Organic-based solar cells potentially offer a photovoltaic module with low production costs and low hazard risk of the components. We report organic dye-sensitized solar cells, fabricated with molecular designed indoline dyes in conjunction with highly reactive but robust nitroxide radical molecules as redox mediator in a quasi-solid gel form of the electrolyte. The cells achieve conversion efficiencies of 10.1% at 1 sun, and maintain the output performance even under interior lighting. The indoline dyes, customized by introducing long alkyl chains, specifically interact with the radical mediator to suppress a charge-recombination process at the dye interface. The radical mediator also facilitates the charge-transport with remarkably high electron self-exchange rate even in the quasi-solid state electrolyte to lead to a high fill factor.

Recently, organic-inorganic halide perovskites have received attention for applications in solar cells. Measurements of high-quality single crystals reveal lower defect densities and longer carrier lifetimes than those of conventional thin films, which result in improved electrical and optical properties. However, single crystal surfaces are sensitive to exposure to ambient conditions, and degrade under long-term storage in air. The surface also shows differences from the bulk in terms of its optical and electronic characteristics. For a heterojunction device, the interface at the single crystal is important. Understanding the difference between the surface and bulk properties offers insights into device design. Here, we prepared non-sliced and sliced formamidinium lead iodide (FAPbI; FA = HC(NH)) single crystals with a bandgap of 1.4 eV, which matches well with the requirements for solar cell photoabsorption layers. We evaluate the energy level diagrams of the surface and bulk regions, respectively. Our data indicate that the valence band maximum of the surface region is at a higher energy level than that of the bulk region. We also discuss hypotheses for the well-known and unexplained phenomena (multiple bandgaps and bandgap narrowing) seen in the absorption and photoluminescence spectra of single crystals. We conclude that these effects are likely caused by a combination of the degraded surface, Rashba-splitting in bulk, and self-absorption by the single crystal itself.

Inverse temperature crystallization (ITC) growth of formamidinium tin iodide (FASnI3) is realized using γ-valerolactone (GVL) solvent, which possesses a low Gutmann acceptor number. The high-crystalline ITC-grown FASnI3 film was fabricated via a dip-coating method featuring a high-temperature substrate and a precursor solution using the GVL solvent. Transmittance measurements and photoluminescence spectroscopy revealed the presence of a narrow band gap in the FASnI3 ITC-grown film, contrasting with the band gap of a conventional single crystal and spin-coated film. Solid-state proton nuclear magnetic resonance spectroscopy demonstrated that formamidinium cations in the FASnI3 ITC-grown film were strongly bound with the SnI6 octahedron and that the motion of the formamidinium molecules was restricted, resulting in the stabilization of a metastable bonding state in the FASnI3 ITC-grown film. On the basis of these results and previous reports, the narrow band gap observed in the FASnI3 ITC-grown film was attributed to an indirect transition state induced by a Rashba splitting effect.

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