Proceedings of 6th International Conference on Hybrid and Organic Photovoltaics (HOPV14)
Publication date: 1st March 2014
Advances in the efficiency of OPVs have been enabled essentially by the discovery of novel organic materials: from polyphenylenevinylene polymers (e.g. MEH-PPV), to thiophene-based materials with greater air stability (e.g. P3HT), and currently, to low band-gap materials of donor-acceptor structure, such as benzothiadiazoles (e.g. PCPDTBT) and diketopyrrolopyrroles (e.g. DPP) [1]. In parallel, the development of better interface or buffer layers permitted the enhancement of device lifetime. Initially, low-work function metals or organic compounds with metallic properties were used, but more recently wide band gap semiconductors, including transition metal oxides, TMOs (e.g. ZnO, TiO2, V2O5, MoO3 and NiO), have shown to provide higher stability. TMOs can be obtained by low-cost solution processing techniques, and their payback time (PBT) is lower if compared to organic semiconductors, which make them promising barrier layer materials for OPVs. In this work, we present the synthesis of transition metal oxides, V2O5 and NiO, obtained by low-cost solution processed methods at low-temperature. In some cases, the oxide can be obtained by synthesis methods in water, and processed temperatures as low as 120 ºC, eliminating the use of toxic solvents and high processing temperatures [2]. Indoor and outdoor stability analyses of the final OPV devices were carried out following the ISOS protocols [3], demonstrating above 1000 h of stability. Contrary to expected, the normal configuration OPV applying the V2O5 oxide demonstrated higher stability than the inverted configuration OPV. The latter has been related to the UV-filtering effect of the oxide layer. The OPVs applying NiO layer was shown to be highly stable with T80 mantained after more 1200 h. The possible degradation mechanisms observed after the first 1000 h by the application of techniques such as XPS, depth-profile Tof-SIMS analyses, among others are also presented.
Figure. 1. OPVs containing water-based, solution- processed V2O5 as the hole transport layer. Comparison of normalised PCE response: (a) regular vs. inverted configuration and (b) Inverted OPV with and without UV filter. The cells were analysed outdoors in Barcelona, Spain (41.30 N, 2.09 W). The PCE values were calculated using the maximum irradiance level per day. Average temperatures: 10 to 15 ºC (day) and 5 to 7 ºC (night). Average RH: 70%. Normal configuration: glass/FTO/V2O5/P3HT:PCBM/ZnO/Ag. Inverted configuration: glass/FTO/ZnO/P3HT:PCBM/V2O5/Ag.
1.- http://www.cost.eu/domains_actions/mpns/Actions/MP1307? 2.- G. Teran Escobar, J. Pampel, J. Caicedo, M. Lira-Cantu. Energy Environ. Sci., 2013, 6, 3088–3098. 3.- Reese, M. O et al. Solar Energy Materials and Solar Cells 2011, 95, 1253-1267.