Low cost, high efficiency, and stability are straightforward research challenges in the development of organic-inorganic perovskite solar cells. Organolead halide is unstable at high temperatures or in some solvents. The direct preparation of a carbon layer on top becomes difficult. In this study, we successfully prepared full solution-processed low-cost TiO2/CH3NH3PbI3 heterojunction (HJ) solar cells based on a low-temperature carbon electrode. Power conversion efficiency of mesoporous (M-)TiO2/CH3NH3PbI3/C HJ solar cells based on a low-temperature-processed carbon electrode achieved 9%. The devices of M-TiO2/CH3NH3PbI3/C HJ solar cells without encapsulation exhibited advantageous stability (over 2000 h) in air in the dark. The ability to process low-cost carbon electrodes at low temperature on top of the CH3NH3PbI3 layer without destroying its structure reduces the cost and simplifies the fabrication process of perovskite HJ solar cells. This ability also provides higher flexibility to choose and optimize the device, as well as investigate the underlying active layers.

Carbon-based ZnO/CH 3 NH 3 PbI 3 /C planar heterojunction perovskite solar cells (PHJ-PSCs) were prepared at low-temperature without using organic hole conductor and metal electrode. When measured via reverse bias scan, rigid and flexible PHJ-PSCs achieved power conversion efficiencies (PCEs) up to 8% and 4% on fluorine-doped tin oxide (FTO)/glass substrates and flexible polymer substrates, respectively. The flexible devices were capable of maintaining 80% of their initial PCEs after 1000 times of bending.

The effect of air exposure on 2H-WSe2/HOPG is determined via scanning tunneling microscopy (STM). WSe2 was grown by molecular beam epitaxy on highly oriented pyrolytic graphite (HOPG), and afterward, a Se adlayer was deposited in situ on WSe2/HOPG to prevent unintentional oxidation during transferring from the growth chamber to the STM chamber. After annealing at 773 K to remove the Se adlayer, STM images show that WSe2 layers nucleate at both step edges and terraces of the HOPG. Exposure to air for 1 week and 9 weeks caused air-induced adsorbates to be deposited on the WSe2 surface; however, the band gap of the terraces remained unaffected and nearly identical to those on decapped WSe2. The air-induced adsorbates can be removed by annealing at 523 K. In contrast to WSe2 terraces, air exposure caused the edges of the WSe2 to oxidize and form protrusions, resulting in a larger band gap in the scanning tunneling spectra compared to the terraces of air-exposed WSe2 monolayers. The preferential oxidation at the WSe2 edges compared to the terraces is likely the result of dangling edge bonds. In the absence of air exposure, the dangling edge bonds had a smaller band gap compared to the terraces and a shift of about 0.73 eV in the Fermi level toward the valence band. However, after air exposure, the band gap of the oxidized WSe2 edges became about 1.08 eV larger than that of the WSe2 terraces, resulting in the electronic passivation of the WSe2.

For the first time, nonstoichiometric WO2.72 was used as a counter electrode (CE) in dye-sensitized solar cells (DSSCs). Oxygen-vacancy-rich WO2.72 nanorod bundles with notable catalytic activity for triiodide and thiolate reduction were prepared in this study. The photovoltaic parameters of dye-sensitized solar cells (DSSCs) with WO2.72 nanorod bundles as CEs are superior compared with those of the WO3-based cells, and nearly the same as those of the precious metal Pt-based cells. In a non-corrosive organic redox couple, the performance of WO2.72 CEs is better than that of Pt and WO3 CEs in DSSCs.

Surface oxygen vacancies (SOVs) are the most relevant surface defects in metal oxides (MOs), and they participate in numerous physical and chemical reactions. However, information on the nature, distribution, formation, and reactivity of SOVs, as well as relationships among SOVs, is lacking. Investigating SOVs is difficult because of disturbance by the crystal phase, morphology of bulk materials, and synergistic effect between substrate and catalyst host. Herein, by clarifying the origin of SOVs and their distribution, onedimensional (1D) tungsten oxide nanowires (NWs) with numerous SOVs were synthesized. Compared with the three-dimensional nanostructure, the high aspect ratio of 1D NW exposed the SOVs on the surface of the nanostructure rather than embedding them in the bulk. To investigate accurately the effect of SOVs on electrocatalytic activity, we clearly identified how SOVs of tungsten oxide catalyst regulate iodide reduction reactions in the solar cell by in situ filling of SOVs in electrodes and maintaining the crystal phase and morphology of NWs. Iodide reduction reaction activity was notably dependent on tungsten oxide catalyst SOVs, which serve as important catalytic site descriptors. These findings may clarify the fundamental features of SOVs on metal oxides and contribute to the rational design of efficient catalysts and supports.

Developing highly effective and stable counter electrode (CE) materials to replace rare and expensive noble metals for dye‐sensitized and perovskite solar cells (DSC and PSC) is a research hotspot. Carbon materials are identified as the most qualified noble metal‐free CEs for the commercialization of the two photovoltaic devices due to their merits of low cost, excellent activity, and superior stability. Herein, carbonaceous CE materials are reviewed extensively with respect to the two devices. For DSC, a classified discussion according to the morphology is presented because electrode properties are closely related to the specific porosity or nanostructure of carbon materials. The pivotal factors influencing the catalytic behavior of carbon CEs are also discussed. For PSC, an overview of the new carbon CE materials is addressed comprehensively. Moreover, the modification techniques to improve the interfacial contact between the perovskite and carbon layers, aiming to enhance the photovoltaic performance, are also demonstrated. Finally, the development directions, main challenges, and coping approaches with respect to the carbon CE in DSC and PSC are stated.

The ideal liquid–solid heterogeneous electrocatalysis should have not only high catalytic activity but also free electron transport.

Crystallization and decomposition of organolead trihalide perovskites (OTPs) are very sensitive to the presence of water in precursor or in ambient conditions. Thus, understanding equilibrium behaviours (crystallization and decomposition) of OTPs in aqueous solution is very critical for OTP solar cells fabricated with water-based precursor solutions. Here, equilibrium behaviours in an aqueous solution of CH 3 NH 3 PbI 3 (MAPbI 3 ) single crystals (MSCs) were studied. Diethyl ether, as an antisolvent, effectively diffused and induced MSC growth by screening different solvents (diethyl ether, tetrahydrofuran, dichloromethane, and chloroform). The structure transforms from the initial PbI 2 to intermediate (H x PbI 2+x $xH 2 O) and finally MSCs were observed by X-ray diffraction. Decomposition of MSCs in aqueous solution was significantly enhanced by potassium iodide coordination and inhibited by CH 3 NH 3 I (MAI) addition. We ascribed this inhibition behaviour to suppressing MAI migration from the MSC crystal structure. Finally, the optical properties of MSC were studied.www.rsc.org/advances 85344 | RSC Adv., 2015, 5, 85344-85349This journal is