Abstract

The polymer solar cell is a promising candidate for the next-generation solar cells because of its light weight, flexibility and easy fabrication. Poly (3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C₆₁(PCBM) are, to date, the most-studied active materials around the world for bulk-heterojunction structure. Recent power-conversion efficiencies are reported up to approximately 13%. This research investigates the optical and morphological properties of P3HT, PC₆₁BM, and P3HT:PC₆₁BM blend thin films deposited on glass substrate by spin-coating technique. Optical absorbance and transmittance measurements were recorded by using a single beam spectrophotometer. Morphological characterization was carried out with scanning electron microscope. Samples were pre-heated at 60°C for 30 min to evaporate the solvent. The samples deposited on the substrate were then annealed at 70 and 80°C for 30 min respectively. Optical properties of thin films were obtained. The sample annealed at 80°C has the highest absorption of photon energy in the visible region of the spectrum for the P3HT: PCBM blend thin film. Observation shows that as the annealing temperature increases, there is increase in absorption of photon energy for PC₆₁BM and P3HT:PC₆₁BM blend films and a reduction for P3HT thin film. Observation of surface morphology shows that: as the temperature increases the surface of the sample roughness increases. This is due to increase in grain size with increase in annealing temperature. It has been demonstrated in the study that gradual thermal annealing, in a controlled manner reveals a more stable and efficient control in tuning the P3HT–based thin film. Furthermore, this work has revealed the significance of employing thermal annealing strategies in control of the bulk of active layer of organic thin film. The results of the study can be used to develop a guideline for improving the design and fabrication of organic solar cells. It is expected that this result will lead to improvement in performance and stability of organic solar cells. 

Key words: Organic thin film, conjugated polymer, morphology, Poly (3-hexylthiophene) (P3HT), 1-(3-methoxycarbonyl) propyl-1-phenyl[6,6]C₆₁ (PCBM), post-annealing.