Applying Graphene to Solar Cells

• Main Authors: Bang-Jian Hong, 洪榜鍵
• Other Authors: Ya-Ping Hsieh
• Format: Others
• Language: en_US
• Published: 2015
• Online Access: http://ndltd.ncl.edu.tw/handle/48758636373662103407

碩士 === 國立中正大學 === 光機電整合工程研究所 === 103 === In this thesis, graphene were applied to two kinds of solar cells. In the first part of thesis, graphene were used as electrode for ultrathin dye solar cell while in the second part of this thesis, graphene is applied as the metal of metal/semiconductor Schottky-junction solar cell. Ultrathin solar cells have the potential to form ubiquitous, cheap, and efficient sources of energy. Practical issues, however, prevent the formation of solar cells below 100 nm. Cracks, voids, and inhomogeneities result in leakage pathways that render ultrathin solar cells unusable. We here demonstrate the use of graphene as the top electrode for ultrathin solar cells with thicknesses between 5 nm and 100 nm. Graphene’s mechanical strength allows suspension over rough surfaces which enable ultrathin solar cell possible and its featureless absorption curve enables characterization of the solar cell’s constituents. These advantages were applied to study the effect of TiO2 thickness on solid state dye sensitized solar cells (DSSCs) performance in the limit of ultra-thin films: investigation of the carrier transport even for 10 nm thin films. Second, optical measurement of dye loading revealed a fast saturating behavior that was attributed to the quasi two-dimensional nature of the thin TiO2 film. Third, carrier recombination was found to be significantly enhanced in these thin films which limit the performance for increasing TiO2 thicknesses. Finally, charge compensation of the photoexcited dye poses limitations on the minimum thickness of the TiO2 which was found to be 10 nm. This work opens up a new route to produce ultrathin solar cells for transparent and flexible applications and also highlight the limits of their scaling. In the second part of this thesis, Graphene/Si Schottky-junction solar was studied. Graphene’s high carrier mobility and ambipolar nature has the potential to improve electronic devices. The absence of a band-gap necessitates heterostructure devices. Schottky-barrier devices consisting of an interface between graphene and a semiconductor represent the simplest heterostructure. Despite its simplicity, graphene-based Schottky barrier devices are not well understood and exhibit low injection efficiencies. We here investigate the impact of graphene/metal interaction on the properties of the Schottky-barrier. Besides the commonly employed Au/graphene we use Pt/graphene contacts. We find that the injection efficiency for Pt is 3 times higher than for Au and systematically study the origin of this behavior. We identify a large difference in the Richardson’s constant due to changes in the density of surface states. The demonstrated ability to increase the injection current was applied to improve the efficiency of graphene-based Schottky solar cells by 13x.