Reduced Singlet–Triplet Annihilation for Low Threshold Amplified Spontaneous Emission from a Blue Polyfluorene Electroluminescent Organic Semiconductor

Abstract

A blue light-emitting conjugated polymer, namely, arylpolyfluorene (aryl-F8), having a high neat-film photoluminescence quantum yield (73%) and a radiative decay rate (2 × 10 9 s −1 ) is reported. Excimer emission from the polymer is significantly reduced in its neat film, unlike many other wide band gap blue emitters achieved as a result of the bulky aryl groups attached to the polymer chain. Amplified spontaneous emission (ASE) under optical excitation is observed in a pristine film at an excitation fluence, E th ∼ 3 μJ cm −2 , which is one of the lowest among polymer-based optical gain media in this spectral range reported so far. The well-separated spectra of stimulated emission and long-lived triplet absorption and very low triplet yields explain the achievement of a low ASE threshold. Under electrical excitation, no singlet−triplet annihilation is detected in the light-emitting diode fabricated with this polymer in a hybrid structure. Thus, aryl-F8 emerges as an attractive optical gain medium for lasing applications.

Internal conversion rate constant calculations considering Duschinsky, anharmonic and Herzberg–Teller effects

“…16–21 Since polyfluorenes have a high fluorescence quantum yield and good thermal stability, OLEDs based on them are expected to be efficient with a long lifetime. 22–24 Calculations of molecular photophysical properties can be very useful when designing the molecular structure of semiconducting polymers in OLED devices. 25…”

“…[16][17][18][19][20][21] Since polyfluorenes have a high fluorescence quantum yield and good thermal stability, OLEDs based on them are expected to be efficient with a long lifetime. [22][23][24] Calculations of molecular photophysical properties can be very useful when designing the molecular structure of semiconducting polymers in OLED devices. 25 Recently, we calculated the vibronic spectra of 9,9,9',9'-tetramethyl-2,2'-bifluorene and 9,9,9',9',9 00 ,9 00 -hexamethyl-2,2'-7',2 00terfluorene, 26 which emit light in the blue region.…”

Carbon Nanodots as Electron Transport Materials in Organic Light Emitting Diodes and Solar Cells

“…The corresponding energy level diagram of the materials embedded into the OLED structure, as well as the chemical structure of F8BT, is illustrated in Figure 6 b. Note that the work functions of ITO, PEDOT:PSS and Al, along with the energy levels of F8BT, were taken from the literature [ 37 , 38 ]. The HOMO and LUMO levels of both carbon nanodots were estimated by cyclic voltammetry and optical measurements, as already mentioned.…”

Study of Intrachain Charge Transfer in a Blue Emissive Polyfluorene Random Copolymer

“…Since the first demonstration of the light emitting diode (LED), based on conjugated polymers, a great amount of research has been carried out in the field of optoelectronics, which established them as excellent electroluminescent materials. − Conjugated polymers are an attractive class of organic semiconductors due to their high photoluminescence quantum yield (PLQY), tunability across a wide range of emission wavelengths, and easy solution processability . Among the various conjugated polymers, polyfluorene-based homopolymers and copolymers are highly explored in electroluminescence applications due to efficient blue emission and high thermal and oxidative stability. − …”