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On page 3780, Q. Liao, H. Fu, and co-workers report a facile one-step solution self-assembly method for synthesizing single-crystalline nanofibers of MAPbI_3-xCl_x, in which substitution of Cl for I takes place at I_flank positions of the halide-octahedra in ab plane of perovskite tetragonal-lattice with Cl concentration from 0 to 25%. All single NF-based devices exhibit excellent photoresponse characteristics under the white-light illumination.

Thermoelectric NaCo₂O₄/TiO₂ coaxial nanofibers are prepared and distributed in the perovskite solar cells. Under illumination, p-type NaCo₂O₄ can convert unwanted heat to thermal voltage, and thus promote the electron extraction and transport with the action of electrostatic force. These advantages collectively contribute to an overall power conversion efficiency improvement of ≈20% (15.14% vs 12.65%).

Alternative low-temperature solution-processed hole-transporting materials (HTMs) without dopant are critical for highly efficient perovskite solar cells (PSCs). Here, two novel small molecule HTMs with linear π-conjugated structure, 4,4′-bis(4-(di-p-toyl)aminostyryl)biphenyl (TPASBP) and 1,4′-bis(4-(di-p-toyl)aminostyryl)benzene (TPASB), are applied as hole-transporting layer (HTL) by low-temperature (sub-100 °C) solution-processed method in p-i-n PSCs. Compared with standard poly(3,4-ethylenedioxythiophene): poly(styrenesulfonic acid) (PEDOT:PSS) HTL, both TPASBP and TPASB HTLs can promote the growth of perovskite (CH₃NH₃PbI₃) film consisting of large grains and less grain boundaries. Furthermore, the hole extraction at HTL/CH₃NH₃PbI₃ interface and the hole transport in HTL are also more efficient under the conditions of using TPASBP or TPASB as HTL. Hence, the photovoltaic performance of the PSCs is dramatically enhanced, leading to the high efficiencies of 17.4% and 17.6% for the PSCs using TPASBP and TPASB as HTL, respectively, which are ≈40% higher than that of the standard PSC using PEDOT:PSS HTL.

Two novel small molecular materials are explored as the hole-transporting layer in p-i-n perovskite solar cells (PSCs), yielding high efficiencies of 17.4% and 17.6%, respectively, which are ≈40% higher than the standard PSCs.

Perovskite solar cells (PSCs) may offer huge potential in photovoltaic conversion, yet their practical applications face one major obstacle: their low stability, or quick degradation of their initial efficiencies. Here, a new design scheme is presented to enhance the PSC stability by using low-temperature hydrothermally grown hierarchical nano-SnO₂ electron transport layers (ETLs). The ETL contains a thin compact SnO₂ layer underneath a mesoporous layer of SnO₂ nanosheets. The mesoporous layer plays multiple roles of enhancing photon collection, preventing moisture penetration and improving the long-term stability. Through such simple approaches, PSCs with power conversion efficiencies of ≈13% can be readily obtained, with the highest efficiency to be 16.17%. A prototypical PSC preserves 90% of its initial efficiency even after storage in air at room temperature for 130 d without encapsulation. This study demonstrates that hierarchical SnO₂ is a potential ETL for fabricating low-cost and efficient PSCs with long-term stability.

Low-temperature hydrothermally grown hierarchical SnO₂ , a mesoporous layer of nanosheet arrays on a compact nanoparticle layer, is used as the electron transporting layer to enhance the long-term stability of perovskite solar cells. A mesoporous device preserves 90% of its initial efficiency, even after storage in air for 130 d without encapsulation.

Three acceptor–acceptor (A–A) type conjugated polymers based on isoindigo and naphthalene diimide/perylene diimide are designed and synthesized to study the effects of building blocks and alkyl chains on the polymer properties and performance of all-polymer photoresponse devices. Variation of the building blocks and alkyl chains can influence the thermal, optical, and electrochemical properties of the polymers, as indicated by thermogravimetric analysis, differential scanning calorimetry, UV–vis, cyclic voltammetry, and density functional theory calculations. Based on the A–A type conjugated polymers, the most efficient all-polymer photovoltaic cells are achieved with an efficiency of 2.68%, and the first all-polymer photodetectors are constructed with high responsivity (0.12 A W⁻¹) and detectivity (1.2 × 10¹² Jones), comparable to those of the best fullerene based organic photodetectors and inorganic photodetectors. Photoluminescence spectra, charge transport properties, and morphology of blend films are investigated to elucidate the influence of polymeric structures on device performances. This contribution demonstrates a strategy of systematically tuning the polymeric structures to achieve high performance all-polymer photoresponse devices.

Three n-type conjugated polymers are synthesized to achieve the most efficient acceptor–acceptor type polymer-based all-polymer photovoltaic cells with an efficiency of 2.68% and the first all-polymer photodetectors with high responsivity (0.12 A W⁻¹) and detectivity (1.2 × 10¹² Jones). This contribution provides a strategy of tuning the polymeric structures to achieve high performance all-polymer photoresponse devices.

On page 4464, M. J. Ko, H. J. Son, and co-workers develop a flexible perovskite solar cell by applying a novel low-temperature solution-processable polymer (PhNa-1T) as a hole-transport material. Compared with conventional PEDOT:PSS, PhNa-1T effectively improves solar cell efficiency and device stability due to the pH-neutral property of PhNa-1T and efficient charge extraction and suppressed charge recombination in solar cell devices.

On page 4653, N. Zhao, M. A. Loi, and co-workers report a giant light-induced enhancement of the photoluminescence intensity in formamidinium lead iodide perovskite thin films. They demonstrate that the “brightening” of the perovskite can be attributed to a moisture-assisted light-healing effect, which can be potentially used to increase and control the quality of hybrid perovskite thin films.

A novel wide-bandgap conjugated polymer PBTA-FPh based on benzodithiophene-alt-benzo[1,2,3]triazole as the main chain and a polar pentafluorothiophenyl (FPh) group in the side chain has been designed and synthesized. In comparison to the pristine polymer PBTA-BO that consists of nonpolar alkyl side chains, the resulting PBTA-FPh exhibits less pronounced aggregation while possessing analogous optical and electrochemical bandgaps. Contact angle measurements demonstrate that the surface energy can be enhanced by incorporating FPh moiety, leading to a better miscibility of PBTA-BO with PC₇₁BM in the presence of a certain amount of PBTA-FPh. The photoactive layer of PBTA-BO:PC₇₁BM:PBTA-FPh with weight ratio of 1:1.2:0.02% exhibits a percolated network with the fibrous features, as revealed by transmission electron microscopy measurements. Of particular interest is the significantly improved photovoltaic performances of polymer solar cell devices for which the power conversion efficiency is enhanced from 6.46% for the control device to 7.91% for device processed with PBTA-FPh as the polymeric additive. These observations indicate that introducing donor–acceptor type of polymeric additive comprising of polar groups in the side chain can be a promising strategy for the fabrication of high-performance polymer solar cells.

A novel wide-bandgap conjugated polymer PBTA-FPh based on benzodithiophene-alt-benzo[1,2,3]triazole as the main chain and a polar pentafluorothiophenyl (FPh) group in a side chain has been designed and synthesized as a polymeric additive. The incorporation of PBTA-FPh can lead to improved miscibility and morphology of PBTA-BO:PC₇₁BM blend films, resulting in obviously improved power conversion efficiency of polymer solar cells from 6.46% to 7.91%.

By creating an effective π-orbital hybridization between the fullerene cage and the aromatic anchor (addend), the azafulleroid interfacial modifiers exhibit enhanced electronic coupling to the underneath metal oxides. High power conversion efficiency of 10.3% can be achieved in organic solar cells using open-cage phenyl C₆₁ butyric acid methyl ester (PCBM)-modified zinc oxide layer.

Organo-lead halide perovskite solar cells (PSCs) have received great attention because of their optimized optical and electrical properties for solar cell applications. Recently, a dramatic increase in the photovoltaic performance of PSCs with organic hole transport materials (HTMs) has been reported. However, as of now, future commercialization can be hampered because the stability of PSCs with organic HTM has not been guaranteed for long periods under conventional working conditions, including moist conditions. Furthermore, conventional organic HTMs are normally expensive because material synthesis and purification are complicated. It is herein reported, for the first time, octadecylamine-capped pyrite nanoparticles (ODA-FeS₂ NPs) as a bi-functional layer (charge extraction layer and moisture-proof layer) for organo-lead halide PSCs. FeS₂ is a promising candidate for the HTM of PSCs because of its high conductivity and suitable energy levels for hole extraction. A bi-functional layer based on ODA-FeS₂ NPs shows excellent hole transport ability and moisture-proof performance. Through this approach, the best-performing device with ODA-FeS₂ NPs-based bi-functional layer shows a power conversion efficiency of 12.6% and maintains stable photovoltaic performance in 50% relative humidity for 1000 h. As a result, this study has the potential to break through the barriers for the commercialization of PSCs.

A bi-functional layer based on hydrophobic ligand capped FeS₂ nanoparticles is an outstanding inorganic hole transporting materials (HTMs) for perovskite solar cells (PSCs). PSCs with FeS₂ bi-functional layer show high photovoltaic performance and improved long-term stability because HTMs based on FeS₂ have excellent hole transport ability and moisture-proof performance. Our approach will contribute to reliability improvement of PSCs.

Longer carrier diffusion length and improved power conversion efficiency have been reported for thin-film solar cell of organolead mixed-halide perovskite MAPbI_{3– x} Cl _x in comparison with MAPbI₃. Instead of substituting I in the MAPbI₃ lattice, Cl-incorporation has been shown to mainly improve the film morphology of perovskite absorber. Well-defined crystal structure, adjustable composition (x), and regular morphology, remains a formidable task. Herein, a facile solution-assembly method is reported for synthesizing single-crystalline nanofibers (NFs) of tetragonal-lattice MAPbI_{3– x} Cl _x with the Cl-content adjustable between 0 ≤ x ≤ 0.75, leading to a gradual blueshift of the absorption and photoluminescence maxima from x = 0 to 0.75. The photoresponsivity (R) of MAPbI₃ NFs keeps almost unchanging at a value independent of the white-light illumination intensity (P). In contrast, R of MAPbI_{3– x} Cl _x NFs decreases rapidly with increasing both the x and P values, indicating Cl-substitution increases the recombination traps of photogenerated free electrons and holes. This study provides a model system to examine the role of extrinsic Cl ions in both perovskite crystallography and optoelectronic properties.

Cl-substitution self-assembled single-crystalline nanofibers (NFs) of tetragonal-lattice MAPbI_{3– x} Cl _x with the Cl-content adjustable between 0 ≤ x ≤ 0.75 are synthesized by a facile solution method. Cl-substitution positions of halide octahedra, optical bandgap, and photoconductivity of MAPbI_{3– x} Cl _x NFs are investigated here, providing a model system to examine the role of extrinsic Cl ions in both perovskite crystallography and optoelectronic properties.

All-vacuum-deposited perovskite solar cells produced by controlling reagent partial pressure in high vacuum with newly developed multi-layer electron and hole transporting structures show outstanding power conversion efficiency of 17.6% and smooth, pinhole-free, micrometer-sized perovskite crystal grains.

A perovskite LED with a perovskite film treated under optimum thermal annealing conditions exhibits a significantly enhanced long-term stability with full coverage of the green electroluminescence emission due to the highly uniform morphology of the perovskite film.

Organolead halide perovskites (e.g., CH₃NH₃PbI₃) have caught tremendous attention for their excellent optoelectronic properties and applications, especially as the active material for solar cells. Perovskite crystal quality and dimension is crucial for the fabrication of high-performance optoelectronic and photovoltaic devices. Herein the controlled synthesis of organolead halide perovskite CH₃NH₃PbI₃ nanoplatelets on SiO₂/Si substrates is investigated via a convenient two-step vapor transport deposition technique. The thickness and size of the perovskite can be well-controlled from few-layers to hundred nanometers by altering the synthesis time and temperature. Raman characterizations reveal that the evolutions of Raman peaks are sensitive to the thickness. Furthermore, from the time-resolved photoluminescence measurements, the best optoelectronic performance of the perovskite platelet is attributed with thickness of ≈30 nm to its dominant longest lifetime (≈4.5 ns) of perovskite excitons, which means lower surface traps or defects. This work supplies an alternative to the synthesis of high-quality organic perovskite and their possible optoelectronic applications with the most suitable materials.

High-quality organic–inorganic perovskite nanoplatelets can be well-controlled synthesized from few-layers to hundred nanometers on Si/SiO₂ substrate. Raman characterizations reveal that the evolutions of Raman peaks are sensitive to the thickness. Furthermore, under the thickness-dependent photoresponse and time-resolved photoluminescence measurements of perovskite devices, the results indicate the most suitable thickness for their possible optoelectronic applications.

Energy-related functionality and performance of organic–inorganic hybrid perovskites, such as methylammonium lead iodide (MAPbI₃), highly depend on their thermal transport behavior. Using equilibrium molecular dynamics simulations, it is discovered that the thermal conductivities of MAPbI₃ under different phases (cubic, tetragonal, and orthorhombic) are less than 1 W m⁻¹ K⁻¹, and as low as 0.31 W m⁻¹ K⁻¹ at room temperature. Such ultralow thermal conductivity can be attributed to the small phonon group velocities due to their low elastic stiffness, in addition to their short phonon lifetimes (<100 ps) and mean-free-paths (<10 nm) due to the enhanced phonon–phonon scattering from highly-overlapped phonon branches. The anisotropy in thermal conductivity at lower temperatures is found to associate with preferential orientations of organic CH₃NH₃⁺ cations. Among all atomistic interactions, electrostatic interactions dominate thermal conductivities in ionic MAPbI₃ crystals. Furthermore, thermal conductivities of general hybrid perovskites MABX₃ (B = Pb, Sn; X = I, Br) have been qualitatively estimated and found that Sn- or Br-based perovskites possess higher thermal conductivities than Pb- or I-based ones due to their much higher elastic stiffness. This study inspires optimal selections and rational designs of ionic components for hybrid perovskites with desired thermal conductivity for thermally-stable photovoltaic or highly-efficient thermoelectric energy harvesting/conversion applications.

The ultralow thermal conductivity of organic–inorganic perovskite (CH₃NH₃PbI₃) (<1 W m⁻¹ K⁻¹) is demonstrated. Such ultralow conductivity results from small phonon group velocities and short phonon lifetimes. Furthermore, the optimal selections and rational designs of the ionic components for hybrid perovskites MABX₃ (B = Pb, Sn; X = I, Br) are further discussed.

Grazing incidence wide and small angle X-ray scattering (GIWAXS and GISAXS) measurements have been used to study the crystallization kinetics of the organolead halide perovskite CH₃NH₃PbI_3–xCl_x during thermal annealing. In situ GIWAXS measurements recorded during annealing are used to characterize and quantify the transition from a crystalline precursor to the perovskite structure. In situ GISAXS measurements indicate an evolution of crystallite sizes during annealing, with the number of crystallites having sizes between 30 and 400 nm increasing through the annealing process. Using ex situ scanning electron microscopy, this evolution in length scales is confirmed and a concurrent increase in film surface coverage is observed, a parameter crucial for efficient solar cell performance. A series of photovoltaic devices are then fabricated in which perovskite films have been annealed for different times, and variations in device performance are explained on the basis of X-ray scattering measurements.

Crystallization of the perovskite CH₃NH₃PbI_3–xCl_x during thermal annealing of a precursor film is studied using in situ grazing incidence wide angle and small angle X-ray scattering measurements. These results can explain the evolution of device performance with annealing time, and optimized films lead to solar cells with average power conversion efficiencies of over 12%.

Transparent conducting electrodes (TCEs) with multi-length scaled structure are promising candidates as a potential replacement for indium tin oxide (ITO). In this work, multi-length scaled silver nanowire (AgNW) grids are demonstrated as TCEs for organic solar cells. The multi-length scale silver nanowire grids are prepared by top-down patterning using a neutral vapor etching process. Patterning AgNW film into multi-length scale grid structures could improve the optical transmittance and enhance the use of incident photons. Based on these multi-length scale AgNW grids, inverted bulk heterojunction polymer solar cells with power conversion efficiency up to 9.02% are fabricated, which are higher than that based on the original AgNW films and comparable to that based on ITO.

Multi-length scaled silver nanowire (AgNW) grids are prepared as transparent conducting electrodes through a neutral vapor etching process. Organic solar cells with power conversion efficiency up to 9.02% are fabricated based on the multi-length scaled AgNW grids, which is higher than that based on original AgNW films and comparable to that based on indium tin oxide.