Co-ligand triphenylphosphine/alkynyl-stabilized undecagold nanocluster with a capped crown structure.

We report the synthesis and crystal structure of novel co-ligand phosphine/alkynyl protected Au nanoclusters, with composition [Au11(PPh3)8(C[triple bond, length as m-dash]CPh-CF3)2](SbF6) (1). The gold atoms in the cluster as a capped crown structure subtend C 3v symmetry with one deriving from a central icosahedron and 10 peripheral Au atoms, and all alkynides are exclusively σ coordination bonding. The mean core diameter is about 5.1 Å and the overall van der Waals diameter can be estimated to be 20.5 Å. The optical absorbance of 1 in solution reveals characteristic peaks at 384 and 426 nm and a shoulder between 450 and 550 nm.

Design of Morphology-Controllable ZnO Nanorods/Nanopariticles Composite for Enhanced Performance of Dye-Sensitized Solar Cells.

A facile one-pot approach was developed for the synthesis of ZnO nanorods (NRs)/nanoparticles (NPs) architectures with controllable morphologies. The concrete state of existence of NPs and NRs could rationally be controlled through reaction temperature manipulation, i.e., reactions occured at 120, 140, 160, and 180 °C without stirring resulted in orderly aligned NRs, disordered but connected NRs/NPs, and relatively dispersed NRs/NPs with different sizes and lengths, respectively. The as-obained ZnO nanostructures were then applied to construct photoanodes of dye-sensitized solar cells, and the thicknesses of the resultant films were controlled for performance optimization. Under an optimized condition (i.e., with a film thickness of 14.7 µm), the device fabricated with the material synthesized at 160 °C exhibited the highest conversion efficiency of 4.30% with an elevated current density of 14.50 mA·cm-2 and an open circuit voltage of 0.567 V. The enhanced performance could be attributed to the coordination effects of the significantly enhanced dye absorption capability arising from the introduced NPs and the intrinsic fast electron transport property of NRs as confirmed by electrochemical impedance spectroscopy (EIS) and ultraviolet-visible (UV-vis) absorption.

Interfacial Modification in Organic and Perovskite Solar Cells.

Organic bulk heterojunction solar cells (OSCs) and hybrid halide perovskite solar cells (PSCs) are two promising photovoltaic techniques for next-generation energy conversion devices. The rapid increase in the power conversion efficiency (PCE) in OSCs and PSCs has profited from synergetic progresses in rational material synthesis for photoactive layers, device processing, and interface engineering. Interface properties in these two types of devices play a critical role in dictating the processes of charge extraction, surface trap passivation, and interfacial recombination. Therefore, there have been great efforts directed to improving the solar cell performance and device stability in terms of interface modification. Here, recent progress in interfacial doping with biopolymers and ionic salts to modulate the cathode interface properties in OSCs is reviewed. For the anode interface modification, recent strategies of improving the surface properties in widely used PEDOT:PSS for narrowband OSCs or replacing it by novel organic conjugated materials will be touched upon. Several recent approaches are also in focus to deal with interfacial traps and surface passivation in emerging PSCs. Finally, the current challenges and possible directions for the efforts toward further boosts of PCEs and stability via interface engineering are discussed.

Retardation of Trap-Assisted Recombination in Lead Halide Perovskite Solar Cells by a Dimethylbiguanide Anchor Layer.

Reaching the full potential of solar cells based on photo-absorbers of organic-inorganic hybrid perovskites requires highly efficient charge extraction at the interface between perovskite and charge transporting layer. This demand is generally challenged by the presence of under-coordinated metal or halogen ions, causing surface charge trapping and resultant recombination losses. These problems can be tackled by introducing a small molecule interfacial anchor layer based on dimethylbiguanide (DMBG). Benefitting from interactions between the nitrogen-containing functional groups in DMBG and unsaturated ions in CH3 NH3 PbI3 perovskites, the electron extraction of TiO2 is dramatically improved in association with reduced Schottky-Read-Hall recombination, as revealed by photoluminescence spectroscopy. As a consequence, the power conversion efficiency of CH3 NH3 PbI3 solar cells is boosted from 17.14 to 19.1 %, showing appreciably reduced hysteresis. The demonstrated molecular strategy based on DMBG enables one to achieve meliorations on key figures of merit in halide perovskite solar cells with improved stability.

A Biopolymer Heparin Sodium Interlayer Anchoring TiO2 and MAPbI3 Enhances Trap Passivation and Device Stability in Perovskite Solar Cells.

Traps in the photoactive layer or interface can critically influence photovoltaic device characteristics and stabilities. Here, traps passivation and retardation on device degradation for methylammonium lead trihalide (MAPbI3 ) perovskite solar cells enabled by a biopolymer heparin sodium (HS) interfacial layer is investigated. The incorporated HS boosts the power conversion efficiency from 17.2 to 20.1% with suppressed hysteresis and Shockley-Read-Hall recombination, which originates primarily from the passivation of traps near the interface between the perovskites and the TiO2 cathode. The incorporation of an HS interfacial layer also leads to a considerable retardation of device degradation, by which 85% of the initial performance is maintained after 70 d storage in ambient environment. Aided by density functional theory calculations, it is found that the passivation of MAPbI3 and TiO2 surfaces by HS occurs through the interactions of the functional groups (COO- , SO3- , or Na+ ) in HS with undersaturated Pb and I ions in MAPbI3 and Ti4+ in TiO2 . This work demonstrates a highly viable and facile interface strategy using biomaterials to afford high-performance and stable perovskite solar cells.

Ambipolar charge transport in a bis-diketopyrrolopyrrole small molecule semiconductor with tunable energetic disorder.

Energetic disorder and activation energy are important parameters that influence the charge carrier mobility in organic semiconductors. Herein, we investigate temperature-dependent ambipolar charge transport alongside its thermal activation energy in organic field-effect transistors (OFETs) based on a diketopyrrolopyrrole (DPP) based small molecule BTDPP2. The determined energetic disorder in BTDPP2 is analogous to those of highly crystalline molecules, such as pentacene, while lower than those of widely used fullerene derivatives (PCBM) or semi-crystalline polymers, such as P3HT. We demonstrate that the energetic disorder and activation energy in BTDPP2 are both impacted by the crystallinity, which is tuned by thermal annealing; moreover, to a certain extent, these two parameters can reduce with increasing the structural order. Moreover, the energetic disorder tends to decrease when BTDPP2 is subjected to thermal annealing. Through comparing the electron transport in BTDPP2 based OFETs and vertical diodes, in which the electron densities differentiate substantially, the different activation energies are roughly described in terms of achievable carrier densities in these two devices. To the best of our knowledge, this aspect has not been addressed on the electron transport in molecular semi-conductive materials. Our results shine light on fundamental understandings of charge transport properties in solution processed small molecules holding promise for opto-electronic applications.

Additive-Enhanced Crystallization of Solution Process for Planar Perovskite Solar Cells with Efficiency Exceeding 19 .

To reduce intrinsic defect density in perovskite films, various additives are often added into the precursor solution. In this paper, a novel solution, in which isopropanol (IPA) was added into the precursor solution, was developed for the preparation of normal planar perovskite solar cells (PSCs). A champion power conversion efficiency (PCE) of 19.70 % was achieved with this approach. By adding IPA into the precursor solution, CH3 NH3 PbI3 perovskite showed better crystallization and stability, indicating improved film quality. Because of the improved morphology of perovskite crystal, the PCE of PSCs increased. In addition, the unencapsulated PSCs retained high output for up to 40 days in air at room temperature, indicating the stability of PSCs. These results may provide a new avenue for manufacturing high efficiency and high stability PSCs.

Incorporating an Inert Polymer into the Interlayer Passivates Surface Defects in Methylammonium Lead Halide Perovskite Solar Cells.

The hysteresis effect and instability are important concerns in hybrid perovskite photovoltaic devices that hold great promise in energy conversion applications. In this study, we show that the power conversion efficiency (PCE), hysteresis, and device lifetime can be simultaneously improved for methylammoniumlead halide (CH3 NH3 PbI3-x Clx ) solar cells after incorporating poly(methyl methacrylate) (PMMA) into the PC61 BM electron extraction layer (EEL). By choosing appropriate molecular weights of PMMA, we obtain a 30 % enhancement of PCE along with effectively lowered hysteresis and device degradation, adopting inverted planar device structure. Through the combinatorial study using Kelvin probe force microscopy, diode mobility measurements, and irradiation-dependent solar cell characterization, we attribute the enhanced device parameters (fill factor and open circuit voltage) to the surface passivation of CH3 NH3 PbI3-x Clx , leading to mitigating charge trapping at the cathode interface and resultant Shockley-Read-Hall charge recombination. Beneficially, modified by inert PMMA, CH3 NH3 PbI3-x Clx solar cells display a pronounced retardation in performance degradation, resulting from improved film quality in the PC61 BM layer incorporating PMMA which increases the protection for underneath perovskite films. This work enables a versatile and effective interface approach to deal with essential concerns for solution-processed perovskite solar cells by air-stable and widely accessible materials.