Ornamenting of Blue Thermally Activated Delayed Fluorescence Emitters by Anchor Groups for the Minimization of Solid-State Solvation and Conformation Disorder Corollaries in Non-Doped and Doped Organic Light-Emitting Diodes

Abstract

Motivated to minimize the effects of solid-state solvation and conformation disorder on emission properties of donor–acceptor-type emitters, we developed five new asymmetric multiple donor–acceptor type derivatives of tert-butyl carbazole and trifluoromethyl benzene exploiting different electron-accepting anchoring groups. Using this design strategy, for a compound containing four di-tert-butyl carbazole units as donors as well as 5-methyl pyrimidine and trifluoromethyl acceptor moieties, small singlet-triplet splitting of ca. 0.03 eV, reverse intersystem crossing rate of 1 × 106 s–1, and high photoluminescence quantum yield of neat film of ca. 75% were achieved. This compound was also characterized by the high value of hole and electron mobilities of 8.9 × 10–4 and 5.8 × 10–4 cm2 V–1 s–1 at an electric field of 4.7 × 105 V/cm, showing relatively good hole/electron balance, respectively. Due to the lowest conformational disorder and solid-state solvation effects, this compound demonstrated very similar emission properties (emission colors) in non-doped and differently doped organic light-emitting diodes (OLEDs). The lowest conformational disorder was observed for the compound with the additional accepting moiety inducing steric hindrance, limiting donor–acceptor dihedral rotational freedom. It can be exploited in the multi-donor–acceptor approach, increasing the efficiency. Using an emitter exhibiting the minimized solid-state solvation and conformation disorder effects, the sky blue OLED with the emitting layer of this compound dispersed in host 1,3-bis­(N-carbazolyl)­benzene displayed an emission peak at 477 nm, high brightness over 39 000 cd/m2, and external quantum efficiency up to 15.9% along with a maximum current efficiency of 42.6 cd/A and a maximum power efficiency of 24.1 lm/W.

Understanding the temporal dynamics of thermally activated delayed fluorescence in solid hosts

“…Multicarbazole TADF structures potentially could be further exploited as low-disorder emitters with high rISC rates. 59–61 The same trend was also shown for TADF compounds with donor units in quasi-axial (low steric hindrance, large disorder) and quasi-equatorial (high steric hindrance, lower disorder) orientations. 50,58 Interestingly, only a minor blue-shift of DF was observed for PTZ-mPYR since the latest DF from the states with large Δ E ST values was quenched.…”

Structure and Conformational Mobility of OLED-Relevant 1,3,5-Triazine Derivatives

“…Numerous studies have focused on devices based on polymers with an incorporated carbazole building block [ 18 , 19 , 20 , 21 ]. Carbazole derivatives are an effective host material for phosphorescent additives in PhOLEDs [ 15 , 22 , 23 , 24 , 25 , 26 , 27 , 28 ], an active component of fluorescent OLEDs with thermally activated delayed fluorescence (TADF) [ 14 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 ] or molecules with a hybridized local and charge state of transfer (HLCT) [ 44 , 45 , 46 , 47 , 48 , 49 , 50 ]. Computer-aided design is also currently being used to suggest more stable and more efficient organic diodes [ 51 ].…”

Marching Toward Long‐Wavelength Narrowband Emissive Multi‐Resonance Delayed Fluorescence Emitters for Organic Light Emitting Diodes

“…[ 26–28 ] To achieve small Δ E ST , the conventional TADF emitter design adopted twisted donor (D)‐acceptor (A) configuration to minimize the orbital overlap between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO). [ 29–49 ] Although, the conventional TADF emitters demonstrated benchmark device performance comparable to the phosphorescent emitters, they are suffering with serious drawbacks. Majorly, as the degree of HOMO‐LUMO separation increases, the radiative transition probability ( f ) decreases for S 1 state, leading to the poor photoluminescence quantum yield (PLQY) for the emitters and inferior EQE of the device.…”

“…Majorly, as the degree of HOMO‐LUMO separation increases, the radiative transition probability ( f ) decreases for S 1 state, leading to the poor photoluminescence quantum yield (PLQY) for the emitters and inferior EQE of the device. [ 29–49 ] Moreover, the D‐A molecular design, which involves strong intramolecular charge transfer (ICT) character leads to the severe structural relaxations in the S 1 state, together with significant vibronic coupling between S 1 and S 0 states. [ 29–79 ] This propensity always leads to the broad emission spectrum with associated large full‐width‐at‐half maximum (FWHM) > 70 nm.…”

“…[ 29–49 ] Moreover, the D‐A molecular design, which involves strong intramolecular charge transfer (ICT) character leads to the severe structural relaxations in the S 1 state, together with significant vibronic coupling between S 1 and S 0 states. [ 29–79 ] This propensity always leads to the broad emission spectrum with associated large full‐width‐at‐half maximum (FWHM) > 70 nm. [ 29–49 ] Thus, additional color filters or microcavities need to be implanted in the OLED to selectively transmit the pure color emission and satisfy the color purity of HD‐OLEDs, which will lead to the increase of fabrication cost, energy loss and low EQE .…”