Temperature Dependence of Transport Properties of Spiro-MeOTAD as a Hole Transport Material in Solid-State Dye-Sensitized Solar Cells

The internal transport and recombination parameters of solid-state dye-sensitized solar cells (ssDSCs) using the amorphous organic semiconductor 2,2′,7,7′-tetrakis(<i>N,N</i>-di-<i>p</i>-methoxyphenylamine)-9,9′-spirobifluorene (spiro-MeOTAD) as a hole transport material (HTM) are investigated using electrical impedance spectroscopy. Devices were fabricated using flat and nanostructured TiO₂ and compared to systems using nanostructured ZrO₂ to differentiate between the transport processes within the different components of the ssDSC. The effect of chemically p-doping the HTM on its transport was investigated, and its temperature dependence was examined and analyzed using the Arrhenius equation. Using this approach the activation energy of the hole hopping transport within the undoped spiro-MeOTAD film was determined to be 0.34 ± 0.02 and 0.40 ± 0.02 eV for the mesoporous TiO₂ and ZrO₂ systems, respectively

Nanocrystalline Rutile Electron Extraction Layer Enables Low-Temperature Solution Processed Perovskite Photovoltaics with 13.7% Efficiency

We demonstrate low-temperature (70 °C) solution processing of TiO₂/CH₃NH₃PbI₃ based solar cells, resulting in impressive power conversion efficiency (PCE) of 13.7%. Along with the high efficiency, a strikingly high open circuit potential (<i>V</i>_OC) of 1110 mV was realized using this low-temperature chemical bath deposition approach. To the best of our knowledge, this is so far the highest <i>V</i>_OC value for solution-processed TiO₂/CH₃NH₃PbI₃ solar cells. We deposited a nanocrystalline TiO₂ (rutile) hole-blocking layer on a fluorine-doped tin oxide (FTO) conducting glass substrate via hydrolysis of TiCl₄ at 70 °C, forming the electron selective contact with the photoactive CH₃NH₃PbI₃ film. We find that the nanocrystalline rutile TiO₂ achieves a much better performance than a planar TiO₂ (anatase) film prepared by high-temperature spin coating of TiCl₄, which produces a much lower PCE of 3.7%. We attribute this to the formation of an intimate junction of large interfacial area between the nanocrystalline rutile TiO₂ and the CH₃NH₃PbI₃ layer, which is much more effective in extracting photogenerated electrons than the planar anatase film. Since the complete fabrication of the solar cell is carried out below 100 °C, this method can be easily extended to plastic substrates

Optimization of Stable Quasi-Cubic FA_<i>x</i>MA_1–<i>x</i>PbI₃ Perovskite Structure for Solar Cells with Efficiency beyond 20%

Complex compositional engineering of mixed halides/mixed cations perovskites has recently fostered a rapid progress in perovskite solar cell technology. Here we demonstrate that when 10% of formamidinium (FA⁺) is simply added into methylammonium lead iodide (MAPbI₃) a highly crystalline and compositionally uniform perovskite is formed, self-organizing into a stable “quasi-cubic” phase at room temperature. We reached power conversion efficiency of over 20.2%, the highest value reported to date for FA_<i>x</i>MA_1−<i>x</i>PbI₃ perovskite

Perovskite Solar Cells with 12.8% Efficiency by Using Conjugated Quinolizino Acridine Based Hole Transporting Material

A low band gap quinolizino acridine based molecule was designed and synthesized as new hole transporting material for organic–inorganic hybrid lead halide perovskite solar cells. The functionalized quinolizino acridine compound showed an effective hole mobility in the same range of the state-of-the-art spiro-MeOTAD and an appropriate oxidation potential of 5.23 eV vs the vacuum level. The device based on this new hole transporting material achieved high power conversion efficiency of 12.8% under the illumination of 98.8 mW cm^–2, which was better than the well-known spiro-MeOTAD under the same conditions. Moreover, this molecule could work alone without any additives, thus making it to be a promising candidate for solid-state photovoltaic application

Impedance Spectroscopic Analysis of Lead Iodide Perovskite-Sensitized Solid-State Solar Cells

Mesoscopic solid-state solar cells based on the inorganic–organic hybrid perovskite CH₃NH₃PbI₃ in conjunction with the amorphous organic semiconductor spiro-MeOTAD as a hole transport material (HTM) are investigated using impedance spectroscopy (IS). A model to interpret the frequency response of these devices is established by expanding and elaborating on the existing models used for the liquid and solid-state dye-sensitized solar cells. Furthermore, the influence of changing the additive concentrations of <i>tert</i>-butyl­pyridine and LiTFSI in the HTM and varying the HTM overlayer thickness on top of the sub-micrometer thick TiO₂ on the extracted IS parameters is investigated. The internal electrical processes of such devices are studied and correlated with the overall device performance. In particular, the features in the IS responses that are attributed to the ionic and electronic transport properties of the perovskite material and manifest as a slow response at low frequency and an additional RC element at intermediate frequency, respectively, are explored

Copper Thiocyanate Inorganic Hole-Transporting Material for High-Efficiency Perovskite Solar Cells

In this Letter we show that the mixed perovskite in the form of (FAPbI₃)_0.85(MAPbBr₃)_0.15 in combination with CuNCS as p-type hole conductor leads to over 16% power conversion efficiency (PCE) under full sun illumination and yields a remarkable monochromatic incident photon-to-electron conversion efficiency of 85%. The devices displayed a short-circuit current density (<i>J</i>_sc) of 21.8 mA/cm², open-circuit voltage (<i>V</i>_oc) of 1100 mV, fill factor (FF) of 0.69, and a PCE of 16.6%. Under similar conditions, the device without CuSCN shows a PCE of 9.5%, with a significant decrease in the <i>J</i>_sc (from 21.8 mA/cm² to 15.64 mA/cm²) and <i>V</i>_oc (from 1100 mV to 900 mV). The high <i>J</i>_sc with CuSCN is mainly due to the effective charge transfer between perovskite and CuSCN, followed by the fast hole transport through CuSCN to the Au. In comparison, the spiro-OMeTAD reference cells showed efficiencies up to 19.65%. Different from most organic hole-transporting materials is the transparency and high hole mobility of CuSCN, which represent a paradigm shift in perovskite solar cells particularly for tandem solar cells

Enhanced TiO₂/MAPbI₃ Electronic Coupling by Interface Modification with PbI₂

Enhanced TiO₂/MAPbI₃ Electronic Coupling by Interface Modification with PbI<sub>2</sub

Unraveling the Dual Character of Sulfur Atoms on Sensitizers in Dye-Sensitized Solar Cells

Cyclometalated ruthenium sensitizers have been synthesized that differ with number of thiophene units on the auxiliary ligands. Sensitizers possessing four (SA25, SA246, and SA285) or none (SA282) sulfur atoms in their structures, were tested in solar cell devices employing I₃^–/I^– redox mediator, enabling an estimation of the influence of sulfur–iodine/iodide interactions on dye-sensitized solar cell (DSC) performance. Power conversion efficiencies over 6% under simulated AM 1.5 illumination (1 Sun) were achieved with all the sensitizers. Consistently higher open-circuit voltage (<i>V</i>_OC) and fill factor (FF) values were measured using SA282. Scrutinizing the DSCs with these dyes by transient absorption spectroscopy (TAS) and electrochemical impedance spectroscopy (EIS) indicate that sulfur atom induced recombination cancels favorable increased regeneration resulting in decreased power conversion efficiencies (PCEs). The data indicate that, to reduce charge recombination channels, the use of sulfur-containing aromatic rings should be avoided if possible in the dye structure when I₃^–/I^– redox mediator is used

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

Photoanode Based on (001)-Oriented Anatase Nanoplatelets for Organic–Inorganic Lead Iodide Perovskite Solar Cell

Photoanode Based on (001)-Oriented Anatase Nanoplatelets for Organic–Inorganic Lead Iodide Perovskite Solar Cel