Fangfang Niu

Film-through large perovskite grains formation via a combination of sequential thermal and solvent treatment

Organic–inorganic halide perovskites have recently attracted strong research interest for fabrication of high-performance, low-cost photovoltaic devices. Recently, we reported a highly reproducible procedure to fabricate high-performance organic–inorganic halide perovskite solar cells. This procedure, based on a one-step, solvent-induced, fast deposition-crystallization method, involves the use of sec-butyl alcohol as a new solvent to induce the CH3NH3PbI3 fast crystallization deposition. In the present study, we propose a reproducible fabrication method to prepare both flat and large-grain perovskite film by adding a pre-annealing step to strengthen the perovskite nucleation, aiming to facilitate the excess CH3NH3I and solvent removal in the sec-butyl alcohol soaking process, in which all films with thickness between 420 nm and 1 μm performed uniformly. The best performing planar device obtained with this procedure had an efficiency of 17.2% under AM 1.5G illumination and an average power conversion efficiency of 16.2 ± 0.5%. We also analyzed the efficiency of halide perovskite planar solar cells as a function of the perovskite film thickness; the efficiency dropped only slightly to 15.7% when the perovskite film thickness was increased to 1 μm.

Synthesis, characterization and application of starburst 9-alkyl-1,3,6,8-tetraaryl-carbazole derivatives for blue-violet to UV OLEDs

A series of starburst carbazole derivatives, which comprise various aryl groups appended to the 1,3,6,8-positions of a 9-alkyl-carbazole core, are synthesized and characterized. These compounds exhibit good thermal stabilities with glass transition temperatures up to 199 °C and thermal decomposition temperatures ranging from 321 to 485 °C. The starburst configuration of the as-prepared compounds results in fluorescence emission in the UV to blue-violet region (λmax = 389–409 nm) with photoluminescence quantum efficiency (ΦPL) reaching 90% and narrow full width half maximum (FWHM) of 43–54 nm in solution. Light-emitting devices are successfully fabricated using these materials as emitters, and emit UV to blue-violet light. With a device structure of indium tin oxide (ITO)/molybdenum trioxide (MoO3)/1,3,6,8-tetraaryl carbazole derivatives/1,3,5-tris(2-N-phenylbenzimidazolyl)benzene (TPBI)/8-hydroxyquinoline aluminum (Alq3)/LiF/Al, maximum external quantum efficiency (EQEmax) of 2.05% and 3.4% have been obtained at UV wavelength of 394 nm with FWHM of 42 nm and blue-violet wavelength of 415 nm with FWHM of 46 nm, respectively.

Star-configured carbazole as an efficient near-ultraviolet emitter and hole-transporting material for organic light-emitting devices

A novel organic material, 9-methyl-1,3,6,8-tetraphenyl-carbazole (MTPC-Me), for use in organic electroluminescent devices has been developed. This star-configured carbazole gives a strong near-ultraviolet (n-UV) emission (λmax=389nm) with a high emission quantum efficiency of 47% and a narrow full width half maximum of 40nm. Two types of high-performance organic light-emitting devices were obtained using MTPC-Me as a n-UV emitter and hole-transporting material with maximum external quantum efficiency, brightness, and turn-on voltage of 1.2%, 1040cd∕m2, and 3.5V for the former and 1.1%, 18000cd∕m2, and 2.4V for the latter, respectively.

LiF Thickness dependence of Electron Injection Models for Alq3/LiF/Al Cathode Structure

We present the experimental evidences showing that three different electron injection models play roles in Alq3 based organic light-emitting diodes in sequence when the thickness of LiF interlayer is changed. It is found that the device with a 0.2 nm LiF layer displays the largest current with declined luminescence. However, the one with a 0.6 nm LiF layer displays the second largest current and the highest luminescence of all. Combining with the photoluminescent test results, three models, namely chemical reaction at ternary interface, dipole effect at binary interface and tunneling enhancement effect, are expected to play roles in sequence when the LiF thickness is increased from 0 nm to 4 nm.

π-Bridge Effect on Symmetric Carbazole-Based Small Molecules for Realizing Ultraviolet Fluorescent Emission

A series of symmetric carbazole derivatives (CzP-H, CzP-CN, CzP-Me, and CzP-OMe), which comprise electron-donating and electron-drawing groups appending on a phenyl core, was synthesized and characterized in detail. These compounds exhibit excellent thermal stabilities, with thermal decomposition temperatures exceeding 400 °C. From the fluorescent spectra in film, CzP-H, CzP-Me, and CzP-OMe showed UV to blue-violet emission, with peaks at 396 nm, 402 nm, and 392 nm, respectively. The E00 energies of CzP-H, CzP-CN, CzP-Me, and CzP-OMe were 3.39 eV, 2.83 eV, 3.50 eV, and 3.35 eV, respectively. From the electrochemical measurements, the highest occupied molecular orbital (HOMOs) energy levels were −5.30 eV, −5.64 eV, −5.46 eV, and −5.24 eV for CzP-H, CzP-CN, CzP-Me, and CzP-OMe, respectively. Through calculations from HOMO energy levels and E00 energies, the lowest unoccupied molecular orbital (LUMOs) energy levels of CzP-H, CzP-CN, CzP-Me, and CzP-OMe were −1.91 eV, −2.81 eV, −1.96 eV, and −1.89 eV, respectively. Therefore, the introduction of different substitutes in phenyl cores would distinctly affect the photophysical properties. These results indicate that the prepared carbazole derivatives could be potential candidates for realizing ultraviolet or blue-violet emission.

Improved performance of organic light emitting devices using triazole/ Cs2CO3/Al cathode

In this study, a new cathode system of triazole (TAZ)/Cs2CO3/Al has been proposed. Obvious efficiency improvement and sharp enhanced electron injection were maintained in the devices with this new cathode. TAZ owns deep the highest occupied molecular orbital (HOMO) and wide band gap, so as to block the hole leakage effectively and confine the exciton in Alq3 emissive layer. An abrupt lowering of the vacuum level was found in the mixture of TAZ and Cs2CO3, which indicated that the electron injection barrier from metal cathode into Alq3 was greatly reduced. Our experimental proved that this new cathode may have a wide application in OLEDs emitting light with short wavelength.

Rational Solvent Annealing for Perovskite Film Formation in Air Condition (July 2017)

The past few years have witnessed a rapid growth in research and development activities for the emerging hybrid perovskite solar cells (PSCs), whose power conversion efficiencies have approached that of the commercialized silicon solar cells. In this regard, we try to develop a rational and accurately controllable solvent annealing method in air condition to maintain cost effective solar energy for PSCs. We have tailored the perovskite crystal growth process by optimizing dimethyl sulfoxide (DMSO) vapor annealing parameters, including the vapor concentration, the annealing time duration, and time point. The result shows that film-through large grains and sharply reduced surface defects can be obtained for perovskite film when the DMSO vapor annealing treatment is set at the initial stage of crystal growth, followed with mere thermal annealing for at least 20 min to reconstruct the perovskite crystal. Our results have supplied strong evidences for the scenario of perovskite film surface dissolution in DMSO vapor and the key role of atmosphere at the initial stage of crystal growth process for the final perovskite film morphology.