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This chapter describes the state of the art of solution-processed, n-type crystalline Si (n-Si) heterojunction with organic thin-layer (HOT) solar cells using conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). A single junction of PEDOT:PSS/n-Si solar cells on a plane n-Si substrate showed a power conversion efficiency (PCE) of 13.3%. Then, the PCE increased to 15.4% by adding an antireflection layer. In addition, the solar cell module using 10 units of series-connected 2 × 2 cm2 (4-in.)-sized PEDOT:PSS/n-Si solar cells exhibited an 0.37 W (7.2 W). Furthermore, PCE increased to 15.8%–18.2% for the front-PEDOT:PSS/n-Si device on textured n-Si substrate using charged mist deposition (CMD) method. In this discussion, we demonstrate the effect of the optical anisotropy of PEDOT:PSS on the photovoltaic (PV) performance of PEDOT:PSS/n-Si heterojunction solar cells, along with the junction property at the PEDOT:PSS/n-Si interface.

Molybdenum ditelluride (MoTe2) is known to have two different crystal phases: α-MoTe2 and β-MoTe2. α-MoTe2 is the polytype stable below 815 °C, and β-MoTe2 is stable above 900 °C. Here, we report changes in the structure and composition of each single crystal during heating in low and high vacuum conditions. Contrary to the results of a recently published paper, we did not observe α-MoTe2 transforming to β-MoTe2 at elevated temperatures. In a low vacuum, the α-MoTe2 sample was mainly oxidized, forming MoO2. In a high vacuum, it partially decomposed with surface oxidation. The recently reported "reversible phase transition" of α-MoTe2 is the misinterpretation of X-ray diffraction data.

Top-contacted organic field-effect transistors (OFETs) with graphene electrodes were fabricated by the laminate transfer method. Graphene electrodes were prepared on a different glass substrate and transferred onto an organic semiconductor layer of poly(3-hexylthiophene) (P3HT) using a double-layer laminate film of polyacrylonitrile (PAN) and poly(methyl methacrylate) (PMMA). The fabricated top-contacted OFET with graphene electrodes covered by the polymer film showed good p-type transistor characteristics with hole mobility of (3.7±0.5)×10-2 cm2 V-1 s-1, and 92% of the mobility was maintained even after exposure to ambient air for 250 h.

The performance of an organic thin film photovoltaic cell was improved using a solution-processed fluorinated surfactant Zonyl® FS-300 (Zonyl) cathode interlayer. Poly[N-9'-heptadecanyl-2,7-carbazole-alt-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) and [6,6]-phenyl C71-butyric acid methyl ester ([70]PCBM) were used as the donor and acceptor, respectively, in the bulk-heterojunction photoactive layer. The spin-casting of the appropriate concentration of the Zonyl solution on the photoactive layer significantly and simultaneously improved the short-circuit current, open-circuit voltage, and fill factor. The optimized Zonyl treatment increased the power conversion efficiency from 3.53 to 4.76%.

Efficient crystalline silicon (c-Si) heterojunction solar cells with conductive poly(ethylene dioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and graphene oxide (GO) composite are demonstrated using a structure of Ag/PEDOT:PSS/PEDOT:PSS:GO composite/c-Si (100)(ρ: 3–5 Ωcm)/Al. The power-conversion efficiency η increased to 10.7% under illumination of AM1.5 100 mW/cm2 simulated solar light by adjusting the PEDOT:PSS and GO mixing concentration ratio. The GO addition to conductive PEDOT:PSS suppressed electron recombination and/or promoted the hole current at the anode. The soluble PEDOT:PSS:GO composite is promising as a hole-transporting transparent conducting layer for c-Si photovoltaic applications.

The observation of reflection high energy electron diffraction (RHEED) oscillations has been proved to be a key to open the nano-world of materials, since it definitely verifies that the film growth proceeds in layer-by-layer mode with each layer thickness controllable by simply counting the number of oscillations. This enabled the fabrication of nano-engineered hetero-junctions and devices as commonly practiced for conventional semiconductors and metals. Here we report on the first observation of clear RHEED intensity oscillation in thin film fabrication of pi-conjugated molecular solid. The observation has been achieved by coupling a novel deposition method using a continuous-wave infrared laser for evaporation and a high sensitive RHEED detector, in addition to the combinatorial optimization of film deposition parameters that facilitated our preceding first success in the layer-by-layer growth of oxide thin films. Some details of system design and experimental conditions are presented to discuss the key factors for atomically controlled film growth of molecular solids. (C) 2008 The Japan Society of Applied Physics.