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This study reports three newly developed dopant-free hole-transporting materials (HTMs) for perovskite solar cells. The design is based on a quinacridone (QA) dye as the core with three different extended end-capping moieties, namely, acenaphthylene (ACE), triphenylamine (TPA) and diphenylamine (DPA), attached to the QA core. These HTMs were synthesized and used to successfully fabricate in mesoscopic TiO2/CH3NH3PbI3/HTM perovskite devices. Under AM 1.5G illumination at 100 mW cm-2, the devices achieved a maximum efficiency of 18.2% for ACE-QA-ACE, 16.6% for TPA-QA-TPA and 15.5% for DPA-QA-DPA without any additives, whereas reference devices with doped spiro-OMeTAD as the HTM achieved a PCE of 15.2%. Notably, the unencapsulated devices based on the novel dopant-free HTMs exhibited impressive stability in comparison with the devices based on doped spiro-OMeTAD under a relative humidity of 75% for 30 days. These linear symmetrical HTMs pave the way to a new class of organic hole-transporting materials for cost-efficient and large-area applications of printed perovskite solar cells.

Bismuth halide compounds as a non-toxic alternative are increasingly investigated because of their potential in optoelectronic devices and their rich structural chemistry. Hard X-ray spectroscopy was applied to the ternary bismuth halide Cs3Bi2I9 and its related precursors BiI3 and CsI to understand its electronic structure at an atomic level. We specifically investigated the core levels and valence band using X-ray photoemission spectroscopy (PES), high-resolution X-ray absorption (HERFD-XAS), and resonant inelastic X-ray scattering (RIXS) to get insight into the chemistry and the band edge properties of the two bismuth compounds. Using these element specific X-ray techniques, our experimental electronic structures show that the primary differences between the two bismuth samples are the position of the iodine states in the valence and conduction bands and the degree of hybridization with bismuth lone pair (6s(2)) states. The crystal structure of the two layered quasi-perovskite compounds plays a minor role in modifying the overall electronic structure, with variations in bismuth lone pair states and iodine band edge states. Density Functional Theory (DFT) calculations are used to compare with experimental data. The results demonstrate the effectiveness of hard X-ray spectroscopies to identify element specific bulk electronic structures and their use in optoelectronic devices.

Perovskite solar cell (PSCs) technology has made a tremendous impact in the solar cell community due to the exceptional performance of PSCs as the power conversion efficiency (PCE) surged to a world record of 22% within the last few years. Despite this high efficiency value, the commercialization of PSCs for large area applications at affordable prices is still pending due to the low stability of the devices under ambient atmospheric conditions and the very high cost of the hole transporting materials (HTMs) used as the charge transporting layer in these devices. To cope with these challenges, the use of cheap HTMs can play a dual role in terms of lowering the overall cost of perovskite technology as well as protecting the perovskite layer to achieve a higher stability. To achieve these goals, various new organic hole transporting materials (HTMs) have been proposed. In this study, we used a unique and novel anthanthrone (ANT) dye as a conjugated core building block and an affordable moiety to synthesize a new HTM. The commercially available dye was functionalized with an extended diphenylamine (DPA) end capping group. The newly developed HTM, named DPA–ANT–DPA, was synthesized in a single step and used successfully in mesoporous perovskite solar cell devices, achieving a PCE of 11.5% under 1 Sun condition with impressive stability. The obtained device efficiency is amongst the highest as compared to that of other D–A–D molecular design and low band gap devices. This kind of low cost HTM based on an inexpensive starting precursor, anthanthrone dye, paves the way for the economical and large-scale production of stable perovskite solar cells.

In this report, highly efficient and humidity‐resistant perovskite solar cells (PSCs) using two new small molecule hole transporting materials (HTM) made from a cost‐effective precursor anthanthrone (ANT) dye, namely, 4,10‐bis(1,2‐dihydroacenaphthylen‐5‐yl)‐6,12‐bis(octyloxy)‐6,12‐dihydronaphtho[7,8,1,2,3‐nopqr]tetraphene (ACE‐ANT‐ACE) and 4,4′‐(6,12‐bis(octyloxy)‐6,12‐dihydronaphtho[7,8,1,2,3‐nopqr]tetraphene‐4,10‐diyl)bis(N,N‐bis(4‐methoxyphenyl)aniline) (TPA‐ANT‐TPA) are presented. The newly developed HTMs are systematically compared with the conventional 2,2′,7,7′‐tetrakis(N,N′‐di‐p‐methoxyphenylamino)‐9,9′‐spirbiuorene (Spiro‐OMeTAD). ACE‐ANT‐ACE and TPA‐ANT‐TPA are used as a dopant‐free HTM in mesoscopic TiO2/CH3NH3PbI3/HTM solid‐state PSCs, and the performance as well as stability are compared with Spiro‐OMeTAD‐based PSCs. After extensive optimization of the metal oxide scaffold and device processing conditions, dopant‐free novel TPA‐ANT‐TPA HTM‐based PSC devices achieve a maximum power conversion efficiency (PCE) of 17.5% with negligible hysteresis. An impressive current of 21 mA cm−2 is also confirmed from photocurrent density with a higher fill factor of 0.79. The obtained PCE of 17.5% utilizing TPA‐ANT‐TPA is higher performance than the devices prepared using doped Spiro‐OMeTAD (16.8%) as hole transport layer at 1 sun condition. It is found that doping of LiTFSI salt increases hygroscopic characteristics in Spiro‐OMeTAD; this leads to the fast degradation of solar cells. While, solar cells prepared using undoped TPA‐ANT‐TPA show dewetting and improved stability. Additionally, the new HTMs form a fully homogeneous and completely covering thin film on the surface of the active light absorbing perovskite layers that acts as a protective coating for underlying perovskite films. This breakthrough paves the way for development of new inexpensive, more stable

Carbon nanotubes (CNTs) are considered promising for a large range of emerging technologies ranging from advanced electronics to utilization as nanoreactors. Here we report a controlled facile synthesis of aligned carbon nanotubes with very small dimensions directly grown on steel grid substrates via two-step catalytic chemical vapor deposition (CCVD) of a molecular catalyst (ferrocene) with ethylene as the carbon source. The system is characterized by resonance Raman spectroscopy and the results show single walled carbon nanotube (SWCNT) arrays composed of 0.80 nm to 1.24 nm semiconducting CNTs, as analyzed using Kataura analysis, which is approaching the lowest diameters attainable for SWCNTs. The G+ and G− mode splitting, G− line shapes and ring breathing modes (RBMs) are analyzed to characterize the CNTs. The approach results in close packed and vertically aligned SWCNT bundles formed into hair shapes, with some contribution from multiwall CNTs (MWCNTs). IR spectroscopy is utilized to characterize the edge/defect states that have the ability to form esters and ether bonds in the as-prepared CNTs. The stepwise deposition of the catalyst followed by the carbon source gives control over the formation of small diameter single walled carbon nanotubes (SWCNTs). The utilization of molecular catalysts for narrow diameter growth directly on steel grid substrates forms a promising approach for producing cost-effective CNT substrates for a plethora of sensing and catalytic applications.

The formation of CH3NH3PbI3 (MAPbI(3)) from its precursors is probably the most significant step in the control of the quality of this semiconductor perovskite material, which is highly promising for photovoltaic applications. Here we investigated the transformation of spin coated PbI2 films to MAPbI(3) using a reaction with MAI in vapor phase, referred to as vapor assisted solution process (VASP). The presence of a mesoporous TiO2 scaffold on the substrate was found to speed up reaction and led to complete conversion of PbI2, while reaction on glass substrates was slower, with some PbI2 remaining even after prolonged reaction time. Based on data from UV-visible spectroscopy, Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy, the formation of an X-ray amorphous intermediate phase is proposed, which is identified by an increasing absorption from 650 to 500 nm in the absorption spectrum. This feature disappears upon long reaction times for films on planar substrates, but persists for films on mesoporous TiO2. Poor solar cell performance of planar VASP prepared devices was ascribed to PbI2 remaining in the film, forming a barrier between the perovskite layer and the compact TiO2/FTO contact. Good performance, with efficiencies up to 13.3%, was obtained for VASP prepared devices on mesoporous TiO2.

Films of the hybrid lead halide perovskite CH3NH3PbI3 were found to react with pyridine vapor at room temperature leading to complete bleaching of the film. In dry air or nitrogen atmosphere recrystallization takes place, leading to perovskite films with markedly improved optical and photovoltaic properties. The physical and chemical origin of the reversible bleaching and recrystallization mechanism was investigated using a variety of experimental techniques and quantum chemical calculations. The strong Lewis base pyridine attacks the CH3NH3PbI3. The mechanism can be understood from a frustrated Lewis pair formation with a partial electron donation of the lone-pair on nitrogen together with competitive bonding to other species as revealed by Raman spectroscopy and DFT calculations. The bleached phase consists of methylammonium iodide crystals and an amorphous phase of PbI2( pyridine)(2). After spontaneous recrystallization the CH3NH3PbI3 thin films have remarkably improved photoluminescence, and solar cell performance increased from 9.5% for as-deposited films to more than 18% power conversion efficiency for recrystallized films in solar cells with planar geometry under AM1.5G illumination. Hysteresis was negligible and open-circuit potential was remarkably high, 1.15 V. The results show that complete recrystallization can be achieved with a simple room temperature pyridine vapor treatment of CH3NH3PbI3 films leading to high quality crystallinity films with drastically improved photovoltaic performance.

An approach to perform controlled acid-catalyzed oligomerization via vapor pressure control of thereactions inside Nafion membranes is presented. The interaction of Nafion with several classes ofaromatic (pyrrol, furan, thiophene) and unsaturated (methyl-acetylene) gas phase monomers was studiedas a function of contact time and temperature by in situ vibrational (FTIR) and electronic (UV-Vis)spectroscopy with the support from theoretical linear response and time dependent DFT calculations tomonitor the vibrations and the effective number of conjugated double bonds. The formation ofH-bonded adduct as seen from IR spectroscopy transforms the hydrogen bonded species into positivelycharged oligomers through an activated proton transfer mechanism where oligomerization progressthrough increasing contact time with the respective gas phase reactants at room temperature. Theactivated proton transfer oligomerization proceeds through the stepwise growth propagation cycles viacarbocationic intermediates, finally leads to the formation of irreversible, conjugated charged oligomersas a product. The colored, conjugated oligomeric Nafion composite products are formed at roomtemperature as a function of reaction time and are irreversible after complete degassing of the gas phasereactants as well stable in ambient environment stored for many days in pure oxygen or air and cannotbe extracted with common solvents, appearing strongly encapsulated inside Nafion membranes. This iscrucial for future applications of the presented route for direct production of conjugated species insideNafion and thus production and control of composite membrane materials of interest in fuel cells andcatalysis.

Organo-metal halide perovskites (OMHPs) are materials with attractive properties for optoelectronics. They made a recent introduction in the photovoltaics world by methylammonium (MA) lead triiodide and show remarkably improved charge separation capabilities when chloride and bromide are added. Here we show how halide substitution in OMHPs with the nominal composition CH3NH3PbI2X, where X is I, Br, or Cl, influences the morphology, charge quantum yield, and local interaction with the organic MA cation. X-ray diffraction and photoluminescence data demonstrate that halide substitution affects the local structure in the OMHPs with separate MAPbI3 and MAPbCl(3) phases. Raman spectroscopies as well as theoretical vibration calculations reveal that this at the same time delocalizes the charge to the MA cation, which can liberate the vibrational movement of the MA cation, leading to a more adaptive organic phase. The resonance Raman effect together with quantum chemical calculations is utilized to analyze the change in charge transfer mechanism upon electronic excitation and gives important clues for the mechanism of the much improved photovoltage and photocurrent also seen in the solar cell performance for the materials when chloride compounds are included in the preparation.