Zinc phthalocyanine - Influence of substrate temperature, film thickness, and kind of substrate on the morphology

Zinc phthalocyanine (ZnPc), C32H16N8Zn, is a planar organic molecule having numerous optical and electrical applications in organic electronics. This work investigates the influence of various deposition parameters on the morphology of vapour thermal evaporated ZnPc films. For this purpose, ZnPc is deposited at different substrate temperatures up to 90 °C and film thickness up to 50 nm onto various substrates. The morphology of this ZnPc layers is characterised by X-ray diffraction (XRD), X-ray reflectivity (XRR) and atomic force microscopy (AFM) methods. XRD measurements show that all ZnPc films are crystalline in a triclinic (α-ZnPc) or monoclinic (γ-ZnPc) phase, independent from the kind of substrate, layer thickness, or substrate temperature. The ZnPc powder, the starting product for the thermally evaporated ZnPc films, is present in the stable monoclinic β-phase. Thus, the stacking of the ZnPc molecules changes completely during deposition. The crystallite size perpendicular to the substrate determined by XRD microstructure analysis is in the range of the layer thickness while the lateral size, obtained by AFM, is increasing with substrate temperature and film thickness. AFM and XRR show an increase of the layer roughness for thicker ZnPc layers and higher substrate temperatures during film deposition. The strain in the ZnPc films decreases for higher substrate temperatures due to enhanced thermal relaxation and for thicker ZnPc films due to lower surface tension. © 2011 Elsevier B.V. All rights reserved

Structure and properties of nanosized electron beam deposited zirconia thin films

Thin films of amorphous zirconium oxide are deposited in high vacuum via the electron gun evaporation of pure ZrO2 within a large scale of vapour incidence angles. A nanosized grain surface structure and columnar growth morphology are revealed by electron optical methods. It is demonstrated that the oblique deposition results in both column inclination and anisotropy of the column cross section that are related to the vapour beam incidence. The revealed structure is found to influence the Knoop microhardness, DC conductivity and the effective refractive index by inducing in-plane anisotropy, which is most pronounced at high vapour incidence angles. The dependence of these properties on the vapour incidence is explained with the nanodimensionality of the film morphological features. Finally, it is shown how an excimer laser processing modifies both the structure and properties of the zirconia films. (C) 2001 Elsevier Science B.V. All rights reserved

Increase in internal quantum efficiency in small molecular oligothiophene: C60 mixed heterojunction solar cells by substrate heating

We present small molecule solar cells with α,ω-bis-(dicyanovinylene)-sexithiophene: C60 mixed heterojunctions, reaching power conversion efficiencies of 4.9±0.2%. We use substrate heating during deposition of the mixed layer to achieve an optimized morphology and show that this significantly improves the internal quantum efficiencies (IQEs) to values approaching 70%. By optical modeling, we evaluate the amount of loss due to absorption in inactive layers and show that IQE of the active layer itself is about 80%. © 2010 American Institute of Physics.</em

Correlation between morphology and performance of low bandgap oligothiophene:C60 mixed heterojunctions in organic solar cells

We investigate the end-capped oligothiophene derivative α,ω -bis-(dicyanovinylene)-sexithiophene with ethyl side chains (DCV6T) as donor material in heterojunctions with C60. The effect of the substrate temperature on the morphology and related photophysical properties of single DCV6T and mixed DCV6T:C60 layers is investigated. Single layers of DCV6T show crystalline features in UV-visible absorption and x-ray diffraction when grown on a substrate heated to 90 °C. Investigations of DCV6T:C60 mixed layers by atomic force microscopy, UV-visible absorption, and photoluminescence measurements reveal that the elevated substrate temperature induces an increased phase separation between the two materials with larger domain size and higher surface roughness. Based on these observations, we present mixed heterojunction solar cells where the power conversion efficiency (ηPCE) is increased from 1.6% to 3.8% by increasing the substrate temperature from 30 to 90 °C, respectively. © 2010 American Institute of Physics

Morphology and molecular orientation of ethyl-substituted dicyanovinyl-sexithiophene films for photovoltaic applications

Enhancement of the efficiency of organic solar cell devices requires knowledge about the structure of the organic layers involved. Films of the donor material dicyanovinyl-sexithiophene bearing four ethyl side-chains at thiophenes two and five DCV6T-Et(2,2,5,5) (DCV6T-Et) are prepared by thermal evaporation in high vacuum at various thicknesses and substrate temperatures. Infrared spectroscopic ellipsometry is used for determination of the molecular orientation in the thin films grown on room temperature (RT) substrate. From simulation of the IR ellipsometric data, the film thickness and the anisotropic optical constants of the DCV6T-Et films are determined. It is found that the optical constants strongly depend on the film thickness. Different average molecular orientations are determined for a few molecules thin (4 nm) and somewhat thicker (20 nm) films. Furthermore, the evolution of the surface morphology of films deposited at elevated substrate temperatures (80°C, 100°C) is studied in comparison to the thick RT-film. Atomic force microscopy images indicate that the growth on heated substrate is accompanied by an increase in grain size and surface roughness of the films. Simultaneously, the measured optical absorption spectra display structured and increased absorption in the red spectral region for the DCV6T-Et films deposited at higher substrate temperatures. The changes in surface topography and optical response relate to improved molecular arrangement induced by the substrate heating. To demonstrate the morphological influence on solar cell performance, we finally discuss DCV6T-Et/C60 planar heterojunction solar cells composed of DCV6T-Et films deposited at different substrate temperatures. © 2012 Elsevier B.V

Effect of film thickness, type of buffer layer, and substrate temperature on the morphology of dicyanovinyl-substituted sexithiophene films

The influence of film thickness, type of buffer underlayer, and deposition substrate temperature on the crystal structure, microstructure, and morphology of the films of dicyanovinyl-substituted sexithiophene with four butyl-chains (DCV6T-Bu4) is investigated by means of X-ray diffraction (XRD) and X-ray reflectivity methods. A neat Si wafer or a Si wafer covered by a 15 nm buffer underlayer of fullerene C60 or 9,9-Bis[4-(N,N-bis-biphenyl-4- yl-amino)phenyl]-9H-fluorene (BPAPF) is used as a substrate. The crystalline nature and ordered molecular arrangement of the films are recorded down to 6 nm film thickness. By using substrates heated up to 90 °C during the film deposition, the size of the DCV6T-Bu4 crystallites in direction perpendicular to the film surface increases up to value of the film thickness. With increasing deposition substrate temperature or film thickness, the DCV6T-Bu4 film relaxes, resulting in reducing the interplane distances closer to the bulk values. For the films of the same thickness deposited at the same substrate temperature, the DCV6T-Bu4 film relaxes for growth on Si to BPAPF to C60. Thicker films grown at heated substrates are characterized by smaller density, higher roughness and crystallinity and better molecular ordering. A thin (up to about 6 nm-thick) intermediate layer with linear density-gradient is formed at the C60/DCV6T-Bu4 interface for the films with buffer C60 layer. The XRD pattern of the DCV6T-Bu4 powder is indexed using triclinic unit cell parameters. © 2011 Elsevier B.V. All rights reserved

Optimization of organic tandem solar cells based on small molecules

Organic solar cells (OSC) have attracted growing attention in recent years and their development has reached a stage at which several companies are preparing to make them commercially available either as standalone products or integrated into other device. There are different production routes for OSC: one very promising approach uses thermal evaporation of small organic molecules in vacuum, i.e. the same approach that is used in all current commercial manufacturing of organic LEDs. We use vacuum processing to create an organic stack in the p-i-n concept. In this concept the intrinsic absorber layers are sandwiched between p- and n-doped wide gap transport layers which leads to a nearly ideal solar cell structure and offers a stable platform both for investigation of fundamental processes and device optimization. In recent years it was found that the device operation crucially depends on the morphology of the bulk heterojunction and that optical interference effects in the organic stack play an important role for light absorption. We show here how the morphology of the organic layers can be controlled in vacuum deposited layers, and describe the optical optimization of tandem solar cells, for which an efficient recombination contact and current matching are essential requirements. Applying these principles and subsequently combining two complementary absorbing subcells lead to a tandem organic solar cell with an independently certified efficiency of 6.07% on 2cm2 device area, i.e. the first OSC over 6% on module relevant dimensions. © 2010 IEEE

Dicyanovinyl sexithiophene as donor material in organic planar heterojunction solar cells: Morphological, optical, and electrical properties

We study the morphology and optical properties of vacuum deposited films of the α-ω-bis-(dicyanovinylen)-sexithiophene, comprising four butyl side chains DCV6T-Bu(1,2,5,6) (DCV6T-Bu). An absorption band showing vibronic substructure indicates ordered molecular arrangement in the solid state. The room temperature (RT) self-organization is confirmed by X-ray diffraction (XRD). For films grown on heated substrates, XRD analysis and atomic force microscopy display increased crystallinity with larger domain size. In correlation to the XRD data, with increasing substrate temperature the absorption of the heated films becomes more structured and continuously shifts to longer wavelengths. Further, the hole mobility in DCV6T-Bu/C60 planar heterojunction (PHJ) devices, utilizing DCV6T-Bu films grown at RT and elevated substrate temperature is investigated using the charge extraction by linearly increasing voltage method. The derived values of the activation energy are consistent with the corresponding DCV6T-Bu film morphology. However, the charge carrier mobility does not increase with improving molecular order, as is evident by the obtained mobility values of 1.0×10-6cm2/Vs for the RT and 3.1×10-7cm2/Vs for the heated device, respectively. Finally, DCV6T-Bu/C60 PHJ solar cells consisting of absorber layers deposited on heated and unheated substrates are compared. © 2011 Elsevier B.V