• Energy Research

CdS nanocrystal-sensitized solar cells (NCSSCs) were investigated by using polyaniline (PANI) as a replacement for conventional platinum counter electrode. The growth time of the NCs significantly affects the solar cell performance. At an optimum growth, the NCSSCs exhibit 0.83% of the conversion efficiency in comparison to 0.13% for the identical cells without CdS NCs. Electrochemical impedance spectroscopy showed that the charge transfer in the solar cells with CdS nanocrystals was improved. The enhanced overall energy conversion efficiency by NCs is attributed to improved light absorption and suppressed recombination rate of interfacial charges with injection, resulting in significantly improved charge transfer and electron life time. In addition, the PANI electrodes with large surface area and ideal corrosion-inertness toward polysulfide redox exhibit promising application potential as a counter electrode for NCSSCs. This study demonstrates that the solution grown CdS nanocrystals and polyaniline are potentially useful for fabricating high performance NCSSCs, which is technically attractive for large scale and economic production.

Graphene has been used as an electrically tunable material for switchable devices. A large area fabrication of Al-doped ZnO/Al2O3/graphene/Al2O3/gold/silicon device was enabled by a spin-processible hydrophilic mono-layer graphene oxide. The graphene was obtained directly from graphene oxide during the atomic layer deposition without other extra steps. A significant shift of Raman frequency up to 360 cm−1 was observed from graphene in the fabricated device, indicating a structural change in graphene. The absorption from the device was tunable with a negative voltage applied on the Al-doped ZnO side. The generated absorption change was sustainable when the voltage was off and erasable when a positive voltage was applied. The sustainability of tuned optical property in the graphene under investigation can lead to a design of device with less power consumption and many other applications.

Abstract Thin film and core–shell nanowire heterostructures containing copper indium gallium sulfide Cu(In,Ga)S2 (CIGS) as light absorber are fabricated by low cost, solution based processes. The unique structures consist of ZnO and CdS or (Zn,Mg)O core–shell nanowire arrays embedded inside CIGS thin films for solar cell applications. Nontoxic (Zn,Mg)O buffer layer deposited by atomic layer deposition is proven to be suitable for replacing solution grown CdS for fabricating CIGS nanowire solar cells. A systemic comparison of device performance is made between the CIGS nanowire solar cells and their thin film counterparties with different final treatment temperatures. Overall, the nanowire devices show higher efficiency than the corresponding planar structures fabricated under similar conditions. Effect of nanowire on the solar cell performance is further studied by using nanowires of various lengths, which reveals that the longer nanowire arrays resulted in a higher short circuit current. This study represents the first working CIGS nanowire solar cells reported so far and provides an opportunity for investigating the mechanism of nanowire solar cells using a well-defined material system.

Abstract Multiple wall carbon nanotube (MWNT) networks form Schottky-like heterojunction on n-type silicon substrate. Acid doping downshifts the Fermi level of the MWNTs, can significantly reduce the internal resistance of the MWNT film. Wetting the MWNT networks with nitric or sulfuric acid can form MWNT-acid–Si photoelectrochemical units. The photoelectrochemical units and the Si–MWNT heterojunctions connected in parallel on the same side of the Si substrate, consequently boosting the power conversion efficiency by more than 10 times.

The open-circuit voltage (V OC) in organic photovoltaic cells has been shown to depend on a number of parameters including the energy levels of the active materials, active layer structure, illumination intensity, and operating temperature. Here we report, a significant increase in V OC from 0.43 to 0.63 V in zinc phthalocyanine (ZnPc)/C60 planar heterojunction photovoltaic cells operated at room temperature under 1 sun AM1.5G solar illumination, when a home-synthesized and purified ZnPc source materials was used instead of a commercially obtained (and home purified) ZnPc source. While the two ZnPc source materials have nearly identical UV–Vis and IR absorption properties, the home-synthesized ZnPc contains chlorinated derivatives and has half of the electrical defect density (on the order of 1016 cm−3) as compared to the commercial ZnPc. The improved V OC in devices with the home-synthesized ZnPc is contributed from both a lower dark current and a higher magnitude of photocurrent. Additional experiments revealed that the different device characteristics are mostly associated with processes occurring at the ZnPc/C60 interface, which we attribute to nongeminate recombination of charges built-up on either side of the interface.

AbstractEffectively releasing the locked polysaccharides from recalcitrant lignocellulose to fermentable sugars is among the greatest technical and economic barriers to the realization of lignocellulose biorefineries because leading lignocellulose pre‐treatment technologies suffer from low sugar yields, and/or severe reaction conditions, and/or high cellulase use, narrow substrate applicability, and high capital investment, etc. A new lignocellulose pre‐treatment featuring modest reaction conditions (50°C and atmospheric pressure) was demonstrated to fractionate lignocellulose to amorphous cellulose, hemicellulose, lignin, and acetic acid by using a non‐volatile cellulose solvent (concentrated phosphoric acid), a highly volatile organic solvent (acetone), and water. The highest sugar yields after enzymatic hydrolysis were attributed to no sugar degradation during the fractionation and the highest enzymatic cellulose digestibility (∼97% in 24 h) during the hydrolysis step at the enzyme loading of 15 filter paper units of cellulase and 60 IU of beta‐glucosidase per gram of glucan. Isolation of high‐value lignocellulose components (lignin, acetic acid, and hemicellulose) would greatly increase potential revenues of a lignocellulose biorefinery. Biotechnol. Bioeng. 2007;97: 214–223. © 2007 Wiley Periodicals, Inc.