Pengju Zeng

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.

sec-Butyl alcohol assisted pinhole-free perovskite film growth for high-performance solar cells

Perovskites have recently emerged as promising materials for high-performance, low-cost photovoltaic devices. Here, we report a one-step, solvent-induced, fast crystallization method based on sec-butyl alcohol to produce flat, uniform CH3NH3PbI3 films. sec-Butyl alcohol has proven to be a suitable solvent not only for the CH3NH3PbI3 fast crystallization deposition process, but also for adjusting the amount of CH3NH3I in CH3NH3PbI3 films. Planar heterojunction solar cells with flat and pinhole-free CH3NH3PbI3 films yielded an average power conversion efficiency (PCE) of 13.1 ± 1.2% and the highest PCE of 14.3%; no hysteresis was observed by changing the scan direction in our devices under standard AM 1.5 illumination. Remarkably, we found that the presence of CH3NH3I in CH3NH3PbI3 films has a positive effect on the photovoltaic device performance.

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.

Fabrication of high-performance and low-hysteresis lead halide perovskite solar cells by utilizing a versatile alcohol-soluble bispyridinium salt as an efficient cathode modifier

A novel alcohol-soluble conjugated bispyridinium salt (FPyBr) is developed and used as a cathode modifier to improve the cathode interface of planar heterojunction perovskite solar cells (PHJ PVSCs). The excellent electron-withdrawing ability of bispyridinium rings endows FPyBr with a favorable energy level alignment with phenyl-C60-butyric acid methyl ester (PCBM) and the cathode (e.g., Al), which leads to an ideal ohmic contact and efficient electron transport and collection. The deep-lying highest occupied molecular orbital energy level of FPyBr can also effectively block hole carriers and thus decrease leakage current and hole–electron recombination at the cathode interface. In addition, FPyBr can n-dope PCBM through an anion-induced electron transfer process, which increases the electron mobility of PCBM drastically, thereby diminishing interfacial resistance and promoting electron transport. As a result, by incorporating an FPyBr cathode interlayer with ethanol solvent, high-performance and low-hysteresis PHJ PVSCs with a maximal power conversion efficiency (PCE) of 19.61% can be realized. In contrast, reference devices without any cathode interlayer display a distinctly worse performance, with a PCE of 16.97%. Therefore, this excellent cathode modifier provides a new opportunity to fabricate high performance multilayer PVSCs using low-temperature solution processing without interfacial erosion/mixing.

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.

Enhanced perovskite morphology and crystallinity for high performance perovskite solar cells using a porous hole transport layer from polystyrene nanospheres

Organic-inorganic metal halide perovskites have led to remarkable advancements in emerging photovoltaics. The rapid increase in the power conversion efficiency (PCE) of PSCs has been mainly achieved by improving perovskite morphology and crystallinity. Herein, we report a simple and effective means to improve perovskite grain sizes using a porous hole transport layer (i.e. , PEDOT PSS in this work). We used polystyrene nanospheres as a sacrificial template to fabricate the porous-PEDOT:PSS. The growth of the CH3NH3PbI3 perovskite film on the porous-PEDOT:PSS substrate yields a dramatic improvement in crystallinity and an enhancement in perovskite grain sizes. When the porous structure was applied as a hole transport layer in PSCs with planar heterojunction structures, the efficiency was significantly enhanced from 15.33% for the planar device to 17.32%. This simple method for enhancing perovskite morphology and crystallinity paves the way for its application to other device architectures for enhanced photovoltaic performance.

Water- and alcohol-soluble cationic phenanthroline derivatives as efficient cathode interfacial layers for bulk-heterojunction polymer solar cells

In recent years, cathode interfacial layers (CILs), as versatile functional layers, have been extensively investigated for use in organic electronics. Many excellent CILs have been designed and prepared, and these have provided a distinct improvement in device performance. In this study, two bathophenanthroline-based cationic CILs (Bphen-Et and Bphen-Pr) were synthesized via a simple intramolecular cyclization reaction with 1,2-dibromoethane and 1,3-dibromopropane, respectively, for use in polymer solar cells (PSCs), and their chemical, thermal, photophysical, electrochemical, and photovoltaic properties were characterized. Bphen-Et and Bphen-Pr exhibited high thermal stability, and their glass transition temperatures exceeded 100 °C. Additionally, Bphen-Et has a glass transition temperature above 181 °C. The electrochemical characterization shows that Bphen-Et and Bphen-Pr have very deep highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels, where the HOMOs and LUMOs of Bphen-Et and Bphen-Pr are −6.63 and −6.67 and −4.16 and −4.18 eV, respectively. These very low HOMOs and LUMOs enhance the photovoltaic performance and decrease interfacial energy loss. The PSCs based on the poly[4,8-bis(2-ethylhexyloxyl)benzo[1,2-b:4,5-b′]dithio-phene-2,6-diyl-alt-ethylhexyl-3-fluorothithieno[3,4-b]thiophene-2-carboxylate-4,6-diyl] (PTB7): [6,6]-phenyl-C71 butyric acid methyl ester (PC71BM) system with Bphen-Et and Bphen-Pr CILs exhibit simultaneous enhancement in open-circuit voltage, short-circuit current density, and fill factor, and their power conversion efficiency increases from 3.98% to 8.05%, relative to that of the bare Al device. This type of cationic aromatic compound shows promise as a candidate CIL for use in PSCs.

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.