Silver nanowires (AgNWs) surrounded by insulating poly(vinylpyrrolidone) have been synthesized by a polyol process and employed as transparent electrodes. The AgNW transparent electrodes can be fabricated by heattreatment at about 200 °C which forms connecting junctions between AgNWs. Such a heating process is, however, one of the drawbacks of the fabrication of AgNW electrodes on heat-sensitive substrates. Here it has been demonstrated that the electrical conductivity of AgNW electrodes can be improved by mechanical pressing at 25 MPa for 5 s at room temperature. This simple process results in a low sheet resistance of 8.6 Ω/square and a transparency of 80.0%, equivalent to the properties of the AgNW electrodes heated at 200 °C . This technique makes it possible to fabricate AgNW transparent electrodes on heat-sensitive substrates. The AgNW electrodes on poly(ethylene terephthalate) films exhibited high stability of their electrical conductivities against the repeated bending test. In addition, the surface roughness of the pressed AgNW electrodes is one-third of that of the heat-treated electrode because the AgNW junctions are mechanically compressed. As a result, an organic solar cell fabricated on the pressed AgNW electrodes exhibited a power conversion as much as those fabricated on indium tin oxide electrodes. These findings enable continuous roll-to-roll processing at room temperature, resulting in relatively simple, inexpensive, and scalable processing that is suitable for forthcoming technologies such as organic solar cells, flexible displays, and touch screens.

Optically transparent nanofiber paper containing silver nanowires showed high electrical conductivity and maintained the high transparency, and low weight of the original transparent nanofiber paper. We demonstrated some procedures of optically transparent and electrically conductive cellulose nanofiber paper for lightweight and portable electronic devices. The nanofiber paper enhanced high conductivity without any post treatments such as heating or mechanical pressing, when cellulose nanofiber dispersions were dropped on a silver nanowire thin layer. The transparent conductive nanofiber paper showed high electrical durability in repeated folding tests, due to dual advantages of the hydrophilic affinity between cellulose and silver nanowires, and the entanglement between cellulose nanofibers and silver nanowires. Their optical transparency and electrical conductivity were as high as those of ITO glass. Therefore, using this conductive transparent paper, organic solar cells were produced that achieved a power conversion of 3.2%, which was as high as that of ITO-based solar cells.

An electronegative conjugated compound composed of a newly designed carbonyl‐bridged bithiazole unit and trifluoroacetyl terminal groups is synthesized as a candidate for air‐stable n‐type organic field‐effect transistor (OFET) materials. Cyclic voltammetry measurements reveal that carbonyl‐bridging contributes both to lowering the lowest unoccupied molecular orbital energy level and to stabilizing the anionic species. X‐ray crystallographic analysis of the compound shows a planar molecular geometry and a dense molecular packing, which is advantageous to electron transport. Through these appropriate electrochemical properties and structures for n‐type semiconductor materials, OFET devices based on this compound show electron mobilities as high as 0.06 cm2 V−1 s−1 with on/off ratios of 106 and threshold voltages of 20 V under vacuum conditions. Furthermore, these devices show the same order of electron mobility under ambient conditions.

New donor–acceptor-type copolymers containing dioxocycloalkene-annelated thiophenes as electron-accepting units have been designed and synthesized for application to p-type organic semiconducting materials in organic photovoltaics. The investigation of their photophysical and electrochemical properties revealed that these copolymers possessed low optical bandgaps (from 1.63 to 1.92 eV) and low-lying HOMO energy levels (from −5.41 to −5.33 eV). Organic field-effect transistor measurements revealed that these copolymers had hole-transporting characteristics with mobilities on the order of 10–7–10–4 cm2 V–1 s–1. The bulk-heterojunction photovoltaic devices fabricated from blends of these copolymers with fullerene derivatives as acceptors showed high power conversion efficiencies of up to 4.87%, with an open-circuit voltage of 0.90 V, a short-circuit current of 11.46 mA cm–2, and a fill factor of 0.48 under air mass 1.5 simulated solar illumination.

In organic photovoltaics (OPVs) using nonfullerene acceptors, the fine-tuning of interfaces between donor and acceptor in the bulk-heterojunction (BHJ) structure has become an important factor to improve the performance. A series of electron-accepting π-conjugated compounds based on benzothiadiazole and arenedicarboximides were systematically synthesized to investigate the impact of structural modification on molecular orientation at donor−acceptor interfaces. X-ray diffraction and surface free energy measurements of these compounds in the film state revealed that the crystallinity correlates with the London dispersion (γ d ) and the polar components of their interfacial energies. BHJ solar cells prepared with our π-conjugated compounds as acceptors and poly(3-hexyl)thiophene as a donor exhibited that the structural modification exerts a significant influence on the photovoltaic characteristics, and afforded the highest power conversion efficiency of 2.05%. Absorption, photoluminescence, and carrier mobility measurements of the blend films showed that the OPV performance of our system are mainly governed by the efficiency of charge-separation into free carrier at the donor−acceptor interfaces. Furthermore, a strong correlation was found between the short-circuit current density of OPV and γ d of acceptors, indicating that this quantity promotes the formation of desirable charge-separated states. The findings provide novel information for the development of nonfullerene acceptors for OPVs.

No abstract

The development of nonfullerene acceptor materials applicable to organic photovoltaics (OPVs) has attracted considerable attention for the achievement of a high power conversion efficiency (PCE) in recent years. However, it is still challenging due to the insufficiency of both the variety of effective electron‐deficient units and certain guidelines for the design of such materials. This work focusses on naphtho[1,2‐c:5,6‐c′]bis[1,2,5]thiadiazole (NTz) as a key electron‐deficient unit. Therefore, a new electron‐accepting π‐conjugated compound (NTz‐Np), whose structure is based on the combination of NTz and the fluorene‐containing imide‐annelated terminal units (Np), is designed and synthesized. The NTz‐Np compound exhibits a narrow optical energy gap (1.73 eV), a proper energy level (−3.60 eV) of the lowest unoccupied molecular orbital, and moderate electron mobility (1.6 × 10−5 cm2 V−1 s−1), indicating that NTz‐Np has appropriate characteristics as an acceptor against poly(3‐hexylthiophene) (P3HT), a representative donor. OPV devices based on NTz‐Np under the blend with P3HT show high photovoltaic performance with a PCE of 2.81%, which is the highest class among the P3HT/nonfullerene‐based OPVs with the conventional device structure. This result indicates that NTz unit can be categorized as a potential electron‐deficient unit for the nonfullerene acceptors.

Vacuum deposition is a simple and controllable approach that aims to form higher-quality perovskite films compared with those formed using solution-based deposition processes. Herein, we demonstrate a novel method to promote the intercalation control of inorganic cesium lead iodide (CsPbI 3 ) perovskite thin films via alternate vacuum deposition. We also investigated the effect of layer-by-layer deposition of PbI 2 /CsI to fabricate efficient planar heterojunction CsPbI 3 thin films and solar cells. This procedure is comparatively simple when compared with commonly used coevaporation techniques; further, precise intercalation control of the CsPbI 3 thin films can be achieved by increasing the number of layers in the layer-by-layer deposition of PbI 2 /CsI. The best control and the highest reproducibility were achieved for the deposition of four double layers owing to the precise intercalation control during the deposition of the CsPbI 3 thin film. A power conversion efficiency of 6.79% was obtained via alternating vacuum deposition of two double layers with a short-circuit current density ( J sc ) of 12.06 mA/cm 2 , an open-circuit voltage ( V oc ) of 0.79 V, and a fill factor (FF) of 0.72. Our results suggest a route for inorganic precursors to be used for efficient perovskite solar cells via alternating vacuum deposition.