Passivation and process engineering approaches of halide perovskite films for high efficiency and stability perovskite solar cells

The surface, interfaces and grain boundaries of a halide perovskite film carry critical tasks in achieving as well as maintaining high solar cell performance due to the inherently defective nature across their regime. Passivating materials and felicitous process engineering approaches have significant ramifications in the resultant perovskite film, and the solar cell's overall macroscale properties as they dictate structural and optoelectronic properties. Herein, we exploit a vast number of defect engineering approaches aiming to increase the performance and the stability of perovskite solar cells, especially against humidity, continuous illumination, and heat. This review begins with the perovskite materials' fundamental structural properties followed by the advances made to induce higher stabilization in perovskite solar cells by fine-tuning materials chemistry design parameters. We continue by summarizing defect passivation strategies based on molecular entities' application, including suitable functional groups that enable sufficient surface, bulk and grain boundary passivation, morphology, and crystallinity control. We also present methods to control the density of defects through the variation of processing conditions, solvent annealing and solvent engineering approaches, gas-assisted deposition methods, and use of self-assembled monolayers, as well as colloidal engineering and coordination surface chemistry. Finally, we give our perspective on how a combined understanding of materials chemistry aspects and passivation mechanisms will further develop high-efficiency and stability perovskite solar cells

Hydrogenated under-stoichiometric tungsten oxide anode interlayers for efficient and stable organic photovoltaics

In this work a hydrogenated under-stoichiometric tungsten oxide is introduced as an efficient anode interlayer in organic photovoltaics (OPVs). The benefits of hydrogen incorporation into the oxide lattice for obtaining desirable properties of tungsten oxides are explored. These benefits include the occupation of gap states near the Fermi level, which may facilitate charge transport, and the maintenance of a high work function, nearly similar to that of the stoichiometric tungsten oxide, which contributes to the formation of a large interfacial dipole at the anode interface and enhances charge extraction. A large improvement was achieved in the operational characteristics-especially in the open-circuit voltage-of bulk heterojunction solar cells based on different polymeric donors, namely poly(3-hexylthiophene), P3HT, or poly[(9-(1-octylnonyl)-9H-carbazole-2,7-diyl)-2,5-thiophenediyl-2,1, 3-benzothiadiazole-4,7-diyl-2,5-thiophenediyl], PCDTBT, and the fullerene acceptor [6,6]-phenyl-C71 butyric acid methyl ester, PC71BM, that incorporated a hydrogenated tungsten oxide as an anode interlayer. This improvement was correlated with the devices' incident photon-to-electron conversion efficiencies (IPCEs) and impedance measurements. Furthermore, an increase in both the device's flat-band voltage (Vfb) and the doping level of the organic semiconductor was measured in P3HT:PC71BM based devices by Mott-Schottky capacitance analysis. Additional benefits are the large process window established for the devices incorporating the hydrogenated tungsten oxide as an anode interlayer and the maintenance of a high PCE (&gt;80% of its initial efficiency) over 50 days, demonstrating good long-term stability, which is much better than that of the conventional devices based on PEDOT:PSS. The results suggest that the interface engineering with hydrogen-treated metal oxide interlayers is an important strategy to develop highly performing and stable organic photovoltaics.</p

Tungsten oxides as interfacial layers for improved performance in hybrid optoelectronic devices

Tungsten oxide (WO3) films with thicknesses ranging from 30 to 100 nm were grown by Hot Filament Vapor Deposition (HFVD). Films were studied by X-Ray Photoemission Spectroscopy (XPS) and were found to be stoichiometric. The surface morphology of the films was characterized by Atomic Force Microscopy (AFM). Samples had a granular form with grains in the order of 100 nm. The surface roughness was found to increase with film thickness. HFVD WO3 films were used as conducting interfacial layers in advanced hybrid organic-inorganic optoelectronic devices. Hybrid-Organic Light Emitting Diodes (Hy-OLEDs) and Organic Photovoltaics (Hy-OPVs) were fabricated with these films as anode and/or as cathode interfacial conducting layers. The Hy-OLEDs showed significantly higher current density and a lower turn-on voltage when a thin WO3 layer was inserted at the anode/polymer interface, while when inserted at the cathode/polymer interface the device performance was found to deteriorate. The improvement was attributed to a more efficient hole injection and transport from the Fermi level of the anode to the Highest Occupied Molecular Orbital (HOMO) of a yellow emitting copolymer (YEP). On the other hand, the insertion of a thin WO3 layer at the cathode/polymer interface of Hy-OPV devices based on a polythiophene-fullerene bulk-heterojunction blend photoactive layer resulted in an increase of the produced photogenerated current, more likely due to improved electron extraction at the Al cathode.</p

Reduced transition metal oxides as electron injection layers in hybrid-PLEDs

Here we report on the improved performance of hybrid polymer light emitting diodes (HyPLEDs) upon inserting an ultra thin layer of partially reduced tungsten oxide (WO2.5) or molybdenum oxide (MoO2.7), deposited (by heating a W or Mo filament while hydrogen was flowing through the chamber) at the polymer/Al cathode interface. Improved current densities, luminances and efficiencies were achieved as a result of improved electron injection and transport at its interface with Al. The observed electron injection improvement is attributed to the lowering of the effective cathode interfacial barrier due to the occupation of gap states with electrons after partial reduction.</p

Porphyrin oriented self-assembled nanostructures for efficient exciton dissociation in high-performing organic photovoltaics

Herein we report on enhanced organic solar cell performance through the incorporation of cathode interfacial layers consisting of self-organized porphyrin nanostructures with a face-on configuration. In particular, a water/methanol-soluble porphyrin molecule, the free base meso-tetrakis(1- methylpyridinium-4-yl)porphyrin chloride, is employed as a novel cathode interlayer in bulk heterojunction organic photovoltaics. It is demonstrated that the self-organization of this porphyrin compound into aggregates in which molecules adopt a face-to-face orientation parallel to the organic semiconducting substrate induces a large local interfacial electric field that results in a significant enhancement of exciton dissociation. Consequently, enhanced photocurrent and open circuit voltage were obtained resulting in overall device efficiency improvement in organic photovoltaics based on bulk heterojunction mixtures of different polymeric donors and fullerene acceptors, regardless of the specific combination of donor-acceptor employed. To highlight the impact of molecular orientation a second porphyrin compound, the Zn-metallated meso-tetrakis(1-methylpyridinium-4-yl)porphyrin chloride, was also studied and it was found that it forms aggregates with an edge-to-edge molecular configuration inducing a smaller increase in the device performance.</p

Solution processed multi-color organic light emitting diodes for application in telecommunications

In this work we present an all solution processing scheme for the fabrication of the three primary colors, (R-G-B), emitting organic light-emitting diodes (OLEDs) via efficient color tuning of a blue organic semiconducting (OSC) thin film, in particular the poly[9,9-di-(2′-ethylhexyl)fluorenyl-2,7-diyl] (PF), in which different color fluorescent emitters are dispersed to define the final emitting color and thus to simplify the different color device fabrication. The transmission speed of the fabricated OLEDs was also examined for possible application in interactive telecommunications. To increase the response speed of the different color devices we altered both the device geometry and the electron injection efficiency. To this end we increased the emissive layer thickness and decreased the device emissive area and we also performed engineering of the cathode interfaces through the incorporation of solution processed porphyrin interlayers in order to lower the electron injection barrier height. The final devices exhibited improved operational characteristics and, consequently, modulation speeds.</p