Highly efficient and stable perovskite solar cells enabled by low-dimensional perovskitoids

Deep traps originated from the defects formed at the surfaces and grain boundaries of the perovskite absorbers during their lattice assembly are the main reasons that cause nonradiative recombination and material degradation, which notably affect efficiency and stability of perovskite solar cells (PSCs). Here, we demonstrate the substantially improved PSC performance by capping the photoactive layer with low-dimensional (LD) perovskitoids. The undercoordinated Pb ions and metallic Pb at the surfaces of the three-dimensional (3D) perovskite are effectively passivated via the Pb-I bonding from the favorably lattice-matched 3D/LD interface. The good stability and hydrophobicity of the LD (0D and 1D) perovskitoids allow excellent protection of the 3D active layer under severe environmental conditions. The PSC exhibits a power conversion efficiency of 24.18%, reproduced in an accredited independent photovoltaic testing laboratory. The unencapsulated device maintains 90% of its initial efficiency after 800 hours of continuous illumination under maximum power point operating conditions.

Selenium-Containing Organic Photovoltaic Materials

ConspectusOrganic photovoltaics (OPVs) with a photoactive layer containing a blend of organic donor and acceptor species are considered to be a promising technology for clean energy owing to their unique flexible form factor and good solution processability that can potentially address the scalability challenges. The delicate designs of both donors and acceptors have significantly enhanced the power conversion efficiency of OPVs to more than 18%. Nonfullerene small-molecule acceptors (NFAs) have played a critical role in enhancing the short-circuit current density (J_SC) by efficiently harvesting near-infrared (NIR) sunlight. To take full advantage of the abundant NIR photons, the optical band gap of NFAs should be further reduced to improve the performance of OPVs. Incorporating highly polarizable selenium atoms onto the backbone of organic conjugated materials has been proven to be an effective way to decrease their optical band gap. For example, a selenium-substituted NFA recently developed by our group has attained a J_SC of approximate 27.5 mA cm^-2 in OPV devices, surpassing those of most emerging photovoltaic systems. Inspired by this advance, we concentrate on the topic of selenium-containing materials in this Account to incite readers' interest in further exploring this series of materials.In this Account, we first compare the differences among chalcogen heterocycles and discuss the influence of fundamental electronic behavior on the collective photoelectrical properties of the resulting materials. The superior features of selenium-substituted materials are summarized as follows: (1) The large covalent radius of selenium can diminish the π-orbital overlap, rendering enhanced quinoidal resonance character and a narrowed optical band gap of resulting materials. (2) The selenium atom is more polarizable than sulfur owing to its larger and looser outermost electron cloud, enabling enhanced intermolecular Se-Se interaction and increased charge carrier mobility of relevant materials in the solid state. We then focus on summarizing the design rules for various categories of selenium-containing materials including polymer donors, small-molecule acceptors, and polymer acceptors, especially those composed of ladder-type polycyclic units. The motivation for incorporating selenium atoms into these materials and the structure-property relationships were thoroughly elucidated. Specifically, we discuss the changes in the optical band gap, charge carrier mobility, and molecular packing induced by selenium substitution and correlate the effects of these changes with the exciton behaviors, energy loss, and nanoscale film morphology of corresponding OPV devices. Furthermore, we point out the intrinsic stability of selenium-containing materials under maximum-power-point tracking and long-term photo- or thermostress and indicate their potential use in semitransparent and tandem solar cells. At the end, the prospect of future research focuses and the possible applications of selenium-containing materials in the OPV field are discussed.

Synthesis of a side-chain hole transporting polymer through Mitsunobu post-functionalization for efficient inverted perovskite solar cells

Mitsunobu post-functionalization was utilized to construct a new efficient dopant-free side-chain hole transporting polymer for inverted perovskite solar cells, exhibiting a power conversion efficiency of 17.75% and a high fill factor over 81%.

Fused selenophene-thieno[3,2-b]thiophene–selenophene (ST)-based narrow-bandgap electron acceptor for efficient organic solar cells with small voltage loss

A novel NIR-light-absorbing fused-ring electron acceptor, STIC, was successfully developed for organic solar cells and achieved a PCE of 9.68%.

Design rules for the broad application of fast (<1 s) methylamine vapor based, hybrid perovskite post deposition treatments

While organo-metal halide perovskite photovoltaics have seen rapid development, growth of high quality material remains a challenge.

Side chain structure affects the molecular packing and photovoltaic performance of oligothiophene-based solution-processable small molecules

Small molecules with alkyl side chains of different lengths were prepared with 2,2′-bithiophene, terthiophene and thiobarbituric acid as the central core, spacer and end-cap. Uniform, shorter chain lengths gave stronger intermolecular interactions, favoring crystallization.

Photovoltaic performance of ladder-type indacenodithieno[3,2-b]thiophene-based polymers with alkoxyphenyl side chains

Ladder polymers deliver over 5% power conversion efficiencies in polymer solar cells made without engaging post-solvent or additive treatments.

Cascading Retro-Diels-Alder Cycloreversion and Sydnone-Maleimide Based Double 1,3-Dipolar Cycloaddition for Quantitative Thermal Cross-Linking of an Amorphous Polymer Solid

A new approach to polymer cross-linking is investigated using a cascading cycloreversion of a maleimide-furan adduct and a double 1,3-dipolar cycloaddition between a sydnone and maleimide. The cross-linking proceeds quantitatively above 63 °C, despite the polymer possessing no observable glass transition temperature. The resulting polymer network possesses a high thermal stability (>300 °C) due to the irreversibility of the sydnone-maleimide cycloaddition, which releases CO₂ during the cross-linking, driving the reaction. The rigid three-dimensional structure of the bis-maleimide-sydnone cycloadduct produced local free volumes in films, decreasing the dielectric constant of the material.

Effects of self-assembled monolayer structural order, surface homogeneity and surface energy on pentacene morphology and thin film transistor device performance

A systematic study of six phosphonic acid (PA) self-assembled monolayers (SAMs) with tailored molecular structures is performed to evaluate their effectiveness as dielectric modifying layers in organic field-effect transistors (OFETs) and determine the relationship between SAM structural order, surface homogeneity, and surface energy in dictating device performance. SAM structures and surface properties are examined by near edge X-ray absorption fine structure (NEXAFS) spectroscopy, contact angle goniometry, and atomic force microscopy (AFM). Top-contact pentacene OFET devices are fabricated on SAM modified Si with a thermally grown oxide layer as a dielectric. For less ordered methyl- and phenyl-terminated alkyl ~(CH₂)₁₂ PA SAMs of varying surface energies, pentacene OFETs show high charge carrier mobilities up to 4.1 cm² V^-1 s^-1. It is hypothesized that for these SAMs, mitigation of molecular scale roughness and subsequent control of surface homogeneity allow for large pentacene grain growth leading to high performance pentacene OFET devices. PA SAMs that contain bulky terminal groups or are highly crystalline in nature do not allow for a homogenous surface at a molecular level and result in charge carrier mobilities of 1.3 cm² V^-1 s^-1 or less. For all molecules used in this study, no causal relationship between SAM surface energy and charge carrier mobility in pentacene FET devices is observed.

Optical and electrical effects of plasmonic nanoparticles in high-efficiency hybrid solar cells

Improved performance was obtained by doping a hole-transporting layer or active layer with Au nanoparticles in PCPDTBT–CdSe QD hybrid solar cells.

Solid-State Densification of Spun-Cast Self-Assembled Monolayers for Use in Ultra-Thin Hybrid Dielectrics

Ultra-thin self-assembled monolayer (SAM)-oxide hybrid dielectrics have gained significant interest for their application in low-voltage organic thin film transistors (OTFTs). A [8-(11-phenoxy-undecyloxy)-octyl]phosphonic acid (PhO-19-PA) SAM on ultrathin AlOx (2.5 nm) has been developed to significantly enhance the dielectric performance of inorganic oxides through reduction of leakage current while maintaining similar capacitance to the underlying oxide structure. Rapid processing of this SAM in ambient conditions is achieved by spin coating, however, as-cast monolayer density is not sufficient for dielectric applications. Thermal annealing of a bulk spun-cast PhO-19-PA molecular film is explored as a mechanism for SAM densification. SAM density, or surface coverage, and order are examined as a function of annealing temperature. These SAM characteristics are probed through atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and near edge X-ray absorption fine structure spectroscopy (NEXAFS). It is found that at temperatures sufficient to melt the as-cast bulk molecular film, SAM densification is achieved; leading to a rapid processing technique for high performance SAM-oxide hybrid dielectric systems utilizing a single wet processing step. To demonstrate low-voltage devices based on this hybrid dielectric (with leakage current density of 7.7×10-8 A cm-2 and capacitance density of 0.62 µF cm-2 at 3 V), pentacene thin-film transistors (OTFTs) are fabricated and yield sub 2 V operation and charge carrier mobilites of up to 1.1 cm2 V-1 s-1.

Dipolar Chromophore Facilitated Huisgen Cross-Linking Reactions for Highly Efficient and Thermally Stable Electrooptic Polymers

Efficient thermal cross-linking protocol for an azide-alkyne-based Huisgen 1,3-dipolar cycloaddition reaction has been developed for making highly efficient electrooptic (EO) polymers. The material system is based on an azide-containing side-chain copolymer and cross-linkable dendronized chromophores that have two pairs of dendritic bispropargyl ether moiety on the periphery. The dendritic chromophore possesses a tetraene conjugating bridge and strong dialkylaminophenyl donor and CF₃-TCF acceptor. This material system not only provides adequate accessibility for propargyl ether to react azido-containing moieties for efficient cross-linking but also gives the rotational freedom needed for electric field poling. The site isolation offered by the bispropargyl ether dendron effectively suppresses the unwanted side reactions that are usually associated with the decompositions of azides and chromophores. Due to these concerted efforts, it allows the Huisgen 1,3-dipolar cycloaddition reactions to be carried out at moderate temperatures in polar media that have a high concentration of dipolar polyene chromophores. Through sequential electric field poling and in situ cross-linking, the poled films exhibit very large EO activity (up to 147 pm/V at 1.31 μm) with a long-term alignment stability at 85 °C.

Novel Divalent Osmium Complexes: Synthesis, Characterization, Tuning of Emission, and use in Organic Light Emitting Diodes

In this work we present the synthesis and characterization of several novel osmium complexes of the form [Os(N-N)2 L-L]2+ 2Ts- or [Os(L-L)2 N-N]2+ 2Ts- designed for organic light emitting device (OLED) applications. In the complex notation N-N represents a derivative of 2,2'-bipyridine or 1,10-phenanthroline and L-L represents a strong π-acid arsine or phosphine ligand. Several counterions have been used and include tosylate, hexafluorophosphate, triflate, heptafluorobutyrate, chloride, bromide, and iodide. The complexes feature 3MLCT emission that ranges from 611-650 nm, which makes them suitable as an emission source for red OLEDs. Phosphorescent quantum yields as high as 45% and emission lifetimes as short as 400 nanoseconds have been reached. The complexes were incorporated into OLED and give off red, orange, yellow, and green electrophosphorescence. Red devices have been created using polyvinylcarb/2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole, polyvinylnapthalene/2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole, and polyfluorene host materials. OLEDs have been created which give brightness up to 3000 cd m-2 and efficiency up to 2.2% at 632 nm emission.