Organic solar cells using a multilayer structure MoO3/Ag/MoO3 as anode

MoO3/Ag/MoO3 structures have been grown and characterized. It is shown that the transmittance of the films increases when the silver thickness increase from 8 to 10 nm, whereas further increase induces transmittance decrease. The study of the variation of the conductivity vs. Ag thickness shows that the MoO3/Ag/MoO3 structures become highly conductive when the Ag thickness reaches 10 nm. Therefore, the optimum structure is obtained when the silver thickness is 10 nm. These MoO3/Ag/MoO3 structures have been used as anode in glass/anode/CuPc (35 nm)/C60(40 nm)/Alq3 (9 nm)/Al (120 nm) organic solar cells. These anodes permit achievable promising results, even if their efficiencies stay slightly smaller than that achieved with ITO based devices

Effect of the deposition conditions of NiO anode buffer layers inorganic solar cells, on the properties of these cells

tNiO thin films deposited by DC reactive sputtering were used as anode buffer layer in organic photovoltaiccells (OPVs) based on CuPc/C60planar heterojunctions. Firstly we show that the properties of the NiOfilms depend on the O2 partial pressure during deposition. The films are first conductive between 0 and2% partial oxygen pressure, then they are semiconductor and p-type between 2 and 6% partial oxygenpressure, between 6 and 9% partial oxygen pressure the conduction is very low and the films seem to be n-type and finally, for a partial oxygen pressure higher than 9%, the conduction is p-type. The morphology ofthese films depends also on the O2 partial pressure. When the NiO films is thick of 4 nm, its peak to valleyroughness is 6 nm, when it is sputtered with a gas containing 7.4% of oxygen, while it is more than double,13.5 nm, when the partial pressure of oxygen is 16.67%. This roughness implies that a forming process,i.e. a decrease of the leakage current, is necessary for the OPVs. The forming process is not necessary ifthe NiO ABL is thick of 20 nm. In that case it is shown that optimum conversion efficiency is achievedwith NiO ABL annealed 10 min at 400◦C

XPS study of the band alignment at ITO/oxide (n-type MoO3 or p-type NiO) interface

While they have different electronic properties n-type MoO3 and p-type NiO are very efficient as buffer layers between the ITO anode and the organic electron donor in organic photovoltaic cells. While it is admitted that MoO3 is n-type, its band structure is still under study. Here, the band alignment at the interface of an ITO/MoO3 heterojunction is studied by X-ray photoelectron spectroscopy (XPS). The same study is realized on the structure ITO/NiO, NiO being a p-type semiconductor. The measurements have been performed on samples obtained under the same experimental conditions as those used to achieve organic photovoltaic cells. The MoO3 (NiO) upper layer was 3 nm thick. The semidirect XPS technique used to measure the band offsets allows us to estimate the band discontinuities at the interface ITO/MoO3: ΔEv = 0.50 eV and ΔEc = 0.90 eV, while at the interface ITO/NiO we have ΔEv = −2.10 eV and ΔEc = −1.90 eV. Therefore, n-type MoO3 and p-type NiO, which are both very efficient anode buffer layers (ABLs), exhibit different band structure at the contact with ITO. However, the measurement, by means of a Kelvin probe, of the work functions of the structures ITO/NiO and ITO/MoO3, shows that they are close and significantly higher than that of ITO alone