A Small‐Molecule Zwitterionic Electrolyte without a π‐Delocalized Unit as a Charge‐Injection Layer for High‐Performance PLEDs

Abstract

Conjugated organic optoelectronic materials have attracted significant attention owing to their application in lightemitting diodes (LEDs), solar cells, thin-film transistors, optically amplified biosensor assays, and other areas. [1] For organic optoelectronic devices, interfacial modification between the organic material and the electrode is a crucial issue and becomes one of the most important research focal points, since charge injection/extraction affect device performance greatly. [2] Excellent charge injection leads to a lower turn-on voltage and higher efficiency in organic LEDs. [3] In the case of organic solar cells, good interface properties result in a higher short-circuit current and a higher open-circuit voltage, which are the most important factors for solar-cell performance. [4] Among the many kinds of solution-processable interfacial-modification materials, [4a,b, 5] conjugated polyelectrolytes (CPEs) and oligoelectrolytes (OEs) containing a p-delocalized backbone with pendant groups that can be ionized have attracted great interest. Their solubility in polar solvents enables the fabrication of multilayer polymer-based devices by the solution-processing approach. [6a-g] The reported performance of devices based on CPEs/OEs is comparable to that of devices based on the conventional Ca/Ba electrode. However, mobile counterions, such as Na + , Br À , and tetrasubstituted borates, [6a,b, 7] which can migrate during device operation, make the device mechanism more complicated and seriously affect the device turn-on time. [1h, 8] In recent years, conjugated zwitterionic materials have attracted great interest as a new kind of charge-injection materials owing to their unique chemical structure, in which both the positive and the negative ions are combined. As the ions are not mobile, the device response time is improved significantly. Furthermore, the solubility of the materials in polar solvents is good enough for multilayer-device fabrication as a result of the presence of the charged groups. [2a] In 2009, Bazan and co-workers reported small-molecule zwitterionic materials synthesized by the addition of iodoalkanes to sodium tetrakis(1-imidazolyl)borate (NaBIm 4 ); the device response time and performance were comparable to those of the Ba/Al electrode. [6d] In a previous study, [9] one of our research groups reported a conjugated polyelectrolyte with a fluorene-based zwitterionic sulfoammonium structure and no free counterions. When this zwitterionic polymer known as "F(NSO 3 ) 2 " (CPE1) was used as the electron-injection layer, devices based on poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) as the emitter showed very fast response times; moreover, both efficiency and brightness were considerably improved (by a factor of more than 2) in comparison with standard calcium-based devices. Huang and co-workers independently reported a fluorene-based zwitterionic polymer, "PF 6 NSO", [3a] the luminance efficiency of which was as high as 23.8 cdA with poly[2-(4-(3',7'-dimethyloctyloxy)phe...

Zwitterionic ammonium and neutral amino molecules as cathode interlayer for inverted polymer solar cells

“…The synthetic routes have been described in a previous publication [45]. M1 and M2 are neutral amino derivates, while S1, S2 and S3 are zwitterionic ammonium derivates.…”

“…Under the guidance of this theory, several specific functional groups are introduced to alcohol-soluble interface materials, such as ethylene oxide [35], phosphonate [36,37], amino [38][39][40][41][42] and ammonium [43,44]. In our previous study, we have reported a series of alcohol-soluble small molecules including amino and ammonium derivatives, and used them as charge-injection materials for polymer lightemitting diodes (PLEDs) [45]. We found that zwitterionic nature of the molecules is crucial important for the charge injection and p-delocalized structure is not absolutely necessary.…”

Enhanced efficiency in polymer solar cells by incorporation of phenothiazine-based conjugated polymer electrolytes

“…It has been reported that the interfacial dipoles which are mainly generated from the modification of the metal electron density tail because of the presence of either ionic or polar groups could reduce the work function of cathode [28,29,20]. To investigate the effect of CPEs on the work function of the Al cathode, we measured the work function of Al and CPE-coated Al by using an ultraviolet photoelectron spectroscopy (UPS).…”

Micellization and Single-Particle Encapsulation with Dimethylammoniopropyl Sulfobetaines

“…Zwitterionic surfactants are electroneutral yet easily dispersed in high ionic strength solutions, and their reputation for biocompatibility is of great interest to the pharmaceutical and cosmetic industries; for example, phosphatidylcholines are often used in emollients to retain body moisture or treat skin diseases. 11 In materials science, zwitterionic surfactants can serve as novel charge injection materials for faster device response times in organic light-emitting diodes, 12 or as electrolytes that can combine high ionic mobility with water retention in solid-state supercapacitors. 13 …”