Highly efficient polyacetylene–based polyelectrolytes as cathode interfacial layers for organic solar cell applications

Abstract

Abstract For application to inverted organic solar cells (IOSCs), three polyacetylene-based polyelectrolytes, poly(N-alkyl/aryl-2-ethynylpyridinium bromide) (P1-P3), were incorporated as cathode interfacial layers (CILs) between the ZnO and one of the following two photoactive layers: poly[4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]-thiophenediyl] (PTB7):[6,6]-phenyl C71-butyric acid methyl ester (PC71BM) or poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b;4,5-b′]dithiophene-2,6-diyl-alt-(4-(2-ethylhexyl)-3-fluorothieno[3,4-b]thiophene-)-2-carboxylate-2-6-diyl)] (PTB7-Th):PC71BM. This interfacial modification of the IOSCs by the CIL insertion enhanced their performance with an increase in power conversion efficiency (PCE) from 7.46% for a ZnO-based control device to 8.29% for PTB7 and to 9.37% for PTB7-Th. Characterizations of the IOSC devices by using electrical impedance spectroscopy (EIS), space-charge-limited current (SCLC) electron mobility, dark current density-voltage (J-V) curves, and film surface morphology study revealed that the CIL incorporation suppressed the charge recombination and thus facilitated charge extraction, as evidenced by the observed improvement of open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF). The devices with CILs showed higher electron mobility, in line with the higher FF and reduced series resistance. These CILs with ZnO exhibited significantly enhanced PCE and also served as an internal shield against oxygen contamination and humidity, which further increased the device stability.