Functional polymers for growth and stabilization of CsPbBr3 perovskite nanoparticles.

We introduce an approach to synthesize polymer-stabilized CsPbBr3 perovskite nanoparticles (NPs) using ammonium bromide-functionalized polymers as both bromide precursors and stabilizing ligands. The polymer-passivated NPs exhibit significant advantages over conventional perovskite NPs owing to their facile dispersion in polymer matrices and enhanced optoelectronic stability.

Chemical Stabilization of Perovskite Solar Cells with Functional Fulleropyrrolidines.

While perovskite solar cells have invigorated the photovoltaic research community due to their excellent power conversion efficiencies (PCEs), these devices notably suffer from poor stability. To address this crucial issue, a solution-processable organic chemical inhibition layer (OCIL) was integrated into perovskite solar cells, resulting in improved device stability and a maximum PCE of 16.3%. Photoenhanced self-doping of the fulleropyrrolidine mixture in the interlayers afforded devices that were advantageously insensitive to OCIL thickness, ranging from 4 to 190 nm. X-ray photoelectron spectroscopy (XPS) indicated that the fulleropyrrolidine mixture improved device stability by stabilizing the metal electrode and trapping ionic defects (i.e., I-) that originate from the perovskite active layer. Moreover, degraded devices were rejuvenated by repeatedly peeling away and replacing the OCIL/Ag electrode, and this repeel and replace process resulted in further improvement to device stability with minimal variation of device efficiency.

Conjugated Polymer Zwitterions: Efficient Interlayer Materials in Organic Electronics.

Conjugated polymer zwitterions (CPZs) are neutral, hydrophilic, polymer semiconductors. The pendent zwitterions, viewed as side chain dipoles, impart solubility in polar solvents for solution processing, and open opportunities as interfacial components of optoelectronic devices, for example, between metal electrodes and organic semiconductor active layers. Such interlayers are crucial for defining the performance of organic electronic devices, e.g., field-effect transistors (OFETs), light-emitting diodes (OLEDs), and photovoltaics (OPVs), all of which consist of multilayer structures. The interlayers reduce the Schottky barrier height and thus improve charge injection in OFETs and OLEDs. In OPVs, the interlayers serve to increase the built-in electric potential difference (Vbi) across the active layer, ensuring efficient extraction of photogenerated charge carriers. In general, polar and even charged electronically active polymers have gained recognition for their ability to modify metal/semiconductor interfaces to the benefit of organic electronics. While conjugated polyelectrolytes (CPEs) as interlayer materials are well-documented, open questions remain about the role of mobile counterions in CPE-containing devices. CPZs possess the processing advantages of CPEs, but as neutral molecules lack any potential complications associated with counterions. The electronic implications of CPZs on metal electrodes stem from the orientation of the zwitterion dipole moment in close proximity to the metal surface, and the resultant surface-induced polarization. This generates an interfacial dipole (Δ) at the CPZ/metal interface, altering the work function of the electrode, as confirmed by ultraviolet photoelectron spectroscopy (UPS), and improving device performance. An ideal cathode interlayer would reduce electrode work function, have orthogonal processability to the active layer, exhibit good film forming properties (i.e., wettability/uniformity), prevent exciton quenching, possess optimal electron affinity that neither limits the work function reduction nor impedes the charge extraction, transport electrons selectively, and exhibit long-term stability. Our recent discoveries show that CPZs achieve many of these attributes, and are poised for further expansion and development in the interfacial science of organic electronics. This Account reviews a recent collaboration that began with the synthesis of CPZs and a study of their structural and electronic properties on metals, then extended to their application as interlayer materials for OPVs. We discuss CPZ structure-property relationships based on several material platforms, ranging from homopolymers to copolymers, and from materials with intrinsic p-type conjugated backbones to those with intrinsic n-type conjugated backbones. We discuss key components of such interlayers, including (i) the origin of work function reduction of CPZ interlayers on metals; (ii) the role of the frontier molecular orbital energy levels and their trade-offs in optimizing electronic and device properties; and (iii) the role of polymer conductivity type and the magnitude of charge carrier mobility. Our motivation is to present our prior use and current understanding of CPZs as interlayer materials in organic electronics, and describe outstanding issues and future potential directions.

Bulk charge carrier transport in push-pull type organic semiconductor.

Operation of organic electronic and optoelectronic devices relies on charge transport properties of active layer materials. The magnitude of charge carrier mobility, a key efficiency metrics of charge transport properties, is determined by the chemical structure of molecular units and their crystallographic packing motifs, as well as strongly depends on the film fabrication approaches that produce films with different degrees of anisotropy and structural order. Probed by the time-of-flight and grazing incidence X-ray diffraction techniques, bulk charge carrier transport, molecular packing, and film morphology in different structural phases of push-pull type organic semiconductor, 7,7'-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b']dithiophene-2,6-diyl)bis(6-fluoro-4-(5'-hexyl-[2,2'-bithiophen]-5yl)benzo[c][1,2,5] thiadiazole), one of the most efficient small-molecule photovoltaic materials to-date, are described herein. In the isotropic phase, the material is ambipolar with high mobilities for a fluid state. The electron and hole mobilities at the phase onset at 210.78 °C are 1.0 × 10(-3) cm(2)/(V s) and 6.5 × 10(-4) cm(2)/(V s), respectively. Analysis of the temperature and electric field dependences of the mobilities in the framework of Gaussian disorder formalism suggests larger energetic and positional disorder for electron transport sites. Below 210 °C, crystallization into a polycrystalline film with a triclinic unit cell symmetry and high degree of anisotropy leads to a 10-fold increase of hole mobility. The mobility is limited by the charge transfer along the direction of branched alkyl side chains. Below 90 °C, faster cooling rates produce even higher hole mobilities up to 2 × 10(-2) cm(2)/(V s) at 25 °C because of the more isotropic orientations of crystalline domains. These properties facilitate in understanding efficient material performance in photovoltaic devices and will guide further development of materials and devices.

Fulleropyrrolidine interlayers: tailoring electrodes to raise organic solar cell efficiency.

A major challenge in organic solar cell design is the trade-off between oxidative stability and work function of the metal cathode. We found that in single-junction polymer solar cells, this problem can be surmounted by solution-based incorporation of fulleropyrrolidines with amine (C60-N) or zwitterionic (C60-SB) substituents as cathode-independent buffer layers. Specifically, a thin layer of C60-N reduced the effective work function of Ag, Cu, and Au electrodes to 3.65 electron volts. Power conversion efficiency values exceeding 8.5% were obtained for organic photovoltaics independent of the cathode selection (Al, Ag, Cu, or Au). Such high efficiencies did not require precise control over interlayer thickness, as devices prepared with C60-N and C60-SB layers ranging from 5 to 55 nanometers performed with high efficiency.

Crystallinity and morphology effects on a solvent-processed solar cell using a triarylamine-substituted squaraine.

2,4-Bis[4'-(N,N-di(4″-hydroxyphenyl)amino)-2',6'-dihydroxyphenyl]squaraine (Sq-TAA-OH, optical bandgap 1.4 eV, HOMO level -5.3 eV by ultraviolet photoelectron spectroscopy) is used as an active layer material in solution processed, bulk-heterojunction organic photovoltaic cells with configuration ITO/PEDOT:PSS/Sq-TAA-OH:PC71BM/LiF/Al. Power conversion efficiencies (PCEs) up to 4.8% are obtained by a well-reproducible procedure using a mixture of good and poor Sq-TAA-OH solubilizing organic solvents, with diiodooctane (DIO) additive to make a bulk heterojunction layer, followed by thermal annealing, to give optimized V(OC) = 0.84-0.86 V, J(SC) = 10 mA cm(-2), and FF = 0.53. X-ray diffraction and scattering studies of pristine, pure Sq-TAA-OH solution-cast films show d-spacing features similar to single-crystal packing and spacing. The DIO additive in a good solvent/poor solvent mixture apparently broadens the size distribution of Sq-TAA-OH crystallites in pristine films, but thermal annealing provides a narrower size distribution. Direct X-ray diffraction and scattering morphological studies of "as-fabricated" active layers show improved Sq-TAA-OH/PC71BM phase separation and formation of crystallites, ∼48 nm in size, under conditions that give the best PCE.

Conjugated polymeric zwitterions as efficient interlayers in organic solar cells.

Conjugated polymeric zwitterions, when utilized as interlayer materials in bulk heterojunction organic solar cells, lead to significantly enhanced power conversion efficiencies. The electrostatic model of self-aligning dipolar side groups in the vicinity of the metal surface rationalizes the effects of reduced cathode work function, a key factor behind the observed enhanced efficiency.

Tailored one- and two-dimensional self-assembly of a perylene diimide derivative in organic solvents.

We report on studies of the tailored self-assembly of the perylene diimide derivative, N,N'-ditridecylperylene-3,4,9,10-tetracarboxylic diimide, into structures with fibrous gel-type, one-dimensional, and two-dimensional morphologies. This approach for producing highly ordered nanostructures of well-defined morphologies utilizes a property of π-conjugated molecules to assemble in poor organic solvents due to π-π interaction between the aromatic cores and takes advantage of the temperature dependence of solubility. The morphology control is based on a fine-tuning of anisotropic, intermolecular solute-solute interactions that are attenuated by the solute-solvent interaction in organic solvents of different chemical structure. We discuss the role of light illumination in the self-assembly process as well as application of ultrasonic treatment as a way of mechanical tailoring of morphology. This approach paves the way toward the molecular-scale tailoring of structural properties of organic semiconducting materials for electronic and optoelectronic applications.