Investigations of Thermally Induced Morphology in P3HT/PCBM Thin Films: Influence of Composition and Thermal Annealing on Photovoltaic Properties

Organic solar cells based on P3HT: PCBM bulk heterojunctions show promise for high power conversion efficiencies. The properties of composite donor polymers and acceptor materials play a significant role; hence the need for optimised bulk heterojunctions active layer morphologies is critical for efficient devices.
To achieve optimised bulk heterojunctions, compositional factors and processing conditions are two primary aspects to focus on. Thermal annealing has been demonstrated to be one of the most successful processing techniques for morphology optimisation in P3HT – based organic solar cells. However the crucial correlation between composite composition and thermal annealing in P3HT – based devices is not fully understood yet.
Combining optical absorption spectroscopy, structural and electrical methods; the properties of P3HT: PCBM blend thin films, with different PCBM percentage weight ratios were studied in this work. Optical absorption spectra results for all three blend ratios, i.e., 1:1, 1:0.8, and 1:0.6, showed that the peak absorption intensity associated with PCBM reduced the most for the 1:1 ratio, after thermal annealing at 175°C. The impact of the correlation between PCBM composition and thermal annealing on photovoltaic performance parameters was demonstrated. For the three different PCBM compositions, the optimum power conversion efficiencies were determined at different optimum thermal annealing conditions. Optimum power conversion efficiency of 3.38% (1:1) was obtained at 175°C, whilst 2.27% (1:0.8) and 1.44% (1:0.6) were demonstrated at 125°C respectively.
To further probe the influence of thermally induced PCBM molecular segregation and aggregations, three different thermal annealing strategies were employed; namely, annealing (i) gradually from 50°C – 175°C, in steps of 25°C, 10 minutes each (ii) at high temperature 175°C, for 10 minutes once, and (iii) at 175°C for a longer time, i.e., 60 minutes . Optical absorption spectroscopy results reveal the dependence of PCBM aggregation on different thermal annealing strategies. Employing Raman spectroscopy mapping methods, the surface of thin films were mapped revealing and confirming PCBM rich regions upon thermal annealing. Furthermore exciton generation rate studies proved useful in establishing a good correlation between the estimated excitons generated, with short circuit current densities. The observed increase in excitons generated was also consistent with the photoluminescence spectra results which showed an enhancement in intensities upon thermal annealing.
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Importantly, this work has shown the significance of employing different thermal annealing strategies in nanomorphology control of the bulk of active layers of organic photovoltaic devices. Importantly, it has also been demonstrated in the work of this thesis that gradual thermal annealing, in a controlled manner revealed a more stable and efficient control in tuning the nanomorphologies of P3HT – based solar cells.
In addition, impedance spectroscopy and capacitance – voltage measurement techniques have been shown to be very useful tools for characterising organic photovoltaic devices. Herein, it was shown that after thermal annealing at the optimum temperature of 150°C, impedance spectroscopy characterisation revealed extended charge carrier lifetimes in devices. This highlights the significance of having an optimised interpenetrating network within active layers of organic solar cells, as this have a critical impact on charge carrier lifetimes. Capacitance – voltage measurements was used to demonstrate the thermally induced vertical segregation of PCBM molecular aggregates also. The decrease in measured built – on potential from 0.68V (at film/cathode interface) from as cast device to about 0.35V after thermal annealing at 150°C, was shown to be indicative of vertical segregation.