ABSTRACT
The formation of stabilized radical anions on organic materials in the solid state is an important issue in radical-based fundamental research and various applications. Herein, for the first time, we report on gas-induced ion-free stable radical anion formation (SRAF) of organic semiconducting solids with high gas selectivities through the use of organic field-effect transistor (OFET) gas sensors and electron spin resonance spectroscopy. In contrast to the previously reported SRAF, which requires either anionic analytes in solution and/or cationic substituents on π-electron-deficient aromatic cores, NDI-EWGs consist of an n-type semiconducting naphthalene diimide (NDI) and various electron-withdrawing groups (EWGs) that exhibit non-ion-involved, gas-selective SRAF in the solid state. In the presence of hard Lewis base gases, NDI-EWG-based OFETs exhibit enhanced conductivity (Current-ON mode) through the formation of an SRAF NDI/gas complex, while in the presence of borderline and soft Lewis base gases, NDI-EWG-based OFETs show decreased conductivity (Current-OFF mode) by the formation of a resistive NDI/gas complex. Organic semiconducting solids with EWGs exhibiting highly gas-selective solid-SRAF constitute a very promising platform for radical-based chemistry and can be used in various applications, such as highly gas-selective probes.
ABSTRACT
A design strategy is proposed for electron-transporting materials (ETMs) with homochiral asymmetric-shaped groups for highly efficient non-fullerene perovskite solar cells (PSCs). The electron transporting N,N'-bis[(R)-1-phenylethyl]naphthalene-1,4,5,8-tetracarboxylic diimide (NDI-PhE) consists of two asymmetric-shaped chiral (R)-1-phenylethyl (PhE) groups that act as solubilizing groups by reducing molecular symmetry and increasing the free volume. NDI-PhE exhibits excellent film-forming ability with high solubility in various organic solvents [about two times higher solubility than the widely used fullerene-based phenyl-C61 -butyric acid methyl ester (PCBM) in o-dichlorobenzene]. NDI-PhE ETM-based inverted PSCs exhibit very high power conversion efficiencies (PCE) of up to 20.5 % with an average PCE of 18.74±0.95 %, which are higher than those of PCBM ETM-based PSCs. The high PCE of NDI-PhE ETM-based PSCs may be attributed to good film-forming abilities and to three-dimensional isotropic electron transporting capabilities. Therefore, introducing homochiral asymmetric-shaped groups onto charge-transporting materials is a good strategy for achieving high device performance.
ABSTRACT
Organic electron-transporting materials (ETMs) with low-temperature solution processability and high electron-transfer/transport characteristics have received significant attention for their use in high-performance perovskite solar cells (PSCs). In contrast to widely investigated fullerene-based organic ETMs for PSCs, only a few types of non-fullerene-based ETMs have been developed. In this Concept article, a representative design concept for non-fullerene ETMs is described for use in normal (n-i-p) and inverted (p-i-n) type PSCs. On the basis of a discussion on ETM requirements for PSCs, recently developed non-fullerene polymeric and small-molecule-based ETMs for PSCs are highlighted, alongside various other approaches for enhancing PSC device performance, such as using additional dopants and introducing amine terminal groups in the ETMs and ETM-based interlayers. This Concept provides a basic design concept for non-fullerene-based ETMs in PSCs.
