• Energy Research

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.

Accurate characterization and reporting of organic photovoltaic (OPV) device performance reniains one of the important challenges in the field. The large spread among the efficiencies of devices with the same structure reported by different groups is significantly caused by different procedures and equipment used during testing. The presented article addresses this issue by offering a new method of device testing using "suitcase sample" approach combined with outdoor testing that limits the diversity of the equipment, and a strict measurement protocol. A round robin outdoor characterization of roll-to-roll coated OPV cells and modules conducted among 46 laboratories worldwide is presented, where the samples and the testing equipment were integrated in a compact suitcase that served both as a sample transportation tool and as a holder and test equipment during testing. In addition, an internet based coordination was used via plasticphotovoltaics.org that allowed fast and efficient communication among participants and provided a controlled reporting format for the results that eased the analysis of the data. The reported deviations among the laboratories were limited to 5% when compared to the Si reference device integrated in the suitcase and were up to 8% when calculated using the local irradiance data. Therefore, this method offers a fast, cheap and efficient tool for sample sharing and testing that allows conducting outdoor measurements of OPV devices in a reproducible manner. (C) 2014 Elsevier B.V. All rights reserved.

Abstract Organic/inorganic hybrid solar cells are an interesting type of polymer based solar cells, which combine beneficial properties of inorganic semiconductors with them of polymer based materials, in particular, the easy processability on flexible plastic substrates. Herein, we present a method to prepare polymer/copper indium sulfide hybrid solar cells on flexible PET substrates via the metal xanthate route for the in situ preparation of ligand-free inorganic nanocrystals directly in the conjugated polymer matrix. The issue that the temperatures needed for the formation of the inorganic nanoparticles were too high to apply this preparation route on flexible substrates, was solved in this study by adding n-hexylamine to the precursor solution which facilitates the formation of higher crystalline nanoparticles at lower temperatures. n-Hexylamine thereby reacts with the xanthate forming the corresponding O-2,2-dimethylpentan-3-yl-N-hexylthiocarbamate. Following this modified route, flexible hybrid solar cells with power conversion efficiencies of 1.6% could be realized using temperatures not higher than 140 °C in the whole fabrication process. Furthermore, we demonstrate that the metal xanthate route is also well suited for the fabrication of larger area solar cells and present hybrid solar cell modules on glass as well as on flexible PET substrates. In addition, the lifetime of the prepared solar cells was investigated. The devices prepared at low temperature exhibited significantly improved stability compared to devices fabricated at 195 °C.

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 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.

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.