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The optical properties of π-conjugated polymers based on 9,10-dihydrophenanthrene units, such as optical absorption and photoluminescence (PL), were studied. The electronic energy structures were determined by optical measurements. Strong PL with high quantum efficiency was clarified in the films, and photo-pumped multimode blue laser emission was demonstrated from cylindrical microcavities with the polymer thin film fabricated inside the optical capillary tube.

The C70 derivative [6,6]-phenyl-C71-butyric acid methyl ester ([70]PCBM) has wider and stronger absorption in the visible light region than the analogous C60 derivative ([60]PCBM). Using [70]PCBM as an n-type semiconductor in combination with poly(3-hexylthiophene) (P3HT), we successfully fabricated a highly efficient organic photovoltaic (PV) cell showing higher short-circuit current density (JSC) and incident photon-to-electron conversion efficiency (IPCE) values than a P3HT:[60]PCBM-blend cell, with a maximum power conversion efficiency (PCE) of 3.8% under standard simulated solar illumination (air mass 1.5 G, 100 mW/cm2). All of the PV characteristics of the P3HT:[70]PCBM and P3HT:[60]PCBM cells were identically improved by thermal annealing, suggesting that changes in the internal structure of the photovoltaic layer and in the contact between the photovoltaic layer and the top electrode of the P3HT:[70]PCBM cell occurred in a similar manner to those of the P3HT:[60]PCBM cell. Therefore, [70]PCBM is an easy-to-replace alternative n-type semiconducting molecule to [60]PCBM, which has a higher PCE.

Enhancing the light-extraction efficiency is the key issue for realizing highly efficient AlGaN-based ultraviolet light-emitting diodes (UV-LEDs). We introduced several features to improve the light extraction: a transparent AlGaN:Mg contact layer, a Rh mirror electrode, an AlN template on a patterned sapphire substrate, and encapsulation resin. The combination of the AlGaN:Mg contact layer and the Rh mirror electrode significantly improved the output power and the external quantum efficiency (EQE) of UV-LEDs. By introducing the aforementioned features, a maximum EQE of >20% at an emission wavelength of 275 nm and a 20-mA direct current was achieved.

Novel small-molecule organic photovoltaic cell structures with pentacene and fullerene multilayers are reported. Pentacene is an attractive material for photovoltaic cells, but it tends to crystallize with molecular orientation during vacuum deposition. Therefore, thus far, it has been difficult to fabricate a bulk-heterojunction active layer with fullerenes by coevaporation, despite the demand for pentacene/fullerene bilayer heterojunction cells owing to their good photovoltaic performance. Here, instead of preparing an active layer by coevaporation, we prepare a pentacene and fullerene multilayer-heterojunction active layer by alternating evaporation. It was found that multilayer cells show superior photovoltaic performance to bilayer cells owing to the improvements in their exciton dissociation efficiency and charge carrier transport properties.

Recently, the power conversion efficiency (PCE) of organic photovoltaic (OPV) cells has been improved through increased photocurrent density. To further increase PCE, open circuit voltage (Voc) should also be increased. Here, we report tandem OPV cells based on low-molecular-weight semiconductors evaporated in vacuum. The tandem system produced a high Voc that is proportional to the number of tandem OPV cells. We obtained a Voc of 2.07 V using a quadruple cell, which was fourfold that of a single cell.

The long-term performance stability of organic solar cells (OSCs), as well as their high power conversion efficiency, is necessary for their practical use. For polymer-based OSCs, poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) is conventionally introduced as a buffer layer. However, there is concern that PEDOT:PSS, which is highly hygroscopic and acidic, accelerates the degradation of OSCs by oxidizing certain components of the OSCs. In this study, we compared the stabilities of three types of cell containing different buffer layers (PEDOT:PSS, molybdenum oxide, and no buffer layer). The OSC with the PEDOT:PSS layer was degraded very rapidly, while the other OSCs were degraded slowly. In addition, we observed that the oxidation of the aluminum electrode occurred at the aluminum/photovoltaic layer interface. These results suggest that PEDOT:PSS accelerates the oxidation of the aluminum electrodes of OSCs and reduces their efficiency. We conclude that the introduction of the PEDOT:PSS layer, despite producing a high "initial" efficiency, is not the best strategy to improve the OSC stability.