Research paperCorrelation between the PL and EL emissions of polyfluorene-based diodes using bilayers or polymer blends
Introduction
Organic light-emitting diodes (OLEDs) based on conducting polymers are attractive due to the possibility of ease of processing and manufacturing, low-cost fabrication and ability to build large-area devices [1], [2], [3]. OLEDs with different colors can be achieved by combining multiple layers in a tandem diode architecture [4], [5], [6]; using a single polymer with multiple functional groups [7], [8], [9], [10]; using mixtures of polymers with quantum dots [11], [12], [13], nanorods/nanotubes [14], [15], [16], and small fluorescent [8], [17], [18] or phosphorescent [19], [20] molecules; using systems with an excimer or exciplex; or preparing blends of conjugated polymers [2], [21], [22], [23].
Polyfluorene derivatives have emerged as a very attractive class of conjugated polymers for display applications because of their efficient electroluminescence coupled with a high charge carrier mobility and good processability [24]. In the blend type of polyfluorene OLEDs, the use of poly(9,9-di-n-octylfluorenyl-2,7-diyl) (PFO), which has a relatively large band gap function, as the host material and the green light-emitting polyfluorene copolymer, poly[(9,9-di-n-octylfluorenyl-2,7-diyl)-alt-(benzo[2,1,3]thiadiazol- 4,8-diyl)] (F8BT), as the dopant material has attracted research interest [1], [25], [26], [27], [28], [29]. The use of polymer blends as emissive layers in OLEDs offers several advantages over single component layers [26], [30] by combining two different polymers with contrasting electrical properties, in which the emission efficiency can be optimized by balancing hole and electron injection [26].
Despite this advantage, the blending of materials also requires additional precautions because of the possibility of nonradiative energy transfer processes (known as the Förster energy transfer mechanism [31], [32]), which occur when a component that emits at a lower energy also absorbs in the spectral range of emission of another component that emits at a higher energy [2], [33]. This phenomenon may quench or decrease the intensity of the higher energy emission band, and when the energy transfer process is very efficient, the higher energy component of the emission is eliminated. Nevertheless, when a judicious concentration of the lower energy component is used, both color tuning and an increase in the luminous performance can be achieved simultaneously [2]. Thus, understanding the dynamics of the emission in the polymer mixture is necessary to explain device performance.
PFO has a high hole mobility [34], and it emits blue fluorescence following ultraviolet excitation with a quantum efficiency [35], [36] above 50%. Additionally, F8BT has a large electron mobility [37], and it emits green fluorescence with a quantum yield of 50% following excitation by blue light [38]. Therefore, light emission can occur from both the host (PFO) and the dopant polymer (F8BT), which can be excited by Förster energy transfer, as explained above [37]. Buckleyet et al. showed via time-resolved spectroscopic measurements that excitons photogenerated on PFO molecules can rapidly be transferred to F8BT molecules over distances of 5 nm via Förster transfer (dipole–dipole coupling) [39].
In this work, the dynamics of Förster energy transfer in donor-acceptor blends and bilayer polyfluorene-based OLEDs were investigated. We also studied the device performance. Two types of polyfluorene-based organic light-emitting diodes (OLEDs) were investigated: blend emissive layer devices consisting of PFO (higher weight percentage, from 98 to 99.8 wt.%) and F8BT (lower weight percentage, from 0.2 to 2 wt.%) and bilayer devices consisting of emissive layers of PFO/F8BT. The luminance–voltage and current density–voltage characteristics and electroluminescent (EL) spectra were recorded. The photophysical properties of the emissive layers were also analyzed to compare the photo- and electro-luminescence characteristics.
Section snippets
Materials
PFO ( = 40,000–150,000 g mol−1) was purchased from American Dye Source (ADS129BE). F8BT ( = 22,000–24,000 g mol−1), polyvinylcarbazole (PVK) ( = 1.1 × 10−5 g mol−1), toluene (Tol) (99.8%) and chlorobenzene (ClBz) (99.8%) were purchased from Sigma-Aldrich. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) was purchased from H.C. Starck (Clevios P VP AI 4083). The ITO-glass substrates (Corning Eagle XG/1737, 7-10 ohm-sq) were cut, cleaned and patterned before use.
PLED assembling using F8BT and PFO mixtures
Two types of emissive
Results and discussion
The nature of the polymer–polymer interfaces in blends or bilayers is of great importance for device performance. In this work, we compared bilayer and blend films based on PFO and F8BT and investigated the influence of the film type and composition on the PL and EL properties.
For the blends, it has been previously discussed that the nature of the solvent used for spin-coating a PFO:F8BT mixture affects the degree of phase separation in the film. It has been demonstrated for blends containing
Conclusions
PFO/F8BT mixtures are versatile systems in which the EL color can be fine-tuned over a range of colors between green and blue shades or white using several tools: variation of the applied voltage, phase separation induced by annealing or variation of the composition (ratio) of the blends. Here, we demonstrated that partial stratification using a bilayer-like structure is yet another means to tune the EL color. Although the optimization of device performance was outside the scope of this paper,
Acknowledgements
The authors acknowledge FAPESP (Grant 2013/16245-2), CNPq, the National Institute of Organic Electronics (INEO) (MCT/CNPq/FAPESP), CTI and UNICAMP/FAEPEX for financial support and fellowships.
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