A Versatile Buffer Layer for Polymer Solar Cells: Rendering Surface Potential by Regulating Dipole

Spin-coated film of poly(vinylidenefluoride-hexafluoropropylene) (P(VDF-HFP)) acts as a cathode/anode buffer layer in polymer solar cells (PSCs) with conventional/inverted device structures. Such devices show optimized performances comparable with the controlled device, making P(VDF-HFP) a good substitute for LiF/MoO3 as a cathode/anode buffer layer. Ultraviolet photoelectron spectroscopy (UPS) and Kelvin force microscope (KFM) measurements show that increased surface potential of active layers improves cathode contact. In piezoresponse force microscopy (PFM) measurement, P(VDF-HFP) responds to applied bias in phase curve, showing tunable dipole. This tunable dipole renders surface potential under applied bias. As a result, open-circuit voltage of devices alters instantly with poling voltage. Moreover, positive poling of P(VDF-HFP) together with simultaneous oxidation of Ag gradually improves performance of inverted structure device. Integer charge transfer (ICT) model elucidates improved electrode contacts by dipole tuning, varying surface potential and vacuum level shift. Understanding the function of dipole makes P(VDF-HFP) a promising and versatile buffer layer for PSCs.