Advanced Functional Materials and Devices (AFMD) Group

Abstract:

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-Bu 4) 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 C 60 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-Bu 4 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-Bu 4 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-Bu 4 film relaxes for growth on Si to BPAPF to C 60. 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 C 60/DCV6T-Bu 4 interface for the films with buffer C 60 layer. The XRD pattern of the DCV6T-Bu 4 powder is indexed using triclinic unit cell parameters. © 2011 Elsevier B.V. All rights reserved.

Abstract:

Efficient single bulk heterojunction organic solar cells based on blends of a fluorinated zinc phthalocyanine as electron donor and fullerene C 60 as electron acceptor are reported. In comparison to the commonly used absorber zinc phthalocyanine, the fluorination of the molecule to F 4ZnPc leads to an increase in ionisation potential and subsequently to an improvement of about 170 mV in the open circuit voltage of organic solar cells, while the short circuit current density and fill factor remain nearly unchanged. Similar to ZnPc:C 60-based devices, the device characteristics of F 4ZnPc:C 60 solar cells can be further enhanced by improving the blend layer morphology by substrate heating during deposition. F 4ZnPc is an efficient donor material that can achieve a 4.6% power conversion efficiency in single heterojunction organic solar cells. Organic solar cells with bulk heterojunctions of C 60 as acceptor and either zinc phthalocyanine (ZnPc), or fluorinated ZnPc (F 4ZnPc) as donor materials are presented. Substrate heating of F 4ZnPc:C 60 devices during film deposition strongly increases fill factor and photocurrent. Furthermore, the replacement of ZnPc by F 4ZnPc in otherwise identical devices leads to an improvement of open circuit voltage of 30%, increasing the efficiency from 3.3% (ZnPc:C 60) to 4.6% (F 4ZnPc:C 60). Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract:

The structural properties and charge carrier mobility of pentacene doped by 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ) and 2,2-(perfluoronaphthalene-2,6-diylidene) dimalononitrile (F6-TCNNQ) are studied by X-ray diffraction, scanning electron microscopy, field effect transistor measurements, and space charge limited currents (SCLC). We observe the presence of polycrystalline and amorphous domains within the doped pentacene film grown under co-deposition conditions. The appearance of the amorphous phase is induced by the molecular dopants F4-TCNQ and F6-TCNNQ. A strong drop of crystallite size is obtained at a doping concentration of around 7 and 4 wt.%, respectively. The loss of the polycrystalline structure is correlated to a strong decrease of the charge carrier mobility in pentacene in horizontal and vertical film structures. We discuss typical scenarios of charge transport for polycrystalline and amorphous thin films in order to explain the observed loss of mobility originated by the doping induced structural phase transition. In this way an optimum doping concentration for highest conductivity with acceptable mobility is determined which can help to improve the performance of organic solar cells and organic high-frequency rectification diodes. © 2011 Elsevier B.V. All rights reserved.

Abstract:

We introduce 2,3,8,9,14,15-hexachloro-5,6,11,12,17,18-hexaazatrinaphthylene (HATNA- Cl 6) as n-dopable electron transport material (ETM) for small molecule organic solar cells. Because of its large optical energy gap of 2.7 eV and its well suited energy level positions, the material can be implemented as a semitransparent spacer layer between the reflecting metal contact and the photoactive C 60 acceptor layer in p-i-n type solar cells. By varying the ETM thickness, it is possible to shift the position of the photoactive area with respect to the interference maximum of the optical field distribution. Applying n-HATNA- Cl 6 instead of the parasitically absorbing reference ETM n- C 60 results in a considerably improved photocurrent density and accordingly in a higher efficiency. At d ETM = 100 nm the power conversion efficiency is more than doubled as it increases from (100 nm n- C 60) 0.5 to (100 nm n-HATNA- Cl 6) 1.1. © 2011 American Institute of Physics.

Abstract:

A series of dicyanovinyl-oligothiophenes are investigated concerning their thermal stability, absorption in thin film, and hole mobility. Due to very high extinction coefficients, these materials are interesting for application as donor in solar cells. The quinquethiophene DCV2-5T, which shows a hole mobility of 2.2×105 cm2/Vs, is used as donor material in a flat heterojunction organic small molecule solar cells. Despite a very thin donor layer of only 6 nm, these devices exhibit in a planar heterojunction with 15 nm C60 an efficiency of up to 2.8% with a fill factor of up to 58%, a short circuit current density of 5.2 mA/cm2, an open circuit voltage of 1.03 V, and an external quantum efficiency of 30% in the green spectral range. © 2011 Elsevier B.V. All rights reserved.