Jingjing Chang

Transparent Conductive Oxide-Free Perovskite Solar Cells with PEDOT:PSS as Transparent Electrode

UNLABELLED Perovskite solar cells (PSCs) have been attracting considerable attention because of their low fabrication cost and impressive energy conversion efficiency. Most PSCs are built on transparent conductive oxides (TCOs) such as fluorine-doped tin oxide (FTO) or indium tin oxide (ITO), which are costly and rigid. Therefore, it is significant to explore alternative materials as the transparent electrode of PSCs. In this study, highly conductive and highly transparent poly(3,4-ethylenedioxythiophene):polystyrenesulfonate ( PEDOT PSS) films were investigated as the transparent electrode of both rigid and flexible PSCs. The conductivity of PEDOT PSS films on rigid glass or flexible poly(ethylene terephthalate) (PET) substrate is significantly enhanced through a treatment with methanesulfonic acid (MSA). The optimal power conversion efficiency (PCE) is close to 11% for the rigid PSCs with an MSA-treated PEDOT PSS film as the transparent electrode on glass, and it is more than 8% for the flexible PSCs with a MSA-treated PEDOT PSS film as the transparent electrode on PET. The flexible PSCs exhibit excellent mechanical flexibility in the bending test.

Controlled Growth of Large-Area High-Performance Small-Molecule Organic Single-Crystalline Transistors by Slot-Die Coating Using A Mixed Solvent System

A slot-die coating technique is used for the crystal alignment of triisopropylsilylethynyl (TIPS)-pentacene in solution-processed field-effect transistors (FETs). The film thickness, uniformity, and crystal growth behavior are well controlled by tuning the coating parameters and by using a mixed solvent system (toluene/anisole). An average saturation regime FET mobility of 1.8 cm(2) V(-1) s(-1) is achieved under ambient conditions.

Thiophene-fused tetracene diimide with low band gap and ambipolar behavior.

The first tetracene diimide derivative fused with four thiophene rings, TT-TDI, was synthesized by an FeCl(3) mediated oxidative cyclodehydrogenation reaction. TT-TDI exhibited a low band gap of 1.52 eV and amphoteric redox behavior. TT-TDI also showed a liquid crystalline property and ambipolar charge transport in thin film field-effect transistors.

Stepwise cyanation of naphthalene diimide for n-channel field-effect transistors.

Stepwise cyanation of tetrabromonaphthalenediimide (NDI) 1 gave a series of cyanated NDIs 2-5 with the monocyanated NDI 2 and dicyanated NDI 3 isolated. The tri- and tetracyano- NDIs 4 and 5 show intrinsic instability toward moisture because of their extremely low-lying LUMO energy levels. The partially cyanated intermediates can be utilized as air-stable n-type semiconductors with OFET electron mobility up to 0.05 cm(2) V(-1) s(-1).

Cyanated diazatetracene diimides with ultrahigh electron affinity for n-channel field effect transistors.

Several diazatetracene diimides with high electron affinity (up to 4.66 eV!) were prepared and well characterized. The LUMO energy level of these electron-deficient molecules was found to be closely related to their material stability. Compound 7 with ultrahigh electron affinity suffered from reduction and hydrolysis in the presence of silica gel or water. The stable compounds 3 and 6 showed n-channel FET behavior with an average electron mobility of 0.002 and 0.005 cm(2) V(-1) s(-1), respectively, using a solution processing method.

Disc-like 7, 14-dicyano-ovalene-3,4:10,11-bis(dicarboximide) as a solution-processible n-type semiconductor for air stable field-effect transistors

Large disc-like ovalene diimides (ODI and ODI-CN) were prepared for the first time mainly via Diels–Alder cycloaddition reactions at the bay regions of bisanthene. The CN-substituted ovalene diimide (ODI-CN) exhibited typical n-type semiconducting behaviour in solution processing OFET devices, showing high electron mobility up to 1.0 cm2 V−1 s−1 in nitrogen atmosphere and 0.51 cm2 V−1 s−1 in air together with good device stability.

Solution-processed LiF-doped ZnO films for high performance low temperature field effect transistors and inverted solar cells.

This paper reports that high performance metal oxide thin film transistors (TFTs) can be achieved by using LiF-doped ZnO thin films processed from aqueous solution. It was found that LiF doping at an appropriate amount enhanced the oxide film carrier concentration. The TFTs based on the 10 mol % LiF-doped ZnO thin films annealed at 300 °C revealed a good device performance with an average electron mobility of 8.9 cm(2) V(-1) s(-1) and a high on/off current ratio of 4 × 10(7), superior to the devices based on the nondoped ZnO TFTs (1.6 cm(2) V(-1) s(-1)). Even when annealed at 150 °C, the device still showed good transistor operation with an electron mobility of 0.54 cm(2) V(-1) s(-1). The inverted bulk heterojunction solar cells based on P3HT:PCBM blend system fabricated using 10 mol % LiF doped ZnO as electron selective layer showed higher power conversion efficiency (η = 3.3%) than that from undoped ZnO thin films (η = 2.94%) due to enhanced short circuit current (Jsc = 10.55 mA/cm(2)). Our results suggest that LiF incorporation can be a useful technique to produce high performance and low temperature solution-processed oxide TFTs and interface layer for solar cells.

Boosting the performance of planar heterojunction perovskite solar cell by controlling the precursor purity of perovskite materials

Perovskite solar cells (PSCs) have received great attention due to their high power conversion efficiency and low fabrication cost. The perovskite layer is usually prepared from a solution of precursors. We found that the PbI2 purity has a significant effect on the crystallinity, charge carrier dynamics, and photovoltaic properties of the perovskite films. Planar heterojunction PSCs using highly pure PbI2 showed a high power conversion efficiency (PCE) of 16.4%, which was higher than that of control PSCs with low purity PbI2 by 30–40%. Steady-state photoluminescence (PL), time-resolved PL (TR-PL) and femtosecond transient absorption measurements (FS-TA) revealed that impurities can lower the electron lifetime and increase the non-radiative recombination. This study implies that the PCEs of the perovskite solar cell devices could be further boosted by controlling the precursor purity.

Antiaromatic bisindeno-[n]thienoacenes with small singlet biradical characters: syntheses, structures and chain length dependent physical properties

Recent studies demonstrated that aromaticity and biradical character play important roles in determining the ground-state structures and physical properties of quinoidal polycyclic hydrocarbons and oligothiophenes, a kind of molecular materials showing promising applications for organic electronics, photonics and spintronics. In this work, we designed and synthesized a new type of hybrid system, the so-called bisindeno-[n]thienoacenes (n = 1–4), by annulation of quinoidal fused α-oligothiophenes with two indene units. The obtained molecules can be regarded as antiaromatic systems containing 4n π electrons with small singlet biradical character (y0). Their ground-state geometry and electronic structures were studied by X-ray crystallographic analysis, NMR, ESR and Raman spectroscopy, assisted by density functional theory calculations. With extension of the chain length, the molecules showed a gradual increase of the singlet biradical character accompanied by decreased antiaromaticity, finally leading to a highly reactive bisindeno[4]thienoacene (S4-TIPS) which has a singlet biradical ground state (y0 = 0.202). Their optical and electronic properties in the neutral and charged states were systematically investigated by one-photon absorption, two-photon absorption, transient absorption spectroscopy, cyclic voltammetry and spectroelectrochemistry, which could be correlated to the chain length dependent antiaromaticity and biradical character. Our detailed studies revealed a clear structure–aromaticity–biradical character–physical properties–reactivity relationship, which is of importance for tailored material design in the future.

Enhancing the planar heterojunction perovskite solar cell performance through tuning the precursor ratio

Perovskite solar cells (PSCs) have attracted great attention due to their high power conversion efficiencies (PCEs) and low fabrication cost. The composition of the precursor solution determines the compositions of perovskite films. Excess precursor(s) may be used in the solution for the fabrication of perovskite films. However, it is still unclear how an excess precursor like PbI2 affects the structure and properties of the perovskite layer and the photovoltaic performance of PSCs. In this work, we investigated the effect of excess PbI2 that has a large bandgap on the electronic structure and properties of perovskite films and the photophysics and photovoltaic performance of PSCs. The presence of slightly excess PbI2 can affect the crystal structure and thus shift the Fermi level of perovskites. It can increase the open-circuit voltage (Voc) and thus the PCE of PSCs. However, the presence of a large amount of excess PbI2 is detrimental to the photovoltaic performance of PSCs. It can shorten the carrier lifetime, increase the resistance of the perovskite films, and decrease the fill factor (FF) and PCE of PSCs.