Christopher Lorch

Solvent vapor annealing on perylene-based organic solar cells†

Diindenoperylene (DIP) and tetraphenyldibenzoperiflanthene (DBP) are two commonly used donor materials in organic solar cell devices. Despite their structural similarities, DIP films are crystalline, exhibiting good charge and exciton transport, whereas DBP films are amorphous and have lower carrier mobility and a short exciton diffusion length. However, DBP reveals a distinctly higher absorption due to the lying orientation of its transition dipole moments. In this paper, we investigate the influence of solvent vapor annealing (SVA) on the solar cell performance of both materials. In general, SVA induces a partial re-solubilization of the material leading to enhanced crystallinity of the treated layer. For DBP, extended annealing times result in a strong aggregation of the molecules, creating inhomogeneous layers unfavorable for solar cells. However, in DIP cells, SVA leads to an increase in fill factor (FF) and also a slight increase in short-circuit current density (JSC) due to interface roughening. The best results are obtained by combining solvent vapor annealed DIP layers with strongly absorbing DBP and C70 on top. Through this device architecture, we obtain the same increase in FF in addition to a higher gain in JSC, elevating the power conversion efficiency by a factor of 1.2 to more than 4%.

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.

Structure formation in perfluoropentacene:diindenoperylene blends and its impact on transient effects in the optical properties studied in real-time during growth

We discuss the result of the competing effects of favourable intermolecular interactions and steric incompatibilities due to the size mismatch of perfluoropentacene (PFP) and diindenoperylene (DIP) on the structure formation and associated optical properties in mixed films. Using real-time grazing incidence X-ray diffraction we investigate the size of coherently scattering islands l(s) as a function of film thickness and mixing ratio. We find that for PFP:DIP 1:2 blends l(s) is by a factor of ~4 smaller than in pure DIP films, while l(s) of the PFP:DIP 2:1 blends is not significantly reduced compared with pure PFP. Yet, we observe an increase in l(s) with film thickness for all of the samples, independent on the mixing ratio. In parallel with the structural characterization we investigate the evolution of the absorption spectra in the visible spectral range and its dependence on l(s) in situ during film growth using differential reflectance spectroscopy. We observe a surprisingly strong effect of changes in the structural order on the shape of ε(2, xy)(E), evident by a pronounced evolution of characteristic peaks in the thickness range from 1.6 nm to 9.6 nm. The combined results of the real-time experiments allow to identify the thickness dependent crystal grain size as the origin of the observed transient effects in the absorption spectra.

Growth and annealing kinetics of α-sexithiophene and fullerene C60 mixed films

Thin films of α-sexithiophene (6T) and C60 mixtures deposited on nSiO substrates at 303 and 373 K were investigated in real time and in situ during the film growth using X-ray diffraction. The mixtures are observed to contain the well known 6T low-temperature crystal phase and the β phase, which usually coexist in pure 6T films. The addition of C60 modifies the structure to almost purely β-phase-dominated films if the substrate is at 303 K. In contrast, at 373 K the low-temperature crystal phase of 6T dominates the film growth of the mixtures. Post-growth annealing experiments up to 373 K on equimolar mixtures and pure 6T films were also performed and followed in real time with X-ray diffraction. Annealing of pure 6T films results in a strong increase of film ordering, whereas annealing of equimolar 6T:C60 mixed films does not induce any significant changes in the film structure. These results lend further support to theories about the important influence of C60 on the growth behaviour and structure formation process of 6T in mixtures of the two materials.

Controlling length-scales of the phase separation to optimize organic semiconductor blends

The length-scale of phase separation in organic semiconductor donor-acceptor mixtures, while being crucially important for applications, is a non-trivial parameter to control in non-equilibrium thin film growth. We present a comprehensive study of all the important parameters that can be used to tailor the length-scale of phase separation in organic semiconductor mixtures. We employed different substrate temperatures, different growth rates, time-dependent deposition rates, and surface functionalization layers. We found not only that the substrate temperature is most prominent in influencing the length-scale of phase separation in the studied parameter range, but also that other routes can be used to tailor this length-scale.

Charge Separation at Nanostructured Molecular Donor–Acceptor Interfaces

Planar and bulk heterojunctions of organic donor and acceptor molecules are used to understand elementary processes in photovoltaic cells. The electronic structure, interface and film morphology, excitonic behavior, device characteristics, and correlations between these properties are reviewed here using a wide range of material combinations.

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.