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