C. Frank

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%.

Structure and morphology of coevaporated pentacene-perfluoropentacene thin films.

The structural properties of coevaporated thin films of pentacene (PEN) and perfluoropentacene (PFP) on SiO(2) were studied using x-ray reflectivity and grazing incidence x-ray diffraction. Reciprocal space maps of the coevaporated thin films with different volume fractions reveal the coexistence of two different molecular mixed PEN-PFP phases together with the pure PEN and PFP crystallites. The crystal structure of PEN:PFP blends does not change continuously with volume fraction, instead the proportion of the appropriate phases changes, as seen from the diffraction analysis. Additional temperature dependent experiments reveal that the fraction of the two mixed PEN-PFP phases varies with growth temperature. The λ-phase (molecular plane parallel to the substrate) is metastable and induced by low growth temperature. The σ-phase (molecular plane nearly perpendicular to the substrate) is thermally stable and nucleates predominantly at high growth temperatures.

Simultaneous in situ measurements of x-ray reflectivity and optical spectroscopy during organic semiconductor thin film growth

Simultaneous in situ real-time measurements of x-ray reflectivity and differential reflectance spectroscopy were conducted during deposition of perfluorinated copper-phthalocyanine thin films on SiO2/Si. We found a continuous spectral change coinciding with structural changes from submonolayer coverage, to standing βbilayer-phase and to β-phase for thicker films. This combined measurement enables us to study the relationship between structural and optical properties of organic semiconductor thin films.

Chain-length dependent growth dynamics of n-alkanes on silica investigated by energy-dispersive x-ray reflectivity in situ and in real-time

We compare the growth dynamics of the three n-alkanes C(36)H(74), C(40)H(82), and C(44)H(90) on SiO(2) using real-time and in situ energy-dispersive x-ray reflectivity. All molecules investigated align in an upright-standing orientation on the substrate and exhibit a transition from layer-by-layer growth to island growth after about 4 monolayers under the conditions employed. Simultaneous fits of the reflected intensity at five distinct points in reciprocal space show that films formed by longer n-alkanes roughen faster during growth. This behavior can be explained by a chain-length dependent height of the Ehrlich-Schwoebel barrier. Further x-ray diffraction measurements after growth indicate that films consisting of longer n-alkanes also incorporate more lying-down molecules in the top region. While the results reveal behavior typical for chain-like molecules, the findings can also be useful for the optimization of organic field effect transistors where smooth interlayers of n-alkanes without coexistence of two or more molecular orientations are required.

Post-growth surface smoothing of thin films of diindenoperylene

We applied in situ x-ray reflectivity and ultraviolet photoelectron spectroscopy to study the impact of annealing on low temperature (200 K) deposited organic thin films of diindenoperylene (DIP) on SiO2 and indium tin oxide (ITO). At 200 K, DIP is crystalline on SiO2 and amorphous on ITO. Upon heating to room temperature, the roughness of DIP is reduced on both substrates, from 1.5 nm to 0.75 nm (SiO2) and from 0.90 nm to 0.45 nm (ITO). The smoothing is accompanied by crystallization of the surface molecules, whereas the bulk structure of the films does not strongly reorganize.

Growth of Competing Crystal Phases of α‑Sexithiophene Studied by Real-Time in Situ X‑ray Scattering

We report on a real-time in situ study of the growth of α-sexithiophene on silicon oxide substrates. Synchrotron-based X-ray diffraction experiments were performed during and directly after the growth in order to monitor the growth process. We observed a coexistence of two different crystal phases for different substrate temperatures. For films prepared at 233 and 308 K a disordered phase (β-phase) seems to be dominant compared to films prepared at 373 K where the so-called low- temperature bulk crystal phase (LT-phase) is dominant. From real-time measurements during growth we observed a temperature and film thickness dependent effect on the fraction of both phases in one sample. At 373 K the film growth begins primarily in the β-phase, and above a certain thickness the film growth proceeds mainly in the LT-phase. However, at 308 K the film growth is dominated by the β-phase for the entire thickness. We show that for kinetically limited growth conditions (high deposition rate and/or low substrate temperature) substrate induced growth effects are dominant.

Analysis of island shape evolution from diffuse x-ray scattering of organic thin films and implications for growth

Understanding the growth of organic semiconducting molecules with shape anisotropy is of high relevance to the processing of optoelectronic devices. This work provides insight into the growth of thin films of the prototypical rodlike organic semiconductor diindenoperylene on a microscopic level by analyzing in detail the film morphology. We model our data, which were obtained by high-resolution grazing incidence small-angle x-ray scattering, using a theoretical description from small-angle scattering theory derived for simple liquids. Based on form-factor calculations for different object types, we determine how the island shapes change in the respective layers. Atomic force microscopy measurements approve our findings.

Real-time X-ray scattering studies on temperature dependence of perfluoropentacene thin film growth

We report on real-time scattering investigations of growth of thin films of Perfluoropentacene (PFP) and its dependence on the substrate temperature, ranging between −120°C and 60°C. All films were grown up to 50 nm on silicon oxide. We find that along with the known thin-film phase, there is also a coexisting molecular arrangement with a unit cell twice the size with respect to the long axis. Furthermore, we observe that even at temperatures as low as −20°C PFP shows a high degree of crystallinity in the out-of-plane direction. The growth of PFP is characterized by a two-stage process, where the molecular lattice experiences a much stronger in-plane relaxation in the thickness regime 0–19 nm compared to the thickness regime 19–50 nm, which can be probed only by in situ real-time scattering measurements.

Influence of C60 co-deposition on the growth kinetics of diindenoperylene–From rapid roughening to layer-by-layer growth in blended organic films

We investigated the growth of the two phase-separating materials diindenoperylene (DIP) and buckminsterfullerene C60 with different mixing ratio in real-time and in situ by X-ray scattering experiments. We found that at room temperature, mixtures with an excess of DIP show a growth mode which is very close to the perfect layer-by-layer limit with DIP crystallites forming over the entire film thickness. An unexpected increase in the island size is observed for these mixtures as a function of film thickness. On the other hand, equimolar and C60 dominated mixtures grow with poor crystallinity but form very smooth films. Additionally, it is observed that higher substrate temperatures lead to an increase in the length scale of phase separation with film thickness.