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.

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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.

Metal‐halide perovskite materials are crystalline semiconductors that can be processed at room temperature using solution‐processible deposition techniques. In only a few years, perovskite‐based solar cell efficiencies have seen a huge increase from 3 % to 22.1 %. These direct‐bandgap materials are potential candidates for other optoelectronic device applications such as photodetectors, light‐emitting transistors, and light‐emitting diodes. In this Review, we present the current state‐of‐the‐art in the research and development of perovskite light‐emitting diodes (PeLEDs) based on metal halide perovskite semiconductors with an emphasis on size of crystallites and its effects on optical properties, device architectures, and PeLED performance parameters. A uniform pinhole‐free morphology with small grain size is essential for high‐performance PeLEDs. For this purpose, a p‐i‐n type device architecture with a p‐type poly(3,4‐ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) layer was found to be more suitable than the n‐i‐p type. In PeLEDs based on bulk‐phase perovskites, the average size of the crystallites is in the range 100–500 nm. The efficiency can be improved further by using perovskite nanoparticles with size <100 nm. Color of the emitted light from these materials can be tuned over a wide range, that is, from NIR (775 nm) to near‐UV (410 nm), simply by changing the halide ion and the stoichiometric ratio between halide ions in the mixed‐halide‐type perovskites. Change of stoichiometry also affects the structural properties and the final film morphology, which in turn influences the radiative and non‐radiative recombination rates of the charge carriers and hence the device performance. Furthermore, we also provide recent developments in PeLEDs based on inorganic perovskite nanocrystals that complement the efforts being made in hybrid PeLEDs.

Efficiency estimations of organic solar cells are observed to be dependent on the dimensions of electrode defining the active area. We address this issue and explore the manner in which efficiency scales in polymer solar cells by studying these devices as a function of electrode area and incident beam size. The increase in efficiency for smaller active areas can be explained by the reduced electrical resistive loss, the enhanced optical effects, and the finite additional fraction of photogenerated carriers in the vicinity of the perimeter defined by the metal electrode

Using soft-imprint nanolithography, we demonstrate large-area application of engineered two-dimensional polarization-independent networks of silver nanowires as transparent conducting electrodes. These networks have high optical transmittance, low electrical sheet resistance, and at the same time function as a photonic light-trapping structure enhancing optical absorption in the absorber layer of thin-film solar cells. We study the influence of nanowire width and pitch on the network transmittance and sheet resistance, and demonstrate improved performance compared to ITO. Next, we use P3HT-PCBM organic solar cells as a model system to show the realization of nanowire network based functional devices. Using angle-resolved external quantum efficiency measurements, we demonstrate engineered light trapping by coupling to guided modes in the thin absorber layer of the solar cell. Concurrent to the direct observation of controlled light trapping we observe a reduction in photocurrent as a result of increased reflection and parasitic absorption losses; such losses can be minimized by re-optimization of the NW network geometry. Together, these results demonstrate how engineered 2D NW networks can serve as multifunctional structures that unify the functions of a transparent conductor and a light trapping structure. These results are generic and can be applied to any type of optoelectronic device.

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