Photovoltaic (PV) panels based on crystalline semiconductors such as silicon are relatively expensive to produce and process for large-area applications. Organic photovoltaic materials offer cheap, large area processing on flexible substrates and are particularly attractive for diffuse lighting and for versatility. For instance, rucksacks are already produced with organic solar panels for charging hand-held devices. Organic PV efficiencies have increased dramatically over the last decade, promising large-scale, wider applications.1 In organic PV devices the Coulombic interaction between the photo-excited electron and hole are poorly screened due to the low dielectric constants of molecular materials. Hence charge recombination is a limiting factor such that exciton generation needs to occur within a diffusion length of around 10 nm of a donor-acceptor interface. Here the free energy gained on charge transfer from the photoexcited donor molecule to an acceptor molecule can overcome the electrostatic attraction between electron and hole (Fig. 1a). A second requirement is a continuous network of donor and acceptor paths for the carriers to percolate to their respective electrode (Fig. 1b). These two requirements involve a delicate balance of intimate mixing of the two components, to ensure exciton dissociation, and local segregation to form the bicontinuous network for carrier conduction to the electrodes.
Figure 1: Schematic of (a) the energy lineup: charge separation as electrons are transferred from the LUMO (Lowest Unoccupied Molecular Orbital, corresponding to the conduction band edge of an inorganic semiconductor) of the donor to that of the acceptor (b) the donor (dark blue) and acceptor (pale blue) bicontinuous percolated paths of the carriers to the electrodes (c) the molecular structure of P3HT and PCBM.
The best studied material system is the combination of regioregular poly-3-hexylthiophene (P3HT) with the methanofullerene, phenyl C61 butyric acid ester (PCBM) (Fig. 1c). A wide range of experimental studies have demonstrated that variations in polymer regioregularity, blend film composition, solvent, deposition conditions, and post deposition treatments such as thermal and vapour annealing lead to variations in organic photovoltaic device performance, apparently as a result of variations in blend film morphology.2-4 Such studies also demonstrate that improvements in solar cell performance are usually associated with increases in the polymer crystallinity. An AFM image of the P3HT/PCBM blend is shown in Figure 2, in which the bicontinuous nature is clearly seen.
Figure 2: An atomic force microscopy image (1x1 µm) of the nano-scale morphology of P3HT: PCBM. The nanofibrillar P3HT can be seen, the darker regions consisting of amorphous P3HT and PCBM.
Grazing incidence X-ray diffraction (GIXRD) is a prime tool for investigating the structure of thin films. We have used it to study the changes in P3HT:PCBM film structure during a thermal annealing process, which is used in solar cell optimisation. When P3HT:PCBM solar cells are thermally annealed, they are usually placed on a pre-heated hot plate at a temperature around 140-150°C to anneal for a period of typically 30 minutes, with various groups employing anneals of between 5 and 60 minutes,. After thermal annealing, devices are moved to a metallic surface at room temperature for fast cooling. These conditions were simulated in our experiments by careful temperature control optimization for rapid heating to 140°C with <1°C overshoot. The cooling transient is slower than that for heating due to the absence of a cooling stage.
Figure 3: GIXRD diffraction pattern for as spun P3HT/PCBM blend thin film.
Figure 3 shows the GIXRD pattern for P3HT:PCBM (50 wt% PCBM) taken on beamline I07. Such images were obtained at 8 second periods throughout the anneal of 50 minutes at 140°C. Line profiles were determined for each image and the position and the full width at half maximum (FWHM) of the (h00) P3HT peaks during annealing were fitted to the profiles. These fitted parameters gave the lattice spacing and ordering of P3HT along the alkyl stacking direction in real time during the anneal (Fig. 4). The change of lattice constant during heating and cooling is extremely anisotropic, yielding approximate thermal expansion coefficients of aT ˜ 5.3×10-4 K-1 and 4.3×10-5 K-1 along the  direction and  directions respectively. The diffraction peak width was found to increase strongly with increasing scattering vector Q, thus preventing simple Debye-Scherrer analysis. The Q-dependent width could be interpreted in terms of paracrystalline disorder, in which there is statistical distribution of inter-chain spacings. A simple analysis procedure enabled the width of this distribution and the domain size to be determined in real time, shown in Figure 4(b,c). During the first few minutes of the anneal a sudden change is observed in both parameters. The P3HT (100) domain size, which can be identified with the width of the crystalline nanofibrils that form in the P3HT component of the blend, increases approximately by a factor of two when annealing commences. Parallel device measurements show that maximum increase in photocurrent generation also occurs within these first few minutes of annealing. Maximum performance is achieved on a longer time scale than P3HT crystallisation, and may be associated with fullerene diffusion. Further experiments are in progress at beamlines I07 at Diamond Light Source and XMaS (BM28) at ESRF to investigate the effect of the substrate on the anneal behaviour and to probe the depth dependence of the structure. Such time-resolved GIXRD measurements can be applied more widely to follow structural changes in real time during in-situ processing in molecular electronics.
Figure 4: The P3HT domain size, corrected for paracrystallinity, and the paracrystalline disorder parameter for peak broadening of the P3HT (h00) peaks.
Lilliu S, Agostinelli T, Pires E, Hampton M, Nelson J, Macdonald J E, Dynamics of Crystallization and Disorder During Annealing of P3HT: PCBM Bulk-Heterojunctions Macromolecules, 44 (8), pp 2725–2734 (2011)
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This work was partially funded by the Engineering and Physical Sciences Research Council (EPSRC) grant number EP/F016255, EP/F023200, and Research Councils UK (RCUK)