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We present a hole injection layer processed from solution at room temperature for inverted organic solar cells. Bis(2,4-pentanedionato) molybdenum(VI) dioxide (MoO2(acac)2) is used as the precursor for MoOx. Small amounts of Nafion in the precursor solution allow it to form continuous films with good wetting onto the active layers. The hydrolysis of MoO2(acac)2and the effects of adding Nafion to the precursor solution are studied by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. The devices with solution-processed MoOxincluding Nafion exhibited comparable performance to the reference devices based on the commonly used hole injection layers such as poly(3,4-ethylenedioxythiophene):polystyrenesulfonate (PEDOT:PSS) or evaporated MoO3. Inverted poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester devices with Nafion-modified MoOxmaintain 80% of their initial power conversion efficiency upon exposure to ambient air for ∼5000 h, outperforming devices with PEDOT:PSS or with evaporated MoO3.

Metal oxide transport layers have played a crucial role in recent progress in organic photovoltaic (OPV) device stability. Here, we measure the stability of inverted and encapsulated polythiophene:fullerene cells with MoO3/Ag/Al composite anode in operational conditions combining solar radiation and 65 °C. Performance loss of over 50% in the first 100 h of the aging is dominated by a drop in the short-circuit current (Jsc). We reveal a concurrent loss in reflectance from 85% to 50% above 650 nm, which is below the optical gap of the used photoactive materials, hence, excluding any major degradation in the bulk of this layer. Correlating the responses of aged devices to a series of test structures comprised of ITO/ZnO cathode, MoO3/Ag, and MoO3/Ag/Al anodes and their combinations with the active layer allowed us to identify that the presence of Al causes the reduced reflectance in these devices, independent of the presence of the active layer. Systematic single-stress aging on the test structures further indicates that elevated heat is the cause of the reflectance loss. Cross-section transmission electron microscopy coupled with elemental analysis revealed the unsuspected role of Al; notably, it diffuses through the entire 150 nm thick Ag layer and accumulates at the MoO3/Ag interface. Moreover, XRD analysis of the aged MoO3/Ag/Al anode indicates the formation of Ag2Al alloy. Depth profiling with X-ray photoelectron spectroscopy advanced our understanding by confirming the formation of Ag–Al intermetallic alloy and the presence of oxidized Al only at the MoO3/Ag interface suggesting a concomitant reduction of MoO3to most probably MoO2. This latter compound is less reflective than MoO3, which can explain the reduced reflectance in aged devices as proven by optical simulations. On the basis of these results, we could estimate that 20% of the loss in Jscis ascribed to reduction of MoO3triggered by its direct contact with Al.

We report on a sol–gel-based technique to fabricate MoO3thin films as a hole-injection layer for solution-processed or thermally evaporated organic solar cells. The solution-processed MoO3(sMoO3) films are demonstrated to have equal performance to hole-injection layers composed of either PEDOT:PSS or thermally evaporated MoO3(eMoO3), and the annealing temperature at which the sol–gel layer begins to work is consistent with the thermodynamic analysis of the process. Finally, the shelf lifetime of devices made with the sMoO3is similar to equivalent devices prepared with a eMoO3hole-injection layer.