Solution-Processed Organic Solar Cells from Dye Molecules: An Investigation of Diketopyrrolopyrrole:Vinazene Heterojunctions

Although one of the most attractive aspects of organic solar cells is their low cost and ease of fabrication, the active materials incorporated into the vast majority of reported bulk heterojunction (BHJ) solar cells include a semiconducting polymer and a fullerene derivative, classes of materials which are both typically difficult and expensive to prepare. In this study, we demonstrate that effective BHJs can be fabricated from two easily synthesized dye molecules. Solar cells incorporating a diketopyrrolopyrrole (DPP)-based molecule as a donor and a dicyanoimidazole (Vinazene) acceptor function as an active layer in BHJ solar cells, producing relatively high open circuit voltages and power conversion efficiencies (PCEs) up to 1.1%. Atomic force microscope images of the films show that active layers are rough and apparently have large donor and acceptor domains on the surface, whereas photoluminescence of the blends is incompletely quenched, suggesting that higher PCEs might be obtained if the morphology could be improved to yield smaller domain sizes and a larger interfacial area between donor and acceptor phases

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

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

Role of Crystallization in the Morphology of Polymer:Non-fullerene Acceptor Bulk Heterojunctions

Many high efficiency organic photovoltaics use fullerene-based acceptors despite their high production cost, weak optical absorption in the visible range, and limited synthetic variability of electronic and optical properties. To circumvent this deficiency, non-fullerene small-molecule acceptors have been developed that have good synthetic flexibility, allowing for precise tuning of optoelectronic properties, leading to enhanced absorption of the solar spectrum and increased open-circuit voltages (<i>V</i>_OC). We examined the detailed morphology of bulk heterojunctions of poly­(3-hexyl­thio­phene) and the small-molecule acceptor HPI-BT to reveal structural changes that lead to improvements in the fill factor of solar cells upon thermal annealing. The kinetics of the phase transformation process of HPI-BT during thermal annealing were investigated through <i>in situ</i> grazing incidence wide-angle X-ray scattering studies, atomic force microscopy, and transmission electron microscopy. The HPI-BT acceptor crystallizes during film formation to form micron-sized domains embedded within the film center and a donor rich capping layer at the cathode interface reducing efficient charge extraction. Thermal annealing changes the surface composition and improves charge extraction. This study reveals the need for complementary methods to investigate the morphology of BHJs

Silicon-Naphthalo/Phthalocyanine-Hybrid Sensitizer for Efficient Red Response in Dye-Sensitized Solar Cells

Introduction of a naphthalocyanine moiety to phthalocyanine allows for a gradual red shift of the absorption spectrum in the resulting chromophore. Using silicon as a core atom allows for the introduction of additional siloxane side chains which mitigate dye aggregation. A dye-sensitized solar cell with this hybrid sensitizer exhibits a broad and flat IPCE of 80% between 600 and 750 nm and high photocurrent densities of 19.0 mA/cm²

Bandgap Tuning of Silicon Quantum Dots by Surface Functionalization with Conjugated Organic Groups

The quantum confinement and enhanced optical properties of silicon quantum dots (SiQDs) make them attractive as an inexpensive and nontoxic material for a variety of applications such as light emitting technologies (lighting, displays, sensors) and photovoltaics. However, experimental demonstration of these properties and practical application into optoelectronic devices have been limited as SiQDs are generally passivated with covalently bound insulating alkyl chains that limit charge transport. In this work, we show that strategically designed triphenylamine-based surface ligands covalently bonded to the SiQD surface using conjugated vinyl connectivity results in a 70 nm red-shifted photoluminescence relative to their decyl-capped control counterparts. This suggests that electron density from the SiQD is delocalized into the surface ligands to effectively create a larger hybrid QD with possible macroscopic charge transport properties

Hydrophobic Organic Hole Transporters for Improved Moisture Resistance in Metal Halide Perovskite Solar Cells

Solar cells based on organic–inorganic perovskite semiconductor materials have recently made rapid improvements in performance, with the best cells performing at over 20% efficiency. With such rapid progress, questions such as cost and solar cell stability are becoming increasingly important to address if this new technology is to reach commercial deployment. The moisture sensitivity of commonly used organic–inorganic metal halide perovskites has especially raised concerns. Here, we demonstrate that the hygroscopic lithium salt commonly used as a dopant for the hole transport material in perovskite solar cells makes the top layer of the devices hydrophilic and causes the solar cells to rapidly degrade in the presence of moisture. By using novel, low cost, and hydrophobic hole transporters in conjunction with a doping method incorporating a preoxidized salt of the respective hole transporters, we are able to prepare efficient perovskite solar cells with greatly enhanced water resistance

Phenyl/Perfluorophenyl Stacking Interactions Enhance Structural Order in Two-Dimensional Covalent Organic Frameworks

A two-dimensional imine-based covalent organic framework (COF) was designed and synthesized such that phenyl and perfluorophenyl structural units can seamlessly alternate between layers of the framework. X-ray diffraction of the COF powders reveals a striking increase in crystallinity for the COF with self-complementary phenyl/perfluorophenyl interactions (FASt-COF). Whereas measured values of the Brunauer–Emmet–Teller (BET) surface areas for the nonfluorinated Base-COF and the COF employing hydrogen bonding were ∼37% and 59%, respectively, of their theoretical Connolly surface areas, the BET value for FASt-COF achieves >90% of its theoretical value (∼1700 m²/g). Transmission electron microscopy images also revealed unique micron-sized rodlike features in FASt-COF that were not present in the other materials. The results highlight a promising approach for improving surface areas and long-range order in two-dimensional COFs

Direct Conversion of Hydride- to Siloxane-Terminated Silicon Quantum Dots

Peripheral surface functionalization of hydride-terminated silicon quantum dots (SiQD) is necessary in order to minimize their oxidation/aggregation and allow for solution processability. Historically thermal hydrosilylation addition of alkenes and alkynes across the Si–H surface to form Si–C bonds has been the primary method to achieve this. Here we demonstrate a mild alternative approach to functionalize hydride-terminated SiQDs using bulky silanols in the presence of free-radical initiators to form stable siloxane (∼Si–O–SiR₃) surfaces with hydrogen gas as a byproduct. This offers an alternative to existing methods of forming siloxane surfaces that require corrosive Si–Cl based chemistry with HCl byproducts. A 52 nm blue shift in the photoluminescent spectra of siloxane versus alkyl-functionalized SiQDs is observed that we explain using computational theory. Model compound synthesis of silane and silsesquioxane analogues is used to optimize surface chemistry and elucidate reaction mechanisms. Thorough characterization on the extent of siloxane surface coverage is provided using FTIR and XPS. TEM is used to demonstrate SiQD size and integrity after surface chemistry and product isolation