Applications of Non-conjugated Polymer Doped with Salts and Metal Chelate Complexes in Polymer Light-emitting Diode

Abstract

Polymer light-emitting diodes (PLEDs) are carrier-injection devices which basically require balance of hole- and electron-injection from the two electrodes along with effective transport and recombination in the emitting layer However for most conjugated polymers holes are more readily injected and transported than electrons Therefore effective electron injection and excition confinement in emitting layer are essential for high performance PLEDs In this study we used non-conjugated polymer poly(vinyl alcohol) (PVA) and hydroxyethyl cellulose (HEC) as electron-injection or hole–blocking layer to improve electron injection and carrier recombination Non-conjugated polymer possesses high polar functional groups in main or side chain which facilitate interfacial dipole formation and facile fabrication by solution process In addition we attempted to dope this non-conjugated polymer layer with some different carbonate salts acetate salts and metal chelate complexes which successfully enhances its electron-injection capability Using neat PVA as an electron-injection layer the maximum luminance and maximum current efficiency of the device (glass/ITO/PEDOT:PSS/HY-PPV/PVA/Al) were significantly enhanced due to promoted electron injection and hole blocking Moreover the device performance was further enhanced by doping the PVA with alkali metal salts (M2CO3 or CH3COOM; M: Na K Cs) and the enhancement is increased with increasing dopant concentration Particularly the PVA doped with 30 wt% alkali metal carbonates revealed the best performance (20214~25163 cd/m2 5 83~6 83 cd/A) The synergistic effect of electro-injecting and hole-blocking PVA electron affinitive alkali metal salts and well-aligned crystallites on layer surface gives rise to the enhancements Water-soluble non-conjugated polyer HEC was also applicable as electron-injection or hole–blocking layer An electron-injection layer using HEC filled with chelate complexes [acetate group (CH3COO)2-M ethylenediaminetetraacetic group EDTA-M; M: Ca2+ Mg2+] was fabricated by spin-coating processes Devices based on HEC doped with EDTA-M provided the superior performance with the maximum luminance and maximum current efficiency enhanced seven- to eight-fold approximately This performance enhancement has been attributed to electron donation from the chelator that reduces metal cations to a “pseudo-metallic state” enabling it to act as an intermediate step to facilitate electron injection These results provide compelling evidence that the chelating capability of EDTA plays an essential role in enhancing performances Finally another metal chelate complex copper(II) phthalocyanine-tetrasulfonated acid tetrasodium salt (TS-CuPc) was investigated Doping 12 wt% TS-CuPc into HEC as a dual functional hole-blocking layer (df-HBL) of device (glass/ITO/PEDOT:PSS/HY-PPV/TS-CuPc-doped HEC/LiF/Al) significantly enhanced maximum luminance maximum current and power efficiency from 10319 cd/m2 2 98 cd/A and 1 24 lm/W of without the df-HBL to 29205 cd/m2 13 27 cd/A and 9 56 lm/W respectively Topography and conductivity AFM images show that doping TS-CuPc increases the interfacial contact area and interfacial conductivity which can overcome the insulating nature of HEC and thus further facilitate electron injection Enhancements in device performance have been attributed to the improved carrier balance and recombination in the presence of df-HBL We successfully demonstrated several effective approach to further achieve PLED performance improvements by using non-conjugated polymers doped with salts or metal chelate complexes These results indicate that thes doped non-conjugated polymers are extremely promising materials for fully solution-processed light-emitting devices which can simultaneously realize electron injection and hole blocking

Date of Award	2017 Aug 21
Original language	English
Supervisor	Yun Chen (Supervisor)