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

Effect of Posttreatment of Titania Mesoscopic Films by TiCl₄ in Solid-State Dye-Sensitized Solar Cells: A Time-Resolved Spectroscopy Study

Posttreatment of mesoporous titanium dioxide films by TiCl₄ solutions is commonly applied during the fabrication of solid-state dye-sensitized solar cells (ssDSCs), as this operation markedly improves the performance of the photovoltaic device. The effect of the posttreatment upon the charge carrier dynamics was scrutinized in an ssDSC aiming at unraveling its mechanism. Kinetic studies carried out using femtosecond and nanosecond transient absorption spectroscopies showed that a biphasic electron injection from the dye excited state is observed, for both treated and nontreated films, whose kinetics is not significantly affected by the surface modification step. However, hole injection in the hole transport material (HTM) spiro-OMeTAD and charge recombination were found to be markedly slower in TiCl₄-treated films. These findings are rationalized by a model describing the interaction at the interface between TiO₂, the dye-sensitizer, and spiro-OMeTAD. Rather than resulting from a modification of the energetics of the conduction band of the oxide, the effect of the TiCl₄ posttreatment appears to be associated with a subtle change of the film morphology. Results emphasize the importance of controlling the contact at the heterojunction between the HTM and the sensitized semiconductor oxide network

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

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

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