NTO/Ag/NTO multilayer transparent conducting electrodes for photovoltaic applications tuned by low energy ion implantation

Abstract

Multilayer structures with optimized Nb (3.7 at.%) doped TiO2 (NTO) having the layers as NTO/Ag/NTO (NAN) were fabricated to obtain high optical transmittance and low electrical resistivity which could be a suitable replacement of the conventional transparent conducting electrodes used in the energy conversion and optoelectronics devices. These optimized pristine films of NAN layers were deposited by sputtering and implanted with 40 keV N+ ions with fluences ranging from 1 × 1014 to 1 × 1016 ions/cm2. The N+ ion implantation leads to the improvement in the electrical conductivity of NAN films, confirmed by the Hall measurement. The resistivity of pristine film is 9.6 × 10−5 Ω cm which decreases to 5.5 × 10−5 Ω cm after ion implantation for the N+ ion fluence of 1 × 1016 ions/cm2. Electrical transport properties were studied in the temperature range of 80–340 K, and the results show stable behavior of films. This substitutional doping causes narrowing of band gap and improvement in the electrical conductivity. The optimized NAN multilayer films show a low sheet resistance of 6.9 Ω/□ and a high transmittance of ∼81% for the 1 × 1016 ions/cm2 fluence. The Haacke figure of merit (FOM) of 18 × 10−3 Ω−1 was obtained for the highest fluence (1 × 1016 ions/cm2). The X-ray photoemission spectroscopy study of the implanted samples revealed substitution of Ti by Nb in NTO film and appearance of Ti3+ state. The work function of pristine NAN films, measured using ultravoilet photoemission spectroscopy (UPS) was found to 4.63 eV, which matches with the work function of active layer of photovoltaic cell. On implantation, the O ions are replaced by N+ ions. These results indicate that the NAN films with N+ ion implantation are suitable for potential transparent conducting electrode (TCE) applications in photovoltaics due to their high transmittance, low electrical resistivity and compatibility for growth of further layers.