Improving the Stability of Bulk Heterojunction Solar Cells by Incorporating pH-Neutral PEDOT:PSS as the Hole Transport Layer

In the application of traditional bulk heterojunction polymer solar cells, to prevent the etching of ITO by the acidic poly­(3,4-ethylenedioxythiophene):poly­(styrenesulfonate) (PEDOT:PSS) and thereby improve the device stability, pH-neutral PEDOT:PSS is introduced as the hole transport layer (HTL). After treating the neutral PEDOT:PSS with UV-ozone and with an oxygen plasma, the average power conversion efficiency (PCE) of the device increases from 3.44% to 6.60%. Such surface treatments reduce the energy level offset between the HTL and the active layer, which increases the open circuit voltage and enhances hole transportation, leading to the PCE improvement. Moreover, the devices with the neutral PEDOT:PSS HTL are more stable in air than those with the acidic PEDOT:PSS HTL. The PCE of the devices with the acidic PEDOT:PSS HTL decreases by 20% after 7 days and 45% after 50 days under ambient conditions, whereas the PCE of the devices with the pH-neutral PEDOT:PSS HTL decreases by only 9 and 20% after 7 and 50 days, respectively. X-ray photoelectron spectroscopy shows that the acidic PEDOT:PSS etches the indium from the indium–tin−oxide (ITO) electrode, which is responsible for the degradation of the device. In comparison, the diffusion of the indium is much slower in the devices with the pH-neutral PEDOT:PSS HTL

All Inkjet-Printed Metal-Oxide Thin-Film Transistor Array with Good Stability and Uniformity Using Surface-Energy Patterns

An array of inkjet-printed metal-oxide thin-film transistors (TFTs) is demonstrated for the first time with the assistance of surface-energy patterns prepared by printing pure solvent to etch the ultrathin hydrophobic layer. The surface-energy patterns not only restrained the spreading of inks but also provided a facile way to regulate the morphology of metal oxide films without optimizing ink formulation. The fully printed InGaO TFT devices in the array exhibited excellent electron transport characteristics with a maximum mobility of 11.7 cm² V^–1 s^–1, negligible hysteresis, good uniformity, and good stability under bias stress. The new route lights a general way toward fully inkjet-printed metal-oxide TFT arrays

High-Efficiency Small Molecule-Based Bulk-Heterojunction Solar Cells Enhanced by Additive Annealing

Solvent additive processing is important in optimizing an active layer’s morphology and thus improving the performance of organic solar cells (OSCs). In this study, we find that how 1,8-diiodooctane (DIO) additive is removed plays a critical role in determining the film morphology of the bulk heterojunction OSCs in inverted structure based on a porphyrin small molecule. Different from the cases reported for polymer-based OSCs in conventional structures, the inverted OSCs upon the quick removal of the additive either by quick vacuuming or methanol washing exhibit poorer performance. In contrast, the devices after keeping the active layers in ambient pressure with additive dwelling for about 1 h (namely, additive annealing) show an enhanced power conversion efficiency up to 7.78% with a large short circuit current of 19.25 mA/cm², which are among the best in small molecule-based solar cells. The detailed morphology analyses using UV-vis absorption spectroscopy, grazing incidence X-ray diffraction, resonant soft X-ray scattering, and atomic force microscopy demonstrate that the active layer shows smaller-sized phase separation but improved structure order upon additive annealing. On the contrary, the quick removal of the additive either by quick vacuuming or methanol washing keeps the active layers in an earlier stage of large scaled phase separation

Coffee-Ring Defined Short Channels for Inkjet-Printed Metal Oxide Thin-Film Transistors

Short-channel electronic devices several micrometers in length are difficult to implement by direct inkjet printing due to the limitation of position accuracy of the common inkjet printer system and the spread of functional ink on substrates. In this report, metal oxide thin-film transistors (TFTs) with channel lengths of 3.5 ± 0.7 μm were successfully fabricated with a common inkjet printer without any photolithography steps. Hydrophobic CYTOP coffee stripes, made by inkjet-printing and plasma-treating processes, were utilized to define the channel area of TFTs with channel lengths as short as ∼3.5 μm by dewetting the inks of the source/drain (S/D) precursors. Furthermore, by introduction of an ultrathin layer of PVA to modify the S/D surfaces, the spreading of precursor ink of the InO_<i>x</i> semiconductor layer was well-controlled. The inkjet-printed short-channel TFTs exhibited a maximum mobility of 4.9 cm² V^–1 s^–1 and an on/off ratio of larger than 10⁹. This approach of fabricating short-channel TFTs by inkjet printing will promote the large-area fabrication of short-channel TFTs in a cost-effective manner

High Efficiency and High <i>V</i>_oc Inverted Polymer Solar Cells Based on a Low-Lying HOMO Polycarbazole Donor and a Hydrophilic Polycarbazole Interlayer on ITO Cathode

In this work, poly­[<i>N</i>-9′-heptadecanyl-2,7-carbazole-<i>alt</i>-5,5-(4,7-di-2-thienyl-5,6-bis­(dodecyloxy)-2,1,3-benzothiadiazole)] (PCDTBT12) was synthesized as the polymer donor for photovoltaic application. PCDTBT12 possesses a band gap of 1.99 eV, a low-lying HOMO of −5.6 eV, and good hole mobility up to 4.1 × 10^–3 cm² V^–1 s^–1. With ZnO as the interlayer on an ITO cathode, a PCDTBT12-based inverted solar cell showed a high open-circuit voltage of 0.98 V and a good power conversion efficiency (PCE) of 5.53%, suggesting that PCDTBT12 would be a promising donor material in the fabrication of a subcell for shorter wavelength absorption in a tandem solar cell. Using PC-P, a homopolymer of 2,7-carbazole with hydrophilic phosphonate side chains, as an interlayer polymer on the ITO cathode could further elevate the efficiency to 6.04% because of increased current (higher efficiency of 6.2% was achieved for a smaller cell area of 0.045 cm²). The efficiencies are the highest ones so far reported for an inverted solar cell with an organic cathode interlayer. It was proposed that the hydrophilic side chains of PC-P supplied a subgap state for electron transport. The two devices showed comparable air stability, and retained over 96% of their initial PCEs after storage in air for more than 1 month. Therefore, a hydrophilic conjugated polymer as the cathode interlayer, already shown in outstanding cathode modifications in conventional polymer solar cells, will play an important role in the future development of high efficiency and air-stable inverted solar cells

Regioisomeric Non-Fullerene Acceptors Containing Fluorobenzo[<i>c</i>][1,2,5]thiadiazole Unit for Polymer Solar Cells

We designed and synthesized two isomeric nonfullerene acceptors, IFBR-<i>p</i> and IFBR-<i>d</i>. These molecular semiconductors contain indacenodithiophene (IDT) as the central unit, adjacent asymmetric 5-fluorobenzo­[<i>c</i>]­[1,2,5]­thiadiazole units, and are flanked with rhodanine as the peripheral units. The orientation of the two fluorine atoms (proximal, <i>p</i>, or distal, <i>d</i>), relative to IDT impacts most severely the film morphologies when blended with the electron-donating polymer PTzBI. Polymer solar cells based on PTzBI:IFBR-<i>p</i> give rise to a power conversion efficiency (7.3 ± 0.2%) that is higher than what is achieved with PTzBI:IFBR-<i>d</i> (5.2 ± 0.1%). This difference is attributed to the lower tendency for (over)­crystallization by IFBR-<i>p</i> and the resulting more favorable morphology of the photoactive layer. These results highlight the subtle impact of substitution regiochemistry on the properties of nonfullerene acceptors through modulation of their self-assembly tendencies

Homogeneous Surface Profiles of Inkjet-Printed Silver Nanoparticle Films by Regulating Their Drying Microenvironment

Obtaining homogeneous surface profiles of inkjet-printed silver nanoparticle films (SNFs) is severely hampered by the classical <i>coffee-ring</i> phenomenon. Traditional approaches to suppress the coffee-ring effect are achieved by improving the uniformity of liquid evaporation rate or introducing inward Marangoni force during ink drying. However, these existing methods involve extra chemicals, treatments, or equipment that will definitely increase the production cost or reduce conductivity. In this study, we demonstrate an inexpensive and efficient method to obtain uniform surface profiles of inkjet-printed SNFs by rationally regulating the drying microenvironment. To this end, a number of surface profiles of printed dots and lines, such as concave, convex, and flat, were first obtained via this method. The underlying principle of this method was then investigated by analytical calculations based on the simplified vapor diffusion model. Consequently, homogeneous surface profiles of inkjet-printed SNFs were achieved on the basis of aforementioned analytical calculations. Our work provides a new possibility to regulate the surface profiles of inkjet-printed nanoparticle-based patterns in the form of low cost and efficiency

High-Performance, Solution-Processed Quantum Dot Light-Emitting Field-Effect Transistors with a Scandium-Incorporated Indium Oxide Semiconductor

Light-emitting field-effect transistors (LEFETs) have attained great attention due to their special characteristics of both the switching capacity and the electroluminescence capacity. However, high-performance LEFETs with high mobility, high brightness, and high efficiency have not been realized due to the difficulty in developing high electron and hole mobility materials with suitable band structures. In this paper, quantum dot hybrid LEFETs (QD-HLEFETs) combining high-luminous-efficiency quantum dots (QDs) and a solution-processed scandium-incorporated indium oxide (Sc:In₂O₃) semiconductor were demonstrated. The red QD-HLEFET showed high electrical and optical performance with an electron mobility of 0.8 cm² V^–1 s^–1, a maximum brightness of 13 400 cd/m², and a maximum external quantum efficiency of 8.7%. The high performance of the QD-HLEFET is attributed to the good energy band matching between Sc:In₂O₃ and QDs and the balanced hole and electron injection (less exciton nonradiative recombination). In addition, incorporation of Sc into In₂O₃ can suppress the oxygen vacancy and free carrier generation and brings about excellent current and optical modulation (the on/off current ratio is 10⁵ and the on/off brightness ratio is 10⁶)

Damage-Free Back Channel Wet-Etch Process in Amorphous Indium–Zinc-Oxide Thin-Film Transistors Using a Carbon-Nanofilm Barrier Layer

Amorphous indium–zinc-oxide thin film transistors (IZO-TFTs) with damage-free back channel wet-etch (BCE) process were investigated. A carbon (C) nanofilm was inserted into the interface between IZO layer and source/drain (S/D) electrodes as a barrier layer. Transmittance electron microscope images revealed that the 3 nm-thick C nanofilm exhibited a good corrosion resistance to a commonly used H₃PO₄-based etchant and could be easily eliminated. The TFT device with a 3 nm-thick C barrier layer showed a saturated field effect mobility of 14.4 cm² V^–1 s^–1, a subthreshold swing of 0.21 V/decade, an on-to-off current ratio of 8.3 × 10¹⁰, and a threshold voltage of 2.0 V. The favorable electrical performance of this kind of IZO-TFTs was due to the protection of the inserted C to IZO layer in the back-channel-etch process. Moreover, the low contact resistance of the devices was proved to be due to the graphitization of the C nanofilms after annealing. In addition, the hysteresis and thermal stress testing confirmed that the usage of C barrier nanofilms is an effective method to fabricate the damage-free BCE-type devices with high reliability

Critical Impact of Solvent Evaporation on the Resolution of Inkjet Printed Nanoparticles Film

We first verify the critical role of solvent evaporation on the resolution of inkjet printing. To confirm our hypothesis, we adjusted the evaporation rate gradient along the surface of adjacent droplets by controlling the drying microenvironment. Uneven solvent evaporation flux caused thermocapillary surface flow inward the space of micrometer-sized droplets and increase the air pressure, which prevented the neighboring droplets from coalescence. When reducing the droplet distance by the solvent evaporation-based method, a uniform profile could be obtained at the same time. This work brings us a step closer to resolving one of the critical bottlenecks to commercializing printed electronic goods