Exciplex‐Forming Cohost Systems with 2,3‐Dicyanopyrazinophenanthrene‐based Acceptors to Achieve Efficient Near Infrared OLEDs

Abstract

Two new 2,3‐dicyanopyrazinophenanthrene‐based acceptors (A) p‐QCN and m‐QCN were synthesized to blend with a donor (D) CPTBF for the exciplex formation. The energy levels of p‐QCN and m‐QCN are modulated by the peripheral substituents 4‐ and 3‐benzonitrile, respectively. Exciplex‐forming blends were identified by the observation of the red‐shifted emissions from various D : A blends with higher ratios of donor for suppressing the aggregation of acceptor. The two‐component relaxation processes observed by time‐resolved photoluminescence support the thermally activated delayed fluorescence (TADF) character of the exciplex‐forming blends. The device employing CPTBF : p‐QCN and (2 : 1) and CPTBF : m‐QCN (2 : 1) blend as the emitting layer (EML) gave EQEmax of 1.76 % and 5.12 %, and electroluminescence (EL) λmax of 629 nm and 618 nm, respectively. The device efficiency can be further improved to 4.32 % and 5.57 % with CPTBF : p‐QCN and (4 : 1) and CPTBF : m‐QCN (4 : 1) as the EML, which is consistent with their improved photoluminescence quantum yields (PLQYs). A new fluorescent emitter BPBBT with photoluminescence (PL) λmax of 726 nm and a high PLQY of 67 % was synthesized and utilized as the dopant of CPTBF : m‐QCN (4 : 1) cohost system. The device employing CPTBF : m‐QCN (4 : 1): 5 wt.% BPBBT as the EML gave an EQEmax of 5.02 % and EL λmax centered at 735 nm, however, the weak residual exciplex emission remains. By reducing the donor ratio, the exciplex emission can be completely transferred to BPBBT and the corresponding device with CPTBF : m‐QCN (2 : 1): 5 wt.% BPBBT as the EML can achieve EL λmax of 743 nm and EQEmax of 4.79 %. This work manifests the high efficiency near infrared (NIR) OLED can be realized by triplet excitons harvesting of exciplex‐forming cohost system, followed by the effective energy transfer to an NIR fluorescent dopant.

Development of novel acceptors with deep LUMO energy levels

“…Recently, more and more researchers have committed to exploring efficient exciplex-based OLEDs and made great progress. For the blue and green emissions, the external quantum efficiencies (EQEs) of some pureorganic exciplex-based devices have approached and exceeded 20%, gradually catching up with the pace of single-molecule TADF OLEDs [5][6][7]. However, the development of pure-organic red exciplex emitters with emission peaks over 600 nm has been almost stagnant.…”

Enhancing Tandem Organic Light-Emitting Diode Performance with Multiple Electroluminescent Units

“…27,34,35 D/A molecules used in the exciplex host system are usually HTM/ETM with high carrier mobility, enabling researchers to arbitrarily adjust the blending D/A ratio to achieve carrier balance and also ensure that the exciton formation zone is effectively allocated in the EML. 27,36 Furthermore, this mixed host system could transport holes and electrons simultaneously and thus expand the recombination zone in the EML, mitigating the exciton quenching. 37−40 enabling OLEDs to meet the stringent requirements for lighting or phototherapy applications.…”

“…On the other hand, reducing the negative impact of intermediate CGLs with uneven charge generation efficiency in tandem devices consisting of three or more EUs results in carrier imbalance in each EU. Exciplex or co-host systems generated by the mixture of specific electron donor (D) molecules and electron acceptor (A) molecules are potential EML designs to maintain the carrier balance and EL efficiency in each EU. ,, D/A molecules used in the exciplex host system are usually HTM/ETM with high carrier mobility, enabling researchers to arbitrarily adjust the blending D/A ratio to achieve carrier balance and also ensure that the exciton formation zone is effectively allocated in the EML. , Furthermore, this mixed host system could transport holes and electrons simultaneously and thus expand the recombination zone in the EML, mitigating the exciton quenching. − Last but not least, the light extraction structure introduced in TOLEDs could substantially improve the light outcoupling of the device and significantly prolong device lifetime compared to conventional devices operating at the same luminance level, enabling OLEDs to meet the stringent requirements for lighting or phototherapy applications. According to our previous studies, an embedded NP-based diffuser structure should have great potential to improve the outcoupling efficiency of the tandem device and reduce the directional emission due to the microcavity effect. − …”