Fast Recovery of the High Work Function of Tungsten and Molybdenum Oxides via Microwave Exposure for Efficient Organic Photovoltaics

In this work, we use microwave exposure of tungsten and molybdenum oxides to improve hole extraction in organic photovoltaics (OPVs). This is a result of fast recovery of the high work function of metal oxides occurring within a few seconds of microwave processing. Using the space-charge-limited current model, we verified the formation of an anode contact that facilitates hole extraction, while Mott–Schottky analysis revealed the enhancement of the device built-in field in the devices with the microwave-exposed metal oxides. Both were attributed to the formation of large interfacial dipoles at the ITO/microwave-exposed metal oxide interface. The power conversion efficiency (PCE) of OPVs using microwave-exposed metal oxides and based on blends of poly­[(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thiophenediyl-2,1,3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl] (PCDTBT) with ([6,6]-phenyl-C₇₁ butyric acid methyl ester, PC₇₁BM) reached values of 7.2%, which represents an increase of about 30% compared with the efficiency of 5.7% of devices using metal oxides not subjected to microwave exposure

Old Metal Oxide Clusters in New Applications: Spontaneous Reduction of Keggin and Dawson Polyoxometalate Layers by a Metallic Electrode for Improving Efficiency in Organic Optoelectronics

The present study is aimed at investigating the solid state reduction of a representative series of Keggin and Dawson polyoxometalate (POM) films in contact with a metallic (aluminum) electrode and at introducing them as highly efficient cathode interlayers in organic optoelectronics. We show that, upon reduction, up to four electrons are transferred from the metallic electrode to the POM clusters of the Keggin series dependent on addenda substitution, whereas a six electron reduction was observed in the case of the Dawson type clusters. The high degree of their reduction by Al was found to be of vital importance in obtaining effective electron transport through the cathode interface. A large improvement in the operational characteristics of organic light emitting devices and organic photovoltaics based on a wide range of different organic semiconducting materials and incorporating reduced POM/Al cathode interfaces was achieved as a result of the large decrease of the electron injection/extraction barrier, the enhanced electron transport and the reduced recombination losses in our reduced POM modified devices

Effect of the Oxygen Sub-Stoichiometry and of Hydrogen Insertion on the Formation of Intermediate Bands within the Gap of Disordered Molybdenum Oxide Films

The electronic structure of disordered-amorphous molybdenum oxide films was investigated near the band gap by optical absorption and photoluminescence (PL) measurements. It was found that, in nearly stoichiometric films, the 3.2 eV wide gap is free of states and a PL band at 3.7 eV was attributed to electronic transitions between the Mo 4d bonding–antibonding orbitals. In sub-stoichiometric films, electronic states appear within the gap giving rise to additional PL emission within the range 3–3.25 eV. The band gap remains at 3.2 eV, and intermediate bands (IB) are formed within the gap for oxygen-deficient hydrogenated samples. A substantial increase of the PL intensity within the range 2.75–3.75 eV was observed, attributed to the IBs and the increase of the density of Mo 4d antibonding states within the conduction band (CB). The band gap decreases to 2.7 eV in sub-stoichiometric and hydrogenated samples, while the rest of the elctronic structure remains unchanged as for hydrogenated samples also giving enhanced PL intensity within the same range

Surface Modification of ZnO Layers via Hydrogen Plasma Treatment for Efficient Inverted Polymer Solar Cells

Modifications of the ZnO electron extraction layer with low-pressure H plasma treatment increased the efficiency of inverted polymer solar cells (PSCs) based on four different photoactive blends, namely, poly­(3-hexylthiophene):[6,6]-phenyl C₇₁ butyric acid methyl ester (P3HT:PC₇₁BM), P3HT:1′,1″,4′,4″-tetrahydro-di­[1,4]­methano­naphthaleno-[5,6]­ullerene-C₆₀ (P3HT:IC₆₀BA), poly­[(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thio­phenediyl-2,1,3-benzo­thiadiazole-4,7-diyl-2,5-thio­phenediyl]:PC₇₁BM (PCDTBT:PC₇₁BM), and (poly­[[4,8-bis­[(2-ethyl­hexyl)­oxy]­benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithio­phene-2,6-diyl]­[3-fluoro-2-(2-ethyl­hexy)­carbonyl]­thieno­[3,4-<i>b</i>]­thio­phenediyl]]):PC₇₁BM (PTB7:PC₇₁BM), irrespective of the donor:acceptor combination in the photoactive blend. The drastic improvement in device efficiency is dominantly attributable to the reduction in the work function of ZnO followed by a decreased energy barrier for electron extraction from fullerene acceptor. In addition, reduced recombination losses and improved nanomorphology of the photoactive blend in the devices with the H plasma treated ZnO layer were observed, whereas exciton dissociation also improved with hydrogen treatment. As a result, the inverted PSC consisting of the P3HT:PC₇₁BM blend exhibited a high power conversion efficiency (PCE) of 4.4%, the one consisting of the P3HT:IC₆₀BA blend exhibited a PCE of 6.6%, and our champion devices with the PCDTBT:PC₇₁BM and PTB7:PC₇₁BM blends reached high PCEs of 7.4 and 8.0%, respectively

The Influence of Hydrogenation and Oxygen Vacancies on Molybdenum Oxides Work Function and Gap States for Application in Organic Optoelectronics

Molybdenum oxide is used as a low-resistance anode interfacial layer in applications such as organic light emitting diodes and organic photovoltaics. However, little is known about the correlation between its stoichiometry and electronic properties, such as work function and occupied gap states. In addition, despite the fact that the knowledge of the exact oxide stoichiometry is of paramount importance, few studies have appeared in the literature discussing how this stoichiometry can be controlled to permit the desirable modification of the oxide’s electronic structure. This work aims to investigate the beneficial role of hydrogenation (the incorporation of hydrogen within the oxide lattice) versus oxygen vacancy formation in tuning the electronic structure of molybdenum oxides while maintaining their high work function. A large improvement in the operational characteristics of both polymer light emitting devices and bulk heterojunction solar cells incorporating hydrogenated Mo oxides as hole injection/extraction layers was achieved as a result of favorable energy level alignment at the metal oxide/organic interface and enhanced charge transport through the formation of a large density of gap states near the Fermi level

Triazine-Substituted Zinc Porphyrin as an Electron Transport Interfacial Material for Efficiency Enhancement and Degradation Retardation in Planar Perovskite Solar Cells

Motivated by the excellent electron-transfer capability of porphyrin molecules in natural photosynthesis, we introduce here the first application of a porphyrin compound to improve the performance of planar perovskite solar cells. The insertion of a thin layer consisting of a triazine-substituted Zn porphyrin between the TiO₂ electron transport layer and the CH₃NH₃PbI₃ perovskite film significantly augmented electron transfer toward TiO₂ while also sufficiently improved the morphology of the perovskite film. The devices employing porphyrin-modified TiO₂ exhibited a significant increase in the short-circuit current densities and a small increase in the fill factor. As a result, they delivered a maximum power conversion efficiency (PCE) of 16.87% (average 14.33%), which represents a 12% enhancement compared to 15.01% (average 12.53%) of the reference cell. Moreover, the porphyrin-modified cells exhibited improved hysteretic behavior and a higher stabilized power output of 14.40% compared to 10.70% of the reference devices. Importantly, nonencapsulated perovskite solar cells embedding a thin porphyrin interlayer showed an elongated lifetime retaining 86% of the initial PCE after 200 h, while the reference devices exhibited higher efficiency loss due to faster decomposition of CH₃NH₃PbI₃ to PbI₂