Degradation mechanisms of methylammonium lead halide perovskite solar cells (PSCs) have drawn much attention recently. Herein, the bulk and surface degradation processes of the perovskite were ...differentiated for the first time by employing combinational studies using electrochemical impedance spectroscopy (EIS), capacitance frequency (CF), and X-ray diffraction (XRD) studies with particular attention on the roles of small polar molecules (MA+ and H2O). CF study shows that short-circuit current density of the PSCs is increased by H2O at the beginning of the degradation process coupled with an increased surface capacitance. On the basis of EIS and XRD analysis, we show that the bulk degradation of PSCs involves a lattice expansion process, which facilitates MA+ ion diffusion by creating more efficient channels. These results provide a better understanding of the roles of small polar molecules on degradation processes in the bulk and on the surface of the perovskite film.
Perovskite is emerging as a novel emitter in solution-processed light-emitting diodes (LEDs). In these LEDs, morphology, especially the grain size of perovskite, plays a key role in determining ...electroluminescence performance. Several studies have shown that sizes of the perovskite grains can be controlled by the contact angle between the perovskite solution and the substrate. In this work, we found that in the quasi-two-dimensional (2D) system, the perovskite grain size can be substantially refined when there are hydrogen bonding between the perovskite’s organic spacer and the substrates. In fact, for quasi-2D perovskite, with the presence of such hydrogen bond, its effects on the perovskite grain size overshadow the contact angle’s effect. We demonstrated that perovskite with refined grains can form amine- or carbazole-based polymers which can form N···H hydrogen bonding with the perovskite’s organic spacer. Using these polymers as hole-transporting layers on poly(3,4-ethylenedioxythiophene):polystyrene sulfonate, external quantum efficiency of CsPbBr3-based LEDs can be enhanced from 1.5 to 10.0% without passivation treatment. This work suggests that bonding between perovskite precursors and the substrate can have significant influence on the morphology of the final perovskite grains and their optoelectronic performance.
Several breakthroughs in organic optoelectronic devices with new applications and performance improvement have been made recently by exploiting novel properties of charge transfer complexes (CTCs). ...In this work, a CTC film formed by coevaporating molybdenum(VI) oxide and pentacene (MoO3:pentacene) shows a strong dipole of 2.4 eV with direction controllability via pre-biasing with an external voltage. While CTCs are most widely known for their much red-shifted energy gaps, there is so far no report on their controllable dipoles. By controlling this dipole with an electrical pre-bias in a MoO3:pentacene CTC based device, current changes over 2 orders of magnitude can be achieved. Kelvin probe force microscopy further confirms that surface potential of the MoO3:pentacene film can be modulated by an external electric field. It is shown for the first time that a dipole of controllable direction can be set up inside a CTC layer by pre-biasing. This concept is further tested by incorporating the CTC layer in organic photovoltaic (OPV) devices. It was demonstrated that by pre-biasing the OPV devices in different directions, their open circuit voltages (V oc) can be significantly tuned via the built-in potentials.
After several decades of intense researches on organic optoelectronics, several types of optoelectronic devices such as organic light-emitting devices (OLEDs) have become commercially viable, and ...performance of organic photovoltaic (OPV) devices has seen significant progress. At the same time, scientists are getting better understanding on the device physics, and the important roles of charge-transfer (CT) states in these devices are widely recognized. CT states are formed upon CT interaction between donor (D) and acceptor (A) molecules. When the extent of CT is high enough, materials with the CT states are specifically referred as charge-transfer complexes (CTCs). Depending on their formation mechanisms, CTCs can be classified into ground-state CTCs (GSCs) and excited-state CTCs (exciplexes). Novel optoelectronic properties of GSCs, including their abnormally high electrical conductivities, long-wavelength photon absorption, charge-generation, etc., have been applied in various optoelectronic devices. On the other hand, attention of excited CTCs is primarily focused on exciplex emission and CT absorption. Recent experimental evidence deepens the understanding of CT states and their new applications are exploited. As there are limited systematic reviews on the application of CTCs in different organic devices, this review intends to fill this gap by giving a brief overview of novel CTC applications and correlating CT properties with device performance.
Two ambipolar host materials, 9,9-bis(9-methylcarbazol-3-yl)-4,5-diazafluorene (MCAF) and 9,9-bis(9-phenylcarbazaol-3-yl)-4,5-diazafluorene (PCAF), comprising two electron-donating carbazole units ...and an electron-accepting 4,5-diazafluorene group, have been designed, synthesized, and characterized. Given the nonplanar structure of the sp3-hybridized C9 atom of the 4,5-diazafluorene unit, MCAF and PCAF exhibit high triplet energy levels of 2.82 and 2.83 eV, as well as high glass-transition temperatures of 187 and 188 °C, respectively. Equipped with ambipolar transport properties as well as suitable highest occupied and lowest unoccupied molecular orbital energy levels, the two compounds excellently perform in blue phosphorescent organic light-emitting devices (PHOLEDs). The MCAF-based blue PHOLED has a very low turn-on voltage of 2.6 V, a high current efficiency of 32.2 cd A–1, a high external quantum efficiency of 17.9%, a high power efficiency of 31.3 lm W–1, and a low efficiency roll-off with a high efficiency of 27.6 cd A–1 even at 10 000 cd m–2. These values are among the highest ever reported for devices doped with iridium(III) bis2-(4′,6′-difluorophenyl)pyridinato-N,C(2′)-picolinate.
We reported a simple one-step, low-temperature solution-processed technique to fabricate perovskite solar cells using lead acetate as the lead source. Solvent annealing was applied for grain growth ...to obtain better morphology. Uniform perovskite films without pinholes can be obtained by solvent annealing for 5 min at 100 °C. Planar perovskite solar cells based on the high quality perovskite films deliver power conversion efficiency up to 12.71% with negligible hysteresis and good reproducibility. In addition, the substrate surfaces have little effect on the crystallization of perovskite when lead acetate was used, leading to uniform films on different substrates, which can provide a wide choice of substrates and interfacial materials.
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•High quality perovskite films can be easily fabricated from lead acetate precursor.•Solvent annealing was applied to increase perovskite grain size and crystallinity.•Planar perovskite solar cells deliver PCE up to 12.71% with negligible hysteresis.•Uniform perovskite films can be formed on different substrates.
Cubic boron nitride (cBN) has strong potential for the applications in high-temperature and high-power electronics and deep ultraviolet devices due to its outstanding combined physical and chemical ...properties. P-type surface transfer doping of heteroepitaxial cBN films was achieved by employing MoO3 and tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) as the surface dopants. The surface conductivities of hydrogenated cBN films increased by 3–6 orders after the deposition of surface dopants. The photoemission spectroscopy (PES) measurements revealed the variation of electronic structures at the interface regions, which suggested that the electron transfer from cBN films to the surface dopants induced hole accumulation at the cBN surface and the increase of surface conductivity. Based on the PES results, the energy level diagrams at MoO3/cBN and F4-TCNQ/cBN interfaces were determined. The achievement provided a potential approach for fabricating cBN-based electronic devices, especially on micrometer and nanometer scales.
Due to their easy availability, low cost and opportunities for exploiting reactions of bromo substituents, 1,3,6,8-tetrabromopyrene has attracted major attention in the organic electronics community ...for designing and constructing novel classes of pyrene based organic semiconducting functional materials. In the present work, 1,3,6,8-tetrabromo pyrene was transformed into the corresponding tetrasubstituted carbazole and phenothiazine derivatives using the classical Suzuki coupling reaction. These newly synthesized materials with a carbazole substituent ( PY-CA ) and a phenothiazine substituent ( PY-PH ) were characterised thoroughly and were successfully used as an active light-emitting layer in organic light emitting diodes which resulted in blue and green emission with promising device performance. PY-CA exhibited the maximum brightness at around 2500 cd m −2 and the power efficiency of 1.5 lm W −1 while that of PY-PH exhibited 2116 cd m −2 and 0.45 lm W −1 respectively.
Organolead halide perovskite devices are reported to be susceptible to thermal degradation, which results from heat-induced fast ion diffusion and structural decomposition. In this work, it is found ...that the performances of degraded low-dimensional perovskite solar cells can be considerably improved (e.g., power conversion efficiency shows ∼10% increase over the fresh device) by a short-time heat treatment (85 °C, 3 min). Capacitance–frequency, X-ray diffraction, and ionic diffusion calculation results suggest that heat treatment can enhance the crystallinity of the degraded low-dimensional perovskite and minimize the detrimental effects caused by water molecules, leading to improved performances. Our results indicate that the heat treatment does not necessarily lead to the accelerated degradation but can also regenerate the degraded low-dimensional perovskite.
Charge-transfer state (CTS) plays a very important role in organic photovoltaic (OPV) devices. Especially the relationship between open-circuit voltage (V OC) and CTS has been widely discussed. It is ...proposed that the CTS energy (E CT) directly determines the V OC value; however, the E CTs measured from different techniques often show considerable discrepancy. Here four methods are applied to probe the E CT values in five different bulk-heterojunction polymer:fullerene OPVs. It is found that linear relationships exist between different E CT’s and V OC values. The detailed energetic meanings of the E CT values measured from different techniques are discussed, and the origin of their discrepancy is analyzed. Lastly, based on a proposed energy model, a relationship is summarized to estimate V OC loss by considering the energetic broadening of CTS, bimolecular recombination, and dielectric effect. The results provide a guideline to forecast the V OC and investigate its loss in OPV.
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