EFFECT OF π-CONJUGATED LIGANDS AND A* SITE ORGANIC CATIONS ON THE ENERGETIC PROPERTIES AND PHOTOVOLTAIC PERFORMANCE OF METAL HALIDE PEROVSKITES

Metal halide perovskites have gained interest in optoelectronic applications such as photovoltaics, lasers, LEDs, transistors, and photodetectors due to their excellent semiconducting properties considering their low cost. Metal halide perovskite (HP) photovoltaics have rapidly increased in power conversion efficiency (PCE), which now exceeds 25%. HPs have gained attention in these applications due to their high tolerance towards defects, long charge carrier diffusion lengths, high charge carrier mobility, high optical absorption, and bandgaps that are tunable over a large range. Even though HP photovoltaic PCEs are improved these are still not commercially available due to them showing lower stability and energy loss due to severe charge recombination at the surface and interfaces in the device. Treating the HP surface with surface ligands has become a promising approach to improve photovoltaic performance, defect passivation, and interfacial energetics. In this dissertation, the influence of ammonium functionalized π – conjugated ligands on MAPbI3 perovskite energetics, photovoltaic performance, and interfacial charge transfer is investigated. With the thiophene ligands, a drastic PCE drop was observed for p-i-n devices, and improved PCE was obtained for n-i-p devices. With PDI surface ligands no significant change was observed for photovoltaic performance. Two-dimensional metal halide perovskites (2D HP) have captured interest in the field due to their improved stability against air, moisture, and light relative to their 3D counterparts. 2D HPs have a layered structure, where the organic spacer cations are sandwiched between layers of inorganic octahedra. This organic layer in 2D HPs adds additional protection against moisture and oxygen ingression and other degradation pathways. These materials are used as the active layer in LEDs and solar cells and as capping layers in 3D HPs. 2D perovskites demonstrate remarkable structural variabilities, where the properties can be modified by changing the layer thickness, the halide anion, and the spacer cation. To make devices with 2D perovskites we need to understand the influence of the organic spacer cations on the optoelectronic properties of these materials. In this work, we investigate the influence of the dipole magnitude and the direction of a series of functionalized PEAI derivatives as organic spacer cations on the ionization energy and the electron affinity of 2D tin halide perovskites. However, determining ionization energy and electron affinity in HPs could be quite difficult as several methods are being used in data interpretation for HPs. In this work, we propose a method to assign the energy levels in 2D HPs by correcting for the instrumental resolution in ultraviolet and inverse photoemission spectroscopy

Probing transport energies and defect states in organic semiconductors using energy resolved electrochemical impedance spectroscopy

Abstract Determining the relative energies of transport states in organic semiconductors is critical to understanding the properties of electronic devices and in designing device stacks. Futhermore, defect states are also highly important and can greatly impact material properties and device performance. Recently, energy‐resolved electrochemical impedance spectroscopy (ER‐EIS) is developed to probe both the ionization energy (IE) and electron affinity (EA) as well as sub‐bandgap defect states in organic semiconductors. Herein, ER‐EIS is compared to cyclic voltammetry (CV) and photoemission spectroscopies for extracting IE and EA values, and to photothermal deflection spectroscopy (PDS) for probing defect states in both polymer and molecular organic semiconductors. The results show that ER‐EIS determined IE and EA are in better agreement with photoemission spectroscopy measurements as compared to CV for both polymer and molecular materials. Furthermore, the defect states detected by ER‐EIS agree with sub‐bandgap features detected by PDS. Surprisingly, ER‐EIS measurements of regiorandom and regioregular poly(3‐hexylthiophene) (P3HT) show clear defect bands that occur at significantly different energies. In regioregular P3HT the defect band is near the edge of the occupied states while it is near the edge of the unoccupied states in regiorandom P3HT

Multifunctional Conjugated Ligand Engineering for Stable and Efficient Perovskite Solar Cells

Surface passivation is an effective way to boost the efficiency and stability of perovskite solar cells (PSCs). However, a key challenge faced by most of the passivation strategies is reducing the interface charge recombination without imposing energy barriers to charge extraction. Here, a novel multifunctional semiconducting organic ammonium cationic interface modifier inserted between the light-harvesting perovskite film and the hole-transporting layer is reported. It is shown that the conjugated cations can directly extract holes from perovskite efficiently, and simultaneously reduce interface non-radiative recombination. Together with improved energy level alignment and the stabilized interface in the device, a triple-cation mixed-halide medium-bandgap PSC with an excellent power conversion efficiency of 22.06% (improved from 19.94%) and suppressed ion migration and halide phase segregation, which lead to a long-term operational stability, is demonstrated. This strategy provides a new practical method of interface engineering in PSCs toward improved efficiency and stability

Low-loss contacts on textured substrates for inverted perovskite solar cells

<p>Inverted perovskite solar cells (PSCs) promise enhanced operating stability compared to their normal-structure counterparts. To improve efficiency further, it is crucial to combine effective light management with low interfacial losses. Here we develop a conformal self-assembled monolayer as the hole-selective contact on light-managing textured substrates. Molecular dynamics simulations indicate cluster formation during phosphonic acid adsorption leads to incomplete SAM coverage. We devise a co-adsorbent strategy that disassembles high-order clusters, thus homogenizing the distribution of phosphonic acid molecules, thereby minimizing interfacial recombination and improving electronic structures. We report a lab-measured power-conversion efficiency (PCE) of 25.3% and a certified quasi-steady-state PCE of 24.8% for inverted PSCs, with a photocurrent approaching 95% of the Shockley-Queisser maximum. An encapsulated device having a PCE of 24.6% at room temperature retains 95% of its peak performance when stressed at 65°C and 50% relative humidity following > 1000 hours of maximum power point tracking under 1-sun illumination. </p&gt

Inverted perovskite solar cells with low-loss hole-selective contact on textured substrates

Inverted perovskite solar cells (PSCs) promise enhanced operating stability compared to their normal-structure counterparts. To improve efficiency further, it is crucial to combine effective light management with low interfacial losses. Here we develop a conformal self-assembled monolayer as the hole-selective contact on light-managing textured substrates. Molecular dynamics simulations indicate cluster formation during phosphonic acid adsorption leads to incomplete SAM coverage. We devise a co-adsorbent strategy that disassembles high-order clusters, thus homogenizing the distribution of phosphonic acid molecules, thereby minimizing interfacial recombination and improving electronic structures. We report a lab-measured power-conversion efficiency (PCE) of 25.3% and a certified quasi-steady-state PCE of 24.8% for inverted PSCs, with a photocurrent approaching 95% of the Shockley-Queisser maximum. An encapsulated device having a PCE of 24.6% at room temperature retains 95% of its peak performance when stressed at 65°C and 50% relative humidity following > 1000 hours of maximum power point tracking under 1-sun illumination

Suppressing phase disproportionation in quasi-2D perovskite light-emitting diodes.

Electroluminescence efficiencies and stabilities of quasi-two-dimensional halide perovskites are restricted by the formation of multiple-quantum-well structures with broad and uncontrollable phase distributions. Here, we report a ligand design strategy to substantially suppress diffusion-limited phase disproportionation, thereby enabling better phase control. We demonstrate that extending the π-conjugation length and increasing the cross-sectional area of the ligand enables perovskite thin films with dramatically suppressed ion transport, narrowed phase distributions, reduced defect densities, and enhanced radiative recombination efficiencies. Consequently, we achieved efficient and stable deep-red light-emitting diodes with a peak external quantum efficiency of 26.3% (average 22.9% among 70 devices and cross-checked) and a half-life of ~220 and 2.8 h under a constant current density of 0.1 and 12 mA/cm2, respectively. Our devices also exhibit wide wavelength tunability and improved spectral and phase stability compared with existing perovskite light-emitting diodes. These discoveries provide critical insights into the molecular design and crystallization kinetics of low-dimensional perovskite semiconductors for light-emitting devices