Changing the Sign of Exchange Interaction in Radical Pairs to Tune Magnetic Field Effect on Electrogenerated Chemiluminescence

Two different electrogenerated chemiluminescence (ECL) systems, Ru­(bpy)₃²⁺/TPrA and Ru­(bpy)₃²⁺/C₂O₄^2–, are chosen to study the relationship between the sign of exchange interaction in radical pairs and magnetic field effects (MFEs) on electrogenerated chemiluminescence intensity (MFE_ECL). A positive MFE_ECL up to 210% is observed for the Ru­(bpy)₃²⁺/TPrA system, while a negative MFE_ECL of only −33% is observed based on the Ru­(bpy)₃²⁺/C₂O₄^2– system. The significant difference on MFE_ECL is ascribed to different signs of exchange interaction in radical pairs [Ru­(bpy)₃³⁺···TPrA^•] and [Ru­(bpy)₃³⁺···CO₂^–•] because they have a distant and proximate separation distance between two radicals of a pair, which result in different magnetic-field-induced intersystem crossing directions between singlet and triplet states. The experimental results suggest that an applied magnetic field can enhance the singlet → triplet conversion rate in radical pairs [Ru­(bpy)₃³⁺···TPrA^•] while facilitating an inverse conversion of triplet → singlet in radical pairs [Ru­(bpy)₃³⁺···CO₂^–•]. The increase/decrease of triplet density in radical pairs stimulated by an applied magnetic field leads to an increase/decrease on the density of light-emitting triplets of Ru­(bpy)₃^2+*. As a consequence, we can tune MFE_ECL between positive and negative values by changing the sign of exchange interaction in radical pairs during an electrochemical reaction

Enhancing Photovoltaic Performance of Inverted Planar Perovskite Solar Cells by Cobalt-Doped Nickel Oxide Hole Transport Layer

Electron and hole transport layers have critical impacts on the overall performance of perovskite solar cells (PSCs). Herein, for the first time, a solution-processed cobalt (Co)-doped NiO<i>_X</i> film was fabricated as the hole transport layer in inverted planar PSCs, and the solar cells exhibit 18.6% power conversion efficiency. It has been found that an appropriate Co-doping can significantly adjust the work function and enhance electrical conductivity of the NiO<i>_X</i> film. Capacitance–voltage (<i>C</i>–<i>V</i>) spectra and time-resolved photoluminescence spectra indicate clearly that the charge accumulation becomes more pronounced in the Co-doped NiO<i>_X</i>-based photovoltaic devices; it, as a consequence, prevents the nonradiative recombination at the interface between the Co-doped NiO<i>_X</i> and the photoactive perovskite layers. Moreover, field-dependent photoluminescence measurements indicate that Co-doped NiO<i>_X</i>-based devices can also effectively inhibit the radiative recombination process in the perovskite layer and finally facilitate the generation of photocurrent. Our work indicates that Co-doped NiO<i>_X</i> film is an excellent candidate for high-performance inverted planar PSCs

Fine-Tuning the Quasi-3D Geometry: Enabling Efficient Nonfullerene Organic Solar Cells Based on Perylene Diimides

The geometries of acceptors based on perylene diimides (PDIs) are important for improving the phase separation and charge transport in organic solar cells. To fine-tune the geometry, biphenyl, spiro-bifluorene, and benzene were used as the core moiety to construct quasi-three-dimensional nonfullerene acceptors based on PDI building blocks. The molecular geometries, energy levels, optical properties, photovoltaic properties, and exciton kinetics were systematically studied. The structure–performance relationship was discussed as well. Owing to the finest phase separation, the highest charge mobility and smallest nongeminate recombination, the power conversion efficiency of nonfullerene solar cells using PDI derivatives with biphenyl core (BP-PDI₄) as acceptor reached 7.3% when high-performance wide band gap donor material poly­[(2,6-(4,8-bis­(5-(2-ethylhexyl)­thiophen-2-yl)-benzo­[1,2-<i>b</i>:4,5-<i>b</i>′]­dithiophene))-<i>alt</i>-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis­(2-ethylhexyl)­benzo­[1′,2′-<i>c</i>:4′,5′-<i>c</i>′]­dithiophene-4,8-dione))] was blended