Interfacial layer material derived from dialkylviologen and sol–gel chemistry for polymer solar cells

Sol–gel processible organosilicate material based on dialkylviologen (1,1-(bis-trimethoxysilane)-[4,4′]bipyridium dibromide (bis-trimethoxypropylsilane)-yl-viologen, PV-Si) was synthesized and used as an interfacial layer material for polymer solar cells based on poly(3-hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM). PV-Si is very good soluble in polar protic solvents because of two pyrinium bromide salts and PV-Si pre-polymer can be easily prepared by sol–gel chemistry under the mild acidic conditions. From the ultraviolet spectroscopy (UPS) study, the reduction of the work function of Al and ITO is observed by the formation of interface dipole, which is induced by the thin film of thermally cured PV-Si pre-polymer (cPV-Si) at 180 °C. The open circuit voltage (Voc) of conventional type polymer solar cell (CPSC) with a structure of ITO/active layer (P3HT:PCBM)/cPV-Si(<5 nm)/Al is 0.58 V, which is higher than the CPSC without cPV-Si (0.55 V). This indicates that the favorable interface dipole is generated by the thin film of cPV-Si. Besides, the power conversion efficiency (PCE) of CPSC with cPV-Si reaches at 2.90%, which is higher than that of the device without cPV-Si (2.69%). Surprisingly, the PCE and the short circuit current (Jsc) of inverted type polymer solar cell (IPSC) with a structure of ITO/cPV-Si (<5 nm)/active layer/WO3/Ag are 2.83% and −9.19 mA/cm2, respectively, which are higher than those of the device with ZnO (2.51% and −8.63 mA/cm2) as an electron transporting/injecting layer. This is due to that the work function of ITO is also reduced by the formation of interface dipole. The IPSC with cPV-Si as an interfacial layer (IFL) shows very good rectification and a contact property as well. From the results, the thin layer of cPV-Si is potential material for an IFL for either CPSC or IPSC. Especially, ZnO can be replaced by cPV-Si because of their improved device performances and pretty low processing temperature.

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► Sol–gel processible organosilicate derived from viologen for polymer solar cells.
► Cured sol–gel processible viologen as interfacial layer in polymer solar cells.
► Favorable interface dipole between active layer and metal or ITO electrode.