Effect of counter-ions on the properties and performance of non-conjugated polyelectrolyte interlayers in solar cell and transistor devices

We have investigated a series of non-conjugated polyelectrolytes (NPEs) which are based on a polyethylenimine (PEI) backbone with various counterions, such as Br(−) I(−) and BIm(4)(−), as interfacial layers at the electrodes of solar cells and transistor devices to improve the power conversion efficiency (PCE) and device performance. This new series of NPEs with different counterions are capable of forming electric dipoles at NPE/metal electrode interfaces; as a consequence tuning of the energy levels, and work function (WF) of the electrodes is possible. Using this approach, the PCE of organic solar cells could be improved from 1.05% (without NPEs) to 6.77% (with NPEs) while the charge carrier mobility and on/off ratio of FET devices could be improved, showing the broad utility of this type of material. This study provides a novel approach towards investigating the influence of ions on interfacial dipoles and electrode WFs in solution-processed semiconducting devices.