Correlating the effective work function at buried organic/metal interfaces with organic solar cell characteristics

The energy level alignment at organic semiconductor/metal interfaces determines the efficiency of
charge injection and collection in organic electronic devices. Back electrodes, applied after the organic
layer is processed, form buried organic/metal interfaces that are not directly experimentally accessible
for measuring the energy level alignment. Here we devise an approach to evaluate the effective work
function (EWF) between an organic solar cell bulk heterojunction (BHJ) and silver electrodes, including
interlayers, and correlate the EWF with solar cell characteristics. The platforms used are self-generated
interlayers formed by migration of additives from the BHJ to the organic/metal interface. The EWFs of
interlayer/Ag interfaces are assessed by step-depositing silver on interlayer films followed by in situ
ultraviolet photoelectron spectroscopy and compared with EWF values of the bare Ag/BHJ interface.
Solar cell characteristics of devices comprising the same interfaces confirm unambiguously the correlation
between the EWF and Voc. We stress that the metal work function is a surface property and cannot be
reliably used to estimate the energy level alignment at organic/metal buried interfaces. Rather, EWF values
should be used to assess the interfacial level alignment. Known EWF values for organic/metal interfaces
with and without interlayers can reveal the contribution of the interlayer to device performance.