs light, energy is emitted back to the original source. This loss mechanism is inevitable to the processes of energy harvesting from thermal radiation. Thus, overcoming this restriction is of fundamental importance to provide opportunities for higher energy harvesting efficiency. With nonreciprocal properties, it is possible to violate the Kirchhoff’s law and channel the emitted light to the direction different from that of the incident light. Then, these emitted photons can be reused in subsequent parts of the harvester to contribute to further energy generation. Especially with a semitransparent structure, the nonreciprocal emitter can completely absorb normally incident light and emit solely to the opposite side under ideal operation. This feature is particularly suitable for multi-junction solar cells with stacked multi-layers. With proper symmetry conditions satisfied, magneto-optical effect and guided-mode resonance enables this functionality, and nearly ideal performance can be achieved from optimization based on temporal coupled mode theory framework. Furthermore, with the use of a recently discovered magnetic Weyl semimetal, it is possible to design a relatively realistic emitter that still reaches a significant emissivity-absorptivity contrast.

6. Yasser Hassan University of Toronto

Ligand-engineered bandgap stability in mixed-halide perovskite LEDs

Lead halide perovskites are promising semiconductors for light-emitting applications, owing to their bright, bandgap tuneable and high colour purity luminescence. Close to unity photoluminescence quantum yields have been achieved for perovskite nanocrystals across a broad range of emission colours, and light emitting diodes with external quantum efficiencies exceeding 20%, which approach commercial OLEDs, have been demonstrated in both the Infrared and green emission channels. However, due to the formation of lower band gap iodide-rich domains in mixed-halide perovskites, achieving colour-stable and efficient red electroluminescence at the desired wavelength is yet to be realised. Here, we report mixed-halide perovskite nanocrystals passivated with multidentate ligands that suppress halide segregation, and enable colour-stable red emission centred at 620 nm, with electroluminescence external quantum efficiencies of 20.3% in light emitting diodes. We use density functional theory, to model that the nature of binding between the ligands and the nanocrystal surface supresses iodide Frenkel defect formation that may inhibit halide-segregation. Our work exemplifies how the functionality of metal halide perovskites is extremely sensitive to the nature of the (nano)crystalline surface and presents a route for controlling surface defect formation and migration. This is critical for achieving band gap stability for light emission, and will also have far-reaching impact for other optoelectronic applications, such as photovoltaics, where bandgap stability is required.