Jun Sakai

Open circuit voltage of stacked bulk heterojunction organic solar cells

We investigate the open circuit voltage (VOC) of stacked bulk heterojunction (BHJ) organic solar cells based on conjugated polymer and fullerene derivative by spin-coating method. Poly[2-methoxy-5-(3′,7′-dimethyloctyloxy)-1,4-phenylene vinylene] (MDMO-PPV) blended with [6, 6]-phenyl C61-butyric acid methyl ester (PCBM) is used as a photoactive layer. The structure is ITO(lower BHJ cell)PEDOT:PSS/MDMO-PPV:PCBM/ITO/(upper BHJ cell)PEDOT:PSS/MDMO-PPV:PCBM/Al. VOC of the stacked cell is 1.34V and is about 1.6 times as large as that of single BHJ organic solar cell. VOC of the stacked cell is not proportional to the number of photoactive layers, but is the sum of the VOC values of the upper and the lower BHJ cells.

Pentacene:Fullerene Multilayer-Heterojunction Organic Photovoltaic Cells Fabricated by Alternating Evaporation Method

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.

Photo-degradation and its recovery by thermal annealing in polymer-based organic solar cells

Organic solar cells (OSCs) have potential advantages in their mechanical flexibility, portability, and low manufacturing cost. The power conversion efficiencies (PCEs) of OSCs based on conjugated polymers and soluble fullerenes have been improved markedly in the recent decade. For practical use, long-term stability of the cell performance, as well as high efficiency is essential. In contrast to considerable efforts on improving the PCE of OSCs, researches on the stability of OSCs had been rarely performed until the last few years so that the degradation mechanism and key technologies for the long-term stability are still unclear. In this study, we investigated the degradation under light irradiation and confirmed the recovery of cell performances by thermal annealing.

Tandem Organic Photovoltaic Cells Based on Low-Molecular-Weight Semiconductors

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.

Optical Properties and Microcapillary Laser of Blue-Emissive π-Conjugated Polymers Based on 9,10-Dihydrophenanthrene Unit

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.

Improvement of photo-current density by alternative deposition method for organic photovoltaic cell

Recently, power conversion efficiency of organic photovoltaic cell (OPV) has been upgraded due to high photo current density. An aggregation of organic molecules causes a reduction of photovoltaic performance owning to decline in film uniformity of organic layer. Here, we investigate the effect of alternative deposition method on controlling the aggregation and crystallization of organic layers. Advantage of the alternative deposition method is to improve photo current density due to the improved quality of p-n heterojunction. We succeeded in improving the photo current density by our alternative deposition method of tetraphenyldibenzoperiflanthene/C60 OPV cells.

Efficient organic photovoltaic tandem cells with poly (3-hexylthiophene): Fullerene active layers and transparent conductive oxide interlayer

Efficient organic photovoltaic tandem cells consist of P3HT:bis[60]PCBM and P3HT:[70]PCBM active layers and transparent conductive oxide interlayer are presented. A short circuit current and a conversion efficiency are estimated at more than 6 mA/cm2 and 5 % from optical simulation and optimization of the both active layer thicknesses. We clarify the almost same characteristics is achieved by the development of the nanoscale phase separation process of the active layers and the interlayer with the Indium Tin Oxide (ITO) sandwiched by MoO3 and LiF buffer layers. This is attributed to the improvement of carrier transport property in the active layers and interlayer.