Energy Landscape of Vertically Anisotropic Polymer Blend Films toward Highly Efficient Polymer Light‐Emitting Diodes (PLEDs)

• Published in: Advanced functional materials Vol. 28; no. 8
• Main Authors: Hassan, Muhammad Umair, Liu, Yee‐Chen, Yetisen, Ali K., Butt, Haider, Friend, Richard Henry
• Format: Journal Article
• Language: English
• Published: Hoboken Wiley Subscription Services, Inc 21.02.2018
• Subjects: anisotropic films; Charge transport; charge traps; Current carriers; energy band diagram; Energy distribution; Excitons; Hole mobility; Hole traps; Light emission; Light emitting diodes; Materials science; Molecular orbitals; Organic light emitting diodes; Polymer blends; Polymer films; Radiative recombination; Raman spectroscopy; Vertical distribution
• ISSN: 1616-301X; 1616-3028
• DOI: 10.1002/adfm.201705903

A blend of two hole‐dominant polymers is created and used as the light emissive layer in light‐emitting diodes to achieve high luminous efficiency up to 22 cd A−1. The polymer blend F81−xSYx is based on poly(9,9‐dioctylfluorene) (F8) and poly(para‐phenylene vinylene) derivative superyellow (SY). The blend system exhibits a preferential vertical concentration distribution. The resulting energy landscape modifies the overall charge transport behavior of the blend emissive layer. The large difference between the highest unoccupied molecular orbital levels of F8 (5.8 eV) and SY (5.3 eV) introduces hole traps at SY sites within the F8 polymer matrix. This slows down the hole mobility and facilitates a balance between the transport behavior of both the charge carriers. The balance due to such energy landscape facilitates efficient formation of excitons within the emission zone well away from the cathode and minimizes the surface quenching effects. By bringing the light‐emission zone in the middle of the F81−xSYx film, the bulk of the film is exploited for the light emission. Due to the charge trapping nature of SY molecules in F8 matrix and pushing the emission zone in the center, the radiative recombination rate also increases, resulting in excellent device performance. Light‐emitting polymer films based on the blend of poly(9,9‐dioctylfluorene) and poly(para‐phenylene vinylene) derivative superyellow exhibit a preferential vertical concentration distribution. A nonuniform energy landscape due to the large difference between the highest occupied molecular levels of both polymers balances the charge transport in the films and leads to achieving highly efficient light‐emitting diodes.