Enhancement of photovoltaic characteristics of nanocrystalline 2,3-naphthalocyanine thin film-based organic devices

• Published in: Applied surface science 2012-10, Vol.259, p.600-609
• Main Authors: Farag, A.A.M., Osiris, W.G., Ammar, A.H.
• Format: Article
• Language: English
• Subjects: Activation energy; Al/NPC/ITO device; Annealing; Condensed matter: electronic structure, electrical, magnetic, and optical properties; Condensed matter: structure, mechanical and thermal properties; Cross-disciplinary physics: materials science; rheology; Devices; Energy gaps (solid state); Exact sciences and technology; Infrared spectroscopy; Nanocrystals; Nanostructured film; Optical band gap; Photovoltaic cells; Physics; Scanning electron microscopy; Solar cells; Thermal annealing; Thin films
• ISSN: 0169-4332; 1873-5584
• DOI: 10.1016/j.apsusc.2012.07.083

Scanning electron microscopy (SEM) image of NPC films: (a) cross section view, (b) surface morphology of the film at 300K, (c) surface morphology of the annealed film at 350K, (d) surface morphology of the annealed film at 400K, (e) surface morphology of the annealed film at 450K, and (f) surface morphology of the annealed film at 500K. [Display omitted] ► The absorption edge shifts to the lower energy for the annealed NPC film. ► The device of Au/NPC/ITO exhibit rectifying characteristics. ► The devices show improvement in photovoltaic parameters. ► The power conversion efficiency of the devices show enhancement under annealing. In this work, nanocrystalline thin films of 2,3-naphthalocyanine (NPC) were successfully deposited by a thermal evaporation technique at room temperature under high vacuum (∼10−4Pa). The crystal structure and surface morphology were measured using X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively. A preferred orientation along the (001) direction was observed in all the studied films and the average crystallite size was calculated. Scanning electron miscroscopy (SEM) images of NPC films at different thermal treatment indicated significant changes on surface level patterns and gave clear evidence of agglomeration of nanocrystalline structures. The molecular structural properties of the thin films were characterized using Fourier transform infrared spectroscopy (FTIR), which revealed the stability of the chemical bonds of the compound under thermal treatment. The dark electrical conductivity of the films at various heat treatment stages showed that NPC films have a better conductivity than that of its earlier reported naphthalocyanine films and the activation energy was found to decrease with annealing temperature. The absorption edge shifted to the lower energy as a consequence of the thermal annealing of the film and the fundamental absorption edges correspond to a direct energy gap. The temperature coefficient of the onset and optical band gaps for the film was calculated to be −4.4×10−4 and −1.76×10−3eV/K, respectively. The effect of thermal annealing on the photovoltaic properties of Al/NPC/ITO devices was also considered. The as-deposited device showed maximum power conversion efficiency about 0.70% under illumination of 100mW/cm2, whereas 2.65% power conversion efficiency was achieved after annealing the samples at 500K for 1h.