Investigation of low resistance transparent MoO3/Ag/MoO3 multilayer and application as anode in organic solar cells
Résumé
Depending on the resistivity and transmittance, transparent conductive oxides (TCO) are widely used in thin film optoelectronic devices. Thus doped In2O3 (ITO), ZnO, SnO2 are commercially developed. However, the deposition process of these films need sputtering and/or heating cycle, which has negative effect on the performances of the organic devices due to the sputtering and heat damages. Therefore a thermally evaporable, low resistance, transparent electrode, deposited onto substrates room temperature, has to be developed to overcome these difficulties. For these reasons combination of dielectric materials and metal multilayer has been proposed to achieve high transparent conductive oxides. In this work the different structures probed were: MoO3 (45 nm)/Ag (x nm)/MoO3 (37.5 nm), with x = 5-15 nm. The measure of the electrical conductivity of the structures shows that there is a threshold value of the silver thickness: below 10 nm the films are semiconductor, from 10 nm and above the films are conductor. However, the transmittance of the structures decreases with the silver thickness, therefore the optimum Ag thickness is 10 nm. A structure MoO3 (45 nm)/Ag (10 nm)/MoO3 (37.5 nm) resulted with a resistivity of 8 × 10− 5 Ω cm and a transmittance, at around 600 nm, of 80%. Such multilayer structure can be used as anode in organic solar cells according to the device anode/CuPc/C60/Alq3/Al. We have already shown that when the anode of the cells is an ITO film the introduction of a thin (3 nm) MoO3 layer at the interface anode (ITO)/organic electron donor (CuPc) allows reducing the energy barrier due to the difference between the work function of ITO and the highest occupied molecular orbital of CuPc [1]. This property has been used in the present work to achieve a high hole transfer efficiency between the CuPc and the anode. For comparison MoO3/Ag/MoO3/CuPc/C60/Alq3/Al and ITO/MoO3/CuPc/C60/Alq3/Al solar cells have been deposited in the same run. These devices exhibit efficiency of the same order of magnitude.