Crown Ether-Substituted Carbazole Dye for Dye-Sensitized Solar Cells: Controlling the Local Ion Concentration at the TiO₂/Dye/Electrolyte Interface

The conduction band edge potentials (<i>E</i>_CB) and electron lifetimes (τ) of the TiO₂ electrodes in dye-sensitized solar cells (DSSCs) are affected by ion concentrations (e.g., Li⁺ and I^–/I₃^–) at the TiO₂/dye/electrolyte interface. To control the local concentrations of these ions in the vicinity of the TiO₂ surface, a novel carbazole-based dye incorporating a 12-crown-4 ether on the carbazole donor (MK-70) was synthesized as a DSSC sensitizer. The interactions between Li⁺/I^–/I₃^– and MK-70 were compared with those between Li⁺/I^–/I₃^– and MK-1, an analogue lacking the crown ether. The crown ether did not affect the <i>E</i>_CB level of TiO₂, but it did decrease τ at a high electrolytic Li⁺ concentration. Results suggest that localized Li⁺ ions associated with the crown ethers electrostatically attract surplus I₃^– from the bulk electrolyte even though the crown ethers are located far from the TiO₂ surface. After the cells were aged, negative shifts in the <i>E</i>_CB levels of the TiO₂ electrode and blueshifts of the MK-70 absorption spectra were observed with electrolytes that included I^–/I₃^– and Li⁺. The aging behavior may be determined by the balance between two attractive forces, <i>K</i>₁ (I^–/I₃^– and Li⁺ at the TiO₂ surface) and <i>K</i>₂ (dye interactions with I^–/I₃^– and Li⁺)

Efficiency Enhancement of ZnO-Based Dye-Sensitized Solar Cells by Low-Temperature TiCl₄ Treatment and Dye Optimization

ZnO is a promising candidate as a low-cost porous semiconductor material for photoelectrodes in dye-sensitized solar cells (DSSCs). However, ZnO-based DSSCs tend to exhibit lower energy conversion efficiencies than do those based on TiO₂. In this study, the performance of ZnO porous electrodes was enhanced using a surface treatment carried out by immersion in cold aqueous TiCl₄ solution that resulted in TiO₂-coated ZnO (<i>Z</i>/<i>T</i>) electrodes. The <i>Z</i>/<i>T</i> electrodes were sensitized with either the Ru complex dye N719 or the organic indoline dye D149. For each dye, the DSSCs with the <i>Z</i>/<i>T</i> photoelectrodes showed the highest open-circuit voltage (<i>V</i>_oc), short circuit current (<i>J</i>_sc), and power conversion efficiency compared to those with ZnO, TiO₂, or TiO₂-coated TiO₂ (<i>T</i>/<i>T</i>) electrodes. To study the effects of the TiCl₄ treatment, the relationships between the electron lifetime (τ), cell voltage, and electron density (<i>n</i>) of the cells prepared with each electrode, with each of the two dyes, or without either dye were assessed. It was found that the TiCl₄ treatment negatively shifted the conduction band edge (CBE) potential of the ZnO electrodes by more than 100 mV for both dyes and also in the absence of a dye. In addition, τ increased with the use of the organic D149 and in the absence of a dye. The DSSC with a D149-sensitized <i>Z</i>/<i>T</i> layer showed the highest efficiency of 4.89% under 100 mW cm^–2 irradiation

Structural Effect of Donor in Organic Dye on Recombination in Dye-Sensitized Solar Cells with Cobalt Complex Electrolyte

The effect of the donor in an organic dye on the electron lifetime of dye-sensitized solar cells (DSSCs) employing a cobalt redox electrolyte was investigated. We synthesized organic dyes with donor moieties of carbazole, coumarin, triphenylamine, and <i>N</i>-phenyl-carbazole and measured the current–voltage characteristics and electron lifetimes of the DSSCs with these dyes. The cell with the triphenylamine donor dye produced the highest open circuit voltage and longest electron lifetime. On the other hand, the lowest open circuit voltage and shortest electron lifetime was obtained with coumarin donor dye, suggesting that the coumarin attracted the cobalt redox couples to the surface of the TiO₂ layer, thus increasing the concentration of cobalt complex. On the other hand, the longest electron lifetime with triphenylamine was attributed to the blocking effect by steric hindrance of the nonplanar structure of the donor

Synthesis of Oligo(thienylene-vinylene) by Regiocontrolled Deprotonative Cross-Coupling

Concise synthesis of oligo­(thienylene-vinylene) with a head-to-tail type structure is achieved by regioselective deprotonative coupling of 3-hexylthiophene. The palladium catalyzed reaction of 3-hexylthiophene with (<i>E</i>)-2-(2-bromoethenyl)-3-hexyl­thiophene takes place to afford head-to-tail type <i>trans</i>-1,2-dithienyl­ethene. Further extension of a vinylthiophene unit is similarly performed in an iterative manner

Crystallization Dynamics of Organolead Halide Perovskite by Real-Time X‑ray Diffraction

We analyzed the crystallization process of the CH₃NH₃PbI₃ perovskite by observing real-time X-ray diffraction immediately after combining a PbI₂ thin film with a CH₃NH₃I solution. A detailed analysis of the transformation kinetics demonstrated the fractal diffusion of the CH₃NH₃I solution into the PbI₂ film. Moreover, the perovskite crystal was found to be initially oriented based on the PbI₂ crystal orientation but to gradually transition to a random orientation. The fluctuating characteristics of the crystallization process of perovskites, such as fractal penetration and orientational transformation, should be controlled to allow the fabrication of high-quality perovskite crystals. The characteristic reaction dynamics observed in this study should assist in establishing reproducible fabrication processes for perovskite solar cells

Adjustment of Conduction Band Edge of Compact TiO₂ Layer in Perovskite Solar Cells Through TiCl₄ Treatment

Perovskite solar cells (PSCs) without a mesoporous TiO₂ layer, that is, planar-type PSCs exhibit poorer cell performance as compared to PSCs with a porous TiO₂ layer, owing to inefficient electron transfer from the perovskite layer to the compact TiO₂ layer in the former case. The matching of the conduction band levels of perovskite and the compact TiO₂ layer is thus essential for enhancing PSC performance. In this study, we demonstrate the shifting of the conduction band edge (CBE) of the compact TiO₂ layer through a TiCl₄ treatment, with the aim of improving PSC performance. The CBE of the compact TiO₂ layer was shifted to a higher level through the TiCl₄ treatment and then shifted in the opposite direction, that is, to a lower level, through a subsequent heat treatment. These shifts in the CBE were reflected in the PSC performance. The TiCl₄-treated PSC showed an increase in the open-circuit voltage of more than 150 mV, as well as a decrease of 100 mV after being heated at 450 °C. On the other hand, the short-circuit current decreased after the treatment but increased after heating at temperatures higher than 300 °C. The treated PSC subjected to subsequent heating at 300 °C exhibited the best performance, with the power conversion efficiency of the PSC being 17% under optimized conditions

Highly Efficient 17.6% Tin–Lead Mixed Perovskite Solar Cells Realized through Spike Structure

Frequently observed high <i>V</i>_oc loss in tin–lead mixed perovskite solar cells is considered to be one of the serious bottle-necks in spite of the high attainable Jsc due to wide wavelength photon harvesting. An amicable solution to minimize the <i>V</i>_oc loss up to 0.50 V has been demonstrated by introducing an n-type interface with spike structure between the absorber and electron transport layer inspired by highly efficient Cu­(In,Ga)­Se₂ solar cells. Introduction of a conduction band offset of ∼0.15 eV with a thin phenyl-C61-butyric acid methyl ester layer (∼25 nm) on the top of perovskite absorber resulted into improved <i>V</i>_oc of 0.75 V leading to best power conversion efficiency of 17.6%. This enhancement is attributed to the facile charge flow at the interface owing to the reduction of interfacial traps and carrier recombination with spike structure as evidenced by time-resolved photoluminescence, nanosecond transient absorption, and electrochemical impedance spectroscopy measurements

Investigation of Interfacial Charge Transfer in Solution Processed Cs₂SnI₆ Thin Films

Cesium tin halide based perovskite Cs₂SnI₆ has been subjected to in-depth investigations owing to its potentiality toward the realization of environment benign Pb free and stable solar cells. In spite of the fact that Cs₂SnI₆ has been successfully utilized as an efficient hole transport material owing to its p-type semiconducting nature, however, the nature of the majority carrier is still under debate. Therefore, intrinsic properties of Cs₂SnI₆ have been investigated in detail to explore its potentiality as light absorber along with facile electron and hole transport. A high absorption coefficient (5 × 10⁴ cm^–1) at 700 nm indicates the penetration depth of 700 nm light to be 0.2 μm, which is comparable to conventional Pb based solar cells. Preparation of pure and CsI impurity free dense thin films with controllable thicknesses of Cs₂SnI₆ by the solution processable method has been reported to be difficult owing to its poor solubility. An amicable solution to circumvent such problems of Cs₂SnI₆ has been provided utilizing spray-coating in combination with spin-coating. The presence of two emission peaks at 710 and 885 nm in the prepared Cs₂SnI₆ thin films indicated coexistence of quantum dot and bulk parts which were further supported by transmission electron microscopy (TEM) investigations. Time-resolved photoluminescence (PL) and transient absorption spectroscopy (TAS) were employed to investigate the excitation carrier lifetime, which revealed fast decay kinetics in the picoseconds (ps) to nanoseconds (ns) time domains. Time-resolved microwave photoconductivity decay (MPCD) measurement provided the mobile charge carrier lifetime exceeding 300 ns, which was also in agreement with the nanosecond transient absorption spectroscopy (ns-TAS) indicating slow charge decay lasting up to 20 μs. TA assisted interfacial charge transfer investigations utilizing Cs₂SnI₆ in combination with n-type PCBM and p-type P3HT exhibited both intrinsic electron and hole transport