Influence of a polyelectrolyte based-fluorene interfacial layer on the performance of a polymer solar cell

Abstract

A new quaternized ammonium polyfluorene polyelectrolyte poly[3,3′-(2-(3-hexyl-5-(7-(4-hexyl-5-methylthiophen-2-yl)benzo[c][1,2,5]thiadiazol-4-yl)thiophen-2-yl)-7-methyl-9H-fluorene-9,9-diyl)bis(N,N-dimethylpropan-1-amine)]dibromide (PFBTBr) is applied as the cathode interfacial layer of a polymer solar cell based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C₆₁-butyric acid methyl ester (PC₆₁BM). Electrostatic force microscopy (EFM) measurements of PFBTBr layers demonstrate the formation of the interfacial dipole between the active layer and the cathode by inserting a PFBTBr interfacial layer. Atomic force microscopy (AFM) measurements of PFBTBr layers with varied concentrations show that the morphology of the PFBTBr layer plays a direct, important role in the contact quality between the active layer and the PFBTBr interfacial layer, which can strongly affect the performance of devices. X-ray photoelectron spectroscopy measurements (XPS) indicate that PFBTBr may serve as a protective agent for the active layer against Al-induced degradation, since it prevents hot aluminum atoms from diffusing into the active layer. The power conversion efficiency (PCE) of the PSCs with the PFBTBr layer reaches 3.9% under the illumination of AM 1.5G, 100 mW cm⁻², which is 1.6 times higher in comparison with that (2.4%) of the device without the PFBTBr layer. The significant increase in efficiency and easy utilization indicate that this interfacial material has promising and practical application prospects.