Ellagic Acid Templating Reveals Molecular Reorientation Mechanisms in Organic Photovoltaics Using the J.A. Woollam RC2 Spectroscopic Ellipsometer

The microstructure of organic small molecule (SM) thin films plays a critical role in the performance of organic photovoltaics, influencing processes such as light absorption, charge transport and recombination. 

Recent research demonstrates how ellagic acid (EA), a naturally derived templating layer, modifies the morphology and optoelectronic properties of vacuum thermally evaporated DCV5T-Me(3,3) donor films. 

The introduction of an EA interlayer induces a molecular reorganisation of the donor material, promoting a transition from an edge-on to a face-on molecular packing orientation. The templating effect extends throughout the donor film, showing how a thin molecular interface can influence film growth and nanoscale organisation beyond the immediate template region. 

Optical characterisation using the J.A. Woollam RC2 Variable Angle Spectroscopic Ellipsometer enabled the analysis of the optical response of the templated films and the determination of their optical constants. 

Scientific Highlights 

• A thin ellagic acid templating layer induces an edge-on to face-on molecular reorientation in DCV5T-Me(3,3) donor films;
• The templating effect extends beyond the donor-template interface, influencing molecular organisation throughout the film; 
• Ellagic acid promotes H-aggregation and modifies the optical response of the donor material;
• A 5 nm EA layer increases short-circuit current density by 12% in bulk heterojunction devices. 

Optical Characterisation of Templated Thin Films with the J.A. Woollam RC2 

Variable Angle Spectroscopic Ellipsometry (SE), performed using the J.A. Woollam RC2, was used to characterise the optical properties of the templated thin films. 

Reflectance measurements were collected at multiple angles of incidence, complemented by transmission-mode measurements to analyse the optical response across an extended wavelength range. 

The experimental data were analysed using CompleteEASE® software with Kramers–Kronig consistent B-spline models to determine the optical constants of the films. Transmission-mode ellipsometry also helped identify the spectral contribution associated with the ellagic acid layer. 

The combination of reflection and transmission measurements provided detailed optical information on the organic semiconductor layers and supported the analysis of the templated thin-film structures. 

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To learn how the J.A. Woollam RC2 can support your research activities, contact our expert Marwan Channab or visit the dedicated product page.