ITO Interface
Modifiers Can Improve <i>V</i><sub>OC</sub> in Polymer Solar
Cells and Suppress Surface Recombination
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Abstract
We use dipolar phosphonic acid self-assembled
monolayers (PA SAMs)
to modify the work function of the hole-extracting contact in polymer/fullerene
bulk heterojunction solar cells. We observe a linear dependence of
the open-circuit voltage (<i>V</i><sub>OC</sub>) of these
organic photovoltaic devices on the modified indium tin oxide (ITO)
work function when using a donor polymer with a deep-lying ionization
energy. With specific SAMs, we can obtain <i>V</i><sub>OC</sub> values exceeding those obtained with the common poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)
(PEDOT:PSS) hole-extraction layer. We measure charge-carrier lifetimes
and densities using transient photovoltage and charge extraction in
a series of devices with SAM-modified contacts. As expected, these
measurements show systematically longer carrier lifetimes in devices
with higher <i>V</i><sub>OC</sub> values; however, the trends
provide useful distinctions between different hypotheses of how transient
photovoltage decays might be controlled by surface chemistry. We interpret
our results as being consistent with changes in the band bending at
the ITO/bulk heterojunction interface that have the net result of
altering the internal electric field to help prevent electrons in
fullerene domains from undergoing surface recombination at the hole-extracting
electrode