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Here, an investigation of three-dimensional (3D) morphologies for bulk heterojunction (BHJ) films based on regioregular poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) is reported. Based on the results, it is demonstrated that optimized post-treatment, such as solvent annealing, forces the PCBM molecules to migrate or diffuse toward the top surface of the BHJ composite films, which induces a new vertical component distribution favorable for enhancing the internal quantum efficiency (ηIQE) of the devices. To investigate the 3D BHJ morphology, novel time-of-flight secondary-ion mass spectroscopy studies are employed along with conventional methods, such as UV-vis absorption, X-ray diffraction, and high-resolution transmission electron microscopy studies. The ηIQE of the devices are also compared after solvent annealing for different times, which clearly shows the effect of the vertical component distribution on the performance of BHJ polymer solar cells. In addition, the fabrication of high-performance P3HT:PCBM solar cells using the optimized solvent-annealing method is reported, and these cells show a mean power-conversion efficiency of 4.12% under AM 1.5G illumination conditions at an intensity of 100 mW cm−2.

Changes in the nanoscale morphologies of the blend films of poly (3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), for high-performance bulk-heterojunction (BHJ) solar cells, are compared and investigated for two annealing treatments with different morphology evolution time scales, having special consideration for the diffusion and aggregation of PCBM molecules. An annealing condition with relatively fast diffusion and aggregation of the PCBM molecules during P3HT crystallization results in poor BHJ morphology because of prevention of the formation of the more elongated P3HT crystals. However, an annealing condition, accelerating PCBM diffusion after the formation of a well-ordered morphology, results in a relatively stable morphology with less destruction of crystalline P3HT. Based on these results, an effective strategy for determining an optimized annealing treatment is suggested that considers the effect of relative kinetics on the crystallization of the components for a blend film with a new BHJ materials pair, upon which BHJ solar cells are based.

Polymer-based photovoltaic cells, with periodic sub-micrometer structures as an efficient light-trapping scheme, are investigated to improve the performance of organic solar cells based on poly(3-hexylthiophene) and 1-(3-methoxycarbonyl)propyl-1-phenyl-(6,6)C61. A soft lithographic approach that uses photoresponsive azo polymer films as masters and poly(dimethylsiloxane) as stamps is used to form surface relief gratings (SRGs) on the active layers. The effect of periodic gratings on solar cell performance is precisely investigated according to various grating conditions such as period, depth, and dimension. The solar cells with 1D and 2D SRGs present improved incident-photon-to-current conversion efficiencies and an overall increase in power conversion efficiencies, primarily resulting from the enhancement of short-circuit current density, indicating that periodic structures induce further photon absorption in the active film.

Despite the relatively low efficiencyin comparison with conventional inorganic solar cells, thepotential of roll-to-roll processing and large-area processa-bility on flexible low-cost substrates renders conjugatedpolymer-based organic solar cells (OSCs) very attractive asa cost-effective solution to the problem of energy shortage.Among the available BHJ systems, poly(3-hexylthiophene)(P3HT) and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6)C

Since the report of a molecular thin-film organic solar cell (OSC) by Tang, organic materials have increasingly become attractive candidates for the fabrication of cost-efficient and flexible photovoltaic cells. In particular, polymer bulkheterojunction (BHJ) solar cells based on interpenetrating networks of an electron donor and an acceptor, with a largearea donor and acceptor interface, resulting in an efficient photo-induced charge separation, have gained considerable interest. Despite of their relatively low efficiency in comparison to conventional inorganic solar cells, the potential of roll-to-roll processing on low-cost and flexible substrates makes polymer solar cells (PSCs) so attractive as a cost-effective solution to the energy problems we are facing today. Among the various BHJ systems, poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61butyric acid methyl ester (PCBM) networks produced by spin-coating a blend combined with a preor post-heat treatment to improve the degree of ordering of P3HT have shown the highest efficiencies up to ca. 4–5 % under 80 or 100 mW cm illumination under AM 1.5 G condition. Because the electrical and optical properties of conjugated polymers are strongly dependent on the structural order of polymers, the processing methods and conditions, which determine the degree of organization of polymers, have a critical influence on the performance of electrical and optoelectrical devices based on these polymers. In addition, considering efficiencies of ca. 5 %, the development of novel solution processes that are compatible with low-cost mass production is one of the crucial requirements for practical device applications. Here, we report on novel solution-processed high-efficiency polymer solar cells based on a blend of P3HT and PCBM, resulting from improved organization of the P3HT. By directly brush painting a blend of P3HT and PCBM on appropriately temperature-controlled substrates, enhanced ordering of the polymers, induced by the shear stress in the direction parallel to the brushing direction, was achieved. This highly ordered active layer facilitates charge transport separated at the interface of the P3HT and PCBM, leading to an increased efficiency, in particularly an improved fill factor. In addition, this novel solution process can be considered a promising method for the fabrication of flexible and large-area polymer solar cells based on high-throughput roll-to-roll manufacturing, which would make the realization of low-cost PSCs possible. First, two types of devices were fabricated by conventional spin-coating and brush painting, respectively, without any preor post-heat treatment, which are usually performed to stabilize a nanoscale interpenetrating network with a crystalline order, resulting in an increase in overall conversion efficiency. Figure 1 shows the schematic of the brushing method and the resulting I–V curves for the two types of devices. The brush painting was performed on a poly(3,4-ethylenedioxythiophene) (PEDOT):poly(styrene sulfonate) (PSS)-coated indium tin oxide (ITO) substrate on a hot plate with a temperature of 50 °C. A general paintbrush made of nylon fibrils was used and the active layer was coated with a speed of ca. 1.5 cm s. It took ca. 2 s to prepare a complete film on the ITO substrate with a size of 1.5 cm × 1.5 cm by brush painting twice. During the brush painting process, an appropriate temperature was necessary to make a smooth and uniform active layer with a high degree of ordering, which is related to the evaporation rate of the solvents. In our case chlorobenzene was used, for which good-quality films could not be obtained when the brush painting was carried out below 50 °C because of too slow evaporation of the solvent at low temperatures. To prepare active layers with the same thickness, the blend was spin-coated at room temperature on the PEDOT:PSScoated ITO substrate at 2000 rpm and the thickness was determined to be ca. 90 nm by means of a surface profiler (Kosaka ET-3000i). The surface morphologies determined by atomic force microscopy (AFM) for the surfaces produced by the brushing technique (with a rms roughness of ca. 0.91 nm) were similar to that of a conventional spin-coated active layer (not shown here). As shown in Figure 1b, the performance was improved when the active layer was prepared by the brush-painting process. In the case of a spin-coated device, a poor performance was observed with VOC (opencircuit voltage) = 0.6 V, ISC (short-circuit current density) = 3.59 mA cm, FF (fill factor) = 32.5 %, and ge (power conversion efficiency) = 0.7 %, which is similar to previous C O M M U N IC A TI O N

The maskless selective wet etching of p-GaN layer with KOH in ethylene glycol (KE) and H3PO4/H2SO4 (HH) acids was developed for the highly efficient light-emitting diodes (LEDs). The p-GaN surfaces textured by the selective wet etching process without using etch mask showed hexagonal and stripe shapes in the KE and HH solutions, respectively. The current-voltage (I-V) characteristics of the LED textured by KE and HH solutions showed improved electrical properties compared to the non-etched LED. This result could be attributed to a reduced contact resistance due to an increased contact area between the metal electrode and p-GaN layer. In addition, the light-output power of the LED textured by KE and HH solutions was improved by 29.4% and 36.8% relative to that of the non-etched LED. This result was attributed to the increase in probability of escaping photons from the LED and the reduction of surface defects by the maskless selective wet etching process. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

1 School of Materials Science and Engineering, Heeger Center for Advanced Materials (HCAM), Gwangju Institute of Science and Technology (GIST), Gwangju 500-712, Korea 2 Korea Institute of Science and Technology (KIST), Institute of Advanced Composite Materials, Jeollabuk-do 565-902, Korea 3 Department of Nano & Chemical Engineering, Kunsan National University, 753-701, Kunsan, Jeollabuk-do, 753-701, Korea 4 School of Nanobio Materials and Electronics, Research Institute for Solar and Sustainable Energies (RISE), GIST, Gwangju 500-712, Korea