ZKC

Poly(3-hexyl thiophene) (P3HT) is widely regarded as the benchmark polymer when studying the physics of conjugated polymers used in organic electronic devices. P3HT can self-assemble via π–π stacking of its backbone, leading to an assembly and growth of P3HT fibrils into 3D percolating organogels. These structures are capable of bridging the electrodes, providing multiple pathways for charge transport throughout the active layer. Here, a novel set of conditions is identified and discussed for P3HT organogel network formation via spin coating by monitoring the spin-coating process from various solvents. The development of organogel formation is detected by in situ static light scattering, which measures both the thinning rate by reflectance and structural development in the film via off-specular scattering during film formation. Optical microscopy and thermal annealing experiments provide ex situ confirmation of organogel fabrication. The role of solution characteristics, including solvent boiling point, P3HT solubility, and initial P3HT solution concentration on organogel formation, is examined to correlate these parameters to the rate of film formation, organogel-onset concentration, and overall network size. The correlation of film properties to the fabrication parameters is also analyzed within the context of the hole mobility and density-of-states measured by impedance spectroscopy.

Poly(3-hexylthiophene) organogels offer exploitable electronic properties. These gels agglomerate into 3D percolating networks that span the active layer of a thin-film device. Novel spin-coating conditions that produce organogels in a rapid deposition process are discussed, including the interplay of kinetics and solution thermodynamics in organogel formation, control of the network size, and the correlation of structure to electronic properties.

Organic–inorganic heterostructures are an emerging topic that is very interesting for optoelectronics. Here, non-conventional p–n junctions are investigated using organic rubrene single crystal and 2D MoS₂ as the p- and n-type semiconducting materials, respectively. The current-rectifying behavior is clearly observed in the junction device. The rectification ratio can be electrically tuned by the gate voltage due to the 2D nature of the heterostructure. The devices also show good photoresponse properties with a photoresponsivity of ≈500 mA W⁻¹ and a fast response time. These findings suggest a new route to facilitate the design of nanoelectronic and optoelectronic devices based on layered inorganics and organics.

Through the marriage of MoS₂ and rubrene, a novel organic–inorganic van der Waals heterostructure is demonstrated with a gate-tunable rectifying behavior. Good photoresponse properties are also obtained from the heterojunction with a photoresponsitivity of 500 A mW^–1 and a fast response time.

The conclusions reached by a diverse group of scientists who attended an intense 2-day workshop on hybrid organic–inorganic perovskites are presented, including their thoughts on the most burning fundamental and practical questions regarding this unique class of materials, and their suggestions on various approaches to resolve these issues.

Organometal halide perovskite and poly(ethylene oxide) composite thin films are studied. Single-layer light-emitting diodes using the composite thin film sandwiched between indium tin oxide and indium-gallium eutectic alloy exhibit a low turn-on voltage and high brightness because of the ionic conductivity of the composite film and the formation of a p-i-n homojunction.

Two-inch-sized perovskite crystals, CH₃NH₃PbX₃ (X=I, Br, Cl), with high crystalline quality are prepared by a solution-grown strategy. The availability of large perovskite crystals is expected to transform its broad applications in photovoltaics, optoelectronics, lasers, photodetectors, LEDs, etc., just as crystalline silicon has done in revolutionizing the modern electronics and photovoltaic industries.

An oxide-free strategy to fabricate extremely thin, flexible, and smooth transparent electrodes is introduced by H. J. Son, J.-W. Kim, and co-workers on page 4580. The developed method, an inverted layer-by-layer process, enables the design and fabrication of highly efficient organic solar cells showing excellent mechanical stability with ultra-small folding radiuses.

The microstructure of solution-processed organometallic lead halide perovskite thin films prepared by the “gas-assisted” method is investigated with synchrotron-based techniques. Using a combination of GIWAXS and NEXAFS spectroscopy the orientational alignment of CH₃NH₃PbI₃ crystallites and CH₃NH₃⁺ cations are separately probed. The GIWAXS results reveal a lack of preferential orientation of CH₃NH₃PbI₃ crystallites in 200–250 nm thick films prepared on both planar TiO₂ and mesoporous TiO₂. Relatively high efficiencies are observed for device based on such films, with 14.3% achieved for planar devices and 12% for mesoporous devices suggesting that highly oriented crystallites are not crucial for good cell performance. Oriented crystallites however are observed in thinner films (≈60 nm) deposited on planar TiO₂ (but not on mesoporous TiO₂) indicating that the formation of oriented crystallites is sensitive to the kinetics of solvent evaporation and the underlying TiO₂ morphology. NEXAFS measurements on all samples found that CH₃NH₃⁺ cations exhibit a random molecular orientation with respect to the substrate. The lack of any NEXAFS dichroism for the thin CH₃NH₃PbI₃ layer deposited on planar TiO₂ in particular indicates the absence of any preferential orientation of CH₃NH₃⁺ cations within the CH₃NH₃PbI₃ unit cell for as-prepared layers, that is, without any electrical poling.

Using a combination of grazing-incidence wide-angle X-ray scattering (GIWAXS) and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, the orientational alignment of CH₃NH₃PbI₃ crystallites and CH₃NH₃⁺ cations are separately probed. The orientation of CH₃NH₃PbI₃ crystallites is sensitive to film thickness, solvent evaporation rate, and the underlying TiO₂ morphology. However, CH₃NH₃⁺ cations exhibit a random molecular orientation that is independent of the TiO₂ architecture and the perovskite film thickness.

Organic–inorganic lead halide perovskite solar cells are promising alternatives to silicon-based cells due to their low material costs and high photovoltaic performance. In this work, thin continuous perovskite films are combined with copper(I) iodide (CuI) as inorganic hole-conducting material to form a planar device architecture. A maximum conversion efficiency of 7.5% with an average efficiency of 5.8 ± 0.8% is achieved which, to our knowledge, is the highest reported efficiency for CuI-based devices with a planar structure. In contrast to related planar 2,2′,7,7′-tetrakis-(N,N -di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD)-based devices, the CuI-based devices do not show a pronounced hysteresis when tested by scanning the potential in a forward and backward direction. The strong quenching of photoluminescence (PL) signal and comparatively fast decay of open-circuit voltage demonstrates a more rapid removal of positive charge carriers from the perovskite layer when in contact with CuI compared to spiro-OMeTAD. A slow response on a timescale of 10–100 s is observed for the spiro-OMeTAD-based devices. In comparison, the CuI-based device displays a significantly faster response as determined through electrochemical impedance spectroscopy (EIS) and open-circuit voltage decays (OCVDs). The characteristically slow kinetics measured through EIS and OCVD are linked directly to the current–voltage hysteresis.

Planar perovskite/copper(I) iodide solar cells with near to no J–V hysteresis, made by employing thin CuI and perovskite layers, result in a record conversion efficiency of 7.5%. The magnitude of dielectric polarization at the perovskite/hole-conductor interface is proposed to influence the degree of J–V hysteresis.