wileyonlinelibrary.comAdv. Funct. Mater. 2011, 21, 932-940 Charge transport in the ribbon phase of poly(2,5-bis(3-alkylthiophen-2-yl) thieno[3,2-b ]thiophene) (PBTTT)-one of the most highly ordered, chainextended crystalline microstructures available in a conjugated polymer semiconductor-is studied. Ribbon-phase PBTTT has previously been found not to exhibit high carrier mobilities, but it is shown here that fi eld-effect mobilities depend strongly on the device architecture and active interface. When devices are constructed such that the ribbon-phase fi lms are in contact with either a polymer gate dielectric or an SiO 2 gate dielectric modifi ed by a hydrophobic, self-assembled monolayer, high mobilities of up to 0.4 cm 2 V − 1 s − 1 can be achieved, which is comparable to those observed previously in terrace-phase PBTTT. In uniaxially aligned, zone-cast fi lms of ribbonphase PBTTT the mobility anisotropy is measured for transport both parallel and perpendicular to the polymer chain direction. The mobility anisotropy is relatively small, with the mobility along the polymer chain direction being higher by a factor of 3-5, consistent with the grain size encountered in the two transport directions.
A probable limiting factor for efficiency and fill factors of organic solar cells originates from the cathode-polymer interface. We utilize various forms of cathode layer such as Al, Ca, oxidized Ca, and low melting point alloys in model systems to emphasize this aspect in our studies. The current-voltage ͑JV͒ response in the fourth quadrant indicates a general trend of convex shaped JV characteristics ͑d 2 J / dV 2 Ͼ 0͒ for illuminated devices with good cathode-polymer interfaces and linear or concave JV responses ͑d 2 J / dV 2 Ͻ 0͒ for inefficient cathode-polymer interfaces.
Solution processed polymer:fullerene solar cells on opaque substrates have been fabricated in conventional and inverted device configurations. Opaque substrates, such as insulated steel and metal covered glass, require a transparent conducting top electrode. We demonstrate that a high conducting (900 S cm−1) PEDOT:PSS layer, deposited by a stamp‐transfer lamination technique using a PDMS stamp, in combination with an Ag grid electrode provides a proficient and versatile transparent top contact. Lamination of large size PEDOT:PSS films has been achieved on variety of surfaces resulting in ITO‐free solar cells. Power conversion efficiencies of 2.1% and 3.1% have been achieved for P3HT:PCBM layers in inverted and conventional polarity configurations, respectively. The power conversion efficiency is similar to conventional glass/ITO‐based solar cells. The high fill factor (65%) and the unaffected open‐circuit voltage that are consistently obtained in thick active layer inverted geometry devices, demonstrate that the laminated PEDOT:PSS top electrodes provide no significant potential or resistive losses.
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