Investigating the role of efficiency enhancing interlayers for bulk heterojunction solar cells by scanning probe microscopy

Detailed optimization of the device architecture of bulk heterojunction organic solar cells is of crucial importance when eventually targeting commercial applications. It has been shown before that the efficiency of such devices can be increased significantly upon incorporation of an imidazolium-substituted polythiophene interlayer. The cause of this increase in efficiency was, however, still unclear and is investigated here by a combination of PeakForce Quantitative Nanomechanical Mapping, PeakForce TUNA and Kelvin Probe Force Microscopy. A link between the local morphological and electrical properties is established. We show that the conjugated polyelectrolyte interlayer acts as a hole blocking layer. Additionally, illuminated Kelvin Probe Force Microscopy indicates that the interlayer creates an extra built-in electric field promoting charge transfer from the bulk heterojunction active layer into the interlayer.