# Solution-Processed Organic Light-Emitting Diodes Using a Lamination Method
## 1 Introduction  
Organic light-emitting diodes (OLEDs) have attracted significant attention for their excellent performance in high-quality full-color displays and lighting applications. Recently, numerous OLED display products have been introduced to the global market, and OLED products for lighting are also being actively developed. However, most OLED manufacturing processes currently rely on vacuum deposition equipment, which has the disadvantages of high manufacturing costs and complex processes. In particular, in the fields of large-screen TVs, automotive displays, and lighting, where large-area production is in progress, the possibility of manufacturing large-size devices at low cost has become important [1]. Accordingly, the solution process is attracting attention as a process advantageous for manufacturing large-area devices at low cost, and various studies are being conducted on spin-coating [2], roll-to-roll [3], slit-coating [4], inkjet printing [5], and laminating [6]. Solution-processed OLEDs have reliability issues such as low brightness and intermediate layer mixing, which makes them exhibit characteristics that are inferior to vacuum deposition methods. OLEDs fabricated by vacuum deposition methods show a luminance of approximately 22,000 cd/m\({}^{2}\)[7], while OLEDs fabricated by solution process spin-coating show a luminance of about 15,000 cd/m\({}^{2}\)[8]. Laminating two substrates can be a simple, low-cost process for producing large-area, flexible lighting, but have a significantly low luminance of 4000 cd/m\({}^{2}\)[9]. In a previous paper, our lab presented the experimental results of mixing PEG into PEI for enhanced characteristics in both adhesion and electrical applications [10]. When 0.1 wt% PEI and 0.01 wt% PEG were mixed, thedevice had a maximum luminance of 2700 cd/m\({}^{2}\). It is encouraging that the mixed-solution layer provided similar luminous characteristics as single- and double-substrate devices. On the other hand, the electrical and luminous characteristics were still insufficient. In our previous research, the other problem was that our device showed different dependency on the composition of the adhesive interlayer for the current density and luminance [10], which was explained by the unbalancing problem of the charge carriers. Therefore, this study focused on improving the composition of the adhesive layer and electron-transport layer to implement high-performance OLEDs. In order to laminate and bond two substrates, the adhesive layer must have appropriate adhesive strength and, at the same time, must have properties that do not hinder electron injection. We used polyethyleneimine (PEI) with sticky properties to meet these conditions. However, PEI had the issue of reduced electron injection capability when its adhesive strength was enhanced [11]. Unlike previous studies that used PEG and D-sorbitol individually [6; 9], we optimized the adhesive layer by blending PEI with PEG or D-sorbitol. This approach simultaneously enhanced adhesion strength and electron injection properties, addressing the limitations of standalone materials. The higher melting point of D-sorbitol (95 \({}^{\circ}\)C) compared to PEG (60 \({}^{\circ}\)C) ensured better thermal stability under high-voltage conditions, while its plasticizing effect minimized surface roughness, improving electron injection and overall device performance. PEG is a polymer electrolyte and is widely used as an adhesive aid, and D-sorbitol is a polyhydric alcohol that strengthens adhesive strength through hydrogen bonding. In particular, D-sorbitol induces a plasticizing effect during heat treatment to promote the flattening of the layer [12]; through this, it can reduce the electron injection degradation that can occur due to the presence of a rough surface.