Photoelectron spectroscopy investigation of thin metal films employed as top contacts in transparent organic solar cells

The performance of transparent metal top contacts in organic solar cells can strongly be improved by employing surfactant layers. We use scanning electron microscopy to investigate the change in morphology upon insertion of an Al surfactant layer between 4,7-diphenyl-1,10-phenanthroline (BPhen) and a silver top contact. UV photoelectron spectroscopy measurements show the changes in energetic alignments at different steps of the organic/metal interface formation. Furthermore, using X-ray photoelectron spectroscopy depth profiling, we compare the differing intermixing processes happening within the two samples. Thereby, we can show that Al binds to BPhen molecules, acting as surfactant for subsequently deposited Ag layers, while Ag without any Al surfactant layer penetrates into and intermixes with the BPhen layer.