Investigations of n/p-junction photovoltaic cells of perylenetetracarboxylic acid diimides and phthalocyanines

Abstract

The preparation of organic photovoltaic cells in the configuration ITO/n-conductor/p-conductor/Au obtained by vapour deposition of the organic compounds is described in detail. As n-conductors perylenetetracarboxylic acid diimides (e.g., dimethyl derivative) and as p-conductors phthalocyanines (e.g., zinc phthalocyanine) were used. The film thicknesses of the layers were varied between 10 and 250 nm. The following investigations were carried out: reproducibility of cell preparation, influence of purity of the organic materials, the type of ITO substrate, deposition rate, substrate temperature and thickness of the organic films. Photovoltaic parameters were determined as follows: short-circuit currents ⩽ 3.5 mA cm^–2, open-circuit voltages ⩽ 685 mV, fill-factors ⩽ 0.52, photovoltaic efficiencies < 0.43% under white light of 100 mW cm^–2. The cells exhibit a high series resistance of around 70 Ω cm² and a low shunt resistance of around 670 Ω cm². Under 1 mW cm^–2 illumination the efficiency increased to 1.6%. In a cell of ITO/n-conductor (10 nm)/p-conductor (50 nm)/Au quantum yields up to 80% were obtained. Results of stability measurements show a 50% decrease in the short-circuit current over time. The results are discussed in detail considering the positions of energy levels in the n- and p-conducting materials.