Advanced Functional Materials and Devices (AFMD) Group

Abstract:

The change of morphology in mixed layers due to different substrate temperature T of organic solar cells containing C60 and zinc phthalocyanine (ZnPc) is studied. Heating the substrate during deposition of the bulk heterojunction C60:ZnPc leads to a significant improvement of solar cell performance, mainly due to an increase in photocurrent and fill factor (FF). This is attributed to improved charge carrier percolation pathways within the C60:ZnPc blend. Using atomic force microscopy, scanning electron microscopy, transmission electron microscopy, organic field effect transistor, X-ray diffraction, and absorption measurements, we observe aggregation, cluster-like, and polycrystalline growth of the heated bulk layer. This provides better transport percolation paths by inducing a phase separation of the molecules. Heated blend layer with thickness of 60 nm shows high performance without loss in FF. When heating the substrate to the optimum temperature of 110 °C, a power conversion efficiency of 3.0% is achieved, compared to 1.4% for an identical device prepared on a substrate held at room temperature. © 2010 Elsevier B.V. All rights reserved.

Abstract:

We show that S-kinks in the current voltage characteristics, which decrease the fill factor significantly, can be caused by a strong imbalance of charge carrier mobilities (hole mobility in donor and electron mobility in acceptor) in planar/flat heterojunction organic solar cells. Electrical simulations according to a drift-diffusion model predict the occurrence of an S-kink for a mobility mismatch factor larger than 100. By combining a low-mobility donor material, (1,2,3,4,9,10,11,12-octaphenyl-diindeno [1,2,3-cd: 1′, 2′, 3′ -lm]perylene), with the acceptors C60 and N, N′ -dimethylperylene-3,4:9,10-dicarboximide, which show different electron mobilities, we experimentally verify the predictions. Our results demonstrate that not only interface effects but also the photoactive material itself can cause S-kinks. © 2011 American Institute of Physics.

Abstract:

We present the material 2,3,10,11-tetrabutyl-1,4,9,12-tetraphenyl- diindeno[1,2,3-cd:1′,2′,3′-lm] perylene (Bu4-Ph4-DIP) as alternative green donor for bulk heterojunction small molecule organic solar cells (SMOSC). It is shown that Bu4-Ph4-DIP exhibits suitable absorption characteristics to be a potential material to fill the absorption gap between the commonly used standard absorbers ZnPc and C60. Devices with bulk heterojunctions of Bu4-Ph4-DIP:C60 display very high open circuit voltages of 0.99 V, high fill factors of up to 57%, and experiments yield promising efficiencies of η>2%. Such green-blue absorbing SMOSC are characterized by current voltage and external quantum efficiency measurements, and material properties are studied. It is shown that the devices are responsive to substrate heating, and that different donoracceptor mixing ratios can increase device performance. Possible influences of mixing ratio and heating on device morphology and electrical properties are discussed.

Abstract:

The performance of transparent metal top contacts in organic solar cells can strongly be improved by employing surfactant layers. We use scanning electron microscopy to investigate the change in morphology upon insertion of an Al surfactant layer between 4,7-diphenyl-1,10-phenanthroline (BPhen) and a silver top contact. UV photoelectron spectroscopy measurements show the changes in energetic alignments at different steps of the organic/metal interface formation. Furthermore, using X-ray photoelectron spectroscopy depth profiling, we compare the differing intermixing processes happening within the two samples. Thereby, we can show that Al binds to BPhen molecules, acting as surfactant for subsequently deposited Ag layers, while Ag without any Al surfactant layer penetrates into and intermixes with the BPhen layer. © 2010 Elsevier B.V. All rights reserved.

Abstract:

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.