• Energy Research

Abstract Pyrite (Eg=0.95 eV) is being developed as a solar energy material due to its environmental compatibility and its very high light absorption coefficient. A compilationof material, electronic and interfacial chemical properties is presented, which is considered relevant for quantum energy conversion. In spite of intricate problems existing within material chemistry, high quantum efficiencies for photocurrent generation (>90%) and high photovoltages (≈500 mV) have been observed with single crystal electrodes and thin layers respectively. The most interesting aspect of this study is the use of pyrite as an ultrathin (10–20 nm) layer sandwiched between large gap p-type and n-type materials in a p-i-n like structure. Such a system, in which the pyrite layer only acts as photon absorber and mediates injection of excited electrons can be defined as sensitization solar cell. The peculiar electron transfer properties of pyrite interfaces, facilitating interfacial coordination chemical pathways, may turn out to be very helpful. Significant research challenges are discussed in the hope of attracting interest in the development of solar cells from this abundant material.

Abstract Structural and compositional properties of Zn(Se,OH)/Zn(OH)2 buffer layers deposited by chemical bath deposition(CBD) on Cu(In,Ga)(S,Se)2 (CIGSS) absorbers are investigated. Due to the aqueous nature of the CBD process, oxygen and hydrogen were incorporated into the ‘ZnSe’ buffer layer mainly in the form of Zn(OH)2 as is shown by X-ray photoelectron spectroscopy and nuclear reaction analysis (NRA) measurements leading to the nomenclature ‘Zn(Se,OH)’. Prior to the deposition of Zn(Se,OH), a zinc treatment of the absorber was performed. During that treatment a layer mainly consisting of Zn(OH)2 grew to a thickness of several nanometer. The whole buffer layer therefore consists of a Zn(Se,OH)/Zn(OH)2 structure on CIGSS. Part of the Zn(OH)2 in both layers (i.e. the Zn(Se,OH) and the Zn(OH)2 layer) might be converted into ZnO during measurements or storage. Scanning electron microscopy pictures showed that a complete coverage of the absorber with the buffer layer was achieved. Transmission electron microscopy revealed the different regions of the buffer layer: An amorphous area (possibly Zn(OH)2) and a partly nanocrystalline area, where lattice planes of ZnSe could be identified. Solar cell efficiencies of ZnO/Zn(Se,OH)/Zn(OH)2/CIGSS devices exceed 14% (total area).

Abstract The development of inorganic solid-state nanostructured solar cells over the last years has been reviewed with respect to concepts and materials. Major attention has been paid to solar cells with extremely thin absorber, solar cells with ultra-thin nano-composite absorber and solar cells with quantum dot absorber layers. The focus has been set to structured transparent electron conductors and absorber materials prepared by mainly low-temperature and wet chemical deposition methods. The great potential of inorganic solid-state nanostructured solar cells is discussed.

Abstract ZnO thin films were deposited on either indium tin oxide-coated glass or copper substrate by the electrodeposition process, using zinc chloride and flowing air as precursors. The effect of pH on the structural and morphological ZnO films was studied and the optimum deposition conditions have been outlined. The kinetics of the growth of the films have been investigated. We note that the rate of deposition of ZnO in an acidic solution was larger than in a basic solution. The structure of the films was studied using X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The surface morphology and thickness of the films were determined using scanning electron microscopy. The X-ray diffraction analysis shows that the films are polycrystalline with hexagonal crystal structure (zincite) at pH 4. The optical transmittance of ZnO decreases with varying film thickness. The optical energy bandgap was found to be 3.26 eV.

Abstract Photoactive epitaxial layers of FeS2 were grown using bromine as a transport agent and a simple closed ampoule technique. The substrates used were (100)-oriented slices of natural pyrite 1 mm thick. A vapor-liquid-solid (VLS) growth mechanism was elucidated by means of optical microscopy. Macrosteps, terrace surfaces and protuberances are often accompanied with the presence of liquid FeBr3 droplets. In the absence of a liquid phase growth hillocks are found. Localized photovoltaic response for the evaluation of carrier collection using a scanning laser spot system has been used to effectively locate and characterize non-uniformities present in the epitaxial thin films.

In this paper, we present the fabrication of Cu2ZnSn(S, Se)4 (CZTSSe) thin film absorbers by a four-step solution process based on ZnS, SnS and Cu3SnS4 nanoparticle precursors and their application in thin film solar cells. The influence of ligand-exchange process on the morphologies of the resulting CZTSSe thin films was studied. Ligand exchange with each sequential spun coat layers leads to cracking films which can be avoided by combining ligand-exchanged and non-ligand-exchanged processes. Moreover, a two-step annealing process yields the most homogeneous films. CZTSSe thin films consisting a large grain and fine nanoparticle grain layered structure was formed. The formation of layered structure for the absorbers was found to be due to the existence of high content of carbon left near the back contact and the out diffusion of Cu and Zn from the bottom layer to the surface layer. As a result, solar cell conversion efficiency was improved from 1.2% to 3.0% upon adoption a two-step annealing process. Temperature dependent I–V characteristic analysis reveals the dominant loss mechanism of the solar cells is the strong CZTSSe and CdS buffer interface recombination.

AbstractThin film solar cells already benefit from significant material and energy savings. By using photon management, the conversion efficiency and the power density can be enhanced further, including a reduction of material costs. In this work, micrometer‐sized Cu(In,Ga)Se2 (CIGS) thin film solar cells were investigated under concentrated white light illumination (1–50×). The cell design is based on industrially standardized, lamellar shaped solar cells with monolithic interconnects (P‐scribe). In order to characterize the shunt and serial resistance profiles and their impact on the device performance the cell width was reduced stepwise from 1900 to 200 µm and the P1‐scribe thickness was varied between 45 and 320 µm. The results are compared to macroscopic solar cells in standard geometry and dot‐shaped microcells with ring contacts. Under concentrated white light, the maximal conversion efficiency could be increased by more than 3.8% absolute for the lamellar microcells and more than 4.8% absolute in case of dot‐shaped microcells compared to their initial values at 1 sun illumination. The power density could be raised by a factor of 51 and 70, respectively. But apparently, the optimum concentration level and the improvement in performance strongly depend on the chosen cell geometry, the used contact method and the electrical material properties. It turns out, that the widely used industrial thin film solar cell design pattern cannot simply be adapted to prepare micro‐concentrator CIGS solar modules, without significant optimization. Based on the experimental and simulated results, modifications for the cell design are proposed. Copyright © 2015 John Wiley & Sons, Ltd.

Abstract Photo-induced electron transfer to FeS 2 ( E g = 0.9 eV) (which can be stabilized efficiently by iodide against corrosion at high photocurrent densities (1–2 A cm −2 )) was studied using rotating ring disc techniques. With decreasing efficiency I −1 , Br − and Cl − compete with water and OH − ions which form photocorrosion products (iron hydroxide, sulfate). I − is very effective in stabilizing n-FeS 2 against corrosion even at high photocurrent densities (100% at 1–2 A cm −2 ). The production efficiency for liberating chlorine (from a 3 M KCl solution at pH 0) is still 73%. Remarkably the electron transfer from Fe 2+ and Mn 2+ is more than one order of magnitude less efficient, suggesting that specific surface interaction between negative electron donors and iron d -states is involved. Evidently photo-induced electron transfer via a complex formation between surface transition metals and electron donating ligands has a clear advantage over the outer sphere electron transfer mechanism.

Abstract In the present contribution we report on recent work covering Zn(S,O) buffer as heterojunction partner layer applied to pilot line low-gap Cu(In,Ga)(SSe)2 (CIGSSe, Eg = 1.03 eV) and production scale wide-gap CuInS2 (CIS, Eg = 1.54 eV). We highlight the crucial role that the processing control of the Zn(S,O) plays for the fabrication of Cu-chalcopyrite solar cells and modules. The analytical information obtained by the correlation with state-of-the art high resolution Transmission electron microscopy, X-ray photoemission and Auger spectroscopy (XPS and XAES) as well as L-edge XAS are discussed. A large number of efficient laboratory-scale solar cells and monolithically interconnected prototype CIGSSe and CIS modules are produced. The efficiencies are comparable to the CdS base line references or even higher. The electrical, electronic properties and the emerging phenomena in Cd-free devices such as light soaking are discussed.