Small Molecule Anchored to Mesoporous ITO for High-Contrast Black Electrochromics

We developed transmissive-to-black electrochromic devices by assembling <i>p</i>-aminotriphenlyamine (pAT) on the surface of mesoporous tin-doped indium oxide (mITO) electrodes. pAT-modified electrodes possess an optical contrast ratio of up to 64%, switching speeds of 1–4 s, and enhanced chemical reversibility as compared to unbound pAT in solution. Gel permeation chromatography (GPC) and inductively coupled plasma mass spectrometry (ICP-MS) were used to analyze the electrochemical degradation products of pAT-modified electrodes and pAT in solution. The rare transparent-to-black electrochromism of pAT makes it a promising small-molecule, anodically coloring material for smart window applications

Polymer–Nanoparticle Electrochromic Materials that Selectively Modulate Visible and Near-Infrared Light

In this manuscript, we describe a class of hybrid electrochromic materials utilizing polythiophenes and tin-doped indium oxide (ITO) nanoparticles that independently modulate visible and near-infrared (NIR) light. By altering the voltage applied across electrodes modified with these composite materials, the electrodes can be repeatedly switched between three distinct modes of operation. These “bright and warm”, “bright and cool”, and “dark and cool” modes vary in their visible and NIR transmission properties and are targeted toward the development of smart windows that can control both solar lighting and heating fluxes. Electrodes containing the composite films, which are fabricated by electropolymerizing polythiophenes on transparent electrodes coated with ITO nanoparticles, possess fast switching times (<90 s), moderate durability, and contrast ratios similar to those of the individual composite components. The maximum contrast ratios of the composite systems are 47% at 700 nm and 39% at 1250 nm. After 200 switching cycles, these contrast ratios degrade by 15–20%. The composite materials developed here represent a new direction of research aimed at modulating light and heat flux in smart windows

Enhancing the Hole-Conductivity of Spiro-OMeTAD without Oxygen or Lithium Salts by Using Spiro(TFSI)₂ in Perovskite and Dye-Sensitized Solar Cells

2,2′,7,7′-Tetrakis­(<i>N,N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD), the prevalent organic hole transport material used in solid-state dye-sensitized solar cells and perovskite-absorber solar cells, relies on an uncontrolled oxidative process to reach appreciable conductivity. This work presents the use of a dicationic salt of spiro-OMeTAD, named spiro­(TFSI)₂, as a facile means of controllably increasing the conductivity of spiro-OMeTAD up to 10^–3 S cm^–1 without relying on oxidation in air. Spiro­(TFSI)₂ enables the first demonstration of solid-state dye-sensitized solar cells fabricated and operated with the complete exclusion of oxygen after deposition of the sensitizer with higher and more reproducible device performance. Perovskite-absorber solar cells fabricated with spiro­(TFSI)₂ show improved operating stability in an inert atmosphere. Gaining control of the conductivity of the HTM in both dye-sensitized and perovskite-absorber solar cells in an inert atmosphere using spiro­(TFSI)₂ is an important step toward the commercialization of these technologies

Chlorine in PbCl₂‑Derived Hybrid-Perovskite Solar Absorbers

Chlorine in PbCl₂‑Derived Hybrid-Perovskite Solar Absorber

Mechanism of Tin Oxidation and Stabilization by Lead Substitution in Tin Halide Perovskites

The recent development of efficient binary tin- and lead-based metal halide perovskite solar cells has enabled the development of all-perovskite tandem solar cells, which offer a unique opportunity to deliver high performance at low cost. Tin halide perovskites, however, are prone to oxidation, where the Sn²⁺ cations oxidize to Sn⁴⁺ upon air exposure. Here, we identify reaction products and elucidate the oxidation mechanism of both ASnI₃ and ASn_0.5Pb_0.5I₃ (where A can be made of methylammonium, formamidinium, cesium, or a combination of these) perovskites and find that substituting lead onto the B site fundamentally changes the oxidation mechanism of tin-based metal halide perovskites to make them more stable than would be expected by simply considering the decrease in tin content. This work provides guidelines for developing stable small band gap materials that could be used in all-perovskite tandems

Ternary Bulk Heterojunction Solar Cells: Addition of Soluble NIR Dyes for Photocurrent Generation beyond 800 nm

The incorporation of a <i>tert</i>-butyl-functionalized silicon 2,3-naphthalocyanine bis­(trihexylsilyloxide) dye molecule as a third component in a ternary blend bulk heterojunction (BHJ) organic solar cell containing P3HT (donor) and PC₆₀BM (acceptor) results in increased NIR absorption. This absorption yields an increase of up to 40% in the short-circuit current and up to 19% in the power conversion efficiency (PCE) in photovoltaic devices. Two-dimensional grazing incidence wide-angle X-ray scattering (2-D GIWAXS) experiments show that compared to the unfunctionalized dye the <i>tert</i>-butyl functionalization enables an increase in the volume fraction of the dye molecule that can be incorporated before the device performance decreases. Quantum efficiency and absorption spectra also indicate that, at dye concentrations above about 8 wt %, there is an approximately 30 nm red shift in the main silicon naphthalocyanine absorption peak, allowing further dye addition to contribute to added photocurrent. This peak shift is not observed in blends with unfunctionalized dye molecules, however. This simple approach of using ternary blends may be generally applicable for use in other unoptimized BHJ systems towards increasing PCEs beyond current levels. Furthermore, this may offer a new approach towards OPVs that absorb NIR photons without having to design, synthesize, and purify complicated donor–acceptor polymers

Minimal Effect of the Hole-Transport Material Ionization Potential on the Open-Circuit Voltage of Perovskite Solar Cells

Hole-transport material optimization is an important step toward maximizing the efficiency of perovskite solar cells. Here, we investigate the role of one hole-transport material property, the ionization potential, on the performance of perovskite solar cells. We employ a device architecture that allows us to systematically tune the ionization potential while avoiding any impact to other device parameters, and we find that for a wide range of ionization potentials the photovoltaic performance is minimally affected. This finding relaxes the requirement for the development of hole-transport materials with particular ionization potentials, allowing for the optimization of hole-transport materials that can improve performance in differing ways such as through increased stability or decreased parasitic absorption

Thermal Stability of Mixed Cation Metal Halide Perovskites in Air

We study the thermal stability in air of the mixed cation organic–inorganic lead halide perovskites Cs_0.17FA_0.83Pb­(I_0.83Br_0.17)₃ and Cs_0.05(MA_0.17FA_0.83)_0.95Pb­(I_0.83Br_0.17)₃. For the latter compound, containing both MA⁺ and FA⁺ ions, thermal decomposition of the perovskite phase was observed to occur in two stages. The first stage of decomposition occurs at a faster rate compared to the second stage and is only observed at relatively low temperatures (<i>T</i> < 150 °C). For the second stage, we find that both decomposition rate and the activation energy have similar values for Cs_0.05(MA_0.17FA_0.83)_0.95Pb­(I_0.83Br_0.17)₃ and Cs_0.17FA_0.83Pb­(I_0.83Br_0.17)₃, which suggests that the first stage mainly involves reaction of MA⁺ and the second stage mainly FA⁺

Hole Transport Materials with Low Glass Transition Temperatures and High Solubility for Application in Solid-State Dye-Sensitized Solar Cells

We present the synthesis and device characterization of new hole transport materials (HTMs) for application in solid-state dye-sensitized solar cells (ssDSSCs). In addition to possessing electrical properties well suited for ssDSSCs, these new HTMs have low glass transition temperatures, low melting points, and high solubility, which make them promising candidates for increased pore filling into mesoporous titania films. Using standard device fabrication methods and Z907 as the sensitizing dye, power conversion efficiencies (PCE) of 2.94% in 2-μm-thick cells were achieved, rivaling the PCE obtained by control devices using the state-of-the-art HTM spiro-OMeTAD. In 6-μm-thick cells, the device performance is shown to be higher than that obtained using spiro-OMeTAD, making these new HTMs promising for preparing high-efficiency ssDSSCs

Molecular Engineering of Organic Dyes for Improved Recombination Lifetime in Solid-State Dye-Sensitized Solar Cells

A major limitation of solid-state dye-sensitized solar cells is a short electron diffusion length, which is due to fast recombination between electrons in the TiO₂ electron-transporting layer and holes in the 2,2′,7,7′-tetrakis­(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene (Spiro-OMeTAD) hole-transporting layer. In this report, the sensitizing dye that separates the TiO₂ from the Spiro-OMeTAD was engineered to slow recombination and increase device performance. Through the synthesis and characterization of three new organic D-π-A sensitizing dyes (WN1, WN3, and WN3.1), the quantity and placement of alkyl chains on the sensitizing dye were found to play a significant role in the suppression of recombination. In solid-state devices using Spiro-OMeTAD as the hole-transport material, these dyes achieved the following efficiencies: 4.9% for WN1, 5.9% for WN3, and 6.3% for WN3.1, compared to 6.6% achieved with Y123 as a reference dye. Of the dyes investigated in this study, WN3.1 is shown to be the most effective at suppressing recombination in solid-state dye-sensitized solar cells, using transient photovoltage and photocurrent measurements