Yin Xiao

Novel hole transporting materials with a linear π-conjugated structure for highly efficient perovskite solar cells

Novel small-molecule hole transporting materials (HTMs) with a linear π-conjugated structure have been synthesized. The perovskite solar cell based on as the HTM exhibits an encouraging power conversion efficiency of 9.1% under AM 1.5 G (100 mW cm(-2)) illumination, which is the first demonstration of an effective perovskite solar cell using a linear structured HTM.

Isomer-Pure Bis-PCBM-Assisted Crystal Engineering of Perovskite Solar Cells Showing Excellent Efficiency and Stability

A fullerene derivative (α-bis-PCBM) is purified from an as-produced bis-phenyl-C61 -butyric acid methyl ester (bis-[60]PCBM) isomer mixture by preparative peak-recycling, high-performance liquid chromatography, and is employed as a templating agent for solution processing of metal halide perovskite films via an antisolvent method. The resulting α-bis-PCBM-containing perovskite solar cells achieve better stability, efficiency, and reproducibility when compared with analogous cells containing PCBM. α-bis-PCBM fills the vacancies and grain boundaries of the perovskite film, enhancing the crystallization of perovskites and addressing the issue of slow electron extraction. In addition, α-bis-PCBM resists the ingression of moisture and passivates voids or pinholes generated in the hole-transporting layer. As a result, a power conversion efficiency (PCE) of 20.8% is obtained, compared with 19.9% by PCBM, and is accompanied by excellent stability under heat and simulated sunlight. The PCE of unsealed devices dropped by less than 10% in ambient air (40% RH) after 44 d at 65 °C, and by 4% after 600 h under continuous full-sun illumination and maximum power point tracking, respectively.

Efficient CH3NH3PbI3 perovskite solar cells with 2TPA-n-DP hole-transporting layers

CH3NH3PbI3 perovskite solar cells with 2TPA-n-DP (TPA = 4,4′-((1E, 1′E,3E,3′E)-[1,1′-biphenyl]-4,4′-diylbis(buta-1,3-diene-4,1-diyl)); DP = bis(N,N-di-p-tolylaniline); n = 1, 2, 3, 4) as hole-transporting materials (HTMs) have been fabricated. After optimization of the mesoporous TiO2 film thickness, devices based on 2TPA-2-DP with power conversion efficiencies (PCEs) of up to 12.96% have been achieved, comparable to those of devices with (2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene) (spiro-OMeTAD) as HTM under similar conditions. Further time-resolved photoluminescence (PL) measurements showed a fast charge transfer process at the perovskite/2TPA-2-DP interface. With the aid of electrochemical impedance spectra, a study of the electron blocking ability of 2TPA-2-DP in the device reveals that the presence of 2TPA-2-DP can greatly increase charge transfer resistance at the HTM/Au interface in the device, thus reducing the recombination. Furthermore, the perovskite solar cells based on these four HTMs exhibit good stability after testing for one month.

A thin pristine non-triarylamine hole-transporting material layer for efficient CH3NH3PbI3 perovskite solar cells

A new non-traditional organic hole-transporting material (HTM), 4-(4-phenyl-4-α-naphthylbutadienyl)-N,N-bis(4-benzyl)-aniline (PNBA), has been employed in CH3NH3PbI3 perovskite solar cells for the first time. The pore filling of PNBA into mesoporous TiO2/CH3NH3PbI3 scaffold is investigated in detail. As high as 11.4% of light-to-electricity conversion efficiency has been achieved, comparable to corresponding spiro-OMeTAD-based devices under the same conditions. It is revealed that the uniform and thin PNBA film is sufficient as a HTM for perovskite solar cells, and can facilitate hole transport to the metal cathode and also block electron transfer from the perovskite to the metal cathode.

Tuning the crystal growth of perovskite thin-films by adding the 2-pyridylthiourea additive for highly efficient and stable solar cells prepared in ambient air

Here, we report a rapid and simple process to prepare perovskite solar cells in ambient air by adding 2-pyridylthiourea in the precursor solution. 2-Pyridylthiourea can not only promote the conversion of 2-D PbI2 to tetragonal CH3NH3PbI3 crystals, but also improve the quality of the perovskite absorber layer via enhancing the uniformity and the size of the crystal grains. The perovskite solar cells with the addition of 2-pyridylthiourea at a concentration of 0.5 mg mL−1 exhibited a remarkable overall power conversion efficiency (PCE) of 18.2%, which is among the highest PCE of CH3NH3PbI3-based devices fabricated in ambient air. It also showed an 18% increase and less hysteresis compared with the cells without additives. Importantly, the devices with 2-pyridylthiourea show relatively better stability compared to reference devices when aged under ambient air of 55 ± 5% relative humidity in the dark and under 65 °C after 30 days. The presented results clearly show that 2-pyridylthiourea can be an additive candidate for tuning the morphology of perovskite thin films, which may be a new direction for large-scale production of PSCs.

The Synthesis, Characterisation, Photophysical and Thermal Properties, and Photovoltaic Performance of 7-Coumarinoxy-4-Methyltetrasubstituted Metallophthalocyanines

The synthesis, characterisation, photophysical and thermal properties of 2(3),9(10),16(17),23(24)-tetrakis(7-coumarinoxy-4-methyl)-phthalocyaninatozinc(ii) (ZnPc-Coumarin) and 2(3),9(10),16(17),23(24)-tetrakis(7-coumarinoxy-4-methyl)-phthalocyaninatocobalt(ii) (CoPc-Coumarin) are reported. The ground state absorbance of ZnPc-Coumarin shows molar extinction coefficients as high as 1.80 × 105 dm3 mol–1 cm–1. The fluorescence spectrum and fluorescence quantum yields of compounds ZnPc-Coumarin and CoPc-Coumarin are also investigated. The photoluminescence decay of the two transition-metal complexes in DMF solution, in poly(methyl methacrylate) (PMMA), and on TiO2 films has been studied with time-resolved emission. This study shows that the electron transfer from the dye to TiO2 is through space. The thermal stability studies indicate that both of the two complexes are stable up to 390°C. The ZnPc-Coumarin achieved a higher overall conversion efficiency than the reported SnPcCl2-Coumarin, InPcCl-Coumarin, and RuPcCl-Coumarin because of its slower charge recombination rate and faster electron injection from the dye to the conduction band of the conducting glass.

Synthesis and properties of new luminescent hole transporting materials containing triphenylamine and carbazole units

Two new blue luminescent hole transporting materials (HTM) containing triphenylamine, carbazole units and olefinic linkers (TM1 and TM2) were synthesized via Wittig reaction and characterized by (1)H NMR, FT-IR, and HRMS. The compounds show good solubility in common organic solvents such as dichloromethane, chloroform, tetrahydrofuran and dimethyl formamide. Their optical, electrochemical and crystalline properties were investigated by using UV-Vis, photoluminescence (PL) spectra, cyclic voltammetry (CV) and differential scanning calorimetry (DSC), respectively. Quantum-chemical calculation was performed to obtain their optimized structures and the electron distribution of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels. The UV-Vis absorption and PL spectra of the two compounds in solid state were found to be similar to that when they were in dilute THF, which suggests that these compounds remain as an amorphous state in solid films. CV measurements show that the two compounds embody suitable HOMO levels (in a range of -5.28 to -5.23 eV) for hole injection, which is consistent with the calculation consequence. Two compounds possess high glass-transition temperature (T(g)) at 96.61 and 90.74 °C for TM1 and TM2, respectively, suggesting the two compounds could form stable amorphous glassy states. The experimental results show that the synthesized compounds have great potential for application in organic light-emitting devices (OLEDs).

Novel photochromic and electrochromic diarylethenes bearing triphenylamine units

Six novel photochromic and electrochromic diarylethenes containing triphenylamine units were synthesized by the McMurry reaction. The properties of photochromism, electrochromism, fluorescence and electrochemistry were investigated in detail. The results showed that these compounds exhibited reversible photochromism, changing from colorless to yellow after irradiation with 302 nm UV light both in solution and in PMMA amorphous film. When arriving at the photostationary state, the fluorescent intensity was quenched to about 40%. In the cyclic voltammetry curves, there were a reversible redox couple and one or two irreversible oxidation peaks. The potential of the reversible oxidation peak is adopted as the excitation voltage of electrochromism. The electrochromic devices could be simulated with an equivalent circuit, R(CR)(CR), by electrochemical impedance spectroscopy, and the transformation between colorless and blue was realized, which required 8.6 to 12.1 s for color switching and 8.9 to 12.8 s for bleaching. The reflectance minima of these compounds are in the range from 709.7 to 781.7 nm after coloring. The results showed that diarylethenes containing triphenylamine units possessed both photochromic and electrochromic properties.

Dopant-free and low-cost molecular “bee” hole-transporting materials for efficient and stable perovskite solar cells

With the dramatic development of the power conversion efficiency (PCE) of perovskite solar cells (PSCs), the device lifetime has attracted extensive research interest and concern. To enhance device durability, developing dopant-free hole-transporting materials (HTMs) with a high performance is a promising strategy. Herein, three new HTMs with a N,N′-diphenyl-N,N′-di(m-tolyl)benzidine (TPD) core: TPD-4MeTPA, TPD-4MeOTPA and TPD-4EtCz are designed and synthesized, showing suitable energy levels and excellent film-formation properties. PCEs of 15.28% were achieved based on pristine TPD-4MeOTPA as the HTM, which is a little lower than that of the p-doped 2,2′,7,7′-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene (spiro-OMeTAD)-based device (17.26%). Importantly, the devices based on the new HTMs show relatively improved stability compared to devices based on spiro-OMeTAD when aged under ambient air with 30% relative humidity in the dark.

Charging behavior of carbon black in a low-permittivity medium based on acid–base charging theory

This work presents an investigation of the charging behavior of carbon black (CB) particles dispersed in a low-permittivity medium Isopar L. Highly charged and stably dispersed CB particles in Isopar L were prepared by employing a nonionic surfactant polyisobutylene–mono-succinimide (T151) with basic functionality to treat CB with acidic functionality (oxidized with concentrated nitric acid). As expected, CB particles were successfully negatively charged and presented a remarkably high electrophoretic mobility of −11.56 × 10−10 m2 V−1 s−1 and a zeta potential of −180.3 mV at the T151 concentration of 2.0 wt%, which are the highest values for CB in nonpolar media reported by far. The dispersion stability of CB particles, dominated by the particle charge, was dramatically improved with T151 concentrations increased in the range from 1.0 to 5.0 wt%. Prototype electrophoretic display device based on 2.0 wt% T151 treated CB particles exhibited highly improved performance compared to that of device using pristine CB particles. The high charge density and excellent dispersibility in apolar solvents of CB nanoparticles imply their potential applications in other electronic fields, such as electrically conductive inks for various printed electronics and conductive materials for various sensors.