Abstract
In situ and ex situ neutron reflectivity is used to characterize annealed regioregular-P3HT/PCBM bilayers. In situ annealing of a 20 nm PCBM/35 nm P3HT bilayer at 170 °C reveals rapid mixing of PCBM and P3HT to produce a polymer-rich layer that contains around 18–20% PCBM. Samples with three different thicknesses of P3HT layer are ex situ annealed at 140 °C. This again reveals migration of PCBM into the P3HT and vice versa, with the polymer-rich layer in the 20 nm PCBM/35 nm P3HT sample containing 19% PCBM. Complete migration of the entire PCBM layer into the P3HT layer is observed for a 20 nm PCBM/80 nm P3HT bilayer. The robustness of fitted model composition profiles, in comparison with real-space imaging of sample surface morphology and previous work on annealed P3HT/PCBM bilayer compositions, is discussed in detail.
Highlights
Organic photovoltaic (OPV) devices offer the potential for low-cost and large-area solar cells.[1]
In this paper we look at the mixing between phenyl C61-butyric acid methyl ester (PCBM) and regioregular P3HT as a function of film thickness and annealing temperature
The interfacial roughness, r, between the two layers is 1.2 nm. This scattering length density (SLD) profile corresponds closely to that of an as-fabricated PCBM/polymer bilayer, and shows that little interdiffusion has occurred during the temperature step from room temperature to 80 °C
Summary
Organic photovoltaic (OPV) devices offer the potential for low-cost and large-area solar cells.[1]. This paper describes a study that focusses on conjugated polymer/fullerene mixing, by looking at the evolution of morphology within a simplified (bilayer) geometry, as a function of the polymer film thickness and temperature. Chen et al, working in thinner bilayers but at similar annealing times and temperatures, observed only partial mixing, even though the PCBM constituted less than 40% of the total. D. Mon et al.: Mixing in PCBM/P3HT bilayers, using in situ and ex situ neutron reflectivity. In this paper we look at the mixing between PCBM and regioregular P3HT as a function of film thickness and annealing temperature. We adopt the robust methodology that we have used in our previous studies of PCBM/ polymer bilayers[26] and use a combination of in situ annealing (annealing a sample in the neutron beam) and ex situ annealing (annealing a sample before measuring the reflectivity). In relation to PCBM-enrichment at the free surface
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