Cost-effective organic photovoltaic (OPV) devices will require use of environmentally stable materials that act as effective charge-collecting electrodes. Developing electron-collecting electrodes has been particularly challenging because low-work function conductive materials tend to be chemically reactive in air. We recently demonstrated that surface modifiers based on polymers containing simple aliphatic amine groups, such as polyethylenimine (PEI, branched) and polyethylenimine ethoxylated (PEIE), can significantly reduce the work function of a wide variety of materials, including metals, metal oxides, and conducting polymers.  These large band gap insulators have been found to physisorb onto a conductor's surface when processed in air from dilute solutions of environmentally friendly solvents such as water or methoxyethanol.Here, we will discuss the properties of these materials and progress towards developing single-cell and tandem OPVs wherein PEIE or PEI are used to coat high work function conductors to produce efficient electron collecting electrodes and charge recombination layers. We will show the realization of high efficiency inverted single cell OPVs having PEIE-coated FTO, ITO, Ag and PEDOT:PSS (PH1000) bottom electrodes.  We will show that the use of PEIE or PEI modified electron-collecting electrodes also leads to conventional geometry OPVs with the same performance than OPVs having Ca/Al electron-collecting electrodes.  We will also discuss how modification of PEDOT:PSS by PEIE or PEI opens the door for the demonstration of all-additive processed all polymeric solar cells and to simpler tandem cell structures wherein this bilayer is used as  charge recombination layer. These inverted tandem OPVs display high efficiencies and very high fill factor values with no loss of open-circuit voltage. The approach herein described is therefore proving to be a versatile and effective replacement for reactive low-work function metals and other interface modifications used to create efficient electron-collecting electrodes and represent a step forward towards developing a cost-effective OPV technology.

Polyethylenimine-modified electron-collecting electrodes in organic photovoltaics

247th National Meeting (2014)

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