Intensity-Modulated Scanning Kelvin Probe Microscopy for Probing Recombination in Organic Photovoltaics

We study surface photovoltage decays on sub-millisecond time scales in organic solar cells using intensity-modulated scanning Kelvin probe microscopy (SKPM). Using polymer/fullerene (poly[<i>N</i>-9″-heptadecanyl-2,7-carbazole-<i>alt</i>-5,5-(4′,7′-di-2-thienyl-2′,1′,3′-benzothiadiazole)]/[6,6]-phenyl C₇₁-butyric acid methyl ester, PCDTBT/PC₇₁BM) bulk heterojunction devices as a test case, we show that the decay lifetimes measured by SKPM depend on the intensity of the background illumination. We propose that this intensity dependence is related to the well-known carrier-density-dependent recombination kinetics in organic bulk heterojunction materials. We perform transient photovoltage (TPV) and charge extraction (CE) measurements on the PCDTBT/PC₇₁BM blends to extract the carrier-density dependence of the recombination lifetime in our samples, and we find that the device TPV and CE data are in good agreement with the intensity and frequency dependence observed <i>via</i> SKPM. Finally, we demonstrate the capability of intensity-modulated SKPM to probe local recombination rates due to buried interfaces in organic photovoltaics (OPVs). We measure the differences in photovoltage decay lifetimes over regions of an OPV cell fabricated on an indium tin oxide electrode patterned with two different phosphonic acid monolayers known to affect carrier lifetime

V₂O₅ as Hole Transporting Material for Efficient All Inorganic Sb₂S₃ Solar Cells

This research demonstrates that V₂O₅ is able to serve as hole transporting material to substitute organic transporting materials for Sb₂S₃ solar cells, offering all inorganic solar cells. The V₂O₅ thin film is prepared by thermal decomposition of spin-coated vanadium­(V) triisopropoxide oxide solution. Mechanistic investigation shows that heat treatment of V₂O₅ layer has crucial influence on the power conversion efficiency of device. Low temperature annealing is unable to remove the organic molecules that increases the charge transfer resistance, while high temperature treatment leads to the increase of work function of V₂O₅ that blocks hole transporting from Sb₂S₃ to V₂O₅. Electrochemical and compositional characterizations show that the interfacial contact of V₂O₅/Sb₂S₃ can be essentially improved with appropriate annealing. The optimized power conversion efficiency of device based on Sb₂S₃/V₂O₅ heterojunction reaches 4.8%, which is the highest power conversion efficiency in full inorganic Sb₂S₃-based solar cells with planar heterojunction solar cells. Furthermore, the employment of V₂O₅ as hole transporting material leads to significant improvement in moisture stability compared with the device based organic hole transporting material. Our research provides a material choice for the development of full inorganic solar cells based on Sb₂S₃, Sb₂(S,Se)₃, and Sb₂Se₃

A Simple Perylene Derivative as a Solution-Processable Cathode Interlayer for Perovskite Solar Cells with Enhanced Efficiency and Stability

A simple alcohol-soluble perylene derivative (i.e., tetramethylammonium salt of perylene-3,4,9,10-tetracarboxylic acid; TMA-PTC) was prepared and applied as a cathode interlayer (CIL) to modify the PC₆₁BM/Ag interface in planar p–i–n perovskite solar cells (PeSCs). As a result, the power conversion efficiency (PCE) of the TMA-PTC-based PeSCs is ca. 30% higher than that of the devices without CIL. It was revealed that the enhancement in PCE might be attributed to the improved electron-transporting and hole-blocking properties of the PC₆₁BM/TMA-PTC/Ag interfaces. Moreover, the TMA-PTC devices show remarkably higher stability than those without CIL probably due to the suppressed corrosion of perovskite on Ag cathode. Our findings thus demonstrate a multifunctional and solution-processable CIL that may be a promising block for the fabrication of low-cost, high-efficiency and stable planar p–i–n PeSCs

Interplay between Interfacial Structures and Device Performance in Organic Solar Cells: A Case Study with the Low Work Function Metal, Calcium

A better understanding of how interfacial structure affects charge carrier recombination would benefit the development of highly efficient organic photovoltaic (OPV) devices. In this paper, transient photovoltage (TPV) and charge extraction (CE) measurements are used in combination with synchrotron radiation photoemission spectroscopy (SRPES) to gain insight into the correlation between interfacial properties and device performance. OPV devices based on PCDTBT/PC₇₁BM with a Ca interlayer were studied as a reference system to investigate the interfacial effects on device performance. Devices with a Ca interlayer exhibit a lower recombination than devices with only an Al cathode at a given charge carrier density (<i>n</i>). In addition, the interfacial band structures indicate that the strong dipole moment produced by the Ca interlayer can facilitate the extraction of electrons and drive holes away from the cathode/polymer interface, resulting in beneficial reduction in interfacial recombination losses. These results help explain the higher efficiencies of devices made with Ca interlayers compared to that without the Ca interlayer

Bottom-Up Synthesis of Metalated Carbyne

Because of stability issues, carbyne, a one-dimensional chain of carbon atoms, has been much less investigated than other recent carbon allotropes such as graphene. Beyond that, metalation of such a linear carbon nanostructure with regularly distributed metal atoms is even more challenging. Here we report a successful on-surface synthesis of metalated carbyne chains by dehydrogenative coupling of ethyne molecules and copper atoms on a Cu(110) surface under ultrahigh-vacuum conditions. The length of the fabricated metalated carbyne chains was found to extend to the submicron scale (with the longest ones up to ∼120 nm). We expect that the herein-developed on-surface synthesis strategy for the efficient synthesis of organometallic carbon-based nanostructures will inspire more extensive experimental investigations of their physicochemical properties and explorations of their potential with respect to technological applications

ITO Interface Modifiers Can Improve <i>V</i>_OC in Polymer Solar Cells and Suppress Surface Recombination

We use dipolar phosphonic acid self-assembled monolayers (PA SAMs) to modify the work function of the hole-extracting contact in polymer/fullerene bulk heterojunction solar cells. We observe a linear dependence of the open-circuit voltage (<i>V</i>_OC) of these organic photovoltaic devices on the modified indium tin oxide (ITO) work function when using a donor polymer with a deep-lying ionization energy. With specific SAMs, we can obtain <i>V</i>_OC values exceeding those obtained with the common poly­(3,4-ethylenedioxythiophene)-poly­(styrenesulfonate) (PEDOT:PSS) hole-extraction layer. We measure charge-carrier lifetimes and densities using transient photovoltage and charge extraction in a series of devices with SAM-modified contacts. As expected, these measurements show systematically longer carrier lifetimes in devices with higher <i>V</i>_OC values; however, the trends provide useful distinctions between different hypotheses of how transient photovoltage decays might be controlled by surface chemistry. We interpret our results as being consistent with changes in the band bending at the ITO/bulk heterojunction interface that have the net result of altering the internal electric field to help prevent electrons in fullerene domains from undergoing surface recombination at the hole-extracting electrode

Partially Oxidized SnS₂ Atomic Layers Achieving Efficient Visible-Light-Driven CO₂ Reduction

Unraveling the role of surface oxide on affecting its native metal disulfide’s CO₂ photoreduction remains a grand challenge. Herein, we initially construct metal disulfide atomic layers and hence deliberately create oxidized domains on their surfaces. As an example, SnS₂ atomic layers with different oxidation degrees are successfully synthesized. <i>In situ</i> Fourier transform infrared spectroscopy spectra disclose the COOH* radical is the main intermediate, whereas density-functional-theory calculations reveal the COOH* formation is the rate-limiting step. The locally oxidized domains could serve as the highly catalytically active sites, which not only benefit for charge-carrier separation kinetics, verified by surface photovoltage spectra, but also result in electron localization on Sn atoms near the O atoms, thus lowering the activation energy barrier through stabilizing the COOH* intermediates. As a result, the mildly oxidized SnS₂ atomic layers exhibit the carbon monoxide formation rate of 12.28 μmol g^–1 h^–1, roughly 2.3 and 2.6 times higher than those of the poorly oxidized SnS₂ atomic layers and the SnS₂ atomic layers under visible-light illumination. This work uncovers atomic-level insights into the correlation between oxidized sulfides and CO₂ reduction property, paving a new way for obtaining high-efficiency CO₂ photoreduction performances

Hierarchical Dehydrogenation Reactions on a Copper Surface

Hierarchical control of chemical reactions is being considered as one of the most ambitious and challenging topics in modern organic chemistry. In this study, we have realized the one-by-one scission of the X–H bonds (X = N and C) of aromatic amines in a controlled fashion on the Cu(111) surface. Each dehydrogenation reaction leads to certain metal–organic supramolecular structures, which were monitored in single-bond resolution via scanning tunneling microscopy and noncontact atomic force microscopy. Moreover, the reaction pathways were elucidated from X-ray photoelectron spectroscopy measurements and density functional theory calculations. Our insights pave the way for connecting molecules into complex structures in a more reliable and predictable manner, utilizing carefully tuned stepwise on-surface synthesis protocols

Refining Defect States in W₁₈O₄₉ by Mo Doping: A Strategy for Tuning N₂ Activation towards Solar-Driven Nitrogen Fixation

Photocatalysis may provide an intriguing approach to nitrogen fixation, which relies on the transfer of photoexcited electrons to the ultrastable NN bond. Upon N₂ chemisorption at active sites (e.g., surface defects), the N₂ molecules have yet to receive energetic electrons toward efficient activation and dissociation, often forming a bottleneck. Herein, we report that the bottleneck can be well tackled by refining the defect states in photocatalysts via doping. As a proof of concept, W₁₈O₄₉ ultrathin nanowires are employed as a model material for subtle Mo doping, in which the coordinatively unsaturated (CUS) metal atoms with oxygen defects serve as the sites for N₂ chemisorption and electron transfer. The doped low-valence Mo species play multiple roles in facilitating N₂ activation and dissociation by refining the defect states of W₁₈O₄₉: (1) polarizing the chemisorbed N₂ molecules and facilitating the electron transfer from CUS sites to N₂ adsorbates, which enables the NN bond to be more feasible for dissociation through proton coupling; (2) elevating defect-band center toward the Fermi level, which preserves the energy of photoexcited electrons for N₂ reduction. As a result, the 1 mol % Mo-doped W₁₈O₄₉ sample achieves an ammonia production rate of 195.5 μmol g_cat^–1 h^–1, 7-fold higher than that of pristine W₁₈O₄₉. In pure water, the catalyst demonstrates an apparent quantum efficiency of 0.33% at 400 nm and a solar-to-ammonia efficiency of 0.028% under simulated AM 1.5 G light irradiation. This work provides fresh insights into the design of photocatalyst lattice for N₂ fixation and reaffirms the versatility of subtle electronic structure modulation in tuning catalytic activity

Oxide Defect Engineering Enables to Couple Solar Energy into Oxygen Activation

Modern development of chemical manufacturing requires a substantial reduction in energy consumption and catalyst cost. Sunlight-driven chemical transformation by metal oxides holds great promise for this goal; however, it remains a grand challenge to efficiently couple solar energy into many catalytic reactions. Here we report that defect engineering on oxide catalyst can serve as a versatile approach to bridge light harvesting with surface reactions by ensuring species chemisorption. The chemisorption not only spatially enables the transfer of photoexcited electrons to reaction species, but also alters the form of active species to lower the photon energy requirement for reactions. In a proof of concept, oxygen molecules are activated into superoxide radicals on defect-rich tungsten oxide through visible-near-infrared illumination to trigger organic aerobic couplings of amines to corresponding imines. The excellent efficiency and durability for such a highly important process in chemical transformation can otherwise be virtually impossible to attain by counterpart materials