Engineering the interfaces between organic semiconductors and electrodes minimizes interfacial resistances and enhances the performance of polymer solar cells. Organic semiconductors have intrinsically low free carrier densities, which can lead to large injection barriers if the work functions of the electrodes are not properly matched to the energy levels of the photoactive layers in electronic devices. One approach to engineer this crucial interface is through the judicious selection of electrode materials. Selecting the electrodes so their work functions match the energy levels of the organic semiconductors within the photoactive layer, however, can often compromise the environmental stability of polymer solar cells. One must thus strive to achieve a balance during device fabrication between the bulk properties of the electrode, such as electrical conductivity, and its interfacial properties, such as the energy alignment between the organic semiconductor and the electrode. Another approach to enhance charge extraction at the organic semiconductor-electrode interface is to adsorb molecular layers (MLs) on the electrode prior to the deposition of the photoactive layer. If the adsorbed molecules are preferentially oriented and they possess a net dipole moment, MLs can be utilized to modify the work function of the electrode so to minimize resistive losses during charge extraction. In this approach, one needs to take into account changes in the morphology of the photoactive layer – which undoubtedly also alters device performance – that result due to differences in the surface energy of the ML-modified electrode. As an alternative to completely replacing the electrode, interfacial modification via ML adsorption offers optimization of the charge extraction efficacy at the organic semiconductor-electrode interface independent of the bulk conductivity of the electrode.
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