In the bulk heterojunction architecture of polymer-based solar cells (PSCs), the separate acceptor-donor phases form a bi-continuous inter-penetrating network by simultaneous casting from solution with morphological control stemming from external parameters such as thermal annealing, co-solvent inclusion, and drying conditions. While such treatments enhance device performance, a fundamental understanding of vertical concentration gradients within the fabricated active layer has been limited. In an effort to understand such morphological changes, several reports have explored 3D bulk heterojunction nanostructure using electron tomography,¹ ellipsometry,² neutron scattering,³ and spectroscopic techniques.⁴ This work, however, has yielded somewhat contradictory conclusions about fundamental network development and the origin of emerging concentration gradients. For example, some studies reported nearly equal blends at the PEDOT:PSS surface of annealed samples,^2,4c while others found P3HT^1,4a,4e or PCBM^3,4b,4d,4f preferentially decorating the buried interface. Several groups^2b,3,4c,4f further reported annealing causes PCBM diffusion towards the exposed surface, suppressing as cast vertical composition gradients. However, Xu et al.^4b detected PCBM migration towards the PEDOT:PSS interface upon annealing, while Xue et al.^4a suggested PCBM diffusion away from both interfaces. Here we report a combined experimental and theoretical analysis of phase segregation. The vertical stratification within a P3HT:PCBM bulk heterojunction solar cell is examined by depth profiling using both x-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS), with the effects of thermal annealing and P3HT:PCBM ratio being explored. In addition, the vertical phase stratification is predicted on thermodynamic grounds based on measured interfacial energies of the PSC constituents. Using these results, a fundamental understanding of the thermodynamic driving force for bulk heterojunction phase segregation and vertical stratification is then presented.

References

1 van Bavel, S.S. et al. Nano Lett.9, 507 (2009).

2 (a) Germack, D.S. et al. Macromolecules 43, 3828 (2010); (b) Campoy-Quiles, M. et al. Nat. Mater.7, 158 (2008).

3 Parnell, A.J. et al. Adv. Mater.22, 2444 (2010).

4 (a) Xue, B. et al. J. Phys. Chem. C114, 15797 (2010); (b) Xu, Z. et al. Adv. Funct. Mater.19, 1227 (2009); (c) Yu, B.-Y. et al. ACS Nano4, 833 (2010); (d) Vaynzof, Y. et al. ACS Nano5, 329 (2011); (e) Wang, H. et al. Chem. Mater.23, 2020 (2011); (f) Germack, D.S. et al. Appl. Phys. Lett.94, 233303 (2009).