• Energy Research

Electron energy-loss spectroscopy (EELS) is a powerful tool for imaging chemical variations at the nanoscale. Here, we investigate a polymer/organic small molecule-blend used as absorber layer in an organic solar cell and employ EELS for distinguishing polymer donor and small molecule acceptor domains in the nanostructured blend based on elemental maps of light elements, such as nitrogen, sulfur or fluorine. Especially for beam sensitive samples, the electron dose needs to be limited, therefore optimized acquisition and data processing strategies are required. We compare data acquired on a post-column energy filter with a direct electron detection camera to data from a conventional CCD camera on the same filter and we investigate the impact of statistical data processing methods (principal components analysis, PCA) on acquired spectra and elemental maps extracted from spectrum images. Our work shows, that the quality of spectra on a direct electron detection camera is far superior to conventional CCD imaging, and thereby allows clear identification of ionization edges and the fine structure of these edges. For the quality of the elemental maps, the application of PCA is essential to allow a clear separation between the donor and acceptor phase in the bulk heterojunction absorber layer of a non-fullerene organic solar cell.

Abstract Solution processable small molecules are an alternative to conjugated polymers in organic photovoltaics and have recently been in the focus of intense research. In this work, organic–inorganic hybrid solar cells with active layers consisting of the solution-processable small-molecule p-DTS(FBTTh 2 ) 2 and copper indium sulfide nanoparticles are presented. The copper indium sulfide nanoparticles are formed in situ directly in the small molecule matrix from metal xanthate precursors. The prepared nanocomposite small molecule/copper indium sulfide films are very smooth, highlighting the good compatibility of p-DTS(FBTTh 2 ) 2 with the in situ preparation of metal sulfide nanoparticles. The formed nanoparticles have diameters of about 3 nm. Hybrid solar cells, exhibiting power conversion efficiencies of 1.3%, are characterized by IV curves, EQE spectra and electron microscopy.

Procedures for testing organic solar cell devices and modules with respect to stability and operational lifetime are described. The descriptions represent a consensus of the discussion and conclusions reached during the first 3 years of the international summit on OPV stability (ISOS). The procedures include directions for shelf life testing, outdoor testing, laboratory weathering testing and thermal cycling testing, as well as guidelines for reporting data. These procedures are not meant to be qualification tests, but rather generally agreed test conditions and practices to allow ready comparison between laboratories and to help improving the reliability of reported values. Failure mechanisms and detailed degradation mechanisms are not covered in this report. © 2011 Elsevier B.V. All rights reserved.

Abstract The performance of organic and hybrid solar cells is, in addition to the properties of the absorber layer, also strongly dependent on electrodes and interlayers extracting the charges generated in the absorber layer. Here, we investigate Ti and TiOx interlayers regarding the performance and stability of polymer/copper indium sulfide hybrid solar cells with aluminum and silver electrodes. For this, TiOx interlayers were prepared via thermal evaporation of titanium under moderate vacuum conditions in oxygen containing atmosphere, whereas Ti interlayers were deposited under high vacuum conditions. The titanium-based interlayers improved the power conversion efficiencies of solar cells with Al as well as with Ag electrodes. Also the stability of solar cells with Al electrodes was improved; however, solar cells with Ag electrodes and TiOx as well as Ti interlayers turned out to be much more stable and showed lifetimes of several thousand hours under continuous illumination.

A large number of flexible polymer solar modules comprising 16 serially connected individual cells was prepared at the experimental workshop at Risø DTU. The photoactive layer was prepared from several varieties of P3HT (Merck, Plextronics, BASF and Risø DTU) and two varieties of ZnO (nanoparticulate, thin film) were employed as electron transport layers. The devices were all tested at Risø DTU and the functional devices were subjected to an inter-laboratory study involving the performance and the stability of modules over time in the dark, under light soaking and outdoor conditions. 24 laboratories from 10 countries and across four different continents were involved in the studies. The reported results allowed for analysis of the variability between different groups in performing lifetime studies as well as performing a comparison of different testing procedures. These studies constitute the first steps toward establishing standard procedures for an OPV lifetime characterization. © 2011 Elsevier B.V. All rights reserved.

Abstract In this contribution we present an in situ method for the preparation of CuInS 2 –poly(3-(ethyl-4-butanoate)thiophene) (P3EBT) nanocomposite layers and their application in nanocomposite solar cells. A precursor solution containing copper and indium salts, thiourea and the conjugated polymer was prepared in pyridine, which was coated onto glass/ITO substrates followed by a heating step at 180 °C. The heating step induced the formation of the CuInS 2 nanoparticles homogeneously dispersed in the conjugated polymer matrix. The formation of the nanocomposite was investigated in situ by X-ray scattering techniques and TEM methods showing that nano-scaled CuInS 2 was formed. By addition of small amounts of zinc salt to the precursor solution, zinc containing CuInS 2 (ZCIS) was formed. ZCIS–P3EBT active layers exhibited higher V OC than CuInS 2 –P3EBT layers and showed efficiencies of about 0.4%. Additionally the stability of the solar cells was tested over a time scale of 172 h.

Abstract Nanoparticles of copper zinc tin sulfide (CZTS, Cu 2 ZnSnS 4 ) and copper zinc tin selenide (CZTSe, Cu 2 ZnSnSe 4 ) were prepared by a synthesis route using corresponding metal salts and sulfur or selenium. Oleylamine was used as solvent and capping agent. This fast and uncomplicated preparation method delivers amine-capped nanoparticles with a diameter of approximately 7–35 nm depending on the reaction conditions, whereas CZTSe nanoparticles are bigger than similarly synthesized CZTS nanoparticles. The nanoparticles are characterized by powder-X-ray diffraction, mass spectrometry, transmission electron microscopy, high resolution transmission electron microscopy, electron diffraction and energy dispersive X-ray spectroscopy. For CZTS the chemical composition – proven by energy dispersive X-ray spectroscopy - is in good accordance with the stoichiometric chemical composition, while in the case of CZTSe the chemical composition depends strongly on the metal salts used for the synthesis. A composition close to the stoichiometric one was achieved by selecting metal salts with appropriate reactivity. Transmission electron microscopy images, as well as high resolution transmission electron microscopy images, reveal perfect crystals for the CZTS sample, in the CZTSe samples defects in the crystal structure are observed.

Abstract Background The abundance of glucuronoxylan (GX) in agricultural and forestry residual side streams positions it as a promising feedstock for microbial conversion into valuable compounds. By engineering strains of the widely employed cell factory Saccharomyces cerevisiae with the ability to directly hydrolyze and ferment GX polymers, we can avoid the need for harsh chemical pretreatments and costly enzymatic hydrolysis steps prior to fermentation. However, for an economically viable bioproduction process, the engineered strains must efficiently express and secrete enzymes that act in synergy to hydrolyze the targeted polymers. Results The aim of this study was to equip the xylose-fermenting S. cerevisiae strain CEN.PK XXX with xylanolytic enzymes targeting beechwood GX. Using a targeted enzyme approach, we matched hydrolytic enzyme activities to the chemical features of the GX substrate and determined that besides endo-1,4-β-xylanase and β-xylosidase activities, α-methyl-glucuronidase activity was of great importance for GX hydrolysis and yeast growth. We also created a library of strains expressing different combinations of enzymes, and screened for yeast strains that could express and secrete the enzymes and metabolize the GX hydrolysis products efficiently. While strains engineered with BmXyn11A xylanase and XylA β-xylosidase could grow relatively well in beechwood GX, strains further engineered with Agu115 α-methyl-glucuronidase did not display an additional growth benefit, likely due to inefficient expression and secretion of this enzyme. Co-cultures of strains expressing complementary enzymes as well as external enzyme supplementation boosted yeast growth and ethanol fermentation of GX, and ethanol titers reached a maximum of 1.33 g L− 1 after 48 h under oxygen limited condition in bioreactor fermentations. Conclusion This work underscored the importance of identifying an optimal enzyme combination for successful engineering of S. cerevisiae strains that can hydrolyze and assimilate GX. The enzymes must exhibit high and balanced activities, be compatible with the yeast’s expression and secretion system, and the nature of the hydrolysis products must be such that they can be taken up and metabolized by the yeast. The engineered strains, particularly when co-cultivated, display robust growth and fermentation of GX, and represent a significant step forward towards a sustainable and cost-effective bioprocessing of GX-rich biomass. They also provide valuable insights for future strain and process development targets.