Structural properties of the active layer of discotic hexabenzocoronene/perylene diimide bulk hetero junction photovoltaic devices: The role of alkyl side chain length

We investigate thin blend films of phenyl-substituted hexa-peri-hexabenzocoronenes (HBC) with various alkyl side chain lengths ((CH 2 )n. n = 6. 8. 12 and 16)/perylenediimide (POl). These blends constitute the active layers in bulk-hetero junction organic solar cells we studi ed recently [1 J. Their structural properties are studied by both scanning electron microscopy and X-ray diffraction measurements. The results support the evidence for the formation of HBC donor-POI acceptor complexes in all blends regardless of the side chain length of the HBC molecule. These complexes are packed into a layered structure parallel to the substrate for short side chain HBC molecules (n = 6 and 8). The layered structure is disrupted by increasing the side chain length of the HBC molecule and eventually a disordered structure is formed for long side chains (n > 12). We attribute this behavior to the size difference between the aromatic parts of the HBC and POl molec ules. For short side chains. the size difference results in a room for the side chains of the two moleeules to fill in the space around the aromatic cores. For long side chains (n >12). the empty space will not be enough to accommodate this increase. leading to the disruption of the layered structure and a rather disordered structure is formed. Our results highlight the importance of the donor-acceptor interaction in a bulk heterojunction active layer as weil as the geometry of the two molecules and their role in determining the structure of the active layer and thus their photovoltaic performance. 1.lntroduction Bulk- heterojunction (BHJ) organic solar cells have recently attracted considerable attention owing to their potential low cost fabrication with currently maximum efficiency exceeds 7% [2-6]. The solution-processed active layer in these cells is formed by bien ding two self-assembling organic semiconductors: a hole-transporting donor (0). and an electron-transporting acceptor (A). Ideally, the two semiconductors have to vertically segregate while maintaining a large interfacial surface area. This ensures an unperturbed transport of positive and negative charge carriers to their respective electrodes and effective charge separation needed to achieve highest power conver­ sion efficiency. Therefore, the structure and nanoscale morphology of the active layer are of major importance for the performance of a BHJ organic solar cell. Another prerequisite of the semiconductors is high charge-carrier mobility. A promising candidate for such organic semi­ conductors is offered by conjugated discotic liquid crystals [7 - 10]. Their flat, disk-shaped aromatic, cores with strong Tl-TI interactions promote their self-assembly into columnar stacks that allow electric