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.

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► Inverted OSCs with a conjugated polyelectrolyte as cathode interlayer.
► WF of ITO is substantially lowered due to ultra-thin CPE layer.
► UV illumination further reduces WF of ITO/CPE cathode.
► CPE as simple alternative to TiOx or ZnO interlayers.
► High efficiency (4.8%) of inverted OSC with CPE interlayer.