Karolien Vasseur

Novel bis-C60 derivative compared to other fullerene bis-adducts in high efficiency polymer photovoltaic cells

We report the application of novel mono- and bis-o-quino-dimethane C60 (oQDMC60) adducts in bulk heterojunction photovoltaic devices. When blended with poly(3-hexylthiophene), the fullerene adducts presented here have an enhanced open-circuit voltage of 640 mV and 820 mV, while preserving high short-circuit current and fill factor, resulting in efficiencies of 4.1% and 5.2%, respectively. Detailed assessment of material properties relevant to photovoltaic devices such as energy levels, charge carrier mobility, absorption and solubility further complements the evaluation. Increased fullerene solubility hindering phase segregation in blends with bis-oQDMC60 has been circumvented by an in-depth morphology optimization assisted by absorption spectroscopy, X-ray reflectivity and atomic force microscopy. This optimized preparation could also serve as a guide for implementation of similar fullerene derivatives. Furthermore, we compare bis-oQDMC60 to previously reported fullerene bis-adducts to provide insight into this emerging class of materials.

Controlling the Texture and Crystallinity of Evaporated Lead Phthalocyanine Thin Films for Near-Infrared Sensitive Solar Cells

To achieve organic solar cells with a broadened spectral absorption, we aim to promote the growth of the near-infrared (NIR)-active polymorph of lead phthalocyanine (PbPc) on a relevant electrode for solar cell applications. We studied the effect of different substrate modification layers on PbPc thin film structure as a function of thickness and deposition rate (rdep). We characterized crystallinity and orientation by grazing incidence X-ray diffraction (GIXD) and in situ X-ray reflectivity (XRR) and correlated these data to the performance of bilayer solar cells. When deposited onto a self-assembled monolayer (SAM) or a molybdenum oxide (MoO3) buffer layer, the crystallinity of the PbPc films improves with thickness. The transition from a partially crystalline layer close to the substrate to a more crystalline film with a higher content of the NIR-active phase is enhanced at low rdep, thereby leading to solar cells that exhibit a higher maximum in short circuit current density (JSC) for thinner donor layers. The insertion of a CuI layer induces the formation of strongly textured, crystalline PbPc layers with a vertically homogeneous structure. Solar cells based on these templated donor layers show a variation of JSC with thickness that is independent of rdep. Consequently, without decreasing rdep we could achieve JSC=10 mA/cm2, yielding a bilayer solar cell with a peak external quantum efficiency (EQE) of 35% at 900 nm, and an overall power conversion efficiency (PCE) of 2.9%.

Correlating the Polymorphism of Titanyl Phthalocyanine Thin Films with Solar Cell Performance.

The structure of titanyl phthalocyanine (TiOPc) thin films is correlated with photovoltaic properties of planar heterojunction solar cells by pairing different TiOPc polymorph donor layers with C60 as an acceptor. Solvent annealing and the insertion of two different templating layers, namely 1H,1H,2H,2H-perfluorodecyltrichlorosilane (FDTS) and CuI, prove to be effective methods to control the TiOPc thin film structure. The crystal phase of TiOPc thin films was identified by combining X-ray reflectivity (XRR) measurements with spectroscopic techniques, including absorption and micro-Raman measurements. Implementation of a donor layer with an absorption spectrum extending into the near-infrared (NIR) led to solar cells with external quantum efficiencies (EQEs) above 27% from λ = 600 - 890 nm, with the best device yielding a power conversion efficiency (PCE) of 2.6%. Our results highlight the need to understand the relationship between processing parameters and thin film structure, as these have important consequences on device performance.

High-speed growth of pentacene thin films by in-line organic vapor phase deposition

Taking another step towards industrial production of devices based on organic semi-conductors, this work presents an extension of the organic vapor phase deposition technique to in-line geometry. A study of the in-line tool operation is carried out. It leads to the definition of a specific in-line deposition rate that qualifies the coating speed. It also allows for an understanding of processing parameter variations that lead to high deposition speeds. As a consequence, pentacene films are grown at in-line deposition rates of up to 1055μm2/s. This corresponds to web speeds of 2.1 m/min, equivalent to an average deposition rate of 105 Å/s in a static system. These films present a high uniformity, with a thickness standard deviation below 1.2% over 4 inch diameter substrates. Moreover, with transistor mobilities of up to 1.5 cm2/Vs, these pentacene films are of excellent electrical quality. This quality is conserved up to the highest deposition rates. Finally, 5-stage ring oscillators on foil based on a pentacene thin film deposited by in-line OVPD achieve a frequency of 24 kHz at a supply voltage of 20 V.