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Polymer solar cells undoubtedly hold tremendous potential for solutionprocessable, inexpensive photovoltaics on flexible plastic substrates. 1Ϫ5 The most promising organic photovoltaics (OPVs) are based on random dispersion of acceptors in the form of organic or inorganic nanostructures within a conjugated polymer matrix, which acts as the donor phase. The interfaces at the acceptor nanostructures and the donor host polymer matrix create bulk heterojunctions (BHJs), which provide a large number of sites for charge separation and bicontinuous pathways for efficient carrier transport. 6Ϫ8 Although numerous combinations of acceptors and donors have been reported, 9Ϫ14 the most popular BHJ polymer solar cells consist of poly(3-hexylthiophene) (P3HT) and fullerene derivative phenyl-C61-butyric acid methyl ester (PCBM) blends. The mechanisms for achieving high photovoltaic efficiencies in excess of 6% in P3HT:PCBM BHJ devices are well-established. 15 However, several key components remain unresolved and must be addressed if the theoretical efficiencies of ϳ10% are to be realized. 16 For example, in a simple BHJ device, both the donor and acceptor phases are in direct electrical contact with the cathode and anode electrodes, leading to recombination of carriers and current leakage. To minimize such detrimental effects, electron blocking and hole transport layers (HTLs) are deposited on top of the transparent and conducting indium tin oxide (ITO) anode. HTLs must be wide band gap p-type materials, and several inorganic materials such as V 2 O 5 and MoO 3 17 have been reported with NiO being the most effective, yielding efficiencies greater than 5%. 18 However, inorganic HTLs are deposited using vacuum deposition

Tuning to G(O) flat: Photoluminescence in graphene oxide (GO) suspensions can be tuned from red to blue emission (see scheme) by gradually changing the amounts of sp2‐ and sp3‐bonded carbon atoms through reduction of the surface oxide groups. Electron–hole recombination from two different types of excited states is proposed to explain the luminescence in GO at varying degrees of reduction.

Hydrogen evolution reaction (HER) from water through electrocatalysis using cost-effective materials to replace precious Pt catalysts holds great promise for clean energy technologies. In this work we developed a highly active and stable catalyst containing Co doped earth abundant iron pyrite FeS(2) nanosheets hybridized with carbon nanotubes (Fe(1-x)CoxS(2)/CNT hybrid catalysts) for HER in acidic solutions. The pyrite phase of Fe(1-x)CoxS(2)/CNT was characterized by powder X-ray diffraction and absorption spectroscopy. Electrochemical measurements showed a low overpotential of ∼0.12 V at 20 mA/cm(2), small Tafel slope of ∼46 mV/decade, and long-term durability over 40 h of HER operation using bulk quantities of Fe(0.9)Co(0.1)S(2)/CNT hybrid catalysts at high loadings (∼7 mg/cm(2)). Density functional theory calculation revealed that the origin of high catalytic activity stemmed from a large reduction of the kinetic energy barrier of H atom adsorption on FeS(2) surface upon Co doping in the iron pyrite structure. It is also found that the high HER catalytic activity of Fe(0.9)Co(0.1)S(2) hinges on the hybridization with CNTs to impart strong heteroatomic interactions between CNT and Fe(0.9)Co(0.1)S(2). This work produces the most active HER catalyst based on iron pyrite, suggesting a scalable, low cost, and highly efficient catalyst for hydrogen generation.