Idealized energy diagram of a P3HT/PCBM solar cell, showing ideal alignment of the molecular orbitals, has the PCBM component of the cell in contact with the aluminum electrode and the P3HT component in contact with the hole conducting layer of PEDOT/PSS.
A cross sectional image of the wet cell holder used for phase-sensitive neutron reflectivity measurements. The path of the neutron beam enters through a silicon “fronting,” encounters the film being studied and exits through to the “backing” reservoir, which was either air or for this experiment.
(a) vs Q for pure films of P3HT (△) and PCBM (○) and (b) corresponding SLD profiles from the best fit lines of the data. The PCBM film was noticeably thinner than the P3HT film due to spin coating conditions. All fitting was performed using the Parratt formalism and both film profiles terminate at an SLD of , that of the pure silicon substrate. The native layer is not seen in the P3HT film profile due to the film thickness and finite Q obtained. The PCBM film is small enough to see the layer, but it is contrast matched almost exactly and therefore blends into the PCBM SLD profile.
Results for the fast grown film spin coated from a 1:1 mixture by weight of P3HT and PCBM in chlorobenzene spin coated at 2500 RPMs. (a) Composite reflectivity data sets obtained from specular reflectivity in both air and . Large error bars at high Q for the air sample are expected as the reflectance here is approaching the background. (b) The real part of the reflection amplitude corresponding to the film as extracted mathematically via the methods described in the text. (c) The fast grown film SLD profiles obtained by simultaneous fitting and direct inversion of Re[r] shown together.
(a) Composite reflectivity data sets obtained for the slow grown film and fits which were determined simultaneously and (b) the slow grown film SLD profile obtained by simultaneous fitting of the corresponding data. As the SLD profiles were simultaneously fitted the profiles overlap until the backing layer was reached and they differ with the air backing film going to and the backing film going to an .
The imaginary part of the reflection amplitude for the film in Fig. 5, Im[r](q), which is sensitive to in-plane inhomogeneities on a length scale of the order of a hundred microns. The presence of well-defined zero crossing points indicates a very homogeneous and good film.
(a) SLD profiles obtained by inverting the real part of the reflection amplitude generated from the two model profiles, one flat and homogeneous and the other with increased SLD at the interfaces. (b) Imaginary part of the reflection amplitude, Im[r] (corresponding to Re[r]), for the two profiles, i.e., the flat homogeneous case (solid line) and the one with increased SLD adjacent to the interfaces (dashed line).
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