Probing the effect of substrate heating during deposition of DCV4T:C60 blend layers for organic solar cells

We present a comprehensive investigation of morphological changes inside the active layer of an organic solar cell induced by substrate heating during layer deposition by thermal evaporation in ultra-high vacuum. To explore the trends observed in solar cell devices, we apply absorption and photoluminescence spectroscopy, atomic force microscopy, X-ray diffraction, and organic field effect transistor measurements. The material combination we use comprises unsubstituted dicyanovinyl end-capped quaterthiophene (DCV4T) as the donor material mixed with C60C60 as the acceptor. The solar cell power conversion efficiency decreases with increasing substrate temperature during film deposition due to changes in the crystalline structure of the oligothiophene phase, leading to a decrease in absorption strength. Photoluminescence measurements show that substrate heating increases the amount of phase separation between the donor and acceptor, and topology and structure investigations reveal large aggregates of polycrystalline DCV4T at the surface. However, the fill factor is increased for higher substrate temperatures due to better transport properties. The highest efficiency obtained with this material combination and stack design is 3.0% under AM1.5g illumination.

Figure optionsDownload as PowerPoint slideHighlights
► Quaterthiophenes are strong green absorbers for use in organic solar cells.
► Thin film morphology is tuned by substrate heating during layer deposition.
► Strong clustering of the oligothiophene causes high roughness and phase demixing.
► XRD results reveal change in molecular packing that causes a decrease of absorption.
► PCE is reduced, so substrate heating procedure must not always be beneficial.