Ultrathin interlayers of a conjugated polyelectrolyte for low work-function cathodes in efficient inverted organic solar cells

Inverted polymer: fullerene solar cells are reported using the conjugated polyelectrolyte poly(3-[6-(N-methylimidazolium)hexyl]thiophene) bromide P3ImHT as ultra-thin cathode interlayer to reduce the work-function (WF) of ITO for improved electron selectivity. Similar to ITO/TiOx cathodes, UV exposure is found necessary in the case of ITO/CPE to achieve a high fill factor on the order of 60%. UV illumination is shown to lead to a reduction of the ITO WF which in combination with WF lowering due to the dipole of the CPE leads to a suitable electron selective cathode. As a result, inverted solar cells with an efficiency of 4.8% are achieved. This is the highest efficiency of an inverted OSC using a conjugated polyelectrolyte as interface modifier directly on top of ITO. (C) 2013 Elsevier B.V. All rights reserved.