Bias-Induced Electrochemical Electron Doping of Organic Semiconductor Contacts

Abstract

It is well-known that thin polyethylenimine and polyethylenimine (ethoxylated) (PEIE) layers can reduce the work function of metals, metal oxides, and conducting polymers by 1 eV or more through an interface dipole, but this is not generally sufficient to produce an ohmic electron contact. Here, it is shown that a short, positive-bias preconditioning can switch the injection characteristics of a self-assembled monolayer of PEIE on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDT:PSSH) from electron-blocking to electron-injecting into poly(N,N′-bis(2-octyldodecyl)-1,4,5,8-naphthalenebis(dicarboxy-imide)-2,6-diyl)-alt-(2,2′-bithiophene)-5,5′-diyl). Despite its low work function (4.0 eV) and favorable energy level alignment, the PEIE-modified contact is initially blocking because the tunneling barrier limits carrier accumulation. The prebias boosts this accumulation density through an interfacial solid-state electrochemical process that positively charges the PEIE, which induces charge compensation by mobile electron carriers in the adjacent semiconductor layer through the field effect. This mechanism to trigger the ohmic transition appears to be general, as similar effects, albeit to a small extent, is found in semiconductors with even smaller electron affinity. This is the first example of ohmic electron injection from PEDT:PSSH into an organic semiconductor, which would be useful for fabricating tandem and other cells.