Inkjet printing of colloidal quantum dots (QDs) is considered a promising technology for application in full‐color quantum dot light‐emitting diode (QLED) displays. However, QLEDs that are inkjet printed in a pixel‐defining bank structure generally exhibit a low performance, mainly due to the nonuniformity in its QD morphology. In this study, an enhanced performance of inkjet‐printing‐based pixelated QLEDs is achieved by introducing small amounts of poly(methyl methacrylate) (PMMA) of different molecular weights into QD inks. When this QD–PMMA composite ink is adopted, uniform droplets are formed, originating from contact line depinning during drying. Inside the bank structure, the inkjet‐printed QD–PMMA composite film shows a smooth surface and little pileup at the bank edges. A pixelated QLED with PMMA with a molecular weight of 8 kDa exhibits the highest luminance of 73 360 cd m−2 and an external quantum efficiency of 2.8%, which are remarkably higher than that of the inkjet‐printed QLED subpixels without PMMA. The result is verified through the observation of the drying process and the QLED subpixel shapes under operation. Thus, inkjet‐printed QD–PMMA composite inks can be a promising strategy for future research on pixelated QLEDs for the fabrication process of full‐color QLED displays.

Charge balance between electrons and holes, which are injected into the quantum dot (QD) emission layer (EML), is critical for realizing stable and efficient QD light‐emitting diodes (QLEDs). ZnO has been widely used as an electron‐transport layer (ETL) because of its superior performance compared to other metal oxides. However, nearly barrier‐free electron injection into the QD EML leads to spontaneous charge transfer and excess electron injection, resulting in reduced device performance. Here, to adjust electron–hole balance and thereby improve the lifetime and efficiency of QLEDs, we introduce an yttrium (Y)‐doped ZnO (YZO) ETL into QLEDs. The sol–gel processed YZO film, with a mobility that could be simply tuned by varying the Y concentration, provides enhanced charge‐transport balance by suppressing excess electron flow to the QD EML. Furthermore, YZO helps suppress QD charging and smooth the surface morphology. Applying the YZO ETL into the inverted‐structure QLED enables us to achieve color‐saturated red emission, an improved external quantum efficiency of up to 8.6%, and an eight times longer lifetime compared to the device with undoped ZnO. This result provides a simple method for enhancing the performance of QLEDs by easily controlling metal doping concentration in the sol–gel processed ZnO ETL.

In this work, operationally and mechanically stable organic field‐effect transistors (OFETs) are demonstrated on aramid fiber‐based paper enabled by a simple and universal surface planarization method. By employing a nanoimprint lithography‐inspired surface smoothening method, rough aramid paper is successfully smoothened from a scale of several tens of micrometers to a sub‐nanometer‐scale surface roughness. Owing to the sub‐nanometer‐scale surface roughness of the aramid paper, the OFETs fabricated on the aramid paper exhibit decent field‐effect mobility (0.25 cm2 V−1 s−1) with a high current on‐to‐off ratio (>107), both of which are comparable with those of OFETs fabricated on rigid silicon substrates. Moreover, the OFETs fabricated on the aramid paper exhibit both high operational and mechanical stability; this is indicated by a bias‐stress‐induced threshold voltage shift (∆VTH ≈ 4.27 V under an excessive gate bias stress of 1.7 MV cm−1 for 1 h 30 min) comparable to that of OFETs on a rigid silicon substrate, moderate field‐effect mobility, and a threshold voltage stability under 1000 bending cycles with a compressive strain of 1%. The demonstration of highly stable OFETs on paper enabled by the simple planarization method will expand the potential use of various types of paper in electronic applications.

We demonstrated a bank structure for inkjet-printed quantum dot light-emitting diodes (QLEDs) fabricated through photolithography process using black photoresist (B-PR). The B-PR banks have low surface energy (13 mJ m -2 ), resulting in well confined quantum dot (QD) ink inside the pixel area. Based on the B-PR bank structure, we demonstrated a QLED with 0.20 % of external quantum efficiency.

We report the influence of post-treatment via the external pressure on the device performance of quantum dot (QD) solar cells. The structural analysis together with optical and electrical characterization on QD solids reveal that the external pressure compacts QD active layers by removing the mesoscopic voids and enhances the charge carrier transport along QD solids, leading to significant increase in JSC of QD solar cells. Increasing the external pressure, by contrast, accompanies reduction in FF and VOC, yielding the trade-off relationship among JSC and FF and VOC in PCE of devices. Optimization at the external pressure in the present study at 1.4-1.6 MPa enables us to achieve over 10% increase in PCE of QD solar cells. The approach and results show that the control over the organization of QDs is the key for the charge transport properties in ensemble and also offer simple yet effective mean to enhance the electrical performance of transistors and solar cells using QDs.