Tris(2-(1<i>H</i>-pyrazol-1-yl)pyridine)cobalt(III) as p-Type Dopant for Organic Semiconductors and Its Application in Highly Efficient Solid-State Dye-Sensitized Solar Cells

Chemical doping is an important strategy to alter the charge-transport properties of both molecular and polymeric organic semiconductors that find widespread application in organic electronic devices. We report on the use of a new class of Co(III) complexes as p-type dopants for triarylamine-based hole conductors such as spiro-MeOTAD and their application in solid-state dye-sensitized solar cells (ssDSCs). We show that the proposed compounds fulfill the requirements for this application and that the discussed strategy is promising for tuning the conductivity of spiro-MeOTAD in ssDSCs, without having to rely on the commonly employed photo-doping. By using a recently developed high molar extinction coefficient organic D-π-A sensitizer and p-doped spiro-MeOTAD as hole conductor, we achieved a record power conversion efficiency of 7.2%, measured under standard solar conditions (AM1.5G, 100 mW cm^–2). We expect these promising new dopants to find widespread applications in organic electronics in general and photovoltaics in particular

Subnanometer Ga₂O₃ Tunnelling Layer by Atomic Layer Deposition to Achieve 1.1 V Open-Circuit Potential in Dye-Sensitized Solar Cells

Herein, we present the first use of a gallium oxide tunnelling layer to significantly reduce electron recombination in dye-sensitized solar cells (DSC). The subnanometer coating is achieved using atomic layer deposition (ALD) and leading to a new DSC record open-circuit potential of 1.1 V with state-of-the-art organic D-π-A sensitizer and cobalt redox mediator. After ALD of only a few angstroms of Ga₂O₃, the electron back reaction is reduced by more than an order of magnitude, while charge collection efficiency and fill factor are increased by 30% and 15%, respectively. The photogenerated exciton separation processes of electron injection into the TiO₂ conduction band and the hole injection into the electrolyte are characterized in detail

Influence of Donor Groups of Organic D−π–A Dyes on Open-Circuit Voltage in Solid-State Dye-Sensitized Solar Cells

In solid-state dye-sensitized solar cells (ssDSCs), the poor pore filling of the mesoporous semiconductor and the short diffusion length of charge carriers in the hole-transport material (HTM) have limited the mesoscopic titania layer to a thickness of 2–3 μm. To increase the amount of light harvested by ssDSCs, organic dyes with high molar extinction coefficients are of great importance and have been the focus of intensive research. Here we investigate ssDSCs using an organic D−π–A dye, coded Y123, and 2,2′,7,7′-tetrakis(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene as a hole-transport material, exhibiting 934 mV open-circuit potential and 6.9% efficiency at standard solar conditions (AM1.5G, 100 mW cm^–2), which is a significant improvement compared to the analogue dyes C218, C220, and JK2 (<i>V</i>_oc values of 795, 781, and 914 mV, respectively). An upward shift in the conduction band edge was observed from photovoltage transient decay and impedance spectroscopy measurements for devices sensitized with Y123 and JK2 dyes compared to the device using C220 as sensitizer, in agreement with the high photovoltage response of the corresponding ssDSCs. This work highlights the importance of the interaction between the HTM and the dye-sensitized TiO₂ surface for the design of ssDSCs

Regulating a Benzodifuran Single Molecule Redox Switch via Electrochemical Gating and Optimization of Molecule/Electrode Coupling

We report a novel strategy for the regulation of charge transport through single molecule junctions via the combination of external stimuli of electrode potential, internal modulation of molecular structures, and optimization of anchoring groups. We have designed redox-active benzodifuran (BDF) compounds as functional electronic units to fabricate metal–molecule–metal (m–M–m) junction devices by scanning tunneling microscopy (STM) and mechanically controllable break junctions (MCBJ). The conductance of thiol-terminated BDF can be tuned by changing the electrode potentials showing clearly an off/on/off single molecule redox switching effect. To optimize the response, a BDF molecule tailored with carbodithioate (−CS₂^–) anchoring groups was synthesized. Our studies show that replacement of thiol by carbodithioate not only enhances the junction conductance but also substantially improves the switching effect by enhancing the on/off ratio from 2.5 to 8

Influence of the Donor Size in D−π–A Organic Dyes for Dye-Sensitized Solar Cells

We report two new molecularly engineered push–pull dyes, i.e., <b>YA421</b> and <b>YA422</b>, based on substituted quinoxaline as a π-conjugating linker and bulky-indoline moiety as donor and compared with reported <b>IQ4</b> dye. Benefitting from increased steric hindrance with the introduction of bis­(2,4-dihexyloxy)­benzene substitution on the quinoxaline, the electron recombination between redox electrolyte and the TiO₂ surface is reduced, especially in redox electrolyte employing Co­(II/III) complexes as redox shuttles. It was found that the open circuit photovoltages of <b>IQ4</b>, <b>YA421</b>, and <b>YA422</b> devices with cobalt-based electrolyte are higher than those with iodide/triiodide electrolyte by 34, 62, and 135 mV, respectively. Moreover, the cells employing graphene nanoplatelets on top of gold spattered film as a counter electrode (CE) show lower charge-transfer resistance compared to platinum as a CE. Consequently, <b>YA422</b> devices deliver the best power conversion efficiency due to higher fill factor, reaching 10.65% at AM 1.5 simulated sunlight. Electrochemical impedance spectroscopy and transient absorption spectroscopy analysis were performed to understand the electrolyte influence on the device performances with different counter electrode materials and donor structures of donor−π–acceptor dyes. Laser flash photolysis experiments indicate that even though the dye regeneration of <b>YA422</b> is slower than that of the other two dyes, the slower back electron transfer of <b>YA422</b> contributes to the higher device performance