Abstract
A series of white polymer light-emitting devices (WPLEDs) were fabricated by utilizing star-shaped white-emission copolymers containing tri[1-phenylisoquinolinato-C2,N]iridium (Ir(piq)3), fluorenone (FO) and poly(9,9-dioctylfluorene) (PFO) as red-, green- and blue-emitting (RGB) components, respectively. In these WPLEDs, a maximum current efficiency of 6.4 cd·A−1 at 20 mA·cm−2 and Commission Internationale d’Eclairage (CIE) coordinates of (0.33, 0.32) were achieved, and the current efficiency was still kept to 4.2 cd·A−1 at the current density of 200 mA·cm−2. To investigate energy transfer processes among the three different chromophores of the star-shaped copolymers in these WPLEDs, the time-resolved photoluminescence (PL) spectra were recorded. By comparing the fluorescence decay lifetimes of PFO chromophores in the four star-like white-emitting copolymers, the efficient energy transfer from PFO units to Ir(piq)3 and FO chromophores was confirmed. From time-resolved PL and the analysis of energy transfer process, the results as follows were proved. Owing to the star-like molecular structure and steric hindrance effect, intermolecular interactions and concentrations quenching in the electroluminescence (EL) process could also be sufficiently suppressed. The efficient energy transfer also reduced intermolecular interactions’ contribution to the enhanced device performances compared to the linear single-polymer white-light systems. Moreover, saturated stable white emission results from the joint of energy transfer and trap-assisted recombination. This improved performance is expected to provide the star-like white-emitting copolymers with promising applications for WPLEDs.
Highlights
In the last few decades, white polymer light-emitting devices (WPLEDs) have gained remarkable attention because of their various applications in full-color displays with color filters, backlights, and solid-state lighting [1,2,3,4,5,6,7,8]
When electrons and holes are injected into emitting layer (EML) from the layers of PEDOT:PSS and TPBi, respectively, the sites of FO and Ir(piq)3 will act as traps for both electrons and holes in EML [26,30,31] and these carriers are trapped by these traps near the interface of EML, hindering the migration of these carriers towards the recombination region [32,33]
More carriers need to be injected into the EML for white emission, which causes a larger current density
Summary
In the last few decades, white polymer light-emitting devices (WPLEDs) have gained remarkable attention because of their various applications in full-color displays with color filters, backlights, and solid-state lighting [1,2,3,4,5,6,7,8]. A high current efficiency of 18 cd·A−1 was obtained in their single-layer devices utilizing the star-like white-emitting polymers as the active layer. Yang and his coworkers [20] developed a series of new two-color star-shaped white-emission single polymers that simultaneously consist of fluorescent and phosphorescent dyes. As these star-shaped white-emitting polymers only comprise two complementary colors, the EL spectra of these polymers cannot cover the whole visible range and result in the limitation of applications
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