A pH-Neutral Polyelectrolyte Hole Transport Layer for Improved Energy Band Structure at the Anode/PTB7 Junction and Improved Solar Cell Performance

摘要

In organic solar cells (OSCs), interfacial properties between the donor phase andhole transport layers (HTLs) are critical factors which govern charge extractionefficiency. Many ionic and polar materials are known to function as effectiveinterfacial layers; however, an understanding of how ionic moieties affect theelectronic band structure and characteristics of OSCs is lacking. Herein, a new,pH-neutral polyelectrolyte is introduced that resolves several problems which areencountered with the commonly used HTL, poly(3,4-ethylenedioxythiopene):polystyrenesulfonate (PEDOT:PSS). An effective p-type polyelectrolyte dopant isdesigned, comprising an anionically charged PSS backbone with easily reducedCu~(2+) counterions (Cu:PSS), and interfacial properties for HTL/donor interfacesby photoelectron spectroscopy are analyzed. The effects of the polyelectrolyte oninterfacial energy levels and charge extraction efficiency between the active layerand HTL are quantified. Using optimized processing conditions, the efficiencycan be improved from 8.31% to 9.28% in conventional OSCs compared with astandard PEDOT:PSS HTL. The energy-level alignment at the HTLs/donorinterface determined by UV photoelectron spectroscopy measurements revealsthe origin of distinct differences in device performances. The reduced ionizationpotential (IP) and hole injections barrier (Φ_h) at the HTL/donor interface play acrucial role in efficient charge extraction in conventional OSCs.