Advanced Functional Materials and Devices (AFMD) Group

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.

Abstract:

In this contribution the effect of changes in the activematerial system and modification of contact properties in organic solar cells is investigated at the example of ZnPc:C60 small molecule solar cells. Devices with different blend mixing ratios and a variation of hole transport layer are prepared by vacuum deposition and compared to drift-diffusion simulation data. It is shown that the open circuit voltage is mainly defined by themixing ratio,whereas the fill factor is strongly influenced by the choice of hole transport layer. Extraction barriers for photogenerated holes lead to S-shaped IV curves. The strength of the S-shape scales with the height of the extraction barrier. The slope of the IV curves at open circuit suggests that the observed increase in open circuit voltage with a higher amount of C60 in the blend might be due to a downshift of the highest occupied molecular orbital of ZnPc. © 2011 Society of Photo-Optical Instrumentation Engineers (SPIE).

Abstract:

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.