Preparation and characterization of ternary blend polymer solar cells using solvent and thermal annealing treatment

Abstract

The main objective of this study is to fabricate ternary blend polymer solar cells (PSC) with an optimized morphology of an active layer using solvent annealing with subsequent thermal annealing treatment. Three phases of experimental works have been designed in order to achieve the above objective. In phase I, the influences of spray coating time on the optical and morphological properties of the thin films were investigated. It was found that 10s to be the most suitable deposition time to produce thin films with an acceptable roughness, root means square (RMS):17.5nm, together with good absorption properties and crystallinity. In order to improve the performance of the PSC, ternary blends of the active layer which consists of poly (2, 5-dihexyloxy-1, 4-phenylenevinylene) (PDHeOPV), poly (3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) were prepared in phase II. The effects of PDHeOPV weight ratio on the optical and morphological properties of the thin films were investigated by varying the weight ratio from 0.5 to 2. The addition of PDHeOPV at higher weight ratio has further disrupted the chain packing of P3HT and thus reduced the crystallinity of P3HT. Nevertheless, the addition of PDHeOPV improved the light harvesting property of the ternary blend as there was an increment in absorbance area. In phase III, the effects of different post treatments on the performance of spray coated P3HT: PDHeOPV: PCBM for an active layer in PSC were studied. The annealing temperatures were varied from 130°C to 150°C while the annealing time was fixed at 10 min. The atomic force microscopy (AFM) results revealed that the samples that underwent solvent and thermal annealing treatment had higher crystallinity of P3HT or phase separation in these samples was mostly favored. The peak absorption for P3HT: PDHeOPV: PCBM thin film was higher compared to P3HT: PCBM thin film. The x-ray diffractometer (XRD) spectra showed that the structure of the samples would evolve at high thermal annealing temperature (150°C) due to weak intermolecular force in P3HT molecules. This contributed to the low crystallinity of the films which consequently affects the absorption properties of the films. The PSC devices were prepared in inverted structure using 1:0.5:1 (P3HT: PDHeOPV: PCBM) active layers in order to observe the performance of the PSC. It was found that, thermal annealing at 130°C lead to an improvement in short circuit current (Isc= 4.08±0.061 mA/cm2) and slightly increased open circuit voltage (Voc= 0.49±0.03V) compared to the ambient PSC. Interestingly, the thermal annealed (150ºC) ternary blends PSC demonstrated a significant performance enhancement under continuous illumination (120 min) without any sign of degradation. Thus, it is concluded that the P3HT: PDHeOPV: PCBM coupled with thermal annealing has a great potential for the fabrication of PSC with improved power conversion efficiency (PCE) and stability at simplicity and shorter manufacturing time compared to a tandem structure.