can be inserted between the active layer and the contact layer to create a new interface with more favorable properties, which is the method explored here. These thin layers are often called interlayers and many types of materials have been investigated, such as low work-function metals, [ 3,4 ] metal salts, [ 5 ] silane-based selfassembled monolayers (SAMs), [ 6 ] phosphonic-acid SAMs, [ 7,8 ] metal oxides, [9][10][11] polymers, [12][13][14][15] and polyelectrolytes. [ 16 ] Other examples are also available in the literature, with a thorough review provided by Ma et al. [ 1 ] The majority of these interlayers improve device performance by reducing electronic barriers for extraction of the desired charge carrier and/or increasing charge selectivity by blocking the undesired charge carrier. Creating interfaces or layers that preferentially extract only one polarity of charge carrier are key to optimizing BHJ devices since the intermixed donor and acceptor do not create a diode.Charge selective layers create a thermodynamic barrier for collection of the undesired carrier (electron or hole). The effects of selective layers on OPV diode characteristics in the dark and under illumination are distinct from changes due to shifts in the contact layer work function ( φ ).[ 17 ] Changes in φ tend to only manifest themselves in the open-circuit voltage ( V oc ), whereas changes in selectivity from thermodynamic barriers (with constant φ ) are evident in the reverse saturation current and shunt resistance, in addition to V oc . Additionally, a study comparing injection and extraction effi ciency in OPV devices with and without selective contacts showed that the dominant mechanism controlling V oc is injection, with charge selective contacts having a much lower rate of charge injection into the active layer. [ 18 ] One of the most commonly used charge selective contact layers in OPV devices is the polymer blend poly(3,4-ethylenedi oxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The nanostructure of PEDOT:PSS layers is complex due to the phase-separated colloidal structure of the PEDOT:PSS ink. In the bulk of the layer, elongated ellipsoids of PEDOT are surrounded by thin shells of PSS. [ 19,20 ] This morphology gives rise to its anisotropic conductivity and refractive index. [ 21,22 ] However, the surface of the PEDOT:PSS has a 3-4 nm thick PSS-rich surface layer. [23][24][25] This PSS-rich layer forms due to surface energy considerations. In blend fi lms, the component with the lowest surface energy