Abstract
Organic semiconductors with bipolar transporting character are highly attractive as they offer the possibility to achieve high optoelectronic performance in simple device structures. However, the continual efforts in preparing bipolar materials are focusing on donor-acceptor (D-A) architectures by introducing both electron-donating and electron-withdrawing units into one molecule in static molecular design principles. Here, we report a dynamic approach to construct bipolar materials using only electron-donating carbazoles connected by N-P=X resonance linkages in a donor-resonance-donor (D-r-D) structure. By facilitating the stimuli-responsive resonance variation, these D-r-D molecules exhibit extraordinary bipolar properties by positively charging one donor of carbazole in enantiotropic N+=P-X- canonical forms for electron transport without the involvement of any acceptors. With thus realized efficient and balanced charge transport, blue and deep-blue phosphorescent organic light emitting diodes hosted by these D-r-D molecules show high external quantum efficiencies up to 16.2% and 18.3% in vacuum-deposited and spin-coated devices, respectively. These results via the D-r-D molecular design strategy represent an important concept advance in constructing bipolar organic optoelectronic semiconductors dynamically for high-performance device applications.
Highlights
Organic semiconductors with bipolar transporting character are of key importance in organic electronics to achieve the balanced hole and electron transportation for highperformance device applications, including organic light emitting diodes (OLEDs) [1,2,3], organic solar cells (OSCs) [4, 5], organic field effect transistors (OFETs) [6, 7], photodetectors [8], memory devices [9], and organic afterglow applications [10, 11]
To take advantages of charge redistribution during the resonance variation, we constructed a type of new-concept bipolar molecules using carbazole as the donor and N-P = X (X = O, S, or Se) as the resonance linkage in a D-r-D configuration with an electronically inert but highly solvent soluble tert-butyl group on phosphine (Figure 1(d))
Good thermal stability and excellent film-forming property of these D-r-D molecules were revealed by thermogravimetry analyses (TGA)/ differential scanning calorimetry (DSC) (Figure S10) and atomic force microscopy (AFM) (Figure S11), respectively; the decomposition temperatures (Tds) up to 296°C and rootmean-square roughness (RMS) lower than 0.281 nm are favorable in the fabrication of thermally and morphologically stable thin films for optoelectronic devices [23, 24]
Summary
Organic semiconductors with bipolar transporting character are of key importance in organic electronics to achieve the balanced hole and electron transportation for highperformance device applications, including organic light emitting diodes (OLEDs) [1,2,3], organic solar cells (OSCs) [4, 5], organic field effect transistors (OFETs) [6, 7], photodetectors [8], memory devices [9], and organic afterglow applications [10, 11]. Compared to the individual D and A components, D-A molecules commonly show narrower bandgaps (Egs) with significant bathochromic shift in emission and lower triplet energies (ET s) with much extended π-conjugation owing to the inevitable intramolecular charge transfer (ICT) interactions between D and A moieties [15] These intrinsic features of D-A. molecules limit significantly their development in highperformance blue emitting molecules with large Eg or host materials with high ET for blue/deep-blue phosphorescent OLEDs (PhOLEDs) [12, 16]. In virtue of the insulating resonance linkage, π-conjugation can be well controlled without reducing much of the excited energy These resonance molecules designed in N-P=O and N-P=S resonance (r) linked D-A architecture (D-r-A) are excellent bipolar host materials of blue PhOLEDs, showing the selectively and remarkably enhanced electronic properties for high device performances. These novel resonance-driven bipolar molecules in D-r-D structure with high device performance illustrate a new way for designing bipolar organic optoelectronic semiconductors with high solubility, Eg, and ET simultaneously, which should be highly instructive via dynamic strategies instead of conventional static approaches
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