Advanced Functional Materials and Devices (AFMD) Group

Abstract:

Procedures for testing organic solar cell devices and modules with respect to stability and operational lifetime are described. The descriptions represent a consensus of the discussion and conclusions reached during the first 3 years of the international summit on OPV stability (ISOS). The procedures include directions for shelf life testing, outdoor testing, laboratory weathering testing and thermal cycling testing, as well as guidelines for reporting data. These procedures are not meant to be qualification tests, but rather generally agreed test conditions and practices to allow ready comparison between laboratories and to help improving the reliability of reported values. Failure mechanisms and detailed degradation mechanisms are not covered in this report. © 2011 Elsevier B.V. All rights reserved.

Abstract:

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 × 10-6 cm2/V s for the RT and 3.1 × 10-7 cm2/V s 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. All rights reserved.

Abstract:

The dissociation of excitons and the creation of charges in the active bulk layer in small molecule organic solar cells is significantly influenced by the morphology of the active layer. Here, we influence the active layer morphology of dicyanovinyl-quinquethiophene (donor): fullerene C60 (acceptor) blend layers by deposition on heated substrates. The signatures of the donor cations and triplet excitons are investigated by photoinduced absorption spectroscopy (PIA) at different measurement temperatures. With increasing measurement temperatures, we observe a decrease in triplet exciton generation rate, accompanied by an increase in cation generation rate. At room temperature we compare the dynamics of donor cations in blend layers deposited at room temperature (Tsub = 30 °C) and blend layers deposited on a heated substrate (Tsub = 80 °C) by PIA. The cation lifetime (≈10 - 100 μs) is significantly increased in the heated layer (Tsub = 80 °C), whereas the cation generation rate is decreased in the heated layer compared to the unheated layer (Tsub = 30 °C). Impedance spectroscopy of heated (Tsub = 80 °C) and unheated (T sub = 30 °C) solar cells exhibits a similar increase in carrier lifetime for the heated layer. Furthermore, we determine the lifetimes of free (1-5 μs) and trapped charges (1 s) by impedance spectroscopy and hence assign the optically detected cation signatures to shallow trap states. © 2011 Elsevier B.V. All rights reserved.

Abstract:

Small molecule organic solar cells were studied with respect to water and oxygen induced degradation by mapping the spatial distribution of reaction products in order to elucidate the degradation patterns and failure mechanisms. The active layers consist of a 30 nm bulk heterojunction formed by the donor material zinc-phthalocyanine (ZnPc) and the acceptor material Buckminsterfullerene (C60) followed by 30 nm C60 for additional absorption. The active layers are sandwiched between 6 nm 4,7-diphenyl-1,10-phenanthroline (Bphen) and 30 nm N,N′-diphenyl-N, N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine p-doped with C60F36 (MeO-TPD:C60F36), which acted as hole transporting layer. Indium-tin-oxide (ITO) and aluminum served as hole and electron collecting electrode, respectively. Time-of-flight secondary ion mass spectrometry (TOF-SIMS) and X-ray photoelectron spectroscopy (XPS) in conjunction with isotopic labeling using H218O and 18O2 provided information on where and to what extent the atmosphere had reacted with the device. A comparison was made between the use of a humid (oxygen free) atmosphere, a dry oxygen atmosphere, and a dry (oxygen free) nitrogen atmosphere during testing of devices that were kept in the dark and devices that were subjected to illumination under simulated sunlight. It was found that water significantly causes the device to degrade. The two most significant degradation mechanisms are diffusion of water through the aluminum electrode resulting in massive formation of aluminum oxide at the BPhen/Al interface, and diffusion of water into the ZnPc:C60 layer where ZnPc becomes oxidized. Finally, diffusion from the electrodes was found to have no or a negligible effect on the device lifetime. © 2011 Elsevier B.V. All rights reserved.