Ming-Fai Lo

Novel Efficient Blue Fluorophors with Small Singlet‐Triplet Splitting: Hosts for Highly Efficient Fluorescence and Phosphorescence Hybrid WOLEDs with Simplified Structure

The exact hosts for F-P hybrid WOLEDs have been first demonstrated following a new design strategy for blue fluorophors with small singlet-triplet splitting. Two novel compounds DPMC and DAPSF exhibit efficient blue fluorescence, high triplet energies and good conductivities. These merits allow us to use new simplified device designs to achieve high efficiency, slow efficiency roll-off and stable emission color.

Limits of open circuit voltage in organic photovoltaic devices

Open circuit voltage (Voc) of organic photovoltaic devices has been interpreted with either the metal-insulator-metal (MIM) model or the energy offset between highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor (HOMOD-LUMOA). To elucidate the relation between Voc and the two models, we have used electrodes of a wide range of work functions to connect the CuPc/C60 organic photovoltaic devices. We found that when the work function difference (Δϕelectrodes) between ITO and Al electrode is in the range −3 and 0 eV, Voc increases linearly with Δϕelectrodes as prescribed by the MIM model. Outside this range, Voc saturates with values close to that given by the HOMOD-LUMOA less the exciton binding energy.

Chlorine Incorporation for Enhanced Performance of Planar Perovskite Solar Cell Based on Lead Acetate Precursor

We show the effects of chlorine incorporation in the crystallization process of perovskite film based on a lead acetate precursor. We demonstrate a fabrication process for fast grain growth with highly preferred {110} orientation upon only 5 min of annealing at 100 °C. By studying the correlation between precursor composition and morphology, the growth dynamic of perovskite film in the current system is discussed. In particular, we found that both lead acetate precursor and Cl incorporation are beneficial to perovskite growth. While lead acetate allows fast crystallization process, Cl improves perovskite crystallinity. Planar perovskite solar cells with optimized parameters deliver a best power conversion efficiency of 15.0% and average efficiency of 14.0% with remarkable reproducibility and good stability.

Formation chemistry of perovskites with mixed iodide/chloride content and the implications on charge transport properties

Although Cl-doping is a common technique for achieving high photovoltaic (PV) performance, the Cl content is negligibly small and cannot easily be tuned. Therefore, we herein study the formation chemistry of Cl-doped perovskites by examining the chemical interactions between thermally evaporated MAI and PbCl2 through X-ray photoemission spectroscopy (XPS). We show that PbCl2 is not stable at the MAI/PbCl2 contact surface. The Cl atom readily detaches from the PbCl2, which subsequently initiates electron transition from Pb to MAI. Via thermal-evaporation, a perovskite with a high PbCl2 content can be prepared and examined. We found that the presence of metallic Pb, associated with increased Cl content, can quench the photogenerated exciton in PV devices. By optimizing the ratio of MAI : PbCl2, a perovskite solar cell with ∼6% efficiency was obtained.

Ambient effects on fullerene/copper phthalocyanine photovoltaic interface

Effects of ambient-air exposure on the energy levels at photovoltaic interface of fullerene (C60)/copper phthalocyanine (CuPc) were studied using ultraviolet photoemission spectroscopy. The junction prepared in ultrahigh vacuum showed flat energy levels with little vacuum level offset, while exposure to ambient air at 10−5 Torr induced p-type doping of C60 with energy levels bend up for 0.27 eV. The energy difference between HOMOCuPc−LUMOC60, describing the theoretical maximum open-circuit voltage, increased from 0.64 to 0.81 eV. The exposure moved the LUMOC60 away from the Fermi level, leading to reduction in carrier concentration and film conductivity.

Influence of the donor/acceptor interface on the open-circuit voltage in organic solar cells

The donor/acceptor interface in a standard CuPc/C60 organic solar cell was modified by insertion of a thin layer of molybdenum trioxide (MoO3). An ultrathin layer of MoO3 between the donor and acceptor increased the open-circuit voltage (VOC) from 0.45 to 0.85 V. The enhancement in VOC is explained by the increase in the energy level offset between the lowest unoccupied molecular orbital of the acceptor and the highest occupied molecular orbital of the donor (EDHOMO-EALUMO). The explanation is supported by the energy level analysis of the donor/acceptor interface by ultraviolet photoemission spectroscopy and x-ray photoemission spectroscopy.

Charge-Transfer Complexes and Their Role in Exciplex Emission and Near-Infrared Photovoltaics

Charge transfer and interactions at organic heterojunctions (OHJs) are known to have critical influences on various properties of organic electronic devices. In this Research News article, a short review is given from the electronic viewpoint on how the local molecular interactions and interfacial energetics at P/N OHJs contribute to the recombination/dissociation of electron-hole pairs. Very often, the P-type materials donate electrons to the N-type materials, giving rise to charge-transfer complexes (CTCs) with a P(δ+) -N(δ-) configuration. A recently observed opposite charge-transfer direction in OHJs is also discussed (i.e., N-type material donates electrons to P-type material to form P(δ-) -N(δ+) ). Recent studies on the electronic structures of CTC-forming material pairs are also summarized. The formation of P(δ-) -N(δ+) -type CTCs and their correlations with exciplex emission are examined. Furthermore, the potential applications of CTCs in NIR photovoltaic devices are reviewed.

Stability enhancement in organic photovoltaic device by using polymerized fluorocarbon anode buffer layer

We show that by introducing a polymerized fluorocarbon film (CFx) on indium tin oxide (ITO), stability of CuPc/C60 organic photovoltaic device can be improved. While the efficiency of a standard device decreases from 1.5% to 0.7% after 2000 h of storage and intermittent operation, a CFx-buffered device shows negligible changes (1.6% throughout 2000 h). We found that C60 is influenced by the ITO substrate. X-ray photoemission studies show that reactive −O• species on the ITO surface were minimized after CFx polymerization. Reduction of such species from organic layers is considered a major cause for the stability enhancement in the CFx-buffered device.

Operation stability enhancement in organic photovoltaic device by a metal doped organic exciton blocking layer

While metal diffusion in organic layers have been considered as causes for performance degradation in organic light-emitting devices, we show that suitable metal doping can instead improve physical stability of organic films. By using a metal doped organic exciton blocking layer (EBL), enhanced stability is demonstrated in unpackaged CuPc/C60 organic photovoltaic devices (OPV). While devices with a pure organic EBL of bathocuproine and tris(8-hydroxyquinolinato)aluminum (Alq3) show over ∼20% decreases in efficiency for first 150 min of operation, the device with magnesium-doped Alq3 EBL shows less than ∼5% variation in efficiency during the same period.

Electronic properties and open-circuit voltage enhancement in mixed copper phthalocyanine:fullerene bulk heterojunction photovoltaic devices

While bulk heterojunctions (HJ) have been used in organic photovoltaic (OPV) devices, there are few studies on their interface electronics and mechanisms on device performance. Here, we studied the electronic structure of a mixed CuPc:C60 and a discrete CuPc/C60 junctions using photoemission spectroscopy. The HOMOCuPc-LUMOC60 energy offset, which controls the theoretical maximum open-circuit voltage (Voc), was increased from 0.64 to 1.13 eV by mixing CuPc with C60. This Voc increase is attributed to the underlying substrate work function and charge transfer between two molecules. The results provide an understanding of the Voc enhancement in OPV devices with bulk HJ.