Unraveling Doping Capability of Conjugated Polymers for Strategic Manipulation of Electric Dipole Layer toward Efficient Charge Collection in Perovskite Solar Cells

Abstract

Developing electrical organic conductors is challenging because of the difficulties involved in generating free charge carriers through chemical doping. To devise a novel doping platform, the doping capabilities of four designed conjugated polymers (CPs) are quantitatively characterized using an AC Hall-effect device. The resulting carrier density is related to the degree of electronic coupling between the CP repeating unit and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), and doped PIDF-BT provides an outstanding electrical conductivity, exceeding 210 S cm(-1), mainly due to the doping-assisted facile carrier generation and relatively fast carrier mobility. In addition, it is noted that a slight increment in the electron-withdrawing ability of the repeating unit in each CP diminishes electronic coupling with F4-TCNQ, and severely deteriorates the doping efficiency including the alteration of operating doping mechanism for the CPs. Furthermore, when PIDF-BT with high doping capability is applied to the hole transporting layer, with F4-TCNQ as the interfacial doping layer at the interface with perovskite, the power conversion efficiency of the perovskite solar cell improves significantly, from 17.4% to over 20%, owing to the ameliorated charge-collection efficiency. X-ray photoelectron spectroscopy and Kelvin probe analyses verify that the improved solar cell performance originates from the increase in the built-in potential because of the generation of electric dipole layer.