Vertical Stratification and Interfacial Structure in P3HT:PCBM Organic Solar Cells

Abstract

Structure and morphology play a critical role in determining the performance of organic photovoltaic devices. In this paper, variation of the postannealing cooling rate is used to create a series of “snapshots” of the vertical and interfacial reorganization processes that occur upon annealing. The data show that slower cooling rates result in significantly enhanced device efficiencies primarily driven by increased short circuit current and fill factor. UV−vis spectroscopy, X-ray diffraction (XRD), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), atomic force microscopy (AFM), and contact angle measurements are used to probe the origin of these improvements. Our results show evidence for a distinct and changing vertical stratification and interfacial structure in the device throughout the annealing process, with both composition and crystallinity varying through the active layer. The implications of these changes are discussed in terms of device properties.

Molecular versus crystallite PCBM diffusion in P3HT:PCBM blends

“…The effect of vertical stratification of P3HT:PCBM blends [18][19][20][21] can be ignored as the film is only ∼100 nm thick; 2 orders of magnitude smaller than the lateral diffusion which extends over ∼10 -20 µm. Figure 1(b) shows the 1-D perpendicular PCBM composition profile from the centre of the phase segregated region.…”

Solution processable interface materials for nanoparticulate organic photovoltaic devices

“…29 Thermal annealing of P3HT:PCBM bulk heterojunction devices is known to result in significant changes to device morphology and, in particular, vertical phase segregation of the components. 30 As such, we speculate that annealing of the Al cathode device results in some phase segregation of the components in the P3HT:PCBM NP layer producing regions of high purity P3HT in which photocurrent generation is reduced. Furthermore, consistent with this hypothesis, unannealed Al cathode devices measured for this study exhibit higher J sc values.…”

Polymer Blends and Composites

“…Reproduced from Ref. [41] with permission of The Royal Society of Chemistry of the blend [50], and the influence of the P3HT regioregularity [51], as well as the use of optical measurements towards the understanding of the vertical phase separation [45,[52][53][54][55][56].…”